A study and application of zinc phosphate coating on AZ91D magnesium alloy

A technology for obtaining the zinc phosphate coating on an AZ91D magnesium alloy was investigated. The main ingredients in the phosphating bath were H3PO4, ZnO and NaF. This coating has typical phosphate microstructure, and the compositions in this coating analyzed by XRD are Zn3(PO4)2·4H2O, Zn, AlPO4 and MgZn2(PO4)2. Scanning electron microscopy (SEM) has shown that very fine zinc particles surrounded phosphate crystals and filled in the interstice of the insoluble phosphate. Salt spray tests have indicated that the zinc phosphate coating has better corrosion resistance due to the presence of crystalline zinc in the coating. The zinc phosphate coating was used as the base layer for further paint. Salt spray showed that the paint plus phosphate coating had good corrosion resistance. The adhesion of paint on the zinc phosphate coating was better than that of paint on chromate conversion coating due to the microporous structure of the phosphate coating.