Abstract:Plasma electrolytic oxidation (PEO) was applied using a pulsed unipolar waveform to produce Al₂O₃-TiO₂ composite coatings from sol electrolytic solutions containing colloidal TiO₂ nanoparticles. The sol solutions were produced by dissolving 1, 3, and 5 g/L of potassium titanyl oxalate (PTO) in a silicate solution. Scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and Raman spectroscopy were applied to characterizing the coatings. Corrosion behavior of the coatings was investigated using polarization and impedance techniques. The results indicated that TiO₂ enters the coating through all types of micro-discharging and is doped into the alumina phase. The higher level of TiO₂ incorporation results in the decrease of surface micro-pores, while the lower incorporation shows a reverse effect. It was revealed that the higher TiO₂ content makes a more compact outer layer and increases the inner layer thickness of the coating. Electrochemical measurements revealed that the coating obtained from the solution containing 3 g/L PTO exhibits higher corrosion performance than that obtained in the absence of PTO. The coating produced in the absence of PTO consists of γ-Al₂O₃, δ-Al₂O₃ and amorphous phases, while α-Al₂O₃ is promoted by the presence of PTO.

Key words: Al alloy; Al₂O₃-TiO₂ coating; plasma electrolytic oxidation; potassium titanyl oxalate; corrosion resistance