Fabrication of TiO₂ Nanotubes Through Electrochemical Anodization and Secondary Oxidation

Titanium dioxide nanotubes (TiO₂ NTs) fabricated by electrochemical anodization have attracted attention because their morphology can be regulated by processing parameters. Although the anodic fabrication of TiO₂ NTs has been widely studied, the morphology evolution of nanotubes on Ti6Al4V during secondary anodization after ultrasonic removal of the first nanotube layer still requires further clarification. In this study, anodic nanotubular oxide structures were fabricated on Ti6Al4V substrates by primary anodization and secondary anodization. The effects of anodization voltage, fluoride ion concentration, oxidation time, and secondary anodization on nanotube morphology were investigated. The results show that increasing anodization voltage promotes nanotube formation and increases tube diameter, whereas excessive oxidation time and high fluoride concentration lead to nanograss formation, tube collapse, and surface damage. Compared with primary anodization, secondary anodization produced smaller nanotube diameters and distinct morphology evolution, which may be associated with changes in the initial surface state after ultrasonic removal of the first nanotube layer. Among the investigated conditions, primary anodization at 40 V in 0.5 wt.% NH₄F electrolyte followed by secondary anodization at 50 V produced relatively regular nanotubular regions, although nanograss was also present on the surface. This work provides a process-oriented understanding of TiO₂ nanotube morphology regulation on Ti6Al4V substrates during primary and secondary anodization.