Porous anodic oxides on titanium and on a Ti–W alloy
Introduction
The excellent corrosion resistance of titanium in many environments is primarily due to the protection afforded by the passive film that is developed upon its surface. The film is based upon TiO2, with a thickness of a few nanometres [1]. The naturally-formed oxide on titanium can be sufficient for many applications requiring low corrosion rates [2]. However, the oxide can also be thickened artificially by anodizing which, for instance, may be relevant to biocompatibility [3], [4], [5]. There is also much current interest in the formation of relatively thick, nanoporous anodic oxides on titanium, with self-ordering pores, typically in electrolytes containing fluoride species that promote dissolution of titania [6], [7], [8].
The present study compares the formation of anodic oxides on titanium and a Ti–W alloy in a glycerol/phosphate electrolyte at 453 K. From previous studies in the same electrolyte, relatively thick, nanoporous oxides can be produced on niobium [9] and tantalum [10], [11], in the absence of dielectric breakdown. Thick oxides have also been reported to form on titanium, although with few details [12]. The particular alloy was selected here because of existing knowledge of the growth of a conventional barrier oxide [13], which assists in understanding the oxidation mechanism.
Section snippets
Experimental
Titanium and Ti–20 at.% W layers, of thickness ∼300 nm, were deposited onto anodized aluminium using an Atom Tech magnetron sputtering system, with 99.9% titanium and 99.9% tungsten targets. The pressure in the chamber prior to deposition was 3 × 10−5 Pa, with sputtering undertaken using 99.995% argon at 0.5 Pa. After masking to define a working area of ∼2 cm2, specimens were anodized at 5 or 10 A m−2 in 10 wt% dibasic potassium phosphate in glycerol at 453 K, using a two-electrode cell with a platinum
Voltage–time responses
The voltage–time response for titanium anodized at 10 A m−2 revealed an initial surge to about 0.5 V, attributable to a pre-existing air-formed oxide, followed by an approximately linear rise at about 0.4 V min−1 (Fig. 1a). The rise then slowed to an average value in the range of 0.3–0.9 mV s−1, depending upon the particular specimen, for the period from 3 to 120 min. The charge passed after anodizing for 120 min was 72 kC m−2, when the voltage had reached 5–9 V. Anodizing of the Ti–20 at.% W alloy at 10 A m
Discussion
The findings of the study reveal development of a nanoporous, amorphous oxide, of composition TiO2 · 0.35WO3, on the Ti–20 at.% W alloy, indicating an increased content of tungsten in the oxide film relative to that in the alloy. In contrast, the oxide on titanium is partially crystalline. The morphology of the oxide on the alloy contrasts with the amorphous, barrier oxide formed by anodizing the alloy at almost 100% efficiency in aqueous borate electrolyte at 298 K [13]. The pore-free, barrier
Conclusions
- 1.
Nanoporous oxide films are formed on titanium and the Ti–20 at.% W alloy during anodizing at 10 A m−2 in glycerol/phosphate electrolyte at 453 K.
- 2.
The anodic films on titanium are composed of amorphous and anatase phases. However, for film growth on a Ti–20 at.% W alloy, the films are amorphous due to suppression of structural transformation by incorporation of units of WO3 into the amorphous TiO2. The efficiency of oxide growth is greatly increased with an amorphous structure.
- 3.
The films are
Acknowledgements
The authors wish to acknowledge the support of a Grant-in-Aid for Scientific Research (B), No. 16360353, from the Japan Society for the Promotion of Science. They also wish to thank Dr. I. Vickridge of the Group de Physique des Solides, Universités paris 7 et 6, for assistance with ion beam analyses.
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