Magnesium alloys are remarkably corroded in chloride environments. The fundamental corrosion behavior of Mg–6 mass% Al alloy cast into copper mold has been conducted through the polarization test and electrochemical impedance spectroscopy. Metallographic characteristics were evaluated by optical microscopy using the sample in the condition of as cast, as solution-treated, as under-aged for 172.8 ks and as over-aged for 864 ks. Corrosion potential was different from microstructure in Mg–6 mass% Al copper mold castings with same composition. Pitting potential was equal in Mg–6 mass% Al alloy castings with different microstructure, and the protection film dissolution potential was same. Charge transfer resistance of Mg–6 mass% Al alloy with single phase microstructure in the condition of as solution-treated and as over-aged was large, on the other hand, with double phase microstructure in the condition of as cast and as under-aged was small.
