Regulating the passive film of NiCoCrMo alloy in hydrofluoric acid solution by small addition of Cu

The effect of 2 wt.% Cu addition on the corrosion resistance of Ni-30Co-16Cr-15Mo-6Fe alloy in hydrofluoric acid was investigated by immersion testing at 100 °C. Microstructural observations of the passive film of samples after immersion testing via various methods revealed that the small addition of Cu yielded a tremendous improvement in corrosion resistance, even after cold forging. In addition, the passivation of Ni-30Co-16Cr-15Mo-6Fe alloy by a Mo-oxide-dominated passive film was completely changed to passivation by a Cu-dominated one in Ni-30Co-16Cr-15Mo-6Fe-2Cu alloy during immersion in hydrofluoric acid solution because the segregation behaviour of Cu was much stronger compared to that of Mo.