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Modifications of the passive film formed on 316L stainless steel (SS) in acidic media were performed under dc or ac polarization. The effects of these ageing treatments on the chemical composition and the electronic structure of the passive film were studied. Chemical analysis with angle resolved X-ray photoelectron spectroscopy revealed that ageing induces a net enrichment in chromium oxide in the inner part of the passive film and an increase of the thickness of the inner oxide. Semiconducting behavior of the passive films was revealed by a Mott-Schottky analysis. The passive film was considered as a dielectric with different trapping levels lying in the bandgap. The observed n- or p-type semiconductivity can be assumed to be the result of the ionization of these trap levels under polarization. The most important modification of the electronic structure observed after ageing was a net decrease of the density of acceptor levels, which is associated with a less disordered inner chromium oxide. In the light of these results, the effect of ageing on the transient electrical coupling of the passive film with an active surface was investigated in free potential conditions. The capacitive response revealed by the potential and current transients was related to the localized states in the bandgap of the passive film. The current transient during the potential drop was related to the repassivation of the active surface. At the same time, electrons from the active site are transiently stored in the capacity formed by trap levels in the inner oxide of the passive film before being consumed by the cathodic reaction. The kinetics of the overall process should be limited by the heights of the potential barriers at the different interfaces and by the energy level of the acceptor-type traps. © 2004 The Electrochemical Society. All rights reserved.

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