Microstructural evolution of NiCoCrMo medium-entropy alloy and its corrosion resistance in hydrofluoric acid

Fluorine-based compounds are indispensable in semiconductor manufacturing, nuclear technology, and aerospace industries, yet developing alloys resistant to fluorine-induced corrosion remains challenging. This study investigates the corrosion behavior of a Ni₃₀Co₃₀Cr₃₀Mo₁₀ medium-entropy alloy (MEA) in hydrofluoric acid (HF) solutions. The alloy, rapidly solidified via centrifugal casting, featured a primary γ phase (Ni- and Co-rich) with a highly refined dendritic microstructure, alongside a Cr- and Mo-rich σ phase uniformly distributed within the interdendritic regions. Immersion tests conducted over 28 days yielded average corrosion rates of 0.077, 0.202, and 0.134 mm/year in 20, 30, and 40 vol% HF solutions, respectively. Microstructural analysis revealed sub-100 nm Ni- and Co-rich corrosion products dispersed across the matrix surface. While a strongly adherent Cr-rich passive film formed on the σ phase, the γ phase developed a loosely structured Co-rich film. Although the corrosion appeared uniform at the macroscopic scale, the primary degradation mechanism was attributed to micro-galvanic interactions between the compositionally distinct γ and σ phases. These results advance our understanding of NiCoCrMo MEAs' corrosion behavior in HF environments and contribute to the development of improved fluorine-resistant alloys for advanced industrial applications.