Non-equilibrium point defect model for time-dependent passivation of metalsurfaces

This work presents an improved point defect model for the time-dependent fo rmation of passive oxide films on metal surfaces. Like previous point-defec t models, the present model assumes that charged defects, or vacancies, car ry current across the growing oxide film. However, we treat the vacancies e xplicitly as material species which participate in oxide formation and diss olution reactions formulated for arbitrary oxide stoichiometry. The model i ncludes boundary conditions, based on jump mass balances from formal contin uum 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 p roblem. Casting the equations in dimensionless form yields the key dimensio nless 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 ni ckel passivation, although the model parameters have not been optimized to achieve good agreement with current density data. The model provides a star ting point for incorporating better descriptions of interfacial reaction ki netics within boundary conditions based on rigorous continuum mechanics. (C ) 2001 Elsevier Science Ltd. All rights reserved.