Pit Growth and Repassivation

Laycock and coworkers developed simulations of pit growth on stainless steel at constant applied potential. The present discussion focuses on their second paper, in which the model was substantially improved by the extension of the simulation domain to include the solution outside the pit cavity. The pit growth model incorporated a hypothesis of the critical condition for passivation within the pit, and the resulting development of the pit shape was evaluated. The metal surface was assumed to be passivated at metal ion concentrations lower than a critical value the dissolution rate increased with metal ion concentration above the critical value. These concentration-dependent dissolution kinetics were closely related to results of experimental studies of artificial pits, and the criterion for passivation was supported by measurements of actual pit growth. It should be mentioned that description of mass transport in the model did not conform to the dilute solution theory as outlined above, since a current continuity equation was used which included contributions from migration but not diffusion hence, the potential gradient in the pit was probably overestimated. However, inaccuracies in the prediction of potential were likely compensated by adjustment of empirical dissolution kinetic parameters in the model. 

In the experiments of Hebert and coworkers, repassivation was initiated by either step or ramp reductions of the applied current during etching. Repassivation in tunnels produces morphological changes of the otherwise flat tip surface, which were clearly apparent in electron microscope images. The current step experiments showed that oxide passivation on the tip occurred within times of 1 ms, and was accompanied by characteristic potential transients 

Hebert s model for tunnel growth predicted the tuimel shape on the basis of the repassivation model just described. Starting from the initial condition of a cubic etch pit, the model calculated the evolution of the pit shape resulting from dissolution and sidewall passivation. The dissolution rate was taken directly from experimental measurements. Since the passivation kinetics were potential-dependent, it was necessary to accmately predict the potential at the tunnel tip. This required the use of concentrated solution transport equations, for the first time in a pitting model. All transport and kinetic parameters used in the model were taken from independent sources. The calculations showed that pits growing at the bulk solution repassivation potential spontaneously transformed into tunnels by sidewall passivation (Fig. 6). The tuimels then grew with parallel walls until the concentration at the tip approached saturation, at 

and 13 p.m were measured experimentally (Reproduced with permission from Ref [40] Copyright 2001, The Eletrochemical Society.) 

The apparently different repassivation concepts embodied in the models of Laycock and White and Hebert may in fact share important common elements. Repassivation occurs in the former model below a critical electrolyte concentration, while in the latter it takes place below a critical potential, the value of which decreases with increasing concentration. Therefore, at a given potential, both concepts indicate that active dissolution occurs only above a minimum concentration. The hypotheses differ in that Hebert additionally accounted for a direct influence of the potential on passivation. 

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Localized corrosion usually involves change in pH and chloride ion concentration around corrosion pits. Therefore, potentiometric SECM tips can be used to study corrosion processes. When a pH microsensor was used as the SECM tip for the in situ measurements during localized corrosion of stainless steel, it was found that pH decreases at the pit initiation stage and increases during pit growth and repassivation (113). 

Two main points should be remembered from this lab. First, the breakdown potential is not necessarily the best measurement of pitting resistance. This is because pitting can occur at potentials below EM, as was demonstrated by metastable pitting in test 4. Ebi corresponds to the potential for stable pit growth and propagation only. Pits can nucleate, however, at any potential above the repassivation potential. Secondly, the effects that additional anions have on the pitting behavior is concentration dependent and not mass dependent. 

A model for the growth and repassivation of pits in Mo-free, Mo and Mo-N containing alloys. (From Falkenberg, F. and Olefjord, I., Proceedings of the Intematkmal Symposium in Honor (fProf sor Norio Sato Passivity and Localized Corrosion, M. Seo, B. MacDougaU, H. Takahashi, and R.G. Kelly, eds.. Electrochemical Society, Pennington, NJ,... 

At the area between the breakdown potential Eb and the critical pitting potential pit local film breakdown occurs, which leads to the creation of pit nuclei. However, these nuclei are immediately repassivated. Consequently, in this potential region it is concluded that breakdown and repair are continuously repeated without creating pit growth. 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

The resistance of many metals and alloys to corrosion depends critically upon the presence of a thin (10-1000 A [168]) passive surface film [169]. In "aggressive environments, this film may become damaged locally via several processes, e.g. surface stress effects (either flow-induced [170,171] or as a result of anion adsorption [168]), the impingement of small particles on the surface [169], spontaneous depassivation [169]. Retention of the protective film by the metal only results if repassivation of the unprotected area is feasible compared with pit growth. 

Fig. 14 Repassivation of small corrosion pits on an iron electrode (A = 0.13 cm ) during potentiostatic steps to E < Ep for t >0.1 s after pit growth at 0.5 V for 25 ms seen by the decrease of the anodic current and following increase due to new nucleation of pits in phthalate buffer pH 5.0 + 0.03 M C - [19].

Metastable pitting may be considered as a precursor of stable pit growth which occurs above the pitting potential. The charge associated with the formation and repassivation of metastable pits can be calculated by integrating the corresponding current spikes. This allows one to estimate the extent of pit growth before they... 

The autocatalytic nature of pit growth is responsible for the fact that the repassivation potential observed potentiodynamic experiments is generally lower than the breakdown potential (Figure 7.49(a)) and that it depends on the details of the experimental procedure the concentration changes induced by the dissolution reaction during sweep reversal determine the ionic concentrations in the developing pit and the ease of repassivation. 

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