Hot corrosion of a plasma sprayed Ni{sub}3Al coating on a Ni-base superalloy

摘要

Ni{sub}3Al metallic powder was prepared by the mechanical mixing of pure nickel and aluminium powders and deposited on a Ni-base superalloy namely Superni 75 by the shrouded plasma spray process. Hot corrosion behaviour of Ni{sub}3Al coated and uncoated superalloy specimens was evaluated in a simulated environment of fossil fuel boilers comprising of Na{sub}2SO{sub}4-60%V{sub}2O{sub}5 by accelerated corrosion tests. The accelerated testing was done under cyclic conditions for 50 cycles, with each cycle consisting of 1 h heating in the silicon carbide furnace and cooling for 20 min in air. The coated superalloy was also subjected to air oxidation for 50 cycles to get an idea about its oxidation behaviour and adherence of the Ni3Al coating and its oxide scale with the substrate superalloy. The oxide scales were characterised by X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray (SEM/ EDAX) analyses and electron probe microanalysis (EPMA). The uncoated superalloy suffered an accelerated corrosion in the form of spallation of its oxide scale, whereas the oxide scale formed on the Ni{sub}3Al coated specimen was intact during the air as well as the molten salt oxidation. Furthermore, the coating was found successful in preventing the internal oxidation of the substrate superalloy. The coating maintained its adherence to the substrate superalloy during the exposure to both the environments of the study. The XRD analysis revealed the presence of oxides like Al{sub}2O{sub}3, NiO and NiAl{sub}2O{sub}4 in the oxide scale of the coated superalloy. The XRD results were further supported by the SEM/EDAX and EPMA analyses.