PEO coatings obtained on an Mg–Mn type alloy under unipolar and bipolar modes in silicate-containing electrolytes

Protective surface layers with high corrosion resistance (Rp = 3.3·105 ohm cm2) and significant microhardness (H = 4.8 GPa), as compared to the substrate material, were obtained on MA8 magnesium alloy by bipolar Plasma Electrolytic Oxidation (PEO) in a silicate–fluoride electrolyte. The phase and elemental composition and morphology of the coatings were investigated. It was found that the application of the bipolar PEO mode enables one to synthesise on the alloy's surface a high-temperature phase of magnesium silicate, forsterite (Mg2SiO4) having good anticorrosion and mechanical properties.