The intergranular corrosion susceptibility of a heavily overaged Al-Mg-Si-Cu alloy

Usually, underaged or peakaged Al-Mg-Si-Cu alloys are susceptible to intergranular corrosion (IGC), whereas overaging can eliminate the IGC susceptibility. However, it is found that the IGC susceptibility can be formed again when the alloys are heavily overaged further. The IGC susceptibility in the heavily overaged condition results from the anodic dissolution of the precipitation-free zones (PFZs) with the closely-neighbored grain boundary Q-phase (Al4Mg8Si7Cu2) precipitates acting as the continuous cathodes. The formation of the closely-neighbored grain boundary precipitates is attributed to their further growth via dissolving the adjacent matrix precipitates based on the Ostwald ripening process during the long-time overaging.