Effect of aging time on microstructure evolution and corrosion behavior of an Al-Mg-Si alloy

This investigation aims at assessing the microstructural features and associated corrosion behavior of a specific type of Al-Mg-Si alloy with various heat-treatment conditions: underaging (T4), peakaging (T6), overaging (T7). The results show that the alloy exhibits localized corrosion in T4 temper. T6 sample exhibits the heaviest intergranular corrosion (IGC), T7 sample exhibits combined corrosion form of IGC and pitting corrosion. The microstructural changes in the size and distribution of the grain boundary precipitates are the main reasons of the transformation of the corrosion form. The pitting corrosion in the underaged condition is associated with the preferential dissolution of MgSi and the anodic dissolution of the solute-depleted zones (SDZs). The heavy IGC susceptibility in T6 temper results from the closely-neighbored grain boundary precipitates and precipitation free zones (PFZs). Coarse precipitates in the grain boundaries with discontinuous distribution and the reduction of composition difference between matrix and PFZ improve the IGC resistance.