Protection of magnesium alloys from faster degradation in physiological environment is of primary importance in order for them to be used as biodegradable implant materials. Present investigation reports the effect of sodium tetra borate (Na2B4O7· 10H2O) and potassium titanium fluoride (K2TiF6) on corrosion behavior of oxide coatings developed on ZM21 magnesium alloy by plasma electrolytic oxidation (PEO) in Na2SiO3-KOH base electrolyte system. The effect of these additives on the microstructure and corrosion properties of coatings was investigated. The surface morphology and phase composition of these ceramic coatings were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Analysis of pores in the coating was done using a three-dimensional (3D) optical profilometer. The corrosion resistance was evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF) environment at a pH of 7.4. It was found that the use of additives significantly altered the pore morphology of the coating. Among the PEO treated samples, the coating formed in silicate based electrolyte system containing 2g/L of Na2B4O7· 10H2O showed less porosity and improved corrosion resistance in SBF environment. © 2013 Elsevier B.V.