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

Abstract

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 °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 °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 × 10−7 A/cm2 and −0.47 V, respectively, whereas the coating heat-treated at 300 °C and then sealed had the lowest corrosion current density (1.8 × 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.