Conversion-coating treatment for magnesium alloys by a permanganate–phosphate solution

A chrome-free conversion coating treatment, a mainly permanganate–phosphate solution, was developed in this study for magnesium and its alloys. The morphology of the conversion-coated layer was observed using SEM; the crystal structure and the composition was analysed and determined by using XRD and XPS. The XRD results indicated that amorphous structures existed that covered the coated specimen. The solution treated specimen showed a greater intensity of Mg17Al12 than the as-received. The XPS results indicated that the coated layer contained products of MgO, Mg(OH)2, MgAl2O4, Al2O3, Al(OH)3, MnO2 or Mn2O3 and amorphous oxy-hydroxides for Mg–Al–Zn alloys, but MgO, Mg(OH)2, MnO2, amorphous oxy-hydroxides and MgMn2O8 for pure Mg. The electrochemical polarisation test results demonstrated that the presented conversion treatment for the series of AZ alloys had an equivalent corrosion protection potential as the JIS H 8651 MX-1 (similar to Dow No. 1) chrome-based method.