Some studies on hot corrosion performance of plasma sprayed coatings on a Fe-based superalloy

NiCrAlY, Ni–20Cr, Ni3Al and Stellite-6 metallic coatings were deposited on a Fe-based Superalloy (32Ni–21Cr–0.3Al–0.3Ti–1.5Mn–1.0Si–0.1C–Bal Fe). NiCrAlY was used as bond coat in all the cases. Hot corrosion studies were conducted on uncoated as well as plasma spray coated superalloy specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermogravimetric technique was used to establish kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive analysis (SEM/EDAX) and electron probe microanalysis (EPMA) techniques were used to analyse the corrosion products. All these overlay coatings showed better resistance to hot corrosion as compared to that of uncoated superalloy. NiCrAlY coating was found to be most protective followed by Ni–20Cr coating. Ni3Al coating was also effective in decreasing the weight gain to about one-third as compared to uncoated superalloy. Stellite-6 coating was least effective but still decreased the weight gain to around 60% of that of the bare superalloy. The formation of oxides and spinels of nickel, aluminium, chromium or cobalt may be contributing to development of hot corrosion resistance in the coatings studied.