In this study, the corrosion behavior of Mg-5Al alloys with different quantities of calcium was investigated by DC and AC polarization and immersion tests. The alloys' microstructure was investigated by optical microscopy and SEM with EDS. It was found that increasing the Ca content from 0.9 to 1.8 wt% improves the corrosion resistance of the alloy; the corrosion resistance of the alloy with 1.8 wt% Ca equals that of the base Mg-5Al alloy. Ca addition causes a reduction of the average grain size, which increases corrosion resistance as the precipitates located at grain boundaries are more continuous and act as a barrier to the advance of corrosion. The precipitates located at grain boundaries contain Al and Ca and are much less cathodic than the beta phase, thus making the microgalvanic effect in the specimen less effective.