Corrosion potential oscillation of stainless steel in concentrated sulphuric acid: I. Electrochemical aspects

The spontaneous oscillation of corrosion potential between the active and passive states of UNS S30403 stainless steel has been studied in 93.5wt.% sulphuric acid at 60°C. Detailed electrochemical analysis of the kinetics of the oscillating potential has suggested the presence of a nickel salt which is stable over a limited potential range. The nickel salt is subject to dissolution at potentials more positive than −0.25V (MSE), while at lower potentials—less than −0.35V (MSE)—a stable salt film is formed. A critical coverage of the salt on the surface is the prerequisite for the mixed potential to be shifted into the passive range by an increase in the exchange current density for the sulphuric acid cathodic reaction. However, when the coverage exceeds a critical value, such as can be produced by a potentiostatic hold, the salt effectively blocks the formation of a passive film.