Growth of zinc phosphate coatings on AZ91D magnesium alloy

Zinc phosphate coating was formed on AZ91D magnesium alloy through a phosphating bath where H3PO4, ZnO, Zn(NO3)2 and NaF were applied. Chlorate (NaClO3) was used as an accelerator of phosphatization to replace nitrite. The coating compositions were hopeite (Zn3(PO4)2·4H2O) and Zn. The growth process of the phosphate film on the magnesium alloy substrate was investigated by SEM observation and XRD analysis. In the early stage (0–60 s) of phosphatization, both hopeite and metallic zinc nucleated and grew together on the substrate to form flower structure. A great number and uniform nucleation of hopeite and metallic zinc implied that they formed on the β and α phases of the AZ91D magnesium alloy simultaneously. Afterwards, because magnesium alloy substrate was almost fully covered, only hopeite deposited continuously to form slab-like structure. The formed zinc phosphate coatings exhibited high corrosion resistance in 5% NaCl solution as shown by polarization measurement.