An experimental confirmation of the pitting potential model of galvele

Corrosion Science, Vol. 28, No. 5, pp. 471-477, 1988 Printed in Great Britain

0010--938X/88 $3.00 + 0.00 Pergamon Press plc

AN EXPERIMENTAL CONFIRMATION OF THE POTENTIAL MODEL OF GALVELE

PITTING

R. C. NEWMAN, M. A. A. AJJAWI, H. EZUBER and S. TURGOOSE Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, U.K. Abstract--The dependence of the pitting potential on ionic activity in chloride solutions has been shown to be consistent with a purely ohmic effect within pit nuclei, as proposed by Galvele. The IR potential drop (q~) within small one-dimensional pits, dissolving at their anodic limiting current densities in sodium chloride solutions, is given by: ~b = A - B l o g a + over the range 10-3 < a± < 1, with B ~ 60 mV for iron and 90 mV for Type 302 stainless steel. Provided that the critical local chemistry required for pitting is close to saturation in FeCl 2, these results can be compared directly with similar relationships for the pitting potential. The high value of B for the stainless steel may be connected with the participation of H + in electro-transport within the pits.

INTRODUCTION THE CRITICALpotential for pitting, or pitting potential, retains its significance despite the widespread recognition that its exact value depends on the method and criteria used to determine it. 1 A notable feature of many pitting potential measurements is a logarithmic dependence on chloride ion activity, as reviewed by Galvele: 2 Ep = A - B log a+

(1)

where B is generally in the range 50-100 mV. Galvele showed that such a relationship could arise from a migration-dominated transport process within a pit nucleus, coupled with a critical dissolved metal ion concentration at its base, and derived a theoretical value for B of 2.3 RT/F, i.e. 59 mV at 25°C. The value of B is not always close to 60 mV, and in particular there are many careful measurements of stainless steels showing a value close to 90 mV. 2-4 Measurement of the dependence of the IR drop on Cl- activity for various materials therefore provides an opportunity to test the model of Galvele, and perhaps to discard other models. If it could be shown that the different values of B reflect different dependences of the IR drop on a_+, this would be a powerful piece of evidence in support of the model. In order to measure the IR drop within small pits, it is necessary to use a one-dimensional (artificial pit) geometry. It is also necessary to use very narrow pits, since otherwise the IR potential drop external to the pit would dominate the system at low CI- activities; the model of Galvele assumes (implicitly) that for very small pit nuclei the dominant resistance is within the pit nucleus, not in the region of semi-infinite 3-D current flow outside the nucleus. These requirements have been

Manuscript received 31 July 1987. 471

R . C . NEWMAN, M. A. A. AJJAWI,H. EZUBER and S. TURGOOSE

472

met by dissolving 10 and 50/~m wires from one end after mounting them in resin. It will be shown that even 50/~m is too large to test the model reliably, since the bulk solution IR drop has already begun to affect the results. A further assumption is necessary regarding the critical dissolved metal ion concentration required to stabilize a pit. If this was different for different materials, then differences in the value of B might arise due to different degrees of non-ideality in the pit solutions. Since it would be an immense task to measure the IR drop as a function of metal ion concentration in the pits, the measurements have been carried out in a limiting current condition, i.e. with a saturated (almost saturated) metal salt solution adjacent to the dissolving surface. Fortunately, differences in the value of B for iron and stainless steel have been detected for this condition, and seem to correspond quite well to the pitting potential data in the literature. EXPERIMENTAL

METHOD

Iron and stainless steel wires, nominally 10 and 50/~m in thickness, were purchased from Goodfellow Metals. The stainless steel wires were nominally a Type 302 (18Cr-8Ni) and were bare; the 10/am iron was glass-coated, and the thickness of the actual metal was only 7/~m. Each wire was mounted in a rod of epoxy resin which was abraded so as to expose one end. The assembled specimens were mounted on the shorter arms of J-shaped glass tubes and the wires threaded through the tubes for electrical contact (made with conducting paint for the 10/~m wires and by soldering for the 50/~m wires). The experimental solutions contained NaCl (10-3-3 M) or in one case HC1 (0.1 M). They were used at room temperature (19 + 3°C). All experiments were done in beakers open to the air, using a saturated calomel reference electrode (SCE) and a stainless steel counter electrode. The potentials of the specimens were controlled by a potentiostat and varied by a ramp generator. The current was measured by a digital electrometer, connected in series with the cell, and the analogue output of the electrometer was recorded on a chart recorder. Each experiment began by polarizing the specimen at a potential where it would dissolve uniformly and establish an anodic diffusion-limited current density. This was trivial for iron, which did not passivate in the test solutions, but for stainless steel it was necessary to polarize at a high potential (500 mV) to initiate pits on the exposed surface. Once uniform diffusion-controlled dissolution of the stainless steel was occurring, the potential was lowered to a value just above the transition from diffusion-controlled to

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Schematic current-potential diagram for the DC pit resistance measurement.

An experimental confirmation of the pitting potential model of Galvele

473

ohmically-controlled dissolution, and the current was allowed to reach its steadily decaying value (i ~ t-in). The DC pit resistance was measured by applying a simple perturbation to the pit as shown in Fig. 1. It was not necessary to make a high frequency AC measurement, as the charge transfer resistance was only a few percent of the solution resistance. The time taken to make the resistance measurement at a scan rate of 50 mV/s was short enough that the interfacial metal ion concentration decreased by only a few percent from its saturation value. The measurements were repeated at intervals during pit growth, and the pit resistance was plotted against the reciprocal of the limiting current. The slopes of these lines (i.e. the IR drops within the pits at their limiting anodic current densities) were plotted against the logarithm of the mean ionic activity for comparison with Galvele's model.

EXPERIMENTAL RESULTS AND DISCUSSION All the plots of pit resistance versus reciprocal limiting current (Figs 2 and 3) were straight lines. T h e behaviour in 0.1 M NaCI and 0.1 M HC1 (Fig. 4) shows that metal and chloride ions dominate the resistance. The lines all had positive intercepts on the resistance axis; the trend of these intercepts with bulk NaCI concentration indicates a complicating effect of the enrichment of dissolved metal ions near, but not in, the pits. Briefly, it appears that the necessity for semi-infinite diffusion and current flow outside the pits is equivalent to an extension of the pit length, but not in a simple linear fashion. The results for the 50 g m electrodes m a y also be affected by convection. The I R versus a+ plots are shown in Figs 5 and 6. It is apparent that the 50/~m iron electrode has significantly non-linear behaviour, whereas the 7-10 g m electrodes show straight lines with slopes of - 6 6 + 14 m V (for iron) and - 9 1 + 9 m V (for stainless steel). If one ignores the point at a_+= 2.1 (i.e. 3 M NaC1), then the slope for iron becomes - 6 2 + 13 mV; this might be justified in that the linear behaviour must b r e a k down at sufficiently high chloride activity. 2.3 R T / F at the test t e m p e r a ture is - 5 8 mV. The rather Z-shaped curve obtained with the 50/~m iron electrode is reminiscent of the dependence of pit repassivation potential on NaCI concentration, as shown by Azzerri et al. 5 and analysed by Newman. 6 In this case we suspect that the result is being dominated by events outside the pit, as would be expected at larger pit diameters. Considering the relatively crude experimental technique, these results are a promising confirmation of the model of Galvele. The larger slope found for the stainless steel m a y indicate that the hydrogen ions produced by Cr 3+ hydrolysis are decreasing the I R drop m o r e rapidly with increasing chloride activity than would be the case for iron. This can perhaps be understood in terms of the charges on the current-carrying ions: at high electrolyte conductivities (low fields) the high diffusivity of H ÷ is effective in reducing the I R drop, whereas at low conductivities (high fields) the multiple charges on the Fe 2÷, Cr 3÷ and Ni 2÷ ions m a k e t h e m m o r e effective current carriers. This is not an attempt to rationalize the absolute differences in I R between iron and stainless steel, only the relative trends with chloride activity. A numerical model is being set up, and results will be reported in due course. CONCLUSIONS . The linear relationship between the I R drop in a small pit and the logarithm of the m e a n ionic activity has been confirmed for small artificial pits dissolving near their anodic limiting current density in NaCI solutions.

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2. The slopes of the IR vs log a+ plots are - 6 6 + 14 mV for a 7 p m iron electrode and - 9 1 _+ 9 mV for 10 or 50 pm electrodes of Type 302 stainless steel. Ignoring very high CI- activities, where the linear behaviour is expected to break down, the result for iron becomes - 6 2 _+ 13 mV. 3. The higher slope found for stainless steel is probably connected with the much higher H ÷ activity in the pit. 4. These results provide support for the pitting potential model of Galvele, since it is unlikely that other models could explain both a 60 mV and a 90 mV slope.

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REFERENCES D. E. WILLIAMS, C. WESTCOTTand M. FLEISCHMANN,J. electrochem. Soc. 132, 1804 (1985). J. R. GALVELE,J. electrochem. Soc 123,464 (1976). H. P. LECKIEand H. H. UHLIG,J. eleclrochem Soc. 113, 1262 (1966). Z. SZKLARSKA-SMIALOWSKA,Pitting Corrosion of Metals. NACE, Houston (1986). N. AZZERRI,F. MANCIAand A. TAMBA,Corros. Sci. 22,675 (1982). R. C. NEWMAN,Corros. Sci. 23, 1045 (1983).