A universal electrolyte for the plasma electrolytic oxidation of aluminum and magnesium alloys

The plasma electrolytic oxidation (PEO) of aluminum and magnesium alloys is carried out in electrolytes which contain the same reactants, but in fundamentally different concentrations. In this research the possibility of the PEO of aluminum and magnesium alloys in a universal electrolyte is studied. The two most commonly encountered alloys, namely, aluminum alloy 5052 and magnesium alloy AZ91D, are chosen. The oxide layers obtained are studied using SEM, EDX, XRD, and a microhardness tester. The corrosion properties are determined using a potentiostat. The effect of variation of the silicate concentration in the electrolyte on the growth kinetics of the coating and its qualitative characteristics is discussed. It is shown that at Na₂SiO₃·5H₂O concentrations in the electrolyte ranging from 3.2 to 32gl^-1, the rate of growth of the oxide layer increases from 15 to 55μmh^-1 with significant variation of the phase composition of the coating. The greatest hardness of an oxide ceramic layer was obtained in the outer sublayer on the magnesium alloys (874.7HV₁₀) and in the inner sublayer on the aluminum alloys (1123HV₁₀). The most favorable combination of physical and chemical properties for both alloys is obtained in an electrolyte containing 12.72gl^-1 Na₂SiO₃·5H₂O.