Corrosion of Aluminum and Aluminum Alloys (#06787G)

Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 1-24 DOI: 10.1361/caaa1999p001 Copyright © 1999 ASM Int...

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Corrosion of Aluminum and Aluminum Alloys J.R. Davis, editor, p 1-24 DOI: 10.1361/caaa1999p001

Copyright © 1999 ASM International® All rights reserved. www.asminternational.org

Chapter 1

Introduction

ALUMINUM became an economic competitor in engineering applications toward the end of the 19th century. The reason aluminum was not used earlier was the difficulty of extracting it from its ore. When the electrolytic reduction of aluminum oxide (Al2O:3) dissolved in molten cryolite was independently developed by Charles Martin Hall in the United States and Paul T. Heroult in France, the aluminum industry was bom. The emergence of three important industrial developments in the late 18008 and early 1900s would, by demanding material characteristics consistent with the unique qualities of aluminum and its alloys, greatly benefit growth in the production and use of the new metal. The first of these was the introduction of the first internal-combustion-engine-powered vehicles. Aluminum would play a role as an automotive material of increasing engineering value. Secondly, electrification would require immense quantities of lightweight conductive metal for long-distance transmission and for construction of the towers needed to support the overhead network of cables that deliver electrical energy from sites of power generation. Within a few decades, a third important application area was made possible by the invention of the airplane by the Wright brothers. This gave birth to an entirely new industry which grew in partnership with the aluminum industry development of structurally reliable, strong, and fracture-resistant parts for airframes, engines, and ultimately, for missile bodies, fuel cells, and satellite components. However, .the aluminum industry growth was not limited to these developments. The first commercial applications of aluminum were novelty items such as mirror frames, house (address) numbers, and serving trays. Cooking utensils were also a major early market In time, aluminum applications grew in diversity to the extent that virtually every aspect of modem life would

be directly or indirectly affected by use. Today, aluminum is surpassed only by steel in its use as a structural material.

Key Characteristics of Aluminum Aluminum offers a wide range of properties that can

be engineered precisely to the demands of specific applications through the choice of alloy, temper, and fabrication process. The properties of aluminum and its alloys which give rise to their widespread usage include the following: • •

• • •

• • • •

•

Aluminum is light; its density is only one-third that of steel. Aluminum and aluminum alloys are available in a wide range of strength values-from highly ductile low-strength commercially pure aluminum to very tough high-strength alloys with ultimate tensile strengths approaching 690 MPa (100 ksi). Aluminum alloys have a high strength-to-weight ratio. Aluminum retains its strength at low temperatures and is often used for cryogenic applications. Aluminum has high resistance to corrosion under the majority of service conditions, and no colored salts are formed to stain adjacent surfaces or discolor products with which it comes into contact. Aluminum is an excellent conductor of heat and electricity . Aluminum is highly reflective. Aluminum is nonferromagnetic, a property of importance in the electrical and electronics industries. Aluminum is nonpyrophoric, which is important in applications involving inflammable or explosive materials handling or exposure. Aluminum is nontoxic and is routinely used in containers for food and beverages.

2 I Corrosion of Aluminum and Aluminum Alloys Strength. Commercially pure aluminum has a tensile strengthof about90 MPa (13 ksi). Thus its usefulness as a structuralmaterial in this form is somewhat limited By working the metal, as by cold rolling, its strength can be approximately doubled. Much larger increases in strengthcan be obtained by alloying aluminum with small percentages of one or more other elements such as manganese, silicon, copper, magnesium, or zinc. Like pure aluminum, the alloys are also made strongerby cold working. Some of the alloys are further strengthened and hardened by heat treatments. Figure 1 shows the range of strength levels of representativealuminumand aluminumalloys. High Strength-to-Weight Ratio. The strengthto-weightratio of aluminum is much higher than that of many common grades of constructional steelsoften double or more (Fig. 1). This property permits design and construction of strong, lightweight structures that are particularly advantageous for anything that moves-space vehiclesand aircraftas well as all types ofland- and water-borne vehicles. Corrosion Resistance. When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin formsimmediately, whichprotectsthe metalfrom further oxidation. This self-protecting characteristic gives aluminum its high resistance to corrosion. Unless exposed to some substance or condition that destroys this protectiveoxide coating, the metal remains fully protected against corrosion. Aluminumis highly resistant to weathering, even in industrial atmospheres that often corrode other metals. It is also corrosion resistant to many acids. Alkalis are among the few substances that attack the oxide skin and thereforeare corrosive to aluminum Although the metal can safely be used in the presenceof certain mild alkalis with the aid of inhibitors, in general, direct contact with alkaline substances should be avoided. The high thermal conductivity of aluminum (about 50 to 60% that of copper) came prominently intoplay in the very firstlarge-scalecommercialapplication of the metal in cooking utensils. This characteristic is important whenever the transfer of thermal energy from one medium to anotheris involved, either heating or cooling.Thus aluminumheat exchangers are

• Aluminumhas an attractive appearancein its natural finish, which can be soft and lustrous or bright and shiny. It can be virtuallyany color or texture. • Aluminumis recyclable. Aluminumhas substantial scrap value and a well-established market for recycling, providing both economic and environmental benefits. • Aluminum is easily fabricated. Aluminum can be formed and fabricated by all common metalworking and joining methods. Table 1 lists the important physical properties of pure aluminum. Table 2 shows the characteristics of aluminumand their importancefor differentend uses. Low Density. Aluminumhas a density of only 2.7 glcm3, approximately 35% that of steel (7.83 glcm3) and 30% of copper (8.93 g/cm') or brass (8.53 glcm3). One cubic foot of steel weighsabout 222 kg (490 lb); a cubic foot of aluminum weighs only about 77 kg (170 lb).

Table 1 Summary of the important physical properties of high-purity (~.95% AI) aluminum Property

Va'"

Thermalneutroncross section Latticeconstant(lengthof unit cube) Density(solid)

Density(liquid) Coefficientof expansion Thermalconductivity Volume resistivity Magneticsusceptibility Surfacetension Viscosity Meltingpoint Boilingpoint Heatof fusion Heatof vaporation Heatcapacity

0.232 ± 0.003bams 4.0496 x 1O-lO m at 298 K 2699kg/m3 (theoretical density basedon latticespacing) 2697-2699 kg/m3 (polycrystalline material) 2357kg/m3 at 973K 2304kg/m3 at 1173K 23 x Io-IK at 293 K 2.37Wlcm· Kat 298 K 2.655 x 10-8 Q. m 16 x 1O-3/m3 g/atomat 298 K 868dyne/cmat themelting point 0.012poiseat themeltingpoint 933.5K 2767K 397Jig 1.08 x 10-4 Jig' K

0.90 Jig . K

Table2 Property combinations important for the use of aluminum in various application areas Typeof semiCabrieated products

Characteristics Goodbeat and electrical

Field ofuse Transport Building Packaging Electrical Household Machines, appliances Chemicals andfood

Lightness

cooductivity

1 2 3 3 2 1

3 1 1 2

2

2

1,veryimportant;2, important;3, desirable

Deeoratiseaspeds

Resioltanee

(with orwithout

to corrosion surface treatment)

2 2

2 1

I

1

2 1 2

Castings or forgings 2

W"U'e

.Formed

Impart

Extruded

aDd

sheet

extmsions

sedions

cable

Foil

2

2 2 2

2 2

2 2 2 2

2

2 2

2

2 2

2

2

3

2

2

2

Introduction I 3 commonly used in the food, chemical, petroleum, aircraft, and other industries. High Electrical Conductivity. Aluminum is one of the two common metals having an electrical conductivity high enough for use as an electric conductor. The conductivity of electric conductor grade (1350) is about 62% that of the International Annealed Copper Standard (lACS). Because aluminum has less than one-third the specific gravity of copper, however, a pound of aluminum will go about twice as far as a pound of copper when used for this purpose. Reflectivity. Smooth aluminum is highly reflective of the electromagnetic spectrum, from radio waves through visible light and on into the infrared and thermal range. It bounces away about 80% of the visible light and 90% of the radiant heat striking its surface.

The high reflectivity gives aluminum a decorative appearance; it also makes aluminum a very effective barrier against thermal radiation, suitable for such applications as automotive heat shields. Nontoxic Characteristica. The fact that aluminum is nontoxic was discovered in the early days of the industry. It is this characteristic that permits the metal to be used in cooking utensils without any harmful effect on the body. Today a great deal of aluminum equipment is used in the food processing industry. Nontoxicity permits aluminum foil wrapping to be used safely in direct contact with food products. Finishability. For the majority ofapplications, aluminum needs no protective coating. Mechanical finishes such as polishing, sand blasting, or wire brushing meet the majority of needs. In many instances, the surface

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3OO3-H18 2014-0 1060-H18 1060-0

HK31A-H24 ZK40A-T5 AZ31B-0

Titanium alloys,

Aluminum alloys,

Magnesium alloys,

-4.5 glcm3

-2.75 glcm3

-1.8 glcm3

-7.9 glcm3

~c ~ f-

3AI-2.5V ASTM A 715 ASTM A 242 AISI 1015

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Titanium alloys

2014-T6 6061-T6 3003-H18 1060-H18 1060.0 Aluminum alloys

~ HK31A-H24 ZK40A-T5 AZ31B-0

Magnesium alloys

Comparison of aluminum alloys with competing structural alloys on • the basis of lal tensile strength and ~bl specific tensile strength ltensile strength, in ksl, divided by density, in g/cm 1

4 I Corrosion of Aluminum and Aluminum Alloys finish suppliedis entirelyadequate withoutfurtherfinishing. Where the plain aluminum surface does not suffice or where additional protection is required, any of a wide variety of surface finishes may be applied. Chemical, electrochemical, and paint finishes are all used. Many colors are availablein both chemical and electrochemical finishes. If paint, lacquer, or enamel is used, any color possible with these finishes can be applied. Vitreous enamels have been developed for aluminum, and the metal can also be electroplated. Ease of Fabrication. The ease with which aluminum can be fabricatedinto any form is one of its most important assets. Often it can compete successfully with cheapermaterialshaving a lower degreeof workability.The metal can be cast by any methodknown to foundrymen. It can be rolled to any desired thickness down to foil thinner than paper; aluminum sheet can be stamped, drawn, spun or roll-formed. The metal also can be hammered or forged. Aluminum wire, drawn from rolled rod, may be stranded into cable or any desired size and type. There is almost no limit to the different profiles (shapes) in which the metal can be extruded. The ease and speed with which aluminum can be machined is one of the important factors contributing to the low cost of finished aluminumparts.The metal can be turned, milled, bored, or machined in other manners at the maximum speeds of which most machines are capable. Another advantage of its flexible machiningcharacteristics is that aluminumrod and bar can readily be employed in the high-speed manufacture of parts by automaticscrew machines. Almost any method of joining is applicableto aluminum: riveting, welding, brazing, or soldering. A wide varietyof mechanicalaluminumfasteners simplifies the assemblyof many products.Adhesivebonding of aluminum parts is widely employed,particularly in joining aircraft components. Table3 lists fabricationcharacteristics of commonly used wroughtaluminumand aluminumalloys. Property Combinations Needed for Specific End Uses. In most applications, two or more key characteristics of aluminum come prominently into play-for example, light weight combined with strength in airplanes, railroad cars, trucks, and other transportation equipment. High resistance to corrosion and high thermal conductivity are important in equipmentfor the chemical and petroleum industries; these properties combine with nontoxicity for food processing equipment. Attractiveappearance together with high resistance to weatheringand low maintenancerequirements have led to extensive use in buildings of all types. High reflectivity, excellent weathering characteristics, and light weight are all important in roofing materials. Light weight contributes to low handlingand shipping costs, whatever the application. Table 2 reviews the material characteristics required for different markets and applications. Additional information can also be found in the section "Applications" in this chapter.

CompetingMetals forLightweight Consbvction. The light (low density) metals and alloys of commercial importance are based on aluminum, magnesium, and titanium. Each of these metals has distinct qualities that make them suitable or preferred for certain applications. With a density of 1.8 g/cm3, magnesium alloys are among the lightest known structural alloys. This is their chief advantage when compared with aluminum and titanium. However, a low yield strength and modulus of elasticity combinedwith poor thermaland electrical conductivity limit their range of application. Figure 1 comparesthe properties of magnesium and aluminum alloys. The combinationof low density (-4.5 g/cm3), outstanding corrosion resistance, and high strength make titanium and titanium alloys popular in the aerospace, chemical processing, and medical (prostheses) industries. However, its high price (due to processing difficulties) has limited the use of titanium to niche markets. Figure 1 comparesthe propertiesof titanium and aluminumalloys.

The Aluminum Industry

Primary Aluminum Production Occurrence. Aluminum comprises about 8% of the earth's crust, making it second only to silicon (-28%). Iron is third at about 5%. The principal ore of aluminum, bauxite,usually consistsof mixturesofhydrated aluminum oxide, either AlO(OH) or Al(OHh. Besides these compounds, bauxite contains iron oxide (whichgives it a reddish-brown color), as well as silicates (clay and quartz), and titaniumoxide. The bauxites used for the production of aluminum typically contain 35 to 60% total aluminumoxide. Extraction or Refining Methods. The most widely used technology forproducing aluminum involves two steps: extractionand purificationof aluminumoxide (alumina) from ores (primarily bauxite although alternateraw materialscan be used), and electrolysisof theoxideafterit hasbeendissolved in fusedcryolite. The Bayer process is almost universally employed for the purificationof bauxite. In this process, which was developedby AustrianKarl JosephBayer in 1892, the crushed and ground bauxite is digested with caustic soda solution, at elevated temperature and under pressure,and the aluminais dissolvedout as a solution of sodium aluminate. The residue, known as "red mud," containsthe oxides of iron, silicon,and titanium and is separated by settling and filtration. Aluminum hydrate is separated from the solution of sodium aluminateby seedingand precipitationand is convertedto the oxide, Al Z03, by calcination. Present practice for aluminum electrolysis involves the use of the Hall-Heroult cell as pictured in Fig. 2. The cell is lined with carbon, which acts as the cathode; steel bars are embedded in the cathode lining to provide a path for current flow.The anodes are also of

Introduction I 5 Table 3 Comparative fabrication characteristics of wrought aluminum alloys Weldabilily(b) Resistance

Cold AHoy

Temper

1050

0 H12 H14 H16 H18 0 H12 HI4 H16 H18 0 H12 H14 H16 H18 0 H12 H14 H16 H18 0 H12 HI4 H16 H18 0 H12,Hlll H14,H24 H16,H26 HI8 T3 T4,T451 T8 0 T3, T4, T451 T6, T651, T651O, T6511 0 T4, T3, T351, T351O,T3511 T361 T6 T861, T81, T851, T851O, T8511 172 T4 T851 T61 172 0 T31, T351, T351O, T3511 T37 T81, T851, T8510, T8511 T87 T61 0 H12 H14 H16 H18 H25

1060

1100

1145

1199

1350

2011

2014

2024

2036 2124 2218 2219

2618

3003

workabilily(a)

Machinability(a)

Gas

An:

spolIIIId seam

Brazeability(b)

Solderability(c)

A A A B B A A A B B A A A B C A A A B B A A A B B A A A B B C B D

E E

C D C D

D D D E E D D D A A A D B B D B B B B

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A D D D D D D D C D D D

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A D D D D B B D B C C C

B A A A A B A A A A B A A A A B A A A A B A A A A B A A A A D D D B B B D B B B B

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A D D D D D D D D D D D

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A C C C C C C C C C C C

B D

B C B

D

B C

D D

D D A A A A D A A A A A A

C A A A A A C A A A A A A

B B C B B A A A A B B A A A A A

C D

D D D E E

D D D E E D D D E E D D D E E

B C D D D

B B B B B

A A B C C B

E E

D D D D

D D D D D D D A A A A A A

C C C NA

NA A A A A A A

(continued) (a)RatingsA throughDforcold workabilityandA throughE formachinability are relativeratingsin decreasingorderof merit.(b)RatingsA through D for weldability and brazeabilityare relativeratingsdefinedas follows:A, generallyweldableby all conunercialproceduresand methods;B, weldable with specialtechniquesor for specificapplications and requiringpreliminarytrials or testing to developwelding procedureand weld performance;C, limitedweldabilitybecauseofcracksensitivityor lossin resistancetocorrosionandmechanicalproperties;D, nocommonlyusedwelding methodshavebeendeveloped (c)RatingsA throughD andNA for solderabilityare relativeratingsdefinedasfollows:A, excellent;B, gond;C, fair; D, poor;NA, not applicable

6 I Corrosion of Aluminum and Aluminum Alloys Table 3 (continued) Weldability(b) Cold workability(a)

AHoy

Temper

3004

0

A

H32 H34 H36 H38

B B C C

0

A

HI2 HI4 HI6 HI8 H25 T6

B B C C B

3105

4032 4043

5005

0 HI2 HI4 HI6 HI8 H32 H34 H36 H38

5050

0 H32 H34 H36 H38

5052

5056

5086

5154

5182 5252

5254

A A

B C C A

H32 H34 H36 H38

B B C C

0

A A

H321,H116 Hll1

B B C 0 0 B C C

0

A

H32,H1116 H34 H36 H38 H1l1

B B C C B

0

A

H32 H34 H36 H38

B B C C

0

0

A

HI9 H24 H25 H28

0 B B

0 H32 H34 H36 H38

5356 5454

B C C B C C

0

Hll1 H12,H32 HI4,H34 H18,H38 HI92 H392 5083

NA A A

0

C A B B C C NA A

IlesIstaoce MathiDability(a)

Gas

Arc

0 0

B B B B B B B B B B B 0

A A A A A A A A A A A

NA A A A A A A A A A A A A A A A A A A A C C C C C C C C C C C C C C C C C C C C C C C A A A C C C C C NA C

NA A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A NA A

C C C E E

0 0 0 D B C E E

0 0 0 E

0 0 0 E

0 0 C C

0 0 C C C

0 0 0 C C B B D

0 0 0 0 C C C

0 0 0 C C C

0 B 0 C C

0 0 C C C B

0

(continued)

B

spot aod seam

Brazeability(b)

Solderability(c)

B

B B B B B B B B B B B 0

B B B B B B B B B B B

A A A A

B B A A A A C NA

NA

NA NA

A A A A NA

B B B B B B B B B B B B B B C C C C C 0 D 0 0 0 0 0 D 0 0 0 0 0 0 0 0 0 0 0 0 0 D 0 C C C 0 0 0 0 0

B B B B B B B B B C C C C C 0 0 D D 0 D D 0 D D 0 0 0 0 D D 0 0 0 0 0 D 0 0 0 D D 0 0 0 D 0 D 0 0 0

NA

NA

B

0

B A A A A A A A A

B A A A A

B A A A A

B A A A A A A

B A A

B A A A A A

B A A A A

B A A A A

B

Introduction I 7 Table 3 (con'nuecl) Weldability(b) AHoy

Temper

5454 (continued) H32 H34 Hlll 5456 0 Hll1 H321,H115 5457 0 5652 0 H32 H34 H36 H38 5657 H241 H25 H26 H28 6005 T5 T4 6009 6010 T4 0 6061 T4, T451, T451O, T4511 T6, T651, T652, T651O, T6511 6063 TI T4 T5, T52 T6 T83, T831, T832 0 6066 T4, T451O, T451I T6, T651O, T6511 6070 T4, T4511 T6 T6, T63 6101 T61,T64 6151 T6, T652 6201 T81 T6, T651, T651O, T651I 6262 T9 T5,T6 6351 T1 6463 T5 T6 7005 T53 7049 T73, T7351, T7352 T76,T7651 T74, T7451, T7452 7050 T76, T761 7072 7075 0 T6, T651, T652, T651O, T6511 T73, T7351 T74, T7452 7175 7178 0 T6, T651, T6510, T6511 7475 T6,T651 T73,T7351, T7352 T76, T765I

Cold workability(a)

B B B B C C A A B B C C A B B C C A B A B C B B B C C B C C B C C B

C D C B B C C D D D D

A

Machinability(a)

Gas

An:

Resistance spot and seam

D

C C C C C C A A A A A A A A A A A A A A A A

A A A A A A A A A A A A A A A A A A A A A A

A A A B A A B B A A A A A A A A A A A B A A

D D D D D D

A A A A A D

A A A A A B B B A A A A

A A A A A B B B A A A A

A A A A A D D

A A A A A A A B C C C C A C C

A A A A A A A B B B B B A B B

A A A A A A A B

C C C C C C C

B B B B B B B

D D

C D D D D

E D D

C C C D D D D

C C C D C C D D

C C C D

C B C C C D

C B B C D

C C A B B B B D D

D D

A A A A A A A A A A A B D D

D D A

D

B

D D

D D

B B

D D

D D D D

B B B B

D D D D D

Brazeability(b)

B C C C C C B B B B A A A A A A

Solderability(c)

NA

NA B D D D D D

NA

NA B B B B B B B B B B NA

D

B B A A

D

D D D

NA NA B NA NA B NA B D D D D

A

A

D

D D

D

D D D D D

D D D

D D D D

(a)RatingsA throughD for cold workabilityand A throughE for machinabilityarerelativeratingsin decreasingorderof merit.(b) RatingsA through D for weldabilityand brazeabilityare relativeratings definedasfollows:A, generallyweldableby all commercialproceduresand methods;B, weldable with special techniques or for specific applicationsand requiring preliminarytrials or testing to develop welding procedure and weld performance;C, limitedweldabilitybecauseofcracksensitivityor lossin resistanceto corrosionandmechanicalproperties;D, no commonlyusedwelding methodshavebeendeveloped.(c)RatingsA throughD andNA forsolderabilityarerelativeratingsdefinedas follows:A, excellent; B, good;C, fair; D, poor;NA, not applicable

8 I CorTosion of Aluminum and Aluminum Alloys carbon and are gradually fed into the top of the cell because the anodes are continually consumed during electrolysis. A group of cellsare connected in series to obtain the voltage required by the particular direct currentpowersource thatis being used. For aluminum, the electrolyte used is cryolite (Na3AlF6) with 8 to 10% Al20 3 dissolved in it. Other additives, such as CaF2 and AlF3, are added to obtain desirable physical properties.The melting point of the electrolyte is approximately 940 °C (1725 "F), and the Hall-Heroult cell operates at temperatures of approximately 960 to 1000 °C (1760 to 1830 oF). At the cathode of the aluminum cell, aluminum is reduced from an ionic state to a metallic state-for example:

with the aluminum. Examples of two common metals associated with aluminumores that fit this description are iron and silicon. It is, therefore, very important that raw materials be as free of these metal oxides as possible. By careful control of raw materials, aluminum with a purity of 99% or higher may be produced. Generally, the purity of aluminumas it comes from the electrolysis cell (i.e., up to 99.9%) is adequate. Highpurity aluminumof at least 99.97% AI content is necessary for certain special purposes (e.g., reflectors or electrolytic capacitors). For such applications, secondstage refmingoperations(Hoopescell electrolysis)are necessary. Aluminum produced in this way is 99.99% pure. Higher purities of up to 99.9999% ("six-nines" aluminum) canbe obtained withzone-refining operations.

Secondary Aluminum Production This is a very simplified representation of the complex reactions that take placeat the cathode. However, it does represent the overall production of molten aluminum, which forms a molten pool in the bottom of the cell. Periodically, the molten pool of aluminum metal is drainedor siphoned fromthe bottomof the celland cast At the anode, oxygen is oxidizedfrom its ionic state to oxygen gas. The oxygen gas in tum reacts with the carbon anode to form carbon dioxide gas, which gradually consumes the anode material.Two types of anodes are in use: prebaked and self-baking. Prebaked anodes are individual carbon blocks that are replaced one after another as they are consumed. Self-baking anodes, as shown in Fig. 2, are made up of carbon paste that is fed into a steel frameabovethe cell. As the anode descends in the cell, it hardens, and new carbon paste is fed continuallyinto the top of the steel frame. Impurities in the Al20 3 raw materialwhich are more noble than aluminum are reduced at the cathode along

Advantages. Aluminum recovered from scrap (secondary aluminum) has been an importantcontributor to the total metal supply since the 1950s. The economics of recycling, together with improved techniques of scrap preparation and melting,which providehigher yields, led to the development of the secondary aluminum industry. The increased concem with, and economic implications of, energy supply in recent years have focusedeven more attention on recycling of aluminum becauseof its energy-intensive nature.The energy required to remelt secondary aluminum preparatory to fabrication for reuse is only 5% of that required to produce new (primary)aluminum Today secondary aluminum accounts for about 35% of the aluminum supply in both the United States and Europe. The Recycling Loop. The reclamation of aluminum scrapis a complex interactive processinvolvingcollectioncenters, primary producers, secondary smelters, metal processors and consumers. Figure 3 depicts the flow of

Anode leads

Steel studs

Cathode cart>oo

Cathodeleads

Fig. 2

Hall-Herault aluminum production cellwithself-baking anode

Imports

Imports

Exports

Imports

Exports

End use Containers and packaging, 21.7% Building and construction, 12.9% UBC processing facility

I

I

I

•I

Total aluminum supply

New scrap generated

Transportation, 29.2% Electrical, 6.9% Consumer durables, 6.8% Machinery and equipment, 6.1%

Scrap recycling industry

Secondary aluminum (molten or ingot)

t

Imports

Fig. 3

Other, 3.1%

Exports

Flow diagram for aluminum in the United States, showing the role of recycling in the industry. Scrap recycling (lower left) includes scrap collectors, processors, dealers and brokers, sweat furnace operators, and dross reclaimers.

i 8

....... '0

10 I Corrosion of Aluminum and Aluminum Alloys

metal originating in primary smelting operations through various recycling activities. The initial reprocessing of scrap takes place in the facilities of primary producers. In-process scrap, generated both in casting and fabricating, is reprocessed by melting and recasting. Increasingly, primary producers are purchasing scrap to supplement primary metal supply; an example of such activity is the purchase of toll conversion of used beverage cans (UBC) by primary producers engaged in the production of rigid container stock. Scrap incurred in the processing or fabrication of semifabricated aluminum products represents an additional source of recyclable aluminum. Traditionally, this form of new scrap has been returned to the supplier for recycling, or it has been disposed of through sale on the basis of competitive bidding by metal traders, primary producers and secondary smelters. Finished aluminum products, which include such items as consumer durable and nondurable goods; automotive, aerospace, and military products; machinery; miscellaneous transportation parts; and building and construction materials, have finite lives. In time, discarded aluminum becomes available for collection and recovery. So-called old scrap (metal product that has been discarded after use) can be segregated into classifications that facilitate recycling and recovery. Process Technologies. Scrapped aluminum products are broken into small pieces and separated from dirt and foreign materials so as to yield feedstock suitable for remelting. This is done using breakers, shredders, magnetic, and settlement/flotation separators. Such scrap typically contains alloys of many types, all mixed together. A more sophisticated kind ofrecycling was developed in the 1970s and 1980s for process scrap and UBCs. By selectively collecting scrap in targeted alloy categories, the goal was to recycle the material back into products similar to those from which it originated. Thus, the casthouses of extrusion plants produce extrusion billets from process scrap and from recycled scrap extrusions. Similarly, the high rate of recovery of UBCs from the consumer enables a large proportion of canstock coils to be made from UBCs. Recovery of UBCs has multiplied repeatedly since the early 1970s. In 1997, some 2,052 million pounds of UBCs were collected in the United States. This constitutes 66.8% of can shipments. In some countries, for example Sweden, recycling rates exceeding 80% are achieved.

and zinc; other elements are also added in smaller amounts for grain refmement and to develop special properties. The total amount of these elements can constitute up to 10% of the alloy composition (percentages given in weight percent unless otherwise noted). Impurity elements are also present, but their total percentage is usually less than 0.15% in aluminumalloys.

Aluminum Alloys

•

Classifications and Designations It is convenient to divide aluminum alloys into two major categories: wrought composition and cast compositions. A further differentiation for each category is based on the primary mechanism of property development. Many alloys respond to thermal treatment based on phase solubilities. These treatments include solution heat treatment, quenching, and precipitation (or age) hardening. For either casting or wrought alloys, such alloys are described as heat treatable. A large number of other wrought compositions rely instead on work hardening through mechanical reduction, usually in combination with various annealing procedures for property development. These alloys are referred to as work hardening or non-heat-treatable. Some casting alloys are essentially not heat treatable and are used only in as-east or in thermally modified conditions unrelated to solutions or precipitation effects. Cast and wrought alloy nomenclatures have been developed. The Aluminum Association system is most widely recognized in the United States. Their alloy identification system employs different nomenclatures for wrought and cast alloys but divides alloys into families for simplification. Wrought Alloy Families. For wrought alloys, a four-digit system is used to produce a list of wrought composition families as follows:

•

•

•

• The mechanical, physical, and chemical properties of aluminum alloys depend on composition and microstructure. The addition of selected elements to pure aluminum greatly enhances its properties and usefulness. Because of this, most applications for aluminum utilize alloys having one or more elemental additions. The major alloying additions used with aluminum are copper, manganese, silicon, magnesium,

•

•

lxx-x: Controlled unalloyed (pure) composition, used primarily in the electrical and chemical industries 2xxx: Alloys in which copper is the principal alloying element, although other elements, notably magnesium, can be specified. 2xxx series alloys are widely used in aircraft where their high strengths (yield strengths as high as 455 MPa, or 66 ksi) are valued. 3xxx: Alloys in which manganese is the principal alloying element, used as general-purpose alloys for architectural applications and various products 4xxx: Alloys in which silicon is the principal alloying element, used in welding rods and brazing sheet 5xxx: Alloys in which magnesium is the principal alloying element, used in boat hulls, gangplanks, and other products exposed to marine environments &xx: Alloys in which magnesium and silicon are the principal alloying elements, commonly used for architectural extrusions. Txxx: Alloys in which zinc is the principal alloying element (although other elements, such as copper, magnesium, chromium, and zirconium, can be

Introduction I 11 specified), used in aircraft structural components and other high-strength applications. The 7xxx series are the strongest aluminum alloys, with yield strengths ~OO MPa (~73 ksi) possible. • &xxx: Alloys characterizing miscellaneous compositions.The &xxx series alloyscan contain appreciable amounts of tin, lithium, and/or iron. • 9xxx: Reservedfor future use A comprehensive listing of composition limits for wroughtaluminumand aluminumalloyscanbe foundin Appendix 1 to thisbook. Cast Alloy Families. Casting compositionsare described by a three-digit system followed by a decimal value. The decimal .0 in all cases pertains to casting alloy limits. Decimals .1 and .2 concern ingot compositions, which, after melting and processing, should result in chemistries conforming to casting specifications requirements. Alloy families for casting compositions include the following: •

• •

• • • •

• •

lxx.x: Controlled unalloyed (pure) compositions, especiallyfor rotor manufacture 2xx.x: Alloysin which copper is the principalalloying element.Other alloyingelements may be specified. 3xx.x: Alloys in which silicon is the principal alloying element. The other alloying elements such as copper and magnesium are specified. The 3xx.x series comprises nearly 90% of all shaped castings produced. 4xx.x: Alloys in which siliconis the principalalloying element 5xx.x: Alloys in which magnesium is the principal alloying element 6xx.x: Unused 7xx.x: Alloys in which zinc is the principalalloying element. Other alloying elements such as copper and magnesiummay be specified. &xx.x: Alloys in which tin is the principal alloying element 9xx.x: Unused

A comprehensive listing of composition limits for cast aluminumand aluminumalloyscan be found in Appendix 2 to thisbook. Temper Designations. The temper designation system adopted by the Aluminum Association and used in the United States is used for all product forms (both wrought and cast), with the exception of ingot. The system is based on the sequencesof mechanicalor thermal treatments, or both, used to produce the various tempers.The temper designation follows the alloy designationand is separated from it by a hyphen. Aluminum alloys are hardened and strengthened by either deformation at room temperature, referred to as strain hardening and designation by the letter H, or by an aging heat treatment designated by the letter T. If a wrought alloy has been annealed to attain its softest condition, the letter 0 is used in the temper designation. If the product has been shaped without any at-

tempt to control the amount of hardening, the letter F (as-fabricated) is used for the temper designation.The strain-hardened and heat-treated conditions are further subdividedaccording to the degree of strain hardening and the type of heat treating. Major subdivisions of basic tempers(i.e., H, T, 0, and F) are indicatedby one or more digits following the letter. A more complete description of the temper designation system for aluminum and aluminum alloys can be found in Appendix 3 to this book.

EIIects of Alloying Additions A brief summary of the effects of the principal alloying additions on aluminum is given here. Emphasis is placed on their influence on strength and response to heat treatment. The effects of alloying elementson the corrosionresistanceof aluminum alloys is discussedin Chapter 2, "Understanding the Corrosion Behavior of Aluminum," as well as in other chapters in this book that deal with specific forms of corrosion (e.g., stresscorrosion cracking). Copper is one of the most important additions to aluminum. It has appreciable solubility and a substantial strengthening effect through the age-hardening characteristics it imparts to aluminum. Many alloys contain copper either as the major addition (2xxx or 2xx.x series) or as an additional alloying element, in concentrationsof I to 10%. Manganese has limited solid solubility in aluminum but in concentrations of about I % forms an important series of non-heat-treatable wrought aluminum alloys (3.ux series). It is employed widelyasa supplementary addition in both heat treatable and non-heattreatable alloys and provides substantial strengthening. Silicon lowers the melting point and increases the fluidity (improves casting characteristics) of aluminum A moderate increase in strength is also provided by silicon additions. Magnesium provides substantial strengthening and improvement of the work-hardening characteristicsof aluminum It has a relatively high solubility in solid aluminum, but AI-Mg alloys containing less than 7% Mg (5xxx series) do not show appreciable heat treatment characteristics. Magnesium is also added in combination with other elements, notably copper and zinc, for even greater improvements in strength. Zinc is employed in casting alloys and in conjunction with magnesium in wrought alloys to produce heat treatable alloys (7:xxx series) having the highest strength among aluminum alloys. Copper and silicon are used together in the commonly used 3xx.x series casting alloys. Desirable ranges of characteristicsand properties are obtained in both heat treatable and non-heat-treatable alloys. Magnesium and silicon are added in appropriate proportions to form Mg2Si, which is a basis for age hardening in both wrought and (6xxx series) and casting (3xx.x series) alloys.


Tin improves the antifriction characteristic of aluminum, and cast AI-Sn alloys (8xx.x series) are used for bearings. Uthium is added to some alloys in concentrations approaching 3 wt% to decrease density and increase the elastic modulus. Examples include Al-Cu-Li alloys (e.g., 2091) containing 1.7 to 2.3% Li and AI-Li-CuMg alloys (e.g., 8090) containing 2.2 to 2.7% Li.

Properties 01 Wrought Alloys Non-heat-treatable wrought aluminum alloys are those that derive their strength from solid-solution or dispersion hardening and are further strengthened by strain hardening. They include lxxx, 3xxx,4xxx, 5xxx, and some 8xxx(AI-Fe and Al-Fe-Ni) alloys. Heat treatable alloys are strengthened by solution heat treatment and controlled aging and include the 2xxx, &xx,7xxx, and some 8xxx (Al-Li-Cu-Mg) alloys. The strength

ranges attainable with various classes of wrought alloys are given in Table 4. Mechanical Properties. Typical mechanical (tensile) properties of some connnonly used wrought aluminum alloys are shown in Tables 5 and 6. In Table 5, mechanical properties are shown for several representative non-heat-treatable alloys in the annealed, half hard and full hard tempers; values for high-purity aluminum (99.99%) are included for comparison. Although pure aluminum can be substantially strain hardened, a mere I % alloying addition produces a comparable tensile strength to that of fully hardened pure aluminum with much greater ductility in the alloy. The alloys can then be substantially strain hardened to produce even greater strengths. While strain hardening increases both tensile and yield strengths, the effect is more pronounced for the yield strength so that it approaches the tensile strength, and they are nearly equal in the fully hard temper. Ductility and workability are reduced as the material is

Table 4 Strength ronges of various wrought aluminum alloys Aluminum Association series

Tensile strength range

Type of alloy

composition

l,UX 2.ux 2.ux 3'ux 4,UX

Al AI-Cu-Mg (1-2.5% Cu) AI-Cu-Mg-Si (3~% Cu) Al-Mn-Mg AI-Si

5'ux 5'ux fu= Txxx 7,UX

AI-Mg (1-2.5% Mg) AI-Mg-Mn (3-6% Mg) AI-Mg-Si AI-Zn-Mg AI-Zn-Mg-Cu Al-U-Cu-Mg

8.ux

Strengthening method

Coldwork Heat treat Heat treat ColdwolK Cold WOIK (some heat treat)(a) Cold work Cold work Heat treat Heat treat Heat treat Heat treat

MPa

ksi

70-175 170-310 380-520 140-280 105-350

10-25 25-45 55-75 20-40 15-50

140-280 280-380 150-380 380-520

20-40 40-55 22-55 55-75 75-90 40-80

5~20

280-560

(a) Alloy 4032 is heat treatable.

TableS Typical mechanical properties of representative non·heot-treatable aluminum alloys Anoy

Nominol COIIlposition

1199

99.99+% AI

1100

99+% Al

Temper

0 HI8

0 HI4 HI8

3003

1.2%Mn

0 HI4 HI8

3001

1.2% Mn, 1.0% Mg

0 H34 H38

5005

0.8%Mg

0 H34 H38

5052

2.5Mg

0 H34 H38

5456

5.1% Mg,0.8% Mn

0 HII6

(a) 500 kg load on 10 mm ball

TemDestrength MPa ksi

Yieldstrength ksi MPa

45 117 90 124 165 110 152 200 179 241 283 124 159 200 193 262 290 310 352

10 110 34 117 152 41 145 186 69 200 248 41 138 186 90 214 255 159 255

6.5 17

13 18 24 16 22 29

26 35 41 18

23 29 28 38 42 45 51

1.5 16 5 17 22 6 21 27 10 29 36 6 20

27 13 31 37

23 37

Elongation in SOmm (2 in.), II>

50 5 35 9 5 30 8 4 20 9 5 25 8 5 25 10 7 24 16

Hardness(a),

DB

23 32 44 28 40 55 45 63 77 28 41 55 47 68 77 90

Introduction I 13

strain hardened, and most alloys have limited formability in the fully hard tempers. The effect of alloying additions on the strength of annealed aluminum is dramatically depicted in Fig. 4 The pseudolinear relationship between yield strength and percent alloying addition extends to the strongest non-heat-treatable commercial alloy, 5456, with approximately 6% Mg plus Mn (minor alloying elements have not been figured into the percent alloying additions). This relationship does not hold for the heat treatable 2xxx and 7xxx series alloys. Table 6 lists typical mechanical properties and nominal compositions of some representative heat treatable aluminum alloys. The strengthening effect of the alloyingadditionsin these alloys is not reflectedin the annealed condition to the same extent as that in the

non-heat-treatable alloys (see Fig. 4), but the full value of the additions can be seen in the aged conditions. Aged heat treatable alloys are significantly stronger than full hard non-heat-treatable alloys and generally retain more ductility. The range of strengths available with commonly used aluminum alloys is shown in Table 4 and Fig. 5. Mechanical Properties at Low Temperatures. Aluminum alloys represent a very important class of structural metals for subzero-temperature applications and are used for structural parts operating at temperatures as low as -270°C (-450 "F). Below zero, most aluminum alloys show little change in properties. Yield and tensile strengths can increase (Fig. 6), and elongation can decrease slightly. Impact strength remains approximately constant Consequently, alurni-

Table6 Typical mechanical properties of representative heat·treatable aluminum alloys NomioaJ composition

Alloy

2219

6.3%Cu, 0.3%Mn

2024

7005

Teusilestrength ksi

VJeldstrength MPa ksi

0

172 393 476 186 469 517 379 124 241 310 193 352 228 572 503

25 57 69 27 68 75 55 18 35 45 28 51 33 83 73

69 317 393 76 324 490 317 55 145 276 83 290 103 503 434

0 T4 T861 T6

0 T4 T6

0

4.6%Zn. 1.4%Mg 5.6%Zn. 2.4%Mg, 1.6%Cu

7075

MPa

T37 T87

4.4% Cu. 1.5%Mg, 0.6%Mn 12.2%Si 1.0%Mg. 0.6%Si

4032 6061

Temper

T6

0 T6 T73

Elongatioo in SOmm (2 in.), %

10 46 57 11 47 71 46 8 21 40 12 42 15 73 63

Hardnoss(a), HB

18 11 10 20 20 6 9 25 22 12 20 13 17 11 13

117 130 47 120 135 120 30 65 95

60 150

(a) 500 kg load on IOmmball

90

621

80

_

Tensilestrength

o

Yieldstrength

552 483

70

414

60 'iii

~

50

o

ui Ul

~

en

~ q~

30

q

0

CO

U'l

IACS

Non-heat-treatable alloys

1100 3003 3004 5005

5052 5456

2.71 2.73 2.72 2.70 2.68 2.66

643--{j57 643--{j54 629--{j54 632--{j54 607--{j49 568--{j38

1190--1215 1190--1210 1165-1210 1170-1210 1125-1200 1055-1180

222 193 163 200 138 117

128 112

59

94

116 80 67.5

42 52 35 29

2.84 2.77 2.68 2.70 2.78 2.80

543--{j43 502--{j38 532-571 582--{j52 607--{j46 477--{j35

1010-1190 935-1180 990--1060 1080-1205 1125-1195 890-1175

121 121 155 167

70 70 90 97

30 30 40 43

130

75

33

50

Heat treatable alloys

2219 2024 4032 6061 7005 7075

(a)Equalvolumeat 20°C (68 "F)

alloys with improved fracture toughness have evolved through microstructure control obtained by increased purity, modified composition, and better fabrication and heat treatment practice. Figure 8 shows the relationship between fracture toughness and yield strength for 2xxx and 7xxx alloys. Physical Properties. Table 7 lists densities, melting points, thermal conductivities, and electrical conductivities for selected wrought aluminum alloys.

Properties

0' Cas'Alloys

Casting alloys cannot, of course, be work hardened and are either used in the as-cast or heat treated conditions. Heat treatable casting alloys include the 2xx, 3xx, and Txx series. Mechanical Propertie.. Table 8 lists typical mechanical properties and nominal compositions of 80Ire

representative cast aluminum alloys. Typical tensile properties and nominal compositions of SOIre representative cast aluminum alloys. Typical tensile properties for commonly used casting alloys range from about 145 to 485 MPa (20 to 70 ksi) for ultimate tensile strength, 70 to 415 MPa (10 to 60 ksi) for yield strength, and :l

s e:l 13 e u.

20 10 0 200

250

300

350

400

450

500

Yield strength. MPa

Fig. 8

The effects of alloy type and aged condition on the strength/fracture toughness relationship for aluminum alloys


majority (-74%) of aluminum shipments in the United States. Aluminum ingot for castings, destructive uses, and exports represent the remainder of net shipments. Wrought aluminum products can be divided into two groups. Standardized wrought products include sheet, plate, foil, rod, bar, wire, tube, pipe, and structural forms. Engineered wrought products are those designed for specific applications and include extruded shapes, forgings, and impacts. Typical examples of wrought products include plate or sheet, which is subsequently formed or machined into products such as aircraft or building components, household foil, and extruded shapes such as storm window frames. Cast Aluminum Products. Aluminum castings are produced in a great variety of shapes and sizes by pressure-die, permanent-mold, green- and dry-sand,

investment, and plaster casting. Process variations include vacuum, low-pressure, centrifugal, and patternrelated processes such as lost foam casting. Table 11 provides shipment statistics for aluminum castings. Transportation is the leading market, and the trend toward increasing use in automotive applications is increasing the importance of castings in the total industry picture. Powder Metallurgy Products. Structural parts made by powder metallurgy (PIM) methods constitute only a very small part of the overall aluminum industry. In fact, the majority of aluminum powder produced is used for nonstructural applications (e.g., paints, pigments, and explosives). Aluminum P/M parts are produced by pressing and sintering of atomized powders or by vacuum hot pressing or hot isostatically pressing aluminum powders into billets, which are subsequently rolled, extruded, or forged.

Table 8 Typicalmechanical properties of representative aluminum casting alloys Alloy

201.0

Nominal cOOlIKJOition

4.6%Cu

355.0

5% Si, 1.3% Cu

356.0

7% Si, 0.3% Mg

380.0 390.0

8.5% Si, 3.5% Cu 17% Si, 4.5% Cu

413.0 B443.0

12%Si 5.2%Si

Tensile strength ksi

Product(a)

Temper

MPa

S

T4 T6 TI T51 T6 T61 TI TIl T51 T6 T62 TI TIl T51 T6 TI TIl T6 T7

365 485 460 195 240 270 265 175 210 290 310 280 250 175 230 235 195 265 220 330 280 300 300 159

S S S S S S S P P P P P S S S S P P D D D D P

F F T5

F F

53 70 67 28 35 39 38 35 30 42 45 40 36 25 33 34 28 38 32 48 41 43 43 23

Yieldstrength ksi

MPa

215 435 415 160 175 240 250 200 165 190 280 210 215 140 165 210 145 185 165 165 240 260 140 62

31 63 60 23 25 35 36 29 24 27 40 30 31 20 24 30 21 27 24 24 35 38 21 9

1IardDess(h),

Elongation, If,

HB

20 7 4.5 1.5 3.0 1.0 0.5 1.5 2.0 4.0 1.5 2.0 3.0 2.0 3.5 2.0 3.5 5.0 6.0 3.0 1.0 1.0 2.5 10.0

95 135 130 65 80 90 85 75 75 90 lOS 85 85 60 70 75 60 80 70 120 125

(a) S, sand; P, permanent mold; D, die cast, (b) 500 kg load on lOmm ball

Table 9 Typicalphysical properties of representative aluminum casting alloys Alloy

DensitygJcm3

201.0

2.80 2.71

355.0 356.0 380.0 390.0 413.0 B443.0

2.69 2.71 2.73 2.66 2.69

(a) Equal volume at 20 °C (68 oF)

Approximatemeltingrange OF "C

571-649 549-621 560-616 521~588

505-650 577-588 577-632

1060-1200 1020-1150 1040-1140 970-1090 945-1200 1070-1090 1070-1170

Thermalcoodudivityat 25 "C (71 oF) Btu/ft . h· OF WJm·K

121 150 150 108 134 154 146

70 87 87 62 77 89 84

Electricalcoodu9,795 479 1,262 619 658 3,473

Percentall" ortotal

5,904

43.4 2.1 5.6 2.7 2.9 15.4 0.6 1.0 73.8 26.2

22,568

100.0

146 232 16,664

one-third that of steel and requires special attention to compression members. However, it offers advantages under shock loads and in cases of minor misalignments. When properly designed, aluminum typically saves over 50% of the weight required by low-carbon steel in small structures; similar savings are possible in long-span or movable bridges. Savings also result from low maintenance costs and in resistance to atmospheric or environmental corrosion. Forming, shearing,sawing, punching, and drilling are readily accomplished on the same equipment used for fabricating structural steel. Since structural aluminum alloys owe their strength to properly controlled heat 1reatment, hot forming or other subsequent thermal operationsareto be avoided Specialattentionmust be given to the strength requirements of welded areas because of the possibility oflocalized annealing effects. Buildin.... Corrugated or otherwise stiffened sheet products are used in roofing and siding for industrial and agricultural building construction. Ventilators, drainage slats, storage bins, window and door frames, and other components are additional applications for sheet, plate, castings, and extrusions. Aluminum products such as roofing, flashing, gutters, and downspouts are used in homes, hospitals, schools, and commercial and office buildings. Exterior walls, curtain walls, and interior applications such as wiring, conduit, piping, ductwork, hardware, and railings utilize aluminum in many forms and finishes. Aluminum is used in bridges and highway accessories such as bridge railings, highway guard rails, lighting standards, traffic control towers, traffic signs, and chain-link fences. Aluminum is also commonly used in bridge structures, especially in long-span or movable bascule and vertical-lift construction. Construction of portable military bridges and superhighway overpass bridges has increasingly relied on aluminum elements. Aluminum is also used for prefabricated pedestrian bridges. Scaffolding, ladders, electrical substation structures, and other utility structures utilize aluminum, chiefly in the form of structural and special extruded shapes. Cranes, conveyors, and heavy-duty handling systems incorporate significant amounts of aluminum. Water

Source:The A1uminwn AssociationInc.

Table 12 U. S.net shipments by majormarketin 1997 Table 11 Shipments of aluminum castings by type in 1996 Sbipmems(a)

Pereentall"0f total

Typeof casting

106 kg

1061b

Diecastings Permanent moldandsemipermanent moldcastings Sandcastings Others Total

1076.1 548.4

2372.3 1209.1

57.7 29.4

153.9 87.9

339.4 193.9 4114.6

8.2 4.7

1866.4

100

(a) Roundedvaluesmightnotadd up to the totalsshown.Source:The AlwninumAssociationIlIC.

millionsof pounds

Pen:eotall" oftotaI

2,921 6,592 1,529 1,561 1,381 4,895 701

12.9 29.2 6.8 6.9 6.1 21.7 3.1

19,580

86.8

Shipment,

Maj... market

Buildingandconstruction Transportation Consumerdurables Electrical Machinery andequipment Containers andpackaging Other Domestic,total Exports Total shipments

2,988

13.2

22,568

100.0

Source:The AluminwnAssociationInc.


storagetanks are often constructedof aluminumalloys to improve resistance to corrosion and to provide attractiveappearance.

Containers and Packaging The food and drug industries use aluminum extensivelybecause it is nontoxic,nonadsorptive, and splinterproof. It also minimizesbacterialgrowth,forms colorless salts, and can be steamcleaned.Low volumetric specific heat results in economies when containers or conveyors must be moved in and out of heated or refrigerated areas. The nonsparkingpropertyof alumi-

num is valuable in flour mills and other plants subject to fire and explosion hazards. Corrosion resistance is important in shipping fragile merchandise, valuable chemicals, and cosmetics. Sealed aluminumcontainers designedfor air, shipboard,rail, or truck shipmentsare used for chemicalsnot suited for bulk shipment. Packaginghas been one of the fastest growing markets for aluminum. Products include household wrap, flexible packaging and food containers, bottle caps, collapsibletubes, and beverage and food cans. Aluminum foil works well in packaging and for pouches and wraps for foodstuffs and drugs, as well as for household uses.

Table 13 Selected applications for wrought aluminum alloys AHoy

Descriptioo and selected applications

1100

Conunerciallypure aluminumhighly resistantto chemical attackand weathering.Low cost, ductile for deep drawing, andeasy to weld. Used for high-purityapplicationssuch as chemicalprocessingequipment.Also for nameplates, fan blades, flue lining, sheet metal work,spun holloware, and fin stock Electricalconductors Screwmachineproducts. Appliance partsandtrim,ordnance, automotive, electronic, fasteners, hardware, machineparts Truckframes,aircraft structures,automotive,cylindersand pistons,machineparts, structurals Screwmachineproducts,fittings,fasteners,machineparts For high-strengthstructuralapplications.Excellent machinabilityin the T-tempers.Fairworlcability and fair corrosionresistance.Alclad 2024combinesthe high strengthof 2024 with the corrosionresistanceof the commerciallypure cladding.Used for truckwheels,many structuralaircraft applications,gearsfor machinery,screw machineproducts,automotiveparts,cylindersand pistons,fasteners,machineparts,ordnance,recreation equipment,screws andrivets Structuraluses at high temperature(to 315°C, or 600 OF). High-strengthweldments Most popular general-purposealloy.Strongerthan 1100 with same good formabilityand weldability.For general use includingsheet metal work,stampings,fuel tanks, chemicalequipment,containers,cabinets, freezerliners, cookingutensils,pressurevessels,builder's hardware, storagetanks, agriculturalapplications,applianceparts and trim, architecturalapplications,electronics,fin stock, fanequipment,name plates,recreationvehicles,trucks and trailers.Used in drawingand spinning. Sheet metalwork, storagetanks,agriculturalapplications, building products, containers,electronics,furniture, kitchenequipment,recreationvehicles,trucksand trailers Residentialsiding, mobilehomes,rain-carryinggoods,sheet metalwork, applianceparts and trim, automotiveparts, buildingproducts,electronics,fin stock, furniture, hospitaland medicalequipment,kitchenequipment, recreationvehicles,trucks and trailers Specifiedfor applicationsrequiringanodizing;anodized coatingis cleanerand Iighterin color than 3003. Uses includeappliances,utensils, architectural, applications requiringgood electricalconductivity,automotiveparts, containers,general sheetmetal,hardware,hospitaland medicalequipment,kitchenequipment,nameplates, and marineapplications. Strongerthan 3003 yet readilyformablein the intermediate tempers.Good weldabilityand resistanceto corrosion. Uses includepressurevessels,fan blades,tanks,electronic panels,electronicchassis,medium-strength sheetmetal parts, hydraulictube,appliances,agriculturalapplications, architecturaluses, automotiveparts, buildingproducts,

1350 2011 2014 2017 2024

2219 3003

3001

3105

5005

5052

AHoy

Descriptioo and selected applications

5052 (continued) chemicalequipment,containers,cooking utensils, fasteners,hardware,highway signs, hospitaland medical equipment,kitchenequipment,marineapplications, railroadcars, recreationvehicles,trucks andtrailers 5056 Cable sheathing,rivetsfor magnesium,screen wire, zippers, automotiveapplications,fence wire,fasteners 5083 For all types of welded assemblies,marinecomponents,and tanksrequiringhigh weldefficiencyand maximumjoint strength.Usedin pressurevesselsup to 65°C (I 50 OF) and in manycryogenicapplications,bridges,freightcars, marinecomponents,TV towers,drillingrigs, transportationequipment,missilecomponents,and dump truck bodies. Goodcorrosionresistance 5086 Used in generallythe same types of applicationsas 5083, particularlywhereresistanceto either stresscorrosionor atmosphericcorrosionis important 5454 For all types of welded assemblies,tanks, pressurevessels. ASMEcode approvedto 205°C (400 of). Also usedin truckingfor hot asphaltroad tankers and dumpbodies; also, for hydrogenperoxideand chemicalstorage vessels 5456 For all types of welded assemblies,storagetanks, pressure vessels,and marinecomponents.Used wherebest weld efficiencyand joint strengthare required.Restrictedto temperatures below 65°C (150 of) 5657 For anodizedauto and appliancetrimand nameplates 6061 Good formability,weldability,corrosionresistance,strength in the T-tempers.Good general-purposealloyused for a broadrange of structuralapplicationsand welded assembliesincludingtruck components,railroadcars, pipelines, marineapplicalions, furniture, agricultural applicalions, aircrafts, architectural applications, automolive parts,buildingproducts, chemicalequipment,dump bodies,electricaland electronicapplications,fasteners, fence wire,fan blades,generalsheet metal,highwaysigns, hospitaland medicalequipment,kitchenequipment, machineparts,ordnance,recreationequipment,recreation vehicles,and storagetanks. 6063 Used in pipe railing,furniture,architecturalextrusions, applianceparts and trim, automotiveparts, building products,electricaland electronicparts, highwaysigns, hospitaland medicalequipment,kitchenequipment, marineapplications,machineparts, pipe,railroadcars, recreationequipment,recreationvehicles,trucksand trailers 7050 High-strengthalloy in aircraftand other structures.Also used in ordnanceand recreationequipment 7075 For aircraftandother applicationsrequiringhighest strengths.Alclad7075 combinesthe strengthadvantages of 7075 with the corrosion-resisting propertiesof commerciallypure aluminum-cladsurface.Also usedin machineparts and ordnance

Introduction I 19 Beverage cans have been the greatest success story of the aluminum industry and market penetrations by the food can are accelerating. Soft drinks, beer, coffee, snack foods, meat, and even wine are packaged in aluminum cans. Draft beer is shipped in alclad aluminum barrels. Aluminum is used extensively in collapsible tubes for ointments, food, and paints.

Transportation Automotive. Both wrought and cast aluminum have found wide use in automobile construction (Table 15). Although aluminum currently accounts for less than 10% of the total weight of a vehicle, this percentage is expected to increase dramatically as average fuel economy mandates and emphasis on recycling continues. As an example of environmental strengths of aluminum for automotive applications, more than 85% of post-eonsumer automotive scrap and virtually all post-manufacturing automotive scrap is recycled. Some 60 to 70% of all automotive aluminum originates from recycled metal. Aluminum sand, die, and permanent mold castings are critically important in engine construction; engine blocks, pistons, cylinder heads, intake manifolds, crankcases, carburetors, transmission housings, and rocker arms are proven components. Brake valves and brake calipers join innumerable other components in

car design importance. Cast aluminum wheels continue to grow in popularity. Aluminum sheet is used for hoods, trunk decks, bright finish trim, air intakes, and bumpers. Extrusions and forgings are finding new and extensive uses. Forged aluminum alloy wheels are a premium option. Because of its lighter weight and corrosion resistance, aluminum is also a prime candidate to replace steel sheet in ''body-in-white'' (structural shell/skin) applications. Aluminum has one-third the density of steel, which means a component can be 1.5 times thicker than a steel version while remaining 50% lighter. It can absorb twice as much energy as steel at the same weight. Aluminum is corrosion resistant, unlike steel, which must be coated with other metals such as zinc (galvanized steel) to improve its resistance to corrosion. The lighter weight and stiffness of aluminum can enhance vehicle acceleration and handling and reduce noise and vibration characteristics. Trvclu. Because of weight limitations and a desire to increase effective payloads, manufacturers have intensively employed aluminum in cab, trailer, and truck designs. Sheet alloys are used in truck cab bodies, and dead weight is also reduced using extruded stringers, frame rails, and cross members. Extruded or formed sheet bumpers and forged wheels are usual. Fuel tanks of aluminum offer weight reduction, corrosion resis-

Table 14 Selected applications for aluminum casting alloys AHoy

Representative applications

AHoy

Representative appUcations

100.0 201.0

Electricalrotorslargerthan 152mm (6 in.) in diameter Structuralmembers;cylinderheadsand pistons;gear, pump,and aerospacehousings General-purpose castings;valvebodies,manifolds,and otherpressure-tightparts Bushings;rneterparts;hearings;hearingcaps; automotivepistons;cylinderheads Sole platesfor electrichand irons Heavy-dutypistons;air-cooledcylinderheads;aircraft generatorhousings Dieseland aircraftpistons;air-cooledcylinderheads; aircraftgeneratorhousings Gearhousings;aircraftfittings;compressorconnecting rnds; railwaycar seatframes General-purpose permanentmoldcastings;ornamental grillesand reflectors Enginecrankcases;gasolineand oil tanks; oil pans; typewriterframes;engineparts Automotiveand heavy-dutypistonpulleys,sheaves Gasmeterand regulatorparts; gearblocks;pistons; generalautomotivecastings Premium-strength castingsfor the aerospaceindustry Sand:aircompressorpistons;printingpressbedplates; waterjackets;crankcases. Permanent: impellers; aircraftfittings;timinggears;jet enginecompressor cases Sand: flywheel castings; automotivetransmission cases; oil pans; pump bodies. Permanent: machine tool parts; aircraft wheels; airframecastings; bridge railings Structuralpartsrequiringhigh strength;machineparts; truckchassisparts Corrosion-resistant and pressure-tight applications High-strengthcastingsfor the aerospaceindustry

360.0

Outboardmotorparts;instrumentcases;coverplates; marineand aircraftcastings Coverplates;instrumentcases; irrigationsystemparts; outboardmotorparts;hinges Housingsfor lawnmowersand radio transmitters; air brakecastings;gearcases Applications requiringstrengthat elevatedtemperature Pistonsand other severeserviceapplications; automatic transmission Internalcombustionengine pistons,blocksmanifolds, and cylinderheads Architectural, ornamental,marine,and food and dairy equipmentapplications Outboardmotorpistons;dentalequipment;typewriter frames;streetlamp housings Cookware; pipe fittings;marinefittings;tire molds; carburetorbodies Fittingsforchemicaland sewageuse; dairyand food handlingequipment;tire molds Permanentmoldcastingof architecturalfittingsand ornamentalhardware Architectural and ornamentalcastings;conveyorparts; aircraftand marinecastings Aircraftfittings;railwaypassengercare frames;truck and busframesections Instrumentparts and otherapplicationswhere dimensionalstabilityis important General-purposecastings that require subsequent brazing Automotiveparts;pumps;trailer parts;mining equipment Bushingsandjournal bearingsfor railroads Rollingmillbearingsand similarapplications

208.0 222.0 238.0 242.0 A242.0 B295.0 308.0 319.0 332.0 333.0 354.0 355.0

356.0

A356.0 357.0 359.0

A360.0 380.0 A380.0 384.0 390.0 413.0 A413.0 443.0 514.0 A5l4.0 518.0 520.0 535.0 A712.0 713.0 850.0 A850.0


tance, and attractive appearance. Castings and forgings are used extensively in engines and suspension systems. TlVck trailers are designed for maximum payload and operating economy in consideration of legal weight requirements. Aluminum is used in frames, floors, roofs, cross sills, and shelving. Forged aluminum wheels are commonly used. Tanker and dump bodies are made from sheet and/or plate in riveted and welded assemblies. Mobile homes and travel trailers usually are constructed of aluminum alloy sheet used bare or with mill-applied baked-enamel fmish on wood, steel, or extruded aluminum alloy frames. Bus manufacturers also are concerned with minimizing dead weight. Aluminum sheet, plate, and extrusions are used in body components and bumpers. Forged wheels are common. Engine and structural

components in cast, forged, and extruded form are extensively used. Bearings. Aluminum-tin and aluminum-silicon alloys are used in medium and heavy-duty gasoline and diesel engines for connecting-rod and main bearings. Cast and wrought bearings can be a composite with a steel backing and babbited or other plated overlay. Railroad Cars. Aluminum is used in the construction of railroad hopper cars, box cars, refrigerator cars, and tank cars. Aluminum is also used extensively in passenger rail cars for mass transit systems. Marine Applications. Aluminum is commonly used for a large variety of marine applications, including main strength members such as hulls and deckhouses, and other applications such as stack enclosures, hatch covers, windows, air ports accommodation ladders, gangways, bulkheads, deck plate, ventilation equipment, lifesaving equipment, furniture,

Table 15 Aluminum alloys used for automotive applications ADoy

'JYpicaJ app6cations

Wrought alloy series 1000series 1100 Trim, nameplates,appliques Extrudedcondensertubesand fins 1200 2000 series Outer and innerbody panels(also suitablefor structural 2008 applications) Outer and inner body panels(also suitablefor structural 2010 applications) 2011(a) Screwmachineparts 2017(a) Mechanicalfasteners Mechanicalfasteners 2024 Outer and inner body panels,load floors, seat shells 2036 2117(a) Mechanicalfasteners 3000series Trim,nameplates,appliques 3002 3003(a) Braze-cladwelded radiatortubes, heatercores, radiator, heaterand evaporatorfins, heaterinlet and outlet tubes, oil coolers, and air conditionerliquid lines Interiorpanels andcomponents 30
6

7

8

Effectsof principal alloying elementson electrolytic solution potential of • aluminum. Potentials are for high.purity binary alloys solution heat treated and quenched. Measured in a solution of 53 giL NaCI plus 3 giL H202 maintained at 25°C (77 °Fl

Understanding the Corrosion Behavior of Aluminum I 29 Table 3 Solution potentials of heat-treatable commercial wrought aluminum alloys ADoy

Temper

Potential(al, V

2014

T4 T6 T3 T4 T6 T8 T3 T4 T6 T8 T4 T8E41 T4 T4 T6 T5 T4 T6 T5 T6 T6 T6 T6 T6 T6 T73 T76 T73 T76 T6 T73 T76 T6 T73 T76 T6

-{l.69(b) -{l.78 -{l.64(b) -{l.64(b) -0.80 -{l.82

Table 5 Solution potentials of aluminum allays and other metals based on ASTM G 69 Lower Metal or a60y

2219

2024

2036 2090 6009 6010 6151 6351 6061 6063 7005 X7016 X7021 X7029 Xn46

7049 7050 7075

7475

7178

-o.sse»

-o.eso» -{l.81 -{l.82

-e.n

-{l.83 -{l.80 -{l.79 -{l.83 -{l.83 -{l.80 -{l.83 -{l.83 -{l.83 -{l.94 -{l.86 -{l.99 -{l.85 -1.02 -{l.84(c) -{l.84(c) -{l.84(c) -{l.84(c)

-o.ase» -{l.84(c) -{l.84(c)

-o.sse: -{l.84(c) -{l.84(c) -{l.83(c)

(a)Potentialversusstandardcalomelelectrodemeasuredin an aqueous solution of 53 gIL NaCI plus 3 gIL HZOz at 25°C (77 oF). (b) Varies ±O.OI V withquenchingrate. (c)Varies ±O.02V with quenching rate

Table4 Solution potentials of cast aluminum alloys ADoy

Temper

Typeofmold(al

Potential(bl, V

208.0 238.0 295.0

F F T4 T6 T62 T4 F F F T4 T6 T6 F F F T4 F

S P SorP SorP SorP SorP P S P SorP SorP SorP S P S SorP S

-{l.n -{l.74 -{l.70 -{l.71 -{l.73 -{l.71 -{l.75 -{l.81 -{l.76 -{l.78 -{l.79 -{l.82 -{l.83 -{l.82 -{l.87 -{l.89 -{l.99

296.0 308.0 319.0 355.0 356.0 443.0 514.0 520.0 710.0

(a) S, sand; P, permanent. (b) Potential versus standard calomel electrode measuredin an aqueous solutionof 53 gIL NaCl plus 3 gIL HzOz at 25 °C (77 OF)

Cr(99.9%) Ni 270 (1 sample) Stainlesssteel316 Cu (99.999%) Bronze(Cu94-Sn6) Ti-6AI4V Brass(Cu63-Zn37) Sn(99.99%) Zr (99.9%) Mn Monel400 (Ni65-Cu33-Fe2) Stainlesssteel321 Fe 2219-T3,T4 2014-T4,2017-T4,2024-T3, T4 2324-T39 2036-T3,T4 2036-T6,209O-T3, T4 Ti(99.7%) 2091-T3,T8 2014-T6,2008-T4 8090-T3, 6010-T4 Pb 2008-T6 2219-T6,T8 2024-T8 6061-T4,6oo9-T4 6013-T6,T8 7075-T6,7178-T6 3003,1100, 6061-T6,6053,6063 5030-T4,209O-T8 AI(99.999%) 7075-T7,809O-T7, 7049-T7, 7050-T7,7475-T7 7055-T77 3004, 1060,5050 5052,5086,Alclad 2024, Alclad2014 5154,5454,5254,5042 5056,7079-T6,5456,5083,5182 7039-T6,T63, 700S 7002, A1c1ad 3003,A1cJad 6061, A1c1ad 7075 7003 Zn Mg

Omit

Uppe!" Erorr , V

Omit

+0.23 +0.12

+0.32

-{l.70

-o.u

-{l.08 -{l.05 -{l.30 -{lAO -{l.20 -{l.80 -{lA3 -{l.31 -{l.6O -{l.56 -{l.62

-{l.76

+0.00

-{l.30 -{l.55 -{l.6O -{l.62 -{l.64 -{l.65 -{l.66 -{l.67 -{l.69 -{l.70

-o.n -{l.74 -{l.73 -{l.73 -{l.74 -{l.76

-{l.76 -{l.77

-{l.71

-o.n -o.n -o.n -{l.73 -{l.74 -{l.74 -{l.74 -{l.75 -{l.75 -{l.75 -{l.75 -{l.76

+0.27 +0.08 -{l.02 +0.27 -{l.08 -{l.19 +0.02 -{lAO -{l.25 -{l.25 -{lA9 -{l.54 -{l.59 -{l.63 +0.14 -{l.66

-{lAS -{l.70 -{l.70

-{l.n

-{l.73 -{l.73 -{l.74

-e.n -{l.87

-{l.78 -{l.84 -{l.87

-1.01

-{l.94 -{l.98 -1.64

Note:Potentialvaluesforaluminumalloysin boldfacewerecalculated fromworkpublishedin Ref7 by adding+92 mY.Source:Ref 8

Table 6 Solution potentials of some secondpha.. constituents in aluminum alloys Pbase

Potential(al, V

Si AI3Ni A13Fe AIZCu

-{l.26 -{l.52 -{l.56 -{l.73 -{l.85 -1.00 -1.24

AI~n

AlzCuMg AlsMgs

(a)Potentialversusstandardcalomelelectrodemeasuredin an aqueous solutionof 53 gIL NaCl plus 3 gIL HZOz at25 °C (77 oF)

30 I Corrosion of Aluminum and Aluminum Alloys potential measurements can determine the effectiveness of solution heat treatment by measuring the amount ofcopper in solid solution. Also, by measuring the potentialsof grain boundaries and grain bodies separately, the difference in potential responsible for intergranular corrosion, exfoliation, and stress-corrosion cracking (SCC) can be quantified. Solution-potential measurementsof alloys containingcopper also show the progress of artificial aging as increased amounts of precipitates are formed and the matrix is depleted of copper. Potential measurements are valuable with zinccontaining (7xxx) alloys for evaluating the effective-

t

More cathodic

--.-"- .. ........... '. '. -- ........ -, '"

'.

'.

.. ....

..

... ... , ..

..

Anodic reaction (AIO - A13+ + 3e)

More anodic

Cathodic reaction

10-S

10-7

10- 5

10-6

10-4

Current density, A1mm2

Fig 5

Anodic-polarization curve for aluminum alloy • 1100. Specimens were immersed in neutral deaerated NaCI solution free 01 cathodic reactant. Pitting develops only at potentialsmore cathodic than the pitting potential E The intersection ol the anodic curve lor aluminum (solid ine)with a curve lor the applicable cofhcdlc reaction (one 01 the representative dashed lines) determines the polential to which the aluminum is polarized, either by calhodic reactionon Ihealuminum itsellor on anothermelal eleclricallyconnected to it. The potentialto which thealumi· numis polarized by a specilic cathodereactiondelermines corrosioncurrentdensityand corrosion rate.

. f

- 0.3 ...--.,.---,-----,.--T""""'--, w J:

!!2 -04 I-....L-F~i;::-l---+_--+---j >

-06

o Bohni and Uhlig L-_-L_--l~ _ _..L..._--L_---'

0.05

0.1

0.2

0.5

1.0

2.0

CI- activity

Fig 6

Elfect01 chlorlde-lonactivityon pitting poten• lial 01 aluminum 1199 in NaCI solutions. Source: Rel10 and 11

ness of the solution heat treatment, for following the aging process, and for differentiating the various artificially aged tempers. These factors can affect the corrosion behavior. In the magnesium-containing (5xxx) alloys, potential measurements can detect low-temperature precipitation and are useful in qualitatively evaluating stress-corrosion behavior. Potential measurements can also be used to follow the diffusion of zinc or copper in alclad products, thus determining whether the sacrificial cladding can continue to protect the core alloy. (Ref 9).

Pitting COn'osion Corrosion of aluminum in the passive range is localized and is usually manifested by random formation of pits. The pitting-potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting (Ref 10-12). Simply stated, pitting potential (Ep) is that potential in a particular solution above which pits will initiate and below which they will not. Four laboratory procedures have been developed to measure Ep• One is based on fixed current, and the other three are based on controlled potential (Ref 13). The most widely used is controlled potential, in which the potential of a specimen, usually immersed in a deaerated electrolyte ofinterest, is made more positive. The resulting current density from the specimen is measured. The potential at which the current density increases sharply and remains high is called the oxide breakdown potential (E br) . With polished specimens in many electrolytes, Ebr is a close approximation of Ep, and the two are used interchangeably. An example is shown in Fig. 5. A specimen of aluminum alloy 1100 was immersed in neutral deaerated sodium chloride (NaCl) solution, and the relationship between anode potential and current density was plotted (solid line, Fig. 5) At potentials more active than Ep, where the oxide layer can maintain its integrity, anodic polarization is easy, and corrosion is slow and uniform. Above Ep, anodic polarization is difficult, and the current density sharply increases. The oxide ruptures at random weak points in the barrier layer and cannot repair itself, and localized corrosion develops at these points. Potential-current relationships for various cathodic reactions are indicated by the dashed lines in Fig. 5. Only when the cathodic reaction is sufficient to polarize the metal to its pitting potential will significant current flow and pitting corrosion start. For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl) is the most frequently encountered in service. The effect of chloride ion concentration on the pitting potential of aluminum 1199 (99.99+% AI) is shown in Fig. 6. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential. In the absence of dissolved oxygen or other cathodic reactant, aluminum will not corrode by pitting because it is not polar-

Understanding the Corrosion Behavior of Aluminum

ized to its pitting potential Generally, aluminum does not develop pitting in aerated solutions of most nonhalide salts because its pitting potential in these solutions in considerably more noble (cathodic) than in halide solutions, and it is not polarized to these potentials in normal service (Ref 12). Pitting potentials for selected aluminum alloys in several electrolytes are reported in Ref. 13. Examples of application of pitting-potential analysis to particular corrosion problems are given in Ref 14 and IS.

I

31

cathodic reactant is depleted. Galvanic corrosion is also low where the electrical resistivity is low, as in high-purity water. Some semiconductors, such as graphite and magnetite, are cathodic to aluminum, and in contact with them, aluminum corrodes sacrificially. The problems associated with galvanic corrosion of aluminumgraphite composites are described in Chapter 10.

Forms 01 COlTOs;on

Ga/van;c Relations Table 7 is a galvanic series of aluminum alloys and other metals representative of their electrochemical behavior in seawater and in most natural waters and atmospheres. As is evident in Table 7, aluminum and its alloys become the anodes in galvanic cells with most metals, and aluminum corrodes sacrificially to protect other metals. Only magnesium and zinc (including galvanized steel) are more anodic and corrode to protect aluminum. Because they have nearly the same electrode potential, neither aluminum nor cadmium corrodes sacrificially in a galvanic cell. The degree to which aluminum corrodes when coupled to a more cathodic metal depends on the degree to which it is polarized in the galvanic cell. It is especially important to avoid contact with a more cathodic metal where aluminum is polarized to its pitting potential because, as shown in Fig. 5, a small increase in potential produces a large increase in corrosion current. In particular, contact with copper and its alloys should be avoided because of the low degree of polarization of these metals. In atmospheric and other mild environments, aluminum can be used in contact with chromium and stainless steel with only slight acceleration of corrosion. In these environments, the two metals polarized highly so that the additional corrosion current impressed onto aluminum with them in a galvanic cell is small. To minimize corrosion of aluminum in contact with other metals, the ratio of the exposed area of aluminum to that of the more cathodic metal should be kept as high as possible. Such a ratio reduces the current density on the aluminum. Paints and other coatings for this purpose can be applied to both the aluminum and the cathodic metal, or to the cathodic metal alone. Paints and coatings should never be applied to only the aluminum because of the difficulty in applying and maintaining them free of defects. Galvanic corrosion of aluminum by more cathodic metals in solutions of nonhalide salts is usually less than in solutions of halide ones because the aluminum is less likely to be polarized to its pitting potential. In any solution, galvanic corrosion is reduced by removal of the cathodic reactant. Thus, the corrosion rate of aluminum coupled to copper in seawater is reduced greatly when the seawater is deaerated. In closed multimetallic systems, the rate, even though it might be high initially, decreases to a low value whenever the

When corrosion of aluminum and aluminum alloys occurs, it is usually of a localized nature and is most commonly caused by pitting or at points of contact with dissimilar metals in a conductive environment

Table7 Galvanic series of metal,expo,ed to seawater Activeend (anodicor least noble) Magnesium Magnesium alloys Zinc Galvanized steel Aluminum 1100 Aluminum 6053 Alciad Cadmium Aluminum 2024 (4.5 Cu, 1.5 Mg, 0.6 Mn) Mild steel Wrought iron Cast iron 13% chromium stainless steel type 410 (active) 18-8 stainless steel type 304 (active) 18-12-3 stainless steel type 316 (active) Lead-tin solders Lead Tin Muntz metal Manganese bronze Naval brass Nickel (active) 76Ni-l6Cr-7Fealloy (active) 6ONi-30Mo-6Fe-IMn Yellow brass Admirality brass Aluminum brass Red brass Copper Silicon bronze 70:30 Cupro Nickel G-bronze M-bronze Silver solder Nickel (passive) 76Ni-1 6Cr-7Fe alloy (passive) 67Ni-33Cu alloy (Monel) 13% chromium stainless steel type 410 (passive) Titanium 18-8 stainless steel type 410 (passive) 18-12-3 stainless steel type 316 (passive) Silver Graphite Gold Platinum

Passiveend (cathodicor most noble)

32

I

Corrosion of Aluminum and Aluminum Alloys

(seawater or road splash containing deicing salts). Corrosion can also be combined with other processes. Examples include the following: •

Mechanically assisted degradation, which includes forms of corrosion that contain a mechanical component (such as velocity, abrasion, and hydrodynamics) and results in erosion, cavitation, impingement, and fretting corrosion • Environmentally assisted cracking, which is produced by corrosion in the presence of static tension stress (stress-corrosion cracking) or cyclic stress (corrosion fatigue)

Uniform or general corrosion of aluminum is rare, except in special, highly acidic or alkaline corrodents. However, if the surface oxide film is soluble in the environment, as in phosphoric acid or sodium hydroxide, aluminum dissolves uniformly at a steady rate. If heat is involved, as with dissolution in sodium hydroxide, the temperature of the solution and the rate of attack increases. Depending on the specific ions present, their concentration, and their temperature, the attack can range from superficial etching to rapid dissolution. Uniform attack can be assessed by measurement of weight loss or loss of thickness. Relationships among some of the units commonly used for measuring uniform corrosion are given in Table 8. Dissolution is most uniform in pure aluminum and then next most uniform in dilute alloys and the nonheat-treatable alloys (Ref 17). Highly alloyed heattreatable alloys often show some surface roughness, especially when thick cross-sections are etched because variable dissolution rates result from throughthickness variations in solid solution concentration of the alloying elements and in the distribution of constituent particles. Localized Corrosion. In environments in which the surface film is insoluble, corrosion is localized at weak spots in the oxide film and takes one of the following forms: • • •

Pitting corrosion Crevice corrosion, including staining corrosion and poultice corrosion Filiform corrosion

•

Galvanic corrosion, including deposition and straycurrent corrosion. (It should be noted that while galvanic corrosion most often appears highly localized, uniform thinning can occur if the anodic area is large enough and a highly conductive electrolyte exists.) • Intergranular corrosion • Exfoliation corrosion • Biological corrosion, which often causes or accelerates pitting or crevice corrosion

Localized corrosion has an electrochemical mechanism and is caused by a difference in corrosion potential in a local cell formed by differences in or on the metal surface. The difference is usually in the surface layer because of the presence of cathodic microconstituents that can be insoluble intermetallic compounds or single elements. Most common are CuA12, FeA13, and silicon. However, the difference can be on the surface because of local differences in the environment A common example of the latter is a differential aeration cell. Another is particles of heavy metal plate out on the surface. Less frequent is the presence of a tramp impurity such as iron or copper embedded in the surface. Other causes of local cell formation have been listed by Mears and Brown (Ref 18). The severity of local cell corrosion tends to increase with the conductivity of the environment Another electrochemical cause of localized corrosion is the result of a stray electric current leaving the surface of aluminum to enter the environment The only type of localized corrosion that does not have an electrochemical mechanism is fretting corrosion, which is a form of dry oxidation. In almost all cases of localized corrosion, the process is a reaction with water:

The corrosion product is almost always aluminum oxide trihydroxide (bayerite). Localized corrosion does not usually occur in extremely pure water at ambient temperature or in the absence of oxygen but can occur in more conductive solutions because of the presence of ions such as chloride or sulfate. An exami-

Table 8 Conversion factors for commonlyused unitsfor measuringuniform corrosion dis metal densityin gramsper cubiccentimeter (g/cm3). FactorroccOIM!rslon to UDit

meld

Milligrams persquaredecimeter

perday(mdd) Gramsper squaremeterper day('E/m2/d) Micronsperyear(/1m1yr) Millimeters per year(mmlyr) Milsperyear (milslyr) Inchesperyear(in./yr) Source:Ref 16

10 0.0274d 27.4d 0.696d 696d

gJm2Jd

IlJIlIyr

IIlIII/yr

miIs/yr

0.1

36.51d

0.0365/d

1.4441d

0.00144ld

I

365/d I 1,000 25.4 25,400

0.365/d 0.001 I 0.0254 25.4

14.4/d 0.0394 39.4 I

O.0144/d 0.0000394 0.0394 0.001

1000

I

0.00274d 2.74d 0.0696d 69.6d

In./yr

Und. .tancling the Corrosion Behavior of Aluminum I 33 nationof the corrosion productcan identify the offending ion andthus causethe corrosion (Ref 19 and 20).

Effect of Composition and Microstructure on Corrosion 1.xxx Wrought Alloys. Pure aluminum (99.00% or purer) is more corrosion resistant than any of the aluminum alloys. Rapid dissolution will occur in highly acidic or alkaline solutions, but in the oxide stablerangeof pH 4 to 9, aluminum is subjectonly to water staining of the surface and to localized pitting corrosion (Ref 17).Pure aluminum doesnot incur any of the more drastic forms of localized corrosion such as intergranular corrosion, exfoliation, or SCC. Wrought aluminums of the lxxx series conformto composition specifications that set maximum individual, combined, and total contents for several elements present as natural impurities in the smelter-grade or refinedaluminum used to produce theseproducts. Aluminums 1100and 1135differsomewhat from the others in this series in having minimum and maximum specified copper contents. Corrosion resistance of all lxxxcompositions is veryhigh,but undermanyconditions, it decreases slightly with increasing alloy content. Iron,silicon,and copper are the elements present in the largestpercentages. The copper and part of the silicon are in solid solution. The second-phase particles present contain either iron or iron and silicon~Fe, AI3Fe, and AI12Fe3Siz-all of which are cathodic to the aluminum matrix. Whentheseparticles are presentat the surface, the oxide film over them is

Mixed oxide

thin or nonexistent. The local cells produced by these impurities promote pitting attack of the surface in a conductive liquid (Fig. 7). The number and/or size of suchcorrosion sites is proportional to the areafraction of the second-phase particles. Not all impurity elements are detrimental to corrosion resistance of lxxx series aluminum alloys, and detrimental elements can reducethe resistance of some typesof alloysbut have no ill effects in others. Therefore, specification limitations established for impurity elements are often based on maintaining consistent and predictable levels of corrosion resistance in various applications rather than on their effects in any specific application. 2.xxxwrought alloys ancl2xx.x casting allGy., in whichcopperis the majoralloyingelement, are less resistant to corrosion than alloysof otherseries, which contain much lower amounts of copper. Alloys of this type were the first heat treatable high-strength aluminum-base materials, dating back to Duralumin developed in Germany in 1919 and subsequently produced in the United States as alloy 2017 (Ref 17). Much of the thin sheet made of these alloys is produced as an alclad composite, but thicker sheet and other products in manyapplications requireno protectivecladding. Electrochemical effects on corrosion can be stronger in these alloys than in alloys of many other types because of two factors: greater change in electrode potential with variations in amountof copper in solid solution (Fig.4) and,under someconditions, the presence of nonuniformities in solid-solution concentration.However, generalresistance to corrosion decreas-

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Aluminum 99.5%

Corrosion of 99.5% pure aluminum in an aggressive solution. Iron-or silicon• containing impurities present at the surface creote local galvanic cells that promote pilling corrosion of the surface. The thin oxide layer covering these secondphase particles exhibits a different chemical composition at areos containing these impu~ities. The aluminu~ oxide ~tsell is a good insulator, but an aluminum oxide containing Iron, lor example. IS a semiconductor that allows the electrons to pass to a certain degree, making galvanic corrosion possible. Source: Rel21

34 I Corrosion of Aluminum and Aluminum Alloys ing with increasing copper content is not primarily attributable to these solid-solution or second-phase solution-potential relationships. The decrease in general corrosion resistance is attributable to galvanic cells created by formation of minute copper particles or films deposited on the alloy surface as a result of corrosion. As corrosion progresses, copper ions, which initiallygo into solution,replate onto the alloy to form metallic copper cathodes. Reduction of copper ions and increasedefficiencyof 0:2 and H+ reductionreactions in the presence of copper increase the corrosion rate. These alloys are invariably solution heat treatedand are used in either the naturallyaged or the precipitation heat treated temper. Developmentof these tempers using good heat treating practice can minimize electrochemical effects on corrosion resistance. The rate of quenching and the temperature and time of artificial aging both can affect the corrosion resistance of the final product. Principal strengthening phases of artificially aged 2xxx alloys are CuAlz for alloys with

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Environmentally Assisted Cracking / 119

tained by blasting with steel grit can be beneficial although the use of round shot obviously is preferable. It is imperative that blasting be uniform and of sufficient intensity to prevent discontinuities in the worked surface. A combination of shot-peening and a good organic coating will prevent stress-corrosion failures almost indefinitely in all but the most adverse conditions of stress and environment. Protective Coatings. When other methods are not feasible, there is always the possibility that a suitable coating can be applied to protect the stressed part. The ideal coating would completely isolate the metal from the corrosive environment. In practice it is not easy to find such a perfect coating or to be sure of its pennanence. It has been demonstrated, however, that coatings can give considerable protection. Anodizing followed by a good paint coating will delay for a considerable time the failure of susceptible specimens, which are stressed to a high degree (Table 5) . Metallized coatings of aluminum alloys (e.g., 7072 alloy) with or without a final coating of paint also provide good protection. As stated in the discussion of "Effects of Product Form," alclad products also provide excellent protection against sec.

Examples 01

secIn theAll'(raIt Industry Because of the longtime and extensive use of aluminum in aircraft, particularly in airframe construction, sec has been a problem in both commercial and military aircraft. The potential for sec as well as other harmful forms of corrosion in aircraft has led to many improvements in alloyltemper development and to new and improved methods for corros ion prevention, monitoring, and inspection.

Many airframe see failures have involved structures that were manufactured from aluminum alloys, especially 7079-T6, which is no longer produced in North America, and 7075-T6. These include the following airframe components that were fabricated from aluminum and that have been observed to fail by sec (Ref 17) (the specific aluminum alloy is provided in parentheses): • A main landing -gear locking cylinder (7079-T6) • A main landing-gear H-link structure (7079-T6). This damaged component is illustrated in Fig. 22. The stress corrosion was induced by the precipitation of magnesium aluminide (Mg2AI3)' which caused the grain boundaries in the aluminum forging to deteriorate anodically. • The front and rear spars of a vertical fin (7079-T6). As shown in Fig. 23, many of the cracks in this failure propagated from fastener holes. These spars had received corrosion-preventive surface treatment However, some of this protection was inadvertently removed during the installation of bolts. Bare metal, therefore, was exposed to a high-humidity environment and sustained high tensile stresses that were produced by the installation of fasteners into these structures. This problem was remedied by the use of 7075-T73 aluminum forgings for the front and rear spars. The latter material provides greater resistance to sec than 7079-T6 provides. • The bearing housing of a vertical stabilizer beam (7079-T6) • A main landing -gear bogie, which has the appearance of a beam or strut-type structure (7075-T6) • The hydraulic cylinders that serve as actuators for a main landing gear door (7075-T6). Views of the fracture surface, including the appearance of inter-

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120

I CotTosion of Aluminum and Aluminum Alloy.

granular fracture, are shown in Fig. 24. This problem could have been alleviatedby the application of a better corrosion-protective treatment in order to minimize the degree of pitting corrosion that occurred during the storage of these cylinders. The use of the more SeC-resistant aluminum alloy 7CJ75-T73 also would have helped to prevent this failure • The fork and strut components of a nose landing gear (7CJ75-T6)

• A fuselage frame structure,in which SCC occurred betweenfastenerholes (7075-T6) • A nose landing gear strut (7076-T6) In addition to these 7xxx-series alloys, aircraft structures that are fabricated from 2017-T4 and 2017-T451 can fail by SCC when sustained stresses are exerted in thetransverse direction relative to the grain structure. The following examples further documentthe occurrence of SCC in high-strength aluminum alloy aircraft components.

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'1 Failure by sec of an Ejection

Seat Swivel. A routine examination on a seat ejection system found that the catapult attachment swivel contained cracks on opposite sides of the part. This swivel, or bath tub, does not experience any extreme loads prior to activation of the catapult system. Some loads could be absorbed, however, when the aircraft is subjected to g loads. The bath tub is fabricated from aluminum alloy 7075-T651 plate. Investigation. Visual examination of the part revealed that cracks were positioned near the base of the bath tub configuration and extended through the wall thickness. One of the cracks was opened (Fig. 25a); this indicated that the fractures initiated on the inner walls of the fixture. Electron optical examination of the fracture at low magnifications revealed a woody appearing topography (Fig. 25b). Further electron optical examination of the fracture at 800 and 2000>< (Fig. 25c and d) showed that the cracking pattern initiated and progressed by an intergranular failure mechanism. This fracture topography indicated that cracking was due to stress corrosion.

Examination of the microstructure near the fracture revealed that the crack was progressing parallel to the transverse grain flow direction and further suggested see. Chemical analysis and hardness tests conducted on the submitted material showed it to be within specification requirements for 7075-T651 aluminum-base material. Conclusion andRecommendation. It was concluded that failure of the catapult attachment swivel fixture occurred by see, and it was recommended that the 7075 aluminum ejection seat fixture be supplied in the T-73 temper to minimize susceptibility to see. Example 21 Cracked Aircraft Wing Spar. A crack (Fig. 26) was found in an aircraft main wing spar flange fabricated from aluminum alloy 7079-T6 during a routine nondestructive x-ray inspection after the craft had logged 300 h. Investigation. Visual examination of the crack edge shown in Fig. 26(a) revealed that the installation of the fasteners produced a fit-up stress, as indicated by the approximate 0.75 mm (0.03 in.) springback of the flange after crack propagated through the hardware. Further inspection of the opened fracture (Fig. 26b)

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I Corrosion of Aluminum and Aluminum Alloys

showed that the crack had been present for some time because a heavy buildup of corrosion products was seen on the fractured surface. The fracture initiated at multiple origins between the arrow shown in Fig. 26(b). Metallographic examination of the flange in the area of fracture initiation showed the presence of end grain exposure (Fig. 26c), which would promote SCC. Electron optical examination of the fracture shown in Fig. 26(b) produced the scanning electron fractographs shown in Fig. 26(d) to (g). Figures 26(d) and (e) show an intergranular topography, while the fractographs in Fig. 26(t) and (g) reveal fatigue striations. This clearly shows the flange was cracking by a mixed mode of stress corrosion and fatigue. Chemical analysis of the flange showed that the material met compositional requirements for 7079 aluminum-base material. Hardness measurement of 85 HRB showed the material was in the heat treat condition. Conclusions. It was concluded that the cracking of the flange occurred by a combination of stress corrosion and fatigue. The cracking was accelerated because of an inadvertent fit-up stress during installation. The age of the crack could not be established. However, a reevaluation of prior x-ray inspections in this area would result in some close estimate of the age of the crack. End grain exposure further promoted SCC.

Example 31 sec of Pitostatic System Connectors. Pitostatic system connectors were found cracked

on several aircraft. The cracks were not restricted to any particular group of aircraft. Two of the cracked connectors were submitted for failure analysis. Both were reportedly made of 2024-1'351 aluminum. The connectors had cut pipelike threads that are sealed with teflon-type tape when installed. Investigation. Longitudinal cracks were located near the opening of the female ends of each connector (Fig. 27a). Both connectors had the same size female end but different size male ends. The connector with the large diameter and longer male end had two cracks, while the connector with the small diameter and shorter male end had only one crack. This size difference was believed to have had no bearing on the cracking. The connector with the large male end was sectioned, and part of the fracture was metallographically examined. The connector exhibited an elongated recrystallized grain structure with cut threads (Fig. 27b). A cross section through the fracture showed intergranular cracking and branching of the crack (Fig. 27c), characteristic ofSCC. Corrosion deposits were chemically removed from one section of the fracture surface, and the surface was examined in the scanning electron microscope. The fracture surface exhibited intergranular cracking of elongated grains (Fig. 27d). A section of the connector with the large male end and some thin transparent film found on the threads of the connector were chemically analyzed. The connector was determined to be either 2014 or 2017 alumi-

num alloy, and the film was determined to be fluorinated hydrocarbon teflon-type tape. Hardness checks on both connectors showed the large male end connector to be 75 HRB and the small male end connector to be 77 HRB. Electrical conductivity checks on both connectors showed the large male end connector to have a conductivity of 31% lACS (International Annealed Copper Standard) and the small male end connector to have a conductivity of 27.5% lACS. The threads of all connector components were incompletely formed with a bottom tap and therefore produced a tapered or pipe-type thread. The large male end connector had only one to two threads cut full depth (Fig. 27e). Conclusions. It was concluded that the pitostatic system connectors failed by SCC. The corrodent involved could not be conclusively determined. The stress was caused by forcing the improperly threaded female nut over its fully threaded male counterpart to effect a seal. The pipelike, incomplete threads produced high hoop stresses when torqued down over a fully formed thread. The one connector tested for chemical composition was not made of 2024 aluminum alloy as reported but of 2017 aluminum. Hardness and conductivity data on both connectors were compatible with a 1'351 condition for a 2024 alloy. Recommendations. It was recommended that the pitostatic system connector manufacturing process be revised to produce full-depth threads rather than pseudo pipe threads. It was also recommended that the wall thickness be increased to increase the hoop stress bearing area if pipe threads were to be used. A determination of proper torque values for tightening the connectors was also suggested.

Corrosion Fatigue Corrosion fatigue is defined as "the sequential stages of metal damage that evolve with accumulated load cycling, in an aggressive environment (gaseous or aqueous) compare to inert or benign surroundings, and resulting from the interaction of irreversible cyclic plastic deformation with localized chemical or electrochemical reactions" (Ref 18). Like SCC, the mechanism of corrosion fatigue involves either hydrogenassisted cracking and/or anodic dissolution. The contribution of each mechanism is controversial and depends on many mechanical, metallurgical, and environmental variables. Some of the more significant variables will be briefly reviewed below. More detailed information on the mechanisms of corrosion fatigue can be found in Fatigue and Fracture, Volume 19 oftheASM Handbook (ASM International, 1996), in various Special Technical Publications published by ASTM (refer to the Selected References listed at the conclusion of this chapter), and in a recent review highlighting modem laboratory methods of characterizing the corrosion fatigue behavior of metals in

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132


normal yield stress, under the conditions where its surface is wetted by some melting liquid metal. Fractures exhibit both brittle intergranular or transgranular modes. Crack growth rates resulting from LME are shown in Fig. 37. Mercury embrittles both pure and alloyed aluminum, decreasing the tensile stress by some 20%. The fatigue life of 7075 aluminum alloy is reducedin mercury, and brittle-to-ductile transition occurs at 200 °C (390 "F), Additions of gallium and cadmium to mercury increase the embrittlementof aluminum.Delayed failureof LME occurs in mercury. Dewettingof aluminum by mercury has been found to inhibit embrittlement; dewetting can be caused by the dissolution of aluminum by mercury, oxidation of fine aluminum particles by air, and formation of aluminum oxide white flowersat the aluminum/mercury interface. Liquid-mercury embrittlementhas been the cause of failures of welded aluminum alloy 5083-0 piping and plate heat exchangersused in ethylene plants (Ref 40). Naturally occurring mercury is occasionally found in unrefined hydrocarbons. The unrefined feedstock can contain mercury at levels as high as 40 ppb. In a plant operation where 2 billion pounds of feedstock can be processed per year, mercury impurity levels in the parts per billion range present a serious threat to the integrity of the aluminum equipment. Mercury-removal systemsare necessaryin such plants. Aluminumalloys also are embrittledby tin-zinc and lead-tin alloys. The embrittlementsusceptibility is related to heat treatment and the strength level of the alloy. Gallium in contact with aluminum severely disintegrates unstressed aluminum alloys into individual grains. Therefore, grain-boundary penetration of gallium is sometimesused to separategrains and to study

topographical features and orientations of grains in aluminum. There is some uncertaintyas to whetherzinc embritties aluminum. However, indium severely embrittles aluminum. Alkali metals, sodium, and lithium also are known to embrittle aluminum Aluminum alloys containing either lead, cadmium, or bismuth inclusions embrittlewhen impact-testednear the melting point of these inclusions; the severity of embrittlement increasesfrom lead to cadmium to bismuth.

REFERENCES 1. TJ. Summerson and D.O. Sprowls, Corrosion Be-

havior of Aluminum Alloys, International Conference of the Hall-Heroult Process Vol ll/ of Con! Proc., (University of Virginia), Engineering Materials Advisory Services Ltd.,p 1576-1662 2. T.D Burleigh, E.H. Gillespie, and S.C. Biondich, "Blowout of Aluminum Alloy 5182 Can Ends Caused by Transgranular Stress Corrosion Cracking," Preprint fromTMS meeting, 1-5 Nov 1992 3. T.D.Burleigh, ThePostulated Mechanisms forStress Corrosion Cracking of Aluminum Alloys: A Review of the Literature 1980-1989, Corrosion, Vol47 (No. 2), 1991,P 89-98 4. R.H. Jones and R.E Ricker, Mechanism of StressCorrosion Cracking, Stress-Corrosion Cracking: Materials Performance andEvaluation, RH. Jones, Ed.,ASM International, 1992,p 1-40 Stress intensity, ksi "IiI. 5

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Types of Corrosive Environments I 141

ering data for casting alloys exposed for the same period of time and at the same sites. Specimens were separately sand-east and permanent mold cast tensile bars, each with a reduced section 12.7 mm (0.5 in.) in diameter. Strength change data for these alloys are summarized in Table 6. Alloys with relatively high coppercontents, such as 295.0-T6, 208.0-F, 319.0-T6, and 319.0-T61, showed the greatest losses. Alloys of the zinc-eontaining Txx:x series generally exhibited larger strength losses than alloys having low zinc or copper contents. In all cases, as for wroughtmaterials, severity of corrosion varied widely, depending on environmental conditions. Comparison with Other Metals. Other metals were exposed to the same weathering environments over the same time periods used to evaluate corrosion of aluminum alloys. Comparative corrosionrates (averageloss in thickness per side calculated from weight losses measured after exposures of 10 and 20 years) are listed in Table 7 for aluminum, copper, lead, and zinc panels. Figure 2 compares losses in tensile strengths at several weathering sites for unprotected low-carbon steel (O.09C, 0.07Cu) and for aluminum alloys. Contact with Dissimilar Metals. The hazards of using dissimilar metals (such as brass, copper, nickel,

formedafter 7 years of exposure (Ref 13).Thirty-four combinations of alloy and temper in the form of 1.27 mm (0.050 in.) thick sheet were exposed at four sites-two seacoast, one industrial, and one rural; Table 3 lists averagevalues of measurements reportedat two of the more aggressive sites. In another ASTM program, 10 yearsof weathering producedthe changes in tensile strength reported in Table 4. Table 5 lists changes in tensile strength of 64 mm (2.5 in.) thick plates of alloys/tempers commonly used in aerospace applications. Test data are presented for 6 months, and 1,2, and 5 years of exposure. Data from these and other weathering programs (Ref 16, 17)demonstrate that differences in resistance to weathering among non-heat-treatable alloys are not great, that alclad products retain their strength well becausecorrosionpenetration is confined to the cladding layer, and that corrosion and resulting strength loss tend to be greater for bare (nonalclad) heat treatable 2xxx and Txxx series alloys. As stated earlier, heat treatable alloys susceptible to exfoliation corrosion or SCC should not be used in marine environments. Data for Casting Alloys. The testing program that was the source of the strength change data for wrought alloys given in Table 4 also providedweath-

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-l--

~_\.

-

--'

..

~_

~

1

,.-:

\

"\' -

;

;

......

Maximum attack, specimens at New Kensington, PA

E E

60

50

40

I Average attack, specimens

L.-&.....,

Correlation 01 weathering data lor specimens 01 alloys 1100, 3003, and 3004 (all in H14 temper) exposed to industrial atmosphere (curves)with service experience with aluminum alloys in various locations

142/ Corrosion of Aluminum and Aluminum Alloys

and steel) with aluminum for atmospheric applications are present but not as acute as they are for application where the combination of metals is exposed to aqueous environments (Ref 8). Galvanic corrosion of aluminum will certainly occur adjacent to the cathodic (more noble) metal, but the severity will vary with the environment and the length of time the aluminum and dissimilar metal parts are wet. Seacoast atmospheres are particularly conducive to galvanic corrosion of aluminum because salt deposits can accumulate and, under damp conditions, can provide a strong electrolyte in which severe galvanic corrosion of aluminum can occur. The hazards of galvanic attack can be substantially reduced through selection of the most compatible metals suitable for the application and by employing protective coatings. Examples of compatible metals are 300 series stainless steels, as well as zinc, cadmium, or chromium plated steels.

Plating thickness is important and will affect the length of the period for which satisfactory service can be expected. A little regarded, but nevertheless important, consideration with dissimilar metals (notably alloys of copper or nickel) is the effect of ''wash'' or drainage from the dissimilar metal onto aluminum. Ions of cathodic metals can be reduced or deposited on aluminum surfaces establishing sites for galvanic corrosion (deposition and galvanic corrosion are further described in Chapter 5). Wash from small dissimilar metal parts might not supply metallic ions in sufficient quantities to be harmful, but from large surface areas (roofing, flashing, coping, etc), it can cause severe corrosion. Dissimilar metal drainage should be avoided whenever feasible; if encountered, the best remedial measure is to coat the second metal or better still, to coat both metal components.

Table 3 Weathering data for 1.27 mm (0.05 in.) thick aluminum alloy sheet after 7 year exposure (ASTM program started in 1958) Average values from Kure Beach, NC,and Newark, NJ A&yand temper

Corrosion rate(a)

mmJyr

Jlin./yr

Non-heat-treatable alloys 11OO-H14 345 1135-HI4 321 1188-H14 250 1199-HI8 205 3003-H14 295 301»-H34 414 4043-H14 335 S005-H34 373 50SO-H34 349 5052-H34 362 5154-H34 326 5454-0 348 5454-H34 342 5456-0 381 5357-H34 292 5083-0 469 S083-H34 375 S086-H34 436

13.6 12.6 9.8 8.1 11.6 16.3 13.2 14.7 13.7 14.3 12.8 13.7 13.5 15.0 11.5 18.5 14.8 17.2

Heat-treatable alloys 2014-T6 644 2024-T3 1022 2024-T81 725 2024-T86 806 6061-T4 378 6061-T6 422 7075-T6 688 7079-T6 635

25.4 40.2 28.5 31.7 14.9 16.6 27.1 25.0

Maximumdepth of attack in 7years mils jUD

jUD

mils

2.6 3.3 4.8 3.8 3.4 4.7 4.1 3.0 4.2 2.4 3.6 3.7 4.1 4.1 5.4 4.0 3.5 4.1

29 37 46 57 52 44 34 27 58 43 65 41 30 37 102 52 56 76

57 98 119 65

3.0 3.0 3.8 3.0 2.2 3.9 4.7 2.6

50 67 76 58 38 42 71 37

2.0 2.6 3.0 2.3 1.5

A1c1ad alloys-heat treatable and non-heat-treatable 2014-T6 358 14.1 43 2024-T3 264 10.4 46 3003-HI4 345 13.6 128 5155-H34 345 13.6 53 6061-T6 14.0 98 356 7075-T6 502 19.8 53 7079-T6 324 72 12.8

1.7

1.8 5.0 2.1 3.9 2.1 2.8

28 27 117 35 25 41 36

1.1 1.1

(a)Basedon weightchange. Source:Ref13

70 83 121

Averagedepth of attack in 7 years

%

86 119 105 76 107 62 91 95 105 104 138 102 88 105 77 76 97 77

1.1

1.5 1.8 2.2 2.0 1.7

1.3 1.1

2.3 1.7 2.6 1.6 1.2 1.5 4.0 2.0 2.2 3.0

1.7

2.8 1.5

4.6 1.4 1.0 1.6 1.4

Lossin !emile strength in 7years, ...

0 0.4 0 3.9 1.1 1.1

2.8 0.9 0.5 0.8 0.9 1.5 0.5 0.4 0.4 1.8 2.2 1.9 1.7 2.0 6.0 6.2 0.4 0.7 1.7 0.5 0 0 0 0 0.7 0.1 0

Types

Contact with Nonmetallics. The weather resistance of aluminum can be seriously affected when aluminum is used in contact with nonmetallic substances that either become saturated by moisture or are hygroscopic (Ref 8). Moist wood, insulation, or masonry in contact with aluminum can stimulate accelerated corrosion simply by keeping the aluminum wet for prolonged periods. These moist materials can also create a poultice which establishes a corrosionconducivedifferently aerated cell. Painting the aluminum and/or the nonmetallic material, where practical, with a good quality coating (free from heavy metal pigmentation) is recommended. In addition, the use of a sealant between the aluminum and the nonmetallic materialcan be considered.

of Corrosive Environments I

143

has formed Technical Committee TCl56 for the purpose of writing atmospheric corrosion testing standards. Useful reviews of the ongoing efforts of ASTM and ISO to evaluate atmospheric corrosion can be found in Ref 19 and 20.

IndoorAtmospheres Indoor air is relativelybenign providedthe temperature is relatively constant (no marked, rapid cooldown) and the air is dehumidified. Metallographers frequently store polished metallographic mounts of aluminum specimens in sealed desiccators for weeks withoutany stainingoccurring. Staining,filiformcorrosion,and other surfacecorrosion can be a serious problem on products stored indoors in unheated buildings, tractor trailers, etc. The problem is condensation on the metal during cool nights after warm, humid days. Airborne pollutants, especiallyS02, dissolvein the condensed vaporresulting in a conducting electrolyte. Serious staining problems can occur quickly.The problems associatedwith water stainingare addressed in Chapter 3. Another significantindoor atmosphere corrosion is that normal humidity, in the typical 40 to 55% relative humidity human "comfort zone," can be a sufficient

Atmospheric: COlTOsion Testing A wide variety of tests have been developed by ASTM for determining the susceptibility of metals to weathering. Tests for flat panels, open-helix specimens, galvanic specimens, and SCC specimens have been standardized. These tests have resulted from the efforts of ASTM Committee G-l and the Subcommittee GOl.04 on Atmospheric Corrosion.In addition,the International Organization for Standardization (ISO)

Table 4(a) Lou in tensile strength for wrought aluminum alloys during various atmospheric exposures (ASTM program) Exposed as 102 x 203 mm (4 x 8 in.) panels. Calculated from average tensile strength of several specimens (usually four) Cbauge in strength, %, during .xpooure orindkated length at Alloy and temper

6mo

State College, PA lyr 3yr 5yr

1.62mm (0.064in.) sheet 2024-T3 I 8 3003-HI4 0 6 -I 3004-H34 6 SOSO-H34 0 6 S052-H34 0 9 -2 606I-T6 5 -I 7075-T6 5

2 2 0 -I -I -2 -3

0 0 0 0 -I -3 0

IOyr

6mo

I

2 4 7 4

I I -I 0 0

-3

-I

-1

-2

6.35 mm (0.25ln.) plate 2014-T4 -3 0 -I 2014-T6 0 606I-T6 -4 0

0 0 -2

0 0 -I

0 0 0

I -I 0

-I

6.35 mm (0.25in.) extruded bar 2014-T4 3 I 2 2014-T6 -I 0 0 606I-T6 0 0 0 -I -I 6063-T5 I -I -3 -I 7075-T6

-I -I -I

-4 0 7

-I

1

1

-2

-3

-I

6.35 mm (0.25in.) aIcIad plate -I 2014-T6 0 2024-T3 0 0 0 7075-T6 0

I

-8 -4 -2 -2

-I

1.62mm (0.064in.) aIcIad sheet -I -I 2014-T6 5 2024-T3 7 -I I 7075-T6 0 6 6

-2

lyr

New York, NY 3yr 5yr

-7 -5 -5 -I -6(a) -7 -5

IOyr

-3 0 2

-4 -2 -2

-I

-I

-I

0 -2

-2 -I -2

-3(a) -I -4

-4 0 -I -2 -I -4 -4

-I

-6 -4 -2

-2

-2(a) 6 6

-I I 2

-I 0 2

-4 0 -I

-2 -I 0

0 -2 -I

-2 -I -2

-I -I 4

-4 -I 3

-4 -2 -4

I -2

-I

0 -I 0

0 -I -I

-12 -I -8

0 0 0

0 -2 I

I -2 -I

-I -2 0

-2 -2 I

-I 2 0

0 0 I

0 I 0

0 2 0

0 I -II(a)

I

I

I -I

0 -2 0 -2 -2

I -I -3 9 -I

-2 -2 -3 II -4

0 -I -I -I -2

0 2 -I 8 -I

-I -I -2 3 0

-I -2 -I 6

-13

-I

I -2 I

-2

0

-5 0 7

I 0

-II(a) -11 (a) 6 -8 -6 5 -7 -5 6 -8 -4 5 -5(a) -7 (a) 6 -4 -11 -8 -6(a) -8(a) 4

Kure Beach, NC 3yr lyr 5yr

-4 -3 -5

4 8 5

-5

6mo

-4 -3 -5

2 0 -2

I

IOyr

-2

-I -2

-I

(a)Averagetensilestrengthvalueswerebelowrequiredminimum. Source:Ref 14

1

-I 6 2 -2

144 I Corrosion of Aluminum and Aluminum Alloys electrolyte to cause SCC in highly susceptible lowcopper or copper-free7xxx alloys, such as 7079-T651. Fortunately, this is now well known and these highly susceptiblealloys are no longer produced.

Corrosion in Waters High-Purity Water. Suitability of the more corrosion-resistantaluminumalloysfor use with highpurity waterat room temperature is well establishedby both laboratory testing and service experience (Ref 21). The slight reaction with the water that occurs initially ceases almost completely within a few days after developmentof a protectiveoxide film of equilibrium thickness. After this conditioning period, the amount of metal dissolved by the water becomesnegligible. Corrosion resistance of aluminum alloys in highpurity wateris not significantly decreasedby dissolved carbon dioxide or oxygen in the water or, in most cases, by the various chemicals added to high-purity water in the steam power industry to provide the required compatibility with steel. These additives include ammonia and neutralizingamines for pH adjustment to control carbon dioxide, hydrazineand sodium

sulfate to control oxygen, and filming amines (longchain polar compounds) to produce nonwettable surfaces. Somewhat surprisingly, the effects of alloying elements on corrosion resistance of aluminum alloys in high-purity water at elevatedtemperaturesare opposite to their effects at room temperature; elements (including impurities) that decrease resistance at room temperature improveit at elevatedtemperatures. At 200 °C (390 OF), high-purity aluminum of sheet thickness disintegrates completely within a few days by reaction with high-purity water to form aluminum oxide. In contrast, aluminum-nickel-iron alloys have the best elevated-temperature resistance to high-purity waterof all aluminum metals; for example,alloy 8001 (1.0Ni-Q.5Fe) has good resistance at temperatures as high as 315°C (600 oF) (Ref 22). Seawater. Service experience with lxxx, 3xxx, 5xxx, and 6xxx wrought aluminum alloys in marine applications, including structures, pipeline, boats, and ships, demonstrates their good resistance and long life under conditions of partial, intermittent, or total immersion. Casting alloys of the 356.0 and 514.0 types also show high resistance to seawater corrosion, and these alloys are used widely for fittings, housings,and other marineparts.

Table 4(b) Loss in tensile strength for wrought aluminum alloys during various atmospheric exposures (ASTM program) Exposure as 102 x 203 mm (4 x 8 in.) panels. Calculated from average tensile strength of several specimens (usually four) Change in strength, ", during exposure orindicated length at Poiol Reyes.CA Freeport. TX

ADoyaDd temper

limo

1.62 mm (0.064 in.) sheet 2024-T3 3003-HI4 3OO4-H34 505O-H34 5052-H34 6061-T6 7075-T6

lyr

3yr

-13(a) 1 -3 2 -1 -3 -3

-19(a) -3 -1 -1

-2 -4 -4

Syr

10yr

limo

lyr

3yr

Syr

-19(a) -1 -I 0 0 -5

-23(a)

3 3 5 5 4 1 1

-2 0 -1 0 -1 -3 -1

-9(a) -5

-8

-13(a)

-4

-5

1 0 0 0 -1 -3

3 6 5

-1 -1 4

-3 -2 -1

-3 0 -1

-2 -3 -2

-1 0 0

-22(a) -2 -2

-4

-4 -4

-4

1 -2 -1 -5 -11 (a)

-4

-4

-1 -3

-3 -6

-6 -8(a) 2

-5 -8(a) 0

-4

0

-2 -2 -2

-1 1 -1

-1 1 0

2 0 2

-1 -1 -1

-8

1 1

3 1 0 2 0

-2 -1 -2 0 -1

-7 (a)

-4

10yr

-2 -3 -1 -3 -8(a)

1.62 mm (0.064 in.) alcIad sheet 2014-T6 2024-T3 7075-T6 6.35 mm (0.25 in.) plate 2014-T4 2014-T6 6061-T6 6.35 mm (0.25 in.) alcIad plate 2014-T6 2024-T3 7075-T6 6.35 mm (0.25 in.) extruded bar 2014-T4 2014-T6 6061-T6 6063-T5 7075-T6

-3 -1 3

-1 -1

-2

-1 -13(a) 1

-4

0 2 1

-1 0 -1

0 -1 0

3

-6

-3 -3 -1 3

-3 0

-4 -1 3 -3

-1 3 -3

-4

-7

1 0

0 7 0

(a) Averagetensile strength values were belowrequired minimwn. Source: Ref 14

11

0

0 0 1

-2

2

8

-5 -3 -2 -1

-1

-4

-2 -1

0

0

Types of ColTOsive Environments I 145

Among the wrought alloys, those of the 5xxx series are most resistant and most widely used because of their favorable strength and good weldability. Alloys of the 3xxx series are also highly resistant and suitable where their strengthrange is adequate. With the 3xxx and 5xxx series alloys, thinning by uniform corrosion is negligible, and the rate of corrosionbased on weight loss does not exceed about 5 lllJlI'year (0.2 mil/year), which is generally less than 5% of the rate for unprotected low-carbon steel in seawater. Corrosion is

mainlyof the pitting or crevice type, characterized by deceleration of penetration with time from rates of 3 to 6 ~m/year (0.1 to 0.2 miVyear) in the first year to average rates over a 10 year period of 0.8 to 1.5 ~m/year (0.03 to 0.06 mil/year), The aluminum-magnesium-silicon 6.ut alloys are somewhat less resistant; althoughno general thinning occurs, weight loss can be two to three times that for 5xxx alloys. The more severe corrosionis reflectedin largerand morenumerous pits.

Table 5 Lou in tensile .trength by corrosion-triplicate un.tre.sec:I.horHransv..... tensile bar. from 64mmplates AIIoyaod temper

2024-T35I 2024-T85I 5456-HI16 6061-T65I 70SO-T7651 70SO-T7451 7075-T65I 7075-T7651 7075-T7351

Decrease in tensile _gtb, \f" during exposure olindicated length in elWinmmellt type ll.uraI(c) Industrilll(b) 2yr limo limo 11' 51' 11' 21' 51' 21'

SellCoost(a)

limo

11'

33 20 4 I 14 12 20

11

46 25 14 8 18 18 23 15

7

11

52 26 19 11 24 21 26 15 16

70 28 20 13 26 25 27 19 23

6 2 5 I 5 8 2 I 3

9 5 7 2 12 9 2 3 5

13 6

20 11 22 8 16

11 2 14 12 3 3 7

17

6 9 9

I 0 0 0 0 0 0 0 0

2 0 2 0 2 I 0 0 0

51'

11

5 0 4 I 4 4 I I 2

6 7 4 5 8 5 5 4

(a)PointJudith,Rl (b) LosAngeles,CA.(c) AlcoaTechnical Center.Source:Ref 15

Table 6(a) Lou in tensile .trength for ca.t aluminum alloy. during various atmospheric exposure. (ASTM program) Exposed as separately cast tensile specimens. Calculated fromaverage tensile strength of several specimens (usuallysixl

AIIoyaod temper

StateCoIIege,PA

limo

11'

31'

5yr

-2 -3 -I I -I 0 -5 -2 -2 -2 -8 3

-2 -2 -3 -2 0 -2 -4 -6 -I -2 -3 -2

-I -4 0 I -I -2

Permanent mold castings -2 319.0-T61 I 355.O-T6 3 0 443.O-F 3 0 705.0-T5 -I -2 707.0-T5 2 -2 711.0-T5 -8 -11 713.0-T5 -2 -2

-I 7 -I -3 -3 -7 0

Sand castings -I 208.0"F 295.0-T6 I 319.0-T6 0 355.O-T6 0 356.0-T6 I 443.O-F 3 520.0-T4 I 705.0-T5 1 707.0-T5 I 710.0-T5 2 712.0-T5 0 I 713.0-T5

Change in strength, \f" during expllSUre oClndic:ated length at NewYork, NY limo 5yr limo 101' 101' 11' 31'

-2 -2 -3 -3 -I -2

-4 -3 -I -2 I

-I 0 -5 -7 -I

-I -2 I I I 0 2 0 2 I 0 -3

-2 2 -I -5 -3

-2 -4 -2 -3 -4 -8 -2

I I I -2 0 2 -I

...{j

...{j

-I

-4

-3 -2 -3 0 -2 -4

...{j

-2 0 -I 3 -I 0 I -3 -2 -4

-11

-4 -6 -6 -3 -2 -2 -I -4 -5 -3 -4 -I

-3 -5 -8 -I -2 -4 -2 -3 -9 -2 -5 -I

0 8 -I -2 -I -5 -2

-4 -2 I -3 -4 -2 -7

(a) Averagetensilestrengthvalueswerebelowrequiredminimum. Source:Ref 14

0 -5 -5 -3 -3 -3

-10 -15 -I -2 -5

-4 -7 0 -7 -7 -6 -2

Kure Beach, NC 11'

31'

-2 -7 -I 2 I -2 -2 I 2 2 -4 -5

-5 -9 -5 2 -I 0 -2 -2 -3 -I -3 -3

-7 -9 -7 0 0 0 -5 -3 -9 -I -8 -8

-5 -2 -I -3 I -2

-3 -7 0 -3 -2

-4 5

...{j

...{j

-11

-12

...{j

...{j

-5 -4

51'

101'

-6

-4 -9 -4 -3 -2 -2

-10 -6 -I -2 -1 -6 -3 -13 -2 -2 -I

-7 -I 2 -9 -7 -6 -4

-4 -18 -I -8 -3

-5 -5 0 -5

-12 -11 -1


Alloys of the 2xxx and Txxx series, which contain copper, are considerably less resistant to seawater than 3xxx, 5xxx, and 6xxx alloys and are generally not used unprotected. Protective measures, such as use of alclad products and coating by metal (thermal) spraying or by painting, provide satisfactory service in certain situations. Aluminum boats operating in salt water require antifouling paint systems because aluminum and its alloys do not inhibit growth of marine organisms. Aluminum is impervious to worms and borers, and the acids exuded from marine organisms are not corrosive to aluminum; however, the accumulation of biofouling on the bottom of the boat impairs performance. Aluminum boats operating in both salt and fresh water,

which alleviates fouling problems, have been able to leave underwater hull areas unpainted (Ref 27). To make antifouling paint systems adhere properly to aluminum, careful surface preparation of the metal is necessary. A thorough precleaning and either a conversion coating or a washcoat primer are required, followed by a corrosion-inhibiting primer and a top coat. The antifouling paint is applied to the top coat. Primers containing red lead should not be used, because this substance can cause galvanic corrosion of the aluminum (paints containing lead also pose a significant health risk). For the same reason, copper-eontaining antifouling paints should not be used on aluminum hulls. The preferred antifouling paints for aluminum are those containing organic tin compounds.

Table 6(b) Lossin tensile strength forcast aluminum alloys during various atmospheric exposures (ASTM program) Exposed as seporately cast tensile specimens. Calculated from average tensile strength of several specimens (usually six) Changein _gth, %, during exposure ofindicated length at AUoyand temper

Point Reyes,CA

Freeport, TX

lyr

3yr

Syr

10yr

6mo

lyr

3yr

Syr

10yr

-11 -13

-13 -15 -14 -8 -I -10 -6 -8 -8 -3 -7 -6

-11

-10 -16 -10 -10 -5 -10

-4 -2 -2 I 2 0 I 6 -I 4 I

-5 -9 -I -I -3 -I -4 3 -5 -I -7

-5 -10 -7 -4 0 -2 -7 -5 -15 -I

-9 -10 -6 -3 -3 -4

-6 -12 -4 -7 -4 -6

-4

--6

-7

-15(a) -2

-14(a) -8 -8 -3 -2 -6 -4

0 4 0 -3 I I -6

-7 -4 -I -5 -3 -4 -9

-4 5 -3 -5 --6 -3 -2

6mo

SWtd castings 208.0-F 295.O-T6 319.O-T6 355.0-T6 356.0-T6 443.O-F 520.0-T4 705.0-T5 707.O-T5 710.0-T5 712.0-T5 713.O-T5

-9 -4 0 -7 -3 3 -5 -I -7 -3

Permaaent mold castings 319.0-T61 355.0-T6 443.0-F 705.0-T5 707.O-T5 711.0-T5 713.0-T5

-7

--6 -7 -5 -3 -5 -9

-17

-11 -7 -2

-10 -7 -6 -7

-4 -8 0

-11 -6 -2 -9 -6

-4 -9 -3 -14 -3 -16(a)

-13 -10 -4 -9 -9 -9

--6

-11 -4 -16 0 -9 -6

-8 -32(a) -2 -9 -9

-5 -2 -2 -8

-5 -7 -2 -14 -24(a) -8 -6

-10 -I 0

(a) Average tensile strength values were below required minimum. Source: Ref 14

Table 7 Atmospheric cOrTOsion rates exposure sites

foraluminum and other nonferrol,ls metals at several

Depthofmeialremowdper side(a),inlIm/yr, during exposure ofindicated lengthfor specimens of Typeof

Location

Phoenix,AZ State College, PA KeyWest,FL Sandy Hook, NJ La Jolla, CA New York, NY Altoona,PA

~~

~~~

uoo~

~~

atmosphere

10yr

20yr

10yr

20yr

10yr

ZOyr

10yr

20yr

Desert Rural Seacoast Seacoast Seacoast Industrial Industrial

0.000 0.025 0.10 0.20 0.71 0.78 0.63

0.076 0.076

0.13 0.58 0.51 0.66 1.32 1.19 1.17

0.13 0.43 0.56

0.23 0.48 0.56

0.10 0.30

0.18 1.09 0.66

1.27 1.37 lAO

0.41 1.43 0.69

0.53 0.38

0.25 1.07 0.53 lAO 1.73 4.8 4.8

0.28 0.63 0.74

1.73 5.6 6.9

(a) Calculated from weight loss, assuming uniform attack, for 0.89 rom (0.035 in.) thick panels. (b) Aluminum 11 00-HI4. (c) Tough pitch copper (99.9% Cu). (d) Commercial lead (99.92% Pb). (e) Prime western zinc (98.9% Zn). Source: Ref 18

Types of Corrosive Environments / 147

The literature on corrosion testing of aluminum alloys in seawater is extensive. Summaries of information are provided in Ref 28 and 29, and in most of the Selected References. Table 8 lists results of 10 year immersion testing of various alloys in the form of rolled plate exposed in three locations. Similar data for extruded products of several 6xxx alloys and one 5xxx alloy are given in Table 9. Direct comparison of the data in Tables 8 and 9 is provided in Table 10, in which corrosion is expressed in terms of average weight loss, and in Fig. 10, which illustrates the decel-

eration of corrosion rate with time that is characteristic of aluminum alloys. Data on corrosion rates, maximum and average depth of pitting, and changes in tensile strength compiled during 10 year tidal and fullimmersion exposure of seven 5xxx alloys and superpurity aluminum 1199 are summarized in Table 11. Full immersion generally resulted in more extensive corrosion than tidal exposure, although the reverse relationship has also been observed. Tensile-strength losses were 5% or less, and yield-strength losses were less than 5% in the panels completely immersed

250 1100 3003 5052

200

50 0

-6051-6061

6051-6061^——i

1

^ T

§ 250

oo n oe in »- en in

100

\

150

-1100 3003 _5052.

i

LSI

:

°> 200 150 51-6061

/

100

6051-6061

6051-6061 50

2**^^

> ■ * " '

0 250 200 150 100 50 »

0

o

250

VI

Al-Mg

Al-M J - S i ,

r

/£*

C^ Al-Mg

Al-Mg

jc |

/

200 150 100

°/

Al-Mg 50 0

i

o / /AI-IV g-Si

-

*&""

-^Al-Mg-Si ""

Al-Mg

/o-1.3 mm/yr

'" ~ 70

ISO

8-

~

Ql

~

E Q)

E Q)

r

'E !

20 c0

0.5

e

0.34% H2 O

(

75

t! ii

~ 00002 ~

0/

0.004

o

.g 500

sFs:

0.006

0.002

.~

1000

Open '----

ii

duced HN03, is available in concentrations from 52 to 99%. Nitric acid over 86% is described as fuming. Nitric acid up to 95% is stored and shipped in type 304 stainless steel. Concentrated acid above 95% is handled in aluminum alloys 1100 or 3003. Figure 15 shows the reason for this; the corrosion rate of type 304 stainless steel increases rapidly above 95% concentration, while that of aluminum 3003 remains essentially constant to 100%. However, if acid concentration falls below 80% or if temperature rises above 40 °C (100 "F), much higher corrosion rates can be expected. This relationship is clearly shown in the isocorrosion diagram for aluminum (Fig. 16). The preferred aluminum alloys for HN03 service are alloys 1100 and 3003 (Ref 63). If higher strength is required, alloys 5052 or 5454can be used. In laboratory tests (Ref 61), the corrosion of aluminum alloys in H2S04 varies with acid concentration and temperature (Fig. 17). The corrosion reaches a maximum at about 80% acid concentration. Above that concentration, attack decreases rapidly until at 98% it becomes mild, less than 125 J.lmlyear (5 mils/year). In other laboratory tests (Ref 61), fuming acids containing 101, 103,107, and 115% H2S04 caused moderate attack of 3003 alloy at ambient temperature.

r 40

100

10

Fig. 16

20

30

SO 60 40 Concentration of HNOJ • %

70

80

Isocorrosion diagram for aluminum alloy 1100 in HN03 . Source: Ref 62

156 I COITOsion of Aluminum and Aluminum Alloys

Organic Acids. Aluminum shows good resistance to many organic acids at room temperature and is widely used for their handling. Some of the higher molecular weight acids cause severe attack of aluminum at highly elevatedtemperatures; therefore,the use of aluminum must be considered for the specific acid and temperature desired. This section deals with three of the more common organic acids: formic acid (HCOOH), acetic acid (CH3COOH), and propionic 6000

150

5000

125 50

4000

y\

100 ~

. 900

-O.79to-O.82 -O.78to-O.79 -0.75 -0.68 to-0.70(c)

3

0

0

\.

-

25

I

90

__ -I~-Hardness

I

.

75

100

III

a:

80 :r

,,; 70 ~ c::

Corrosron . potenttalI 60 ~ :r 50 I I I

50

Distance from weld centerline, mm

leI

2319 4145

-0.63 to-O.65(c)

(a) Potential of all tempers is the sante unless a specific temper is designated. (b) Measured in an aqueous solution of 53 g NaCI + 3 g H20 2 per liter at 25°C (77 "F). (c) Potential varies with quenching rate during fabrication.

Fig 1

Effect of Ihe heat of welding on microstructure, • hardness,and corrosion potential of welded assembliesof three aluminum alloys. Thedifferences in cerrosian potential between the heat-affected zone (HAll ond the base metal can lead to selective corrosion. (al Alloy 5456-H321 base metal with alloy 5556 filler; three-pass metalinertgos weld. lbl A1loy2219-T87 base metolwith olloy 2319 filler; two-pass tungsten inert gas weld. lcl Alloy 7039-T651 base metol with olloy 5183 filler; two-poss tungsteninert gas weld

Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 163

that was highly anodic to both base metal and weld metal. This behavior was attributed to the formation of the equilibrium ~ (AlLi) phase and resulted in severe pitting in the HAZ. In contrast, a 2090-type alloy showed a continuously increasing (cathodic) potential when going from base metal to weld metal and was resistant to pitting attack. This behavior was attributed to the absence of the ~ phase due to a higher copperlithium ratio. Filler Alloy Selection. Although aluminum alloys can be welded autogenously (without the addition of a filler metal), the use of a filler metal is preferred to avoid weld cracking during welding and to optimize corrosion resistance. Table 2 summarizes filler alloy selection recommended for welding various combinations of base metal alloys to obtain maximum properties, including corrosion resistance. Table 3 lists the chemical composition and melting range of standard aluminum filler alloys. The corrosion data in Table 2 are based on performance in fresh or salt water and do not necessarily

apply to other exposure conditions. Therefore, care must be taken not to extrapolate the corrosion performance ratings indiscriminately. Corrosion behavior ratings generally pertain only to the particular environment tested, usually rated in continuous or alternate immersion in fresh or salt water. For example, the highest corrosion rating (A) is listed for use of filler alloy 4043 to join 3003 alloy to 6061 alloy. In strong (99%) nitric acid (RN03) service, however, a weldment made with 4043 filler alloy would experience more rapid attack than a weldment made using 5556 filler metal. With certain alloys, particularly those of the heat treatable 7xxx series, thermal treatment after welding is sometimes used to obtain maximum corrosion resistance (Fig. 2) (Ref 5-7). When postweld solution heat treating and aging is carried out on 7xxx base metals, aluminum-magnesium filler alloys containing more than 3.5% magnesium should not be used because the fusion zone can be sensitized to SCC. Effect of Chemistry Control. Some chemical exposures or special circumstances can require special

Table 2 Relative rating of selected aluminum filler alloys used to fillet weld or butt weld two component base alloys Data are for welded assemblies that were not heat treated aher welding. See Table 3 for filler alloy compositions.

BaseaDoys to be joined ABoyl 319.0,333.0, 354.0,355.0 C355.0, 3&>.0

Alloy 2

1060,1350

FiDeralloy cbaraderistic(a) FiDer aDoys W S D C T M

4043 4145 4043 1100 4145 2014,2036 2319 4043 4145 2219 2319 4043 4145 3003, Alclad 3003 4043 4145 4043 3004 4145 Alclad3004 4043 4145 5005,5050 4043 4145 5052,5652 4043 5083,5456 4043 5086,5356 4043 514.0, A514.0, 4043 B514.0, F514.0, 5154,5254 4043 5454 6005,6063,6101, 4043 6151,6201, 4145 6351,6951 4043 6061,6070 4145 7005,7021,7039, 4043

B A B A B C A B C A B A

B A B A B A A A A A

A A A A A C B A C B B A B A B A B A A A A A

A A A A B A A A A A A A B A A A A A A A B C A A C B A A A A A A B C A A C B A A A A A A B A A A A A A A B A A A A A A A B A A A A A A A B A A A A A A A A A A A A A A A A

ABoyl

BaseaDoys to be joined Alloy2 7046,7146, A712.0, C712.0 413.0,443.0, 444.0,356.0, A356.0, A357.0, 359.0 319.0,333.0, 354.0,355.0, C355.0, 380.0 1060,1350

413.0,443.0, 444.0,356.0, A356.0,A357.0, 1100 359.0 2014,2036

A A A A A A B B A A A A A A B A A A B B A A A A A A B A A A B B A A A A (continued)

FiDeralloy cbaraderistic(a) FiDer aDoys W S D C T M 4145 A A 4043 B 4145 A

B A A A

B A A A A A B A A A

2319 B A A A A A 4145 A B B B A A

4043 4145 4043 4145 4043 4145 2219 4043 4145 3003, Alclad 3003 4043 4145 3004 4043 Alclad3004 4043 5005,5050 4043 5052,5652 4043 5356 5083,5456 4043 5356 5086,5356 4043 5356 514.0,A514.0, 4043 B514.0, F514.0, 5356 5154,5254

A A A A B A B A A A A A A A B A A A A A A

A A A A A B B A A A A A A B B A B A A A A B A A B A A A A B A A A A A A A B B A A A A A A A A A A A A A B A A A A B B B B A A A A B B A A A A B B A A A B

A A A A A A A A A A A A A A A A

(a), A, B, C, and D represent relative ratings (where A is hest and D is worst) of the performance of the two component base alloys combined with each group of selected filler alloys. W, ease of welding (relative freedom from weld cracking); S, strength of welded joint in as-welded condition (rating applies specifically to fillet welds, but all rods and electrodes rated will develop presently specified minimum strengths for but welds); D, duetility (rating based on freebend elongation ofthe weld); C, corrosion resistance in continuous or alternate immersion of fresh or salt water, T, performance in service at sustained temperatures >65 °C (> 150 "F); M, color match after anodizing. (b) No filler suitable. Note: Combinations having no ratings are not recommended. Source: Aluminum Company of America

164

I


Table 2 (continued)

ABoyl

BaseaUoys 10be joined Alloy2

5454 413.0,443.0, 444.0,356.0, A356.0,A357.0, 6005,6063,6101, 6151,6201, 359.0 6351,6951 (continued) 6061,6070

FiDeralloy FiDer chanll:leristic(a) aDoy" W S 0 C T M

4043 5356 4043 4145

4043 4145 7005,7021,7039, 4043 7046,7146, 4145 A712.0,C712.0 5356 413.0,443.0, 4043 444.0,356.0, 4145 A356.0, A357.0, 359.0 7005,7021,7039, 1060,1350 4043 7046,7146, 5183 A712.0,C7120 5356 5556 1100 4043 5183 5356 5556 2014,2036 4043 4145 2219 4043 4145 3003, Alclad 3003 4043 5183 5356 5556 3004 4043 5183 5356 5554 5556 5654 Alclad3004 4043 5183 5356 5554 5556 5654 5005,5050 4043 5183 5356 5554 5556 5654 5052,5652 4043 5183 5356 5554 5556 5654 5083,5456 5183 5356 5556 5086,5356 5183 5356 5556 514.0,A514.0, 5183 B514.0,F514.0, 5356 5154,5254 5554 5556 5654 5454 5183

ABoyl

BaseaDoys 10be joined ABoy2

A B B A A A A A A B A A B A A A A A A B B A A A A A A A A

B A B A A B A

A B B B A A B

A B A B A A B

A A A A A A B A A A

A B B B A B B

A A A A A A A A B A B A B A A A D A B C A C D A B C A C B A A A A A D A B C A C A B A A B A A B C A C A

C B A B C B A B A B A B C B A B C B A A B A C B A A B A C B A A B A C B A A B A B A B B A B B A A B A B

A A A A A A A A A A A A A A A A B A A A A A B A A A A A B A A A A A B A A A A A A A A A A A A A A A A A

A

B A B A A B B B A B B C B C A B B C B C A B B C B C B A A B A B A A A A A A A A B A B A

6005,6063,6101, 6151,6201, 6351,6951

6061,6070

A A A

7005,7021,7039, 7046,7146, A712.0, C712.0

A A A A A A A A A

6061,6070

A A A

1060,1350

llOO

A A A A A A B A A A A A A B A A A A A A A A A A A A A A A A A A A A A A A A A A (continued)

2014,2036 2219 3003, Alclad 3003

3004

Alclad3004

5005,5050

5052,5652

5083,5456

FiDeralloy chara65 °C (>150 "F); M, color matchafter anodizing. (b) No filler suitable.Note: Combinationshaving no ratingsare not recommended. Source:AluminumCompanyof America

168


controls within the elements of an alloy. In the case of hydrogen peroxide exposure, the manganese and copper impurities have been controlled to low limits in 5652 and 5254 base alloys, as well as in 5654 filler alloy. In some cases, a high-purity aluminum alloy is chosen for special exposure. A filler alloy of equal or higher purity to that of the base alloy is generally acceptable in these cases, and filler alloy 1188 would meet most of these requirements. Crevice COr1"osion. As with many other alloy systerns, attention must be given to the threat of crevice corrosion under certain conditions. Strong (99%) HN03 is particularly aggressive toward weldments that are not made with full weld penetration. Although all of the welds shown in Fig. 3 appear to be in excel-

lent condition when viewed from the outside surface, the first two welds (Fig. 3a and b), viewed from the inside, are severely corroded. The weld made using standard GTAW practices with full weld penetration (Fig. 3c) is in good condition after 2 years of continuous service. Knife-Une Attack. Researchers have shown that aluminum alloys, both welded and unwelded, have good resistance to uninhibited HN03 (both red and white) up to 50°C (120 OF). Above this temperature, most aluminum alloys exhibit knife-line attack (a very thin region of corrosion) adjacent to the welds. Above 50°C (120 OF), the depth of knife-line attack increases markedly with temperature. One exception was found in the case of a fusion-welded 1060 alloy in which no

Table 2 (continued)

AlloyI

3004

BaseaHoys to be joined Alloy2

FiBeralloy FiBer cbamcteristic(a) alloys W S D C T M

3003, Alclad 3003 1100 4043 4145 5183 5356 5556 3004 4043 5183 5356 5554 5556 Alclad3004 4043 5183 5356 5554 5556 1060,1350 1100 4043 4145 5183 5356 5556 1100 1100 4043 4145 5183 5356 5556 2014,2036 4043 4145 2219 4043 4145 3003, Alclad 3003 1100 4043 4145 5183 5356 5556 4043 3004 5183 5356

C A B C C C A B B C B A B B C B D A B C C C D A B C C C B A B A C A B C C C A B B

C B B A A A D A B C A D A B C A B A A A A A B A A A A A B A B A C B B A A A B A B

A C D B B B D C B A C D C B A C A C D B B B A C D B B B A B A B A C D B B B D C B

A A B C C C A C C B C A C C B C A A B

A A B

A A A A A A B C C C A C C

A A A A A A A A A A A A A A A A A A A A A A A B B B A A A A B B B A A A A A A A A A A A A A A

AlloyI

Basealloysto be joined Alloy2

FiBeralloy cbamcteristic(a) FiBer alloys W S D C T M

5554 5556 3003,Alclad3003 1060,1350 1100 4043 4145 1100 1100 4043 4145 4043 2014,2036 4145 2219 4043 4145 3003, Alclad 3003 1100 4043 4145 2219 1060,1350 4043 4145 1100 4043 4145 2014,2036 2319 4043 4145 2319 2219 4043 4145 2014,2036 1060,1350 4043 4145 4043 1100 4145 2319 2014,2036 4043 4145 1060,1350 1100 1100 4043 1100 1100 4043 1060,1350 1060,1350 1100 1188 4043 5554 5556

C B B A A B A A B A B A B A A B A B A B B A A B A B A B A C B A B A B A B C A C B

C A B A A B A A A A A A B A A A A A A A C B A C B A A A A A C B B A B A B C A C A

A C A B C A B C A B A B A B C A B A B A B C A B C A B A B A B C A B A B A A B A C

B C A A B A A B A A A A A A B A A A A A C B A C B A A A A A C B A A A A A A A B C

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A B A A A A A A

(a), A, B, C, and D represent relative ratings (where A is best and D is worst) of tile performance of the two component base alloys combined with each group of selected filler alloys. W, ease of welding (relative freedom from weld cracking); S, strength of welded joint in as-welded condition (rating applies specifically to fillet welds, but all rods and electrodes rated will develop presently specified minimum strengths for but welds); D, duetility (rating based on freebend elongation of the weld); C, corrosion resistance in continuous or alternate immersion of fresh or salt water; T, performance in service at sustained temperatures >65 °C (> 150 'F); M, color match after anodizing. (b) No filler suitable. Note: Combinations having no ratings are not recommended. Source: Aluminum Company of America

Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 169 knife-line attack wasobserved evenat temperatures as high as 70°C (160 OF). In inhibited fuming RN03 containing at least OJ % hydrofluoric acid (HF), no knife-line attack wasobserved for anycommercial aluminum alloyor weldment evenat 70°C (160 "F),

Avoiding SCC. As explained in Chapter 7, wrought alloys usually havegreater resistance to SCC in the longitudinal orientation (direction of working) than in the transverse orientation or in the shorttransverse orientation (through the thickness). Because

Table 3 Nominal composition and melting range of standard aluminum filler alloys Aluminum alloys

1100 1188 2319 4009(a) 401O(b) 4011 (c) 4043 4047 4145 4643 5183 5356 5554 5556 5654 C355.0 A356.0 A357.0

Cu

Si

Mn

Nominalcomposition, wt 'JJ Mg Cr

11

AI

Others

~99.oo

0.12

~.88

5.0 7.0 7.0 5.25 12.0 10.0 4.1

6.3 1.25

0.30 0.35 0.58

0.12

4.0 0.75 0.12 0.75 0.75

5.0 7.0 7.0

0.15

0.50

1.25

0.20 4.75 5.0 2.7 5.1 3.5 0.50 0.35 0.58

0.15 0.12 0.12 0.12 0.25

0.13 0.12 0.12 0.10

0.12

bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal

0.18 Zr; 0.10 V

0.55 Be

Approximatemeltingnmge OF DC

643--657 657-660 543--643 54f>-621 557-613 557-613 574-632 577-582 521-585 574-635 579-638 571-635 602-646 568-635 593--643 54~621

0.55 Be

557-613 557-613

1190-1215 1215-1220 1010-1190 1015-1150 1035-1135 1035-1135 1065-1170 1070-1080 970-1085 1065-1175 1075-1180 1060-1175 1115-1195 1055-1175 1100-1190 1015-1150 1035-1135 1035-1135

(a) Wrought alloy with composition identical to cast alloy C355.0. (b) Wrought alloy with composition identical to cast alloy A356.0. (c) Wrought alloy with composition identical to cast alloy A357.0

lal

Fig. 2

(b l

Welded assemblies of aluminum alloy7005 with alloy5356 filler metal aher a oneyear exposureto seawater. (a)As-welded assemblyshowsseverelocalizedcorrosion inthe HAl. (blSpecimen showing the beneficial effects ofpostweld aging. Corrosion potentials of different areas of theweldments are shownwheretheywere measured. Electrochemical measurements performed in53 gil NaCIplus3 gil H20 2 versus a 0.1 N calomel reference electrodeand recalculated to a saturatedcalomel electrode(SCEI


of this, welding of the Txxx series alloys near a basemetal edge can result in a tensile stress in the shorttransversedirection sufficientto cause SCC in the exposed edge. "Buttering" the edge with weld metal providescompressivestress at the edge and overcomes the SCC problem. Resistance spot welding has been used in aircraft and other applications (Ref 8), including (more recently) the automotive industry (Ref 9). Generally, the resistance to corrosion of resistance spot-welded aluminumis high, but in the case ofhigh-strengtb2xn and Txxx alloys, selective attack of the welds can develop in corrosive service, as a result of changes in microstructurethat occur during welding. Protectionto alloys of this type should be provided when they are used under severe environmental conditions. Crevice corrosion can occur in spot-weldedassemblies. One approach used to solve this problem is a procedurecalled weld bond (Ref 10--12) that combines adhesive bonding with resistance spot welding. Usu-

(8)

ally, the pieces to be joined are first bonded by adhesivesthat seal the crevices,followedby resistancespot welding. A more recent developmentin resistance spot welding involvesjoining aluminum to dissimilar metals by the use of transition joints. In this case, aluminum is first spot welded to a compatible metal that in turn is joined to the dissimilar metal. This procedure improves resistanceto galvanic corrosion by minimizing dissimilar metal contact and also eliminates brittle intermetallic compounds that form at the joint interface (Ref 13, 14). In the automotiveindustry, the dissimilar metals of interest are principally aluminum and steel. The aluminum/steel transition sheet is typically made by rolling sheetsof the two metals togetherunder high roll forces such that when the sheets elongate, the oxides on their contacting surfaces are disrupted, exposing bare clean metal. A metallurgical bond results. There is no crevice; that is, potential galvanic corrosion will be confined to the sheet edges, which can be painted or sealed to prevent corrosion. The direct aluminum-to-steel metallurgically bonded joint has high mechanical integrity. Outside the transition joint, the aluminum side is joined to an aluminum automotive component,and the steel side is joined to a steel component Figure 4 shows the principle of a clad transition metal. Spot welded assembliesmade using transition materialsinclude (Ref 13, 14): • Lap joints of 1008 low-carbonsteel to 7046 aluminum that is spot welded with a low-carbon steelclad 7046 aluminum(40 to 60 ratio) transition • Lap joints of 1006 low-carbonsteel to 6111 aluminum that is spot welded with a low-carbon steelclad 5052 aluminum (60 to 40 ratio) transition • Lap joints of electrogalvanized 1006 low-carbon steel to 6111 aluminum that is spot welded with electrogalvanized 1006 steel-clad 5052 aluminum (60 to 40 ratio) transition

Corrosion of Brazed Joints (b)

(e)

Fig. 3

Corrosion of threealuminum weldments in HNOJ service. (aJ and (b) Gas tungstenarc (GTAI and oxyacetylenewelds, respectively, showing crevice corrosionon theinside sur· lace. (c) Standard GTA weld with lull penetration is resistant 10 crevicecorrosion.

Brazing of aluminum can be divided into two general types. One uses a flux, and the other is fluxless. Brazing that is performed in air or other oxygen-containingatmospheres requiresthe use of a chemicalflux to promote wetting and flow of the filler metal. These fluxes, whether used in torch, furnace, or dip brazing, contain chlorides and/or fluorides , which are corrosive. They must be completely removed after joining, or severecorrosioncan occur in service. Cleaningproceduresfor flux removalare described in the following paragraphs. Assemblies joined by fluxless vacuum brazing have improvedcorrosion resistanceover those brazed with a flux. Ba.. Metals. The aluminum alloys that are the most successfully brazed are the Ixxx and 3xxx series and the low-magnesium members of the 5xxx series. The commonly brazed heat treatable wrought alloys are the 6xxx series.Becausethe 2xn and Txxx series of

Corrosion of Welded, Brazed, Soldered, and Adhesive-Bonded Joints I 111 aluminum alloys have melting points that are too low, they are not normally brazeable. Exceptions are the 7072 alloy, which is used for cladding material only, and the 7005 alloy. Filler Metals. Alloys used in flux brazing usually contain between 7 and 12% silicon-balanced aluminum, and tramp metals are controlled to levels below 0.2 % (fable 4). Alloys employed in fluxless brazing use higher percentages of silicon (>9%) and have varying additions of magnesium to enhance oxide film modification to promote wetting, as well as to reduce the partial pressures of oxygen bearing gases in the chamber. These alloys are primarily found in clad form, Some are serniproprietary to processes used in the automotive heat-exchanger industry, e.g., "long-life" aluminum brazing alloys for automotive radiators (Ref 15). The vacuum (fluxless) alloys, BAlSi-6 through BAlSi-ll, are identified in Table 4. The alloys BAlSi-3 through BAlSi-5 also can be used with the fluxless process if modifications related to magnesium are made, either in the base metal or as an addition to the furnace. Brazing sheet is usually made by roll bonding the filler metal to the base metal. It can be single clad (on one side only) or double clad and is an extremely

Aluminum

Low-carbon steel ______ Transition

Low-carbon steel Aluminum

----0

Fig 4

Illustration of a steel-clod aluminum transition • material insert used lor joining aluminum to co rbon steel

useful form for applying filler metal, particularly in assemblies where many joints must be brazed simultaneously. Examples include: •

Brazing sheet No. 11 and 12: 3003 clad (BAlSi-2 in Table 4) on one side (No. both sides (No. 12) • Brazing sheet No. 23 and 24: 6951 clad (BAlSi-5 in Table 4) on one side (No. both sides (No. 24)

with 4343 II) or on with 4045 23) or on

Flux Removal. As stated in the preceding paragraphs, fluxes used in brazing aluminum alloys can cause corrosion if allowed to remain on the parts. Therefore, cleaning of joints after brazing is essential. A thorough water rinse followed by a chemical treatment is the most effective means of complete flux removal. As much flux as possible should be removed by immersing the parts in an overflowing bath of boiling water just after the filler metal has solidified. If such a quench produces distortion, the parts should be allowed to cool in air before immersion to decrease the thermal shock. When both sides of a brazed joint are accessible, scrubbing with a fiber brush in boiling water removes most of the flux. For parts too large for water baths, the joints should be scrubbed with hot water and rinsed with cold water. A pressure spray washer can be the best first step. A stream jet is also effective in opening passages plugged by flux. Any of several acid solutions (fable 5) can remove flux that remains after washing. The choice depends largely on the thickness of the brazed parts, the accessibility of fluxed areas, and the adequacy of flux removal in the initial water treatment. A pitting or intergranular type of attack on parts can result as chlorides from the flux build up in the acid solution. Some solutions have a greater tolerance for these chlorides than others before parts are attacked. The degree of flux contamination tolerable for the five typical flux removal solutions listed in Table 5 is given in the table footnotes. It should be noted that because of disposal!

Table 4 Compositions and solidus, liquidus, and brazing temperature ranges of brazing filler metals for use on aluminum alloys Am.man WeldingSociety dassillcation

BAISi-2 BAISi-3(b) BAISi-4 BAISi-5(c) BAISi-6(d) BAISi-7(d) BAISi·8(d) BAISi.9(d) BAISi-IO(d) BAISi-lI(d)(e)

Composition(a), \I> Si

Cu

Mg

6.S-8.2 0.25 9.3-10.7 3.3-4.7 0.15 11.0--13.0 0.30 0.10 9.0-11.0 0.30 0.05 6.S-8.2 0.25 2.0--3.0 9.0-11.0 0.25 1.0--2.0 11.0--13.0 0.25 1.0--2.0 11.0-13.0 0.25 0.10-0.5 10.0--12.0 0.25 2.0--3.0 9.0--11.0 0.25 1.0--2.0

Zn

Mn

F.

Solidus OF °C

0.20 0.20 0.20 0.10 0.20 0.20 0.20 0.20 0.20 0.20

0.10 0.15 0.15 0.05 0.10 0.10 0.10 0.10 0.10 0.10

0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8

577 521 577 577 559 559 559 562 559 559

1070 970 1070 1070 1038 1038 1038 1044 1038 1038

T.mporalure Liquidus OF °C 613 585 582 591 (jJ7 596 579 582 582 596

1135 1085 1080 1095 1125 1105 1075 1080 1080 1105

Brazing

°C

OF

599~21

1110--1150 1060--1120 1080--1120 1090--1120 1110--1150 1090--1120 1080--1120 1080--1120 1080--1200 1080--1120

571~()4 582~()4 588~()4

599~21 588~()4 582~()4 582~()4

582~()4 582~()4

(a) Principalalloyingelements. (b) Contains 0.15% Cr.(c) Contains0.20%Ti. (d) Solidusand liquidus temperature ranges varywhenused in vacuum. (e)Contains0.02-{).20% Bi

172


environmental problems, many companies avoid using chromates as inhibitors. Testing for complete flux removal is done by monitoring the presence of chlorides in the final rinse water. An acidified solution of 5% silver nitrate is used to check the final rinse water for clarity. If white chloride residues cloud the water, then salt is still present. After several tests on subsequent rinses, the salt is considered removed if the water remains clear. Although this is a simple test, it is quite accurate. COlTOsion Resistance. The aluminum alloys best suited for brazing are also among those most resistant to corrosion. Corrosion resistance of aluminum alloys generally is unimpaired by brazing if a fluxless brazing process is used or if flux is completely removed after brazing. If flux removal is inadequate, the presence of moisture can lead to interdentritic attack on the filler metal at joint faces and to intergranular attack on the base metal. When two aluminum alloys are brazed together, exposure to salt water or some other electrolyte can result in attack on the more anodic alloy. This condition is aggravated if the anodic part is relatively small compared with the other piece; therefore, the anodic aluminum alloy should be the larger of the two members. Torch-brazed alclad 3003 and alclad 3004 show excellentcorrosionresistance.Furnace or dip brazing, however, causes a certain amount of silicon diffusion from the clad surface, which limits application of these methods with conventional alclad products. A brazing sheet with filler metal on one side and alclad with a special alloy on the other performs well in furnace or dip brazing. Commercial filler metals of the aluminum-silicon type have high corrosion resistance, comparable to that

of the base metals usually brazed. Filler metals containing substantial amounts of copper or zinc are less corrosion resistant, but they are usually adequate, except for service in severe environments. Joints brazed with aluminum-silicon filler metals (BAlSi-2, BA1Si-4, and BAlSi-5) show a potential of -0.82 V with respect to a O.1N calomel reference electrode in an aqueous solution of 53 gIL of sodium chloride and 3 gIL of hydrogen peroxide. This potential is barely cathodic to the frequently brazed base metals, for which the value is -0.83 V for 1100, 3003, 6061, and 6063. Therefore, little electrolytic action occurs in assemblies of these base metals that are brazed with the usual filler metals. The potential of joints brazed with filler metal BAlSi-3 (alloy 4145), which contains copper in addition to aluminum and silicon, depends on the cooling rate after brazing. For slow cooling, these joints have about the same potential as joints brazed with the aluminum-silicon filler metals (-0.82 V). If the cooling is rapid enough to retain a substantial amount of copper in solid solution, the potential is lower; a potential of -0.73 V has been found for T-joints in 1.6 mm (0.064 in.) sheet brazed with BAlSi-3 filler metal and rapidly cooled. Although considerable undissolved silicon-containing constituent is evident in brazed joints, it polarizes strongly (except in acid chloride environments) and has little influence on the potential of the brazed joint and its corrosion resistance. Table 6 shows the results of long-time exposure in a highly corrosive environment of various sheet alloys that were furnacebrazed with filler metal BAlSi-3. The good performance can be considered typical of a variety of brazing combinations.

Table 5 Solutions for removing brazing flux from aluminum parts Type of

Cooamtration Component(a)

solution

Amount

Nitricacid

Sgal 34 gal 4 gal Iqt 36 gal IOpt 40 gal

SS--62% HN0 3 Water SS--62% HN03 48% HF(I.1S sp gr) Water 47%HF Water

11;2 gal 71;41b 40 gal 4\1.igal 321b 36 gal

8S%H3P04

Nitric-hydrofluoric acid

Hydrofluoric acid

Phosphoricacid-chromium trioxide

Nitricacid-sodiumdichromate

oo,

Water SS--62% HN03 Na2Cr20T2H20 Water

Operating

temperature, OF

Procednre(b)

Roomtemperature Immersefor 10-20 min;rinse in hot or cold water(c) Roomtemperature Immersefor Io-IS min;rinse in coldwater,rinse in hot water;dry(d) Roomtemperature Immersefor 5-10 min;rinse in cold water;dip in nitricacidsolutionshownat topoftable; rinsein hot or cold water(d) Immersefor Io-IS min;rinsein hot or cold ISO water(e) 140

Immersefor 5-30 min;rinse in hot water(f)

(a)Allcompositions in weightpercent, (b) Beforeusinganyof the solutions,it is recommendedthat the assemblyfirst be immersedin boilingwater to removethe majorportionof the flux. (c)Fluxcontaminationin acid shouldnotexceedS gIL of chlorideexpressedas sodiumchloride.Solutionis not recommendedfor use on base metalsless than 0.020 in. thick. (d) Fluxcontaminationin acid shouldnot exceed3 gIL of chlorideexpressedas hydrochloric acid. Solutionis aggressiveand not recommended for base metalsless than 0.020 in. thick. (e) Tolerancefor flux contaminationis in excessof 100 gIL and permissiblelimitis probablygovernedby cleaningability.Iflarge pocketsof fluxare present,solutionpromotesintergranular attackat thepocket.Recommended forfinalcleaningof thin-gageparts,whenmostof thefluxcan be removedeasilyin water.(f) Exceptionallyhigh flux tolerance.Recommended for cleaningthin-gageassemblies, if adequacyof watercleaningis doubtful. Licenserequired.


Table 6 Results of microscopic examination of fumace brazed specimens exposed 2 yr to 3.5% sodium chloride intennittent spray Specimens weresmall inverted T-joints 011.6 mm (0.064 ln.] sheet; liller metal used wasBAISi·3. Sheet (basemetal)

Joint (IIIIer meta\)

Depth of attack, in.

Sheet alloy

3003 5052 6053 6061

Type of attack

Maximum

Average

Type of attack

Pitting Pitting Pitting,intergranular Pitting,slightintergranular

0.0098 0.0182 0.0126 0.0126

0.0022 0.0042 0.0028 0.0033

Pitting Pitting Pitting Pitting

Corrosion of Soldered Joints Soldering, by definition, involves temperatures below 450 °C (840 "F); therefore, aluminum alloy filler metals are not used in soldering aluminum. Most solders for aluminum fall into one of three categories: (a) low-temperature lead-tin types, (b) intermediate temperature zinc-cadmium or zinc-tin types, and (c) hightemperature zinc or zinc-aluminum types. Compositions and characteristics of solders for aluminum are given in Table 7. Soldering aluminum differs from soldering other common metals in several ways. The tenacious, refractory oxide film of aluminum requires active fluxes; rosin fluxes are not satisfactory. Soldering temperature must also be controlled more closely. With aluminum, resistance to corrosion depends much more on solder composition than it does with copper, brass, or ferrous metals. All soldered alumi-

Depth of attack, in. Maximum Average

0.0011 0.0014 0.0008 0.0014

0.0014 0.0042 0.0012 0.0042

num joints have a lower resistance to corrosion than joints that are welded or brazed have. The corrosion resistance of soldered joints in aluminum depends on the solder composition, flux composition, joint design, protective coating, and environment. Base alloy composition and temper have relatively little effect on the corrosion resistance of soldered joints. In dry atmospheres, such as indoor exposure, unprotected low-temperature soldered joints can provide excellent service. In humid or marine atmospheres without protection, these joints can fail in a short time. Environment is much less critical for zinc-soldered joints, but even these can require protection in the more corrosive industrial and marine atmospheres. In the presence of an electrolyte, or in a moist atmosphere, electrochemical corrosion can occur because of galvanic cells created between the aluminum, the various solder phases, and the diffusion layer formed at the aluminum-solder interface. When such

Table 7 Compositions and properties of typical solders for use with aluminum Solder

type

Sn

Zn Zn Zn Zn Zn Zn-Cd Zn-Cd Zn-Cd Sn-Zn Sn-Zn Sn-Zn Sn-Pb Sn-Zn Sn-Pb Sn-Zn Sn-Zn Sn-Zn Sn-Ph Sn-Ph

70 30 40 60 63 69.3 80 91 36.9 34

Sn-Ph So-Ph Sn-Cd Sn-Cd Pb-Bi

31.6 40 20 50 0.5

2

20

Zn

Ag

Compositioo, % AI Cd

100 94 95 90 79.6 90 60 17.5 15 30 70

4 5 5 10

0.8

Bi

Cu

2

0.4 3 10 40 82.5 64.2

60 0.1 37 2.0

39.4 28 20 9

Pn

0.7

5 5

0.5

3.8 59.3 63

3 9 15 15

8 0.8 0.8

1.5

64.2

...

51 44.2

0.4

... 50 79.3 18.7

Meltingrange, °C Solidus Liquidus

Meltingrange, OF Solidus liquidus

419 382 377 382 275 265 265 265 110 199 199 183 199 183 196 199 199.4 143 195

419 393 377 382 399 404 335 265 277 311 377 238 341 216 335 277 199.4 232 256

787 720 710 720 527 509 509 509 230 390 390 361 390 361 385 390 391 290 383

787 740 710 720 750 760 635 509 530 592 710 460 645 420 635 530 391 450 492

139 168 110 182 246

252 357 277 216 271

282 335 230 360 475

485 675 530 420 520

COITOsion

Wetting

ability

Flux type

resistance

Good Good Good Good

React. React. React, React.

V.good V.good V.good V.good

Good V.good

React. React.

Fair Fair

Fair Good

React. React.

Fair Good

Good

React

Good

Fair

Organic

Poor

Poor

Org.-react. React.

Poor

Poor Good

Org.-react. Organic

Poor Good

V. good,verygood;React.,reaction,(chloride-containing inorganic salt);Org.,organic;andOrg.-react., organicor reaction


cells are established, the material with the highest negative electrode potential corrodes preferentially to protect the remainder of the assembly. The interfacial layer is anodic to aluminwn and to any metals present in the solder with the exception of zinc. Figure 5 illustrates the difference in electrode potential across a low-temperature soldered joint. In such joints, the interfacial layer corrodes preferentially to protect both the aluminwn and the solder. Because the cross section and the total amount of interfacial layer are very small in comparison to the remainder of the assembly, this area can corrode rapidly, and the corrosion resistance of low-temperature soldered joints is relatively poor. In zinc-soldered joints, however, the solder is the most anodic area and corrodes preferentially, protecting both aluminum and the interfacial layer (Fig. 5). Because there is a much greater volume of solder than volume of interfacial layer, zinc-soldered joints endure much longer thanlow-temperature soldered joints in a specific environment Pure zinc or zinc containing small amountsof aluminum, copper,nickel,or other high- melting metals has the highest corrosion resistance. The usual composition variations of the chloridefree, low-temperature soldering fluxes have little or no effect on joint corrosion resistance. However, a reaction flux that deposits zinc is preferable to on depositing tin or other low-melting heavy metals. These lowmelting metals, whether introduced by flux or by solder, can reduce corrosion resistance markedly. Flux composition also can have a pronounced effect if residues are not completely removed. Those from chloride-containing reaction fluxes can cause severe corrosion if trapped in assemblies. Those from chloridefree organic fluxes generally cause little or no corrosion. All flux residues must be removed when foil or small wire is soldered. Inaccessible or terminal joints where complete flux removal is not possible can be protected by first eliminating moisture, then sealing so that moisture cannot enter the joint. The time-to-failure of soldered joints increases with the corrosion path. Corrosion is influenced also by

accessibility of the most anodic area to a corrosive medium Simple lap joints provide several points of entry for moisture. Hence, corrosion of either the interface or the solder can progress from different directions simultaneously. Moreover, these joints can be separated by resultant corrosion products. Lock-seam, socketed-tube, and terminal-lug joints allow relatively limited accessibility to corrosive mediums. These joints are constructed so that they will not open under normal conditions, and they can be sealed by corrosion products. Protection. Prior electroplating of aluminum improves the corrosion resistance of low-temperature soldered joints. Platings of copper, iron, or nickel prevent formation of a high-potential interface between the solder and the aluminum These platings have potentials lower than that of aluminwn (Fig. 5); hence, aluminum corrodes preferentially protecting both plating and solder. To provide maximum resistance to corrosion, only those areas covered by solder should be plated, thus allowing maximwn exposure of aluminum surface. Protective coatings can seal off areas of differing potential, thereby inhibiting electrolytic corrosion. An effective coating must be continuous, inert to both solder and base metal, and resistant to the specific environmental conditions.

Corrosion of Adhesive-Bonded Joints Adhesive bonding is widely used to join aluminum alloys to themselves, each other, other metals, and many nonmetals, including all forms of paper products, insulation board, wood-particle board, plaster board, plywood, fiberglass, and various polymers and organic-matrix composites. Both laminated structures and honeycomb structures (e.g., thin aluminum sheets surrounding a low-density core material) are produced. Key application areas employing adhesive bonding of aluminwn include the aircraft/aerospace, automo-

>

-1.5,.--------, iii ~ lD -1.0

1---'.

(5

0lD

"0

-0.5

e

~

m

O L . . - - - - - -...

Low-temperature solder

Aluminum

Fig. 5

Interface

Aluminum

Zinc solder

Interface

Approximate electrodepotentials across solderedaluminum joints

Low-temperature solder

Copperplating


tive, building products, and sporting equipment industries. Commonly used structural adhesives for bonding aluminum alloys include nitrile epoxy, epoxy polyamide, epoxy phenolic, nitrile phenolic, and unmodified epoxy. Adhesives not compatible with aluminum include the alkaline water-based latex adhesives, acetic anhydride adhesives, and 'adhesives that have been made electrically conductive by the addition of copper or silver. Although adhesive bonding eliminates many of the corrosion problems associated with welding, brazing, and soldering-most notably metallurgical changes along the joint line and/or galvanic effects--environmental susceptibility of bonded structures is of concern. As is described in the following paragraphs, stable oxide surface preparation is an essential part of the bond foundation. Improper surface treatment can result in an unstable oxide layer, which may allow entry of moisture, delamination, and crevice corrosion (Fig. 6). COlTOsion Resistance. Migration of water to adhesive joints is the most common form of bond degradation. Figure 7 compares the loss of strength in aluminum-epoxy joints after exposure in a hot-wet environment with the excellent durability that can be achieved under dry conditions. When adhesive joints are exposed to wet environments, water molecules will migrate and be preferentially adsorbed into the interface region. This is because joint substrates, such as metal or metal oxides, have very high surface energies and water permeates through all organic adhesives. Water can enter either by diffusion through the bulk adhesive layer or by wicking along the adhesive-adherend interface. Ingression or wicking along the interface becomes important in a system where water can readily displace adhesives from the substrate, and the displacement is augmented by preexisting microcracks or debonded areas at the

interface, which originate from poor wetting by the adhesives. However, in a typical structuraljoint, such as epoxyaluminum, many researchers (Ref 16-18) have found that water generally enters a joint system by diffusion through the epoxy rather than by passage along the interface. Another way water can degrade the strength of adhesive joints is through hydration of the metal oxide layer at the interface. Common metal oxides, such as aluminum and iron oxides, undergo hydration. The resulting metal hydrates become gelatinous, and they act as a weak boundary layer because they exhibit very weak bonding to their base metals. In aluminum alloy (2024-T3)-epoxy joints, for example, the initial oxide produced on the aluminum substrate is usually amorphous A1203. Upon exposure to moisture, Al203 is converted to aluminum hydroxide with a chemical composition between that of boehmite (AI203·H20) and pseudoboehmite (AI203·2H20). Failure surface analysis reveals that the hydroxide layer is normally attached to the adhesive side, suggesting that adhesion of the hydroxide to aluminum is very weak. Thus, once a hydroxide is formed, it is separated easily from the substrate, causing failure of the joint. As the crack opens up during its propagation, the freshly exposed aluminum metal surface will undergo further hydration through a corrosion reaction that can be described as:

Figure 8 is the schematic representation of the model proposed by Venables and coworkers (Ref 19) to illustrate the mechanism of adhesion strength loss through the hydration of aluminum oxide. The stability of the oxide surface against hydration was found to vary significantly depending on the type of metal treatment employed (Ref 20-23). For ex-

o [a]

1\

Hot-dry desert site

\ ~ot-wettroPical

r-;

(bl 100

Fig 6

Schematics illustrating the causes of adhesive • delomlnotlon for a metal adherend. (a) Results of moisture entry in the unstable oxide. (b) Corrosion of clcladding and base aluminum. A, adhesive primer system; B, oxide; C, alcladding; D, base aluminum

o

Fig. 7

1

site

t--

2 3 Exposuretime, years

4

Effectof outdoor weathering on the strength of aluminum alloy/epoxy-polyamide joints

176/ Corrosion of Aluminum and Aluminum Alloys

Aluminum hydroxide fonnedduring wedge test

Original

Crack extension

FPL

oxide

Aluminum hydroxide fonnedalter crack propagation

Fig 8

Schematic drawing of the failuremechanism in an aluminum/polymer joint system during wedge testing in • humidenvironment. The original oxlde isconverted to hydroxide,whichadheres poorly to thealuminum subs/ra/e. FPl,Forest Producls labora/ory processed. Source: Ref19

(a)

0.5 11m -10 nm

-1

(8)

-400 nm

-5nm

\

-Aluminum

-Aluminum (b)

Fig 9

Forest Products. laboratory (FPL) 2024 alumi• numsurface./aJHigh-resolulion slereo electron micrograph. (b)Isometric drawing

Fig. 10

Phosphoric acid anodization (PM) 2024 aluminum surface. (a)Stereo micrograph.(b) Isometric draWing


ample, in the aluminum case, phosphoric acid anodization (PAA) produces an oxide surface that outperforms the surfaceproducedby the Forest Products Laboratory (FPL) process in joint integrity as well as in long-term durability. In the FPL process, the surface is typically decreased and alkaline cleaned, followed by immersion in a solution containing Na2Cr2~·2H20, H2S04, and H20 in a 1 to 10 to 30 ratio by weight for 15 to 30 min. in the PAA process, the surfaceis first treatedby the FPL processand then anodized in an aqueous solution containing 10% by weightof H3P04 for about 25 min. The better performance in the PAA-treated surface is attributed to the oxide morphology, which contains a thicker hexagonal cell structure with longer whiskerlike protrusions (10 x 100 nm, or 0.4 x 4.0 uin.) than the FPL-treated surface. This provides a polymer-oxide interface similarto the fiber-reinforced structure with a more effective mechanical interlocking. Figures 9 and 10 comparethe oxide morphology ofthe FPL and PAAsurfacetreatments. Hydration studieson the PAA oxide reveal that improveddurability is due to the presenceof a monolayer of AlP0 4 that is adsorbed on the porous aluminum oxide. Hydration of the oxide is precededby absorption of water in AlP0 4 and subsequent dissolution of phosphates. This prolongs the overall incubation time of oxide hydration and thus improves durability. Application of Primers and Sealants. A primer solution can be applied to the aluminum surfaceprior to bondingwith the adhesive to improve wet strength. Primers are low viscosity fluids that are typically a 10% solution of the adhesive in an organic solvent, whichcan wet out the aluminumsurface. This leavesa coating on which the adhesive can readily flow and attain intimate contact. Virtually all adhesive suppliers recommend a primer for their paste of film adhesives when they are used to bond aluminum. Sealantsare often appliedto the edgesof adhesively bonded joints for prevention of water ingress into the joint. The sealant sometimes can be the same material

Fig 11 Corrosion

01 aldad 2024-T3 adhesive• bonded panels (opened to show corrosion products) alter exposure to 4480 h saltspray test

as the adhesive or anotherfluid-resistant material (e.g., thermoset plasticssuch as anaerobics). Alclad Products. Becausecorrosionof alclad aluminum spreads in the plane of the sheet, corrosion of alcladpartsresultsin the delamination of bondedpanels (hence, the expression "clad is bad"). Figure 11 shows corrosionin adhesive-bonded clad-alloy joints. Becauseof the problem, alclad sheet is often avoided for honeycomb facesheets.

REFERENCES 1. M.B. Shumaker, RA Kelsey, D.o. Sprowls, and 1.G. Williamson, ''Evaluation of Various Techniques for Stress-Corrosion Testing Welded Aluminum Alloys," presented at ASTMStress-Corrosion Testing Symposium, June 1966 2. S.L.Wohler and 0. Schliephake, UbereinemGers., durh Potentialmessunger das Aufgregen der Verb. Mg2Si zu Bestatigen, Z Anorg. Chem., 1962,P 151 3. GW. Akimow and A.S. Oleschko, Gmelins Handbuch der anorganischen Chemie, (No.8), Auflage, 1942,p393 4. G.Beverini, "Investigations oftheCorrosionCharacteristics of Al-LiandAl-Li-Cu Weldments in a 3.5% NaCI Solution," master's thesis T-3508, Colorado School of Mines, 1988 5. Welding Aluminum, American Welding SocietyTheAluminum Association, 1972 6. 1.G. Young, BWRAExperience in the Welding of Aluminum-Zinc-Magnesium Alloys, Weld. Res. Suppl., Oct 1968 7. "AlcoaAluminum Alloy 7005," AlcoaGreen Letter, Aluminum Company of America, Sept 1974 8. 1.G. Young, BWRA Experience in the Welding of Aluminum-Zinc-Magnesium Alloys, Weld. Res. Suppl., Oct 1968 9. "Tentative Guide to Automotive Resistance Spot Welding of Aluminum," The Aluminum Association,Washington, DC,TIO,Oct 1973 10. Weldbonding-An Alternate Joining Method for Aluminum AutoBodyAlloys," The Aluminum Association, Washington, DC, rr; 1978 11. T.L.Wilkinson andW.H. Ailor," Joining andTesting Bimetallic Automotive Panels," SAE Technical Paper, Series 780254, Society of Automotive Engineers, 1978 12. 1.1. Bethkeand SJ. Ketcham, Polysulfide Sealants forCorrosion Protection of SpotWelded Aluminum Joints, Adhes. Age. Nov 1979,p 17 13. G. Haynes and R. Baboian, Laboratory and Field Corrosion Test Results on Aluminum-TransitionSteelSystems on Automobiles, Corrosion and Corrosion Control ofAluminum andSteel inLightweight Automotive Applications, E.N.Soepenberg, Ed.,National Association of Corrosion Engineers, 1985, p 383-1 to 383-13 14. R. Baboian andG. Haynes, Corrosion Resistance of Aluminum-Transition-Steel Jointsfor Automobiles, Paper268, in Proc. of the Sixth Automotive Corro-

178


sionPrevention Coni, Society of Automotive Engineers,l993 15. AC. ScottandRA Woods, ''Long-Life'' Aluminum Brazing Alloysfor Automobile Radiators -A TenYear Retrospective, Paper No. 544, Corrosion 98, NaceInternational 16. AW. Bethune, SAMPEJ., Vol 11 (No. 14), 1978, p4 17. D.M.Brewis, J. Comyn,andJ.L.Tegg, Int. J. Adhes. Adhes., Vol1, 1980,P 35 18. RA G1enhill and AJ. Kinloch, Environmental Failure of Structural Adhesive Joints, 1. Adhes., Vol 6, 1974,p315 19. J.D. Venables, D.K. McNamara, J.M. Chen, B.M. Ditchek, T.I. Morgenthaler, T.S.Sun,andRL. Hopping,Proc. ofthe12th National SAMPETechnological Co'lf, Society for the Advancement of Material and Process Engineering, Oct 1980, P909 20. J.D.Venables, Review-Adhesion andDurability of Metal-Polymer Bonds,J. Mater. Sci., Vol 19, 1984, p2431 21. G.S. Kabayashi and OJ. Donnelly, Report 0041517,The BoeingCompany, Feb 1974 22. J.S.Ahearn,G.D.Davis, T.S.Sun,andJ.D.Venables, Correlation of Surface Chemistry and Durability of AluminumIPolymer Bonds, Adhesion Aspects of Polymeric Coatings, KL. Mittal, Ed., Plenum Press, 1983,p288

23. H.W. Eichner and W.E. Schowalter, Report 1813, ForestProducts Laboratory, 1950

SELECTED REFERENCES Welding

• P.B.Dickersonand B. Irving,WeldingAluminum: It's Not as Difficult as It Sounds, Weld. J., Vol 71 (No.4), 1992,P 45 • ''StructuraI Welding Code-Aluminum," ANSUAWS 01.2-97, AmericaWeldingSociety, 1997 • Welding Aluminum: Theory and Practice, H.L. Sanders,Ed., the AluminumAssociation, 1991 Brazing and soldering

• Aluminum Brazing Handbook, The AluminumAssociation, 1990 • Aluminum Soldering Handbook, The Aluminum Association, 1996 Adhesive bonding

• Adhesive Bonding ofAluminum Alloys, E.W.Thrall and RW. Shannon,Ed., MarcelDekker,Inc., 1985 • Handbook of Aluminum Bonding Technology and Data, The Aluminum Association, 1993



Chapter 10

Corrosion of Aluminum Metal-Matrix Composites

METAL-MATRIX COMPOSITES (MMCs) are a class of materials with potential for a wide variety of structural and thermal management applications. Metalmatrix composites are capable of providing highertemperature operating limits than their base metal counterparts, and they can be tailored to give improved strength, stiffuess, thermal conductivity, abrasion resistance, creep resistance, or dimensional stability. Unlike resin-matrix composites, they are nonflammable, do not outgas in a vacuum, and suffer minimal attack by organic fluids such as fuels and solvents. The principle of incorporating a high-performance second phase into a conventional engineering material to produce a combination with features not obtainable from the individual constituents is well known. In an MMC, the continuous, or matrix, phase is a monolithic alloy, and the reinforcement consists of highperformance carbon, metallic, or ceramic additions. Most of the commercial work on MMCs has focused on aluminum as the matrix metal. The combination oflight weight, environmental resistance, and useful mechanical properties has made aluminum alloys very popular; these properties also make aluminum well suited for use as a matrix metal. The melting point of aluminum is high enough to satisfy many application requirements, yet it is low enough to render composite processing reasonably convenient. Also, aluminum can accommodate a variety of reinforcing agents, as will be described below.

composite. Continuous-fiber or filament reinforcements for aluminum include graphite, silicon carbide (SiC), boron, and aluminum oxide (Al2~)' Fabrication techniques for these composites vary from vapor deposition coating of the fibers, liquid-metal infiltration, and diffusion bonding to liquid-metal infiltration and direct casting to near-net shape. Discontinuous reinforcements consist mainly of SiC in whisker form, particulate types of SiC and Al203 , and short or chopped fibers of Al20) or graphite. These MMCs are produced primarily by stir (vortex/mixing) casting and powder metallurgy (PM) processing although liquid-metal infiltration, squeeze casting, rheocasting (semisolid casting), and spray deposition have also been used. Figure I compares the performance and cost characteristics of both continuous and discontinuous aluminum MMCs. Higher performance composites are produced by more expensive, continuous-fiber

SiC (continuous)1 Alumina ...-'=====;-' (COnlinuOuS)',..b'==0==0==0=;----'

I

I

~ E

't:D.~

I

onen metal

D

SiC (Whiskers) Powder metallurgy ORA

ORACIJAlum;na fiber ORA

Structural Characteristics

Cost (logarithmic scale)

Reinforcements, characterized as either continuous (fiber reinforced) or discontinuous (particle or whisker reinforced), can constitute from 10 to 70 vol% of the

Fig 1

The malerial cost versus performance • of various clumlnum-malrlx composites. DRA, discontinuously reinforced aluminum

180 I Corrosion of Aluminum and Aluminum Alloys reinforcements. At the opposite end of the cost and performance spectrumare the particle-reinforced molten (or cast) metal composites. Additional information on the processing and properties (physical and mechanical) of aluminum MMCs can be found in Ref 1 and 2.

Corrosion Characteristics Although the incorporationof the second (reinforcing) phase into a matrix material can enhance the physical and mechanical propertiesof that material, it can also significantly change the corrosion behavior. Composites, by their nature, combinematerialshaving considerably different corrosion properties. A likely source of corrosion is thereforegalvanic corrosion between the reinforcement and the matrix. Crevices and pores can result in preferentialsites for localizedcorrosion as well. Results that range all the way from no increase to a significant increase in the corrosion rates of composites compared to the matrix have been reported. Indeed, it is a complex issue and depends on the particular matrix-reinforcement system, the anodic film produced, and the interfacial characteristics between the matrix and the reinforcement. In addition, the fabrication processesare critical. This chapterdiscusses the ambient-temperature corrosion characteristics of aluminum MMCs. Emphasis is placed on marine environments. Coatings and design criteriafor optimum protectionof MMCs are also discussed.

Fig. 2

Cross section of a contlnuous-flber reinforced boron/aluminum composite. Shown here are 142 11m diameter boron filaments coated with SAC in a 6061 aluminum alloy matrix

Corrosion Behavior of Boron/Aluminum Composites Boron-reinforced aluminum is a technologically mature continuous-fiber MMC (Fig. 2). Applications for this composite include tubular truss members in the midfuselage structure of the space shuttle orbiter and cold plates in electronic microchip carrier multilayer boards. Fabrication processes for boron/aluminum composites arebasedon hot-press diffusion bondingof alternating layers of aluminum foil and boron fiber mats (foil-fiber-foil processing) or plasma-spraying methods. The com»sion properties of boron/aluminumcomposites are extensively reviewedin Ref3. This section summarizes the significantfindings. Boron/aluminum MMCs experience severe corrosion in chlorideenvironments and are significantly less corrosionresistantthan unreinforcedaluminumalloys. The concentration of corrosion in these compositeshas been found at fiber/matrix interfaces and at the bonds between foils (Ref 4, 5). The acceleratedcorrosion at these sites has been attributed to imperfect bonding and fissures in the composite and emphasizesthe need for eliminating fabrication flaws to reducecorrosionof boron/aluminum MMCs in chloride environments (Ref 4). Corrosion at the fiber/matrix interfaces has also been attributed to the presence of aluminum borideformed during fabrication (Ref 5).

Corrosion Behavior of Graphite/Aluminum Composites The development of continuous-fiber reinforced graphite/aluminum MMCs was initially promoted by the commercial appearanceof strong and stiff carbon fibers in the 1960s. Carbon fibers offer a range of properties, includingan elasticmodulus up to 966 OPa (140 psi x 106) and a negative coefficient of thermal expansion down to -1.62 x l0-6rc (-D.9x 1O-6rF). However, carbon andaluminum in combination aredifficult materials to process into a composite. A deleterious reaction betweencarbon and aluminum,poor wetting of carbon by molten aluminum, and oxidation of the carbon are significanttechnicalbarriers to the production of thesecomposites. Two processes arecurrently used for making commercial aluminum MMCs: liquidmetal infiltration of the matrix on spread tows and hotpress diffusion bonding of spread tows sandwiched between sheets of aluminum foil. The precursor graphite/aluminum wires are fabricated by the titanium-boron vapor deposit (Ti-B VO) method of manufacture that uses TIC4 and BCl3 (g) for the deposition of a TI-B coating on the graphite fibers. The coating improves wettability of the fibers in molten aluminum. Becausethe TI-B YO method is a source of residual chloride, this processing route can have a deleterious effecton corrosionperformance, for example, disbonding of precursor wires and exfoliation of

Corrosion of Aluminum Metal-Matrix Composites I 181

graphite/aluminum plates (Ref 6). Figure 3 shows a transverse cross section of a graphite/6061 aluminum alloyMMC. An alternative method that is being considered for preparing graphite/aluminum composites is to deposit the aluminum matrix directly onto the individual graphite fibers by physical vapor deposition (PVD) or by magnetron sputtering (Ref 7, 8). The flexible alloycoated fibers are arranged in the desired orientations and consolidated by diffusion bonding. The PVD or sputtering processes allow virtually any matrix alloy to be deposited onto the graphite fibers and eliminates residual microstructural chlorides associated with the liquid-metal infiltrationffi-B VD process . Matrix alloys currently under development include a series of aluminum-molybdenum (AI-Mo) alloys containing from 10 to 25 at.% Mo (Ref 7, 8). Corrosion Properties. In addition to corrosion problems such as pitting and exfoliation faced by conventional (monolithic) aluminum alloys, galvanic corrosion caused by the potential difference between the graphite fibers and the aluminum matrix is a reason for concern in these composites. As shown in Table 1, graphite appears at the cathodic (noble) end of the galvanic series while aluminum is at the anodic (active) end of the series. Graphite/aluminum composites have been shown to corrode 80 times faster than monolithic aluminum alloys in an aerated 3.15 wt% sodium chloride (N aCl) at room temperature (Ref 8). Graphite/aluminum composites exhibit accelerated corrosion in marine environments when graphite fibers and aluminum are simultaneously exposed . Assuming that the edges of the graphite/aluminum composite are masked off to prevent exposure of both the graphite and the aluminum, only the aluminum surface foils will initially be exposed to the environment. The aluminum surface foils will pit at an average rate of 0.025 to 0.035 mm1year (1.0 to 1.4 mils/year) in

~ w ires

Alum in um

500 J1m

Fig. 3

Cross section of a graphite/aluminum compositein 6061 alloymatrix. The fibers were precooled with titanium and boron. Fiber bundles were impregnaled by liquid-metal infiltration with 6061. The composite was consolidated by diffusion bonding wilh 6061 foil.

seawater and at 0 .5 to 0.76 urn/year (0.02 to 0.03 mils/year) in the marine atmosphere (1100, 6061, and 5000 series aluminum alloys). Pits may also be present with depths much greater than the average rates reported (Ref 9). Crevice corrosion of the aluminum foils may also occur at the edges because of the crevice formed between the aluminum surface foil and the masking material. The pitting- and crevice-corrosion processes eventually penetrate the foils and result in exposure of the graphite/aluminum composite matrix below, at which point the corrosion rate becomes extremely accelerated. Corrosion has been shown to proceed preferentially along foil/foil, wire/wire, and wire/foil interfaces in the composite (Ref 10). Severe exfoliation occurs because of wedging of the hydrated alumina (Al2(OHh) corrosion products within the composite. Figure 4 shows an example of severe graphite/aluminum corrosion (known as catastrophic failure). This catastrophic condition can occur within 30 days in seawater after exposure of the graphite-aluminum matrix . Catastrophic failure in the marine atmosphere and in splash/spray environments is less rapid than in seawater, Table 1 Galvanic series of selected metals and alloys in seawater Noble or cathodic Platinwn Gold Graphite Titanium Silver Chlorimet3 (Ni-18Cr.18Mo) HastelloyC (Ni·17Cr-l5Mo) 18·8 stainlesssteel with molybdenum(passive) 18·8 stainlesssteel (passive) Chromiwn stainless steel 11·30%Cr (passive) Ioconel(passive) Nickel(passive) Silversolder Monel 400 Cupronickels(Cu40Ni to Cu·1ONi) Bronzes(Cu-Sn) Copper Brasses (Cu-Zn) Chlorimet2 (Ni-32Mo-IFe) HastelloyB (Ni-30Mo-6Fe· IMn) Ioconel (active) Nickel(active) Tin Lead Lead-tin solders 18.g stainless steel with molybdenum(active) 18·8 stainlesssteel (active) Ni-Resist(high-nickel cast iron) Chromiumstainless steel. 13% Cr (active) Cast iron Steelor iron Alwninum aUoy 2024 Cadmiwn AluminumaUoy1100 Zinc Magnesiumand magnesiwnalloys Active or anodic

182


but can occur within six months (Ref 11). This accelerated corrosion is believed to result from the aluminum carbides that are formed at the reinforcement/matrix interface during fabrication , which alter the properties of the aluminum surface film at these locations and render the composite more susceptible to breakdown (Ref 10, 12). The aluminum surface foils alone provide reasonably good corrosion protection to the composites. Marine exposure tests of graphite/aluminum MMCs with 6061, 5056, and 1100 aluminum alloy surface foils (graphite/aluminum edges masked) revealed no pitting penetration through the foils to expose the graphite/ aluminum composite wires below during a 20 month exposure (Ref 11). Pitting of the foils, which occurred on most of the graphite/aluminum panels, ranked as light pitting in the splash/spray zone and marine atmosphere and as localized pitting in filtered seawater. In summary, graphite/aluminum composites undergo extremely severe corrosion in marine environments when the graphite and the aluminum are mutually exposed. Aluminum surface foils have provided 20 months of protection to MMCs, assuming there is no graphite-aluminum exposure. However, the composite will start to fail upon foil penetration by the deepest pit. Service life can be extended by applying corrosion-resistant coatings . Primary emphasis should be placed on preventing exposure of both the graphite and the aluminum, and the graphite/aluminum composite should be frequently inspected while the component is in service.

Corrosion Behavior of Silicon Carbide/Aluminum Composites SiC/aluminum MMCs are produced in both continuous (fiber reinforced) and discontinuous (particle or whisker reinforced) forms. Continuous fiber reinforced MMCs (Fig. 5) can be produced by stacking rows of SiC fibers (plasma sprayed with aluminum) and aluminum foils and diffusion bonding to yield the composite. Alternatively, fiber-reinforced aluminum MMCs can be produced by hot molding, a low-pressure, hot-pressing process designed to fabricate parts at significantly lower cost than is possible with a diffusionbonding/solid-state process. The hot-molding process is analogous to the autoclave molding of graphiteepoxy, in which components are molded in an openfaced tool. The mold in this case is a self-heating, slip-cast ceramic tool that contains the profile of the finished part. A plasma-sprayed aluminum preform is laid into the mold, heated to near molten aluminum temperature, and pressure consolidated in an autoclave by a metallic vacuum bag. Discontinuous reinforced aluminum MMCs are produced primarily by casting or PIM processing. In the stir (vortex/mixing) casting process , the pretreated and prepared reinforcement filler phase is introduced in a continuously stirred molten matrix and then cast by sand, permanent mold, or pressure die casting. Melting under an inert gas cover combined with Ar-SF6 gas mixtures for fluxing and degassing is essential to avoid the entrapment of gases. Mixing can be affected ultrasonically or by reciprocating rods, centrifuging, or zero-gravity processing. Figure 6 shows a typical microstructure of a cast aluminum MMC. Powder metallurgy processing of aluminum MMCs involves both SiC particulates and whiskers . Processing involves the following: (1) blending of the gasatomized matrix alloy and reinforcement in powder form (2) compacting (cold pressing) the homogeneous blend to roughly 80% density (3) degassing the preform (which has an open interconnected pore struc-

SiC fiber

-

Alum inum

100 ,.,.m

Fig 4 •

Catastrophic failure of a graphite/alum inum MMC aher 6 months in a marine atmosphere

Fig. 5

Cross sectionofa contjnuous-fiber sitICOn carbide/ aluminum composite

Corrosion of Aluminum Metal-Matrix Composites I 183

ture) to remove volatile contaminants (lubricants and mixing and blending additives), water vapor, and gases and (4) consolidation by vacuum hot pressing or hot isostatic pressing. The hot-pressed cylindrical billets can be subsequently extruded, rolled , or forged. Corrosion Properties. Marine corrosion of silicon carbide/aluminum composites is much less severe than that observed on graphite/aluminum MMCs. Discontinuous silicon carbide/aluminum MMCs, however, are susceptible to localized corrosion . Mild-to-moderate pitting has been reported on SiC whisker- and particulate-reinforced composites containing 6061 and 5000 series aluminum matrices exposed for a maximum of 42 months in splash/spray and marine atmospheric environments. The degree of corrosion present on the composites is slightly accelerated compared to that on unreinforced aluminum alloys. Silicon carbide/aluminum composites immersed in natural seawater are susceptible to significantly more severe corrosion than is typical for silicon carbide/ aluminum MMCs in the aforementioned environments (splash/spray and marine atmosphere). Silicon carbide/ aluminum panels in seawater undergo pitting that is both localized at the edges and distributed uniformly across the surface. The extent of pitting varies from minimal attack through 33 months of exposure to extensive corrosion that is equivalent to a rate as high as 0.25 mm1year (9.8 mils/year). Corrosion rates for silicon carbide/aluminum MMCs in seawater are also generally higher than is typical for unreinforced aluminum alloys. This is documented in Ref 11 for discontinuous SiC in 6061 and 5000 series aluminum matrices and in Ref 13, which reports that silicon carbidel2024 aluminum corroded approximately 40% faster than 2024 aluminum in sodium chloride (NaCl) solution. Figure 7 shows

plots of weight loss versus test duration for aluminummatrix composites and the corresponding weight losses for the matrix alloys in 3% NaCI. Discontinuous silicon carbide/aluminum MMCs are believed to corrode at the silicon carbide/aluminum interfaces (Ref 11, 13, 14). Concentration of the corrosion at these interfaces is presumably due to the crevices formed there, which are preferential sites for pitting. Evidence of the pitting concentrated at the silicon carbide/aluminum interfaces in both whisker and particulate composites is shown in Fig. 8. Electrochemical studies of discontinuous silicon carbide/aluminum MMCs containing 6061 and 5000 series aluminum alloy matrices demonstrated that the presence of the SiC does not increase the susceptibility of the composite to pit initiation (Ref 12, 15). Research on silicon carbidel2024 aluminum did show a more electropositive pitting potential for the composite relative to the 2024 aluminum (Ref 15); however, this difference in pitting potential might be due to the difference in microstructure between the composite matrix and the 2024 aluminum (Ref 3). Continuous-fiber silicon carbide/aluminum composites also undergo localized corrosion (Ref II ). These composites are susceptible to both crevice corrosion and pining. Seawater entry into the silicon carbide/aluminum composite matrix will result in crevice corrosion at the fiber/matrix interfaces, which accelerates the corrosion rate and eventually results in delamination of the aluminum surface foils. However, the rate of silicon carbide/aluminum corrosion is much less severe than is typical for graphite/aluminum. Figure 9 contrasts the extent of corrosion evident on the silicon carbide/aluminum panels described above.

3.5 . - - - - , - - . - - - , - - - - - , - - . - - - - - , 3.0 1---+---+-:7"""'--+----+-

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'"E

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77 (no pits) tp < 22 te < 7

tp > 102 (no pits) tp < 11 te < 3

tp , pitting time; te, crevicetime. The compositecontained25 vol% of

10urn SiC patticulates,which were mixedwith 6061aluminumpowder and processedby extrusion.Specimenswere immersedin aerated 0.5 N NaCI.(a) Sulfuricacid anodizedfollowedby hot water sealing. Source:Ref 26

Table 4 Exposure test results for anoalZed and hotwater sealed and anodized and alChromate sealed aluminum metal-matrix composites (MMCs) Matrix/reinfon:ement(a)

Exposure time, dnys

Number of pits

79 28 28 28 21 28

0 2 15 15 14 5

28 32 28 28

0 0 0 3

HWS

Al 6061 Al 6061115% SiC Al 6061120% SiC Al 6061120% AI20 3 A356115% SiC Al 2009120% SiC DS

Al 6061115% SiC Al 6061120% SiC A356115% SiC AI2009/20%SiC

HWS, anodized and hot water sealed; DS, anodized and dichromate sealed. The specimenswere immersedin 0.5 N NaC!.(a) AlloyA356 MMC samples were cast; Al20 3/aluminum alloys MMCs were cast and extruded; all other samples processed by PIMmethods. Source: Ref 27

188 I COITOsion of Aluminum and Aluminum Alloys

Blistering

•

• (a)

(b)

corrosion performance. Table 2 compares pitting times for CeCl 3 treated and untreated SiC/6061 alloy MMC.

Design for Corrosion Prevention Zinc corrosion products

(c)

Fig 12

Coated discontinuous silicon carbide (porlicu• latel/aluminum MMCs aher seawater exposure. (al Cocted with ion vapor deposited aluminum; 4 month exposure. (bl Coaledwith plasma-sprayed aluminum oxide; 18 month exposure. (cl Coctedwith arc-sprayed zinc;9 month exposure

For long-term use of MMC components in service, effective coating protection must be employed. Consequently, MMC design should take into consideration the ease of initial coating applications as well as coating maintenance (Ref28). A simple component design is optimum for assuring effective coating application; the more complicated the design, the more difficult it is to obtain an adherent, uniform coating. Areas that are difficult to coat, such as sharp edges and comers, overlaps, rivets, fasteners, and welds, should be eliminated as much as possible during design. Also, recesses or low spots should be avoided, because these areas will collect water and lessen the corrosion resistance of the coating. For maintenance considerations, it is imperative that all areas to be coated be readily accessible.

REFERENCES l. Aluminum-Matrix Composites, ASM Specialty

Handbook: Aluminum and Aluminum Alloys, lR. Table 4 compares the corrosion protection afforded by hot-water sealing versus dichromate sealing on the pitting resistance of various anodized aluminum MMCs. The dichromate seal deposits cf>+, a corrosion inhibitor, in the pores of the anodized layers. The inclusion of this inhibitor provides better corrosion protection for MMCs than hot-water sealing. As with continuous graphite/aluminum MMCs, CeCl 3 passivation treatments also provide improved

Davis, Ed, ASM International, 1993, p 160-179 2. Metal-Matrix Composites, Metals Handbook Desk Edition, 2nd ed., lR. Davis, Ed, ASM International, 1998, p 674-680 3. M. Metzger and 5.0. Fishman, lnd. Eng. Chem: Prod. Res. Dev., Vol 22, 1983, p 296 4. AJ. Sedriks, lA.S. Green, and D.L. Novak, Metall: Trans., Vol 2, 1971, p 871 5. S.L. Pohlman, Corrosion, Vol 34, 1978, p 156

Corrosion of Aluminum Metal-Matrix Composites I 189 6. L.H. Hiharaand RM. Latansion, Localized Corrosion Induced in Graphite/Aluminum Metal-Matrix Composites by Residual Microstructural Chloride, Corrosion, Vol47 (No.5), 1991,P 335-340 7. T.R Schrecengost, B.A. Shaw, RG. Wendt, and W.C. Moshier, Corrosion, Vol 49 (No. 10), 1993, p842-849 8. R.G. Wendt, W.e. Moshier, B. Shaw, P. Miller, andD.L. Olson, Corrosion, Vol 50 (No. 11), 1994, P 819-826 9. W.K Boyd and EW Fink, Corrosion of Metals in Marine Environments, Metals and Ceramics InformationCenter, 1978,p 44, 57-67, 85-87 10. WH. Pheifer, in Hybrid and Select Metal Matrix Composites: A State of theArt Review, WJ. Renton, Ed.,American Institute ofAeronautics andAstronautics, 1977, p 231-252 11. D.M. Aylorand PJ. Moran,Preprint 202, presented at Proc. of the Corrosion/86 Symposium, National Association of Corrosion Engineers, 1986 12. D.M.AylorandPJ. Moran, J. Electrochem. Soc.• Vol 132, 1985, P 1277 13. H.M. Dejarnette and C.R. Crowe, Naval Surface Weapons Center, unpublished research, 1982 14. O.P. Modi, M. Saxena, B.K. Prasad, AK Jha, and AH. Yegneswaran, Corrosion, Vol54 (No.2), 1998, P 129-133 15. P.P. Trzaskorna, E.M. McCafferty, and C.R Crowe, 1. Electrochem: Soc., Vol130, 1983,P 1804 16. AR. Champion, WH. Krueger, H.S. Hartmann, and AK. Dhingra, in Proc. of the Second International Conference on Composite Materials, B. Notonet.al., Ed.,TheMetallurgical Societyof AIME,1978, P 883 17. IY. Yang and M. Metzger, University of lllinois, unpublished research, 1980 18. D. Nath and T.K.G. Narnboodhiri, Corrosion Science, Vol29 (No. 10),1989,P 1215-1229

19. D.A.Davis,M.G. Vassilaros, and J.P.Gudas,Mater. Perfom, Vol21, 1982,p38 20. WL. Phillips, "SharpNotchSCC ofB/AI and Gr/AI Composites," Report3616, NavalResearchLaboratory,Oct 1977 21. C.R. Crowe and D.E Hasson, in Proc. of the Sixth International Con! on the Strength of Metals and Alloys, Vo12, 1982,P 859 22. S.-S.Yau,Ph.D. dissertation, NorthCarolinaUniversity,1983 23. MJ. Snyderand J.H. Payer,"The Engineering Development of Graphite FiberReinforced Aluminum Composites," Report 74-4312A, Launch Vehicle Materials Technology Program, BattelleColumbus Laboratories, Dec 1976 24. D.M.AylorandRM. Kain,Mate.r. Perform, Vol23, 1984,p 32 25. E Mansfeld, S. Lin, and S. Kim, and H. Shih, Electrochim. Acta, Vol34 (No.8), 1989,P 1123-1132 26. S. Lin,H. Greene, H.,Shih,and E Mansfeld, Corrosion, Vol8 (No.1), 1992,P 61-67 27. HJ. GreeneandE Mansfeld, Corrosion, Vo153 (No. 12),1997,P 920-927

SELECTED REFERENCES • L.H. Hihara, "Corrosion of Aluminum-Matrix Composites," Ph.D. dissertation, Massachusetts Instituteof Technology, 1985 • L.H. Hihara, Metal Matrix Composites, Corrosion Tests and Standards: Application and Interpretation, R. Baboian,Ed., ASTM, 1995,P 531-542 • KA Lucas and H. Clarke, Corrosion of AluminwnBasedMetal-Matrix Composites, ResearchStudies Press Ltd. and John Wiley & Sons, Inc., 1993



Chapter 11

Corrosion Prevention Methods

THE INCREASE IN COST attributed to corrosion problems in the United States has been significantfrom $5.5 billion in 1947 to a more recent estimate of approximately $300 billion in 1995. These figures are associated with the direct cost of replacement of the corroded structure; additional costs are associated with maintenance and repair (e.g., maintenance and repair costs military aviation more than $3 billion annually), increased regulatory demand (e.g., environmental issues involving certain coating processes), and lost production. Not only is there a direct and indirect fmancial burden, but corrosion also has a considerable societal impact. Crucial industries such as energy, aerospace, transportation, food, agriculture, electronics, marine, and petrochemical rely on the safety and availability of their infrastructures. In many instances, these requirements have been compromised by corrosion events that have led to loss of life, environmental pollution, and loss of power to industry. However, between 25 and 50% of the economic impact (depending on the specific industry) could have been prevented by the use of well-accepted materials selection and corrosion prevention measures. The purpose of this chapter is to give the reader an overview of the prevention methods commonly associated with aluminum and aluminum alloys. These methods range from relatively straightforward measures, such as proper handling and storage of aluminum products, to advanced early warning corrosion monitoring systems for military aircraft.

stage are far less expensive than subsequent changes, repairs, and stopgap procedures made on a faulty product. If an existing part is being replaced or improved, a good place to start the design process is to determine why the prior material was inadequate or failed and what the possible misapplications were. The basic factors that most influence design for corrosion resistance are summarized in Table 1. Each factor plays a unique yet not always unrelated role with

Table 1 Corrosion factorsthat can inftuence design considerations Type

Environment

Stress Shape

Compatibility

Movement

Temperature

Control

Design Considerations Proper design of a product or assembly is the most important way to prevent corrosion of aluminum. Correction and improvements made during the design

Source:Ref 1

Fact...

Natural Chemical Storage/transit Residualstressfromfabrication Operatingstress-static, variable, alternating Joints. flanges Crevices,deposits Liquidcontainmentand entrapment Metalswith metals Metals with othermaterials Qualitycontrol Flowingfluids Parts movingin fluids Two-phasefluids Oxidation,scales Heat-transfereffects Moltendeposits Condensationand dewpoint Surfacecleaningand preparation Coatings Cathodicprotection Inhihitors Inspection Plannedmaintenance

192


other factors. It should be noted that some of these factors are not specific to aluminum, but rather they apply to a variety of structural metals, most notably steels.

Design Details 'fhatAccelerate Corrosion Location. Exposure to winds and airborne particulates can lead to deterioration of structures. Designs that leave structuresexposed to the elementsshouldbe carefully reviewed, because atmospheric corrosion is significantly affected by temperature, relative humidity, rainfall,and pollutants.Also importantare the season and location of on-site fabrication, assembly, and painting. Codes of practice must be adapted to the location and the season. Shape. Geometrical form is basic to design. The objectiveis to minimize or avoid situationsthat worsen corrosion.These situationscan range from stagnation (e.g., retained fluids and/or solids; contaminated water used for hydrotesting) to sustained fluid flow (e.g., erosion/cavitation in components moving in or contacted by fluids, as well as splashing or droplet impingement). Common examples of stagnationinclude nondraining structures, dead ends, badly located components, and poor assembly or maintenancepractices (Fig. I). General problems include localized corrosion associated with differential aeration (oxygen concentration cells),crevicecorrosion, and deposit corrosion. Movement. Fluid movement need not be excessiveto damage a material. Much dependson the nature of the fluid and the hardnessof the material.A geometric shape can create a sustaineddeliveryof fluid or can locallydisturb a laminarstreamand lead to turbulence. Replaceable baffle plates or deflectors are beneficial where circumstances permit their use; they eliminate the problem of impingement damage to the structurally significantcomponent. Careful fabrication and inspection should eliminate or reduce poor profiles (e.g., welds, rivets, and bolts), rubbing surfaces(e.g., wear and fretting), and galvanic effects due to the assembly of incompatible components. Figure 2 shows typical situations in which geometricdetails influenceflow. Galvanic Compatibility. In plant environments, it is often necessary to use different materials in close proximity. Sometimes, componentsthat were designed in isolation can end up in direct contact in the plant (Fig. 3). In such instances, the ideals of a total design concept become especially apparent, but usually they appear in hindsight.Direct contactof dissimilarmetals introduces the possibility of galvanic corrosion, and small anodic (corroding) areas should be avoided whereverthis contact is apparent. Designers, when aware of compatibility effects, need to exercisetheir ingenuityto minimizethe conditions that most favor galvanic corrosion. Table 2 provides some relevantparametersin this context.

The most common design details relating to galvanic corrosion include jointed assemblies (Fig. 3). Where dissimilar metals are to be used, some considerationshouldbe given to compatiblematerialsknown to have similar potentials (for more information, refer to Chapters 2 and 5). Care should be exercised in that galvanic series are limited and refer to specific environments. Where noncompatible materials are to be joined, it is necessaryto use a more noble metal in a joint (Fig. 3). Effective insulation can be useful if it does not introduce crevice corrosion possibilities. Some difficulties arise in the use of adhesives, which might not be sealants. The relative surface areas of anodic and cathodic surfaces shouldnot be underestimated, because instances of corrosion failure can result from a combination of galvanic and crevice attack. Corrosion in a small anodic zone can be several hundred times greater than that in similar bimetallic components of similar area. Anodic components on occasion can be overdesigned (thicker)to allow for the anticipatedcorrosion loss. In other cases, easy replacement is a cost-effective option, given an awareness by the designer of such information. Where metallic coatings are used, there is always a risk of galvanic corrosion, especially along the cut edges. Roundedprofiles and effectivesealantsor coatings can be beneficial. Transition joints can be introduced when differentmetals will be in close proximity (see Fig. 3b and also refer to Chapter 9 for more information on transitionjoints). Another aspect of corrosion preventionis the coating of the cathodic material for corrosion control. Ineffective painting of an anode in an assembly can significantly reduce the desired service lifetime because local defects will effectively multiply the risk of anodic sites and localized corrosion. Less obvious examples of galvanic corrosionoccur when ion transfer results in the deposition of active and noncompatible deposits on a metal surface. For Table 2 Galvanic cOlTOsion sources and design considerations Designconsiderations

Soun:e

Metallurgical sources(both withinthe metalandfor relativecontactbetween dissimilarmetals)

Environmental sources

Differencein potentialof dissimilar materials;distanceapart;relative areasof anodeandcathode; geometry(fluidretention); mechanical factors(forexample, cold work,plasticdeformation) Conductivity andresistivity of fluid;changesin temperature; velocityanddirectionoffluid flow;aeration; ambient environment (seasonalchanges, etc.)

Miscellaneous sources

Straycurrents;conductivepaths; composites (forexample, aluminum-graphitecomposites)

Corrosion Prevention Methods / 193

example, an aluminum stirrer plate used in water was extensively pitted because the water bath was heated by a copper heater coil (Fig. 3e). The pits resulted from deposition of copper ions from the heater element More rapid, but similar, damage occurred when a dental aspirator (Teflon-coated aluminum) was attacked by mercury from a tooth filling. These two metals rank as a high risk combination for galvanic corrosion. The aluminum section was rapidly pitted once the Teflon had first been worn away by sharp fragments of tooth enamel. Anodic components can on occasion be overdesigned (made thicker) to allow for the anticipated corrosion loss. In other instances, easy replacement is a cost-effective option. Mechanical Factors. Environments that promote metal dissolution can be considered more damaging if stresses are involved (see the discussion on stresscorrosion cracking in Chapter 7). In such circumstances, materials can fail catastrophically and unexpectedly. Safety and health can be significantly affected. Figure 4 shows cases in which design detail is used to minimize stress. Perfection is rarely attained in general practice, and some compromise on materials limitations, both chemical and mechanical, is necessary. The difficulty is that mechanical fault can contribute to corrosion and that corrosion (as a corrosive environment) can initiate or cause mechanical failure. Quality control and assurance can eliminate the former condition. Designs that introduce local stress concentrations directly or as a consequence of fabrication should be carefully considered. Of particular importance are stress levels for the selected material; the influence of tensile, compressive, or shear stressing; alternating stresses; vibration or shock loading; service temperatures (thermal stressing); fatigue; and wear (fretting, friction). Profiles and shapes contribute to stressrelated corrosion if material selection dictates the use of materials susceptible to failure by stress-corrosion cracking or corrosion fatigue. Materials selection is especially important wherever critical components are used. Also important is the need for correct procedures at all stages of operation, including fabrication, transport, startup, shutdown, and normal operation. Less obvious cases of failure can arise from vibration transfer, poor surface fmish, nonuniform application of surface coatings, or the application of coatings to poorly prepared surfaces. Surfaces. Corrosion is a surface phenomenon, and the effects of poorly prepared surfaces, rough textures, and complex shapes and profiles can be expected to be deleterious. Figure 5 shows some examples in which design specification could have considerably reduced the onset of corrosive damage. Design limitations include surfaces exposed to deposits, retained soluble salts (because of poor access for preparation before painting), nondraining assemblies, poorly handled components (distortion, scratches, and dents), and

poorly located components (relative position to adjacent equipment, and so on). Painting and surface-coating technology have advanced in recent years and have provided sophisticated products that require careful mixing and application. Maintenance procedures frequently require field application; in such cases, control is not anticipated. This is significant, for example, in the offshore locations of the oil and gas industry. Inspection codes and procedures are necessary, and total design should incorporate these wherever possible. In critical areas, design for on-line monitoring and inspection will also be important. The human factor in such procedures is often overlooked. The need for better techniques, standardization, and mechanization or full automation has been stated, and adequate training and motivation are of primary importance. Insulation represents another area for potential corrosion attack, although the form and requirements for insulating media differ considerably. Moistureabsorbing tendencies will vary, as will the extent of crevicing from compaction and shrinkage or chloride buildup for certain materials. Wet-dry cycling can lead to concentration effects that can result in pitting of aluminum in contact with the insulation barriers. Figure 6 shows some typical examples in which design and installation procedures could have been improved.

Care of Aluminum

Handling and Storage Because of the excellent corrosion resistance of the lxxx, 3xxx, 4xxx, 5xxx, and 6xxx series alloys, users occasionally have not employed good practice in the handling and storage of these alloys. This can result in water stains or in pitting. Water Staining. As described in Chapter 3, water stain is superficial corrosion that occurs when sheets of bare metal are stacked or nested in the presence of moisture. The source of moisture can be condensation from the atmosphere that forms on the edges of the stack and is drawn between the sheets by capillary action. Aluminum should not be stored at temperatures or under atmospheric conditions conducive to condensation. When such conditions cannot be avoided, the metal sheets or parts should be separated and coated with oil or suitable corrosion inhibitor. Once formed, water stain can be removed by either mechanical or chemical means, but the original surface brightness can be altered. Outdoor storage of aluminum, even under a tarpaulin, is generally not desirable for long periods of time; this varies with the alloy, the end product, and the local environment. Moisture can collect on the surface, sometimes at relative humidities below the dew point, because of the hygroscopic nature of the dust or particles that deposit on the metal from the atmosphere.

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Examples of how design and assembly can affect localized corrosion by creating crevices and traps where corrosive liquids can accumulate. (a) Storage containers or vesselsshould allow complete drainage; otherwise, corrosive species can concentrate in vessel bottom, and debris can accumulate if the vessel is open to the atmosphere. (b) Structural members should be designed to avoid retention of liquids; l-shoped sections should be used with open side down, and exposed seams should be avoided. (c) Incorrect trimming or poor design of seals and gaskets can create crevice sites. (d) Drain values should be designed with sloping bottoms to avoid pitting of the bose of the valve. (e) Nonhorizontal tubing can leave pools of liquid at shutdown. (ij to (il Examples of poor assembly that can lead to premature corrosion problems. (~ Nonvertical heat exchanger assembly permits dead space that is prone to overheating if very hot gases are involved. (g) Nonaligned assembly distorts the fastener, creating a crevice that can result in a loose fit and contribute to vibration, fretting, and wear. (h) Structural supports should allow good drainage; use of a slope at the bottom of the member allows liquid to run off, rather than impinge directly on the concrete support. (i) Continuous weld for horizontal stiffeners prevents traps and crevices from forming. (j) Square sections formed from two L-shapemembers need to be continuously welded to seal out the external environment.

n

o

ici" ::::I

"1lI

ii. o ::::I

f I...... -0

U'I

:.....-

~)" J,

..-:: .-'...n>J..)

Overflowing fluid

'777/77777/// ///~

?zz,[gja;;

-FiiM-- f\ l~t

dJ~ Poor

Collection

=

Better

Better

!.J

Underlying structure

Poor

Ee

"'d~--'iI" I

-W:!.w Poor (d)

Fig 2

solution

_

:'\.

Vessel

Dilute solution

Poor

l Dilute solution

a a

a a

o//azzzw~~

-

~(T0

777777~/7777777T/777777/,

Better (e)

~

.../~

~ (1)

Effect of design features on flow. (al Disturbances to flow can create turbulence and cause impingement damage. (b) Direct impingement should be avoided; deflectors or baffle plates can be beneficial. {cllmpingement from Auid overflowing from a collection tray can be avoided by relocating the structure, increasing the depth of the tray, or using a deflector. (dl Splashing of concentrated fluid on container walls should be avoided. (e)Weld backing plates or rings can create local turbulence and crevices. (ij Slope or modified profiles should be provided to permit Aow and minimize fluid retention.

°

c a So c

~

l !

!L

c aSo c

Concentrate concentra;jed fluid

jO

~

(c)

(b)

(8)

tra~

r

~III

Corrosion Prevention Metf10ds I 197 The resulting staining or localized pitting, although of little structural consequence in the lxxx, 3xxx, 4xxx, 5xxx, and 6xxx alloys, is undesirable if the aluminum will be used for an end product for which surface finish is critical. The 2.xxx and Txxx bare alloys are susceptible to intergranular attack under these conditions, and for these alloys, use of strippable coatings, protective wrappers, papers, or inhibited organic films is advisable when adverse conditions cannot be avoided. Mechanical damage can be easily avoided by good housekeeping practices, proper equipment, and proper protection during transportation. When transporting flat sheets or plates, the aluminum should be oiled or interleaved with approved paper to prevent traffic marks, where fretting action at points of contact causes surface abrasion. Practices to avoid these defects are described in Ref 2.

Cleaning and Deactivation of Con-osion Without cleaning or maintenance, aluminum acquires a gray appearance in many applications, as a result of natural weathering or superficial corrosion. The acceptability of this appearance is governed by the desires of the user, the service for which the metal is intended, and the type of finish that was initially applied. In industrial roofing and siding sheet, bridge railings, lighting standards, and similar applications, the natural weathered appearance of aluminum alloys can be completely acceptable. For uses such as store fronts and automotive trim, more lustrous, cleaner surfaces are sometimes desired. The products formed during weathering of unprotected aluminum alloys in most natural environments are insoluble and provide protection to the underlying metal, thus establishing the self-limiting type of corrosion described in Chapter 8. These adherent corrosion products provide a base for accumulation of airborne particles of dirt and soil. In relatively dirt-free environments-such as along the seacoast and in rural locations-c-oaly the gray patina of naturally weathered aluminum will result from exposure, whereas in industrial areas the metal can tum dark or even black (in the case of roofing or other horizontal surfaces) as a result of soil accumulation. Finishes are applied to aluminum alloys for aesthetic reasons and to provide protection against corrosion. These finishes include nonfinished (bare) aluminum, anodized aluminum, conversion coatings, painted aluminum, porcelain finishes, and plated finishes. In retarding corrosion, these finishes naturally minimize the buildup of corrosion products to which airborne dirt particles can adhere. In most applications, aluminum with an applied finish presents no maintenance or cleaning problem for a considerable period of time; subsequently, when cleaning is required, it is much easier to do. Many (if not most) of these finishes

are applied to enhance appearance. Where this is the case, cleaning procedures must be carefully chosen and used in order to avoid marring the surface. Cleaning and maintenance procedures can be required for sanitation (as in cooking utensils or food service equipment) or to retain or regain the original attractive appearance (as in architectural applications or on automotive trim). Other needs are to prolong the service life of aluminum by retarding corrosion, as encountered in chemical processing equipment, or to remove scale, as in equipment such as heat exchangers. Probably the most important factor in any cleaning or maintenance program is the selection of the proper cleaning procedures for the job. Improper cleaning methods or unsatisfactory materials can result in objectionable discoloration, staining, or pitting of either the finish or the metal surface itself. Type. of Cleaning. Aluminum and its alloys can be cleaned as a final step of manufacture or construction, for periodic maintenance and soil removal, or for restoration. As produced commercially, most aluminum alloys need little in the way of cleaning prior to use, unless they are to be finished. Cleaning of the metal surfaces prior to finishing, covered in Surface Engineering, Volume 5 of the ASM Handbook (ASM International, 1994), is very important. Although, from a functional standpoint, mill-finished, unprotected aluminum needs little cleaning prior to use, it is often advisable to clean the surfaces with an organic solvent to remove residual lubricants and to provide a more uniform appearance. For periodic maintenance cleaning, to retain the original appearance of the metal surface or finish, procedures are simple and are governed by the frequency of maintenance and the types of cleaners employed. Restorative cleaning is done on aluminum surfaces that have been allowed to weather or oxidize in some other way for an extended time without maintenance. This can be the most difficult of cleaning procedures. If the aluminum alloy has suffered appreciable corrosion in the form of localized pitting or surface roughening, no cleaning procedure will remove the damage. Only refinishing will restore the surface. General Recommendation•. Several basic concepts in cleaning aluminum (or any other metal) are generally applicable. It is important to know the finish that has been employed on the aluminum surface to be cleaned, so that the appropriate cleaning procedure can be selected. For example, some cleaners can affect organic coatings adversely, whereas others can have deleterious effects on anodic coatings. It is important to read and follow the directions supplied by the cleaner manufacturer. Different cleaners should not be mixed on anyone job. Cleaners are especially formulated to accomplish a particular type of cleaning. Adding other cleaners or chemicals can alter the efficiency of the cleaner and can even constitute a health hazard to the user. Surfaces should not be cleaned when they are hot. Heated surfaces cause rapid evaporation of the cleaner,

-0

00

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n

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~

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Fastener material should be more noble

t

Coppe r

[ Aluminum

,

!

r:

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7

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~~

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Poor

Poor

-

~

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Poor

,d ~ ~nt

:T

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(b)

(a)

c 3 S· c 3 a

i

~

c 3 S· c

3 ~ROI/ing direction ~

E~grain ~ Poor

=::,/ (c)

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Weld over end

rp

l!;.:J Good

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;tt-a

C8lhode_~

~

Aluminum

Poor

~~::w

1" I

Poor

Glass vessel

Good (d)

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(e)

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Poor

Better

of III

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_ _ _

_

_

Cladding

Timber

~~ ~. v;;r; Better

Poor

---\0J) Condensation ~

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droplets

0

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collection tray

.:c::.::.l:..-:..'1/

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II

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~~;,."".,.""'" Aluminum structure

(f)

(h)

(g)

n o

Fig. 3

Design details that can affect galvanic corrosion. (a) Fasteners should be more noble than the components being fastened; undercuts should be avoided, and insulating woshers should be used. (b) Weld filler metals should be more noble than base metals. Transition joints can be used when a galvanic couple is anticipated at the design stage, and weld beads should be properly oriented to minimize galvanic effects. (c) Local damage can result from cuts across heavily worked areas. End grains should not be left exposed. (dJ Galvanic corrosion is possible if a coated component is cut. When necessary, the cathodic component of a couple should be coated. (e) Ion transfer through a fluid can result in galvanic attack of less noble metals. In the example shown at left, copper ions from the copper heater coil could deposit on the aluminum stirrer. A nonmetallic stirrer would be better. At right, the distance from 0 metal container to a heater coil should be increased to minimize ion transfer. Wood treated with copper preservatives can be corrosive to certain nails, especially those with nobility different from that of copper (e.g., aluminum). Aluminum cladding can also be at risk. (g) Contact of two metals through a fluid trapcan be avoided by using a collection tray or a deflector.

m

iis" ::::I

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f Is-

......


Poor Better Keyways

~ Stress raisers

~

Poor

Better

Better

Poor (b)

(a)

F Poor

~~

or braze

Vibration

•

Long overhang

,

Fatigue

Better

(d)

(e)

Fluid

fI

+

'f~_oo 3~_~ (e)

Fig 4

Deslqn details that can minimize local stress concentrations. la) Cornersshould be given a generous • radius. lb) Welds should be continuous to minimize sharpcontours. lc) Sharpprofilescan be avoided byusingalternative fastening systems.ld) Too long an overhangwithout a supportcan lead to fatigue at the junction. Flexiblehosecan helpalleviatethissituation. Ie)Side-supply pipeworkcan be too rigid to sustain thermalshock froma recurring sequence that involves (1) air underpressure, (21 steam, and (31 cold water.

resulting in insufficient cleaning, or streaking or staining of the metal surface. Conversely, cleaning should not be attempted in freezing weather or where the metal is so cold as to encourage condensation of atmospheric moisture on the surfaces. With any cleaner, it is important that a small area of the aluminum surface be test cleaned prior to any

IIlr Poor

Poor

(a)

Poor

Poor

Better

(b)

Fig 5

Effects of design on effectiveness of cleaning • or painting. (a)Pooraccess in some structures makes surfacepreparation and painting difficult; access to the types of areas shown should be maintained at a minimum of 45 mm 11 3/4 ln.], or one-third of the heightof the structure. (b) Sharp corners and profiles should be avoided if the structure is to be painted or coated.

large-scale cleaning. This will permit an evaluation of the cleaning efficiency and also will determine the acceptability of the appearance of the cleaned surface. The recommended concentration of cleaner should be permitted to remain on the surface of the metal no longer than the length of time necessary for adequate cleaning. No attempt should be made to speed up the cleaning action by increasing the cleaner concentration above that recommended. Cleaners used on aluminum surfaces should not be splashed onto adjacent materials as they can cause damage to those surfaces. Similarly, cleaners used on other surfaces adjacent to aluminum should not be splashed or drained onto the aluminum surfaces. After cleaning, the aluminum surfaces should be thoroughly rinsed and allowed to dry completely. An exception is cleaning with abrasive polishes, which are removed with clean dry cloths. For convenience, the Aluminum Association has divided aluminum cleaners into five groups (Ref 2): • • • • •

Mild soaps and detergents and nonetching cleaners Solvent and emulsion cleaners Abrasive cleaners Etching cleaners Special-duty cleaners (steam, rotary wire brushes, and abrasive blasting)

Table 3 ranks the aggressiveness of these cleaners and matches them to the various finishes previously mentioned. It is always desirable to try mild cleaners before


proceeding to those having more drastic action. If it is found necessary to use the more aggressive cleaners, cleaning procedures should be tried in the order of increasing aggressiveness until satisfactory results are obtained. Proprietary products for the care and cleaning of aluminum have been provided by member companies of the Aluminum Association and are listed in Ref 2.

I

Alclad Products Aluminum products sometimes are coated on one or both surfaces with a metallurgically bonded, thin layer of pure aluminum or aluminum alloy (Fig. 7). If the combination of core and cladding alloys is selected so that the cladding is anodic to the core, it is called

aJ~ ~ m

11 \("""__

cladding _ _.....

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Insulation layer (8)

•

(b)

f..

Chloride-containing insulation Metaloutercladding cut 100 short Cracks and pitsin susceptible material

_ - Outercladding

;i:!F~ -.....

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Moisture

Steel pipe

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(c)

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./

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./

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;'

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Fig 6

201

Corrosion problemsassociated with improper useof insulationand cladding. (allncorrect overlap in lobster• back cladding doesnot allow fluid runoff. (b)Poorinstallationletta gap in theinsufation thatallows easyaccess to the elements. (c) Outer metalcladding was cut too short,leaving a gap with the inner insulationexposed. (d)Insufficient insulation can allow water to enter;chloride in someinsulation can result in pitting or stress-eorrosion cracking of susceptible materials. (e)Overtightenedstrapping can damage the insulation layer and cause fluid "dams" on vertical runs.


alclad. The cladding of alclad products electrochemically protects the core at exposed edges and at abraded or corroded areas. When a corrosive solution is in contact with the product, current from the anodic cladding flows through the electrolyte to the cathodic core, and the cladding tends to dissolve preferentially, thus protecting the core (Fig. 8). Sustained protection is dependent on obtaining the optimum quantity of current (which is influenced by the potential difference between the cladding and core), the conductivity of the corroding medium, film formation, and polarization. The corrosion potentials of cladding and core alloys are important in selecting a coating that is sufficiently anodic to electrochemically protect the core. Copper in solid solution in aluminum is less anodic as copper

content increases. Consequently, pure aluminum is anodic to aluminum-copper-magnesium alloys in the naturally aged T3x and T4x tempers by about 0.154 V and is used as the cladding for most alclad 2xxx products. Increasing zinc in solid solution increases the anodic potential of aluminum alloys, while Mg2Si and manganese have little effect. Alloy 7m2, AI-IZn, has a more anodic potential than pure aluminum and is used as the cladding for Alclad 3003, 5052, 6061, and 7075, as well as others. The most widely used alclad products are sheet and plate, although wire, tube, and other forms are also produced. The most generally accepted method of fabricating alclad sheet and plate consists of hot rolling to pressure weld the cladding slabs to a scalped core ingot. In fabricating alclad products, the temperature

Table 3 Recommended cleaners for various aluminum finishes

Finish

Bare specular Bare satin Anodized Chemical conversion Painted Plated Porcelain

Mild Mild soaps, detergents,

Solvents and

andnon-etchcleaners

emulsions

S S S S

S S S S

T S S

T S S

Ab.....ivecleaner Abr... ives polishes (moderate duty)

T U

T U U

T S(b)

Abrasives (heavy duty)

Etching

Aggressive Special

Steelwool(a)

cleaners

cleaners

T T U U

T T T T

U S(b) S(b) T

U

U

U S(b) S(b) U

T T S

U

U

U

T S(b)

T S

T T

T T

T T T

S, normallysafe, should not damagefinish; T, spot test beforeusing; U, not usually used and can damagefinish. (a) Stainlesssteel wool preferable. (b) Rublightlyin directionof grain,if visible.Note:Informationin this tableis providedmerelyfor quick and easyreference.In general, the mildest cleanersshould be tried first and the moreaggressivecleanersused onlyif necessary.It is suggestedthat the readerconsult Ref 2 before selectinga cleaner.

Fig 7

Alloy 2024-T3 sheet clodwith alloy1230 (5% • perside),solution heattreated. Normal amount of copper and magnesium diffusion from bose metal into cladding (lop). Keller's reagent. 100x

Fig 8

Alloy 7178-T76 sheet clod with 0.125 mm (0.005 in.) of alloy 7072 (3.2 mm, or 0.125 in., 10101 thickness). Sacrificial corrosion of cladding prevented corrosion of sheet during soil fog testing in 5% sodium chloride fortwoweeks. Keller's reegent.75x •


and time of thermal treatments should be minimized to avoid extensive diffusion of soluble elements from the core. This is particularly important in the 2xxx alloys, as diffusion of copper in the cladding makes it less anodic. It is less important in alloys containing zinc and magnesium, because these elements make the cladding more anodic. The percentage of cladding thickness is determined principally by the thickness of the finished part. Because the objective is to provide an adequate abso-

I 203

lute thickness, the percentage of thicker parts need not be as great as the percentage for thinner parts. A listing of the most widely used alclad products is given in Table4.

Anodizing Aluminum anodizing is an electrochemical method of converting aluminum into aluminum oxide (Al2 O:3)

Table 4 Specifiedrequirements for alclad products

Desigoatioo

CompooeotaBoys(a) Core Cladding

Alclad 2014 sheet and plate

2014

6003

Totalspecilled tbid

Z

Results

10'

10' 20

10

0

25

30

35

Original anodic coaling thickness, um

g 10 F.O •

Source: Ref 5

Number of corrosion pits in anodized olumlnum 1100 as a fu nction of coating thickness.

Extrusions 6351-T6 6061-T6 6063-T5 6070-T6 7039-T6

No visiblepitting Edge pittingonly No visiblepitting No visiblepitting No visiblepitting No visiblepitting Edge pittingonly Edge pittingonly Edge pittingonly No visiblepitting No visiblepitting No visiblepitting No visiblepitting Scatteredsmallpits

H2S0 4 anodic coatings 23 um (0.9 mil) thick were sealed in boiling water on lest panels 100 x 150 mm (4 x 6 in.) cut from sheet and extrusions.

25 ....::--,-'6--.----.,..-----,----.--,-----,---,------,1.0

i

.li u

20 1---+---+---"""L7-"":::O""'=:l-~o_~--+---t---+-----i 0.8

~

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~

£

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'+=0

~

=

§ ~

~

8

0.6

15

~

~

~

.~

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E

~

~

10 '-_--'-_ _-'-_ _'--_-'-_ _-.L._ _- ' - - _ - ' -_ _- ' - - _ - - - ' 0.4 o 4 12 20 ~ 16 24 28 32 Exposure time, yr

Fig. 11

Weathering data for anodically coated aluminum in an industrial atmosphere


Anodic coatings, unless used as part of a protective system that includes such other measures as shot peening or painting, are not reliable for protection against stress-corrosion cracking (SCC) of susceptible alloys. Data obtained with short-transverse direction specimens from plate of alloy 7075-T651 and other susceptible alloys show that the anodic coating can retard, have no effect, or even accelerate SCC, depending on the level of stress and, to some extent, on whether or not the stress was present before anodizing. High stresses applied after anodizing crack the coating. The effects of several applied protective measures on lifetimes of specimens in industrial and seacoast environments under relatively high elastic strain are shown in Fig. 12, in which the relatively small protective value of anodic coatings is apparent (Ref 6). Example 11 Corrosion of an Anodized 7075· T6 Wing Panel. New aircraft wing panels extruded from 7075-T6 aluminum were reported to be discolored, exhibiting an unusual pattern of circular, black, interrupted lines (Fig. 13a). The black marks were coherent with the metal and could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrant inspected, shot peened, sulfuric acid anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. During processing, the extrusions were machined on the flat side, oiled, deburred, hot formed, cleaned, penetrant inspected, covered with oil, and then shot peened. They were then recoated with oil, shipped to a second vendor, hand wiped with a solvent, alkaline cleaned, acid desmutted, sulfuric acid anodized, and hot water sealed. The panels were studied using the scanning electron microscope and microprobe analysis. Both conven-

tional energy-dispersive and Auger analyzers were employed. Figures 13(b) and (c) illustrate the contention that the anodic coating was applied over an improperly cleaned and contaminated surface. It was evident that the expanding corrosion product had cracked and in some places had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack (Fig. 13d). The depth of attack was measured to be 0.035 mm (0.0014 in.). Microprobe analysis of the corrosion product did not reveal any clues concerning the reason for or origin of the corrodent. A high sulfur concentration was found to be associated with the corrosion product and on surface areas away from the products. It was suspected that the origin of the sulfur was the hydrocarbon oil. When the anodized layer was stripped from the panels using a phosphoric-chromic acid solution, the evidence of sulfur disappeared. The same stripping procedure did not remove the black corrosion product. Energy-dispersive analysis of the corrosion product revealed the presence of iron, calcium, phosphorus, and chromium in excess. No chlorides were detected. Auger spectroscopy revealed the presence of large amounts of carbon and nitrogen. The MIL-C-27725 coating was removed from a portion of the rib side by using a paint stripper. No corrosion or discoloration of the aluminum was observed. It was concluded that the corrosion of the anodized panel probably resulted from improper and insufficient cleaning prior to anodizing. The preservation oils used during the various steps of manufacture and their incomplete removal prior to anodizing were highly suspect. The recommendations were as follows: • •

99

• •

#-

.."

.~

:;

• •

E

:>

u

.; ~ "iii >

.~

Use a vapor degreaser during cleaning prior to anodizing. Use a hot inhibited alkaline cleaner during cleaning prior to anodizing. Dichromate seal the panels after anodizing. Use deionized water during the dichromate sealing operation . Use an epoxy primer prior to shipment of the panels. Most importantly, monitor the anodizing process itself, including continual monitoring of bath acid concentration, solution cleanliness, temperature control, and voltage/amperage control.

:>

en

Conversion Coatings 1 year

2 years

3 years

4 years

Duration of exposure

Fig. 12

Relative effectiveness of various protective systems in preventing SCC of susceptible aluminum alloys. Combined data lor highly elastically strained specimens of alloys 2014-T651 and 7079·T651 exposed at Pt.Judith, RI;Comfort, TX;and New Kensington, PA. Anodized specimens include the proprietary Alumilite (sulfuriC acid) process.

Conversion coatings are adherent surface layers of low-solubility oxide, phosphate, or chromate compounds produced by the reaction of suitable reagents with the metallic surface. These coatings affect the appearance, electrochemical potential, electrical resistivity, surface hardness, absorption, and other surface properties of the material. They differ from anodic coatings in that conversion coatings are formed by a chemical oxidation-reduction reaction at the surface of


the aluminum, whereas anodic coatings are formed by an electrochemical reaction. Conversion coatings are excellent for achieving the following: •

Improved corrosion resistance, particularly when used conjointly with an organic coating • Improved adhesion for organic finishes • Mild wear resistance • Enhanced drawing or forming characteristics • Decorative purposes, when colored or dyed Conversion coatings are used interchangeably with anodic coatings in organic finishing schedules. One use of conversion coating is as a spot treatment for the repair of damaged areas in anodic coatings. Because of their low strength, conversion coatings should not be used on surfaces to which adhesives will be applied. Anodic coatings are stronger than conversion coatings for adhesive bonding applications. From a corrosion resistance standpoint, chromate conversion coatings are superior when compared to either oxide or phosphate coatings; hence emphasis in this section will be on the uses and performance of chromate coatings. Information on oxide and phosphate coatings, which are commonly used as a paint

(a)

base for aluminum, can be found in Volume 5 of the

ASM Handbook (ASM International, 1994).

Chromate Conversion Coatings Chromate conversion coatings are generally used to increase the corrosion resistance of aluminum. Most conversion coatings slowly dissolve in water and provide limited protection in this mediwn; however, they furnish excellent protection in marine atmospheres and in high-humidity environments. The protection provided by chromate coatings increases directly with thickness up to a certain point, after which the protective nature is sacrificed due to the formation of a porous, nonadherent film. The high corrosion resistance offered by chromate films is attributed to the presence of both hexavalent and trivalent chromiwn in the coating. Analyses of coatings by wet chemical methods and with surfacesensitive techniques have shown that both hexavalent chromium, cr6+ or Cr(Vl), and trivalent chromium, er.3+ or Cr(lll), are present in the films. The trivalent chromium is believed to be present as an insoluble hydrated oxide, whereas the hexavalent chromium imparts a "self-healing" character to the film during oxidative (corrosive) attack by species such as chloride

(b)

-1 .,1 Ie)

Fig 13

I 201

(d)

Alloy 7075-T6 aircrah wing panel (0) showing unusual surface appearance. (b) Crocked anodized coat• ing on the panel surface. Scanning electron microscopy. 16Ox. lc) Anodized coating flaking away and corrosion deposit under the coating. Scanning electron microscopy. 85x (d) Cross section of corrosion site on panel showing depth 01 intergranular allack. 265x


ion. The hexavalentchromium is reduced during corrosion to form an insolubletrivalentchromiumspecies that terminates the oxidativeattack. Applications. Chromate conversion coating treatments are used on five principal types of aluminum parts: aircraft and aerospace structural components, coil (for construction applications such as guttering and siding), extrusions (for window and door frames), heat exchangerparts, and containers(mainlybeverage cans). A considerable amount of aluminum is also used in the automotive industry, but most receives a crystallinephosphate treatmentbecause the aluminum is treated at the same time as the steel frame. Types of Chromate Treatments. The four types in use are alkaline oxide, chromium phosphate, chromate, and no-rinse. Alkaline Oxide. The alkaline chromating process was the first chemicaltreatmentfor aluminum, and it is still used for some appliances and for military equipment. The coatings are applied by immersionin alkali chromate/carbonate baths of pH 10 to 11 for up to 20 min at temperatures approaching95°C (200 "F). Typical coating weights are between 100 and 500 mglft2, with colors ranging from light to brownishgreen. Chromium phosphate were first introducedin 1945. The coatings are used as a paint base for architectural extrusionsin doors, windows,and other exteriorapplications. Becausethe coatings do not contain cr6+, they are widely used for aluminumcan end stock and rigid aluminum food containers made from prepainted coil sheet. The coatings are applied by spray or immersion from processing baths that contain chromic acid (H2Cr04), phosphoric acid (H3P04), and fluoride ion (F"), and that usually have a pH less than 2. The coating weightsrange from 5 to 500 mg/ft2, and the colors range fromcolorless to emeraldgreen. Paintbase coatings are applied for 5 to 60 s at 25 to 50°C (80 to 120 OF), depending on the coating weight required. Decorative coatings require dwell times of 1 to 3 min and temperatures of 40 to 60 "C (l00 to 140 OF). The can stock coatings are usually applied in the 5 to 15 mg/ft2 range, are colorless,and provide excellent lacquer adhesion. In architectural applications, coating weights from 15 to 100 mg/ft2 form an excellent base for paint. The higher coating weights, up to 500 mg/ft2, have good bare corrosion resistance and are also suitable for decorative applications. Recent work on the composition of chromium phosphate films has shownthat they consistprimarilyof hydratedchromium phosphate (CrP04), Cr203, and aluminum oxides. Chromate coatings were first introducedin the early 1950s and are now widely accepted by the alurninumfinishing industry for such applications as domestic appliances, small parts, aircraft and electronic equipment, and continuouscoil coating of architectural aluminum. The films provide excellent paint adhesion and superior painted and unpainted corrosion resis-

tance. The low contact resistance of bare films is useful in spot welding. The processing baths contain H2Cr04, HF, other mineral acids, and accelerators; they are typically run between 6 and 30 points. The original acceleratorwas Fe(CN)~-. Other accelerators, such as molybdate (MoO~-, have recentlybecome more acceptedbecause theyeliminate theproblem of Fe(CN)t waste treatment and disposal. Coating weights range from 15 to 200 mg/ft2, with colors ranging from iridescent yellow to brown. For most paint base applications, coating weightsare from 15 to 30 mglft2. The coatingsfrom the Fe(CN)taccelerated process were characterizedby x-ray photoelectron spectroscopy (XPS) and reported to consist of microcrystallites of hydrated chromium oxides covered with an adsorbed monolayer of the accelerator. The MoOi--accelerated coatings have similar compositions, with the accelerator uniformly distributed through the film. Paint base coatings are applied within 5 to 60 s at 25 to 60°C (80 to 140 "F), Longer times can be required for bare corrosion coatings applied by immersion. No-rinse processes are finding increasinguse in the coil coatingof aluminum. In terms of corrosionprotection and adhesion, these processes can often provide qualityequaling that of conventionalprocesses. The applied compositions contain Cr6+ and cr3+ as well as other ingredients, such as P- or pot. Some fonnulations include organic compounds. For mostpaint base applications, the coating weights range from 5 to 25 mg/ft2. Because the process does not include rinsing after the treatment, the coating weights are directly proportional to the thickness of the applied wet film and the solidscontent of the coating solution. Specifications. The major specifications that cover the performance of chromate conversion coatings are listed in Ref 7. The type of specifications used will depend on the end use of the fabricatedpart, which in turn will dictate the properties of the coating being sought. For example, in order to be used on military aircraft, aluminum alloy parts (such as those made from highly corrosivecopper-containing aluminumalloy 2024-T3 or 7075-T6) must pass governmentspecifications MIL-C-5541 and MlL-C-81706, which require that the unpaintedchromated alloy must survive 336 h of salt fog testing (ASTM B 117). In addition, various tests are used to ascertain paint adhesion and underpaint corrosion under salt fog conditions. Aerospace companies use specifications similar to those used by the government. A boiling water test is often used in the container industry to detect the effectiveness ofthe chromatetreatmentin preventingdiscoloration caused by underpaintcorrosion.

A1tematives to Chromate Conversion Coatings The use and disposal of chromium and chromium compounds have received much regulatory attention


because of the toxicity of chromium and indications that it is a cancer-causing agent. Hexavalent chromium compounds appear to pose the greatestthreat. Inhalation of such compounds can produce tumors of the lungs and nasal cavity (Ref 8). As a result, many organizations have begun to apply considerable effort towardreducing or eliminating their use of chromium in metalfinishing operations. Alternative technologies that have receivedconsiderable attention in the open literature and/or have reachedthe trial stagesin variousaluminum industries includeorganic-based conversion coatings, multivalent metals conversion coatings (rare earth, manganesebase, and trivalent cobalt), and lithium-inhibited hydrotalcite conversion coatings. Each of these will be briefly reviewed in succeeding paragraphs. Table 6 lists a number of experimental and developmental technologies that can lead to breakthroughs with respect to replacement of chromium in conversion coatings in someapplications. Organic-Based Coatings. Given that a large number of water-soluble organic corrosion inhibitors are known to exist (Ref 9-12), conversion coatings based on organic molecules are logical alternatives to

chromium. The difficulty in making organic-based conversion coatings of sufficient thickness is that organic species such as chromic acid are normally poor oxidizing agents. This prevents film thickening because of aluminumoxidation and formation of insoluble oxide and hydroxide species. Typical inhibitorformed films have thicknesses of only 100 Aor less, making their use in severely corrosive environments impractical. In addition, the timerequired to form such films can be hours or more (Ref 12) unless it is possible to accelerate their deposition through use of surfaceactivators such as fluoride ion. Aqueous solubility can alsobe a limitation for some molecules. Even though these films can be thin, they have application in areas such as the treatmentof architectural aluminum (since this material is not usually continuously exposedto corrosive environments). In addition, organic-based conversion coatings have the potential of being excellent undercoats for organic (paint) finishes, for adhesion can be expected to be strong between similar types of molecules. Both sets of molecules containvarious activefunctional groupsthat can interact (e.g., through hydrogen bonding or possibly formation of cross-linked or intertwined structures). It

Table6 Altemative conversion coating technologies Process description

Trivalentchromiumconversioncoatings

Status

Meets no corrosionin 500 h requirement(ASTMB 117 salt spray test) Still containschromium Electrolyticprocess Hydrated aluminacoating Poor paint adhesion Meets no corrosionin 500 h requirement(ASTMB 117 salt spray test) Hydrated metalsaltcoating (Mg, Ni, Mn, Sn, Ti, Does not meet salt sprayrequirement Fe, Ba, Cu, Co, Cal Poor adhesion Peroxideoxidantcoating Does not meet salt sprayrequirement Poor adhesion Unstablechemicalbaths Oxyanion analogs(molybdates,tungstates, Moderatecorrosionresistance vanadates,and perrnanganates) Poorpaint adhesion Molybdateswith borate seembest Expensive Moderatecorrosionprotection(168 h) Potassiumpermanganatecoating Poor wet tape adhesion Does not work well on 2024or 7075 Requiresboilingdeionizedwater Multistep process,expensive Rare earth metalsalts (cerium) Corrosionprotectionclose to that of chromium Goodpaint adhesion Unstablechemicalbath Expensive Has good futurepotential Zirconiumoxide/yttriumoxide in aqueouspolymeric Goodpaint adhesion solution Moderatesalt sprayprotection(100 h) Commerciallyused for> I0 yr One step Expensive Silanes or titanates Good adhesion Moderatecorrosionresistance Containflammablesolvents Thicknessdependent,must be cured Difficultto disposeof lithium-inhibited hydrotalcitecoatings Good corrosionprotectionon 1000-,3000-, and 6000-seriesalloys Poor wet paint adhesion Single processbath Environmentallybenign Verypromising

210

I


is likely that organic-based treatments will find some application in replacing chromium-base systems, but great difficulties exist in attempting to produce treatments that can pass the rigors of 168 and 336 h exposure to salt spray, as required by MIL-C-5541 and MIL-C-81706 on active aluminum alloys such as 7075-T6 and 2024-TI. Rare Earth Metals. The most logical method for obtaining a chromium replacement is to choose another transition, or even a rare earth metal, that has at least two stable oxidation states, is a good oxidizing agent, and has high corrosion resistance. Treatments based on Ce(lll) and other rare earth metals were examined first in Australia (Ref 13-15) and later in the United States (Ref 16-18). Coatings in excess of 1000 A in thickness and rich in cerium + oxygen species were formed on aluminum alloy 7075 after a 20 day exposure to a 100 ppm CeCl3 solution at pH 5.8 (Ref 19). X-ray photoelectron spectroscopy indicated that the film contained both Ce(IV) and Ce(lII) species, which likely existed as Ce02, Ce(OH)4, and Ce(OH)3 (Ref 19). X-ray absorption near edge structure (XANES) studies likewise indicated the presence of a mixed cerium valence film (Ref 20). Coating process time was decreased to 10 min by adding hydrogen peroxide, lowering pH, and increasing the solution temperature (Ref 21). Immersion of the film in NaCI solution converted all ofthe Ce(l!) to Ce(IV) (Ref 19). Measured corrosion rates of treated 7075 indicated that a 50% reduction in corrosion rate from that of an untreated substrate can be obtained (Ref 21). No mention of its effect on pitting corrosion was made, but excellent paint adhesion (comparable to that on chromated surfaces) was observed. The development of "stainless aluminum" has also been claimed for cerium-treated pure aluminum and aluminum alloy 6061-T6 (less satisfactory behavior was obtained for aluminum alloy 2024-T3) (Ref 22). The treatment involves a 2 h exposure to three separate solutions: boiling 10 m M Ce(N03h, boiling 5 m M CeCI3, and anodic polarization in the passive region in deaerated 0.1 M Na2Mo04' Excellent corrosion resistance was found upon immersion of treated samples in 0.5 N NaCI. Scratched surfaces also showed excellent resistance. No mention of salt spray testing of the cerium-base treatments was made, however. Ce(III) molybdate has shown some promise as a corrosion inhibitor in an epoxy/polyamide primer but still does not match the performance of strontium chromate pigmented primers (Ref 23). Manganese-base treatments for aluminum and aluminum alloys have recently been patented (Ref24--26). One of the treatment steps involves exposure of the aluminum alloy surface to permanganate ion, which contains manganese in the +7 oxidation state. Like chromate, the permanganate ion is an excellent oxidizing agent, suggesting that the mechanism of film formation is similar to that of chromate. Although no information on film thickness or composition is given in the patents, one would expect that the manga-

nese found in the film is in some reduced oxidation state (probably either +4 or, more likely, +2). This is a multistep treatment in which many of the steps require elevated temperatures. The last step, which involves a seal with alkali metal silicate, is probably necessary to block the pores created in the film during deposition. Good corrosion resistance, as evidenced from salt spray exposure, has been observed for high-coppercontaining aluminum alloys. Trivalent Cobalt. The final system is based on the use of basic solutions containing complexes of trivalent cobalt, for example, Co(NH3)~+ (Ref 27). CoCl2 has shown some promise as an inhibitor for aluminum alloy corrosion (Ref 28). It is likely that Co(ll) compounds have been examined in the presence of fluoride, for CoF 2 possesses appreciable solubility in water. This new system deposits a corrosion-resistant cobalt-containing film on aluminum alloys. Preliminary examination of this coating with electrochemical impedance spectroscopy (EIS) indicates that the coating has corrosion-resistant properties similar to those of a chromate treatment on aluminum alloy 2024-TI (Ref 29). Good corrosion and paint adhesion properties are also claimed (Ref 27).

Organic Coatings Aluminum is an excellent substrate for organic coatings if the surface is properly cleaned and prepared. For many applications, such as indoor decorative parts, the coating can be applied directly to a clean surface. However, a suitable prime coat, such as a wash primer or a zinc chromate primer, usually improves the performance of the finish coat. For applications involving outdoor exposure, a surface treatment such as anodizing or chemical conversion coating is required prior to the application of a primer and a finish (top) coat, such as an epoxy or polyurethane. Some new one-step, self-priming polyurethane top coats are also available as are low volatileorganic compound (VOC) high-performance primers (e.g., epoxy polyamide). The final coating often is tailored to the application. Examples are high gloss on auto bodies and selfcleaning paint on residential siding. Optimum procedures for both surface preparation and painting of aluminum often differ from those for steel, particularly for electrostatic painting. Compromises have to be made when painting a multimetal (material) product, e.g., an auto body, and designers can be limited to using existent paint line conditions. Maximum protection depends on maintaining an unbroken paint envelope, and repairs should be made when needed. This depends greatly on the application and life expectancy desired. For example, painted jet airliners are stripped of their coating and completely repainted on a regular basis. Automobiles are repainted as needed, usually for appearance purposes. Dents and scratches in residential siding are rarely


even repaired, whereas rain-carrying systems (gutters and down spouts) often are less expensive to replace than to repair and repaint. Antifouling paints to prevent growth of algae, barnacles, and other sea organisms must be tailored to use on aluminum. The common antifouling paints for steels are not suited for use with aluminum because they contain leachable heavy metals, such as arsenic, copper, and lead, that can plate out on the aluminum and cause severe local corrosion. In certain applications, the finishing coat can be replaced by adhesively bonded applique films. These flexible films provide a durable, weather-resistant finish when applied over standard, corrosion-resistant primers. Material in coil form can be coated very economically. Here, the strip is first pretreated, rinsed, and dried, and then the paint is applied and baked in one continuous process. Strip speeds are usually in the range of 60 to 150 m/min (200 to 500 ft/min). As a rule a two-coat system is used, consisting of the primer (about 5 lim) and finishing (about 20 lim) coats. The back side of the strip is usually given a protective coating. Often, however, both sides are simultaneously given the same coating treatment. In the same installation it is possible to apply a laminating glue and to laminate a plastic film onto the strip surface. The coated strip can be decoratively embossed and shaped (for example, roll-formed into corrugated sheet). For this reason, not only must the coating be able to withstand such forming, but the aluminum itself must have very good formability. In coil coating, the paint system is selected according to the requirements of the product. Alkyd, acrylic, vinyl, epoxy, epoxy-phenolic, polyester, silicone polyester, polyvinyl fluoride, and polyvinylidene fluoride type finishes are commonly used. Clear protective coatings (lacquers) are used to provide protection while retaining a glossy metallic appearance. All beverage and food containers are coated for prolonged shelf life and to prevent contamination of the food product. A hole-free coating is required. These coatings can be color tinted to identify that the

Table 7 Melted oxide compositions of frits for enamels for aluminum CoostitueDl

PbO Si0 2 Na20 K20 U 20 B203 A1203 BaO P20 S F2 Ti0 2 Sbz°s

Lead-base enamel

Compositioowt% Noo-Iead-base enamel

14-45 30-40 14-20 7-12 2-4 1-2

30-40 20-25 7-11 3-5 1-2

2-{) 2-4

3-5 2-4

15-20

15-20 2-5

Barium enamel

25 20 25 15 3 12

metal has indeed been coated or to color code the type of coating applied. Clear coatings are employed in the protection of anodized aluminum surfaces on commercial and residential buildings. Clear lacquers also facilitate cleaning procedures. Other notable examples of organic coatings for aluminum are polytetrafiuorethylene (PfFE, or Teflon) used on cooking utensils and pressure-sensitive tapes and/or strippable plastic coatings for temporary protection of aluminum sheets or extrusions used in buildings. Once construction is completed, these temporary protective coatings should be removed because time, heat, and sunlight harden and degrade them and make them increasingly difficult to remove.

Porcelain Enameling Porcelain enamels are glass coatings applied to products to improve appearance and protect the metal surface. Porcelain enamels are distinguished from other ceramic coatings by their predominantly vitreous nature and the types of applications for which they are used. They are distinguished from paint by their inorganic composition and the fusion of the coating matrix to the substrate metal. Aluminum products, including tanks and vessels, architectural panels, cookware, and signs, can be finished by porcelain enameling to enhance appearance, chemical resistance, or weather resistance. The common porcelain enameling alloys for the various forms of aluminum are the following:

• Sheet: 1100,3003, and 6061 • Extrusions: 6061 • Castingalloys: 443 and 356 The basic material of the porcelain enamel coating is frit, a special glass of small friable particles produced by quenching a molten glassy mixture. Because porcelain enamels are usually designed for specific applications, the compositions of the frits from which they are made vary widely. Table 7 gives the compositions of several frits used for aluminum The high-lead enamels for aluminum have a high gloss, good acid and weather resistance, and good mechanical properties. The phosphate enamels generally are not alkaliresistant or water-resistant, but they can have good acid resistance. They melt at relatively low temperatures and are useful in many applications. The barium enamels are not as low-melting as the lead or phosphate glasses, but they do have good chemical durability.

Plating Electroplating. Electroplated coatings are applied to aluminum alloys to obtain a specific metallic appearance; increased resistance to wear, abrasion or erosion; increased electrical conductivity; improved solderability; or improved frictional properties. Although electroplated metal coatings are occasionally

212


used to provide resistance to corrosion, other finishes such as anodic coatings provide higher resistance to corrosion in most atmospheric environments, and therefore they are much more widely used for this purpose. Examples of applications of plated aluminum are given in Table 8. Aluminum-base materials are more difficult to electroplate than the common heavier metals because aluminum has a high affinity for oxygen, which results in

a rapidly formed, impervious oxide film, and because most metals used in electroplating are cathodic to aluminum, so that voids in the coating lead to localized galvanic corrosion. Electroless Plating. For a variety of applications in the aircraft/aerospace industry and the electronics industry, nickel is chemically plated on aluminum parts of shapes for which electroplating is not practical. However, electroless plating is too expensive

Table 8 Applications using electroplated coatings on aluminum products Preplaling Product

Form

Automotive applications Bearings Sheet Castings Bumperguards

treatment

None Buff and zincate

Shell

Extrusion

0.25-1.25 0.1+2+0.03

Cu+Ni+Cr

0.8+20+ 1.3

0.03 + 0.8 + 0.05

HardCr

51

2

Machineand zincate

Cu flash s-Cu +hardCr

2.5 +25 +76

0.1 + 1+3

Wear resistance

Conductive rubbercoating Doublezincate

Ni

203

8

Cu flash + Cd(a)

8-13 (a)

0.3-0.5(a)

Resistanceto corrosion and erosion Dissimilar-metal protection

Cu flash + Cu + Ag(b) 8 + 5(b)

IntermediateDie castings Zincate frequencyhousings

Cu flash + Cu + Ag + Au(c)

13+ 13+0.6(c)

Die castings Zincate

Cuflash + Cu+ Ag+

0.25 + 13 +0.5

Rh

Terminalplates

Sheet

Zincate

Cuflash

General hardware Screws;nuts; bolts

Castings

Buff and zincate

Cd (on threads)

Buff and zincate

Die cast sprayguns and compressors Diecast windowand door hardware Household appliances Coffeemaker Sheet

Reasonfor plating

6-32 2.5+51 +0.8

Electrical and electronics applications Busbars;switchgears Extrusions Zincate

Microwavefittings

mils

jUD

Pb-Sn-Cualloy Cu+Ni+Cr

Lampbrackets; Diecastings Buffandzincate seeong-colomnceps Tire molds Castings None Aireraft applications Hydraulic parts; Forgings landinggears; small enginepistons Forgings Propellers

Thickness

Electroplating system

Prevent seizing Appearance;corrosion resistance Appearance;corrosion resistance Appearance;corrosion resistance

0.3 + 0.2(b)

Nonoxidizedsurface; solderability; corrosionresistance 0.5 + 0.5 + 0.025(c) Surfaceconductivity; solderability; corrosion resistance 0.01 + 0.5 + 0.02 Smooth,nonoxidized interior; corrosion resistanceofexterior Nonoxidizedsurface; solderability; corrosionresistance 0.5; 0.2 on tbreads Corrosionresistance

HardCr

13;0.5 on threads 51

2

Appearance

Barrel burnish andzincate

Brass(d)

8(d)

0.3(d)

Appearance;lowcost

Buff and zincate

Cr

5

0.2

Cu+Ni+Cr

2.5+ 13+0.8

0.1 + 0.5 + 0.03

Appearance;cleanness; resistanceto food contamination Appearance;cleanness; resistanceto food contamination

Refrigeratorhandles; Die Buff and zincate salad makers;cream castings dispensers Personal products Compacts;fountain pens Hearingaids

Sheet

Buffandzincate

Cu flash + brass

5

0.2

Appearance;lowcost

Sheet

Zincate

Cu flash + Ni + Rh

19 +0.25

0.75 +0.01

Jewelry

Sheet

Buff and zincate

Brass+Au

8+0.25

0.3+0.01

Nonoxidizingsurface; low cost Appearance;low cost

(a)Chromatecoating appliedafter cadmiumplating.(b)Solderingoperationfollowssilverplating.(c) Bakedat 200 °C (400 "F) after copperplating and after silverplating.Solderingoperationfollowsgoldplating. (d) Brassplatedin barrelor automaticequipment

Corrosion Prevention Methods I 213

to be used when conventional electroplating is feasible. As with electroplating, flaws or pores in an electroless coating can lead to galvanic corrosion of the less-noblealuminumsubstrate. Example 21 Corrosion of a Plaled Aluminum Antenna Marker Beacon. A corroded antenna marker beacon (Fig. 14) was removed from an aircraft so that the source of corrosion could be determined. The beacon was x-rayed, and corrosion sites were visible in the radiograph (Fig. 14b). The antenna was

opened and found to contain a polyamide foam (Fig. 14c). The antenna blade was removed from the foam (Fig. 14d) and observed to be corroded also. Both the housing and blade were found to be aluminum. The housing was found to be plated with electroless nickel (Fig. 14e), and the blade was plated with a copper strike followedwith a tin plate (Fig. 14t). It is believed thatboth the nickelplate and the copper/ tin plate wereporous.This permitted the penetrationof moisture to the plating and aluminum interface. The

(a)

(b)

(e)

(d)

(e)

(I)

Fig. 14

Aircrah antenna marker beacon (a) that failed by corrosion. (b) X-ray radiograph of beacon shOWingareas of corrosion. (c) Polyamide foam inside the beacon. (d) Corroded aluminum antenna blade removed from housing. (e) X-ray map shOWing thickness of nickel plate (arrowl on aluminum beacon houslnq. (~X-ray map showing the thickness of copper/tin plate (arrows) on aluminum antenna blade.

214

I


presence of moisture and the anodic relationship of aluminum to nickel or tin resulted in a galvanic cell that caused pitting corrosion in the aluminum. These plating systems were used to maintain a high electrical surface conductance on the aluminum components. The corrosion resulted in nonconductive surfaces that affected the electrical performance of the antenna. It was recommended that, instead of plating the aluminum, a chromate conversion coating be used.

Inhibitors Inhibitors can be used to control corrosion of aluminum alloys. Chromates, silicates, polyphosphates, soluble oils, and others, are in common use. Inhibitors, however, must be used with care in order to achieve the desired result. Chromates are effective inhibitors if used in sufficiently high concentrations. However, if concentration is insufficient, corrosive attack may be intensified. In addition, chromates impart conductivity to the electrolyte, thereby enhancing galvanic effects and altering the solution potential relationships between cladding and core in alclad alloys. Research has also been focused on the need for inhibitors that will function adequately in recirculated systems made up of a variety of metals (and perhaps nonmetals as well). Typical systems of this type include jacket water cooler circuits, which may include cast iron parts, copper-bound gaskets, lead-tin or silver solder joints, and brass or aluminum heat exchanger tubes or both. The inhibitive system must provide protection for all the different metals and not stimulate galvanic action between them. Films that may be deposited must be sufficiently thin so heat transfer is not seriously reduced. Such proprietary water treatments involve combinations of several ingredients, for example, two or more of the following types of inhibitor ingredients: polyphosphates, nitrites, nitrates, borates, silicates, and mercapto-benzo-thiazole (MET). An additional complication in cooling tower systems is that pH of the circulating water must be maintained sufficiently close to the neutral range to avoid delignification of red wood. Bacteriacidal treatment given to cooling tower lumber is a possible cause of corrosion. Copper salts leached from tower lumber can plate out on ferrous or aluminum parts, thereby establishing galvanic cells. Mercury salts can cause serious pitting of aluminum or lead parts and stress corrosion of copper base alloys. Inhibitors have also been developed to inhibit aluminum corrosion in hydrochloric acid. Some of these are n-, di-n- ad tri-n-butylarnines. The mechanism of inhibition is based on reduction of anode current density by reduction of the cathode area as the result of adsorption on cathodic surfaces.

Corrosion Monitoring and Inspection Methods Corrosion monitoring has become an important aspect of the design and operation of modem industrial plants because it enables plant engineers and management personnel to be aware of the damage caused by corrosion and the rate of deterioration. A large variety of techniques are available for corrosion monitoring in plant corrosion tests, and much has been written on the subject in recent years (Ref 30-33). The most widely used and simplest method of corrosion monitoring involves the exposure and evaluation of the corrosion in actual test coupons. The ASTM standard G 4, "Standard Method for Conducting Corrosion Coupon Tests in Plant Equipment," was designed to provide guidance for this type of testing. Other nondestructive and/or electrochemical methods used for monitoring corrosion in the processing (chemical and refming) industries include the following: • • • • • • •

Electrical resistance probes Ultrasonic thickness measurements Polarization resistance measurements Measurement of corrosion potentials Alternating current impedance measurements Hydrogen probes Analysis of process streams, e.g., atomic absorption analysis for detecting heavy metals in a process stream

Supplementing these monitoring methods are nondestructive inspection techniques such as visual inspection (e.g., borescopes), eddy current inspection, radiography, infrared thermography, magnetic particle inspection, and liquid penetrant inspection. The instrumentation for a variety of corrosion monitoring techniques is presented in Table 9, and the characteristics of these techniques are identified in Table 10. Another critical area for corrosion monitoring and nondestructive inspection is in the aircraft/aerospace industry (Ref 34). The aging of both civil and military aircraft is becoming of increasing concern. As economic pressures force operators to continue flying aircraft well beyond their original design lives, the aviation industry and government agencies must develop technologies, methods, and procedures to ensure the continued airworthiness of these aircraft at a reasonable cost. Advanced nondestructive inspection and testing of material properties such as fatigue, corrosion, and cracking include thermography, magneto-optic imaging (MOl), and a mobile automated ultrasonic scanner (MADS). The flexibility, reliability, and sensitivity of each technology varies because of inherent limitations as well as the degree of development. The MADS and MOl have been extensively developed and currently constitute rugged and easy-to-operate systems. Thermography has been around for some time, but only recently has infrared camera perfor-

Corrosion Prevention Methods I 215

mance and computing power become both sensitive and affordable. A handheld thermographic system is being developed for the detection of corrosion in metallicstructures, and resultshave been very promising. Another technology under development is that of a meandering winding magnetometer (MWM) array. This is a new surface mountable eddy-current sensor array that can monitor early stage fatigue damage, crack initiation, and crack growth. The MWM is suit-

able for monitoring on-line fatigue tests of coupons and complex components, as well as difficult-toaccess locations on aircraft. The sensor is thin and lightweight. It can be surface-mounted like a strain gage but does not require an intimate mechanical bond. This capability permits the use of compliant adhesives, which improves durability. The MWM is a planer, conformable eddy current sensor designed to support quantitative and autono-

Table 9 Instrumentation for corrosion monitoring Method

Measures or detects

Notes

Use

Linearpolarization Corrosionrate is measuredby the (polarization resistance) electrochemical polarizationresistance methodwithtwo or threeelectrodeprobes Electricalresistance

Potentialmonitoring

Corrosioncoupontesting

Analytical

Analytical

Analytical

Radiography Ultrasonics

Eddy-current testing Infraredimaging (thermography) Acousticemission

Zero-resistance ammeter

Hydrogensensing

Sentinelholes(a)

Suitablefor mostengineeringalloys,if process Frequent fluidis of suitableconductivity. Portable instrumentsat modestcost to moreexpensive automaticunitsare available. Integratedmetalloss is measuredby the Suitablefor measurements in liquidor vapor phase Frequent resistancechangeof a corrodingmetal on mostengineeringmetalsand alloys.Probes, element.Corrosionratescan be calculated. as well as portableand moreexpensive multichannel units,areavailable. Potentialchangeof monitoredmetalor alloy Measuresdirectlystateof corrosionof plant Moderate (preferablyplant)withrespectto a reference (active,passive,pitting,stress-corrosion cracking)viause of a voltmeterand reference electrode electrode. Averagecorrosionrate overa knownexposure Most suitablewhencorrosionis a steadyrate. Frequent periodby weightloss or weightgain Indicatescorrosiontype.Moderatelycheap method,withcorrosioncouponsand spools readilymade. Concentration of thecorrodedmetalions or Can identifyspecificcorrodingequipment.Wide Moderate concentrationof inhibitor rangeof analyticaltools available.Specificion electrodesreadilyused. pH of processstream Commonlyusedin effluents,Standardequipment Frequent availablethroughrobustpH responsive electrodes,suchas antimony,platinum,or tungsten, can be preferableto glass electrodes. Solid AglAgClis usefulreferenceelectrode. Oxygenconcentration in processstream Usefulwhereoxygencontrolagainstcorrosion Moderate usingoxygenscavengerssuch as bisulfiteor dithioniteis necessary. Electrochemical measurement. Raws and cracksby penetrationofradius and Veryusefulfor detectingflawsin welds.Requires Frequent specializedknowledgeand carefulhandling. detectionon film Thicknessof metaland presenceof cracks, Widelyusedfor metalthicknessand crack Frequent pits, etc. by changesin responseto ultrasonic detection. Instrumentation is moderately waves expensive,but simplejobs can be contractedout at fairlylowcost. Uses a magneticprobeto scansurface Detectssurfacedefects,such as pits and cracks, Frequent withbasic instrumentation of only moderatecost. Spot surfacetemperatures or surface Used mosteffectivelyon refractoryand insulation Infrequent temperature patternas indicatorof physical furnacetubeinspection.Requiresspecialized state of object skill. Instrumentation is costly. Leaks,collapseof cavitation,bubbles,by A techniquecapableof detectingleaks,cavitation, Infrequent vibrationlevelin equipment.Cracks,by corrosionfatigne,pitting,and stress-corrosion detectionof the soundemittedduringtheir crackingin vesselsand lines. propagation Galvaniccurrentbetweendissimilarmetal Indicatepolarityand directionof bimetallic Infrequent electrodesin suitableelectrolyte corrosion.Usefulas dewpointdetectorof atmospheric corrosionor leak detectionbehind linings. Hydrogenprobeused to measurehydrogengas Used in mildsteel corrosioninvolvingsulfide, Frequentin liberatedby corrosion cyanide,and otherpoisonslikelyto cause petrochemical hydrogenembrittlement. industry Infrequent Indicateswhencorrosionallowancehas been Usefulin preventing catastrophicfailuredue to erosionat pipe bends,etc. Leakinghole indicates consumed corrosionallowancehas been consumed.

(a) A sentinelhole is a hole that is drilledfromthe outsideof a vesseland thatpartiallypenetratesthe vesselwall.As the corrosionattackof thevessel proceeds,the vesselwall thins. Whenthe corrosionallowancehas beenconsumed,a sentinelholethat is drilledto that depth willindicatethat condition by the leakageof fluidfrom it.


mous data interpretation methods. These methods, called grid measurement methods, permit crack detection on curved surfaces without the need for crack standards. They provide quantitative images of absolute electrical properties (conductivity and permeability) and coating thickness without requiring field reference standards because, in effect, electrical properties are calibrated in air. As an example, significant changes in conductivity have been observed in aluminum alloy 2024 as a func-

tion of percentage of fatigue life depleted. Experiments ended when the first observable crack developed in the coupon. For Al 2024, the MWM begins to detect significant reductions in conductivity after about 60% offatigue life has been depleted. New instrumentation and software permit rapid scanning of these specimens at speeds over 75 mm/s (3 in.ls) for aluminum fatigue monitoring. Speed limits depend on the minimum flaw-size detection required. This rapid scanning capability should provide substan-

Table 10 Characteristics of cOlTOsion monitoring techniques Tecbniques

'I1mefor individual

Typeof

Speed of respooses to

measurement

information

0.1~.5oo

>0.500-1.500

>1.500

Drawntube

g).loo >O.l~.5OO

7049-T73,T7352 70SO-T74(b), T7452(b) 7075-T73,T7352 7175-T74(b),T7452(b), T7454(b)

Forgings

All

measurements(a)

Surfaceof tensile sample

Surfaceof tensile sample Subsurfaceafter removingapproximately 10%of thicknessof tensile sample Surfaceof tensile sample Surfaceoftension sample(d) Subsurfaceafter removingapproximately 10%of thickness of tensile sample Subsurfaceof approximatelycenter of thickness on a plane parallelto the longitudinalcenter line of tbe material Subsurfaceon test coupon surfacethat is closestto the center of the thicknessand on a planeparallelto theextrusionsurface. Surfaceoftensile sample(d) Subsurfaceafter removingapproximately 10%of thickness of tensile sample Surfaceoftensile sample

(a)For curvedsurfaces,the conductivityis measuredon amachinedflat spot.(b)T74typetempers,althoughnot previouslyregistered,have appeared in the literatureand in some specificationsas T736 tempers. (c) Also applies to alclad sheetand plate; however, the cladding must be removed and the electrical conductivitydetennined on tbe core material. (d) For smaller sizes of tube, a cut-out portion is flattened and tbe surface measured. Source: Tbe AluminumAssociationInc.


pitting unless uniform corrosion is slight and pitting is fairly severe. If there is significant uniform corrosion, the contribution of pitting to total mass loss is small. Mass loss should not be ignored in every case, however. For example, measurement of mass loss, along with visual comparison of pitted surfaces can be sufficient to rank the relative resistances of alloys in laboratory tests. Pit depth measurement is generally a better indicator of the extent of pitting than mass loss. Pit depth measurement can be accomplished by several methods, including metallographic examination, machining, use of a micrometer or depth gage, and the microscopical method. In the microscopical method, a metallurgical microscope is focused on the lip of the pit and then on the bottom of the pit. The difference between the initial and final readings on the finefocusing knob of the microscope is the pit depth. Evaluation of Pitting. Pitting can be described in several ways. ASTM G 46 includes procedures for the use of standard charts, metal penetration, statistical analysis, and loss in mechanical properties to quantify the severity of pitting damage. More than one of these methods can be used. In fact, it is often found that no one method is sufficient by itself. Because pitting tends to be nonuniform, the pitting factor (the ratio of maximum to average penetration) described in ASTM G 46 is not useful for aluminum.

Tests for Intergranular Corrosion

Corrosion Resistance of Heat Treatable Aluminum Alloys by Immersion in Sodium Chloride + Hydrogen Peroxide Solution," is a method that was developed for testing 2xxx and 7xxx alloys, although it can be used for other aluminum alloys (e.g., 6xxx alloys), including castings. This practice consists of immersing etched specimens in a NaCI + H 20 2 solution for a period of at least 6 h. Longer exposure times of 24 h or more can be used for more corrosion-resistant alloys/tempers. Shorter exposure times (less than 6 h) can be used for very thin sheet. After immersion, metallographic sections are examined at magnifications of 100 to 500x. The allowable extent and depth of intergranular corrosion are in accordance with criteria established between producer and producer. Other standardized procedures for determining the susceptibility of high-strength aluminum alloys to intergranular corrosion include U.S. Federal Test Method, Standard No. 151b, Method 822.1, "Intergranular Corrosion Test for Aluminum Alloys," and U.S. Military Specification MIL-H-6088, "Heat Treatment of Aluminum Alloys." The latter specification covers tests that are required for periodic monitoring of 2xxx and 7xxx series alloys in all rivets and fastener components as well as sheet, bar, rod, wire, and shapes under 6.4 mm (0.25 in.) thick. Strain-Hardened 5xxx Alloys. Alloys in this series that contain more than about 3% Mg are rendered susceptible to intergranular attack (sensitized) by certain manufacturing conditions or after being

As described in Chapter 4, susceptibility to intergranular corrosion depends primarily on the type of alloy and fabrication process and can occur in most environments. Non-heat-treatable lxxx, 3xxx, and 5xxx alloys containing less than 3% Mg are not susceptible to intergranular attack. Heat-Treated High-Strength Alloys. ASTM G 110, "Standard Practice for Evaluating Intergranular

A Density

D D 0

2.5 x 103 /m'

2

4

1 x 10 /m'

(bl

(a)

.'. :

3

..

.:

0. '

5 x 104 /m'

(d)

(e)

4

[J] ........ 0°

0

0

.0'

°

°

0

,

5

1 x 10 /m' (Horizontal) (f)

Fig 1

(Vertical)

Variations inthecross-sectional shapeofpits. (al • Narrow and deep. (b) Elliptical (cl Wide and shallow. (d) Subsurface. (e) Undercutting. (n Shapes determined bymicrostructural orientation. Source: ASTM G 46

5

Depth

0.5 mrn"

O.4mm

• 2.0 rnrn"

•

8.0mm'

•

12.5 rnrn?

II e ::~:~:::::.:::.,

5 x 105 /m'

Fig. 2

C

B Size

24.5 mrn"

•

0.8mm

1.6mm

3.2mm

6.4mm

Standard rating chartfor pits. Source: ASTMG 46

Corrosion Testing /225

subjected to elevated temperatures up to about 175°C (350 OF). This is the result of the continuous grainboundary precipitation of the highly anodic Mg 2Al 3 phase, which corrodes preferentially in most corrosive environments. The ASTM standard G 67, ''Test Method for Determining the Susceptibility to Intergranular Corrosion of 5xxx Series Aluminum Alloys by Mass Loss after Exposure to Nitric Acid (NAMLT Test)," is a method that provides a quantitative measure of the susceptibility to intergranular attack of these alloys. This method consists of immersing test specimens in concentrated HN03 at 30°C (85 "F) for 24 h and determining the mass loss per unit area as the measure of intergranular susceptibility. When this second phase is precipitated in a relatively continuous network along grain boundaries, the preferential attack of the network causes whole grains to drop out of the specimens. Such dropping out causes relatively large mass losses of the order of 25 to 75 mg/cm2 , although specimens of intergranularresistant materials lose only about I to 15 mg/cm2. Intermediate mass losses occur when the precipitate is randomly distributed. The parallel relationship between the susceptibility to intergranular attack and to SCC and exfoliation of these particular alloys makes ASTM G 67 a useful screening test for alloy and process development studies. A problem arises, however, in selecting a pass-or-fail value in relation to the performance of intermediate materials in environments other than HN03. Other T.... for Aluminum Alloys. The volume of hydrogen evolved upon immersion of etched 2xxx series aluminum alloys in a solution containing 3% sodium chloride (NaO) and 1% hydrochloric acid (HCl) for a stipulated time has been used as a quantitative measure ofthe severity ofintergranular attack. A problem with this approach (which is quite valid) was that the correlation between the amount (or the rate) of hydrogen evolved is influenced by a number of factors, including alloy composition, temper, and grain size (Ref 2, 3). Applied current or potential in neutral chloride solutions (for example, 0.1 N NaCl) provides another direct method of assessing the degree of susceptibility to intergranular attack when accompanied by a microscopic examination of metallographic sections (Ref 2, 4, 5). More sophisticated electrochemical approaches for studying systems involving active-path corrosion use potentiodynamic methods.

Tests for Filiform Corrosion Filiform corrosion is a special type of corrosion that occurs under coatings (usually organic) on metal substrates (usually aluminum, magnesium, or steel) and is characterized by a threadlike or ''worm-track'' morphology and directional growth. Filiform corrosion normally occurs between 20 and 35°C (70 and 95 OF) with a corresponding humidity range of 60 to 95%;

above 95% humidity, blistering (scab corrosion) rather than filiform corrosion can occur. Laboratory T.sts. ASTM D 2803, "Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal," describes three procedures for determining the susceptibility of organically coated metal substrates to the formation of filiform corrosion. In procedure A, scribed panels are subjected to a preliminary exposure in a salt spray cabinet (per ASTM B 117 for salt spray testing as mentioned above) to initiate corrosion, rinsed, and placed in a humidity cabinet that operates at 25 ± 2 "C (77 ± 3 OF) and 85% relative humidity. In procedure B, which is based on ISO 4623, "Paints and Varnishes-Filiform Corrosion on Steel," scribed panels are either exposed to salt spray or dipped in a salt solution but not rinsed prior to being placed in the humidity cabinet. In procedure C, scribed specimens are exposed as in procedure A except the humidity cabinet is operated at 40 ± 2 "C (l05 ± 3 "F). Depending on the test method selected, test periods range from as little as 4 h to as much as 6 weeks (refer to ASTM D 2803 for details). Although a standard method of rating failure due to filiform corrosion is not available, the traditional method for quantifying filiform corrosion damage has been to measure the maximum length or to count the number of filiform sites. Photographs of filiform corrosion are preferred for recording test results.

T.s" for Firlform Corrosion in the Automotiv. Industry. Because of the current interest in aluminum for automotive body sheet, together with the need to maintain an aesthetically pleasing painted surface, increased attention has been given to test procedures that compare the corrosion performance of painted aluminum sheet as determined from various laboratory methods and in-service (e.g., seacoast) exposure. One study (Ref 6) compared the results of a variety of filiform corrosion tests (outdoor exposure, in-service exposure, and laboratory exposure) that were carried out on 0.9 mm (0.035 in.) thick sheet specimens of alloys 2008-T4, 2036-T4, and 6111-T4 and 0.5 mm (0.02 in.) thick sheet of alloy 5182-0. Coatings systems applied to these alloys consisted of a zinc phosphate layer, cathodic electrocoat, a primer/surface coat, and a top coat Two phosphate coatings weights were employed: unmodified "low phos" coatings were roughly 80 mg/ft 2 (0.09 mg/cm2) whereas modified (by fluoride additions) "high phos" coatings were on the order of 190 mg/ft 2 (0.2 mg/cm2). Details on the test procedures employed during the study follow (Ref 6).

• Outdoor exposure-Alcoa Technical Center (ATC) with saltspray: The Alcoa Technical Center is located approximately 20 miles from Pittsburgh, PA. The ambient environment near ATC is considered a ruralI industrial environment The setting is mral,but the typical rainwas a pH of =4.Sulfates, nitrates,and traces of chloride are also detected. Three times per week, the specimens were subjected to a 5% NaCI solution applied with a hand spray bottle. Panel evaluation


16

ATC with 3 times/week salt spray 22 months

160

14

140

12

120

10

100

~

2

ro

5

o

0

o

.-

60

0

0.6

~ 0

0.4

30 -,

c-

Z-

i-

~I"

27 31 3.7

i-

5.8

25 If---J----j,~bc....----j---+---+-----+--+----i 84

'~

7.6

20

~

10.4

15

Vpl = 3.3 x 10-

V-

10

114

mls

I

10

I---+----+----+---+---t---I------l O~

o

__

. . l __ __ ' __ _~_ __ ' __ ____'

400

800

1200

1600

2000

~

2400

_

~ >Z'iii ~

.~

,/

l~,;>

46

64

30

U

(1.0)(2.0) (1.5)(1.5) (2.0)(1.5) (1.5)-

166

15.1

264

24.0

51

46.4

.~ '"

'" E ci.i

____'

2800

Exposure time. h

Fig 23

Crack depth and stress intensity versus time curves for double-beam specimens of aluminum alloys 7075· • T651, 7079-T651, and 7075·T7351 having nearly identical deflections and starting crack depths. Specimenswith S-L orientation (seeFig. 111measuring 25 x 25 x 127 mm (1 x 1 x 5 in.) bolt loaded to pop-in and wetted three times daily with 3.5% sodium chloride. Vpl' plateau velocity. Source: Ref 44


higher resistance to SCC becomes complicated when only small amounts of crack growth occur in normal exposure periods. In such cases, the initial penetration of SCC, even at near-critical stress intensities, can be delayed by an initiation (incubation) period and then can begin at small independent sites along an uneven mechanical crack front The crack measurements are erratic, and the interpretation of the crack growth curves is subjective. Comparisons among relatively resistant materials are difficult. Figure 24 shows a number of crack growth curves for several resistant materials. It is evident that the estimated plateau velocities are quite variable and do not correlate consistently with the total crack growth in a given exposure time. For these more SCC-resistant materials, average growth rates for the first 15 days of exposure appear to relate much better to the actual amount of crack growth and to smooth specimen ratings according to ASTM G 64 (Table 6). The performance of alloy 2 in Table 6 indicates another potential problem with tests performed at

very high stress intensities: that is, with some very resistant materials, environmental crack growth will possibly be the result of mechanical fracture rather than SCc. Therefore, it is necessary when testing SCC-resistantmaterials to verify thatthe crack growth is in fact SCC. The determination of threshold stress intensities by the arrest method is frequently complicated by corrosion product wedging, which changes the stress state at the tip of the crack and invalidates the calculation of effective stress intensities from the crack lengths. With low-resistance alloys, such as 7075-T651, an arrest can never occur, because the crack is continually driven ahead by the advancing wedge of insoluble corrosion products (Ref 45,48). An indication of this was shown by the initiation of SCC in precracked specimens exposed with no applied load for just a few months in a seacoast atmosphere (Ref 45). Experimental evidence of a thresholdstress intensity will depend on the amount of corrosion occurring in a given alloy/environment system (Fig. 25).

Table 6 Correlation of sec plateau crack velocities with smooth specimen sec ratings Plateaucrack velocity Smoothspecimen

First growth

Aventge(0to 15days) in. x lo-5Jh

ratiog(a)

mJs

in. x lo-5/b

mls

5

A A B B B

6 7

D

6 X 10- 10 7 X 10-9 2.1 X 10-9 4.2 X 10-9 7 X 10-9 6.3 X 10-9 1.1 X 10-8

10 100 30 60 100 90 160

2 X 10- 10 1.8 X 10-9 1.2 X 10-9 1.3 X 10- 9 2.1 X 10- 9 4.2 X 10- 9 8.4 X 10-9

Anoy

I 2 3 4

e

3 27(b) 19 20 30 60 120

S-L (see Fig. II) double-beamspecimensbolt-loadedto pop-in and wettedthreetimesdaily with 3.5% sodiumchloride.(a) Short-transverseratings per ASTMG 64. (b) Fractographicexaminationrevealedmechanicalfracturemtherthan theintergranularsee verifiedin all othermaterials.Source: Ref51

05

12.5 E E

100

720 h

~

U

U

50

E c

e 's

Ol

.¥ U

e

~

E Q)

~

e

03

75

.¥ U

;;;

.~

~

I

0

m

04

I

~

2 3

2.5

c

;;; 02 E Q) E c

0.1

w

4

w

e

's c

0 0

800

400 Exposure time, h

see

Fig 24

see.

Examples of crackgrowthin variousaluminum alloyswith relativelyhigh resistance to S-L lsee • Fig. 11) double-beam specimens bolt-loaded to pop-in and wetted with 3.5% sodium chloridethreetimes daily; relativehumidity45%. Thenumbers 1 to 7 indicatedifferenttest materials listedin orderof decreasing resistance to lsee Table6); dashed linesindicateplateauvelocities. The alloy 2 specimen Failed bymechanical fracture rather Source: Ref 45, 51 thanintergranular

see

see.

Corrosion Testing/245 With intermediate-resistance materials, the growth curves can develop prominent steps indicative of temporary arrests. Figure 26 shows some of the various curves that can be obtained, depending on the resistance to corrosion and see of the test material, the corrosivity of the test medium, the magnitude of the applied stress intensity, and the length of exposure. The significant portion of the curve is that which goes from the beginning of the test to the first appreciable

cessation of the crack growth. It is assumed that if it were not for the intervention of the corrosion product wedging, the curve would proceed to an arrest. The threshold stress intensities determined by this method can be useful for ranking materials, but usually cannot be considered valid. Therefore, they cannot be used in design calculations based on fracture mechanics. Displays of complete V-K curves provide convenient comparisons of various materials, as shown in Fig. Tl.

Stressintensity, ksi fm.

o

4

12

8

16

20

I

24

I

I

r---t---t--·~t-_·_--

I

i

I

/

I

: I

/

II 'I

I K,scc (KOhl

I

/

! : I

10"

/

II I I I I

i

I

I K,,-fracture toughness of test material

--+I-+t---t-----1-r---t---t-----i

I

1~H1I~:

I i

I I

I

10- 11 L -_ _-'---'--_-'--_ _-'--_ _-'---'--_..L-.--L_---'-_ _- ' -_ _.....

o

4

12

8

16

20

Stressintensity, MPa

Fig 25

see

vm

24

28

32

Effect of corrosive environment on velocity and threshold stress intensity for 7079-T651 plate • \64 mm, or 2.5 in., thick) stressed in the short-transverse direction (S-L; see fig. 11). Double-beam specimens bo t-locded to pop·in. No occurred during 3 years 01 exposure to dry air in a desiccator; however, the plateau velocity (horizontal part 01 each curve) and the apparent threshold stressintensity (K1SCC or K,.,] vary with the environment. Dashed portions 01 the curves represent the ellect of corrosion product wedging. Source: Rel51

see

246 I Corrosion of Aluminum and Aluminum Alloys The testing of longitudinal (L-T, L-S in Fig. II) and long-transverse (T-L, T-S in Fig. 11) specimens presents special problems with materials having typical directional grain structures. Stress-corrosion cracking growthis small and tends to be in the L- T plane, which is perpendicular to the plane of the precrack (Ref 45, 53). Such out-of-planecrack growth invalidates calculations of the plane-strain threshold stress intensity KISCC' On the other hand, the testing of materialshaving an equiaxedgrain structure also presents problems with stress intensity calculations because of gross crack branching; this would be applicable to specimens of any orientation. The most widely used corrodent for testing precracked specimens is 3.5% NaCI solution applied dropwise to the precrack two or (usually) three times daily (Ref 43-46). This intermittentwetting technique accelerates SCC growth, but it also causes troublesome corrosion of the mechanicalprecrack. Less corrosive corrodents that have been used include substitute ocean water (ASTM D 1141)and an inhibitedsalt solution containing 0.06 M NaCI, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4 (Ref 45, 46, 50). Some investigators have tested 7000 series alloys in distilledwater(Ref 47) and in water vapor at 40 "C (104 "F) (Ref 54). Typicaltest durations that havebeenusedrangefrom200to 2500h. With low-resistance alloys,both of the first two corrodents listed in the precedingparagraphranked alloys similarlyand in agreementwith exposure to a seacoast and an inland industrial atmosphere. Plateauvelocities in the laboratory tests were about five to ten times faster than in the seacoast atmosphere and ten times faster than in the industrial atmosphere. In these K decreasinglaboratorytests, corrosionproductwedging effects dominated after exposure periods of about 200 to 800 h. The length of exposure time before the intervention of corrosionproduct wedging varies with several factors, including the magnitude of starting stress intensity, KIo, and the inherent resistance to crevice corrosion of the test material in the corrosiveenvironment (Ref45,51).

Dynamic Loading: Slow Strain Rate Testing Anotherrecently developedmethod for accelerating the SCC process in laboratory testing involves relatively slow strain rate tension testing of a specimen during exposure to appropriate environmental conditions. The applicationof slow dynamic strain exceeding the elastic limit assists in the SCC initiation. The accelerating technique is consistent with the various proposed general mechanismsof SCC, most of which involvehydrogeninducedcrackingor film rupture due to anodic dissolution. Slow strain rate tests can be used to test a wide variety of product forms, including parts joined by welding. Tests can be conducted in tension, in bend-

ing, or with plain, notched, or precracked specimens. The principal advantage of slow strain rate testing is the rapidity with which the SCC susceptibility of a particularalloy and environmentcan be assessed. Slow strain rate testing is not terminated after an arbitraryperiod of time. Testing always ends in specimen fracture, and the mode of fracture is then compared with the criteriaof SCC susceptibility for the test material. In addition to its timesaving benefits, less scatteroccursin the test results. Typicalstrainrates range between 1O--5/s and 1O-7/s, but for most materials the typical strain rate is at 1O-6/s. The strain sensitivity to SCC can change for different alloys, even of the same metal. Figure 28 shows that for the 2000 series aluminum alloys, the critical strain rate for the highest susceptibility to cracking is 10-6/s, whereas no such critical strain rate exists for the 7000 series aluminumalloys.This difference in slow strain rate behavior of the two alloys can indicate different mechanisms for stress-eorrosion cracking. The slow strain rate behavior indicates that the principal mechanism for cracking of the 2000 series alloys is film rupture anodic dissolution model, while the predominant mechanism for cracking of the 7000 series alloys is hydrogen embrittlement. The parameters that are typically measured in slow strain rate testing to determine the susceptibility to SCC are the following: • Tune to failure • Percentelongation • Percentreductionin cross-sectional area at the fracture surface • Reduction in ultimate tensile strength and yield strengthtensile stress • Presence of secondary cracking on the specimen gage section • Appearance of the fracturesurface

1

AlloV A

Exposure time--+

Fig 26

Schematic 01 thevariableeffects 01 corrosion • product wedgingon see growth curves ina K-decreasing lesl. Solid lines: measured curve. Dashed lines: estimated truecurveexcluding the effect 01 corrosion product wedging. Asterisks indicate lemporarycrack arrests.

Corrosion Testing /247 In order to assess the susceptibility of a material to SCC, the resultsof the slow strainrate test in a particular environment must be compared with those in an inert environment, such as dry nitrogen gas. Slow strain rate testing is described in ASTM G 129, "Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking." Additional information of this technique can be found in Ref 55 to 58.

REFERENCES 1. Corrosion Tests andStandards: Application andInterpretation, R. Baboian, Ed., ASTM, 1995 2. EA. Champion, Corrosion Testing Procedures, 2nd ed.,John WIley & Sons, 1965,p 365,366 3. GJ. Schafer, 1 Appl. Chern., Vol 10, 1960,p 138 4. S. Ketchamand W. Beck, Corrosion, Vol 16, 1960, p37 5. M.K.Buddand EE Booth,Corrosion, Vol18, 1962,

Stress intensity (K,l. ksivTri: 10

40

30

20

0.1

10- 61-

+-~"

of:

.!!!

E

i'0

0.01 10-

7

r:::

i'0 0

0

a;

a;

>

>

.>

c

1:::I

i'

0.10 0.05 1.2-2.0 1.2-2.0 1.2-1.9

0.7Si+Fe 0.25 0.6 0.40 0.50 0.15 0.20 0.12 0.10

7072 7472 7075 7175 7475

0.20 0.20 0.10 0.10 0.10 0.05

1.2-1.9 1.2-1.9 1.3-1.9 2.0-2.6 1.9-2.5 2.0-2.6 I.S-2.4

0.10 0.05 0.30 0.10 0.06

...

...

0.35 0.20 0.12 0.15 0.15 0.15 0.15

0.25 0.15 0.10 0.12 0.12 0.10 0.12

7049 7149 7249 7050 7150 7055 7064

...

0.03 0.10 0.Q3 0.1O-Q.4O 0.1O-Q.4O 0.30

0.5O-Q.9 0.5O-Q.9 0.5O-Q.9 0.10 0.10 0.25

0.12 0.30 O.OS 0.8-1.4 0.40 0.40 0.40

0.10 0.15 0.06 0.30 0.30 0.20 0.20

...

0.8-1.4 0.8-1.4 1.2-1.8

7029 7129 7229 7031 7039 7046 7146

0.05--Q.20

0.10 0.9-1.5 2.1-2.9 2.1-2.9 1.9-2.6

2.0-2.9 2.0-2.9 2.0-2.4 1.9-2.6 2.0-2.7 I.S-2.3 1.9-2.9

1.3-2.0 1.3-2.0 1.3-2.0 0.10 2.3-3.3 1.0-1.6 1.0-1.6

...

1.0-1.6

O.1S--{).2S O.IS--{).2S 0.IS--Q.25

...

O.1O-Q.22 O.1O-Q.22 O.12--{).IS 0.04 0.04 0.04 0.06-0.25

...

0.15--{).25 0.20

...

0.10

... ...

0.05

... ...

...

0.1O-Q.30 1.0-1.5 0.03 0.05 0.10

0.18--{).35 0.05 0.06-0.20 O.12--{).25

0.05 0.10 0.45-1.0 0.50-1.1 0.25

0.20 0.7 0.12 0.30 0.40

0.15 0.6 0.10 0.15 0.25

7011 7013 7016 7116 7021

... ...

0.40 0.10 0.15 0.15

... ...

...

0.10

0.45--{).9 0.45--{).9 0.45--{).9 0.8-1.4 0.50-1.2 0.8-1.2 0.8-1.2 2.6-3.4 1.0-2.0 1.0-1.8 0.7-1.4 0.7-1.4

1.6-2.6 0.05 0.10 0.05 0.05

0.40 0.35 0.40 0.10 0.10

0.35 0.25 0.35 0.10 0.10

7001 7004 7005 700S 71OS

0.10 0.05 0.Q3 0.6-1.1 0.40-1.0 0.15 0.15

... ...

,

..

'"

5.1~.1

(continued)

5.2~.2

5.1~.1

...

... ... ... ... ...

...

...

...

... ...

...

...

...

0.03

... ...

0.03

... ...

...

...

...

...

... ... ... ... ...

...

... ... ...

...

...

Ga

O.S-1.3 1.3-1.9

7.6-8.4 6.S-S.0

...

...

...

5.7~.7

7.2-8.2 7.2-S.2 7.5-S.2

4.2-5.2 4.2-5.2 4.2-5.2 0.S-1.8 3.5-4.5 6.6-7.6 6.6-7.6

4.0-5.5 1.5-2.0 4.0-5.0 4.2-5.2 5.0-6.0

6.8-S.0 3.S-4.6 4.0-5.0 4.5-5.5 4.5-5.5

0.10 0.05 0.03 0.25 0.25 0.25 0.25

0.25 0.25

Zn

Composition, wt%

5.9~.9

... ...

..,

...

...

...

...

... ...

..,

..,

...

'"

...

... ... ..,

... ... ... .., ...

... ...

...

...

0.10 0.04-0.14

0.7-1.1 0.8-1.2

Ni

Cr

Mg

0.20 0.2O-Q.7 0.2O-Q.7 0.05 0.05

0.10 0.20 0.04--{).16 0.7-1.2 0.15--{).4O 0.15--Q.4O 0.7-1.0

0.35 0.15 0.08 0.50 0.50 0.7 0.30

0.2O-Q.6 0.2O-Q.6 0.2O-Q.6 0.9-1.8 1.0-1.7 0.40--0.8 0.40--0.8

6063 6463 6763 6066 6070 6091 6092

0.10 0.15

0.20 O.15--Q.4O

0.50 0.7

0.40--0.8 0.40--0.8

Mn

Cu

6162 6262

Fe

SI

AANo.

TableA-l (continued)

...

...

... ... ... ...

...

... ...

... ...

... ... ... ...

0.05 0.05 0.05

...

0.05 0.05

(y)

...

...

0.OS--Q.15Zr 0.OS--{).15 Zr 0.08--{).25Zr 0.1O-Q.50 Zr(x)

... ... ...

0.1 O-Q.IS Zr 0.1 O-Q.IS Zr

...

... ...

... ...

O.08--{).IS Zr

...

...

.. .

...

...

0.12--Q.25Zr

...

0.1 O-Q.20 Zr 0.OS--Q.20Zr

...

(u) (u)

...

... ...

...

(w)

...

...

... ...

... ...

...

0.05

...

... ...

V

Specified other elements

...

0.20 0.10 0.06

... ...

0.10 0.10 0.06 0.06 0.06 0.06

0.10 0.06 0.06

...

0.05 0.05 0.05

0.03 0.05 0.10

0.05

0.20 0.05 0.01--{).06 0.05 0.05

0.20 0.15 0.15 0.15

...

0.10

0.10 0.15

n

0.05 0.05 0.05 0.05 0.05

0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.Q3 0.05 0.Q3 0.05 0.05 0.05 0.05

0.05 0.05 0.Q3 0.05 0.05

0.05 0.05 0.05 0.05 0.05

0.05 0.05 0.Q3 0.05 0.05 0.05 0.05

0.05 0.05

0.15 0.15 0.15 0.15 0.15

0.15 0.15 0.15 0.15 0.15 0.15 0.15

0.10 0.15 0.10 0.15 0.15 0.15 0.15

0.15 0.15 0.10 0.15 0.15

0.15 0.15 0.15 0.10 0.15

0.15 0.15 0.10 0.15 0.15 0.15 0.15

0.15 0.15

Unspecified other elements Each Total

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Al,mln

~

'"

.f

~

C

3 S· 3

C

~

aa

3 S· C 3

C

~

S.

i·

flt

§

n

0.....

en

0.50 0.40 0.12 0.12 0.12

0.17 0.40 0.40 1.7-1.9 0.40 0.30-1.J

1.0 0.30 0.30 0.10 0.20 0.10 1.2-1.4

0.10 0.15(ft)

1.0Si+Fe 0.10 0.6-{).9 0.03-{).15 0.40-1.0 0.10 0.1()-'{).4O 0.10 0.25-{).45

0.05-{).30 1.0-2.0 0.7 0.20

7277 7178 7090 7091 7093

8001 8006 8007 8009 8010 8111

8112 8014 8015 8017 X8019 8020 8022

8030 8130

8040 8076 8176 8077 8177

8079 8280 8081 8090

0.05 0.7-1.3 0.7-1.3 1.0-1.6

0.10 0.10 0.10

'"

0.05

0.6 0.2()-.{).6 0.1()-'{).4O ... 0.05 0.005 0.10

0.30-1.0 0.30-1.0 0.10 0.1()-.{).8 0.10

... 0.30

0.3()-.{).8

Mn

0.18-{).35 0.18-{).28

Cr

... 0.6-1.3

... 0.08-{).22 ... 0.1()-'{).30 0.04-{).\2

0.05 ..,

...

...

0.9-1.3

0.04-0.16

Ni

0.10

0.2()-.{).7 ...

0.10...

0.7 0.20 0.10... 0.10 ... 0.0l-{).05...

0.10 0.10... 0.10 0.1()-.{).50 0.20 0.05 0.05

1.7-2.3 2.4-3.\ 2.0-3.0 2.0-3.0 2.0-3.0

1.2-2.0

Mg

1.1-1.5

0.10 0.05 0.05 0.25

0.20 0.05 0.10 0.05 0.05 0.03

...

0.05 0.10... ...

...

1.0... 0.10 ... 0.10... 0.05...... 0.05 0.005... 0.05 0.25 0.4O-{).8

0.05 0.10 0.8-1.8 0.25 0.40 0.10

." 5.5-7.0Sn 18.0-22.0Sn 0.04-0.16 Zr (hh)

0.1()-,{).30Zr 0.04 B ... 0.05B(gg) 0.04 B

O.ool-{).04B ...

0.04B,0.003Li (cc) (dd) (ee)

(bb)

(aa)

1.0-1.9 Co (z) 0.2()-.{).60Co(z) 0.08-{).20 Zr (u)

TI

0.10 0.10 0.10

...

...

0.05 ... 0.10

0.20 0.10

0.10 0.10 0.08

0.10 0.20

Specified other elements

3.7-4.3...... 6.3-7.3...... 7.3-8.7 5.8-7.1 8.3-9.7

v 0.20

...

Ga

...

7.0-8.0

Zn

Composition, wt%

0.05 0.05 0.05 0.05

0.05 0.03 0.05 0.03 0.03

0.03 0.03

0.05 0.05 0.05 0.03 0.05 0.03 0.05

0.05 0.05 0.05 0.05 0.05 0.05

0.05 0.05 0.05 0.05 0.05

0.05

0.15 0.15 0.15 0.15

0.15 0.10 0.15 0.10 0.10

0.10 0.10

0.15 0.15 0.15 0.10 0.15 0.10 0.15

0.15 0.15 0.15 0.15 0.15 0.15

0.15 0.15 0.15 0.15 0.15

0.15

Unspecified other elements Each Total

:a.

------------------------------------------------------ ....

'"

~

... ......

)CO

A.

1

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bal

Al,min

(a) 0.0008% max Be for welding electrode and filler wire only. (h) 0.14% max Si + Fe. (c) 0.6% max O. (d) 0.2()-.{).6% Bi, 0.2()-.{).6% Pb. (e) 0.2()-.{).8% Bi, 0.1()-.{).50% Sn. (I) 0.2()-.{).7% Bi, 0.2()-.{).6% Sn. (g) A Zr + Ti limit of 0.20% max can be used with this alloy designation for extruded and forged products only, but only when the supplier and purchaser have mutually so agreed. (h) 0.40% max Si + Fe. (i) 0.005% max Be, 0.2()-.{).50% O. (j) 1.9-2.6% Li. (k) 1.7-2.3% Li. (I) 0.25-{).6% Ag, 0.7-1.4% Li. (m) 0.25-{).6% Ag, 0.7-1.5% Li. (n) 0.25-{).6% Ag, 0.8-1.2% Li. (0) 0.25-{).6% Ag, 1.3-1.9% Li. (p) 1.2-1.8% Li. (q) 1.3-1.7% Li. (r) 0.6-1.5% Bi, 0.05% max Cd. (s) Fonnerly inactive alloy 4245 reactivated as 4048. (I) 1.0-1.3% C, 1.2-1.4% Li, 0.2()-.{).7% O. (u) 0.05-{).50% O. (v) 45-65% of actual Mg. (w)0.4O-{).7%Bi, 0.4O-{).7%Pb. (x) 0.1()-'{).4O% Co, 0.05-{).30% O. (y) A Zr+ Ti limit of 0.25% max can be us~ with this alloy designation for extruded and forged products only, but only when the supplier and purchaser have mutually so agree~. (z) 0.2()-.{).50% O. (aa) 0.001% max B, 0.003% max Cd, 0.001 % max Co, 0.008% max Ll. (bb) 0.30% max O. (cc) 3.5-4.5% Ce, 0.2()-.{).50% O. (dd) 0.1()-.{).50% B1,0.1()-.{).25% Sn. (ee) 0.05-{).20% O. (ft) 1.0% max S. + Fe. (gg) 0.02-{).08% Zr. (hh) 2.2-2.7% Ll. Source: Aluminum Association Inc.

0.7-1.3 0.7 0.7 0.30

0.15-{).30 0.05-{).15

0.3()-.{).8 0.40-1.0(ft)

0.20 0.04 ... 0.05 0.04

0.40 0.20 0.10 0.\()-.{).20 ... 0.005 ...

0.1()-.{).30 0.10

0.15 0.30 0.10

0.8-1.7 1.6-2.4 0.6-1.3 1.1-1.8 1.1-1.9

0.30-1.0

Cu

1.0 1.2-1.6 0.8-1.4 0.55-{).8 7.3-9.3 0.10 6.2-6.8

0.45-{).7 1.2-2.0 1.2-2.0 8.4-8.9 0.35-{).7 0.40-1.0

0.7 0.50 0.15 0.15 0.15

0.6

0.40

7076

Fe

Si

AANo.

Table A·l (continued)

A24O.0,AI4O A240.I,AI4O 142

Hiduminium 350 Hiduminium 350 A-U5GT A-U5GT

A201.2

Designation Former

OI: 10.1361/coaaaa1999p259

100.1 130.1 150.1 160.1 170.1 201.0 201.2 A201.0 A201.l B201.0 203.0 203.2 204.0 204.2 206.0 206.2 A206.0 A206.2 240.0 240.1 242.0

AANo.

S,P

Ingot

S

Ingot

S,P

Ingot

S,P

Ingot

S.P

Ingot

S S

Ingot

S

Ingot

S

Ingot Ingot Ingot Ingot Ingot

Producls(a)

(e)

0.15 0.10 0.10 0.07 0.05 0.50 0.35 0.35 0.10-0.20 0.15 0.10 0.10 0.D7 0.50 0040 1.0

(e)

0.10 0.10 0.05 0.05 0.05 0.30 0.20 0.20 0.15 0.10 0.10 0.05 0.05 0.50 0.50 0.7

(c)

(d) 0.25(d)

0.6-c

B:

-t 1:::J

A360

A360.2 361.0 361.1 363.0 363.1 364.0 364.2 369.0 369.1 380.0(0) 380.2 A380.0(0) A380.1 A380.2 B380.0 B380.1 C380.0 C380.1 D380.0 D380.1 383.0 383.1 383.2 A383.0 A383.1 384.0 384.1 384.2 A384.0 A384.1 B384.0 B384.1 C384.0 C384.1 385.0 385.1 390.0 390.2 A390.0 A390.1 B390.0 B390.1 392.0

392

B384.0, 384 B384.1, 384 390 390 A390 A390

384 384 384 384 384

363 363 364 364 SpecialK-9 Special K-9 380 380 A380 A380 A380 A380 A380

Former

AANo.

Designation

TableA-2 (continued)

D

Ingot D Ingot

S,P

Ingot D Ingot D Ingot D Ingot

D

Ingot

D

Ingot Ingot

D

Ingot

D

Ingot D Ingot Ingot

D

Ingot

D

Ingot

D

Ingot Ingot

D

Ingot

D

Ingot

D

Ingot

D

Ingot

S,P

Ingot

D

Ingot

Producls(a)

9.0-10.0 9.5-10.5 9.5-10.5 4.5-6.0 4.5-6.0 7.5-9.5 7.5-9.5 11.0-12.0 11.0-12.0 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 7.5-9.5 9.5-11.5 9.5-11.5 9.5-11.5 9.5-11.5 9.5-11.5 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 10.5-12.0 11.0-13.0 11.0-13.0 16.0-18.0 16.0-18.0 16.0-18.0 16.0-18.0 16.0-18.0 16.0-18.0 18.0-20.0

SI

0.6--1.0 0.50 0.40 1.3 1.0 1.5

1.3

1.0 2.0 1.1

1.3

1.0

1.3

1.0

1.3

1.0 0.6--1.0

1.3

1.0

1.3

1.0 0.6--1.0

1.3

1.0

1.3

1.0

1.3

1.0

1.3

1.0 0.6

1.3

1.0 2.0 0.7-1.1

1.3

0.6 1.1 0.8 1.1 0.8 1.5 0.7-1.1

Fe

0.10 0.50 0.50 2.5-3.5 2.5-3.5 0.20 0.20 0.50 0.50 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 3.0-4.0 2.0-3.0 2.0-3.0 2.0-3.0 2.0-3.0 2.0-3.0 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 3.0-4.5 2.0-4.0 2.0-4.0 4.0-5.0 4.0-5.0 4.0-5.0 4.0-5.0 4.0-5.0 4.0-5.0 0.40-0.8

Cu

0.10 0.10 0.35 0.35 0.50 0.10 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.10 0.10 0.10 0.10 0.50 0.50 0.20-0.6

(P) (P)

0.05 0.25 0.25

Mn

(continued)

0.45-{).6 0.40-0.6 0.45-{).6 0.15-{).4O 0.20-0.40 0.20-0.40 0.25-{).4O 0.25-{).45 0.30-0.45 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10-0.30 0.15-{).30 0.10-0.30 0.15-{).30 0.10 0.10 0.10 0.10-0.30 0.15-{).30 0.10 0.10 0.10 0.10 0.10 0.10-0.30 0.15-{).30 0.10-0.30 0.15-{).30 0.30 0.30 0.45-{).65(s) 0.50-0.65(s) 0.45-{).65(s) 0.50-0.65(s) 0.45-{).65(s) 0.50-0.65(s) 0.8-1.2

Mg

Composition,wt% NI

0.10 0.10 0.50

0.20-0.30 0.20-0.30 (p) 0.25 (P) 0.25 0.25-{).50 0.15 0.25-{).50 0.15 0.30-0.40 0.05 0.30-0.40 0.05 0.50 0.10 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.30 0.30 0.10 0.30 0.30 0.50 0.50 0.10 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50

0.20-0.30 0.20-0.30

Cr

0.05 0.50 0.40 3.0-4.5 3.0-4.5 0.15 0.15 1.0 0.9 3.0 0.10 3.0 2.9 0.10 1.0 0.9 3.0 2.9 1.0 0.9 3.0 2.9 0.10 3.0 2.9 3.0 2.9 0.10 1.0 0.9 1.0 0.9 3.0 2.9 3.0 2.9 0.10 0.10 0.10 0.10 1.5 1.4 0.50

Zn

0.20 0.20 0.20 0.20 0.20 0.20 0.20

0.20 0.20 0.20 0.20

TI

0.30

0.35 0.35 0.35 0.35 0.35 0.35 0.15 0.15 0.10 0.15 0.15 0.35 0.35 0.10 0.35 0.35 0.35 0.35 0.35 0.35 0.30 0.30

0.10 0.10 0.25 0.25 0.15 0.15 0.10 0.10 0.35 0.10 0.35 0.35

Sn

Others

0.10 0.10 0.10 0.10 0.10 0.10 0.15

0.05

0.05(r) 0.05(r) 0.05 0.05

(q) (q)

0.05 0.05 0.05

Each

0.15 0.15 0.15 0.30 0.30 0.15 0.15 0.15 0.15 0.50 0.20 0.50 0.50 0.15 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.20 0.50 0.50 0.50 0.50 0.20 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.20 0.20 0.20 0.20 0.20 0.20 0.50

Total

bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal

AI,min

~

~

'"

~

~

C

aS· C a

~

"a

S·

a C a

C

!.

'"i·

a

0

n

.....

~

0-

392 Vanasil Vanasil Vanasil

392.1 393.0 393.1 393.2 408.2(u) 409.2(u) 411.2(u) 413.0(0) 413.2 A413.0(0) A4I3.1(o) A413.2 B413.0 B413.1 435.2(v) 443.0 443.1 443.2 A443.0 A443.1 B443.0 B443.1 C443.0 C443.1 C443.2 444.0 444.2 A444.0 A444.1 A444.2 445.2(u) 511.0 511.1 511.2 512.0 512.2 513.0 513.2 514.0 514.1 514.2 515.0 515.2

A344 B444.2 F514.0, F214 F514.I, F214 F514.2, F214 B514.0, B214 B514.2, B214 A514.0, A214 A514.2,A214 214 214 214 L514.0, L214 L514.2, L214

A344

43 43 43 43(0.30 max Cu) 43(0.30 max Cu) 43(0.15 max Cu) 43(0.15maxCu) A43 A43 A43

13 13 AI3 AI3 AI3

Former

AANo.

Designation


Ingot

Ingot D

Ingot Ingot D Ingot D Ingot Ingot S,P Ingot Ingot S,P Ingot Ingot S Ingot S,P Ingot D Ingot Ingot S,P Ingot P Ingot Ingot Ingot S Ingot Ingot S Ingot P Ingot S Ingot

Ingot

Ingot S,P,D Ingot Ingot

Products(a) Fe

18.0-20.0 J.1 21.0-23.0 1.3 21.0-23.0 1.0 21.0-23.0 0.8 8.5-9.5 0.6-1.3 9.0-10.0 0.6-1.3 10.0-12.0 0.6-1.3 11.0-13.0 2.0 11.0-13.0 0.7-1.1 11.0-13.0 1.3 11.0-13.0 1.0 11.0-13.0 0.6 11.0-13.0 0.50 11.0-13.0 0.40 3.3-3.9 0.40 4.5-6.0 0.8 4.5-6.0 0.6 4.5-6.0 0.6 4.5-6.0 0.8 4.5-6.0 0.6 4.5-6.0 0.8 4.5-6.0 0.6 4.5-6.0 2.0 4.5-6.0 1.1 4.5-6.0 0.7-J.1 6.5-7.5 0.6 6.5-7.5 0.13-0.25 6.5-7.5 0.20 6.5-7.5 0.15 6.5-7.5 0.12 6.5-7.5 0.6-1.3 0.30-0.7 0.50 0.30-0.7 0.40 0.30-0.7 0.30 1.4-2.2 0.6 1.4-2.2 0.30 0.30 0.40 0.30 0.30 0.35 0.50 0.35 0.40 0.30 0.30 0.50-1.0 1.3 0.50-1.0 0.6-1.0

Si 0.40-0.8 0.7-J.1 0.7-J.1 0.7-J.1 0.10 0.10 0.20 1.0 0.10 1.0 1.0 0.10 0.10 0.10 0.05 0.6 0.6 0.10 0.30 0.30 0.15 0.15 0.6 0.6 0.10 0.25 0.10 0.10 0.10 0.05 0.10 0.15 0.15 0.10 0.35 0.10 0.10 0.10 0.15 0.15 0.10 0.20 0.10

Cu 0.20-0.6 0.10 0.10 0.10 0.10 0.10 0.10 0.35 0.10 0.35 0.35 0.05 0.35 0.35 0.05 0.50 0.50 0.10 0.50 0.50 0.35 0.35 0.35 0.35 0.10 0.35 0.05 0.10 0.10 0.05 0.10 0.35 0.35 0.10 0.8 0.10 0.30 0.10 0.35 0.35 0.10 0.40-0.6 0.40-0.6

Mn

(continued)

3.5--4.5 3.6--4.5 3.6--4.5 3.5--4.5 3.6--4.5 3.5--4.5 3.6--4.5 3.5--4.5 3.6--4.5 3.6--4.5 2.5--4.0 2.7--4.0

0.10 0.10 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.05 0.10 0.05 0.05 0.05 0.05

0.10

om

0.9-1.2 0.7-1.3 0.8-1.3 0.8-1.3

Mg

Composition, wt%

0.25

0.25 0.25

0.25 0.25

Cr

0.50 0.50

0.50 0.10 0.50 0.50 0.05 0.05 0.05

0.50 2.0-2.5 2.0-2.5 2.0-2.5

Ni 0.40 0.10 0.10 0.10 0.10 0.10 0.10 0.50 0.10 0.50 0.40 0.05 0.10 0.10 0.10 0.50 0.50 0.10 0.50 0.50 0.35 0.35 0.50 0.40 0.10 0.35 0.05 0.10 0.10 0.05 0.10 0.15 0.15 0.10 0.35 0.10 1.4-2.2 1.4-2.2 0.15 0.15 0.10 0.10 0.05

Zn

0.25 0.25 0.20 0.25 0.20 0.20 0.20 0.25 0.25 0.20

0.25 0.20 0.20 0.20 0.20

0.25 0.25 0.20 0.25 0.25 0.25 0.25

0.25 0.25

0.20 0.10-0.20 0.10-0.20 0.10-0.20

Ti

0.15 0.15

0.15 0.10 0.15 0.15 0.05

0.30

Sn

Others

0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.05 0.05

0.05

0.05 0.05 0.05

0.15 0.05(1) 0.05(1) 0.05(1) 0.10 0.10 0.10

Each 0.50 0.15 0.15 0.15 0.20 0.20 0.20 0.25 0.20 0.25 0.25 0.10 0.20 0.20 0.20 0.35 0.35 0.15 0.35 0.35 0.15 0.15 0.25 0.25 0.15 0.15 0.15 0.15 0.15 0.15 0.20 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15

Total

bal bal bal bal bal bal bal bal hal bal hal bal bal bal bal hal bal bal hal hal hal hal hal hal bal hal bal hal bal hal bal bal bal bal bal bal bal hal bal hal hal hal hal

AI,min

::J

~ W

.......

...,

8: )C

.r1

218 218 218 220 220 Almag35 Almag35 A218 A218 B218 B218 603, Temalloy 5 603, Temalloy 5 607, Temalloy 7 607, Temalloy7 A712.0,A612 A712.I,A612 C712.0, C612 C712.1, C612 0712.0, 0612, 40E 0712.2, 0612, 40E 613, Tenzaloy 613, Tenzaloy Precedent71A Precedent 71 A B771.0, Precedent 71B B771.2, Precedent71B 750 750 A850.0, A750 A850.1,A750 B850.0, B750 B850.1, B750 XC850.0, XC750 XC850.2, XC750

AANo.

516.0 516.1 518.0 518.1 518.2 520.0 520.2 535.0 535.2 A535.0 A535.1 B535.0 B535.2 705.0 705.1 707.0 707.1 710.0 710.1 711.0 711.1 712.0 712.2 713.0 713.1 771.0 771.2 772.0 772.2 850.0 850.1 851.0 851.1 852.0 852.1 853.0 853.2

0 Ingot 0 Ingot Ingot S Ingot S Ingot S Ingot S Ingot S,P Ingot S,P Ingot S Ingot P Ingot S Ingot S,P Ingot S Ingot S Ingot S,P Ingot S,P Ingot S,P Ingot S,P Ingot

Products(a) Fe

0.35-1.0 0.35--{).7 1.8 1.1 0.7 0.30 0.20 0.15 0.10 0.20 0.15 0.15 0.12 0.8 0.6 0.8 0.6 0.50 0.40 0.7-1.4 0.7-1.1 0.50 0.40 1.1 0.8 0.15 0.10 0.15 0.10 0.7 0.50 0.7 0.50 0.7 0.50 0.7 0.50

Si

0.30-1.5 0.30-1.5 0.35 0.35 0.25 0.25 0.15 0.15 0.10 0.20 0.20 0.15 0.10 0.20 0.20 0.20 0.20 0.15 0.15 0.30 0.30 0.30 0.15 0.25 0.25 0.15 0.10 0.15 0.10 0.7 0.7 2.0-3.0 2.0-3.0 0.40 0.40 5.5-6.5 5.5-6.5 0.30 0.30 0.25 0.25 0.10 0.25 0.20 0.05 0.05 0.10 0.10 0.10 0.05 0.20 0.20 0.20 0.20 0.35--{).6 0.35--{).6 0.35--{).6 0.35--{).6 0.25 0.25 0.40-1.0 0.40-1.0 0.10 0.10 0.10 0.10 0.7-1.3 0.7-1.3 0.7-1.3 0.7-1.3 1.7-2.3 1.7-2.3 3.0-4.0 3.0-4.0

Cu

Mg

2.5-4.5 2.6-4.5 7.5-8.5 7.6--8.5 7.6--8.5 9.5-10.6 9.6--10.6 6.2-7.5 6.6--7.5 6.5-7.5 6.6--7.5 6.5-7.5 6.6--7.5 1.4-1.8 1.5-1.8 1.8-2.4 1.9-2.4 0.6--{).8 0.65--{).8 0.25--{).45 0.30--{).45 0.50--{).65(s) 0.50--{).65(s) 0.20--{).50 0.25--{).50 0.8-1.0 0.85-1.0 0.6--{).8 0.65--{).8 0.10 0.10 0.10 0.10 0.6--{).9 0.7--{).9

Mn

0.15--{).4O 0.15--{).4O 0.35 0.35 0.10 0.15 0.10 0.10--{).25 0.10--{).25 0.10--{).25 0.10--{).25 0.05 0.05 0.4O--{).6 0.4O--{).6 0.4O--{).6 0.4O--{).6 0.05 0.05 0.05 0.05 0.10 0.10 0.6 0.6 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.50 0.10

Composition, wt%

0.4O--{).6 0.4O--{).6 0.35 0.35 0.06--{).20 0.06--{).20 0.06--{).20 0.06--{).20 0.7-1.3 0.7-1.3 0.30--{).7 0.30--{).7 0.9-1.5 0.9-1.5

6.0-7.0 6.0-7.0 6.0-7.0 6.0-7.0 5.0-6.5 5.0-6.5 7.0-8.0 7.0-8.0 6.5-7.5 6.5-7.5 6.0-7.0 6.0-7.0

2.7-3.3 2.7-3.3 4.0-4.5 4.0-4.5

0.15 0.15

0.20 0.20 0.15 0.15

0.25--{).4O 0.25--{).4O 0.15 0.15 0.05 0.15 0.10

Zn

Nl

0.20--{).4O 0.20--{).4O 0.20--{).4O 0.20--{).4O

Cr

0.25 0.20 0.10--{).25 0.10--{).25 0.25 0.25 0.10--{).25 0.10--{).25 0.25 0.25 0.25 0.25 0.25 0.25 0.20 0.20 0.15--{).25 0.15--{).25 0.25 0.25 0.10--{).20 0.10--{).20 0.10--{).20 0.10--{).20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

0.10--{).20 0.10--{).20

Ti

5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0 5.5-7.0

0.10 0.10 0.15 0.15 0.05

Sn

0.05 0.05 0.05(x) 0.05(y) 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.05 0.05 0.05 0.05

0.05(w) 0.05(w)

Each

Others

0.25 0.25 0.10 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.20 0.20 0.25 0.25 0.15 0.15 0.15 0.15 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30

Total

bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal bal

Al,min

(a) 0, die casting. P, permanent mold. S, sand. Other products might pertain to the composition shown even though not listed. (b) 0.025% max Mg + Cr + Ti + V. (c) FeiSi ratio 2.5 min. (d) FeiSi ratio 2.0 min. (e) FeiSi ratio 1.5 min. (f) 0.40-1.0% Ag. (g) 0.50-1.0% Ag. (h) 0.50% max Ti + Zs. (i) 0.20--{).30% Sb, 0.2O--{).30% Co, 0.10--{).30% Zr. (j) Primarily used for making metal-matrix composite. (k) If Fe exceeds 0.45%, Mg content will not be less than one-half Fe content. (I) 0.04--{).07%Be. (m) 0.10--{).30%Be. (n) 0.15--{).30% Be. (0) A36O.1,A380.1, and A413.1 ingot is used to produce 360.0 and A36O.0; 380.0 and A380.0; 413.0 and A413.0 castings, respectively. (P) 0.8% max Mg + Cr. (q)0.25% max Pb. (r)0.02--{).04% Be. (s)The number of decimal places to which Mg percent is expressed differs from the norm. (t)0.08--{).15%V. (u)408.2, 409.2,41 1.2, and 445.2 are used to coat steel. (v) Used with Zn to coat steel. (w) 0.10% max Pb. (x) 0.003--{).007%Be, 0.005% max B. (y) 0.003--{).007% Be, 0.002 max B. Source: Aluminum Association Inc.

Designation Former


w

'"

of

~

3 S· C 3

C

~

l

3 S· C 3 a

C

~

0

-

s·'"

§

n

t .......



Appendix 3

Temper Designations for Aluminum and Aluminum Alloys

THE TEMPER DESIGNATION SYSTEM used in the United States for aluminum and aluminum alloys is used for all product forms (both wrought and cast), with the exception of ingot. The system is based on the sequences of mechanical or thermal treatments, or both, used to produce the various tempers. The temper designation follows the alloy designation and is separated from it by a hyphen. Basic temper designations consist of individual capital letters. Major subdivisions of basic tempers, where required, are indicated by one or more digits following the letter. These digits designate specific sequences of treatments that produce specific combinations of characteristics in the product. Variations in treatment conditions within major subdivisions are identified by additional digits. The conditions during heat treatment (such as time, temperature, and quenching rate) used to produce a given temper in one alloy can differ from those employed to produce the same temper in another alloy.

H, Strain-Hardened (Wrought Products Only). This indicates products that have been strengthened by strain hardening, with or without supplementary thermal treatment to produce some reduction in strength. The H is always followed by two or more digits. W, Solution Heat Treated. This is an unstable temper applicable only to alloys whose strength naturally (spontaneously) changes at room temperature over a duration of months or even years after solution heat treatment. The designation is specific only when the period of natural aging is indicated (for example, W 1;2 h). T, Solution Heat Treated. This applies to alloys whose strength is stable within a few weeks of solution heat treatment. The T is always followed by one or more digits.

Basic Temper Designations

System for Strain-Hardened Products

F, As-Fabricated. This is applied to products shaped by cold working, hot working, or casting processes in which no special control over thermal conditions or strain hardening is employed. For wrought products, there are no mechanical property limits. 0, Annealed. 0 applies to wrought products that are annealed to obtain lowest-strength temper and to cast products that are annealed to improve ductility and dimensional stability. The 0 can be followed by a digit other than zero.

Temper designations for wrought products that are strengthened by strain hardening consist of an H followed by two or more digits. The first digit following the H indicates the specific sequence of basic operations. H 1, Strain-Hardened Only. This applies to products that are strain hardened to obtain the desired strength without supplementary thermal treatment. The digit following the HI indicates the degree of strain hardening.

266 / Corrosion of Aluminum and Aluminum Alloys

H2, Strain-Hardened and Partially An-

nealed. This pertains to products that are strain-hardened more than the desired final amount and then reduced in strength to the desired level by partial annealing. For alloys that age soften at room temperature, each H2x temper has the same minimumultimate tensile strength as the H3x temper with the same second digit. For other alloys, each H2x temper has the same minimum ultimate tensile strength as the Hlx with the same second digit, and slightlyhigherelongation.The digit followingthe H2 indicatesthe degreeof strain hardening remaining after the product has been partiallyannealed. H3, Strain-Hardened and Stabilized. This applies to products that are strain-hardened and whose mechanicalpropertiesare stabilizedby a low-temperature thermal treatmentor as a result of heat introduced during fabrication. Stabilizationusually improvesductility. This designation applies only to those alloys that, unless stabilized, gradually age soften at room temperature. The digit following the H3 indicates the degree of strain hardening remaining after stabilization. H4, Strain-Hardened and Lacquered or Painted. This applies to products that are strain-hardened and that are also subjected to some thermal operation during subsequent painting or lacquering. The number followingthis designationindicatesthe degree of strain-hardening remaining after the product has been thermally treated as part of the painting/lacquering cure operation. The corresponding H2x or H3x mechanicalpropertylimits apply. Additional Temper Designations. The digit following the designation HI, H2, H3, and H4 indicates the degree of strain-hardening as identified by the minimum value of the ultimate tensile strength. The numeral 8 has been assigned to the hardest tempers normally produced. The minimum tensile strength of tempers Hx8 can be determined from Table I and is based on the minimum tensile strength of the alloy (given in ksi units) in the annealed temper. However, temper registrations prior to 1992 that do not conform to the requirements of Table I shall not be revisedand registrations of intermediate or modified tempers for such alloy/tempersystems shall conform to the registration requirements that existedprior to 1992. Tempers between 0 (annealed) and Hx8 are designated by numerals I through 7 as follows:

• Numeral 4 designates tempers whose ultimate tensile strength is approximately midway between thatof the o temper andthatof the Hx8 tempers. • Numeral2 designates tempers whose ultimate tensile strengthis approximately midwaybetweenthat of the 0 temper and that of the Hx4 tempers. • Numeral 6 designates tempers whose ultimate tensile strengthis approximately midwaybetween that of the Hx4 tempers and that of the Hx8 tempers. • Numerals 1,3, 5, and 7 designate, similarly, tempers intennediate betweenthosedefined above.

• Numeral9 designatestemperswhose minimum ultimate tensile strengthexceeds that of the Hx8 tempers by 2 ksi or more. The ultimate tensile strength of intermediate tempers, determined as described above,when not endingin 0 or 5, shallbe rounded to the next higher0 or 5. When it is desirable to identify a variation of a two-digit H temper, a third digit (from I to 9) can be assigned. The third digit is used when the degree of controlof temper or the mechanicalproperties are different from but close to those for the two-digitH temper designation to which it is added, or when some other characteristic is significantly affected. The minimum ultimatetensilestrengthof a three-digitH temper is at least as close to that of the corresponding twodigit H temper as it is to either of the adjacent twodigit H tempers. Products in H tempers whose mechanicalpropertiesare below those ofHxI tempers are assignedvariations of Hx I. Some three-digitH temper designations have already been assigned for wrought productsin all alloys: Hx 11 appliesto products that incur sufficientstrain hardening after final annealing to fail to qualify as o temper, but not so much or so consistent an amount of strain hardening to qualify as Hxl tem-

•

per. • Hl12 pertains to products that can acquire some strain hardening during working at elevated temperature and for which there are mechanicalproperty limits. • H temper designations assignedto patternedor embossed sheet are listed in Table 2.

System for Heat Treatable Alloys The temperdesignationsystem for wroughtand cast products that are strengthened by heat treatment employs the W and T designations described in ~e s~ tion "Basic Temper Designations." The W designation denotes an unstable temper, whereas the T designation denotes a stable temperother than F, 0, or H. The T is Table 1 Minimum tensilerequirements for the Hx8tempers Minimumtensile strength in annealed temper, ksi ~6

7-9 10-12 13--15 16-18 19-24 25--30 31-36

37-42 ;"43 Source: ANSIH35.1-1997

locreasein tensilestrength to Hx8temper, ksi

8 9 10 11

12 13 14 15 16 17

Appendix 3 I 267 followed by a number from I to 10, each number indicating a specific sequence of basic treatments.

treatment and for which mechanical properties have been stabilized by room-temperature aging. It also applies to products in which the effects of cold work, imparted by flattening or straightening, are accounted for in specified property limits.

plies to products that are not cold worked after an elevated-temperature shaping process such as casting or extrusion and for which mechanical properties have been stabilized by room-temperature aging. It also applies to products that are flattened or straightened after cooling from the shaping process, for which the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.

T4, Solution Heat Treated and Naturally Aged to a Substantially Stable Condition. This

T2, Cooled from an Elevated-Temperature Shaping Process, Cold Worked, and Naturally Aged to a Substantially Stable Condition. This

TS, Cooled from an Elevated-Temperature Shaping Process and Artificially Aged. T5 in-

n, Cooled from an Elevated-Temperature Shaping Process and Naturally Aged to a Substantially Stable Condition. This designation ap-

variation refers to products that are cold worked specifically to improve strength after cooling from a hotworking process (such as rolling or extrusion) and for which mechanical properties have been stabilized by room-temperature aging. It also applies to products in which the effects of cold work, imparted by flattening or straightening, are accounted for in specified property limits.

T3, Solution Heat Treated, ColdWorked, and Naturally Aged to a Substantially Stable Condition. 1'3 applies to products that are cold worked specifically to improve strength after solution heat

Table 2 Htemper designations for aluminum and aluminum alloy pattemed or embossed sheet P_medor embossed sheet

H114 H124 H224 H324 H134 H234 H334 H144 H244 H344 H154 H254 H354 H164 H264 H364 H174 H274 H374 H184 H284 H384 H194 H294 H394 H195 H295 H395 Source: ANSI H35.l-1997

Temper ofsheet from which textured sheet was fabricated

o Hil H21 H31 H12 H22 H32 HI3 H23 H33 Hl4 H24 H34 HI5 H25 H35 H16 H26 H36 HI7 H27 H37 HI8 H28 H38 HI9 H29 H39

signifies products that are not cold worked after solution heat treatment and for which mechanical properties have been stabilized by room-temperature aging. If the products are flattened or straightened, the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.

cludes products that are not cold worked after an elevated-temperature shaping process such as casting or extrusion and for which mechanical properties have been substantially improved by precipitation heat treatment. If the products are flattened or straightened after cooling from the shaping process, the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.

T6, Solution Heat Treated and Artificially Aged. This group encompasses products that are not cold worked after solution heat treatment and for which mechanical properties or dimensional stability, or both, have been substantially improved by precipitation heat treatment. If the products are flattened or straightened, the effects of the cold work imparted by flattening or straightening are not accounted for in specified property limits.

T7, Solution Heat Treated and Overaged or Stabilized. 17 applies to wrought products that have been precipitation heat treated beyond the point of maximum strength to provide some special characteristic, such as enhanced resistance to stress-corrosion cracking or exfoliation corrosion. It applies to cast products that are artificially aged after solution heat treatment to provide dimensional and strength stability.

T8, Solution Heat Treated, Cold Worked, and Artificially Aged. This designation applies to products that are cold worked specifically to improve strength after solution heat treatment and for which mechanical properties or dimensional stability, or both, have been substantially improved by precipitation heat treatment. The effects of cold work, including any cold work imparted by flattening or straightening, are accounted for in specified property limits.

T9, Solution Heat Treated, Artificially Aged, and Cold Worked. This grouping is comprised of products that are cold worked specifically to improve strength after they have been precipitation heat treated.

no, Cooled from an Elevated-Temperature Shaping Process, Cold Worked, and Artificially Aged. Tl 0 identifies products that are cold worked specifically to improved strength after cooling from a hot-working process such as rolling or extrusion and for which mechanical properties have been substan-

268 I Corrosion of Aluminum and Aluminum Alloys tially improved by precipitation heat treatment. The effectsof cold work,including anycold workimparted by flattening or straightening, are accounted for in specified propertylimits. Additional T Temper Variations. When it is desirableto identifya variation of one of the ten majorT tempers described above, additional digits, the first of whichcannotbe zero,can be addedto the designation. Specific setsof additional digitshave been assigned to the following wrought products that have been stress relievedby stretching, compressing, or a combination of stretching and compressing: Product form Plate

Rolledor cold-finishedrod and bar Extrudedrod, bar,profiles(shapes),and tube Drawn tube Dieor ringforgingsand rolledrings

Permanentset, %

IYz-3

1-3 1-3 Yz--3

1-5

Stress relieved by stretching includes the following. Tx51 applies specifically to plate, to rolled or coldfinished rod and bar, to die or ring forgings, and to rolled rings when stretched to the indicated amounts after solution heat treatment or after cooling from an elevated-temperature shapingprocess. These products receiveno furtherstraightening afterstretching. Tx5lOappliesto extrudedrod, bar, shapes, and tubing, and to drawn tubing when stretched to the indicated amounts after solution heat treatment or after coolingfroman elevated-temperature shapingprocess. Products in this temperreceiveno furtherstraightening after stretching. Tx511 applies to extruded rod, bar, profiles (shapes), and tube and to drawntube whenstretched to the indicated amounts after solution heat treatment or after cooling from an elevated temperature shaping process.These products can receive minor straightening after stretching to complywithstandard tolerances. Stress relieved by compressing includes the following.

Tx52 applies to products that are stress relieved by compressing after solutionheattreatment or aftercooling from a hot-working process to produce a permanent set of I to 5%. Stress relieved by combined stretching and compressing includes the following. Tx54 applies to die forgings that are stress relieved by restriking cold in the finishdie. Solution HeatTreated from 0 or F Temper. Temper designations have been assigned to wroughtproducts heat treatedfromthe 0 or the F temper to demonstrate response to heattreatment. T42 meanssolutionheat treatedfrom the 0 or the F temperto demonstrate response to heat treatment and naturally agedto a substantially stablecondition. T62 meanssolution heat treatedfrom the 0 or the F temper to demonstrate response to heat treatment and artificially aged. TIx2 means solution heat treated from the 0 or F temperand artificially overagedto meet the mechanical properties and corrosion resistance limits of the TIxtemper. Temper designations T42 and T62 also can be applied to wroughtproducts heat treated from any temper by the user when such heat treatment resultsin the mechanical properties applicable to these tempers.

System for Annealed Products A digit following the 0 indicates a product in annealedcondition havingspecialcharacteristics. For example, for heat treatable alloys,OJ indicates a product that has been heat treated at approximately the same time and temperature required for solutionheat treatment and then air cooled to room temperature; this designation applies to products that are to be machined prior to solution heat treatment by the user. Mechanical propertylimitsare not applicable.

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