The effect of Cu and Sc on the localized corrosion resistance of Al-Zn-Mg-X alloys

In the present study, the microstructure and localized corrosion behavior of three Al-Zn-Mg-X alloys with Sc addition (MSc), Cu addition (MCu) and Cu + Sc addition (MCuSc) under T74 (121 °C/6 h + 163 °C/24 h) condition have been investigated. Electron backscattered diffraction and transmission electron microscope were used to analyze the microstructure of the three alloys. Pitting corrosion, intergranular corrosion, stress corrosion cracking, ASTM B117 salt spray exposure testing, and electrochemical tests, were carried out to investigate the corrosion behavior of the three alloys. Results show that the addition of Sc and/or the removal of Cu can greatly improve the localized corrosion resistance of the Al-Zn-Mg-X alloys. The pitting corrosion in the MCu and MCuSc alloys is severer than that in the MSc alloy, which is mainly attributed to the dealloying effect occurred in the Al₇Cu₂Fe and Al₂CuMg instead of Al₃(Sc_xZr_1-x) particles. The MSc alloy shows the highest resistance to pitting corrosion, EXCO and SCC. The modified GB features and the decreased potential difference between the GB precipitates and matrix were found to be the key factors to improve corrosion resistance of Al-Zn-Mg-X alloys. This provides a theoretical and experimental basis for the design of new Al-Zn-Mg-X alloys with excellent corrosion resistance and mechanical properties.