Simplified Point Defect Model for Growth of Anodic Passive Films on Iron

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We present a simplified point defect model to describe the growth of the primary passive oxide film on the surface of iron. The model postulates a reduced set of elementary interfacial reactions to describe the formation and dissolution of the oxide film. By casting the model in dimensionless form, we obtain a relatively small set of parameters that must be assigned values. Parameter values are set by matching the film thickness predicted by the model with one experimental data point. The model is then used to predict variations in film thickness with time, temperature, and applied potential, yielding reasonable agreement with experimental data. The model also gives the correct qualitative trend in the dependence of film thickness on electrolyte pH. Although the model parameters used in our comparisons are probably not unique, they suggest that physical picture embodied in the model provides a suitable starting point for modeling the growth of passive films on Fe.