c. blawert

Plasma electrolytic oxidation was employed for producing alumina, alumina/titania composite and alumina/ titania composite coatings containing sodium tungstate additive. These coatings were produced in a silicatebased electrolyte on 7075 aluminum alloy using unipolar and bipolar waveforms with anodic/cathodic duty aspect ratios of 1 and 0.5. The results showed that the morphology of the coatings surfaces depends on the used waveform. By increasing the cathodic duty cycle, pancake-like morphology was converted to crater-like one. The incorporation of TiO2 nano-particles into the coatings, led to the widening the micro-pores when unipolar waveforms were used. The results revealed that the incorporation of TiO2 nano-particles doesn’t depend on the used waveforms. Titania nano-particles were incorporated into the coatings via a physical entrapping mechanism, while the adsorption mechanism of tungstate ions was electrophoretic. The corrosion resistance of the coatings was increased by increasing the cathodic duty cycle. Furthermore, the use of titania nanoparticles led to a reduction of corrosion resistance and the use of tungstate additive in bipolar waveform compensated the reduced resistance. The corrosion currents recorded 3, 5 and 3 nano-amperes for simple, composite and additive contained composite coatings respectively.