The oxide film resistance (RP) and capacitance (CCPE) 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 TiO2 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. RP for the Ti-15Zr-4Nb-1Ta and Ti-15Zr-4Nb-4Ta alloys was proportional to d obtained by FE-TEM. CCPE was proportional to 1/d. RP tended to decrease with increasing CCPE. RP was large (maximum: 13 MΩ·cm2) and CCPE was small (minimum: 12 μF·cm-2·sn-1, n = 0.94) for the Ti-15Zr-4Nb-(0 to 4)Ta alloys. The relative dielectric constant (εr) and resistivity (kOX) of the oxide films formed on these alloys were 136 and 2.4 × 106-1.8 × 107 (MΩ·cm), respectively. The Ta-free Ti-15Zr-4Nb alloy is expected to be employed as an implantable material for long-term use.
Keywords: Ti–15Zr–4Nb alloy; capacitance; electrochemical impedance spectroscopy; implantable metals; oxide film resistance.