Microstructure and corrosion behaviour in biological environments of the new forged low-Ni Co–Cr–Mo alloys

Corrosion behaviour and microstructure of developed low-Ni Co–29Cr–(6, 8)Mo (mass%) alloys and a conventional Co–29Cr–6Mo–1Ni alloy (ASTM F75-92) were investigated in saline solution (saline), Hanks’ solution (Hanks), and cell culture medium (E-MEM+FBS). The forging ratios of the Co–29Cr–6Mo alloy were 50% and 88% and that of the Co–29Cr–8Mo alloy was 88%. Ni content in the air-formed surface oxide film of the low-Ni alloys was under the detection limit of XPS. The passive current densities of the low-Ni alloys were of the same order of magnitude as that of the ASTM alloy in all the solutions. The passive current densities of all the alloys did not significantly change with the inorganic ions and the biomolecules. The anodic current densities in the secondary passive region of the low-Ni alloys were lower than that of the ASTM alloy in the E-MEM+FBS. Consequently, the low-Ni alloys are expected to show as high corrosion resistance as the ASTM alloy. On the other hand, the passive current density of the Co–29Cr–6Mo alloy with a forging ratio of 50% was slightly lower than that with a forging ratio of 88% in the saline. The refining of grains by further forging causes the increase in the passive current density of the low-Ni alloy.