Metastable Pitting of Aluminum and Criteria for the Transition to Stable Pit Growth

Abstract

Analysis of current spikes associated with pitting events was performed on high purity A1 loop wire electrodes over a range of C1- concentrations (10-5 to 1M) and potentiostatieally applied potentials. A distribution ofpitting and repassiva-tion potentials was observed at each C1 concentration. Factors controlling the transition from metastable to stable pitting were identified by comparing the electrochemical behaviors of stable pits at elapsed times equal to the mean lifetime of metastable pits. This comparison also provided insight on the origins of statistical distributions of pitting potentials. The key differences were that (i) stable pits had a faster rate of rise in pit current which implies a faster growth rate and, subsequently, arger pit radii at times equal to metastable pit lifetimes, and (if) stable pits satisfied the criterion I p J rp i t> 10-2 A/em, at all times during growth, indicating that a concentrated A1C13 solution must be maintained for pit survival. However, the pit growth rates were ohmically limited near the pitting potential. Chromate inhibitor decreased the metastable pit nucleation rate and minimized pit growth rates so that the 10-~A/cm criterion was difficult o achieve; hence the chance for pit stabilization was reduced. Studies of the pitting corrosion of aluminum can be di-vided into two basic categories. In one case, breakdown and repassivation potentials have been either related to