High-temperature corrosion behaviour of air plasma sprayed Cr3C2-25NiCr and NiCrMoNb powder coating on alloy 80A at 900 °C

The air plasma spray process is extensively adopted by many industries because of versatility, cost-effectiveness and good quality of the coatings produced. The present study compares the high-temperature corrosion behavior of air plasma sprayed Cr3C2-25NiCr and NiCrMoNb powders coatings on alloy 80A (Ni80TiAl). The corrosion study was carried out by air and molten salt environment of Na2SO4 + 60%V2O5 under cyclic conditions for 50 cycles at 900 degrees C. Thermogravimetric analysis was used to analyze the hot corrosion (air and molten salt environments) of uncoated and coated alloy 80A. Microstructure, porosity, thickness, and hardness of the coatings were evaluated in the cross section of the coatings. XRD and SEM/EDS analysis were carried out to evaluate the corrosion product at the end of the 50th cycle. The microstructure shows the coatings are laminar structure with uniform, adherent and dense microstructure with porosity less than 4% and thickness of 150 +/- 10 mu m. The microhardness of both coatings found to be higher than the substrate. Thermogravimetric analysis shows the air oxidation both coated and uncoated alloy 80A obey the parabolic rate law of oxidation. Uncoated alloy 80A shows the very less weight gain compared to other coated substrates. NiCrMoNb coated with molten salt environment substrates shows the higher weight gain. Also, molten salt environment shows higher mass gain compared to the corresponding air oxidation environment substrates. Presence of NiO, Cr2O3, and spinel oxide NiCr2O4 provides the excellent corrosion resistance in the air oxidation environment both coated and uncoated substrates. The high Mo element content in NiCrMoNb coating is causing more weight gain at 900 degrees C in molten salt (Na2SO4 + 60%V2O3) environment by forming a volatile MoO3 oxide. The results conclude that NiCrMoNb coating may be used as a protective coating in air oxidation environment and the Cr3C2-25NiCr coating may be used as a protective coating in the molten salt environment.