The effects of post-treatment on the hot corrosion behavior of the HVOF-sprayed Cr3C2–NiCr coating

In the present paper hot corrosion behavior of high velocity oxy-fuel (HVOF) sprayed Cr3C2–NiCr coating on ASME-SA213-T91 boiler steel has been investigated. In addition, influence of post treatment of the coating by two different methods, i.e. sealing and heat treatment, on the hot corrosion behavior has also been investigated. Hot corrosion studies were conducted on bare steel, as-deposited Cr3C2–NiCr coating and on post treated Cr3C2–NiCr coating on T91 steel after exposure to a molten salt (Na2SO4–60% V2O5) environment at 900 °C under cyclic conditions. Each cycle consisted of 1 h heating in the silicon carbide tube furnace followed by 20 min cooling in air. The mass gain 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 analyze the corrosion products.Coated steel showed better resistance to hot corrosion as compared to the uncoated counterpart. As-deposited Cr3C2–NiCr coating experienced two stages of hot corrosion, i.e. initiation stage and propagation stage. Once the coating entered the propagation stage, the protective scale rendered ineffective and substrate became vulnerable to hot corrosion. Post treated coating resulted to be more effective than as-deposited coating in enhancing the corrosion resistance of T91 steel.

► HVOF can be successfully used to deposit Cr3C2–NiCr coating on T91 steel.
► Coating provided corrosion resistance to T91 steel in molten salt environment.
► Bare steel suffered intense spalling, sputtering, and peeling-up of its oxide scale.
► As-deposited coating experienced initiation and propagation stage of corrosion.
► Post treatment of Cr3C2–NiCr coating proved more effective against hot corrosion.