ASPECTS OF THE EFFECTS OF ELECTROLYTE-COMPOSITION ON THE OCCURRENCE OF METASTABLE PITTING ON STAINLESS-STEEL

The rate of formation of metastable pits on type 304 stainless steel surfaces in chloride solution is examined as a function of the chloride concentration and pH. The frequency of pitting events decreases as the chloride concentration is reduced; the effect arises from a concomitant reduction in the number of available sites at which metastable pitting can take place. The electrolyte pH has no observable effect on the frequency of metastable pitting, and no effect on the number of available sites. The results are consistent with the hypothesis that the ability of a metastable pit to develop at a particular site depends on the geometry of the site. Preliminary experiments on the effects of aeration show that dissolved air reduces the frequency of occurrence of metastable pitting current transients. The effect of oxygen remains unexplained.