Effect of Mo and Pd on Corrosion Resistance of V-free Titanium Alloys for Medical Implants

Abstract

The corrosion resistances of Ti-15%Zr-4%Nb-4%Ta-0.2%Pd-0.2%O-0.05%N and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd-0.2%O alloys were compared with pure Ti grade 2, Ti-6%Al-4%V ELI (Extra low interstitial), Ti-6%Al-2%Nb-1%Ta and β type Ti-15%Mo-5%Zr-3%Al alloys for medical implants by anodic polarization and immersion tests at 310 K in various pseudo saline solutions.
The change in current density was small up to passivity zones in 1% lactic acid, PBS(−), calf serum and eagle’s MEM+bovine serum solutions except 5%HCl. The current density change of Ti-15%Zr-4%Nb-4%Ta-0.2%Pd-0.2%O-0.05%N alloy was lowest among the whole Ti alloys. Current density at higher voltage region more than 2 volt tend to be higher in the following relation; Ti-6%Al-4%V ELI>Pure Ti grade 2>Ti-6%Al-2%Nb-1%Ta>Ti-15%Sn-4%Nb-2%Ta-0.2%Pd-0.2%O>Ti-15%Zr-4%Nb-4%Ta-0.2%Pd-0.2%O-0.05%N alloy in α+β type alloys. Otherwise the passive current density of the β type Ti-15%Mo-5%Zr-3%Al alloy was higher than that of α+β type alloys. Passive films formed on Ti-15%Mo-5%Zr-3%Al alloy in the calf serum consisted mainly of TiO₂, ZrO₂, Al₂O₃ and Mo as demonstrated by X-ray photoelectron spectroscopy (XPS).
The rate of Ti ion released from Ti alloys in a deaerated 5%HCl solution decreased with increasing Zr, Nb, Ta, and Pd contents. Especially, the rate of Ti ion released from Ti-15%Zr-4%Nb-(2∼4%)Ta-Pd alloys containing Pd in the range from 0 to 0.2% decreased remarkably compared to 0.2 to 0.5%Pd. The passive films formed on the Ti-Zr-Nb-Ta-Pd and Ti-Sn-Nb-Ta-Pd alloys containing 0.2%Pd in the immersion test consist mainly of TiO₂, ZrO₂, SnO₂, Sn, Nb₂O₅, Ta₂O₅ and Pd or PdO. The rate of released Ti increased by the following relation; Ti-15%Sn-4%Nb-2%Ta-0.2%Pd-0.2%O≥Ti-6%Al-2%Nb-1%Ta≥Ti-6%Al-4%V ELI≥Pure Ti grade 2>Ti-15%Zr-4%Nb-4%Ta-0.2%Pd-0.2%O-0.05%N≥β type Ti-15%Mo-5%Zr-3%Al alloy.