HOT CORROSION STUDIES ON PLASMA SPRAY COATINGS OVER SOME Ni- AND Fe- BASED SUPERALLOYS

Abstract

Hot corrosion is an accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposits, which is mostly observed in boilers, gas turbines, internal combustion engines, fluidized bed combustion, industrial waste incinerators, diesel engines and chemical plants. The hot corrosion of an alloy usually occurs in the environments where molten salts such as sulphates (Na2SO4), chlorides (NaCI) or oxides (V205) are deposited onto the surfaces. Sodium vanadyl vanadate (Na20.V204.5V205), which melts at a relatively low temperature 550°C is found to be the most common salt deposit on boiler superheaters. Superalloys are well known candidates for the above mentioned high temperature applications. Although the superalloys have adequate mechanical strength at elevated temperatures, yet often lack resistance to oxidizing/corroding environments during long time exposures. One possible way to combat this problem of hot corrosion and oxidation constitutes the use of protective coatings. Among the various techniques used for deposition of coatings, plasma spraying is a versatile technology that has been successful as a reliable cost-effective solution for many industrial problems. In the present study, oxidation behaviour of some superalloys designated as Superni 75, Superni 600, Superni 601, Superni 718 and Superfer 800H as per the manufacturer's catalogue, has been investigated with and without the application of plasma sprayed coatings in air as well as Na2SO4-60%V205 environment. These superalloys are being developed by Mishra Dhatu Nigam Limited, Hyderabad (India) for the high temperature applications such as boilers and gas turbine parts, heat exchangers and piping in chemical industries, jet engines, pump bodies, high temperature furnace parts and heat treatment jigs. Argon shrouded plasma spray process was utilised to develop four types of coatings viz. Ni-22Cr-1OAI-1Y (NiCrA1Y), Ni-20Cr, Ni3AI and Stellite-6 on the given superalloys. Ni3Al alloy powder was prepared by mixing nickel and aluminium powders in the stochiometric ratios. The NiCrA1Y was also used as bond coat of approximately 150 .tm thickness before the final coating in all cases. As-sprayed coatings were characterised by metallography, SEM, EDAX and EPMA before conducting oxidation studies.