From the 1980s through the early 2000s, building on pioneering work by Galvele and others, we developed some simplified models for pitting corrosion using the artificial pit electrode (pencil electrode) as the main experimental tool. Some of the main conclusions of this research are listed below, using CPT to denote the critical pitting temperature –

- The critical pit chemistry for simple stainless steels, at room temperature, is not lower than 60% of saturation in FeCl₂, and may be considerably higher, as the artificial pit surface tends to split into active and passive areas [1]. Very careful experimentation is required to detect this splitting phenomenon, and to reject conclusions that do not take it into account.

- The variation of the pitting potential with log [Cl^-], as discussed by Galvele, can be rationalized rather exactly by plotting a quantity called the transition potential (E_T) against log [Cl^-]. One can dial-in a limiting current density (dependent on pit depth) to harmonize both potentials – pitting potential and transition potential [2]. A similar procedure works for alloying and (low) temperature effects.

- The CPT is a kind of active-passive transition potential where (essentially) a developing pit repassivates, no matter how concentrated the local solution [3-5], but with many small complexities.

- The complex morphology of real pits involves active-passive transitions within a developing 3D cavity, leading to lacy metal covers and other observations [6-8].

This topic has recently become quite fashionable after a period of some years. In this presentation, the original observations and simplified models will be reiterated and defended.

References:

1. G.T. Gaudet, W.T. Mo, J. Tilly, J.W. Tester, T.A. Hatton, H.S. Isaacs and R.C. Newman, Mass transfer and electrochemical kinetic interactions in localized pitting corrosion, AIChE Journal, 32, 949-958 (1986).
2. N.J. Laycock and R.C. Newman, Localized dissolution kinetics, salt films and pitting potentials. Corros. Sci., 39, 1771-1790 (1997).
3. N.J. Laycock, M.H. Moayed and R.C. Newman, Metastable pitting and the critical pitting temperature. J. Electrochem. Soc., 145, 2622-2628 (1998).
4. M.H. Moayed and R.C. Newman, Analysis of current transients and morphology of metastable and stable pitting on stainless steel near the critical pitting temperature. Corros. Sci., 48, 1004-1018 (2006).
5. M.H. Moayed and R.C. Newman, The relationship between pit chemistry and pit geometry near the critical pitting temperature. J. Electrochem. Soc., 153, B330-B336 (2006).
6. P. Ernst and R.C. Newman, Pit growth studies in stainless steel foils - I Introduction and growth kinetics. Corros. Sci., 44, 927-941 (2002).
7. P. Ernst and R.C. Newman, Pit growth studies in stainless steel foils - II Effect of temperature, chloride concentration and sulphate addition. Corros. Sci., 44, 943-954 (2002).
8. N.J. Laycock and S.P. White, Computer simulation of single pit propagation in stainless steel under potentiostatic control, J. Electrochem. Soc., 148, B264–B275 (2001).