On the Mechanism of Localized Corrosion of Iron and Stainless Steel

During pitting of iron and stainless steels the potential drop across the interface of the dissolving surface within the pit is in the region of active metal dissolution, even when the potential of the specimen surface is controlled at potentials as high at +1.4 (SHE), as well as during natural corrosion. This conclusion is supported by measurements of the potential of the electrolyte within pits and by the observation that hydrogen gas is produced within the pits. From calculations of the concentration and potential gradients within the electrolyte in a pit it is further concluded that existing models of pit growth are inconsistent with these data. A modified model of pit growth is presented which involves growth by active dissolution and includes a high resistance path resulting from a constriction caused by a hydrogen bubble. This model of pit growth is also believed to apply to the propagation of crevices and, in some instances, of intergranular attack.