Passive Film Evolution on Stainless Steels with the Medium Composition: Electrochemical Studies in the Presence of Chloride and Molybdate Ions

In order to obtain information concerning the mechanisms of corrosion in presence of the passive film of stainless steels, and in the changes of the properties of this film due to medium modifications, we have performed a.c. impedance measurements in several media containing sodium chloride and sodium molybdate, to separate the conditions where adsorption, transport, or diffusion can be obtained. The static potential values were chosen in three domains: close to the rest potential, at a potential where the passivity is stable or in transpassivity conditions. The results obtained have shown that the limiting step of the corrosion process was not the same at the three potentials: close to the rest potential, the corrosion is limited by a diffusion process; when the passive film is stable, the limiting step is due to the existence of a capacity, and the passivity breakdown shows short circuits in the film. The influence of the added molybdate ions is not identical in the pitting domain of concentrations or in the passivating one: the changes in the passive film modify the thickness and the properties of the passive film.