METASTABLE PITTING OF ALUMINUM AND CRITERIA FOR THE TRANSITION TO STABLE PIT GROWTH

Analysis of current spikes associated with pitting events was performe d on high purity Al loop wire electrodes over a range of Cl- concentra tions (10(-4) to 1M) and potentiostatically applied potentials. A dist ribution of pitting and repassivation potentials was observed at each Cl- concentration. Factors controlling the transition from metastable to stable pitting were identified by comparing the electrochemical beh aviors of stable pits at elapsed times equal to the mean lifetime of m etastable pits. This comparison also provided insight on the origins o f statistical distributions of pitting potentials. The key differences were that (i) stable pits had a faster rate of rise in pit current wh ich implies a faster growth rate and, subsequently, larger pit radii a t times equal to metastable pit lifetimes, and (ii) stable pits satisf ied the criterion I(pit)/r(pit) > 10(-2) A/cm, at all times during gro wth, indicating that a concentrated AlCl3 solution must be maintained for pit survival. However, the pit growth rates were ohmically limited near the pitting potential. Chromate inhibitor decreased the metastab le pit nucleation rate and minimized pit growth rates so that the 10(- 2) A/cm criterion was difficult to achieve; hence the chance for pit s tabilization was reduced.