The morphology, porosity and corrosion resistance of MAO coatings on as-extruded Mg-Ca alloys were analysed using SEM, XRD, EDS and electrochemical tests. The roles of non-through and through-pores in the MAO coating were identified. The results demonstrated that the corrosion resistance of the MAO-coated Mg-Ca alloys is related to the corrosion resistance of the substrates, the porosity of the MAO coating and the galvanic effect. Chemical and electrochemical corrosion alternately occurred on the MAO coatings. The alternating chemical and electrochemical corrosion mechanisms for the MAO coating are discussed. Display omitted •The change in open circuit potentials leads to alternating degradation mechanisms.•Degradation mechanisms include chemical and electrochemical corrosion.•Chemical corrosion of the coatings may be related to the non-through pores.•Electrochemical corrosion is due to through-pores and micro-cracks of the coating.•Through-pores in the microarc oxidation coating designate a galvanic effect.