Effect of Cu content and aging conditions on pitting corrosion damage of 7xxx series aluminum alloys

The corrosion behavior and microstructure of AA7055 alloy with a high Cu content, and a Cu-free Al-Zn-Mg alloy in various aging conditions were studied. Polarization measurements and ASTM B117 salt-spray exposure were used to assess the corrosion characteristics. Transmission electron microscope and atom probe tomography were used to evaluate the microstructure and the composition of precipitates. The matrix composition of AA7055 changed with aging because of precipitation. Thus, the galvanic relationship between Al₇Cu₂Fe particles and the surrounding matrix correspondingly changed, which affected the susceptibility to pitting corrosion attack. The susceptibility to pitting corrosion damage of AA7055 was found to decrease according to: over-aged samples > under-aged samples > peak-aged sample. However, aging had only a small influence on pitting corrosion damage for the Cu-free Al-Zn-Mg alloy. Furthermore, despite the more active breakdown potential of the Cu-free Al-Zn-Mg alloy, this alloy exhibited less severe pitting corrosion damage than AA7055, in particular for the over-aged samples. This was attributed to the weaker galvanic coupling between Al₃Fe particles and the surrounding matrix. Although Cu in the intermetallic particles is detrimental to pitting resistance, the galvanic effect can be reduced by increasing the matrix Cu content.