Abstract
A new chromium free zinc–phosphate–molybdate composite conversion coating on AZ91D magnesium alloy has been developed using an acidic bath mainly containing zinc phosphate, sodium molybdate and sodium fluoride. The examinations of the microstructure and phase compositions reveal that the composite conversion coating is composed of nanocrystalline metallic Zn, Zn3(PO4)2.4H2O and MgMoO4. It has been found that the addition of molybdate in the conversion bath can refine the microstructure of the composite conversion coating and eliminate the formation of microcracks. Corrosion tests demonstrated that the zinc–phosphate–molybdate composite coating possesses a better corrosion resistance than conventional zinc–phosphate conversion coatings in 3%NaCl and NaOH (pH 13) solutions. This newly developed composite conversion coating has a low weight loss during electrocoating process and can effectively improve the adhesion and corrosion resistance of the electrocoat on AZ91D alloy.