HVOF SPRAYED NiCrAIFeMo COATINGS FOR HIGH TEMPERATURE APPLICATION

Abstract

The superalloy based service components used at high temperature and corrosive environments posses necessary mechanical strength but the corrosion and somewhat erosion resistant properties can be efficiently achieved by coatings. The most common technique adopted for coating is HVOF high-velocity oxy-fuel. It is normally used to apply coatings to components for protection. against erosion and corrosion and also has a potential to build up worn components. Due to flexibility and cost effectiveness of HVOF process the demand is increasing day by day. The present investigation aims to the study of cyclic oxidation behaviours of HVOF sprayed NiCrAIFeMo (8%Cr+7%Ni+5%A1+5%Mo+Balance Ni) coating on Ni-based (Sn-75) and Fe-based (800H) superalloys in a molten salt (75%Na2SO4+25% K2SO4) environment as well as in air at 800°C. This aggressive environment is most commonly met with boiler tubes experiencing temperature above 700°C. The superalloys selected for the present investigation are mostly used in the boiler tube application. The cyclic oxidation tests were performed for the base and HVOF coated superalloys both in air as well as molten salt environments to compare their performance. The thermogravmetric technique was used to establish the kinetics of corrosion. The X-ray diffraction, scanning electron microscopy (SEM) and electron probe microanalysis techniques (EPMA) were used to characterize the sprayed coatings and corrosion products. The bare superalloys showed initial spalling and later sputtering of the scale. Superni-75 was found to undergo less corrosion as compared to Superfe 800H. In the molten salt environment, the corrosion was very severe. The NiCrAIFeMo coating was found to be very effective in decreasing the corrosion rate in the given molten salt environment at 800 °C. The good corrosion resistance may be attributed to the formation of different oxide scales like A1203, Cr2O3 and spinels of Ni and Cr which grow very slowly, adherent to surface and are thermodynamically stable.