Characterization of highly corrosion-resistant nanocrystalline Ni coating electrodeposited on Mg-Nd-Zn-Zr alloy from a eutectic-based ionic liquid

A dense nanocrystalline Ni coating was successfully electrodeposited on Mg-3.0Nd-0.2Zn-0.4Zr (wt.%, NZ30K) alloy from a choline chloride-urea (1:2 molar ratio) eutectic-based ionic liquid (1:2 ChCl-urea IL) on the basis of an appropriate pretreatment. A Cu underlayer was pre-plated from a pyrophosphate based solution to avoid the severe corrosion and replacement reaction of NZ30K alloy in the IL electrolyte. The evolution of surface morphologies, compositions and phase structures of the pretreatment films and Ni coating were studied in detail. The uniform, smooth and dense nanocrystalline Ni coating possessing good adhesion to substrate has been proved by thermal shock test. Potentiodynamic polarization test in 3.5 wt.% NaCl solution showed that the Ni coating significantly improved the corrosion resistance of NZ30K alloy by nearly two orders of magnitude. Open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) during long-term immersion test further demonstrated that the Cu/Ni coating could provide an effective protection for NZ30K alloy from corrosion up to ∼336 h. The high corrosion resistance of the coating can be attributed to the dense nanocrystalline Ni structure and the synergistic effect of electrochemical protection between Ni coating and Cu underlayer.