A characterization of the inhibiting effect of Cu on metastable pitting in dilute Al–Cu solid solution alloys

Copper additions to aluminum decrease susceptibility to pit initiation provided that Cu is retained in solid solution. This can be observed as an increase in pitting potential with increasing Cu content in an alloy. To further understand this effect, metastable pitting of high purity Al, Al–0.2Cu and Al–2.0Cu exposed to 0.1 M NaCl solutions has been examined in detail. Results show that 0.2 wt.% Cu additions decrease the metastable pit initiation rate by more than an order of magnitude and slow the pit growth rate mainly by decreasing the peak pit current attained. In an Al–2.0 wt.% alloy, metastable pitting events were too rare for rigorous study. Repassivation of metastable pits occurs by a two-stage process in Al–0.2 wt.% Cu alloy. The repassivation rate during the first stage is identical to that of high purity Al and appears to be completely unaffected by Cu in the alloy or in the pit solution. In the second stage, repassivation is slow, but is not believed to affect ultimate pit stability. Overall, Cu additions decrease the probability of stable pit formation by decreasing metastable pit initiation and growth rates.