Characterisations and Corrosion Resistance of HVOF Cr₃C₂-NiCr on Ni-Base Superalloys at 900°C in Molten Salt Environment

In this study, high velocity-oxy fuel (HVOF) technique was used to deposit Cr₃C₂-NiCr coating on the Ni-base superalloys for their hot corrosion applications. The coatings were characterised with regard to coating thickness, porosity, microhardness and microstructure. The hot corrosion behaviours of the bare and Cr₃C₂-NiCr coated superalloys were studied after exposure to molten salt (Na₂SO₄-60%V₂O₅) at 900°C under cyclic conditions. Optical microscopy, XRD, SEM/EDAX and EPMA techniques were used to characterise the coatings. The thermogravimetric technique was used to establish kinetics of corrosion. The structure of the as sprayed Cr₃C₂-NiCr coating mainly consisted of γ-nickel solid solution with very low intensity peaks of Cr₇C₃ and Cr₂O₃ phases. Some porosity (less than 1.5%), inclusions, unmelted and semi-melted powder particles were observed in the structure of the coatings. Coating microhardness values were found to be in the range of 850-900 Hv (Vickers hardness). The Cr₃C₂- NiCr coating was resistant to hot corrosion in the given molten salt environment at 900°C. The hot corrosion resistance imparted by Cr₃C₂-NiCr coatings may be attributed to the formation of oxides of nickel, chromium, and spinels of nickel and chromium.