Mehri Hashemzadeh

This work evaluates the effect of sodium meta-silicate pentahydrate (SMS) and potassium hydroxide concentrations on properties of Al₂O₃-TiO₂ coatings produced through plasma electrolytic oxidation in a solution containing 3 g L⁻¹ potassium titanyl oxalate, (PTO), using a unipolar waveform with constant current density. The surface and cross-section characteristics of PEO coatings including morphology, elemental distribution, and phase composition were evaluated using FESEM, EDS, and XRD techniques. Voltage-time response indicated the concentration of SMS and KOH had a significant effect on the duration of each stage of the PEO process. More cracks and pores were formed at the higher concentrated solutions that resulted in the incorporation of solution components especially Si into the coating inner parts. Ti is distributed throughout the coatings, but it had a dominant distribution in the Si-rich areas. The coating prepared in the electrolyte containing no silicate consisted of non-stoichiometric γ-Al₂O₃ and/or amorphous Al₂O₃ phase. Adding silicate into the coating electrolyte resulted in the appearance of α-Al₂O₃ besides the dominant phase of γ-Al₂O₃. The corrosion behaviour of the coatings was investigated using the EIS technique. It was found that the coating prepared in the presence of 3 g L⁻¹ SMS and 2 g L⁻¹ KOH, possessed the highest barrier resistance (~10 MΩ cm²), owing to a more compact outer layer, thicker inner layer along with appropriate dielectric property because this layer lacks the Si element. It was discovered that the incorporation of Ti⁴⁺ and especially Si⁴⁺ in the coating makes the dielectric loss in the coating.