Hot corrosion and cyclic oxidation behavior of CoMoCrSi + Cr₃C₂ composite coatings on MDN 420 steel by HVOF spray process

The CoMoCrSi + Cr₃C₂ coating was applied to MDN 420 steel using High-Velocity Oxy-Fuel (HVOF) spraying after subjecting the CoMoCrSi feedstock to high-energy ball milling (HEBM). This process enhanced the formation of hard intermetallic Laves phases and reduced particle size to 60.12 μm. The study focused on the coating’s performance in its as-sprayed condition and after exposure to oxidation and hot corrosion with molten salts at 700 °C for 50 cycles in static air. Characterization methods included X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS), along with measurements of surface roughness, microhardness, and porosity. The coating exhibited a surface roughness of 2.995 ± 0.25 μm, a thickness of 210.53 ± 15 μm, and a low porosity of 2.160 ± 0.10%. Microhardness increased from 158.6 ± 6 Hv in the substrate to 547.5 ± 15 Hv in the coating, due to the dispersion of Cr₃C₂ particles. The coated samples showed lower parabolic rate constants (Kp) of 0.434 × 10− 15 g2⋅cm− 4⋅s− 1 and 0.330 × 10− 15 g2⋅cm− 4⋅s− 1, indicating improved oxidation and hot corrosion resistance, respectively. XRD analysis revealed that the phases of Co3Mo2Si, Co7Mo6, Co3Mo, and Co2Mo3 were the intermetallic laves phases generated in the CoMoCrSi feedstock through the HEBM process. During oxidation, the formation of dense and adherent Cr₂O₃ and SiO₂ scales provides the primary protection by restricting oxygen diffusion and sealing splat boundaries. While at hot corrosion, with molten salts, leads to the formation of secondary phases such as Na₂MoO₄, NaVO₃, CrMoO₄, and Cr₃(VO₄) ₂ through fluxing reactions with Mo- and Cr-based oxides.