Oxide coatings were formed on AZ91D Mg alloy by microarc oxidation (MAO) in a sodium aluminate (NaAlO2)-based electrolyte, and the effects of additive electrolytes, KF, NaOH, and KOH, on the physical and chemical properties of the MAO coatings were investigated, particularly focusing on their corrosion resistance determined by potentiodynamic and potentiostatic tests. The anodizing characteristics (breakdown voltage and ignition time) and the phase, surface morphology, and thickness of the MAO coatings were strongly dependent on the additive electrolytes. The intense and continuous sparking in the KF-NaAlO2 electrolyte resulted in a thick MgAl2O4 coating containing F- ions, whereas the MAO coating obtained in either NaOH or KOH-NaAlO2 electrolyte was thin and poorly crystalline, resulting from the unstable and discontinuous sparking. The MAO coating obtained in the KF-NaAlO2 electrolyte exhibited the highest corrosion potential, the lowest corrosion current density, and the highest polarization resistance, which were closely related to the crystalline MgAl2O4 phase and the relatively thick dense inner barrier layer. The pitting corrosion occurred in AZ91D Mg alloy, and the pitting was significantly protected by the MAO coating obtained in the KF-NaAlO2 electrolyte. The pitting potentials for the MAO coatings have been determined by potentiostatic tests.