On the kinetics of growth of anodic oxide firms

Two theories, the high field model (HFM), and variants thereof, and the point defect model (PDM), have been advanced to account for the properties of thin anodic passive films (barrier layers). The HFM states that, under potentiostatic conditions, the electric field strength in a passive film decreases as the film thickens, while the PDM proposes that the field strength remains constant but that the potential drop across the metal/film interface decreases as the film grows. These different hypotheses lead to different kinetic laws for film growth. In this work, diagnostic criteria have been derived to determine the electric field-thickness relationship and hence to identify the appropriate kinetic law. Experiments on the growth of anodic oxide films on zirconium, tungsten, and tantalum under potentiostatic conditions in phosphate buffer solution show that the PDM provides a better account of the experimental data than does the HFM.