An Al coating film, electrodeposited on a Mg alloy from aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3-EMIC) ionic liquid, effectively prevents the substrate from rapid corrosion in a hostile environment. The thickness of the Al film can be easily determined by controlling the total cathodic charge applied, because the current efficiency of the electrodeposition reaction is close to 100%. Heat treatment at 450°C under an argon atmosphere for 10min causes an inter-diffusion at the Al/Mg interface, optimizing the protective performance of the coating film. Prolonging heating leads to a Mg17Al12 intermetallic phase and a Mg solid solution phase to be formed at the expense of the deposited Al film. This phase transformation gives rise to a degradation in the corrosion resistance of the Al-coated sample.