The effect of Ca additions on the microstructure and corrosion resistance of Mg–9Al and Mg–9Al-0.5Mn alloys is investigated. Model alloys were used instead of a commercial AZ91 alloy in order to avoid masking effects from other alloying elements. Grain size and secondary phases were characterised with special interest in their Volta potential. Corrosion performance was evaluated by hydrogen evolution and electrochemical methods. A synergetic effect was found when Ca and Mn were added in small amounts (0.5 wt%), reaching corrosion rates up to two orders of magnitude lower than the model Mg–9Al alloy. Such improvement was associated with Ca decreasing the Volta potential of the β-phase and with Ca and Mn decreasing the galvanic activity of Fe-rich intermetallics. Precipitation of Al₂Ca decreased the amount of β-phase and had a negative impact on the corrosion resistance due to its cathodic behaviour.