304 stainless steel oxidation in sulfate and sulfate plus bicarbonate solutions

The electrochemical characterization of 304 stainless steel in 0.1-0.5 M Na2SO4 and Na2SO4 + NaHCO3 aqueous solutions at pH similar to 8 was done in combination with SEM surface analysis. Passivation of the surface film without any pitting events is observed for the current-potential and current-time experiments, and no anodic current spikes, which are associated with depassivation events, are distinguishable above the background current level. However, SEM pictures of an electrode surface polarized at potentials above 0.4 V show microscopic pit nucleation. Even when the metal may be regarded as passive from a current-time or current-potential characteristics, passivity is not stable and localized film breakdown can still occur. The events are ascribed to pit nucleation at the active inclusion sites. The pits do not grow even into the metastable state but die through repassivation by metal salt precipitation immediately after birth. The effect is ascribed to the solubility of metal salts with the electrolyte produced by dissolution in the nucleation sites. The results show that pit nucleation and pit growth are two distinct processes. The importance of solution composition and the protective effect of bicarbonate ions is also discussed.