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

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