Fabrication of corrosion resistant hydrophobic ceramic nanocomposite coatings on PEO treated AA7075

Abstract

Ceramic composite coatings were fabricated on AA7075 by plasma electrolytic oxidation method in silicate + KOH electrolyte with α-Al2O3 or m-ZrO2 nanoparticles. The significant differences between α-Al2O3 and m-ZrO2 nanoparticles incorporation on PEO coating morphology, composition and thickness were analysed using X-ray diffractometer, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy. Corrosion behaviour of the coatings was evaluated by potentiodynamic polarisation (PDP) and salt spray test. The results showed that the hard α-Al2O3 incorporated coating showed better corrosion resistance than m-ZrO2 incorporated coating. The presence of hard α-Al2O3 in the coating structure and higher thickness has played a key role in improving the corrosion resistance of the α-Al2O3 particles incorporated coating. To further enhance the corrosion resistance, hydrophobic surfaces were fabricated on α-Al2O3 incorporated PEO treated AA7075 and m-ZrO2 incorporated PEO treated AA7075 by stearic acid treatment and myristic acid treatment. The coatings morphology and wettability were assessed using SEM and contact angle goniometer. The corrosion behaviour of the hydrophobically modified PEO treated samples was studied through PDP test and electrochemical impedance spectroscopy (EIS) method in 3.5wt% NaCl solution. The results indicated that the corrosion resistance of the nanoparticle incorporated PEO coatings significantly improved by the hydrophobic treatments. Among the modified coatings, the stearic acid modified α-Al2O3 incorporated PEO treated sample exhibited highest Ecorr and lowest icorr values. The sealed pore morphology has a significant role in enhancing the corrosion resistance of the stearic acid modified α-Al2O3 incorporated PEO sample.