Improved corrosion resistance of Mg alloy by surface AI electrodeposition

A metallic aluminum (Al) layer was successfully electrodeposited onto a magnesium (Mg) alloy in a Lewis acidic aluminum chloride-1-ethyl-3- methylimidazolium chloride (AICI₃-EMIC) ionic liquid under a potentiostatic condition at room temperature. Effects of the deposition potential on material characteristics of the deposited layers were explored by means of a scanning electron microscope and an X-ray diffractometer. In addition, the improvement in corrosion resistance of the Mg alloy due to the AI coating was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization measurements. The electrochemical impedance spectroscopic data indicated that a bare Mg alloy had a polarization resistance of only 500 Ω-cm² in 3.5 wt% NaCl solution while the Al-coated Mg sample showed its resistance as high as 5000 Ω-cm² in the same environment. Moreover, it was also found that the AI layer deposited at a lower overpotential was more compact and uniform (as compared with that deposited at a higher overpotential); consequently, this coating revealed a superior protection capability for the Mg substrate against corrosion.