Comparative evaluation of surface characteristics of 21 SRx titanium alloy [implant material]

This study was conducted to determine the effects of alloying and surface treatments on the surface oxide and its resulting corrosion behavior. Polished (0.3 μm) discs of cp Ti, Ti-6Al-4V, and Ti-15Mo-2.8Nb-0.2Si (21SRx) were subjected to the following surface treatments; C=cleaned ultrasonically in a series of solvents; P=C+passivated (ASTM B-86); and HT=C+P+heat treated at 350°C in air for 1 hour. Auger electron spectroscopy (AES) was used to determine the elemental composition and thickness of the resulting surface oxides. Polarization testing was used to determine corrosion potential (Ecorr), corrosion rate (Icorr), and passive current density (Ip). Electrochemical impedance spectroscopy (EIS) spectra were fit to a Randles equivalent circuit and its circuit element values calculated. Duncan´s multiple range test (α=0.05) was used to determine significant differences between the sample groups. The AES results showed typical spectra with dominant Ti and O peaks for all sample groups. Small Al and Mo peaks were detected throughout the surface oxides of the Ti-6Al-4V and 21 SRx specimens, respectively. No significant difference in the oxide thickness was shown for the cleaned and passivated groups, which ranged from 3.0 to 5.4 Mg but the heat treated groups were significantly thicker than all the cleaned and passivated groups, ranging from 8.8 to 16.8 nm. Corrosion results showed passivated and heat treated groups to have significant decreases in Icorr and more noble Ecorrs compared to the cleaned groups. All impedance spectra fit into the Randles equivalent circuit model. All sample groups exhibited ideal capacitive behavior (φ=90°) expected for titanium and its alloys. In summary, passivation significantly decreased the Hcorr, while subsequent thermal treatment, which significantly increased the oxide thickness, did not reduce the Icorr further. Thus, the reduced Icorr cannot be attributed to increased oxide thickness alone, Other factors such as oxide stoichiometry and crystallinity need to be evaluated to further investigate these phenomena