Spectroelectrochemical studies of passivation and transpassive breakdown reactions of stainless steel

Passivation and its transpassive breakdown reactions of stainless steel in neutral sodium chloride solutions have been studied using cyclic voltammetric and spectroelectrochemical experiments. The results indicate that the main chemical reactions of the passivation process are those involved in iron oxidation to various oxides/hydroxides. The composition of the passive film is identified as the mixture of iron oxides and hydroxides of various valences along with Cr2O3 already present on the surface, which changes as a function of the applied potential. The breakdown of the passivity is caused by oxidation of the Cr2O3 layer to a soluble species, CrO42-. The species generated at various potentials have been identified from spectroelectrochemical experiments. The spectro electrochemical behavior of the stainless steel electrode is somewhat similar to that of pure iron, which is the most abundant component in the alloy.