Initiation of Crevice Corrosion: II. Mathematical Model for Aluminum in Sodium Chloride Solutions

Salient features of initial stages of aluminum crevice corrosion in a dilute chloride solution have been expressed with mathematical equations in order to relate corrosion behavior to transport and electrochemical phenomena. The features included in the model equations are metal dissolution, metal-ion hydrolysis, oxygen reduction, diffusion along the crevice, and ohmic resistance effects. The system of nonlinear differential equations was solved with a digital computer. Computations based on experiments described in the first part of this series were compared with experimental data including pH, potential, and current distributions. Theoretical predictions of the time at which breakdown occurred were found to be about one-half of the initiation time observed experimentally. The calculations gave trends in system behavior which correctly tracked observations upon variation of crevice gap and solution conductivity, but not crevice depth. A critically important but highly uncertain feature of the model was the mechanism of passivity breakdown of aluminum in dilute chloride solution. Improved understanding of the breakdown mechanism would enhance predictive capabilities of the model.