Effect of fluoride on plasma electrolytic oxidation of AZ61 magnesium alloy

Plasma electrolytic oxidation has been carried out on AZ61 magnesium alloy in a silicate–phosphate electrolyte under pulsed current conditions. It is shown that the addition of fluoride to the electrolyte reduced localized corrosion that occurred near Al–Mn intermetallic particles before sparking begins. Such corrosion affected the uniformity of the PEO coating. On the other hand, fluoride ions caused significant loss of coating material during coating growth. Irrespective of the fluoride addition, the coatings contained Mg3(PO4)2, amorphous material and possibly MgO. However, fluoride strongly affected the rate of coating growth and the morphology of the coating. The loss of coating material is suggested to be due to the effects of fluoride ions on the intensity of the discharges and melting of the coating.