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The corrosion behavior of 7xxx aluminum alloys with various Cu content was investigated using polarization and electrochemical impedance spectroscopy (EIS) techniques. Two breakdown potentials were found in Cu-containing alloys in deaerated chloride solution, and the values increased logarithmically with increasing Cu content. However, in aerated chloride solution, polarization resistance as determined by EIS decreased with increasing Cu content. The first breakdown potential corresponded to transient dissolution associated with attack of the fine hardening particles and the surrounding solid solution in a thin surface layer. The second breakdown potential was associated with combined intergranular and selective grain attack. The correlation between the microstructure, especially Cu content, and corrosion behavior was made by composition analysis of the grain boundary regions including precipitate free zone and grain boundary precipitates as well as the matrix by scanning transmission electron microscopy. The corrosion potential increased with increasing alloy Cu content in aerated chloride solutions because of enhanced rates of oxygen reduction. This degraded the corrosion resistance despite the increase in the breakdown potentials with Cu content.
Meng et al. (Thu,) studied this question.