One-step preparation of molybdate-stannate-tungstate composite conversion coating on magnesium alloy AZ91D and its microstructure and corrosion resistance

A molybdate-stannate-tungstate (Mo-Sn-W) ternary metal salt conversion coating (TMSCC) was prepared on the surface of magnesium alloy (MA) AZ91D through the method of one-step chemical conversion. For Mo-Sn-W@TMSCC, it microstructure was observed and constructed by scanning electron microscope and scanning tunneling microscope, respectively; it composition was detected by energy dispersion spectroscopy and X-ray photoelectron spectroscopy; it anti-corrosion was evaluated by electrochemical measurement and saline immersion. Mo-Sn-W@TMSCC exhibited a hemispherical microstructure and was composed of Mg, Mo, Sn, W and O; the average thickness of Mo-Sn-W@TMSCC was 4.83 µm. In 3.5 wt% NaCl solution, the complete failure of Mo-Sn-W@TMSCC consumed 192 h approximately; further, compared with binary metal salt conversion coating (BMSCC), Mo-Sn-W@TMSCC provided better anti-corrosion for the substrate of MA AZ91D than Mo-Sn@BMSCC, Mo-W@BMSCC and Sn-W@BMSCC did.