Steady‐State Passive Films: Interfacial Kinetic Effects and Diagnostic Criteria

The point defect model for steady‐state passive films formed anodically on metals in aqueous environments has been extended to include irreversible dissolution of the film and the irreversible generation and annihilation of cation and oxygen vacancies at the metal/film and film/solution interfaces. The model yields a number of diagnostic criteria that can be used to identify the majority (vacancy) charge carrier and to characterize the kinetic nature of the interfacial vacancy generation and annihilation processes. We use these criteria to show that the steady‐state passive film that forms on nickel in acidic phosphate buffer solutions is a cation conductor and that cation transport from the metal to the solution involves irreversible ejection of cations from the film. On the other hand, the passive film that forms on tungsten in the same environment under steady‐state conditions is found to be an oxygen ion conductor with the passive current being determined by kinetically controlled film dissolution at the film/solution interface. These different behaviors are rationalized in terms of the relative energies for generation of cation and anion vacancies in the passive films.