Effect of alloy cations on corrosion resistance of LDH/MAO coating on magnesium alloy

Micro-arc oxidation (MAO) can improve corrosion resistance of magnesium alloys, while the further promotion of corrosion resistance is significantly restricted due to the existence of pore and micro crack. In this study, three types of LDH/MAO coating were prepared via covering layered double hydroxide (LDH) on MAO pre-treated AZ31 alloy, and the effects of these three types of LDH on the corrosion resistance of AZ31 alloy were investigated for the first time. Detailed morphology and composition of the LDH/MAO coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Besides, the corrosion resistance of AZ31 alloy with different LDH/MAO coatings was also tested by electrochemical work station. The results from polarization curves and electrochemical impedance spectroscopy indicate that AZ31 alloy coated with Ni-LDH/MAO has the higher impedance modulus and lower corrosion current density than Zn-LDH/MAO and Al-LDH/MAO. The anti-corrosion property of AZ31 alloy covering Ni-LDH/MAO coating is remarkably enhanced during long-term immersion in 1 M NaCl solution, since Ni-LDH film growing on the original MAO coating can hinder the penetration of chloride ions and effectively adsorb a large number of NO3− to exchange Cl− in corrosion solution.