High corrosion resistance multilayer nickel coatings on AZ91D magnesium alloys

The multilayer coatings consisting of electroless Ni–P, Ni–P–ZrO₂ and electroplating Ni coatings were investigated. The surface and cross-section morphologies of coatings were observed with SEM. The microstructures were analysed by XRD. The corrosion resistance was evaluated by potentiodynamic polarisation curves and salt spray tests. The results showed that the electroplating Ni coating was crystal structure, whereas the electroless Ni–P and Ni–P–ZrO₂ coatings displayed amorphous characteristic. The electroplating Ni coating exhibited more positive corrosion potential E_corr and lower corrosion current density I_corr than that of electroless Ni–P and Ni–P–ZrO₂ coatings. The samples covered with multilayer coatings of Ni–P–ZrO₂/Ni/Ni–P (from substrate to surface) indicated the best corrosion resistance among six samples, and the salt spray tests were carried out for more than 1000 h. According to the analysis of corrosion mechanism, the upper Ni–P layer served as sacrificial protection anode due to its more negative corrosion potential E_corr in comparison with the middle electroplating Ni layer. When the upper Ni–P layer was consumed, the middle Ni layer went to prevent from corroding. Until the corrosion pits penetrated all of the protective coatings, the substrate of magnesium alloys was destroyed completely.