Corrosion Behavior of HVAF-Sprayed WC-10Co-4Cr Coatings in H₂SO₄ and HNO₃ Environments

Three WC-10Co-4Cr coatings with different WC grain sizes were prepared by high-velocity air-fuel (HVAF) spraying. The corrosion behaviors were systematically evaluated in 0.2 mol/L H₂SO₄ and 0.4 mol/L HNO₃ solutions through immersion tests and electrochemical measurements. The results reveal that WC grain size governs coating microstructural integrity, mechanical properties, and corrosion resistance. Among the three coatings, the medium-grained (MG) coating exhibits an optimized balance between compact microstructure, high microhardness, superior fracture toughness, and the best corrosion resistance in both acidic environments. The coarse-grained (CG) coating exhibits the worst corrosion resistance owing to its wide grain boundaries and high porosity, while the fine-grained (FG) coating is similarly compromised by slightly higher porosity and residual stress-induced microcrack networks that facilitate electrolyte penetration. The corrosion proceeds via preferential dissolution of Co in the CoCr binder phase driven by micro-galvanic coupling with WC, followed by WC particle detachment and pit formation. In a 0.4 mol/L HNO₃ solution, the strong oxidizing nature accelerates both binder dissolution and direct WC oxidation.