In this paper, the microstructures and corrosion behaviors of as-cast Mg−5Sn−xGa alloys with varying Ga content (x = 0, 0.5, 1, 2, 3 wt %) were investigated. The results indicated that Ga could not only adequately refine the grain structure of the alloys, but could also improve the corrosion resistance. The microstructures of all alloys exhibited typical dendritic morphology. No Ga-rich secondary phases were detected when 0.5 wt % Ga was added, while only the morphology of Mg2Sn phase was changed. However, when the addition rate of Ga exceeded 0.5 wt %, an Mg5Ga2 intermetallic compound started to form from the interdendritic region. The volume fraction of Mg5Ga2 monotonically increased with the increasing Ga addition level. Although Mg5Ga2 phase was cathode phase, its pitting sensitivity was weaker than Mg2Sn. In addition, the standard potential of Ga (−0.55 V) was lower than that of Sn (−0.14 V), which relieved the driving force of the secondary phases for the micro-galvanic corrosion. An optimized composition of 3 wt % Ga was concluded based on the immersion tests and polarization measurements, which recorded the best corrosion resistance.
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