Electrochemical impedance spectroscopy of ceramic coatings on Ti-6Al-4V by micro-plasma oxidation

Compound ceramic coatings on Ti-6Al-4V alloy were prepared by pulsed bi-polar micro-plasma oxidation (MPO) in NaAlO2 solution. The phase composition of the coatings was studied by X-ray diffraction, while the morphology and the element distribution in the coating were examined through scanning electron microscopy and JEOL SUPERPROBE 733 Electron Probe Micro-Analyzer. The thickness of the coatings was measured by eddy current coating thickness gauge. Electrochemical impedance spectroscopy (EIS) were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution. The results show that the ceramic coating consists of Al2TiO5, α-Al2O3 and rutile TiO2, of which Al2TiO5 is the main crystalline. The content of Al2TiO5, α-Al2O3 and rutile TiO2 increases with increased reaction time. Ti element in the coating is less than that of substrate, whose content in the inner layer is more than that in the outer layer. Al in the coating is more than that of substrate, and the content of Al in the coating first increases, and then decreases after reaching the maximum from the substrate to the surface. EIS and the established 'equivalent circuit' illustrated the double-layer structure of the coatings and its changing regularity. Extending the MPO time, the surface roughness of the coatings increased, whereas the thickness and the compactness of the coatings were improved. Besides, the electric charge transfer resistance in the equivalent circuit can be used to assess the corrosion resistance of the coatings for different time, which is consistent with the result of the polarizing curves test