Abstract
The present paper reports on the formation of self-organized nanotube oxide layers on β-type Ti–29Nb–13Ta–4.6Zr alloy in (NH4)2SO4 containing a small amount of NH4F. Highly ordered nanotube layers can be formed on the alloy under a wide range of applied potentials. Initiation and self-organization of pores is a potential and time dependent process. Layers removed from the electrolyte in an early growth state consist of two different morphologies—an outer nanoporous structure and an underneath ordered nanotube layer. For extended anodization, the outer nanoporous layer is completely dissolved. Within subsequent growth stages multi-scale ordering of the nanotube arrays with two discrete geometries can be observed. The different stages and morphologies depend significantly on the anodizing potentials. Clearly – compared with anodic tubes on pure Ti – a much broader range of nanotube geometries i.e. “structural flexibility” can be achieved with the Ti–29Nb–13Ta–4.6Zr alloy.