Structure And Anti-Corrosion Mechanism Of Ceramic Coatings On Titanium Alloy By Micro-Plasma Oxidation

In this paper, the ceramic coatings were prepared in situ on Ti-6Al-4V alloy by micro-plasma oxidation (MPO). The phase composition, microstructure and corrosion resistance of the ceramic coatings were studied in detail and the technology of corrosion resistant coatings was optimized. The dissolution of the substrate and the changes of the elements in the electrolyte during the MPO process were studied to discuss the growing characters and the structure of the ceramic coatings. The electric circuit model was established and the relation of the electric circuit model, the structure and the corrosion resistance was discussed. Further tests of corrosion resistance of the coatings under the optimized technology were carried out and the corrosion resistance mechanism was analyzed.The phase composition, morphology of the coatings was studied by X-ray diffraction (XRD) and the scanning electron microscopy (SEM). The elements and the distribution characters were examined by electron probe microscopy (EPMA), energy dispersive spectrum (EDS) and X-Ray Fluorescence Spectrometer (XRF). The accelerated electrochemical methods (polarizing curve and potentiodynamic scanning) were used to evaluate the corrosion of the coated samples. The solution of the substrate and the elements in the electrolyte were measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES). Electrochemical Impedance Spectroscopy was used to analysis the relation between the structure and the corrosion resistance of the coated samples. The corrosion resistance of coated samples was assessed by acid corrosion resistance test of the coated samples, galvanic corrosion resistance, and salt spray corrosion test and high-temperature oxidation.The results show that the coating prepared on Ti-6Al-4V alloy by MPO in K₂ZrF₆ system is composed of m-ZrO₂, t-ZrO₂ and zirconium phosphate. Ti in the coating is low, while the content of Zr and P is much high in the coating. Increasing the reaction time, zirconium phosphate becomes the main crystalline of the coating. The coating surface is coarse, loose and two many cracks and micro-holes exist in the section image of the coatings. The ceramic coatings...