The influence of aging temperature on microstructure and intergranular corrosion of Al-Mg-Si alloys with low concentrations of Cu and Zn

The influence of the aging temperature on intergranular corrosion (IGC) of Al-Mg-Si alloys with minor Cu and Zn additions has been investigated to clarify the correlation between the alloy composition, grain boundary chemistry and corrosion resistance in an acidified NaCl solution. Microstructural analysis using scanning transmission electron microscopy (STEM) indicates the presence of Cu-rich films and Cu/Zn containing Mg-Si particles along grain boundaries in an underaged condition of an alloy with 0.05 wt% Cu and 0.06 wt% Zn, which is characterized by high susceptibility to IGC. A peak-aged condition of this alloy is also found to be highly susceptible to IGC. An alloy containing only ~0.03 wt% Cu exhibits lower IGC resistance compared to the alloy containing both Cu and Zn, which suggests that balanced Cu and Zn additions affect corrosion pathways differently from additions of either Cu or Zn alone. High-resolution STEM reveals structural β′′ units and local Q′/C and B′ configurations in an overaged sample containing both Cu and Zn. In addition, bright atomic columns are detected within intragranular precipitates and at precipitate/Al interfaces indicating the presence of Cu and/or Zn atoms. These findings demonstrate that even low Cu and Zn additions can significantly influence grain boundary chemistry and particle composition, thereby affecting the IGC resistance. The results provide useful insights into the influence of compositional tuning and aging temperature on the IGC resistance of Al-Mg-Si alloys. • IGC resistance decreases with increased Cu/Zn ratio. • Aging temperature significantly affects grain boundary chemistry, even with minor Cu/Zn additions. • Cu/Zn additions significantly influenced the precipitate structures within the grain interiors.