The growth kinetics of passive films and the role of defects

Abstract

The mass transport through the passive film which is necessary for film growth may occur either by a cation- or an anion-vacancy (or the corresponding interstitial) mechanism; thus the currents depend on the defect concentration. If it is assumed that vacancies are produced at the metal/oxide or at the oxide/electrolyte interface, the vacancy concentration can depend on the current densities. Consequences for film growth and film decomposition will be discussed and an explanation for the presence or absence of an overshooting of the potential during a current pulse will be given. Similar considerations can be made for an interstitial mechanism, because it is assumed only that the current through a passive film is increased by increasing both the electric field and the defect concentration. If the defects are generated at the boundaries of the film only, a relaxation time is required to distribute the defects within the film. During this relaxation time higher fields are necessary, in order to increase the velocity of the smaller number of defects until the externally imposed current density is reached.