Improved corrosion resistance of AZ91D magnesium alloy by a zinc–yttrium coating

• A zinc–yttrium coating can be formed on the surface of AZ91D magnesium alloy.
• The coating contains a large amount of intermetallic compound Mg5Al2Zn2.
• The microhardness values of the coating are much higher than that of the substrate.
• The coating can improve the corrosion resistance of the substrate effectively.

A zinc–yttrium coating on AZ91D magnesium alloy was conducted by diffusion treatment in order to improve its corrosion resistance and wear resistance. The microstructures and phase constituents of the zinc–yttrium coating were investigated using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrum (EDS). The results reveal that a zinc–yttrium coating has been formed on the surface of magnesium specimens by the solidification of the liquid layer formed between the AZ91D magnesium alloy and the Zn, Y mixed powders. The microstructure of the zinc–yttrium coating is typical eutectic structure, which contains a large amount of intermetallic compound, such as Mg5Al2Zn2. In addition, the microhardness values of the intermetallic compounds are much higher than those of the substrate and this would greatly contribute to the enhancement of wear resistance. The results of electrochemical corrosion tests in 3.5 wt.% NaCl solution show that the corrosion resistance of the coated specimens has been increased significantly.