Influence of the frequency on the structure and corrosion resistance of ceramic coatings on Ti–6Al–4V alloy produced by micro-plasma oxidation

The influence of the working frequency on the structure, morphology and corrosion resistance of ceramic coatings on Ti–6Al–4V by pulsed bi-polar micro-plasma oxidation in NaAlO2 solution has been studied. The produced coatings were composed of Al2TiO5, α-Al2O3 and rutile TiO2, of which Al2TiO5 was the main crystalline phase. An increase in frequency did not change the content of Al2TiO5, however, the content of rutile TiO2 increased gradually while the content of α-Al2O3 decreased drastically. The coating can be divided into two layers: a porous outer layer and a dense inner layer. The thickness of the coatings reduced as the frequency increased. The corrosion potential increased with decreasing working frequency. The corrosion current density of the coated samples decreased with decreasing working frequency up to 60 Hz and then increased with further decrease of the frequency. Therefore, the best corrosion resistance was attained in the coated sample, which was prepared under the frequency of 60 Hz.