Abstract
Plasma electrolytic oxidation was employed to produce Al2O3 and Al2O3/TiO2 composite coatings from a silicate based electrolyte on 7075 aluminum alloy using unipolar and bipolar waveforms with cathodic duty cycle of 20 and 40%. The results showed that the surface morphology of the coatings is dependent on the applied waveform. Pancake like morphology was converted to crater like by altering waveform from unipolar to bipolar. Higher thickness, lower porosity, and thus, higher corrosion protection were achieved using the bipolar waveform at higher cathodic duty cycle of 40%. The incorporation of TiO2 nano-particles in the coatings decreased the thickness, increased the micro-cracks and widened the micro-pores on coating surface when unipolar waveform was applied. It was found that TiO2 nano-particles have been incorporated into the coatings in their original crystalline structure, i.e. rutile, which was categorized in the “inert incorporation” mode and this incorporation has not changed the matrix micro-structure, i.e. γ-alumina. In addition, the incorporated amounts of TiO2 nano-particles were constant and showed no reasonable relation with the applied waveform. The corrosion results indicated that although the composite coating produced at unipolar waveform shows the highest corrosion resistance at short periods of immersion due to repairing mechanism, it degrades at a higher rate. However, for the composite coatings produced using the bipolar waveform with the higher cathodic duty cycle of 40%, the maximum corrosion protection was achieved at long term immersion. Repairing mechanism is plugging the micro-pores in the inner compact layer which was found more effective for the coatings with lower porosity. Accordingly, it was concluded that the corrosion protection of the coatings with higher “intrinsic resistance” get more benefit from the repairing mechanism too.
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