The microstructures and corrosion resistance of magnesium-5 wt% aluminum-0.3 wt% manganese alloys (Mg-Al-Mn) with different Ca additions (0.2-4 wt%) were investigated. Results showed that with increasing Ca addition, the grain of the alloys became more refined, whereas the corrosion resistant ability of the alloys initially increased and then decreased. The alloy with 2 wt% Ca addition exhibited the best corrosion resistance, attributed to the effect of the oxide film and (Mg,Al)(2)Ca phases which were discontinuously distributed on the grain boundaries. These phases acted as micro-victims, they preferentially corroded to protect the alpha-Mg matrix. The oxide film formed on the alloy surface can hinder the solution further to protect the alpha-Mg matrix. (C) 2016 Elsevier B.V. All rights reserved.