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 beta-Ta nanocrystalline coating, composed of equiaxed beta-Ta grains with an average grain size similar to 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 (D-o), of the passive film formed on the beta-Ta coating were compared to those for both uncoated Ti-6Al-4V and commercially pure Ta in Ringer's physiological solution at 37 degrees C, using an array of complementary electrochemical techniques. The results showed that the fi-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 degrees C. Mott Schottky analysis revealed that the passive film formed on the beta-Ta coating had a lower donor density and flat-band potential than commercially pure Ta. The calculated values of D-o for the beta-Ta coating (1.45 X 10(-16) cm(2)/s) are comparable to that for commercially pure Ta (1.13 X 10(-16) cm(2)/s), both of which are 1 order of magnitude lower than that for uncoated Ti-6Al-4V (2.73 X 10(-15) cm(2)/s). Compared with commercially pure Ta, the higher corrosion resistance of the beta-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.