The corrosion and protection of steel in saturated Ca(OH)2 contaminated with NaCl

CorrosionScience,Vol.20, pp. 1241to 1249

© Pergamon Press Ltd. 1980. Printed in Great

Britain.

0010-938X/80/1201-1241$02.00/0

THE CORROSION AND PROTECTION OF STEEL IN SATURATED Ca(OH)2 CONTAMINATED WITH NaCI* J. F. HENRIKSEN'~ Norwegian Dcfence Research Establishment, P.O. Box 25, N-2007 Kjeller, Norway Abstract--The corrosion of steel in saturated Ca(OH)z containing different concentrations of NaCI has been investigated at different potentials, oxygen contents and temperatures. The initiation of pits takes place only above the pitting potential. Repassivation of developed pits is shown to follow the threshold concentration for the initial corrosion step. Above the threshold concentration, existing pits can continue to grow at potentials below the pitting potential, and partial cathodic protection with a potential change of 50 mV is needed to arrest the corrosion. An explanation of the corrosion process is proposed. The corrosion rate is shown to be under anodic control. The growth of pits below the pitting potential is caused by the change in pH of the pit solution, which reaches a steady state regulated by the hydrolysis of the solution within the pit, and by the diffusion rate of hydroxyl ions into the pit from the bulk solution. INTRODUCTION FOR YEARS there has been some d o u b t a b o u t the criteria used for cathodic protection o f steel in alkaline e n v i r o n m e n t s . F o r concrete H a u s m a n 1 has proposed - - 710 mV vs a Cu/CuSO4 electrode for corroding steel a n d below - - 400 m V vs Cu/CuSO4 for steel which has n o t yet started to corrode. However, m a n y engineers are still using the same criterion as used in sea water a n d soil, n a m e l y - - 850 m V vs Cu/CuSO4. While this criterion is safe, a variation o f the potential along the structures must be expected causing overprotection at some parts. Cathodic protection m u c h below - - 850 m V is reported 2 to cause cracking, a n d should therefore be avoided. A better u n d e r s t a n d i n g o f the processes going o n should m a k e it possible to define protection criteria for different concrete mixtures, a n d keep the potentials at a range where cracking is impossible. EXPERIMENTAL METHOD The apparatus used is described in an earlier report, a The electrolyte used was saturated Ca(OH)s containing different quantities of NaCl. The NaC1 concentrations selected were from 0.002N to IN. Except for certain experiments, temperatures were room temperature, i.e. 22 + I°C. The solution was saturated with air but in a separate experiment the oxygen content was changed by bubbling different gas mixtures of Os and N2 through the solution. The mixture varied between 0.1% Os and 99.9~0 Ns to 20~0 02 and 80~0 Ns. The concentration of oxygen in solution at equilibrium with this mixture was 0.04, 0.2, 0.8, 1, 2.3, 3.9, and 8 ppm. The procedure for pitting investigations In order to get the same passive film on all the electrodes, the oxide scale was first machined away. The specimens were then etched in 40% HsSO4 and rinsed three times with distilled water. They were stored in a solution of saturated Ca(OH)~ until the potential stabilized [(ca. - 200 mV(SCE)I which normally took about or~ day. The electrode was then transferred to the experimental cell and, while the potential was kept at a selected value, it was scratched with a stainless steel pick every 10 rain for half an hour. The current was watched. Below the pitting potential the current soon

*Manuscript received I October 1979. "['Now at the Norwegian Institute for Air Research, P.O. Box 130, 2001 Lillestrom, Norway. 1241

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J . F . HENRIKSEN

stabilized at the original level. Above the pitting potential the current rose and a visible corrosion spot developed. The lowest potential found where the current continued to increase was taken as the pitting potential Ep. Procedure./'or measuring the corrosion rate vs temperature Experiments were made at 5, 10, 22, 50 and 65°C with a NaCI concentration of 0.005N. The potential was kept 50 mV anodic to the pitting potential. The pit was initiated and the total current at that potential was measured as a function of time. Since the current neoued to keep the passive area at the selected potential was insignificant, the total current could be used as a measure for the corrosion rate within the pit. Procedure for the cathodic protection investigation A corroding pit was developed in the same manner as for the corrosion rate experiments. The forced corrosion at the high anodic potential was stopped when the current had reached 2 mA. The specimen was allowed to stabilize until the change in the potential was less than 5 mV during half a n hour. On the basis of this steady state potential E,, usually between - 450 to - 500 mV(SCE), a partial cathodic protection AE was supplied. The new potential E, + AE was maintained for a predetermined time before the current was turneo off and the potential of the specimen was watched. If the potential came back to the passive potential ( - 200 mV(SCE)) or above the pitting potential, the electrode was considered to be repassivated. If the potential remained below the pitting potential, the electrode still had a n active corrosion spot on it and protection had not been achieved. EXPERIMENTAL

RESULTS

AND DISCUSSION

Steel corrosion in alkaline solutions has an initiation step controlled by the pitting potential Ep. The initiation step is followed by a corrosion propagation step. The developed pits are able to continue to corrode at a lower potential than Ep, because of changes in the local environment within the pits.

Pitting initiation The change in the pitting potential with the NaCI concentration for St-37 is shown in Fig. 1, and these results are close to those found by Hausman 1 at high NaCI con-

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The corrosion and protection of steel

1243

centrations. At lower concentrations the difference in the two sets of results is somewhat greater and the rise in the curve at 0.02N NaCI does not occur when using the potentiostatic technique. The figure also shows that the steel quality has an effect on the pitting potential. The results with two different steels vary by about 100 mV, the hot-formed steel St-37 being more positive than the cold-rolled steel. The results are in agreement with a work by Defrancq 4 who shows that annealing of steel plates changes the pitting potential in the positive direction. The variation of the pitting potential with the temperature is shown in Fig. 2. The variation is linear with a change of 25 mV for every 10°C, which is of the same order as that found with stainless steel type 304 and type 430. 5 Assuming that the mechanism of pitting is due to an adsorption of chloride followed by a penetration of the oxide film, as proposed by Hoar,6 one of these two steps would normally be the rate-controlling step in the reaction. It will be shown that temperature will affect the two steps in a different way and that the experimental values of Ep vs temperature should therefore give information al~out the rate determining step. The adsorption of C1- ions would follow Gibbs' equation F = a/RT dS/da, in which 17 = excess concentration at the film/solution surface, a -- activity of C1- in the solution, 8 = interfacial tension, R = gas constant, T = temperature. This adsorption of CI- ions will not change much in the temperature range under consideration and will not be linear. The penetration rate of absorbed chlorides through the oxides films is effected by concentration differences and potential differences through the film. Concentration differences will cause diffusion through the film following Fick's law. 7 Under condition where only the temperature is changed the main effect will be a change of the diffusion coemcient shown by Glasstone s to vary linearly with the temperature. In the same way the potential interaction will be effected by the change in the ion mobility in the oxide film empirically shown to increase linearly with the temperature. The change in Ep as a function of the temperature seems therefore most likely to be related to the penetration rate through the oxide film as the rate determining step. In Fig. 3 it is shown that Ep increases with the oxygen content below ~ 1 ~ O~, which indicates that passive films are formed on steel in alkaline solution even when oxygen is absent. Sato et al.9 have provided a model of the passive film in alkaline solution consisting of an inner layer of anhydrous Fe203 with an outer layer of hydrous oxides of FeOOH and Fe(OH)a. Supposing that the normal passive film formed is Fe2Oa, the film formed below 0.4 ppm 02, corresponding to 1 ~O~ in the O~-N2 mixture, must be FeO or Fe30~. Fe304 is known to give passivity and is the more reasonable one. A complete answer to the question about the type of oxide film formed is not given, but the results show that the oxide film formed at low oxygen content is more noble than the one formed at normal oxygen concentrations.

Repassivation Leckie and Uhlig 1° have defined a boundary between pitting and inhibition of steel in alkaline solutions containing chlorides often called the threshold chloride concentration below which pitting corrosion will not occur. In Fig. 1, Hausman's results give a threshold concentration of 0.02N NaC1 in saturated lime solution. Baumel and Engell ix have found the threshold at 0.035N NaC1 for steel in the same

1244

J.F. HENRIKSEN

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solution. In Fig. 4 it is shown that the threshold concentration is due to self-passivation of pits and that self-passivation occur up to a concentration of 0.03N NaC1; the two curves for 0.03N NaCl show the shortest and longest time for repassivation found in these experiments. It is generally known that during corrosion the solution is acidified within the pits and that the pH will tend to decrease with increasing corrosion rate or when the diffusion of the hydroxyl ions is slowed down by blocking of corrosion products in the pit. When the pH decreases below pH 10 steel will not form a passive film and active pits will develop at a corrosion potential similar to steel in neutral conditions.

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The corrosionand protectionof steel

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The measured potential of the steel in the bulk solution will change in the negative direction. By using a potentiostatic method, where the potential is kept at a high level, the current needed to keep the potential will increase. By using a galvanostatic method, the development of a pit will give a drop in the potential. In free corrosion the situation is more like that of the galvanostatic method. When the corrosion of steel in saturated Ca(OH)~ contaminated with NaC1 starts, the steady state potential has to be above the pitting potential. Pits will initiate and a galvanic couple will be formed between the passive areas and the extremely small pits. The size of the passive areas will dominate, and the anodic polarization of the pits and the current density within them will be high; this will cause hydrolysis of the pit solution and turn it acidic. The potential will drop below the pitting potential and prevent the steel from forming new pits. However, the existing pits are still able to grow if the chloride concentration is above the threshold concentration. When the pits increase in size the galvanic couple will change. The pits will become depolarized and the current density will decrease. Within the pits a steady state is formed between the hydrolysis of the pit solution and diffusion of hydroxyl ions from the bulk solution. In chloride solutions the corrosion rate will decrease at chloride concentrations below 0.1N NaC1. The steady state solution within the pits will therefore increase in pH as the NaC1 concentration decreases. Above the threshold concentration the corrosion rate is still high enough to maintain an active pit and corrosion will continue. Below the threshold concentration, hydrolysis will be too low to keep the pit solution corrosive. The solution will change back to a pH above 10 and repassivation of the tiny pits will take place. Under normal conditions or during galvanostatic tests there will be a threshold concentration as the limit for corrosion.

1246

J.F. HENRIKSEN

By using a potentiostatic method there will be the same threshold concentration as the limit for repassivation of the pits. Corrosion rate In Figs. 5 and 6 the corrosion rate, measured as the current needed to maintain the passive potential on the specimen, is given for different temperatures and after different exposure times. Up to 50°C the corrosion rate follows an Arrhenius plot (Fig. 6) with the rate doubling for every 10°C, but between 50°C and 65°C the change in the rate decreases. If the anodic and cathodic reactions follow the Tafel equation i = i 0 exp((l -- ~)Fq/RT), where ~ is the symmetry factor in the Helmholtz double layer and rl is the overpotential, the equation follows an Arrhenius plot if T is the only variable. In these experiments the corroding area on every specimen was the same at the same time so that the measured current can be used, therefore, instead of current density. The only way the total current could give an Arrhenius plot is if the corrosion reaction is dominated by one of the electrode reactions. Since the corroding area increase slowly and the cathodic area decrease with time iron dissolution must be the rate depending step. Cathodic control should give a decrease or nearly constant corrosion with time, anodic control will give increasing corrosion rtae with both time and temperature. Above 50°C the control changes to mixed control and since the

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oxygen content in water decrease with temperature the reaction is expected to be under cathodic control when the temperature approaches 100°C. The increase in the corrosion rate with the time shown in Fig. 7 follows an equation I ---- kP where b is c a . 0.3. A linear slope on a log-log scale is quite normal for pitting

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1248

J.F. HENRIKSEN

corrosion, but most of the publications give b above 1. However, in the results presented here there is one basic difference. Normally the tests are made on specimens with a large number of pits, whereas in the results presented here, corrosion took place at just one spot making a large deep pit. It is normally believed that the corrosion rate in one pit decreases with time. This was shown by Rosenfeld and Danelov 1~ and our results show the same trend.

Cathodic protection To protect steel in saturated Ca (OH)2 solution against pit initiation the potential needs to be kept below the pitting potential Ep. In Figs. l, 2 and 3 the change of Ep with chloride concentration, temperature and oxygen content is shown. If the steel quality and temperature at the steel are known, and if the upper limit for the chloride concentration can be estimated, the pitting potential can be defined. Figure 4 shows that above 0.03N NaC1 a pit once started will continue to grow even at a potential below the pitting potential. One specimen was followed for 2000 h to be sure that selfpassivation did not occur. In order to stop the growth of existing pits at chloride concentrations above 0.03N, the potential needs to be changed in the negative direction. The corrosion rate will then be slowed down so much by partial cathodic protection that the hydrolysis rate in the pit is lower than the diffusion rate of hydroxyl ion from the bulk solution. Thus the pH in the pit solution will increase with consequent repassivation. In Fig. 8 the potential change AE added to the corrosion potential of the corroding specimen is shown as a function of the time. The value of AE is quite high if the protection is needed immediately, but is not more than 30 mV if the time is above 100 h. The potential change of AE -- 30 mV is not a fundamental value, but is influenced

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The corrosion and protection of steel

1249

by several factors. The most important are the geometry and size of the pit, the different diffusion rates through different corrosion layers, and the differences in the p H and chloride concentrations in the pits. Our results show that these factors can have quite large influences during the first hours but they seem to become smaller with time. After 100 h it should be possible, therefore, to define a limiting AE for stopping the growth of pits. To include a safety factor, AE W 50 mV is chosen as the magnitude of the partial cathodic protection required and practical experience has proved this to be sufficient. CONCLUSION Cathodic protection of steel in alkaline solution containing chloride is related to two criteria. For protection against nucleation of pits the potential of the steel must be below the pitting potential. The pitting potential varies with the chloride concentration, the pH, the temperature and, to a limited extent, the oxygen content. Repassivation of active pits will take place below 0.03N chloride and above if partial cathodic protection is applied. The change in the potential required is at least 50 mV from the steady state corrosion potential. Since the pitting results reported here correspond with those obtained by Hausman 1 in a lime--concrete solution, the results and the method used should also be valid for steel in concrete. As long as the lime solution in concrete is saturated at a p H 12.5, the above criteria can be used for cathodic protection of steel in concrete. Acknowledgement--The author wishes to thank Dr. J. A. H. Carson, D.R.E.P., for stimulating

discussions during the work with this report. REFERENCES 1. D. A. HAUSMAN,Materials Protection 8, 23 (1969). 2. B. POULSON,L. C. H~NRIKS~Nand H. AstJP, British Corr. J. 9, 91 (1974). 3. R. EvtK and J. F. I-IENRItSEr~,7th Scandinavian Corr. Congr., Trondheim (1975). 4. J. N. DEI~P.ANCQ,Corros. ScL 14, 461 (1974). 5. Z. SZ~:LAnSKASUaALOWSKA,Corrosion 27, 223 (1971). 6. T. P. HOAR,R. B. MEAns and G. P. ROTrIWELL,Corros. ScL 5, 269 (1965). 7. J. H. BROPHY,R. M. ROS~ and J. WULFV, The Structure and Properties of Materials, Vol. II: Thermodynamics of Structures, pp. 75-80. John Wiley, New York (1967). 8. S. GLASS'tONE,Textbook of Physical Chemistry, p. 1257. Van Nostrand, Princeton (1946). 9. N. SATO,T. NODAand K. Ktnao, Electrochem. Acta 19, 471 (1974). 10. H. P. LECKIEand H. H. UHLIG,J. electrochem. Soc. 113, 1262 (1966). 11. A. BAUMF.t.and H. Z. ENGELL,Archivfiir des Eisenhiittenwesen 30, 417 (1959). 12. I. L. ROSENEELDand I. S. DANXLOV,Zasbchita Metallov 6, No. 1 (1970).