Characteristics of magnesium phosphate coatings formed on AZ31 Mg alloy by plasma electrolytic oxidation with improved current efficiency

Highlights: • High current efficiency was obtained at 800 A.m^−2. • Incorporated Na in the coating played role in the corrosion mechanism. • The PEO coating enhanced surface hardness by fivefold. • The corrosion resistance improved by two orders magnitude. A combination of short processing time and high current density is proposed to enhance the current efficiency of the plasma electrolytic oxidation (PEO) process. Magnesium phosphate coating was synthesized on AZ31 Mg alloy using the PEO method for 3 min at a current density of 600, 800, and 1000 A.m⁻². The current efficiency was predicted by the mass gain. The hardness and corrosion behavior were investigated by micro-Vickers hardness and electrochemical measurements, respectively. The resulting magnesium phosphate coatings enhanced the specimen hardness five times and improved the corrosion resistance by two orders of magnitude. The efficiency of the PEO process was relatively high in the range 21.3–53.3%. The highest efficiency was attained at 800 A.m⁻², where an intense micro-arc dominated the PEO process over initial sparking and post arcing. The proposed parameter proved to be beneficial in improving the efficiency of the PEO process.