Repetitive nucleation of corrosion pits on stainless steel and the effects of surface roughness

The nucleation of corrosion pits on stainless steel in chloride solution at constant potential is described through observation of minute current transients. Pit nucleation transients are of form different from transients due to metastable pit propagation. Nucleation is a sharp, microscopically violent event. Current transients due to metastable pit propagation are initiated by nucleation transients. Most pit nucleation transients show no sign whatsoever of propagation into the metastable pitting regime, but die through repassivation immediately after nucleation. The frequency of occurrence of both types of transient decays with time of polarization, implying that the pits are nucleated at specific sites, and these sites are progressively annihilated. These processes depend strongly on the surface roughness of the metal. A smoother surface shows a smaller frequency of metastable pitting in comparison with a rougher one. The surface with the smoother finish, however, also shows a far higher frequency of nucleation events. This apparently paradoxical phenomenon is ascribed to the repetitive nucleation of pits from individual sites of pitting. Those sites which nucleate, but fail to propagate as metastable pits may renucleate many times. Final annihilation of the site can arise from metastable propagation so that the site is dissolved away. Alternatively, annihilation of the site of potential pitting can occur through repetitive nucleation in the absence of propagation. Prepassivation of the metal surface in chloride-free solution does not affect this. (C) 2001 The Electrochemical Society.