An electrochemical study on self-ordered nanoporous and nanotubular oxide on Ti–35Nb–5Ta–7Zr alloy for biomedical applications

Abstract

Highly ordered nanoporous and nanotubular oxide layers were developed on low-rigidity β Ti–35Nb–5Ta–7Zr alloy by controlled DC anodization in electrolyte containing 1 M H₃PO₄ and 0.5 wt.% NaF at room temperature. The as-formed and crystallized nanotubes were characterized by electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffraction. The electrochemical passivation behavior of the nanoporous and nanotubular oxide surfaces were investigated in Ringer’s solution at 37 ± 1 °C employing a potentiodynamic polarization technique and impedance spectroscopy. The diameters of the as-formed nanotubes were in the range of 30–80 nm. The nanotubular surface exhibited passivation behavior similar to that of the nanoporous surface. However, the corrosion current density was considerably higher for the nanotubular alloy. The surface after nanotube formation seemed to favor an immediate and effective passivation. Electrochemical impedance spectra were simulated by equivalent circuits and the results were discussed with regard to biomedical applications.

Introduction

Recently it has been shown that nanoscale porous as well as tubular oxide layers on titanium alloys can increase the bioactivity of an implant material [1], [2], [3]. Such titanium oxide tubular structures find potential applications in various other fields such as catalysis, sensors, solar energy conversion, etc., due to their peculiar semiconducting and photoelectrochemical properties [4], [5]. In recent years, electrochemical anodization technique in F⁻ containing electrolytes was projected as an efficient and economic approach for the production of highly ordered porous structures on valve metals [6], [7]. Depending on the metal substrate and electrochemical conditions, the anodic oxide film may exhibit a compact, porous or a tubular structure. Such nanostructure formations have been achieved electrochemically on Ti [8], [9], Zr [10], Nb [11], and Ta [12]. Anodization of Ti and Zr resulted in distinctly separated hollow cylinder shaped nanotubes; however, porous oxide layers resulted in the case of Nb and Ta [13]. Nanotube growth has been reported on binary, ternary and quaternary titanium alloys such as TiNb [14], TiZr [15], Ti–6Al–7Nb [16], Ti–30Ta–XZr [17] and Ti–29Nb–13Ta–4Zr [18]. In the anodization of Ti, the dissolution is enhanced by fluoride containing electrolytes which form soluble complexes with titanium, resulting in pore or nanotube formation [13], [19]. However, selective dissolution of less stable elements or different reaction rates of different alloy phases can hinder nanotube formation.

Quaternary β-titanium alloys of the system Ti–Nb–Ta–Zr are of current research interest due to their excellent mechanical properties such as very low elastic modulus, coupled with superior biocompatibility and corrosion resistance [20], [21]. The aim of developing such low modulus titanium alloys was to decrease the elastic modulus difference between the bone (10–30 GPa) and the implant material, thereby promoting load sharing between them [22]. When insufficient load sharing occurs, natural bone resorption and loosening of the joint may occur [23]. Major quaternary alloys of this kind investigated include Ti–35Nb–5Ta–7Zr [20], Ti–4Nb–4Ta–15Zr [21], and Ti–29Nb–13Ta–4.6Zr [24]. Among these, Ti–35Nb–5Ta–7Zr alloy have the lowest elastic modulus (55 GPa) and can be considered as one of the best choices for orthopedic implants [25]. No reported information is available on nanotubular oxide formation on this alloy. Reported works on electrochemical corrosion behavior of porous oxide grown titanium alloys is limited [19]. Also, no comprehensive reported information is available on electrochemical corrosion behavior of titanium alloys after nanotubular oxide formation.

Hence in the present work, with a view to study the effect of nanoporous and nanotubular oxide layer formation on the electrochemical corrosion behavior of β Ti–35Nb–5Ta–7Zr alloy, highly ordered nanoporous and nanotubular oxide layers were produced on the alloy surface using controlled anodization in electrolyte containing 1 M H₃PO₄ and 0.5 wt.% NaF at room temperature. The nanotubes formed were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy/energy dispersive X-ray spectrometry (TEM/EDS). Electrochemical behavior of the nanoporous and nanotubular alloy was investigated using potentiodynamic polarization and impedance spectroscopy measurements in Ringer’s solution at 37 ± 1 °C.