Effects 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 mass% lactic acid, PBS (Phosphate-buffered saline solution) (-), calf serum and Eagle's MEM (Minimum essential medium) + 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. The current densities in a region higher than 2 V tend to increase 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 the Ti-15%Mo-5%Zr-3%Al alloy in the calf serum consisted mainly of Ti4+, Zr4+, Al6+ and Mo as demonstrated by X-ray photoelectron spectroscopy (XPS). The rate of Ti ions released from Ti alloys in a deaerated 5% HCl solution decreased with increasing Zr, Nb, Ta, and Pd contents. Especially, the rate of Ti ions 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 consisted mainly of Ti4+, Zr4+, Sn4+, Sn, Nb5+, Ta5+ and Pd. 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.