In situ microscopy and electrochemical characteristics of initiation of intergranular corrosion of aging-treated Al–1Mg–0.65Si–0.8Cu alloy in 0.1 M NaCl

This study investigated the initiation site of intergranular corrosion (IGC) in aging-treated Al–1Mg–0.65Si–0.8Cu alloy in 0.1 M NaCl. Polarization and open-circuit potential measurements were performed. Further, in situ observations were made using a small electrode area of 300 µm × 300 µm. In potentiodynamic polarization, the breakdown potential was −0.4 V vs. SHE, leading to pitting corrosion, which in turn initiated IGC. The OCP under naturally aerated conditions increased from approximately −1.2 to −0.4 V over time, and IGC was initiated at −0.4 V. Even in the case of the small electrode area (300 µm × 300 µm), IGC occurred at −0.4 V in potentiostatic polarization; high-spatial-resolution in situ observations were then performed. IGC initiated from randomly distributed Mg 2 Si particles and generation of gas bubbles. Moreover, the Mg 2 Si particles at the initiation site of IGC were located on or near the grain boundaries, and trenching was observed at the Mg 2 Si/alloy matrix boundaries. In addition to the exposure of the bare alloy surface by trenching, the acidification inside the trench likely contributed to the initiation of IGC. The role of Mg 2 Si in the initiation of IGC was examined based on the results of in situ observations and the polarization behavior. • Mg 2 Si particles (≥1 µm) were randomly distributed in Al–1Mg–0.65Si–0.8Cu alloy. • Intergranular corrosion (IGC) initiated at Mg 2 Si located on or near grain boundary. • Trenching between Mg 2 Si and alloy matrix provided the sites for IGC. • An acid in the trench likely dissolves the Cu-depleted zone and leads to IGC. • Corrosion tunnels around the Mg 2 Si might also be related to the IGC initiation.