2001 W.R. Whitney Award Lecture: Understanding the corrosion of stainless steel

In order to predict corrosion damage on passive metals, it is essential to use statistical methods and semi-empirical models, but at the same time we must maintain active inquiry into the fundamental deterministic processes that occur during localized corrosion. If it were the case that atomistic events occurring within the intact passive film were responsible for, say, the beneficial effect of alloyed molybdenum, then we would have a gigantic job to do. Luckily, it appears that the quality of the passive film mainly affects the nucleation frequency of pits and has little or no bearing on the effects of environmental or metallurgical variables: T, Cl-, Br-, H2S; Mo, N, sigma.... We find that the anodic kinetics of the metal in the already-developed microenvironment of a pit can account for the effects of a large number of variables in pitting corrosion. Specifically. above the critical pitting temperature (CPT), the potential required to precipitate an anodic salt film in a cavity of relevant size is susceptible to straightforward modeling and experimentation and provides a robust predictor of the pitting potential. The CPT itself is associated with the inability of the metal to maintain active dissolution because passivation intervenes, even in the most aggressive possible microenvironment. Crevice corrosion is easier than pitting because the associated diffusion length is longer and the required anodic current densities are smaller. Chloride-induced stress corrosion cracking (SCC) always initiates in actively growing corrosion sites and will occur whenever the rate of localized corrosion is lower than the rate of crack growth, the latter being governed mainly by the alloy composition and structure and by temperature. Critical temperatures for SCC arise naturally from this approach, which was first developed by Tsujikawa. In sour environments, H2S activates anodic dissolution within pits and cracks, but is easily depleted in deep cavities, accounting for various threshold effects.