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

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