Abstract

 

High velocity oxy-fuel (HVOF) spraying process is a relatively new and rapidly developing technology, which can yield high density coatings with porosity less than 1%, having high hardness and adhesion values, and good erosion, corrosion and wear resistance properties. In this study, HVOF technique was used to formulate Cr₃C₂-NiCr coatings on the Ni- based superalloys for hot corrosion applications. The coatings were characterised in relation 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 900ºC under cyclic conditions. X-Ray Diffraction, optical microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDAX) and electron probe microanalysis (EPMA) techniques were used to characterise the coatings and corrosion products. . 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 phase. Some porosity (less than 1.8%), 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 850-990 Hv on different superalloys. The Cr₃C₂-NiCr coating was found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900⁰C. 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.