A model describing the growth kinetics of passivating oxide film prepared under potentiostatic conditions

Abstract

A model has been developed describing the movement of a metal cation vacancy through the passivating oxide film under the presence of an electric field which contributes dominantly to the film growth kinetics. The present model accounts for the formation of metal and oxygen vacancies at the oxide film/electrolyte interface. By discounting this formation, an alternative model named the point defect model would involve only the diffusion and migration (transport) of oxygen vacancy. The theoretical equation of current vs thickness was derived from the present model involving Fickian diffusion and electrical migration of cation vacancy. The field strength and film thickness vs time for passivating oxide films on iron and nickel were determined from the derived equation fitted to the experimental results of current vs time reported previously. This indicated that the field strength across the passivating films on iron and nickel increased from 3.3 × 10 5 to 7.0 × 10 5 V cm −1 and from 7.5 × 10 4 to 9.5 × 10 5 V cm −1 as applied potential increased from 580 to 813 mV/ sce and from 200 to 600 mV/ sce, respectively. These data were compared with those obtained from the point defect model. The calculated relationship between the film thicknesses and time for the passivating films on iron and nickel is in accordance with those values obtained experimentally.