High-temperature degradation investigations of CoMoCrSi+WC-12Co coatings subjected to a combined thermo-oxidative environment and molten salt corrosion

The engineering components in high-temperature applications, such as gas turbines, boilers, aerospace, and power generation systems, result in rapid degradation of the material. This paper has studied CoMoCrSi+WC-12Co composite coating on MDN420 steel by the high-velocity oxy-fuel (HVOF) spraying process and investigated the high-temperature oxidation and hot corrosion performance of the coated material. The coating integrity was excellent as the as-sprayed coating has a dense and homogeneous microstructure with low porosity (2.1 ± 0.85%), high microhardness (909.21 ± 25H_V), and a moderate roughness of the surface Ra = 4.77 ± 0.18 µm). Simulation of severe service conditions was performed by cyclic oxidation and hot corrosion tests conducted at 700 °C with 20 min of cooling at room temperature between each cycle. The Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) were used to study the change in phases, redistribution of the elements, and degradation process. XRD showed that, cyclically oxidized coating at 700°C in air, an adherent continuous oxide scale leads to Cr₂O₃, CoO, Co₃O₄, and CoCr₂O₄ spinel phases formed with SiO₂ playing a role in densifying the oxide scale. Hot corrosion with molten salt (80%Na₂SO₄ + 20%V₂O₅ at 700°C), the coating formed spinel oxide phases Na₂CrO₄, CoV₂O₆ and minor WO₃ traces, and retained WC partially. The parabolic rate constants of oxidation and hot corrosion relative to MDN420 substrate were 1.21 × 10⁻⁹ g²cm⁻⁴s⁻¹ and 7.6 × 10⁻⁸g²cm⁻⁴ s⁻¹, respectively, indicating the suitability of the coating in high temperature surface protection. Thus, the resulting HVOF-sprayed CoMoCrSi+WC12Co can be utilized as sturdy surface protection coatings of components that have to sustain in the high-temperature corrosive conditions.