The behaviour of galvanized steel in chloride-containing alkaline solutions-I. The influence of the cation

Corrosion Science, Vol. 30, No. 4/5, pp. 393--407, 1990 Printed in Great Britain

0010-938X/90 $3.00 + 0.00 Pergamon Press plc

THE B E H A V I O U R OF G A L V A N I Z E D STEEL IN CHLORIDE-CONTAINING ALKALINE SOLUTIONS--I. THE INFLUENCE OF THE CATION A. MACIAS and C. ANDRADE Instituto Eduardo Torroja de la Construccion y del Cemento, C.S.I,C. Madrid, Spain Abstract--The behaviour of galvanized reinforced bars (rebars) in chloride-contaminated concrete is a very controversial matter not yet resolved. An extensive study of the behaviour of galvanized steel in alkaline solutions which simulate the aqueous phase present in concrete pores has been undertaken. This paper describes the modifications that the presence of chloride has on the corrosion behaviour of galvanized steel in alkaline solutions, as well as the influenceof the accompanyingcation. It was confirmed that the corrosion behaviourof galvanized steel in the presence of chlorides is controlled by the medium pH, which depends on the calciumor sodium salt which provides the chloride ions. INTRODUCTION THE PRESENCE of chlorides is one of the major causes of corrosion produced on reinforcements embedded in concrete, since chlorides can destroy the passive film that normally protects the metal in the alkaline medium found in dense concretes. Thanks to the experience achieved in other aggressive media, different to concrete, it has been found that Zn resists far better the attack of chlorides than steel. In the case of concretes, agreement on this point is not total, according to the contradictory data found in the bibliography on this topic. It should be remarked that the cases in which galvanized steel in contact with chlorides showed bad behaviour occurred in structures where water retention and temperature were aggravators of the corrosion process on reinforcements.1 Wide practical experience 2-5 shows that the use of galvanized steel involves a durability improvement of the structures when they are in contact with chlorides. The results obtained in laboratory tests are also contradictory, probably due to the different measurement procedures and materials used. Most of the data is obtained from Zn or galvanized steel in Ca(OH)2 saturated solutions with different additions. Unz, 6 working with both galvanized and non-galvanized steel plates, totally or partially immersed in Ca(OH)2 saturated solutions with additions of NaC1, explains how galvanic currents produced in non-uniform exposure conditions, leads to local accumulations of chlorides with intense pitting corrosion, independently of the existence of passivating films. Griffen, 7 working with specimens exposed to marine environments found no advantages in the employment of galvanized steel. Ishikawa, s however, using Ca(OH)2 saturated solutions with additions of NaC1 from 0.2 to 2.0%, found that galvanized steel resists higher chloride concentrations than bare steel. This opinion is also supported by other authors, 9-11 though their respective results differ in the Manuscript received 3 December 1987;in amended form 30 August 1988and 24 April 1989. 393

394

A. MACIAS and C. ANDRADE

minimum chloride concentration needed to provoke pitting corrosion. This lack of clarity is largely based on the fact that hardened concrete is a complex electrolyte in which to study metallic corrosion processes. The authors of this paper have carried out extensive work 12-1a on the behaviour of galvanized steel in alkaline solutions which simulate the aqueous phase present in concrete pores. It has been possible to relate these results to those obtained from concrete specimens. By means of this simplification, it is to assert that the galvanized steel becomes passivated in contact with electrolytes whose pH value is <13.3 + 0.1 and that, above this value the galvanized coating is completely dissolved until it totally disappears within a period shorter than 33 days. Similarly, it was confirmed that the passivation product of the galvanized steel in these media was calcium hydroxyzincate, Ca(Zn(OH)3)2 • 2H20. This paper describes the modifications that the presence of chloride has on the corrosion behaviour of galvanized steel in alkaline solutions previously mentioned, as well as the influence of the accompanying cation. Solutions chosen were representative of those found in concrete pores viz. Ca(OH)2 saturated solution and the Ca(OH)2 solution plus 0.1 M, 0.2 M, 0.25 M and 0.5 M KOH, so that, two of them have pH values below and two above the pH limiting value of 13.3 above which there is no passivation of the galvanized steel. Chloride ion was added at a concentration of 0.3, 0.6 and 0.9 M in the form of NaC1 and CaC12. The next part of the work will study the pitting process provoked and the relation between the pit nucleation potential, En, that is, the potential at which pitting is first noticed in an anodic polarization curve and the solution pH value. EXPERIMENTAL

METHOD

Materials Reinforcing corrugated 6 m m nominal diameter and 8 cm long steel bars, having a 60-80 tzm coating produced by hot dip galvanizing at 450°C, were used. The area exposed to solution was 6.5 cm 2. The composition, pH and [Ca 2÷] values of the 22 solutions employed are given in Table 1. As in previous work, 12-14 cells of polyethylene (instead of the glass, which is less resistant to such alkaline media as those that were employed in the test) were used. A coat of liquid paraffin was spread on the solution to avoid carbonation of the m e d i u m and the localized attack along the water-line. The solutions were thermostatically at 25°C and the test period was 33 days. Procedures Solution analysis, p H M e a s u r e m e n t s were carried out by m e a n s of a range 0-14 combined electrode and a digital p H - m e t e r O R I O N model D I G I T 501. The a m o u n t of calcium in solution was estimated by titration with E D T A . 15 The C1- concentration was determined by m e a n s of the Mohr method, however, results are not given because of their total coincidence with the C I - concentration added. Polarization resistance measurements. The polarization resistance m e a s u r e m e n t s were carried out by m e a n s of an A M E L potentiostat, Model M E T A L L O S C A N . Rp was estimated potentiodynamically (10 m V rain-l), drawing the polarization curve section around the Ecorr (AE = +10 mV). The corrosion intensity was evaluated applying the Stern and Geary formula: 16A7 i.... = B/Rp;

B = fla" flc/2"3(fla + tic).

A B value for the metal's corrosion in the active state (intensity higher than 1/xA cm-2), was chosen as 6 × 10 -3 V and 5.2 x 10 -2 V for the estimation of the icorrin passive state (intensity lower than 1/zA cm-2). The total weight loss was obtained by integrating the icorr-time curve. In order to transcribe the measured area to weight loss (AWelectrochemical)the following formula was employed:

Galvanized steel in CI--containing alkaline solutions TABLE 1.

395

pH VALVESANt) [Ca2+ ] FORCa(OH)2 SAXURATEOSOLUTIONSWITH C l - ADDITIONS

Composition Cell number 1

2 3 4 5 6 7 8 9 11) 11 12 13 14 15 16 17 18 19 20 21 22

Molarity NaCI 0.3

Molarity CaCI 2 --

0.6 0.9 1).3 [).3 0.3 0.3 0.6 0.6 11.6 11.6 0.9 11.9 0.9 11.9 --------

--

--------------1).15 11.30 (t.45 0.45 0.45 t!.45 /I.45

AWclcctmchcmk.al =

Molarity KOH

--O. 10 0.2/) 0.25 0.50 O. 10 0.20 0.25 0.5(I 0.10 0.20 0.25 0.50 ---O. 10 0.20 0.25 0.5t)

pH 12.59

12.54 12.54 12.84 13.10 13.19 13.46 12.86 13.(17 13.19 13.36 12.82 13.1t4 13.13 13.41 12.16 11.93 11.83 11.91 11 . 9 0 11.90 11.99

Ca 2+ (g dm 3) 1.11841)

1.1640 1.1721/ /).2880 11.137/) 0.0973 0.0020 11.4100 0.1550 0.133(I 0.0044 //.4120 0.15811 O. 1006 0.0028 6.5600 12.4000 18.2400 18.0800 16.5600 15.6400 11.76011

0.029264 !j'j i~orr(t) " dt,

which is inferred from the Faraday law. EXPERIMENTAL

RESULTS

Chloride addition as NaC1 F i g u r e 1 s h o w s t h e icorr a n d Ecorr v a l u e s v e r s u s t h e t i m e o f i m m e r s i o n in t h e C a ( O H ) 2 s o l u t i o n w i t h a d d i t i o n o f 0.3, 0.6 a n d 0.9 M o f N a C I . It c a n b e o b s e r v e d for t h e s e c o n c e n t r a t i o n s t h a t t h e b e h a v i o u r p a t t e r n o f t h e g a l v a n i z e d s t e e l is a n a l o g o u s to t h e o n e r e c o r d e d f o r a C a ( O H ) 2 s o l u t i o n ~2 w h o s e r e s u l t s a r e g i v e n f o r c o m p a r i s o n in Fig. 2. T h i s m e a n s t h a t , a f t e r a s h o r t p e r i o d o f initial d i s s o l u t i o n w i t h h y d r o g e n e v o l u t i o n , t h e g a l v a n i z e d s t e e l b e c o m e s p a s s i v a t e d . N e v e r t h e l e s s , initial c u r r e n t d e n s i t i e s a r e l o w e r w i t h c h l o r i d e s , w h e r e a s , t h e final o n e s a r e a l m o s t 10 t i m e s h i g h e r , a n d m o r e n e g a t i v e final p o t e n t i a l s a r e r e g i s t e r e d . C u r r e n t d e n s i t y f l u c t u a t i o n s a r e p r o b a b l y d u e to l o c a l i z e d a t t a c k . It c a n b e s e e n t h a t h i g h e r c h l o r i d e c o n c e n t r a t i o n s l e a d to slightly h i g h e r initial c o r r o s i o n c u r r e n t d e n s i t i e s . T h e d r a s t i c d e c r e a s e in icorr a n d t h e l a r g e p o s i t i v e shift o f Ecor~ t a k e p l a c e slightly a f t e r w a r d s f o r i n c r e a s i n g c h l o r i d e c o n c e n t r a t i o n s . L i k e w i s e , t h e final v a l u e o f E c o ~ is i n c r e a s i n g l y n e g a t i v e t h e h i g h e r t h e c h l o r i d e c o n c e n t r a t i o n b u t , p r o b a b l y b e c a u s e o f icorr i n s t a b i l i t y , it has n o t b e e n p o s s i b l e to r e c o r d d i f f e r e n c e s

396

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FIG. 1.

icorr and Ecorr as a function of time for the Ca(OH)2 solutions with addition of 0.3, 0.6 and 0.9 M of NaC1.

between the final corrosion current density values as function of the chloride concentration. Figures 3-5 show the electrochemical results obtained for Ca(OH)2 solution plus 0.3, 0.6 and 0.9 M NaCI with 0.1, 0.2, 0.25 and 0.5 M K O H added to each concentration of NaCI. If these results are compared to the those of Fig. 2 for equal K O H concentrations added to the Ca(OH)2 matrix solution, it can be observed once again, how the presence of chloride does not largely change the corrosion behaviour of galvanized steel in contact with these solutions. It is observed again, however, that when chloride is present, the corrosion current densities are slightly lower during the first days whereas the final Ecorr are more negative. Final values of icorr are two or three times higher in solutions of 0.1 and 0.2 M K O H with CI- and reach similar values for concentrations of 0.5 M K O H .

Galvanized steel in Cl--containing alkaline solutions

397

Nevertheless, for Ca(OH)2 solutions plus 0.25 M KOH with additions of CI-, the final corrosion intensities are equal or even lower to the ones recorded for analogous solutions without CI- additions. Galvanized steel in Ca(OH)2 solution plus 0.6 M NaC1 plus 0.25 M KOH showed an especially remarkable behaviour (Fig. 4). In the cell with 0.25 M KOH and no CI- the metal remains active until the end of the test, but in the presence of chloride icorr drops to values lower than 1 ~A cm -2 at the 9th day. This value decreases further until the end of the test, when it reaches especially values lower than 0.3/.~A cm -2. It seems that the higher the chloride concentration the higher the icorr measured, though this effect is weak.

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+ 0 . 1 M KOH + 0 . 2 M KOH ~rO.25M KOH + O . 5 M KOH

icorr and Ecorr as a function of time for the Ca(OH)2 solutions with addition of 0, 0.1, 0.2, 0.25 and 0.5 M of KOH.

398

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icorr and Ecorr as a function of time for the Ca(OH)2 + 0.3 M NaCI solutions with addition of 0.1, 0.2, 0.25 and 0.5 M of K O H .

Chloride addition as CaCI2

Figure 6 shows the electrochemical results obtained for Ca(OH)2 solutions plus Cl--concentrations equal to those employed for NaCI, but this time the C1- is added as CaCI 2. After a short initial period of corrosion, passivation of the galvanized steel occurs as previously. In the presence of CaC12, values of icorr increase with time, giving corrosion current densities about four times higher than those obtained for NaC1 additions. In addition, Ecorr is more cathodic. With CaCI2 additions, the aggressiveness increment as the salt concentration increases is even less evident, since the icorr with time records for these concentrations are crossing constantly. For this reason galvanized steel was only tested, in Ca(OH)2 solutions plus K O H additions with the highest concentration of chloride

Galvanized steel in Cl--containing alkaline solutions

399

(0.45 M CaCl2). The electrochemical behaviour is shown in Fig. 7. This behaviour is curiously different to that obtained for solutions with analogous concentration of CIas NaCI. For all cases with CaCI2, after a period of initial dissolution with hydrogen evolution, the galvanized steel becomes passivated and values of 1 /xA cm -2 are reached at periods no longer than 6 days. The initial period of dissolution is longer with lower KOH concentration. This behaviour is similar to that recorded for a Ca(OH)2 saturated solution (Fig. 6).

Chloride addition after the galvanized passivation In order to simulate the practical case of concrete mixes, without chloride, but immersed in a marine environment or in contact with de-icing salts, the effect of

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icorr and Ecorr as a function of time for the Ca(OH)2 + 0.6 M NaCI solutions with addition of 0.1, 0.2, 0.25 and 0.5 M of KOH.

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adding 0.9 M chloride after the passivation of galvanized steel in Ca(OH)2 solution by the formation of a calcium hydroxyzincate layer was studied. In order to study the behaviour to long times, the test was prolonged to a year. As shown in Fig. 8, when chloride is added, icorr increases from values around 0.3-9.0 # A cm -2, and then after a few hours, it decreases to values slightly higher than 1/~A cm -2, which are values similar to those measured for equal times if C1- is present initially (Fig. 1). After 30 days, a gradual increase of the icorr is recorded possibly as a result of activation of pits previously formed.

Comparison between gravimetric and electrochemical measurements The agreement between the gravimetric weight losses and the electrochemical ones calculated from the icorr values presented in the previous figures, is shown in Fig. 9.

Galvanized steel in Cl--containing alkaline solutions

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of time for the Ca(OH)2

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DISCUSSION Generally speaking, little importance has been attached to the nature of the cation in tests carried out to examine the resistance to chloride attack of a metal. Recently, however, it has been shown is that the corrosion rates of steel electrodes in cement pastes containing equivalent dosages of CaCI2 or NaC1 were higher in the former case. This appeared largely to result from differences in the effects of the two salts on the early stages of cement hydration, which influenced the extent of the passive film breakdown during the first week of curing. In the case of galvanized steel, the results of the tests carried out lead to two main deductions: First it has been shown that the formation conditions of the calcium hydroxyzincate layer which is responsible for passivating the galvanized steel in alkaline media, are not altered by the presence of chloride coming from a calcium or

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F1c. 7.

CaCL 2 CaCL2 CoCL2 CaCL 2

(days)

* 0 I M KOH ÷ 0 . 2 M KOH ÷ 0 . 2 5 M KOH ÷ O.SM KOH

icorrand Ecorr as a function of time for the Ca(OH)2 + 0.45 M CaCI2 solutions with addition of 0.1, 0.2, 0.25 and 0.5 M of KOH.

a sodium salt. Thus, as observed in Fig. 10, the pH limit of 13.3 + 0.1, above which this protective layer is not formed homogeneously, is maintained in presence of chlorides. Secondly, it has been shown that the different behaviour of the galvanized steel in solutions of equal concentration of Ca(OH)e, KOH or CI- depends on the salt NaCI or CaCIe employed. This different behaviour is due to the different pH modification in the solution depending on the salt used. In a Ca(OH)2 solution plus CaCIa, the following equations should be considered: ,~ Ca(OH)2 ~ Ca 2+ + 2OH+ CaC1 z --~ 2C1- + C a 2÷

+t $ Ca(OH)a.

Ksp = 3.53 x 10 -5 mol dm-3) 3

Galvanized steel in CI -containing alkaline solutions

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The value of Ksp is obtained from the experimental data of [Ca 2+] = 0.892 g dm 3 and [ O H - ] = 1"0- 1 4 (pH = 12.6) measured in a Ca(OH)2 solution at 25°C. The CaCI 2, as a strong electrolyte, is totally dissociated. Ca 2+ precipitates along with O H - in form of Ca(OH)2 resulting in a decrease in pH. For Ca(OH)2 solutions plus different concentrations of K O H and addition of CaC12:

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Comparison between gravimetric and electrochemical weight loss of the galvanized bars (numbers as in Table 1).

added as CaCI2 and the rest modifies the pH in the same way as occurred in the former case. Theorical results of pH and [Ca 2+] obtained from these chemical balances, along with the experimental results are given in Table 2. It should be pointed out that in these solutions the amount of CaC12 added has a buffer effect on the solution pH, since it admits alkali additions for certain concentration without a change in the pH value. However when the addition of chloride is in the form of NaCl, the balances are: ~, Ca(OH)2 ~ Ca 2+ + 2OH-, NaCI-+ C1- + Na +. In this case, the addition of NaCI produces an increase of the solution ionic strength which only leads to a small variation of the ion activities. If the results shown in Tables 1 and 312 are compared, it can be seen that a pH decrease of around 0.1 takes place in the presence of NaC1 which helps to explain the especially good behaviour of the galvanized steel in a Ca(OH)2 solution plus 0.25 M KOH and 0.6 M NaC1. With CaC12 in the solutions and for the alkali concentration added, the pH values reached were always below 12.2, and the galvanized steel is always exposed to the necessary conditions for passivation by means of the calcium hydroxyzincate layer formation. With NaC1, which hardly alters the medium pH, the galvanized steel will passivate or corrode depending on the KOH concentration, which is mainly responsible for the medium pH value. Finally, it should be noted that the presence of chloride attacks the calcium hydroxyzincate protective layer locally, producing pits and an increase of the corrosion current densities, topics which will be studied in the second part of this work. Summing up, the behaviour of galvanized steel in the presence of chlorides is

Galvanized steel in CI -containing alkaline solutions

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406

A. MAC1ASand C. ANDRADE TABLE2. THEORETICAL ANDEXPERIMENTALpH VALUESFORCa(OH)2 SATURATED SOLUTIONSWITHDIFFERENTADDITIONSOFCaCl 2 Molarity CaCI2

Molarity KOH

0.15 0.30 0.45 0.45 0.45 0.45 0.45

---0.10 0.20 0.25 0.50

pH(exp.)

2+ Ca(exp. ) pH(thcor.) (gdm -~)

12.16 11.93 11.83 11.91 11.90 11.90 11.99

TABLE3.

pH

12.18 12.03 11.94 11.97 12.00 12.01 12.12

VALUES

Ca(OH)2 SATURATED

AND

6.56 12.40 18.24 18.08 16.56 15.64 11.76

[Ca2+]

2+ Ca(theor.) (g dm -3)

6.32 12.24 18.21 16.22 14.22 13.23 8.28

FOR

SOLUTIONS WITH K O H

ADDITIONS

Molarity KOH

pH

Ca2+ (g dm -3)

-0.10 0.20 0.25 0.50

12.60 12.97 13.24 13.34 13.59

0.892 0.258 -0.080 0.053

controlled by the m e d i u m p H , which is modified differently d e p e n d i n g on the salt which provides the chloride ions. This fact helps to explain some of the discrepancies on results previously given by several authors and also it m a y help to clarify arguments about the effectiveness of galvanizing as a m e t h o d of protecting reinforcing steel in concrete i m m e r s e d in saline environments. CONCLUSIONS T h e main conclusions f r o m the present results are as follows: (1) Chloride ions do not alter the p H limit of 13.3 + 0.1 above which the protective layer of calcium hydroxyzincate is not f o r m e d uniformly. (2) This passivating layer of calcium hydroxyzincate m a y be altered by the existence of pitting p r o m o t e d by the C I - ions. (3) T h e b e h a v i o u r of galvanized steel in the presence of chlorides d e p e n d s on the parent salt: CaCI2 or NaCI. T h e latter hardly alters the m e d i u m p H , but for CaCI2, the Ca 2+ ions have a certain buffering effect and the p H value remains inside a n a r r o w range a r o u n d 12. (4) T h e resistance of the protective calcium hydroxyzincate layer to chloride attack seems to be similar w h e t h e r it has been f o r m e d in the presence of chloride or not.

Galvanized steel in C1 -containing alkaline solutions

407

REFERENCES 1. C. E. MANGE, Mater. Performance, (7) 34-36 (1977). 2. The Bermuda Experience: "'Galvanized Rebar Performs". Newsletter, Vol. 9, (1). American Hot Dip Galvanizers Association, Washington D.C. (1974). 3. D. STARKand W. PERENCHIO,Construction Technology Laboratoires, Final Report, Portland Cement Association, Skokie, IL (1975). 4. F. C. PORTER, Bldg Specification, (2) 51 (1980). 5. D. TONINI and A. COOK, NACE, Corrosion-78, Paper no. 75, Houston, Texas. 6. M. UNZ, A C I J . , (3) 91 (1978). 7. D. F. GRIFFEN, US Naval Civil Engineering Laboratory, Technical Note N-1032 (1969). 8. T. 1SHIKAWA,I. CORNET and B. BRESLER, Proceedings of the 4th International Congress on Metallic Corrosion (1969). 9. H. KAESCHE, Gutachten A2, 4571 (1968). 10. I. CORNETand B. BRESLER, Corros. Protection, 1, 21 (1970). 11. R. DUVALand G. ARLI6UXE,Mere. Sci. Rev. Metals 71,719 (1976). 12. A. MACIASand C. ANDRADE, Br. Corros. J. 18, 82-87 (1983). 13. M. T. BLANCO, C. ANDRADE and A. MACIAS,Br. Corros. J. 19, xxx (1986). 14. A. MACIASand C. ANDRADE, Br, Corros. J. 22, 113 (1987). 15. J. CALLEJA, F. TRlVI~O and J. Fz. PARIS, Manuales y Normas. IETcc (1964). 16. M. STERN and A. L. GEARY,J. electrochem. Soc. 104, 56 (1957). 17. M. STERN and E. D. WEtSERT, Proc. ASTM 59, 1280 (1959). 18. C. ANDRADE and C. L. PAGE, Br. Corros. J. 21, 49 (1986).