Effect of retrogression treatment on microstructure, mechanical properties, and corrosion behavior in Al–Zn–Mg–Cu alloy

Tailoring the microstructure through heat treatment is crucial for improving both the mechanical properties and corrosion resistance of Al-Zn-Mg-Cu alloys. In this study, we investigate the effect of retrogression and re-aging (RRA) at two different retrogression temperatures that is, 170 and 190 o C on the microstructure, hardness, electrical conductivity, tensile properties, and corrosion behavior of Al-Zn-Mg-Cu alloys. Compared with the conventional T6 alloy, the RRA alloys increased the volume fraction of the η' phase, resulting in higher hardness and tensile strength. Electrochemical analyses revealed that the T6 alloy exhibited the lowest corrosion rate, whereas the RRA alloys exhibited elevated corrosion rates. The higher retrogression temperature used in the RRA process increased the Cu content of precipitates at the expense of solute atoms in the matrix. In RRA alloys, the increased Cu content within the precipitates formed a strong galvanic coupling at the grain boundaries, leading to severe intergranular corrosion, eventually resulting in exfoliation corrosion, surface detachment, and a higher corrosion rate. By contrast, fine precipitates with lower Cu content in the T6 alloy initially promoted localized corrosion attack, but also facilitated the formation of a stable passive oxide layer, contributing to a reduced corrosion rate. These findings deepened our understanding of the effect of retrogression treatment on the microstructure, mechanical properties, and corrosion behavior of Al-Zn-Mg-Cu alloys, paving the way for further process optimization.