Influence of alloy chemistry and overaging on mechanical properties and corrosion resistance in aluminium crossover alloys

This study examines the impact of alloy composition and overageing heat treatments on mechanical properties and corrosion behaviour of aluminium crossover alloys, with a particular focus on the AlMgZn(Cu,Ag) system. Mechanical properties were evaluated through hardness and tensile testing, while corrosion resistance was examined in a solution of NaCl through mass loss tests, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). Mass loss tests indicated that addition of noble elements (Cu,Ag) has an adverse effect on corrosion resistance, which was further corroborated by PDP and EIS. These observations are not in accordance with effects commonly observed in classical 5xxx and 7xxx alloys, from which the alloy originates. To establish a correlation between mechanical and corrosion properties and the microstructure, scanning and transmission electron microscopy (SEM,S/TEM) were employed. We resort to characterization techniques to investigate the influence of grain size, texture, grain boundary precipitate, precipitate-free zone, and bulk precipitate characteristics on the observed corrosion response. The results demonstrate that the presence of Cu and Ag primarily weakens the protective oxide layer and creates a galvanic coupling, resulting in increased corrosion currents between matrix and precipitates. Double-step overageing primarily causes precipitate coarsening. While the addition of Cu and Ag can enhance mechanical properties by refining strengthening precipitates, their addition deteriorate corrosion resistance in novel 5xxx/7xxx aluminium crossover alloys.