Synthesis and growth mechanism of ceramic coatings on an Al-Cu alloy using plasma electrolytic oxidation in molten salt

•Al2O3 coating was developed by plasma electrolytic oxidation in molten salt.•Effect of PEO current frequency on structure and performance was examined.•The highest PEO coating properties were obtained using current frequency of 300 Hz.•The oxide double-layered coating contains inner compact and outer porous layer.•Two mechanisms, at high and low current frequencies, were suggested and described.