Preparation and corrosion resistance of black micro-arc oxidation coating on A356 aluminum alloy
[Introduction]The effect of mass concentration of Na2WO4 as a colorant on microstructure and properties of the micro-arc oxidation(MAO)coating formed on A356 aluminum alloy in a Na2SiO3-Na3PO4 composite electrolyte was studied.[Method]The dosage of Na2WO4 was determined by a proportional batching method.The color,thickness,micromorphology,element distribution,and phase composition of the MAO coatings were characterized by using color meter,eddy current thickness meter,scanning electron microscope(SEM),energy-dispersive spectrometer(EDS),and X-ray diffractometer(XRD).The corrosion resistance of MAO coatings was evaluated through HNO3 dropping corrosion test and potentiodynamic polarization curve measurement in 3.5%NaCl solution.[Result]After adding Na2WO4 to the electrolyte,the color of MAO coating was gradually deepened.The MAO coating prepared with 22.05 g/L of Na2WO4 had the deepest color,while that prepared with 19.98 g/L of Na2WO4 had the best corrosion resistance.[Conclusion]After adding Na2WO4 as a colorant to the electrolyte,tungsten oxides are deposited in the MAO coating,making the MAO coating appear darker and improving its compactness and corrosion resistance.
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