High temperature hot corrosion behaviour of NiCr and Cr3C2–NiCr coatings on T91 boiler steel in an aggressive environment at 750 °C

80Ni–20Cr and 75Cr3C2–25(Ni–20Cr) coatings were deposited on T91 boiler tube steel by high velocity oxy-fuel (HVOF) process to enhance high-temperature corrosion resistance. Hot corrosion studies were conducted on bare and HVOF-coated steel specimens after exposure to a molten salt (Na2SO4–60%V2O5) environment at 750 °C under cyclic conditions. Each cycle consisted 1 h of heating in the silicon carbide tube furnace followed by 20 min of cooling in air. The weight change measurements were performed after each cycle to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDAX) and X-ray mapping techniques were used to analyse the corrosion products. The bare steel experienced higher weight gain, which may be attributed to the formation of unprotective Fe2O3 dominated oxide scales. The 80Ni–20Cr coating was found to be more protective than the 75Cr3C2–25(Ni–20Cr) coating. The phases revealed in the oxide scale of the coated specimens were mainly oxides and spinels of nickel and chromium, which are reported to be protective against the hot corrosion.

► HVOF can be successfully used to deposit NiCr and Cr3C2–NiCr coatings on T91 steel.
► Coatings provided corrosion resistance to T91 steel in molten salt environment.
► Bare steel suffered intense spalling, sputtering, and peeling-up of its oxide scale.
► NiCr coating provided better hot corrosion resistance than the Cr3C2–NiCr coating.
► Oxides and spinels of Ni and Cr were found protective against the hot corrosion.