High Temperature Corrosion in Water Vapor of Fe–2.25Cr–0.54Mo Coated with Ni-Based Alloy Containing WC–Co Using an HVOF Spraying Technique

The high temperature corrosion of Fe-2.25Cr-0.54Mo steel coated with WC–Co/NiCrFeSiB using a high-velocity oxy-fuel spraying technique was investigated. Coated and uncoated steel samples were tested in air and in a humidified atmosphere consisting of N₂-50%, O₂-10%, and H₂O at 750 °C for 120 h. Microstructural and phase analyses of the studied samples were performed by scanning electron microscopy equipped with energy-dispersive spectroscopy and X-ray diffraction. When compared to oxidation in air, the oxidation rate of the uncoated sample in the humidified atmosphere was faster. This occurred because there was a thicker and denser iron oxide layer at the outer subscale, and the thicker layer of inner iron oxide subscale contained chromium (Cr). Moreover, the WC–Co/NiCrFeSiB coating greatly suppressed the rates of oxidation in both the air and the humidified oxygen atmospheres. This occurred because the formation of magnetite (Fe₃O₄) was suppressed, while the protective oxides, especially nickel–chromium (Ni–Cr) spinel and chromia (Cr₂O₃) were formed during oxidation. Water vapor in the atmosphere enhanced the oxidation rate of the coated steel, with higher iron-containing oxide forming as a subscale at the outer coating.