Design and Fabrication of a Sandwichlike Zn/Cu/Al–Zr Coating for Superior Anticorrosive Protection Performance of ZM5 Mg Alloy

A new three-layered film was fabricated on magnesium (Mg) alloy via electroplating to guard against corrosion in a chloride aqueous environment, which consisted of an underlying double-layered zinc/copper (Zn/Cu) and a top aluminum–zirconium (Al–Zr) layer. The Zn/Cu underlayers not only impeded the galvanic corrosion between the Al–Zr coating and Mg alloy but also improved the adhesive ability between the substrate and the upper Al–Zr layer. Herein, we discussed the nucleus sizes of Al–Zr coatings at the stage of nucleation carried out with different contents of ZrCl4 in AlCl3–1-butyl-3-methylimidazolium chloride ionic liquid. The sandwichlike three-layered Zn/Cu/Al–Zr coatings were systematically investigated by surface morphology, phase structure, hardness, anticorrosion performances, and first-principles calculations. The corrosion current density declined from 1.461 × 10–3 A·cm–2 of bare Mg to 4.140 × 10–7 A·cm–2 of the Zn/Cu/Al–Zr3 sample. Neutral salt spray testing demonstrated that the Zn/Cu/Al–Zr3 sample showed no evident signs of corrosion after 6 days of exposure. The enhancement of the corrosion protection property was related to the fact that the application of the Cu layer changed the corrosion direction from initial longitudinal corrosion to extended lateral corrosion and the top Al–Zr coating hindered the transmission of aggressive ions. In addition, upon increasing the Zr content in the alloy films, the Fermi energy reduced initially and then increased. The Al–Zr3 alloy with 8.3 atom % Zr showed the lowest Fermi energy (−3.0823 eV), which exhibited the most efficient corrosion protection. These results showed that the prepared three-layered coating provided reliable corrosion protection to Mg alloy and may thus promote its practical applications.