Fabrication and characterization of anodic oxide nanotubes on TiNb alloys

Titanium and its alloys have been extensively used as implant materials owing to their high specific strength, good biocompatibility and excellent corrosion resistance. Oxide nanotubular array layer can be formed on Ti alloy surface by electrochemical anodization treatment. In this work, the morphology of nanotubes formed on Ti–Nb alloys (Nb content of 5 wt%, 10 wt%, 20 wt%, 30 wt% and 40 wt%) was investigated using an electrolyte containing ethylene glycol and NH₄F. Oxide layers consisting of highly ordered nanotubes with a range of diameters (approximately 40–55 nm for the inner diameter and 100–120 nm for the outer diameter) and lengths (approximately 10–20 μm) can be formed on alloys in the Ti–xNb system, independent on the Nb content. The nanotubes formed on the Ti–Nb alloy surface were transformed from the anatase to rutile structure of titanium oxide. The oxide nanotubular surface is highly hydrophilic compared with the intact TiNb foil. The surface wettability varies with the nanotube diameter. As the nanotube diameter increases while the nanotube layer thickness remains constant, the capillary wetting of the nanotube surface decreases and the surface becomes less hydrophilic. Annealing changes the nanotubular surface wettability further and establishes less hydrophilic surface conditions due to the removal of hydroxyl groups and residue fluoride-containing species. It is believed that the surface wettability is changed due to the decreasing content of hydroxyl groups in ambient atmosphere. This work can provide guidelines for improving structural and environmental conditions responsible for changing surface wettability of TiNb surfaces for biomedical applications.