Preparation and Characterization of Plasma Electrolytic Oxidation Coating on 5005 Aluminum Alloy with Red Mud as an Electrolyte Additive

Abstract

A coating with red mud as an electrolyte additive was applied to 5005 aluminum alloy using plasma electrolytic oxidation (PEO). The phase composition of the coating was investigated using X-ray diffraction. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS) was used to determine the microstructure and composition profiles of the coating. The coating/substrate adhesion was determined by scratch testing. The corrosion behaviors of the substrate and coating were evaluated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). The results indicated that the PEO coating with red mud consisted mainly of α-Al2O3 and γ-Al2O3, with small amounts of Fe2O3, CaCO3, and CaTiO3. The surface of the coating was the color of the red mud. The coating had a uniform thickness of about 80 μm and consisted of two main layers: a 6-μm porous outer layer and a 74-μm dense inner layer, which showed typical metallurgical adhesion (coating/substrate adhesion strength of 59 N). The coating hardness was about 1142 HV, much higher than that of the substrate (60 HV). The corrosion potential E corr and corrosion current density i corr of the coating were estimated to be −0.743 V and 3.85 × 10−6 A cm−2 from the PDP curve in 3.5 wt pct NaCl solution, and the maximum impedance and phase angle of the coating were 11 000 Ω and −67 deg, respectively, based on EIS. PEO coating with red mud improved the surface properties and corrosion resistance of 5005 aluminum alloy. This study also shows a potential method for reusing red mud.