Metastable pitting of aluminum and Al-Cu alloys

Metastable pitting behavior and the transition from metastable to stable pitting on high purity aluminum and two Al-Cu alloys (aged Al-2%Cu and AA2024-T3) were investigated. Experiments on pure Al were done potentiostatically, and nominally identical electrodes galvanic coupling experiments were conducted with the Al-Cu alloys. Applied potential, chloride concentration, and exposure time were the variables used to establish the critical potentials for pit stabilization and signature time series of metastable and stable pitting events. A distribution of pitting and repassivation (transition) potentials were observed at each Cl{dollar}sp-{dollar} concentration. Factors controlling the transition from metastable to stable pitting were identified by comparing the electrochemical behavior 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. A key criterion for pit stabilization is that individual pits exceed a threshold of I{dollar}sb{lcub}rm pit{rcub}{dollar}/r{dollar}sb{lcub}rm pit{rcub} >{dollar} 10{dollar}sp{lcub}-2{rcub}{dollar} A/cm at all times during pit growth. This parameter was linked to pit growth and mass transport conditions leading to the formation of a depassivating pit chemistry.; Methods from electrochemical noise analyses were compared using the galvanic coupling time series. I{dollar}sb{lcub}rm pit{rcub}{dollar}/r{dollar}sb{lcub}rm pit{rcub}{dollar} analysis of the metastable pit current spikes provided a better indication of the transition to stable pitting than the pitting index and electrochemical noise resistance. Power spectral density plots of the current and potential time series provided qualitative information on pit susceptibility.; Temporal analyses of the galvanic coupling current time series provided evidence that some correlation may exist between individual metastable pitting events. The metastable pitting events were found to deviate from Poisson behavior with increasing activity. Metastable pitting events may have an influence on subsequent pitting events at higher activity.