Semiconductor property of passive films and corrosion behavior of Fe-Cr alloys

Abstract

The semiconductor structure of passive films formed in a borate buffer solution and in a sulphuric acid solution are discussed in terms of their electronic structure and growth and corrosion behaviour. A passive film formed in a borate buffer solution is composed of an inner oxide layer and a covering hydroxide layer, and both are n-type semiconductors with optical band gap energy, Eg, of 3.4-3.5 eV and 2.4 eV, respectively. The two n-type semiconductor layers, which consist of a passive film formed in a borate buffer solution, are both in depleted states. These conditions allow the migration of ionic species and electrons, which results in the continuous growth of the film. On the other hand, passive films on Cr and Fe-Cr alloys formed in a sulphuric acid solution consist of a p-type semiconductor inner oxide layer and n-type semiconductor outer hydroxide layer. The field assisted growth for a film grown in an acidic solution is difficult in the steady state and the inner oxide layer grows in the initial stage of passivation when the inner layer exhibits n-type semiconductor properties. This difference in semiconductor properties coincides with passive film growth kinetics reported in the literature.