This work presents an improved point defect model for the time-dependent formation of passive oxide films on metal surfaces. Like previous point-defect models, the present model assumes that charged defects, or vacancies, carry current across the growing oxide film. However, we treat the vacancies explicitly as material species which participate in oxide formation and dissolution reactions formulated for arbitrary oxide stoichiometry. The model includes boundary conditions, based on jump mass balances from formal continuum mechanics, that relate vacancy fluxes to the interfacial reaction rates as well as the motion of the film boundaries. Thus, unlike previous models, this model treats the film growth process formally as a moving boundary problem. Casting the equations in dimensionless form yields the key dimensionless groups. The dependence of the film growth rate on these groups can be rationalized in simple physical terms. The predicted trends in film growth rate and current density agree qualitatively with experimental data for nickel passivation, although the model parameters have not been optimized to achieve good agreement with current density data. The model provides a starting point for incorporating better descriptions of interfacial reaction kinetics within boundary conditions based on rigorous continuum mechanics.