Electrochemical Corrosion Behavior of Nanocrystalline β-Ta Coating for Biomedical Applications.

To explore its potential as a highly corrosion-resistant coating for biomedical titanium alloys, a novel β-Ta nanocrystalline coating, composed of equiaxed β-Ta grains with an average grain size ∼22 nm, was deposited onto Ti-6Al-4V substrate using a double glow discharge plasma technique. The newly developed coating exhibited an extremely dense and homogeneous microstructure, exhibiting a strong (002) preferred orientation. The electrochemical behavior and semiconducting properties, such as donor density, flat-band potential, and diffusivity of point defects (Do), of the passive film formed on the β-Ta coating were compared to those for both uncoated Ti-6Al-4V and commercially pure Ta in Ringer's physiological solution at 37 °C, using an array of complementary electrochemical techniques. The results showed that the β-Ta coating not only provided Ti-6Al-4V with good corrosion protection but also endowed a higher resistance to corrosive attack than commercially pure Ta in Ringer's physiological solution at 37 °C. Mott-Schottky analysis revealed that the passive film formed on the β-Ta coating had a lower donor density and flat-band potential than commercially pure Ta. The calculated values of Do for the β-Ta coating (1.45 × 10-16cm2/s) are comparable to that for commercially pure Ta (1.13 × 10-16 cm2/s), both of which are 1 order of magnitude lower than that for uncoated Ti-6Al-4V (2.73 × 10-15 cm2/s). Compared with commercially pure Ta, the higher corrosion resistance of the β-Ta coating results from its nanosized grains and crystallographic orientation, which promotes the rapid formation of a robust passive film with a more compact structure.