An environmentally friendly molybdate/phosphate black film on Mg–Zn–Y–Zr alloy

An environmentally friendly molybdate/phosphate black film for light absorption application was developed as an alternative to the chromate system on Mg–Zn–Y–Zr alloy. The microstructure, surface morphology, chemical composition, light absorbance and protection property of the black film were investigated. The initiation and growth mechanisms of the black film were also discussed. The black film consists of MoO2 and a fraction of Mg3(PO4)2. The MoO2 contributes to the black color and the Mg3(PO4)2 aims to improve the protection property. The experimental results indicate that the black film can provide not only high light absorbance but also corrosion protection to the Mg–Zn–Y–Zr substrate. The formation mechanism of the black film is associated with the electrochemical heterogeneity of the Mg–Zn–Y–Zr substrate. At the cathodic sites, MoO42− is reduced to MoO2 accompanying with the hydrogen evolution reaction. At the anodic sites, the dissolution of Mg2+ bonds with PO43− to form Mg3(PO4)2.