AbstractIn order to form calcium phosphate (CaPh) coatings in the changing chemical environment, a drop wise addition procedure is performed during electro-deposition process. Two kind of electrolytes of KH2PO4 and K2HPO4, are added into Ca(NO3)2 solution respectively where plasma electrolytic oxidized (PEO) magnesium is placed. Results indicate that the phase composition and morphologies of the CaPh coatings are modified due to the changing chemical composition of electrolyte. Only calcium hydrogen phosphate dihydrate (DCPD) is formed on the PEO surface in the presence of KH2PO4 electrolyte, while a combination of hydroxyapatite (HA) and DCPD is obtained on the PEO surface in the electrolyte containing K2HPO4. The latter coating exhibits superior corrosion resistance in simulated body fluid, which implies the feasibility of the proposed strategy in fabricating biodegradable coating for Mg alloys.