Studies on the role of HVOF coatings to combat hot corrosion, oxidation and erosion of materials used in turbine components

Abstract

Degradation by high temperature oxidation, hot corrosion and erosion are the main failure modes of components in the hot sections of gas turbines, boilers, industrial waste incinerators, metallurgical furnaces, petrochemical installations, etc. The present research work explores the possibility of use of HVOF sprayed cermet coatings on the materials used in gas turbine systems. The coatings are investigated for their resistance to hot corrosion, oxidation and erosion, under laboratory conditions. Three types of feed stock powders, namely, 10% Al2O3+CoCrAlTaY , 25% (Cr3C2- 25(Ni20Cr)) + 75%NiCrAlY and Stellite-6 were sprayed on three alloys used in turbine related applications, namely Ti-6Al-4V (Ti-31), Cobalt based superalloy (Superco-605) and Fe-based special steel (MDN-121). The microstructure and mechanical properties of the coatings were characterized. It is followed by the investigation of their cyclic hot corrosion, cyclic oxidation and erosion of the coating behaviour is compared with the uncoated substrate alloys. Hot corrosion resistance of the coatings and substrates were investigated in a molten salt environment of Na2SO4-50%V2O5 at 800 C for 50 cycles. Each cycle consisted of heating at 800 C for 1 hour followed by 20 minutes of cooling in air. The analysis indicated that the carbide coated substrate alloys showed maximum resistance to hot corrosions, it was followed by alumina coating. The analysis also showed that the better resistance of carbide coating is due to the formation of oxides of alumina and chromium. Similarly, the resistance cyclic oxidation is investigated at 800 C for 50 cycles, Again each cycle consisted of heating at 800 C for 1 hour which is followed by 20 minutes of air cooling. The investigation indicated that the carbide coating exhibits maximum resistance to cyclic oxidation which is followed by Stellite-6 coatings. All three coatings showed better resistance compared to uncoated alloys both under cyclic oxidation and cyclic hot corrosion conditions. Solid particle erosion studies were conducted using silica sand as the erodent. Amongst three coatings, Stellite-6 coating performs better under sand erosion conditions. The 25% (Cr3C2-25(Ni20Cr)) + 75%NiCrAlY coating undergoes composite erosion mode, whereas the 10%Al2O3+CoCrAlTaY and Stellite-6 coatings undergo damage by brittle mode. Erosion behaviour of the substrate materials is ductile and resistance is better than the coating material.