Effect of additional Cu and Mg in Al-Mn-Si alloy on intergranular corrosion susceptibility corrosion characteristics has been investigated by SWAAT after heating at 600°C, followed by reheating at 200°C. Intergranular corrosion occured in Al-Mn-Si-Cu alloy after the heating for 1 to 10 h, while pitting corrosion was observed in Al-Mn-Si-Cu-Mg alloy. Intergranular corrosion in the former alloy was cause by selective dissolution of lacking zone containing less content of Cu. In the latter alloy precipitates including Cu were observed not only in grain boundary but also in grain even after shorter heating time at 200°C. Therefore,decreasing intergranular corrosion susceptibility of the alloy was caused by smaller potential difference between lacking zone and grain matrix. In conclusion, addition of Mg in Al-Mn-Si-Cu alloy has corrosion resistance improving effect after heating at higher temperature.

This study investigated the effect of post-brazing cooling rate and Si addition on the intergranular corrosion (IGC) susceptibility of brazed Al-Mn-Cu alloys by electrochemical analysis and microstructure observation. Waterquenched samples after brazing exhibited no IGC susceptibility, whereas slowly-cooled samples were prone to IGC. The results suggest that IGC is caused by precipitation during cooling. In addition, it was observed that IGC susceptibility depended on the Si content. An alloy sample with a low Si-additive content exhibited high IGC susceptibility because Mn/Cu-depleted zone was formed near the grain boundaries as a result of the preferential precipitation of Al 6 (Mn, Fe) and CuAl 2 on the grain boundaries. In contrast, moderate Si addition inhibited IGC because the decrease of the Mn content in the grain interiors due to enhanced precipitation of Al 15 (Mn, Fe) 3 Si 2 in the grain. Additionally, Cu-depleted zone also disappeared because preferential precipitation of CuAl 2 on the grain boundaries was prevented. The excess-Si alloy exhibited high IGC susceptibility because Si-depleted zone formed around the grain boundaries as a result of the preferential precipitation of coarse Si particles on the grain boundaries although the Mn/Cu-depleted zones were not formed.