The effect of gas nitriding process on the corrosion behavior of TiO2 coating formed by plasma electrolytic oxidation

This study aimed to investigate the corrosion behavior of TiO2-N duplex coating formed by Plasma Electrolytic Oxidation (PEO) and gas nitriding. A TiO2 film formed on the titanium substrate by PEO in electrolyte containing sodium carbonate and sodium hydroxide in the first step. In the second coating process, the titanium substrate and the titanium oxide coated substrate was nitrided in a tube furnace at 1000 oC for 6 hours to compare the corrosion properties of the obtained coatings. X-ray diffraction, scanning electron microscopy of top-surface and cross-sectional structure of the layers, and potentiodynamic polarization along with electrochemical impedance spectroscopy was used to investigate the properties of the coatings. The XRD results showed that the titanium oxide coating consisted of a rutile phase, and the nitrified samples consisted of titanium nitride and TiO0.34N0.74 phases. The morphology of the coatings showed that the titanium oxide sample coating had micropores known as the pancake structure with a diameter of 4.1 microns on the surface. Also, the surface morphology of nitrided oxide-coated titanium indicates a slight change in the surface and a reduction in pore diameter of 2.8 microns due to the penetration of nitrogen in the titanium oxide coating. Finally, the results of impedance and polarization showed that the titanium oxide sample, due to its insulating and dense oxide structure, prevented the transfer of more corrosive ions to the metal surface, and its resistance was improved up to 10 times compared to other samples.