Effect of Positive Voltage on the Structure and Corrosion Resistance of Red Mud Plasma Electrolytic Oxide Coating

A composite ceramic coating was prepared on the surface of 5005 aluminum alloy by plasma electrolytic oxidation (PEO) technique with red mud (RM) as electrolyte additive. The effects on the thickness, microstructure and corrosion resistance of the coatings at various positive voltages were studied. The results indicated that with the increase of positive voltage, the coating grew faster and the thickness increased. When the oxidation time was 20 min, the maximum thickness could be 35.33 μm; surface roughness of the coating increased continuously, with a minimum of 0.68 μm and a maximum of 4.21 μm; surface porosity of the coating first decreased and then increased, with a minimum of 24.36%. The coating consisted mainly of γ-Al2O3 and a small amount of amorphous phase, and α-Al2O3 and a trace amount of RM minerals. However, increasing the positive voltage can not effectively promote the RM particles participation in the coating formation. The experimental results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) showed that corrosion resistance of the coating increased first and then decreased with the increase of positive voltage. When the positive voltage was 475-525 V, the coating showing a good corrosion resistance with lower corrosion current density and corrosion rate, and higher impedance. The existence of RM particles has improved the structure and corrosion resistance of the coating to some extent. © 2019, Materials Review Magazine. All right reserved.