Hot Corrosion Behaviour of Plasma Sprayed Coatings on a Ni-based Superalloy in Na₂SO₄-60%V₂O₅ Environment

Abstract

NiCrAlY, Ni-20Cr, Ni₃Al and Stellite-6 metallic coatings were deposited on a Ni-based Superalloy (18.5Fe-19Cr-0.5Al-0.9Ti-3.05Mo-0.18Mn-0.18Si-5.13Ta-0.15Cu-0.04C-Balance Ni). NiCrAlY was used as bond coat in all the cases. Hot corrosion studies were conducted on uncoated as well as plasma spray coated superalloy specimens after exposure to molten salt at 900°C under cyclic conditions. The thermogravimetric technique was used to establish kinetics of corrosion. X-Ray Diffraction, SEM/EDAX and EPMA techniques were used to analyse the corrosion products. The uncoated superalloy suffered a catastrophic corrosion in the form of intense spalling of the scale accompanied by disintegration of the scale with cracking sounds. NiCrAlY coated specimen showed a minimum weight gain, whereas the Stellite-6 indicated a maximum weight gain among the coatings studied. All the coatings were found to be successful in reducing spalling of the superalloy substrate. It is concluded that the formation of oxides and spinels containing nickel, aluminium, chromium or cobalt contributes to an improved hot corrosion resistance.