Effects of negative voltage on microstructure and corrosion resistance of red mud plasma electrolytic oxidation coatings

Coatings were prepared on 5005 Al alloy surfaces at various negative voltages by plasma electrolytic oxidation, with addition of red mud (RM, 2 g L-1). The effects of the negative voltage on the coating composition, structure, and corrosion resistance were investigated. Various effects were observed with increasing negative voltage. The current density on the sample surface increased, the breakdown function was enhanced, and the RM concentration in the coating increased. The color of the ceramic layer changed from white to reddish brown and continuously deepened, consistent with the RM color. The coating grew faster and the thickness increased, and the overall compactness first increased and then decreased. The coating surface roughness gradually decreased and the surface porosity fluctuated greatly. A negative voltage of 100 V gave the best surface porosity and overall compactness. The XRD patterns showed that the coating consisted mainly of gamma-Al2O3 when the negative voltage was small, an increase in the negative voltage led to formation of alpha-Al2O3 and CaCO3, Fe2O3, CaTiO3, and SiO3 were identified in the coatings by a combination of XRD and EDS analyses, and their contents increased. The coating corrosion resistance first increased and then decreased; a negative voltage of 100 V gave the best corrosion resistance. The presence of RM constituents in the coating with a dense composite structure effectively reduced the corrosion depth of the coating.