Improvement of the properties of AZ91D magnesium alloy by treatment with a molten AlCl3–NaCl salt to form an Mg–Al intermetallic surface layer

A process for surface modification of AZ91D magnesium alloy has been carried out in AlCl3–NaCl molten salts melt at 573 K for 7 h. The process was used to form a diffusion alloying layer on the surface of the sample to improve corrosion resistance. The diffusion alloying layer was mainly composed of Mg–Al intermetallic compounds. The continued immersion test in 5% NaCl solutions displayed that molten salt diffusion-treated specimen had better corrosion resistance compared to the untreated AZ91D specimen. The polarization test indicated that the Mg–Al intermetallic compounds of the layer were an effective corrosion barrier to decrease the corrosion rate for AZ91D magnesium alloy when exposed to 3.5% NaCl solutions. The microhardness values of the Mg–Al intermetallic compounds were higher than those of the substrate.