Continuous intermetallic compounds coatings on AZ91D Mg alloy fabricated by diffusion reaction of Mg-Al couples

► Intermetallics coatings were fabricated on AZ91D by Mg-Al diffusion reaction. ► The coating showed nearly two times higher hardness than that of AZ91D substrate. ► The corrosion resistance of AZ91D was dramatically improved by intermetallics. ► The intermetallics layers exhibited passive behavior in 3.5% NaCl solution.