Investigation on phosphate films on the magnesium alloys surface

The solution which mainly contained Zn (H2PO4)2 was used to prepare phosphate films on AZ31 and AZ61 magnesium alloys surface. Through orthogonal tests, the process parameters and the solution composition was defined. The microstructure and compositions of the phosphate films were analyzed by optical microscope, SEM, spectrum and XRD. The anti-corrosion ability of the films was researched by NSS and some other methods. The possible conversion process of the phosphate reactions and anti-corrosion mechanism of the films were discussed. The results show that the crystals of the conversion films is very fine, and the major composition of the films is [Zn3(PO4)2 ·4H2O]·[Zn2 Mg(PO4)2·4H2O]. The binding force of the coatings with the films can reach grade 1. There is no obvious change on the coatings either in 3% NaCl solution for 360 h, or in neutral salt spray test for 144 h.