Plasma electrolytic oxidation (PEO) on a newly designed ACM522 magnesium die-casting alloy has been conducted in an aqueous phosphate solution and the morphology and corrosion resistance of the PEO films were investigated in detail. By covering the whole surface of the substrate with the PEO film, the corrosion resistance was significantly improved; the modified surface endured a salt spray test for 168h. It was also clarified that traces of local breakdown caused by excessive PEO can be a cause of pitting corrosion, resulting in significant decline in corrosion resistance with PEO time.