Electrochemical characterization of passivity and pitting in Al-Cu solid solutions

When Cu is alloyed with Al in an Al-Cu solid solution, a susceptibility to pit initiation decreases. Results from this study, show that resistance to pit initiation, as measured by the critical pitting potential, is dramatically increased for Al-Cu alloys if they are subject to potentiostatic polarization in passive region for times ranging 'from 50 to 500 minutes. To better understand why Cu has this effect a detailed electrochemical characterization of individual and ensemble metastable pitting events was performed. Experiments followed the methods of Sato and Newman to obtain characteristics of metastable pitting events such as the peak pit current (I _peak), pit growth time (t _g), and pit repassivation time (t _r). The results showed that alloying Cu in Al-Cu solid solutions reduces a pit initiation frequency (i.e. passivating effect) as well as a pit growth rate and peak current (i.e. ennobling effect) but increases pit repassivation time.