Model for corrosion of metals covered with thin electrolyte layers: Pseudo-steady state diffusion of oxygen

A one-dimensional mathematical model is presented for the free corrosion of a bare metal surface (devoid of any oxide film) under a thin electrolyte layer using mixed potential theory where anodic metal dissolution is controlled by oxygen diffusion through the electrolyte layer and by the oxygen reduction at the metal surface. A pseudo-steady state is considered wherein the oxygen diffusion is at steady state while the metal and hydroxyl ions keep accumulating in the thin electrolyte layer due to a decoupling arising from the assumed Tafel laws for corrosion kinetics. Under free corrosion the oxygen diffusion is shown to depend on a non-linear boundary condition with a non-integer power on oxygen concentration at the metal surface which makes the model non-trivial. Analytical and numerical results for the oxygen concentration at the metal surface, corrosion potential, and corrosion current density are reported which depend on several kinetic, thermodynamic and transport parameters in the system. The model is applied to iron and zinc systems with input data taken from the literature. The experimental utility of the model for gathering thin-film corrosion parameters from a study of the corrosion current and potential as a function of the thickness of the electrolyte layer is discussed. Precipitation and passivity, though not the main object of study in this work, are briefly discussed.