Characterization and Performance Assessment of HVOF Sprayed Tribaloy Coating Subjected to Cyclic Oxidation and Hot Corrosion Conditions

In high-temperature gas turbine applications, Co-based CoMoCrSi (Tribaloy T-400) coating was sprayed onto MDN420 steel with the help of a thermal spraying process (high-velocity oxy-fuel (HVOF)). Cyclic Oxidation and Hot Corrosion were tested at 700°C in a hot molten salt environment of 80 wt.% Na 2 SO 4 20 wt.% V 2 O 5 with each cycle taking 1 h exposure time and 20 min cooling to ambient temperature. The microstructure of the as-sprayed coating was dense and uniform with a surface roughness of 5.20 ± 0.35 µm, porosity of 2.25 ± 0.28%, and microhardness of 507.26 ± 20 H V , compared to the uncoated MDN420 substrate of 153.91± 12 H V . Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) confirmed uniform splat morphology and homogeneous distribution of Co, Mo, Cr, and Si throughout the coating. Phase analysis using X-ray diffraction (XRD) identified a γ-Co solid-solution matrix being strengthened by Laves-type intermetallic phases, Co 3 Mo 2 Si, with trace Co 7 Mo 6 , Cr 3 Si and some amorphous phases. Cyclic Oxidation and hot corrosion kinetics provided parabolic rate constants (Kp) of 1.2 ×10 -9 and 7.6 ×10 -8 g 2 cm -4 s -1 respectively, lower than the rate constants of bare MDN420 steel. In the process of oxidation, uninterrupted Cr 2 O 3 and SiO 2 scales grew and inhibited the oxygen movement. When molten salts were exposed to coating constituents, there was a reaction between molten salt and coating constituents to give mixed oxides and vanadates like CoMoO 4 , Co 3 V 2 O 8 and NaVO 3 . The effect of stable oxide scales and thermally stable intermetallic phases was combined to significantly reduce the penetration of molten salts and degradation at high temperatures.