B0746 Corrosion of reinforcement in concrete (EFC 31) (matsci)

European Federation of Corrosion Publications NUMBER 31

Corrosion of Reinforcement in Concrete Corrosion Mechanisms and Corrosion Protection Papersfrom EUROCORR '99

Edited by J. MIETZ,R. POLDER AND B.

ELSENER

Published for the European Federation of Corrosion by IOM Communications

Book Number 746 Published in 2000 by IOM Communications Ltd 1 Carlton House Terrace, London SWlY 5DB

IOM Communications Ltd is a wholly-owned subsidiary of The Institute of Materials

02000 IOM Communications Ltd All rights reserved

ISBN 1-86125-133-5

Neither the EFC nor The Institute of Materials is responsible for any views expressed in this publication

Typesetting by spiresdesign

Made and printed in Great Britain

Reinforced concrete is such a widely used construction material that in most of the world the cost of damage and repair is an important issue despite the fact that the majority of structures are undamaged and have a long service life. In fact, only a small fraction of the total stock is responsible for most of the problems with reinforcement corrosion as the main cause. Owners, engineers and researchers are trying to find solutions to the problems that are involved. Although the most important factors in the corrosion of reinforcing steel are well known necessary work continues in the understanding of rate controlling mechanisms, in developing test methods to assess the severity of existing corrosion and in producing computer based diagnostic systems. Various methods to protect structures have been applied for many years and new ideas have been introduced in recent decades. Cathodic protection of reinforcement is a successful method to stop ongoing corrosion and this application is being extended to new types of structures, to the use of new materials and to links with conventional repairs. Other new technologies in the field are the use of stainless steel, inhibitors and water repellent treatments for improved service life. This volume in the EFC series brings together the full papers presented in the successfulsession ”Corrosion of Steel in Concrete’’ at E UROCORR ’99 held at Aachen, Germany. Thirteen papers were accepted after peer review, and included contributions from The Czech Republic, Denmark, Germany, Italy, The Netherlands, Norway, Poland and Switzerland. The papers are grouped under two headings: Corrosion Mechanisms and Corrosion Measurements Corrosion Protection of Reinforced Concrete Structures

We thank all authors who were willing to share their valuable ,.nowlecge with others and who participated in discussions. The editors hope these papers will encourage readers to apply the ideas and results that were presented to their own problems. The work reported in this volume can be read in conjunction with other EFC publications from this Working Party, for example: EFC 18 Stainless Steels in Concrete: State of the Art Report EFC 24 ElectrochemicalRehabilitationMethods for Reinforced Concrete Structures: State of the Art Report EFC 25 Corrosion of Reinforcement in Concrete: Monitoring, Prevention and Rehabilitation and in conjunction with further volumes in preparation on the subjects of inhibitors for use in concrete, and embeddable reference electrodes for concrete. J. MIETZ Chairman of the EFC Working Party on Corrosion in Concrete R. POLDER B. ELSENER

European Federation of Corrosion Publications Series Introduction

The EFC, incorporated in Belgium, was founded in 1955 with the purpose of promoting European co-operation in the fields of research into corrosion and corrosion prevention. Membership is based upon participation by corrosion societies and committees in technical Working Parties. Member societies appoint delegates to Working Parties, whose membership is expanded by personal corresponding membership. The activities of the Working Parties cover corrosion topics associated with inhibition, education, reinforcement in concrete, microbial effects, hot gases and combustion products, environment sensitive fracture, marine environments, surface science, physico-chemical methods of measurement, the nuclear industry, computer based information systems, the oil and gas industry, the petrochemical industry, coatings, automotive engineering and cathodic protection. Working Parties on other topics are established as required. The Working Parties function invarious ways, e.g.by preparing reports, organising symposia, conducting intensive courses and producing instructional material, including films. The activities of the Working Parties are co-ordinated, through a Science and Technology Advisory Committee, by the Scientific Secretary. The administration of the EFC is handled by three Secretariats: DECHEMA e. V. in Germany, the Sociktk de Chimie Industrielle in France, and The Institute of Materials in the United Kingdom. These three Secretariats meet at the Board of Administrators of the EFC. There is an annual General Assembly at which delegates from all member societies meet to determine and approve EFC policy. News of EFC activities, forthcoming conferences, courses etc. is published in a range of accredited corrosion and certain other journals throughout Europe. More detailed descriptions of activities are given in a Newsletter prepared by the Scientific Secretary. The output of the EFC takes various forms. Papers on particular topics, for example, reviews or results of experimental work, may be published in scientific and technical journals in one or more countries in Europe. Conference proceedings are often published by the organisation responsible for the conference. In 1987 the, then, Institute of Metals was appointed as the official EFC publisher. Although the arrangement is non-exclusive and other routes for publication are still available, it is expected that the Working Parties of the EFC will use The Institute of Materials for publication of reports, proceedings etc. wherever possible. The name of The Institute of Metals was changed to The Institute of Materials with effect from 1January 1992. The EFC Series is now published by the wholly-owned subsidiary of The Institute of Materials, IOM Communications Ltd.

A. D. Mercer EFC Series Editor, The Institute of Materials, London, UK

. I .

Vlll

Series Introduction

EFC Secretariats are located at: Dr B A Rickinson European Federation of Corrosion, The Institute of Materials, 1 Carlton House Terrace, London, SWlY 5DB, UK Mr P Berge Federation Europeene de la Corrosion, Societe de Chimie Industrielle, 28 rue SaintDominique, F-75007 Paris, FRANCE Professor Dr G Kreysa Europaische Foderation Korrosion, DECHEMA e. V., Theodor-Heuss-Allee 25, D-60486, Frankfurt, GERMANY

Contents

Series Introduction

uii

Preface

ix

Foreword

X

Part I - Corrosion Mechanisms and Corrosion Measurements

1

1.Oxygen Reduction on Mild Steel and Stainless Steel in Alkaline Solutions S. JAGGI, B. ELSENERA N D H . BOHNI

3

2. Investigations on Cathodic Control of Chloride-Induced Reinforcement Corrosion M. RAUPACH A N D J. GULIKERS

13

3. Critical Factors for the Initiation of Rebar Corrosion L. ZIMMERMANN, B. ELSENERAND H.BOHNI

25

4. Field Tests of Chloride Penetration into Concrete with Microsilica 0. VENNESLAND A N D J. HAVDAHL

35

5. Comparison of Electrochemical Data and Mass Loss Corrosion Rate Measurements for Steel Reinforcement in Concrete P. NOVAKAND R. MALA

41

Part 2 - Corrosion Protection of Reinforced Concrete Structures

49

6. Corrosion and Protection in Reinforced Concrete: A Computerised System for Studying its Phenomenology, Causes, Diagnosis and Remedies P.PEDEFERRI

51

vi

Contents

7. Organic Corrosion Inhibitors for Steel in Concrete B. ELSENER, M. BUCHLERA N D H.BOHNI

61

8. Corrosion Protection of Reinforcement by Hydrophobic Treatment of Concrete R. B. POLDER,H . B O R S ~AEN D J , DE VRIES

73

9. Cathodic Protection of Concrete Ground Floor Elements with Mixed-in Chloride G. SCHUTEN, J. LEGGEDOOR A N D R. B. POLDER

85

10. SacrificialAnodes for Cathodic Prevention of Reinforcing Steel Around Patch Repairs Applied to Chloride-Contaminated Concrete G. SERGIA N D C . L. PAGE

93

11.Layer Zinc Anodes in Cathodic Protection of Steel Reinforcement W. BOHDANOWICZ

101

12. Lifetime Extension of Thermally Sprayed Zinc Anodes for Corrosion Protection of Reinforced Concrete Structures by Using Organic Top-coatings I. SPRIESTERSBACH, A. MELZER, 1. W I S N I E W S K I , A. WINKELS A N D M.KNEPPER

109

13. Practical and Economical Aspects of Application of Austenitic Stainless Steel, AIS1 316, as Reinforcement in Concrete 0.KLINGHOFFER, T. FRaLUND, B. KOFOED,A. KNUDSEN,F.M.J E N S E N A N D T. SKOVSGAARD

121

List of Abbreviations

135

Index

137

Part 1

Corrosion Mechanisms and Lorrosion

Measurements

1

Oxygen Reduction on Mild Steel and Stainless Steel in Alkaline Solutions S. JAGGI, B. ELSENER and H. BOHNI Institute of Materials Chemistry and Corrosion, Department of Civil Engineering Swiss Federal Institute of Technology, ETH Honggerberg, CH-8093 Zurich, Switzerland

ABSTRACT The cathodic polarisation curve of steel in alkaline solutions always shows three regions: (1)oxygen reduction with a Tafel behaviour at potentials cathodic to the open circuit potential followed by (2) a diffusion limited current of oxygen reduction at more negative potentials and (3)hydrogen evolution at very negative potentials. The diffusion limited region of the cathodic current density is controlled both by the oxygen concentration in solution and the flow rate whereas in the Tafel region (charge transfer) the temperature and the pretreatment of the sample determine the intensity of the current density and the slope of the Tafel line. On stainless steels the cathodic reduction currents are lower then on mild steel. It can be concluded that under usual corrosion conditions for steel in concrete the cathodic oxygen reduction is not diffusion limited but charge transfer controlled.

1. Introduction The corrosion reaction, i.e. the anodic dissolution of steel in concrete, has to be sustained by a corresponding cathodic reaction: in general, this is the reaction of oxygen with water producing hydroxyl ions: 0, + 2H,O

+ 4e- -+ 40H-

(1)

The availability of oxygen at the steel surface and the reaction kinetics of oxygen reduction are thus key factors in the corrosion of steel in concrete. Quite often the reduction of oxygen at the steel surface in alkaline environments is called diffusion limited although only a few papers report results on kinetics and mechanism of oxygen reduction on passive iron or steel in alkaline solutions [1,2]. The influence of oxygen on corrosion of steel in concrete has been studied [3-51. In this work, the influence of oxygen content, temperature and ageing of the passive film on the oxygen reduction reaction on normal and stainless steel in alkaline solutions has been studied. The results are discussed with respect to the mechanism of oxygen reduction and the importance for corrosion of steel in concrete.

4

Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection

2. Experimental Potentiodynamic polarisation curves (scan rate 1 mVs-l) were recorded in an electrochemical flow cell (Fig.1) at defined temperatures and hydrodynamic conditions. The flow velocity was regulated by the flux of solution, 1 mLs' corresponds to a flow velocity of ca. 1.4 mms-I. The counter electrode was a platinum wire spiral and the reference electrode was a saturated calomel electrode. The materials tested (working electrode) were mild steel and DIN 1.4301 stainless steel cylinders with a diameter of 8 mm, embedded in resin and mounted in the flow cell. For each experiment the samples were freshly ground with 180 grit emery paper in water, cleaned with ethanol in an ultrasonic bath, rinsed with deionised water and immersed for 24 h in the alkaline test solution open to air to form the passive film. As electrolytes 0 . 1 NaOH ~ and synthetic pore solution (Table 1) were used. The temperature (5-47OC), oxygen concentration (open to air or saturated) and flow velocity (stagnant or 1mLs-l) were varied; in addition, experiments with prolonged immersion times (ageing of the passive film up to 4 months) of the samples were performed.

Fig. 2 Schematic representation of the electrochemical pow-cell that allows the registration of cathodic polarisation curves under controlled temperature, potential and oxygen content.

Oxygen Reduction on Mild Steel and Stainless Steel in Alkaline Solutions

5

Table 1. Composition of the Synthetic concrete pore solution Concrete pore solution

Ca(OH),

KOH

Na,SO,

NaOH

mgL-'

9.6

13967.8

3121.6

616.8

3. Results The cathodic polarisation curves, starting from the open circuit potential, in solutions ~ with different oxygen contents (open to air, saturated) are shown in of 0 . 1 NaOH Fig. 2(a). The diffusion limited current density increases by a factor of about two in 0, saturated conditions. The slope of the Tafel region of the curve remains constant at ca. 250 mV per decade, although the higher oxygen content results in a slightly higher cathodic current density in the Tafel region. The influence of flow velocity at constant oxygen content (solution open to air) is shown in Fig. 2(b).As expected, the diffusion limited current density, io Dl increases with higher flow velocity, the current densities of oxygen reduction in tAe Tafel region of the polarisation curve are not influenced by the flow velocity. The influence of temperature on the cathodic polarisation curves of oxygen reduction was studied in more detail in synthetic pore solution, with solutions open to air and flow velocity 0.1 mLs-l. As is shown in Fig. 3, the diffusion limited current

6

Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection

-1400 -1200 -1000 -800 -600 -400 -200

0

Potential (mV, SCE) Fig, 2(b)Typical catkodicpolarisation curves of mild steel in 0 . 1 NaOHsolufions ~ as a function of solutionflow rate. Solutions open to air.

Potential (mV, SCE) Fig. 3 Catkodicpolarisation curves of mild steel in synthetic pore solution (pH 13.4) a t different temperatures, flowrate 1 rnLs-l, open to air.

Oxygen Reduction on Mild Steel and Stainless Steel in Alkaline Solutions

7

density, ioZD,remains practically constant. The Tafel region of the polarisation curve is slightly shifted to more negative potentials, the slope of the Tafel region increases slightly with increasing temperature (Table 2). Prolonged exposure to the test solution (ageing of the passive film) showed the most pronounced effect on the cathodic polarisation curves (Fig. 4). An increase of this pre-passivation time results in a marked increase of the slope of the Tafel region, the diffusion limited current density remaining unchanged. Finally, as can be seen from Fig. 5, the electrode material influences the cathodic polarisation curve: with the same surface preparation the cathodic current density of DIN 1.4301 stainless steel is about 4 times lower compared to mild steel. Platinum shows practically uninhibited oxygen reduction with a Tafel slope of -60 mV.

4. Discussion 4.1. Kinetics of the Cathodic Oxygen Reduction

As it is well known from literature, cathodic oxygen reduction (e.g. Fig. 2) shows a Tafel line at low overpotentials and a diffusion limiting current density at high overpotentials (more negative potentials). The thermodynamic equilibrium potential E corresponding to eqn (1)depends, according to the Nernst equation, 02

E

02

= +1.27-2.3RT/F pH+2.3RT/4F logp

02

(2)

on pH, oxygen content and temperature of the solution and E = +0.23 V SCE 02 would result theoretically. On platinum, a potential E of -0.05 V SCE was 02 measured. On passive steel in alkaline solutions, the equilibrium potential E 02 cannot be measured; since due to the interaction with the anodic dissolution of the passive film mixed open circuit potentials around -0.2 V SCE are found which influenced by the magnitude of the cathodic current densities and the passive film dissolution rates (e.g. Fig. 5). Table 2. Cathodic Tafel slopes b,, charge transfer coefficient a and exchange current densities for the oxygen reduction on passive mild steel in synthetic pore solutions at different temperatures

8

Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection

-1400

-1200

-1000

-800

-600

-400

-200

0

Potential (mV, SCE) Fig. 4 Influence of the ageing time of the passivefilm in synthetic pore solution on cathodic polarisation curves (flow rate 1 mLs-I, open to air).

1l000.00

100.00

10.00

1.oo

0.10

0.01 -1400

-1200

-1000

-800

-600

-400

-200

Potential (mV, SCE) Fig.5 Cathodic polarisation curves ofmild steel, 1.4301 stainless steel and platinum in synthetic pore solution atflow velocity of 1 mLs-I compared to results of Pedeferri 161.

0

Oxygen Reduction on Mild Steel and Stainless Steel in Alkaline Solutions

9

On increasing the overpotential in the cathodic direction a Tafel behaviour is found corresponding to the charge transfer controlled region of oxygen reduction. In the experiments on passive metals, this Tafel line could be observed only at E

0

E -300

u

0

-400

10'

a

-500 1oo

-600 0

10

20

30

40

50

60

70

80

t [days] Fig. 3 Repeated experiments of corrosion potential and polaristion resistance of rebars in satd Ca(OH), containing 10% inhibitor. After 50 days the cell was opened by removing the plug in the couer (MCI 2000 is the commercial amine-based inhibitors).

n

Organic Corrosion Inhibitors for Steel in Concrete

65

100

80 60 40

-+ Series -1 --B-

20

Series 2 Series 3 Series 4

+

*

0 0

50

100

150

200

250

300

350

t [days1 Fig. 4 Percentage of corroding samples plotted against the time of cyclic wetldry treatment. Inhibitor content: series 1, 0; series 2, 0.35 kgm”; series 3, 1.75 kgm” (recommended dosage); series 4, 8.75 kgm”.

the inhibitor-free-samples (Fig. 4). No significant differences between the different inhibitor contents can be observed. This result from electrochemical measurements was confirmed by measuring the mass loss of all the samples studied [ l o ] . Samples with active chloride-induced corrosion in saturated. Ca(OH), + IMNaCl solutions showed an increase of the polarisation resistance by a factor 3-4 after addition of 10% of inhibitor blend (Fig. 5). The effect of adding inhibitor to chloridecontaining mortar samples with corroding steel bars (Fig. 6), simulating a restoration treatment with surface applied inhibitor was to show a slight increase of the open circuit potential but no significant change in polarisation resistance.

4. Discussion 4.1. Corrosion Initiation

When sufficiently high concentrations are present at the steel surface, the organic inhibitor blend demonstrates a marked effect on pit initiation in the solutions containing 1~NaC1: no pitting corrosion occurs at an inhibitor content of 10% but at

--*

:

'

- -

P

1

/

9-

'

iii

> -300

-Ea

1 o4

2

-200 -

I

1

-a3

4

0

Y

'=C

-

:

-400 -

103 Y

a, 0

a

-500 -

-600

~

'

"

'

"

"

'

"

"

'

"

"

'

"

"

'

'

~

'

~

'

'

Id

1 o5

-0.2

-0.3

-.-

P

-0.4

U

CI

3

9

io4

(d

c

dJ

C

a -0.5

c

0

a

-0.6

o3

-0.7

1

0 Fig. 6 Polarisation resistance and corrosion potential of steel in mortar samples. Time period 1: cyclic treatment with I M NaCl solution; Time period 2: 80% humidity. After 12 days the samples were dried and soaked for 24 h in pure inhibitor.

Organic Corrosion Inhibitorsfor Steel in Concrete

67

concentrationsof 1%pitting corrosion is initiated similar to the non-inhibited solution. For the inhibition of pit initiation, both constituents of the inhibitor blend have to be present on the steel surface, as has been shown by the separate investigation of the two fractions of the inhibitor [ l o ] . Despite the comparatively high concentration of 9.576, the volatile fraction (mainly dimethylethanolamine) cannot prevent the onset of pitting corrosion. This result is in agreement with the relatively poor inhibiting properties reported for the p u r e dimethylethanolamine [7,11] and surface analytical studies, where no specific adsorption of dimethylethanolamine on passive steel in alkaline solution was found [12,13]. The presence of the non-volatile fraction is thus crucial for the observed inhibiting effect of the inhibitor blend, but this fraction alone also cannot prevent steel from the initiation of pitting corrosion. Evaporation of the volatile fraction (after opening of the cell) results in a decrease of the dimethylethanolamine concentration in solution and chloride-induced corrosion that starts (Fig. 3). Hence, for the inhibition of the corrosion initiation in chloridecontaining solutions a sufficiently high concentration and the presence of both inhibitor fractions are required. Experiments in mortar with inhibitor demonstrated clearly (Fig. 4) that the corrosion initiation is delayed. However, unlike the experiments in solutions, initiation of pitting corrosion cannot be prevented. The evaluation of the chloride content yielded comparable values for all sample series after 380 days of cyclic treatment in chloride solution. Thus, the inhibitor blend does not influence the chloride transport and the observed delaying effect on the initiation of pitting corrosion is caused by its inhibiting properties. 4.2. Corrosion Propagation

Besides the inhibition of the corrosion initiation, corrosion inhibitors can also influence the corrosion rate after the onset of pitting. In the present work the corrosion rate was obtained by determination of the polarisation resistance. The use of these values for absolute comparison can be critical, as the area of the active corrosion site is not known. The polarisation resistance is normalised with the whole sample surface, which is of course mainly passive. Therefore, differences in the polarisation resistance could be exclusively due to a different pit size instead of a different propagation rate of the pit. Nevertheless, the obtained polarisation resistances are useful for a qualitative comparison of the effect of the corrosion inhibitor. In chloride-containing solutions the inhibitor indeed has an influence on the corrosion rate after the initiation of pitting. According to Table 1 a concentration of 1%inhibitor cannot prevent the initiation of pitting corrosion, but the polarisation resistance is about four times higher, indicating a lower corrosion rate. Contrary to the experiments in solution, no effect of the inhibitor on the corrosion rate was found in mortar experiments. This result was obtained by polarisation resistance measurements and was confirmed by determination of the mass loss of the rebar [lo]. Hence, the inhibitor added to the mortar mix does not affect the corrosion rate in mortar after initiation. For application on new structures it can thus be concluded that the inhibiting effect is limited only to some retardation of the corrosion initiation.

68 Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection Table 1. Polarisation resistance of the samples immersed in Ca(OH), solution with different content of inhibitor after the addition of chlorides Mass% inhibitor

R, (kRcm2)

10

490+ EO

0.1

2+1

10

3+1

5. Pre-corroded Samples No reduction of the corrosion rate was found when applying the inhibitor blend on chloride-containingmortar samples where pitting corrosion of the rebars was ongoing (Fig. 6), despite a significant reduction found in solution (Fig. 5) and a high diffusion rate reported for the inhibitor [14,17].A similar result was reported in recent work of Page and Ngala [a] which studied another proprietary, alkanolamine-based blended inhibitor known to contain ethanolamine and an inorganic phosphate. Repeated ponding and drying according to the manufacturer’s dosage caused only a modest reduction in the corrosion rates of pre-corroded steel bars embedded at 12 mm depth in concrete with 0.65 w / c and with low to modest levels of chloride contamination and was apparently ineffective in cases of high chloride content (2.4%by mass of cement). A possible explanation of this discrepancy may be that the blended inhibitor studied in this work may have fractionated and only the volatile part of the inhibitor (hydroxyalkylamine) shows a high diffusion rate and reaches the steel surface [lo]. The same inhibitor as studied in [a] was included in a field test with surface applied inhibitors on chloride-contaminated structures and no reduction in the corrosion rate (macrocell current) was found [la].

6. Inhibitor Concentration The inhibition or retardation of the initiation of pitting corrosion requires a comparatively high concentration of 10%. Typical amine concentrations for the inhibition of uniform corrosion in acidic solutions are in the region of lo4 mol L-*. This contrast is a result of the completely different corrosion mechanisms which are taking place for example, in acidic solutions where the bare metal surface is in contact with the electrolyte and the adsorption of specific inhibitor molecules on the metal can result in a strong decrease of the corrosion rate. In alkaline solutions iron is protected against uniform corrosion by the passive film. Corrosion is initiated locally in the presence of chlorides leading to a heterogeneous system with separated anodes and cathodes, adverse mass transport conditions in the pit, and migration of ions. It is beyond the scope of the present work to discuss the detailed mechanisms, but it is

Organic Corrosion Inhibitors for Steel in Concrete

69

a generally observed phenomenon that inhibitors of pitting corrosion require high concentrations. For nitrite [ 151 and monofluorophosphate (MFP) [16] the ratio between inhibitor and chloride concentration is reported to be in the order of 1. Considering the molar weight of the dimethylethanolamine of 89.14 gmol-* and the chloride concentration of 1 mol L-I used in this work, a ratio of inhibitor/Cl- of about 1is also obtained.

7. Stability and Long-term Efficiency The efficiency of the inhibitor blend was investigated in accelerated tests to obtain information within a reasonable time scale. The acceleration was obtained by a strongly enhanced penetration of chlorides due to the cyclic treatment. However, the obtained results are only correct when no other relevant time-dependent processes are taking place or when they are accelerated in the same way. Surface application of organic inhibitor on mortar samples with corroding steel did not show any reduction in the corrosion rate (Fig. 6) in agreement with long term laboratory studies [8] and with field tests on chloride-contaminated structures [ 181. On considering the loss of inhibiting effect observed in experiments performed in solutions (Fig.3) when evaporation of the inhibitor occurs, the question arises whether this process might have an influence on the long-term performance of the commercial inhibitor blend when admixed in mortar or concrete structures. The fast diffusion of the hydroxyalkylamine through the concrete has been demonstrated [14,171. The diffusion direction follows the concentration gradient of the hydroxyalkylamine. As the hydroxyalkylamine concentration can be expected to be very low on the concrete surface, it can be postulated that the substance should evaporate from concrete structures, which must cause a decrease of concentration over time. Measuring the amine concentration in airtight compartments containing a mortar sample of the series 2 and 3 respectively after 350 days cyclic treatment showed that the amine concentration (determined qualitatively with an amine electrode (Orion; model 9512)) in the air surrounding the samples is increasing with time (Fig. 7). Additionally the evaporation rate is higher for samples with higher inhibitor concentrations which is in agreement with the expected steeper concentration gradient. Hence, the main component of the inhibitor is leaving the concrete structure over time and the question arises whether the observed retardation of the corrosion initiation will still occur under realistic conditions (after several years of service).

8. Conclusions Experiments in solutions are useful for the characterisation of the inhibitor efficiency and the determination of influencing parameters. However, experiments in mortar are more severe and thus necessary to obtain results relevant for practical applications. In chloride-containing alkaline solution, the inhibitor can prevent steel from corrosion initiation at sufficiently high concentrations (10%). At lower

70 Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection

0.7~1 1

1

'

1

I

I

8

I

I

I

I

t

1

,

- 1

0.6 0.5

0.4 0.3 0.2

0.1

0 0

10

5

15

t ways1 Fig. 7 Relative amine concentration of the air in a closed compartment containing mortar samples with admixed inhibitor.

concentrations pitting corrosion initiated, but the corrosion rate is lowered by the presence of the inhibitor. Contrary to the experiments in solution, in mortar neither complete inhibition of the initiation of pitting corrosion nor an influence on the corrosion rate is obtained. Nevertheless, the inhibitor can delay the corrosion initiation. The inhibitor reduces the corrosion rate of pre-corroded samples in chloridecontaining alkaline solutions, no reduction in corrosion rate was found when the inhibitor was surface-applied on mortar samples. Recent field tests on chloride-contaminated structures gave the same result. The volatile constituent of the inhibitor was found to evaporate from solutions and from mortar with a consequent loss in inhibiting properties. The long term efficiency of the admixed inhibitor in field application is questionable.

8. Acknowledgements The authors are pleased to acknowledge the financial support of Holderchem for this project and Prof. Chris Page for making available his paper [8] in advance.

Organic Corrosion Inhibitors for Steel in Concrete

71

References 1. B. Elsener, Corrosion Inhibitors for Steel in Concrete - European Federation of Corrosion State of the Art report, to appear in the EFC series published by The Institute of Materials, London. 2. Y. I. Kusnetsov, Organic Inhibitors of Corrosion of Metals. Plenum, New York, 1996. 3. B. Elsener, M. Buchler and H. Bohni, Corrosion of Reinforcement in Concrete - Monitoring, Prevention and Rehabilitation (J. Mietz, B. Elsener and R. Polder, eds). Publication No. 25 in European Federation of Corrosion series published by The Institute of Materials, London, 1998, pp.154-69. 4. A. Eydelnant, B. Miksic and L. Gelner, ConChem-I., 1993, 2,38. 5 . U. Mader, in Proc. Conf. on Corrosion and Corrosion Protection of Steel in Concrete, Sheffield UK, 1994, Vol. 2, p.851. 6. Concrete Bridge Protection and Rehabilitation: Corrosion Inhibitors and Polymers, SHRP Report S-666, National Research Council Washington DC, 1993. 7. N. S. Berke, M. C. Hicks and P. G. Tourney, in 12th Int. Corros. Congr., Houston, Tx, USA, 1993,5, p. 3271. 8. C. L. Page and V. T. Ngala, Corrosion inhibitors in concrete repair systems, Mag. Concr. Res., 2000,52,25-37. 9. B. Elsener, H. Wojtas and H. Bohni, in Corrosion and Corrosion Protection of Steel in Concrete, (R.N. Swamy, Ed.). Sheffield Academic Press, 1994, 1,p.236. 10. B. Elsener, M. Buchler, F. Stalder and H. Bohni, A migrating corrosion inhibitor blend for reinforced concrete: Part I, Prevention of corrosion, Corrosion, 1999, 55, 1155. 11.A. Phanasgaonkar, B. Cherry and M. Forsyth, in Con$ on Understanding Corrosion Mechanisms of Metals in Concrete - A Key to Improving Infrastructure Durability, MIT Boston, USA, 1997. 12. A. Rossi, B. Elsener, M. Textor and N. D. Spencer, Analusis, 1997,25, M30. 13.A. Rossi, B. Elsener, M. Textor and N. D. Spencer, XPS study of the absorption of inhibitor on iron in alkaline solutions, Proc. EUROCORR '96, Nice, 1996, Vol. 1, paper 11. 14. D. Bjegovic, L. Sipos, V. Ukrainczyk and B. Milksic, in Corrosion and Corrosion Protection of Steel in Concrete (R. N. Swamy, Ed.). Sheffield Academic Press, 1994,2 p. 865. 15. B. B. Hope and A. K. C. Ip, ACIM I., 1989,86,602. 16. C. Andrade, C. Alsono, M. Acha and B. Malric, Cem. Concr. Res., 1992, 22, 869. 17. B. Elsener, M. Buchler, F. Stadler and H. Bohni, A migrating corrosion inhibitor blend for reinforced concrete - Part 11, Inhibitor as repair strategy, Corrosion, 2000,56, 727-732. 18. Y. Schiegg, F. Hunkeler and H. Ungricht, The Effectiveness of Corrosion Inhibitors - A Field Study, submitted to IABSE Congress 'Structural Engineering for Meeting Urban Transportation Challenges', Lucerne, 18- 21 September, 2000.

8 Corrosion Protection of Reinforcement by Hydrophobic Treatment of Concrete R. B. POLDER, H. BORSJE and J. de VRIES*

TNO Building and Construction Research, P.O. Box 49 AL - 2600 AA, Delft, The Netherlands *Ministry of Transport, Civil Engineering Division, Utrecht, The Netherlands

ABSTRACT Penetration of de-icing salts into concrete bridge decks may cause corrosion of reinforcement, even with an asphalt overlay. Hydrophobic treatment of concrete was studied in the laboratory as additional protection. It was shown that hydrophobic treatment strongly reduces chloride ingress, both during semi-permanent contact and in wetting/drying situations. The protection remains effective for at least five years under full exposure to outside conditions. Carbonation of concrete is not significantly accelerated. Hydrophobic treatment does not stop corrosion if initiation by chlorides has already taken place before the treatment. Methods and criteria for testing hydrophobic products are available. Hydrophobic treatment is an effective, low cost preventative measure against corrosion of reinforcement in chloride-contaminated environment. It has become standard for all new concrete bridge decks in The Netherlands.

1. Introduction Exposure to de-icing salts of concrete bridge decks is a potential cause of damage because chloride ions may promote corrosion of steel reinforcement. Bare concrete bridge decks under de-icing salt load may show severe corrosion damage, as has occurred on a large scale in the United States. In Europe, most decks are provided with a layer of dense asphalt. In many countries, some type of additional protection is applied, like a membrane between concrete and asphalt. In The Netherlands, this is not the usual practice: the asphalt is applied directly on the concrete. In the 1990s, the favourable road surface properties of open asphalt (better visibility in wet conditions, lower noise production) led to the decision to use open asphalt on all highways and bridges in these highways. Because the open asphalt requires a higher amount of de-icing salts and is more permeable to chlorides, the need was felt to introduce some form of additional protection of those bridges. Hydrophobic treatment of the concrete with silicone compounds seemed promising. Such treatment makes concrete water repellent, so theoretically water and dissolved chlorides would no longer be absorbed. It was expected that corrosion initiation would be prevented or at least postponed. Hydrophobic treatment is relatively cheap (5 to 10 Euro/m2), so it seemed an economically attractive way to improve the corrosion protection of bridge decks. A research programme was carried out to investigate the effectiveness and to select suitable hydrophobic products. First, a test procedure and criteria were

74 Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection drawn up. Several commercial products were tested [1,2]. Some products complied with the requirements, others failed. Only products specifically developed for use on concrete appeared suitable; products for brick masonry did not comply with the test requirements. The test set-up and the requirements were evaluated, slightly modified and subsequently laid down in a Rijkswaterstaat (Ministry of Transport) Recommendation. Application of hydrophobic treatment with an approved product has become standard practice on all new bridge decks.

2. Theoretical Background When water comes into contact with a porous material such as concrete, it is transported rapidly into the pores by capillary action. The rate of absorption depends on the surface tension, the viscosity, and the density of the liquid, on the angle of contact between the liquid and the pore walls and on the radius of the pores. Since the characteristics of the liquid (water) and of the concrete are given constants, the most important factor is the contact angle (theta). In normal concrete, the contact angle is small ( 3

1300

repair (strategy 2)

1100

+- Concrete repair with

c C

8 e! Q

stainless steel (strategy 3) 900

700

4-

a,

=

500

0

1

2

3

4

5

6

7

8

9

10

discount rate [%]

Fig. 7 Example of the coastal bridge. Net present values (50 years remaining lifetime)for differenf discount rates.

132 Corrosion of Reinforcement in Concrete: Corrosion Mechanisms and Corrosion Protection

No serious sign of corrosion of the stainless steel reinforcement embedded in the concrete was found. However, corrosion was detected on the freely exposed reinforcement (no cover), as could be expected for this grade of stainless steel in a marine environment. For reinforcement with a cover larger than approx. 20 mm, there was no significant corrosion on the bars, despite the extremely high chloride contents of up to 1.9% C1- of dry concrete weight. This is at least 10 times of that normally regarded as a critical chloride concentration for the initiation of corrosion of ordinary carbon steel. For a reinforced concrete structure in marine environment with ordinary carbon steel, the lack of routine maintenance for a 60-year period would in many cases result in serious chloride- or / and carbonation-induced corrosion problems. This is clearly shown by the deterioration of the neighbouring pier located to the west of the inspected pier. The unambiguous conclusion is, therefore, that the use of AISI 304-grade stainless steel as reinforcement has contributed significantly to the good durability of the Progresso pier.

10.Conclusions The following conclusions can be drawn based on the experience gained from this work: The coupling of corroding carbon steel with austenitic stainless steel, AISI 316, is without risk and provides lower corrosion current (corrosion rate) compared to the coupling to passive carbon steel, which always surrounds the corroding areas. Stainless steel has a higher overvoltage for cathodic reaction of oxygen reduction with respect to carbon steel. Therefore, the increase in corrosion rate on carbon steel embedded in chloride-contaminated concrete due to galvanic coupling with stainless steel is significantly lower than the increase brought about with passive carbon steel. Welding, which also decreases the chloride threshold value for initiation of corrosion, can destroy the low cathodic activity of stainless steel. For this reason the influence of welding will be further investigated in the future. The influence of cold working processes on the corrosion properties of stainless steel will also be investigated. However, the evidence obtained so far, shows that carbon steel and stainless steel can be coupled with beneficial results regarding corrosion protection in chloride-contaminated concrete.

Practical and Economic Applications of AlSl316 as Reinforcement in Concrete

133

11. Acknowledgement Part of this paper was presented at the International Conference o n Corrosion a n d Rehabilitation of Reinforced Concrete Structures, held a t Orlando, Florida, USA, 7-11 December 1998.

References 1. S. R. Kilworth and J. Fallon, Stainless steels for reinforcement, development of the paper, ’Fusion Bonded Coated Reinforcement in the Gulf‘, in Int. Conf on Corrosion and Protection of Reinforced Concrete, Dubai, 1994. 2. British Concrete Society, ‘Guidance on the Use of Stainless Steel Reinforcement’, Technical report No. 51, 1998. 3. U. Nurnberger, W. Beul and G. Onuseit, Corrosion behaviour of welded stainless reinforced steel in concrete, Otto-Graf-Journal, 1993. 4. Stainless Steel in Concrete -State of the Art Report (U. Nurnberger, ed.).Publication No. 18, in the European Federation of Corrosion Series. Published by The Institute of Materials, 1996. 5. Materials for Corrosion Cell Cathodes’, Internal Report by the Danish Corrosion Centre as a part of a report regarding Brite/Euram Contract 102 D. 1990. 6. L. Bertolini, M. Gastaldi, T. Pastore, M. P. Pedeferri and P. Pedeferri, Experiences on stainless steel behaviour in reinforced concrete.Book of Abstracts, EUROCORR’98, The Dutch Corrosion Centre, Utrecht, The Netherlands. Published 1998. 7. S. Jaggi, B. Elsener and H. Bohni, Oxygen reduction on passive steel in alkaline solutions, Book of Abstracts, EUROCORR ’99, GFK, Germany. Published 1999. 8. Nordtest standard, ’ConcreteTesting, Hardened Concrete. Chloride Penetration’. NT BUILD 443,1996. 9. L. Bertolini, M. Gastaldi, M. P. Pastore and P. Pedeferri, Experiences on stainless steel behaviour in reinforced concrete, in EUROCORR’ 98, The Dutch corrosion Centre Utrecht, 1998. 10. B. Elsener, 0.Klinghoffer, Frolund, T. Rislund, E. Schiegg and Y. H. Bohni, Assessment of reinforcement corrosion by means of galvanostatic pulse method, in Proc. Int. Conf on Repair of Concrete Structures, Svolvzx, Norway, 1997, pp. 391400. 11.J. Mietz and B. Isecke, in Electrochemical Potential Monitoring on Reinforced Concrete Structures using Anodic Pulse Technique, Non-destructive Testing in Civil Engineering (H. Bungey, ed.). The British Institute of NDT, 1993, 2, pp. 567. 12. K. R. Gowers and S. G. Millard, Corrosion and Corrosion Protection of Steel in Concrete (W. Swamy, ed.). Sheffield Academic Press, 1994, pp. 186. 13. Life-cycle Cost Case Study. River Crossing Highway Bridge (Schafjhausen Bridge, Switzerland). Published by Euro Inox, 1997. 14. E. Stoltzner, A. Knudsen and B. Buhr, Durability of marine structures in Denmark, in Proc. Int. Con. on Repair of Concrete Structures, Svolvm-, Norway, 1997, pp. 59-68. 15. A. Knudsen, F. M. Jensen, 0. Klinghoffer and T. Skovsgaard, Cost effective enhancement of durability of concrete structures by intelligent use of stainless steel reinforcement, in Proc. Int. Con&on Corrosion and Rehabilitation of Reinforced Concrete Structures, Florida, USA, December 1998. 16. Arminox Aps, Internal Report, ’Evaluation of the Stainless Steel Reinforcement of Pier of Progresso, Mexico ‘, March 1999.

List of Abbreviations

The following abbreviations occur in the text and in the Index of contents. ASTM

American Society for Testing and Materials

MIP MS MTBF

BFSC

Blast furnace slag cement

CP CSN

Cathodic protection Czechoslovak National Standard

DIN

German National Standard

es.

Electrochemical

FHWA

Federal Highways Agency (US)

GCP

Galvanic Cathodic Protection Ground Granulated Blast furnace Slag Gross National Product

GGBS GNP

ICCP

IS0

(U)LCC(A)

Mercury intrusion porosimetry Microsilica Mean time between failures (in power supply)

NMR

Nuclear magnetic resonance

OPC

Ordinary Portland cement

PVC

Polyvinyl Chloride

RC RH RILEM

Impressed current cathodic protection International Standards Organisation

Reinforced concrete Relative humidity Reunion Internationale des Laboratoires d’Essais et de Recherches sur les Materiaux et les Constructions (International Union of Testing and Research Laboratories for Materials and Structures)

SCE

Saturated (KC1)calomel electrode

(Updated) Life cycle costs (Analysis)

WAC

Water absorption coefficient Water / cement ratio

w/c

INDEX

Index Terms

Links

A Acidification of cement at anodes

103

Anodes external Zn

103

layered Zn

101

Sacrificial, in patch repairs thermally deposited Zn

94 109

C Carbonation (of concrete) of ground floor elements

85

in hydrophobised concrete

83

Cathodic control (see also Oxygen reduction) of rebar corrosion

13

Cathodic prevention (of corrosion) around patch repairs

94

experimental evaluation of

95

Cathodic protection (see also Sacrificial anodes) of concrete reinforcement, principles of

102

of ground floor elements

85

life time consideration of

88

using layer Zn anodes

101

Chloride induced corrosion of rebar hydrophobic treatment and –

to OH ratio and pit initiation

13

25

81 31

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

Chloride (Cont.): penetration into microsilica concrete

35

profiles in exposed cement

36

Corrosion prevention (see also Cathodic protection, Hydrophobic treatment, Inhibitors) of rebars by cathodic protection

85

computer system for

51

by hydrophobic treatment

73

by organic corrosion inhibitors

73

Corrosion protection potential value in rebar corrosion

46

Corrosion of steel (rebar) in concrete computerised system for

51

critical factors for inhibition of

25

reasons of

10

13

101

Corrosion rate determination by e.c. methods by galvanostatic pulse method by mass loss method

41 127 41

D De-icing salts (see Chlorides)

F Free chloride definition of

25

H Hydrophobic treatment agents for

76

and carbonation

83

chloride penetration and

78

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Index Terms

Links

Hydrophobic treatment (Cont.): of concrete for rebar protection

73

durability of

79

I Inhibitors experiments with

62

use in concrete

61

M Macrocell corrosion e.c. background to

14

in experimental tests

26

numerical simulation of

18

role of

21

Mass loss data (for rebar corrosion) vs e.c. data

41

Macrosilica in concrete

35

O Organic corrosion inhibitors (see Inhibitors) Organic top coats on thermally sprayed zinc

109

Oxygen reduction (see also Cathodic control) in corrosion of steel

3

kinetics of

7

on platinum

9

on stainless steel Tafel behaviour of

15

10 9

This page has been reformatted by Knovel to provide easier navigation.

Index Terms

Links

P Patch repairs application of

93

Potentiodynamic polarisation of steel in NaOH and in synthetic pore solution

4

effect of flow rate on

7

effect of temperature on

5

R Repair procedures options for

86

patch, with CP

93

S Sacrificial anodes (see Anodes) Stainless steel rebars case history

130

corrosion aspects of

123

life cycle cost analysis of

130

oxygen reduction on

10

practical and economic aspects of

121

in repair systems

130

results of corrosion tests with

124

Synthetic pore solutions composition of

5

polarisation tests in

4

26

Z Zinc anodes (see Anodes)

This page has been reformatted by Knovel to provide easier navigation.