Cyclic Oxidation Behaviour of Plasma Sprayed Stellite-6 Coated Ni-Base Superalloys

Stellite-6 coatings were deposited on two Ni-base superalloys namely Superni 75 and Superni 600 by shrouded plasma spray process and the oxidation behaviour of these coatings has been investigated in air and in aggressive environment of Na₂SO₄ - 60%V₂O₅. Accelerated oxidation tests were conducted in a silicon carbide tube furnace under cyclic conditions at an elevated temperature of 900°C for 50 cycles. Each cycle consisted of one hour heating and 20 minutes cooling in air. Thermogravimetric technique was used to approximate the oxidation kinetics. The corrosion scales were characterized by XRD, SEM/EDAX and EPMA analyses. It has been observed that coatings followed the parabolic rate law of oxidation, although minor deviations were observed. Overall weight gain conceived in air was just 51% of that in the aggressive environment in case of coated Superni 75, whereas in case of coated Superni 600, the cumulative weight gain in air was found to be one-third of that in the Na₂SO₄ -60% V2O5 atmospheres. Further the coating showed very good adherence to both of the substrates during the course of air oxidation, whereas some cracks were observed near the edges of the specimens during the molten salt induced oxidation. XRD analysis revealed the presence of phase like CoO, CoCr₂O₄ and Cr₂O₃ in the oxide scales, which are reported to be protective oxides. The XRD results were further supported by SEM/EDAX and EPMA.