Effects of partially substituting cobalt for nickel on the corrosion resistance of a Ni–16Cr–15Mo alloy to aqueous hydrofluoric acid

•Substituting cobalt for nickel of a Ni–Cr–Mo Alloy.•Co enhanced the wear resistance.•Co is not detrimental to corrosion resistance.•Co enhanced the formation of Mo-rich passive film.•Precipitation of Mo-rich μ phase is detrimental to corrosion resistance.

This study examined the effect of partially substituting cobalt for nickel in Ni–16Cr–15Mo alloy on corrosion resistance to 5.2 M HF acid solution at 100 °C. Microstructural characterisations revealed that cobalt substitution did not have a detrimental effect on corrosion resistance, but rather, it seems to enhance the formation of a compact and thin Mo-rich passive film by selective dissolution of elements. Heat-annealing drastically lowered the corrosion resistance due to Mo-rich precipitate of alloy, which lowered the concentration of Mo in bulk matrix, in particular close to precipitate sections, leading to inhibition of formation of homogenous and compact Mo-rich passive film.