On the Kinetics of the Breakdown of Passivity of Preanodized Aluminum by Chloride Ions

The kinetics of passivity breakdown and nucleation of pitting of preanodized aluminum by chloride ions has been investigated using aluminum supporting oxide films of reasonably well‐known thickness and structure. The kinetics of passivity breakdown at the steady‐state, critical pitting potential is influenced by chloride ion concentration, temperature, and oxide film thickness; it was found to be independent of solution pH in the range 5–10. It is postulated that passivity breakdown and nucleation of pitting at the critical pitting potential occurs by a process of Cl− adsorption (assisted by the field at the oxide‐solution interface) on the hydrated oxide surface and formation of a soluble, basic chloride salt with the lattice cation which readily goes in solution. This process of localized dissolution of the hydrated oxide film via formation of a soluble, basic, aluminum chloride salt once initiated is likely to continue in an "autocatalytic" fashion until the oxide is locally "penetrated" and dissolution of the substrate metal begins.