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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. 936 (1989).

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Abstract-The influence of M light (300 nm) on the nucleation of meta-stable pits on type 316 stainless steel in a neutral 0.5 mol dmm3 NaCl solution using current-time measurements is described. A significant increase in the induction periods and a decrease in the rate of pit nucleation were observed for specimens pre-passivated under illumination conditions, indicating that illumination leads to a modification of the passive film that persists even after irradiation is removed. This increased resistance to pitting attack is explained in terms of the semiconducting nature of the passive film and the Point Defect Model (PDM) for the growth and breakdown of passive films. It is proposed that generation of electron-hole pairs leads to a quenching of the electric field strength and consequent modification of the vacancy structure, leading to a decrease in the flux of cation vacancies across the barrier layer. Good agreement was observed between the ratios of experimentally measured and theoretically calculated induction periods for specimens pre-passivated in the dark and in the light.