Theory of Steady‐State Passive Films

A theory is developed for the steady‐state properties of passive films that form on metals and alloys in aqueous environments. This theory is based on the point defect model developed earlier, and predicts that the steady‐state thickness of the barrier film and the log of the steady‐state current will vary linearly with applied voltage. These relationships may be used to estimate empirical parameters that describe the dependencies of the potential drop across the barrier film/ environment interface on the applied voltage and pH and to estimate kinetic parameters for dissolution of the film. If dissolution at the film‐solution interface occurs very slowly, the primary passive film is envisaged to consist of a rigid oxide sublattice that transmits cations from the metal to a gel‐like, precipitated upper layer. If dissolution at the barrier film/ environment interface occurs rapidly, then a steady‐state thickness is achieved by a balance between the rate of dissolution of the film at the film‐solution interface and the rate of growth of the film into the underlying metal phase, due to the outward movement of oxygen vacancies (i.e., inward movement of oxygen ions) through the barrier layer. The model is found to account for many experimental data that have been published in the literature on the quasi steady‐state properties of passive films.