Fabrication of a model specimen for understanding micro-galvanic corrosion at the boundary of α-Mg and β-Mg₁₇Al₁₂

A model specimen with a single boundary of the α/β phase simulating Mg-Al alloys was successfully fabricated by spark plasma sintering. A small electrode area of α phase or β phase was prepared using the model specimen, and the OCPs (open-circuit potentials) of each phase and a small electrode area containing the α/β phase boundary in 0.1 M NaCl at pH 8.0 were compared: the β phase exhibited a higher potential, and the α phase showed a lower potential. The OCP of the small area containing the α/β phase boundary was the intermediate value of these phases. In a small area containing α/β phase boundary, discoloration and gas bubbles were observed on the α phase, but no bubble generation was detected on the β phase. The gas bubbles were initially generated on the α phase near the β phase, but as the discoloration (corrosion) of the α phase approached the β phase, the bubbles were generated on the β phase. In micro-galvanic corrosion of the α and β phases, the β phase did not always function as the preferred cathode. The α phase partially corroded (or discolored) and became the anodes, so that the surrounding areas were most likely to be the cathodes. When corroded areas (anodes) in the α phase approached the β phase, the β phase would become cathodes. In addition to the micro-galvanic corrosion mechanism, the role of Al in corrosion resistance at the α/β phase boundary was determined by surface analysis.