High corrosion resistance of electroless composite plating coatings on AZ91D magnesium alloys

The process of electroless plating Ni–P on AZ91D magnesium alloys was improved. The Ni–P–ZrO2 composite coatings and multilayer coatings were investigated based on the new electroless plating process. The coatings surface and cross-section morphologies were observed with scanning electron microscopy (SEM). The chemical compositions were analyzed by EDXS. The corrosion behaviors were evaluated by immersion, salt spray and electrochemical tests. The experimental results indicated that the Ni–P–ZrO2 composite coatings suffered attack in NaCl solution but displayed passivation characteristics in NaOH and Na2SO4 solutions. The corrosion resistance of Ni–P–ZrO2 coatings was superior to Ni–P coatings due to the effect of ZrO2 nano-particle. The multilayer coatings consisting of Ni–P–ZrO2/electroplating nickel/Ni–P (from substrate to surface) can protect magnesium alloys from corroding more than 1000 h for the salt spray test.