Preparation of Ceramic Coating by Plasma Electrolytic Oxidation on Ti-6Al-4V Substrate and Investigation of its Corrosion Behavior in Simulated Body Fluid (Hanks)

In this study, the influence of the sodium phosphate electrolyte concentration on the coating microstructure, phase composition and corrosion resistance of Ti-6Al-4V alloy coated by plasma electrolytic oxidation (PEO) was investigated. For this purpose, alkaline-phosphate electrolytes with different concentration were applied as follows: 8, 12, 16 g/L. The morphology and phase composition of these coatings were analyzed by a scanning electron microscope (SEM), and X-ray diffraction (XRD) technique. Corrosion properties of the coating were studied under simulated body fluid (Hanks) by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Through the PEO process, an increase in the concentration of sodium phosphate led to increment in the electrolyte electrical conductivity, average pore size and thickness of the coatings. As a consequence of increasing the electrical conductivity, the breakdown voltage in V-t curves decreased. The XRD results showed that with increase in the concentration of sodium phosphate, the metastable anatase phase transformed to the stable rutile phase. The results of the electrochemical tests indicated that the corrosion resistance of the coating produced by PEO process, in the electrolyte with 12 g/L sodium phosphate, was significantly better in comparison to the other specimens. The maximum measured corrosion potential of the coating (391 mV) together with the minimum corrosion current density (6.18×10-8 A cm-2) in the electrolyte with 12 g/L sodium phosphate leads to the highest corrosion resistance of twofold layer, that is, 8.8 MΩ. Corrosion resistance of other samples is 2 to 6 orders of magnitude smaller than sample with 12 g/L sodium phosphate.