Intergranular Corrosion Susceptibility of Al-Mn-Cu Alloys, Heat-Treated after Simulated Brazing

The development of air-conditioners using CO₂ refrigerants in place of freon-based refrigerants is underway. In these air-conditioners, the temperature of the heat-exchange cycle increases to 453 K [1]. The intergranular corrosion susceptibility of the Al-Mn-Cu alloy was investigated using the anode galvanic dissolution test after performing brazing and heat treatment at 453 K.

The intergranular corrosion susceptibility was dependent on the amounts of Mn and Cu, and heat treatment at 453 K resulted in an increase in the intergranular corrosion susceptibility. In the case where brazing was not followed by heat treatment at 453 K, TEM observation revealed Al-Cu intermetallic compounds with Al-Mn precipitates on the grain boundary. On the other hand, when heat treatment was carried out at 453 K, the growth of both Al-Cu intermetallic compounds and their precipitates was observed on the grain boundary with no Al-Mn precipitates. It is believed that the Cu-depleted zone along the grain boundaries increases with the precipitation of Cu on the grain boundary. This is believed to be the reason for an increase in the intergranular corrosion susceptibility with an increase in both the duration of heating at 453 K and the amounts of Mn and Cu.