Hot corrosion of some superalloys and role of high-velocity oxy-fuel spray coatings - a review

Hot corrosion is a serious problem in power generation equipment, gas turbines, internal combustion engines, fluidized bed combustion, industrial waste incinerators and paper and pulp industries. No alloy is immune to hot corrosion attack indefinitely although there are some alloy compositions that require a long initiation time at which the hot corrosion process moves from the initiation stage to the propagation stage. Superalloys have been developed for high-temperature applications. However, these alloys are not always able to meet both the high-temperature strength and high-temperature corrosion resistance simultaneously, so the need to be protected from corrosion. The high-temperature protecting system must meet several criteria, provide adequate environment resistance, be chemically and mechanically compatible with the substrate, be practically applicable, reliable and economically viable. This paper briefly reviews the hot corrosion of some Ni- and Fe-base superalloys to understand the phenomenon. Comprehensive reviews of the corrosion of coatings have appeared regularly since early 1970; the purpose of this paper is not to repeat the published materials but rather to focus on research trends and to point out some research prospects. High-velocity oxy-fuel (HVOF) spraying is a new and rapidly developing technology in combating high-temperature corrosion and is now challenging the vacuum plasma spraying technique (VPS), which is very expensive (capital costs approximately 2 million US$). HVOF coatings have very low porosity, high hardness, high abrasive resistance, good wear resistance with a strong ability to resist high-temperature corrosion resistance. The purpose here is to summarize the performance of such coatings on various substrates. The effect of coatings thickness, residual stresses induced in the substrates, pre and postheat treatment of the HVOF coatings have been reviewed with the aims of summarizing their high-temperature corrosion resistance properties. (c) 2004 Elsevier B.V. All rights reserved.