Growth behaviour of low-energy plasma electrolytic oxidation coatings on a magnesium alloy

Plasma electrolytic oxidation (PEO), a promising surface treatment method to improve the corrosion and wear resistance of magnesium and its alloys, operates at high voltages, resulting in a relatively high energy cost. To make the PEO process more economically viable, its energy efficiency needs to be improved. This study investigates the growth behaviour and microstructural characteristics of low-energy PEO coatings on an AM50 magnesium alloy in a concentrated electrolyte containing sodium tetraborate. The surface morphology of the coatings was different from typical PEO coating morphologies and a large voltage oscillation was observed during treatment. Using different characterisation techniques, and based on a micro-discharge model, a correlation was made between the voltage-time behaviour, micro-discharge characteristics and the composition and microstructure of the coated samples. The results suggest electrolyte chemistry can somewhat control discharge behaviour, which plays an important role in PEO coating growth.