Differential Aeration Corrosion of a Passivating Metal under a Moist Film of Locally Variable Thickness

A theoretical model is developed to predict the current distribution along a metal surface which exhibits passivity during differential aeration corrosion. The process is modeled by assuming that oxygen diffuses through a moist film of variable thickness to the corroding surface whereupon cathodic reaction proceeds in accordance with a Tafel rate expression which includes oxygen concentration dependence. The anodic dissolution rate depends solely upon the potential difference across the electrolyte-metal interface, behavior characteristic of active-passive metals. The model consists of a second-order nonlinear ordinary differential equation which was solved numerically. The influence of the system parameters upon the corrosion process was investigated. The results indicate that multiple steady-state solutions may exist for a single set of system parameters. It is usually found that current is conducted along the moist film between net anodic and net cathodic regions of the corroding surface. It is shown that in certain regions of parameter space the anodic dissolution rate can became highly localized.