A zinc–manganese composite phosphate conversion coating for corrosion protection of AZ91D alloy: growth and characteristics

Abstract

A zinc-manganese composite phosphate conversion coating (Zn–Mn PCC) was deposited on AZ91D alloy to enhance its corrosion resistance. The growth mechanism and characteristics of the Zn–Mn PCC were elucidated by adding different ZnO contents in the phosphate bath and for different phosphating times. The results indicate the PCC has a mixed phase of Mg, Mn(OH)2, Mn3(PO4)2‧3H2O, Zn, and Zn3(PO4)2‧4H2O. The Zn–Mn PCC grown on the AZ91D substrate with different ZnO contents had three structure types: island-like, clump-like, and flower-like. Increasing the ZnO content in the phosphate bath increased the coverage, thickness, elemental concentration of Zn, and reduced the cracks and pores of the coating. The contact angle result also revealed that the hydrophobic property of the coating increased with the increase in ZnO content in the phosphate bath. With ZnO modification, the corrosion resistance of the Zn–Mn PCC on the AZ91D alloy improved significantly compared with the bare alloy due to fewer cracks and pores, after especially adding 4 g/L of ZnO into the phosphate bath.