Characterization of Oxide Film of Implantable Metals by Electrochemical Impedance Spectroscopy

The oxide film resistance (R(P)) and capacitance (C(CPE)) diagrams of implantable metals (commercially pure Ti, four types of Ti alloys, Co–28Cr–6Mo alloy, and stainless steel) were investigated by electrochemical impedance spectroscopy (EIS). The thin oxide film formed on each implantable metal surface was observed in situ by field-emission transmission electron microscopy (FE-TEM). The Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys had higher oxygen concentrations in the oxide films than the Ti–6Al–4V alloy. The thickness (d) of the TiO(2) oxide films increased from approximately 3.5 to 7 nm with increasing anodic polarization potential from the open-circuit potential to a maximum of 0.5 V vs. a saturated calomel electrode (SCE) in 0.9% NaCl and Eagle’s minimum essential medium. R(P) for the Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys was proportional to d obtained by FE-TEM. C(CPE) was proportional to 1/d. R(P) tended to decrease with increasing C(CPE). R(P) was large (maximum: 13 MΩ·cm(2)) and C(CPE) was small (minimum: 12 μF·cm(−2)·s(n−1), n = 0.94) for the Ti–15Zr–4Nb–(0 to 4)Ta alloys. The relative dielectric constant (ε(r)) and resistivity (k(OX)) of the oxide films formed on these alloys were 136 and 2.4 × 10(6)–1.8 × 10(7) (MΩ·cm), respectively. The Ta-free Ti–15Zr–4Nb alloy is expected to be employed as an implantable material for long-term use.