  A variant of the point defect model originally enunciated by Macdonald and
co-workers is advanced and its theoretical implications for the steady state
current density, barrier layer thickness and the concentration of metal vacancy
at the metal/film interface are deduced. The differences between the original
point defect model and the present variant are also highlighted. The empirical
parameters alpha and beta in the original point defect model are replaced with
two physical parameters Rcont and Rhof which represent respectively the
electronic contact resistance at the metal/film interface and the electronic
resistivity of the oxide film. The present variant correctly describes the
annihilation of the metal vacancies at the metal/film interface and also
enforces the conservation of particle and defect volumes during the solid-state
reactions leading to the natural inclusion of the famous Pilling-Bedsworth
ratio RPB into the model. Diagnostics which help to check the model predictions
with experiments are given. Use of this variant to describe stress-induced
failure of the barrier oxide leading to pitting is also discussed.
