Kinetic Model for Aluminum Dissolution in Corrosion Pits

The kinetics of aluminum dissolution in etch pits and tunnels, in a I M HCI-3 M H 5 SO 4 solution at 70°C, were investigated. Dissolution current densities during growth of tunnels and pits, at potentials of roughly -0.8 and I V vs. Ag/AgCl respectively, were found to be approximately 6 A/cm 2 . Transient experiments using current step reductions during pitting, or anodic current pulses during tunnel growth, revealed strongly potential-dependent current densities up to 300 A/cm 2 . The results suggested that the dissolution rate is potential-dependent when measured on times scales of ∼1 ms after potential disturbances, but insensitive to potential in quasi-stationary experiments. A kinetic model was presented assuming a monolayer or multilayer chloride layer on the aluminum surface, including kinetic expressions for transfer of AI +3 and Cl - ions at the film/solution interface, and ionic conduction in the film. In agreement with experiments, the model yields constant or potential-dependent dissolution rates following a Butler-Volmer relation, depending on the time scale of experimental measurements. The large current densities in anodic transient experiments derived from high rates of Cl - incorporation during film growth.