Corrosion and passivity of Ti-13% Nb-13% Zr in comparison to other biomedical implant alloys

Corrosion and passivation of solution-treated (ST), solution-treated and aged (STA), and diffusion-hardened (DH) variants of Ti-13% Nb-13% Zr (Ti-13-13) were examined in a stimulated physiological (lactated Ringer's) and simulated occluded cell (5 M hydrochloric acid (HCl]) environment. Commercially pure (CP) Ti (Grade 2), Ti-6% Al-4% V, Ti-35% Zr-10% Nb, ST and STA Ti-15% Mo-3% Nb-3% Al (Ti-15-3-3), and Ti-55% Ni also were tested. In the Ringer's solution, the Ti-based alloys all exhibited spontaneous passivity and similar passive current densities (i(p) similar to 5 x 10(-7) A/cm(2)). The Ti-based implant alloys exhibited active-to-passive transitions indicative of incomplete passivity resulting from the stability of the titanium anion (Ti3+) or titanium chloride anion (TiCl2+) at deaerated open-circuit potentials (OCP) in 5 M HCl. In contrast DH Ti-13-13 exhibited spontaneous passivity in 5 M HCl as a result of its 0.8-mu m-thick thermal oxide. Anodic polarization and mass-loss investigations of Ti-Nb, Ti-Zr, Ti-Mo, and Ti-Nb-Zr-based alloys in 5 M HCl revealed that additions of Nb, Zr, and Mo improved the corrosion resistance and passivity of Ti, with spontaneous passivity possible with sufficient alloying. The inferior corrosion behavior of STA Ti-15-3-3, with beta + alpha microstructure, in comparision to ST Ti-15-3-3, with a single phase beta solid solution microstructure, was attributed in part to beta and alpha stabilizer partitioning during the metastable beta (body-centered cubic [bcc]) --> beta + alpha phase transformation, which resulted in an Al-rich alpha phase and a Mo- and Nb-rich beta phase in the aged microstructure. This was confirmed by anodic polarization experiments of Ti-Al binary alloys, as well as by energy dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) nanostructural analysis of corroded STA alloy surfaces, Isothermal aging of Ti-13-13 resulted in the partial phase transformation from metastable beta and diffusionless alpha' (martensitic hexagonal close-packed [hcp]) to a beta + alpha' + alpha microstructure. The slight partitioning of Nb into the beta phase, the presence of significant concentrations of Zr in the beta and or phases, and lack of Al accounted for the improved passivity and corrosion resistance of STA Ti-13-13 compared to STA Ti-15-3-3.