Influence of Mo concentration on corrosion resistance to HF acid solution of Ni-Co-Cr-Mo alloys with and without Cu

The corrosion resistance of Ni-30Co-16Cr-6Fe-xMo and Ni-30Co-16Cr-6Fe-2Cu-xMo (x = 7-15 wt%) alloys was investigated by immersion tests in 5.2 M HF solution at 100 °C. The weight loss of the Ni-30Co-16Cr-6Fe-xMo alloys relative to Mo concentration after immersion followed a Boltzmann equation. With decreasing Mo concentration, the Ni-30Co-16Cr-6Fe-xMo alloys first lost the Mo protection of the grain boundaries, then the annealing twin boundaries, and finally the grain matrix. In contrast, decreasing Mo content did not cause a significant increase in Ni-30Co-16Cr-6Fe-2Cu-xMo alloy weight loss, but led to a slightly preferential dissolution of both grain and annealing twin boundaries.