Influence of C_3H_8O_3 in the electrolyte on characteristics and corrosion resistance of the microarc oxidation coatings formed on AZ91D magnesium alloy surface

摘要

Ceramic coatings were fabricated on AZ91D Mg-alloy substrate by microarc oxidation in Na_2SiO_(3~-) NaOH-Na_2EDTA electrolytes with and without C_3H_8O_3 addition. The effects of different concentrations of C_3H_8O_3 contained in the electrolyte on coatings thickness were investigated. The surface morphologies, RMS roughness, phase compositions and corrosion resistance property of the ceramic coatings were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and electrochemical corrosion test respectively. It is found that the addition of C_3H_8O_3 into silicate electrolyte leads to increase of the unit-area adsorptive capacity of the negative ions at anode-electrolyte interface and thus improves the compactness and corrosion resistance of the MAO coating. The coating thickness decreases gradually with the increase of concentrations of C_3H_8O_3 in the electrolyte. The oxide coating formed in base electrolyte containing 4 mL/L C_3H_8O_3 exhibits the best surface appearance, the lowest surface RMS roughness (174 nm) and highest corrosion resistance. In addition, both ceramic coatings treated in base electrolyte with and without C_3H_8O_3 are mainly composed of periclase MgO and forsterite Mg_2SiO_4 phase, but no diffraction peak of Mg phase is found in the patterns.