Nanoscale Corrosion Mechanism at Grain Boundaries of the Al-Zn-Mg Alloy Investigated by Open-Loop Electric Potential Microscopy

Grain boundary (GB) corrosion of Al alloy is often ascribed to the anodic dissolution of nanoscale MgZn2 precipitates formed along the GB. However, this reaction has not been confirmed by direct observation and does not explain the subsequent process to form a large-scale pit. Here, we clarify these points by directly visualizing changes in nanoscale structures and potential (similar to local corrosion cell) distributions during the GB corrosion in the H2SO4 solution by open-loop electric potential microscopy (OL-EPM). The potential images unambiguously confirm the anodic dissolution of the MgZn2 precipitates induced by immersion into the H2SO4 solution. In addition, we found that erosion of the GB after MgZn2 dissolution is driven by hydrogen embrittlement (HE) until another underlying MgZn2 precipitate is exposed. These nanolevel understandings should help improve the susceptibility of Al alloys to stress corrosion cracking originating from the GB corrosion.