Influence of cathodic duty cycle on the properties of tungsten containing Al2O3/TiO2 PEO nano-composite coatings

Abstract

Current waveforms with different cathodic duty cycles of 0, 20 and 40% were applied to coat 7075 aluminum alloy in a silicate based electrolyte containing titania nano-particles. Using a constant current mode, the recorded voltage-time response showed that the required voltage for PEO coating process is lower in the presence of sodium tungstate as additive. By increasing the cathodic duty cycle, pancake surface morphology was converted to crater-like. Phase and chemical composition of the coatings were investigated by using grazing angle XRD and SEM/EDS. The tungsten concentration decreased linearly with the increase of cathodic duty cycle, which is dissimilar to the titania content. It was proposed that physical entrapping has been associated with electrophoretic incorporation of titania nano-particles, while the adsorption mechanism of tungstate anions is purely electrophoretic. Corrosion performance of the coatings was evaluated by potentiodynamic polarization and EIS measurements. Addition of sodium tungstate was detrimental for corrosion performance of the coating in the case of unipolar waveform while the corrosion resistance was improved when bipolar waveforms were used. Long-term corrosion behavior of the coatings was investigated by EIS up to 16 weeks. The results showed remarkable difference between corrosion performance of the coatings produced by unipolar and bipolar waveforms, but the difference was negligible for the coatings produced by the bipolar waveforms with different cathodic duty ratios.