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% Na2SO4 20 wt% V2O5 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 ± 20HV, compared to the uncoated MDN420 substrate of 153.91 ± 12HV. 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, Co3Mo2Si, with trace Co7Mo6, Cr3Si 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 g2 cm−4 s−1 respectively, lower than the rate constants of bare MDN420 steel. In the process of oxidation, uninterrupted Cr2O3 and SiO2 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 CoMoO4, Co3V2O8 and NaVO3. 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.