CYCLIC OXIDATION BEHAVIOUR AND CHARACTERIZATION OF HVOF SPRAYED WC-NiCrFeSiB COATING

Abstract

Superalloys are used in turbines, boilers and coal conversion plants. From the application point of view, mechanical strength of oxide scales, growth stresses in the scales, oxide spallations and the loss of scale adherence to the substrate represent important factors in the determining the usefulness of metals and alloys as high temperature construction materials. It is their application at high temperature which makes the study of their behaviour at high temperature much more important. HVOF coating technology is capable of depositing a very wide range of compositions without significantly heating the substrate. The present work is a comparative study of HVOF sprayed WC-NiCrFeSiB coating on a Ni-based superalloy (SN 75) and Fe-based superalloy (800H) in the different environments, such as simple oxidation (in presence of air only) and a molten salt environment(of 75% Na2SO4-25%NaC1) at 800 °C under cyclic conditions. Oxidation studies were conducted on uncoated superalloys as well. The thermo gravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction, scanning electron microscopy analysis and electron probe microanalysis techniques were used to characterize the as sprayed coatings and corrosion products. The bare superalloys, in general, suffered accelerated corrosion under the given environmental conditions and showed spalling/sputtering of the oxide scale. The hot corrosion resistance of Ni-based superalloys was found to be better than that of Fe-based superalloys. The WC-NiCrFeSiB coating was found to be very effective in decreasing the corrosion rate in the given molten salt environment at 800 °C. The hot corrosion resistance imparted by WC-NiCrFeSiB coatings may be attributed to the formation of oxides of chromium and nickel and spinels of nickel and chromium.