Effect of Cu content on quenching sensitivity relative to exfoliation corrosion susceptibility of Al-Zn-Mg-(Cu) alloys

Abstract

The effect of Cu content on quenching sensitivity relative to exfoliation corrosion susceptibility of Al-Zn-Mg-(Cu) alloys was investigated by electrochemical test, immersion test, optical microscopy, electron backscattered diffraction, scanning electron microscopy, scanning transmission electron microscopy and scanning Kelvin probe force microscopy. As quenching rate decreases from 400 °C/s to 4 °C/s, exfoliation corrosion (EXCO) susceptibility increases for Cu-containing alloys (0.5–2.6 wt%); while for the Cu-free alloy, EXCO susceptibility first increases and then decreases, reaching the lowest at the quenching rate of 70 °C/s, and then continues to increase. Quenching sensitivity of the Cu-free alloy is the lowest, and that of Cu-containing alloys is similar. The decrease of quenching rate or the increase of Cu content leads to a larger difference in the Volta potential between grain boundary precipitates (GBPs) and the matrix. Corrosion tends to propagate only along grain boundaries at high quenching rates, but along both grain boundaries and subgrain boundaries below a critical quenching rate, which increases with increasing Cu content. The corrosion behavior has been discussed primarily based on the features of GBPs and precipitates free zones, and the Volta potential difference between GBPs and the matrix. • Quenching sensitivity is the lowest for Al-Zn-Mg and similar for Al-Zn-Mg-Cu. • EXCO susceptibility of Al-Zn-Mg-Cu is higher at a lower quench rate. • EXCO susceptibility of Al-Zn-Mg is lowest at the quench rate of 70 °C/s. • Corrosion path is related to quench rate and Cu content. • Potential difference is larger at a lower quench rate or a higher Cu content.