Abstract
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.
Highlights
The aerospace and automotive industries have extensive potential to use aluminium and its alloys owing to their high strength to weight ratio, good shape-ability, and nonmagnetic characteristics
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
Plasma electrolytic oxidation (PEO) as a cost-effective and environmentally friendly process is a powerful technique to apply a ceramic coating on light metals, Al and its alloys, and improve their hardness, wear and corrosion resistance significantly [3,4]
Summary
The aerospace and automotive industries have extensive potential to use aluminium and its alloys owing to their high strength to weight ratio, good shape-ability, and nonmagnetic characteristics. The weak corrosion resistance, intergranular and pitting corrosion resulting from intermetallic compounds in the Al alloy matrix has largely confined the broad use of aluminium alloys [1,2]. Plasma electrolytic oxidation (PEO) as a cost-effective and environmentally friendly process is a powerful technique to apply a ceramic coating on light metals, Al and its alloys, and improve their hardness, wear and corrosion resistance significantly [3,4]. Exertion of a high voltage during PEO creates spark discharges and various electrochemical and plasma chemical reactions, resulting in a corrosion-resistant dense layer next to the interface followed by a porous layer [5].
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