Combined effect of heat treatment and sealing on the corrosion resistance of reactive plasma sprayed TiNx/TiOy coatings

A titanium nitride/titanium oxide composite coating was deposited on a Q235 steel substrate by reactive plasma spraying (RPS). The microstructure and phases of the coatings were analysed by optical microscopy, scanning electron microscopy, and X-ray diffraction. The corrosion performance of the coatings was evaluated through electrochemical measurements. The as-sprayed coating is composed of TiN, TiN0.3, and Ti3O phases. Ti2O and TiO2 were generated after heat treatment at 300 and 400 degrees C, respectively. Heat treatment reduced the porosity of the coating. Sealing the coating before and after the heat treatment led to improved corrosion resistance. The coating heat-treated at 300 degrees C and then sealed had the lowest porosity (0.8%), with a reduction in porosity of 55.6% compared to that of a sealed as-sprayed coating (1.8%). The corrosion current density and corrosion potential of the sealed as-sprayed coating were found to be 6.03 x 10-7 A/cm2 and -0.47 V, respectively, whereas the coating heat-treated at 300 degrees C and then sealed had the lowest corrosion current density (1.8 x 10-8 A/cm2) and the highest corrosion potential (- 0.19 V). Thus, the results indicate that sealed heat-treated coatings have much better corrosion resistance than the sealed as-sprayed coating.