Difference in the Precursory Process of the Intergranular Corrosion of Aged Al-Cu and Al-Cu-Mg Alloys in 0.1 M NaCl

Real-time in situ optical microscopy observations of the initiation behavior of intergranular corrosion on artificially aged Al-4.5Cu and Al-4.5Cu-1.5Mg were performed in naturally aerated 0.1 M NaCl at pH 6.0. For both alloys, the discoloration of intermetallic particles occurred before intergranular corrosion, and a discolored coarse intermetallic particle on the grain boundary acted as the initiation site for intergranular corrosion (Al₂Cu for Al-4.5Cu and Al₂CuMg for Al-4.5Cu-1.5Mg). The discoloration of Al₂Cu particles was localized and occurred only on a small number of particles. However, almost all Al₂CuMg particles were discolored; the overall surface of the particles was discolored uniformly. The discoloration of Al₂Cu on Al-4.5Cu led to micropitting. In contrast, the discoloration of Al₂CuMg on Al-4.5Cu-1.5Mg caused the trenching of particles. The difference in the initiation behavior of intergranular corrosion was discussed in terms of these precursory phenomena.