Nano ZnO-assisted formation of zinc phosphate conversion coating for improving corrosion protection of AZ91D magnesium alloy

In this work, micron and nano ZnO powders were used to assist in the formation of zinc phosphate coatings to improve the corrosion resistance of AZ91D magnesium alloy, respectively. The results showed that the ZnO particles acted as the nucleation sites to promote the formation of flower-like Zn 3 (PO 4 ) 2 · 4H 2 O phase, which was the main component of the zinc phosphate conversion coatings. In addition, the micron ZnO-assisted zinc phosphate conversion coatings (MACC) reduced the corrosion rate of AZ91D alloy from 3.36 to 3.20 mm · a −1 . Besides, the corrosion resistance nano ZnO-assisted zinc phosphate conversion coating (NACC) was four times as high as that of MACC, due to that the NACC had the finer grains and more compact structure. The more compact structure resulted from that the nano ZnO particles not only increased the number of nucleation sites but also decreased the concentration of Zn 2+ in the phosphatization solution to inhibit the growth of Zn 3 (PO 4 ) 2 · 4H 2 O phase.