Double-layered manganese phosphate conversion coating on magnesium alloy AZ91D: Insights into coating formation, growth and corrosion resistance

► A new Mn-PO4 based conversion coating system was developed for magnesium alloy AZ91D. ► The coating was a highly protective double-layered coating system against corrosion. ► pH and temperature both have a critical effect on coating properties. ► The coating generated at 80 °C and pH 4 had the lowest corrosion rate.