Effects of sodium citrate on MAO process and characteristics of coatings fabricated on AZ91D magnesium alloy

Microarc oxidation (MAO) coatings were prepared on AZ91D magnesium alloy in a novel dual electrolyte containing various concentrations of sodium citrate (C6H5Na3O7). The surface and cross-sectional morphologies, coating thickness and surface roughness were characterized by scanning electron microscopy (SEM), layer thickness meter and digital microscope, respectively. The corrosion resistance of coatings was evaluated by electrochemical impedance spectroscopy (EIS). The results showed that both the striking voltage and final voltage decreased gradually with the increase of C6H5Na3O7 concentration. When the C6H5Na3O7 concentration increased, surface roughness increased gradually, while thickness of the coating increased at first and then decreased. The coating formed in the electrolyte containing 5g/L C6H5Na3O7 had a much more uniform and compact microstructure with less micropores and micro-cracks. EIS test showed that the concentration of C6H5Na3O7 mainly affected the corrosion resistance of inner dense layer, and the coating with best corrosion resistance was obtained in the electrolyte containing 5g/L C6H5Na3O7.