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Effect of Load Deflection on Corrosion Behavior of NiTi WireInstitute of Oral Medicine, National Cheng Kung University, Tainan, Taiwan, ROC Correspondence: * corresponding author, tmlee{at}mail.ncku.edu.tw
For dental orthodontic applications, NiTi wires are used under bending conditions in the oral environment for a long period. The purpose of this study was to investigate the effect of bending stress on the corrosion of NiTi wires using potentiodynamic and potentiostatic tests in artificial saliva. The results indicated that bending stress induces a higher corrosion rate of NiTi wires in passive regions. It is suggested that the passive oxide film of specimens would be damaged under bending conditions. Auger electron spectroscopic analysis showed a lower thickness of passive films on stressed NiTi wires compared with unstressed specimens in the passive region. By scanning electron microscopy, localized corrosion was observed on stressed Sentalloy specimens after a potentiodynamic test at pH 2. In conclusion, this study indicated that bending stress changed the corrosion properties and surface characteristics of NiTi wires in a simulated intra-oral environment.
Key Words: NiTi wire • bending stress • corrosion • passive film • orthodontic
Equiatomic nickel-titanium (NiTi) shape-memory alloys have been widely used for the production of orthodontic archwires in which superelastic and shape-memory properties are necessary. In clinical applications, NiTi orthodontic wires provide continuous forces in the leveling and alignment stages of tooth movement. Although the passive film, mainly composed of titanium and oxygen elements on the surface of NiTi, could provide protection from the attack of an aggressive environment, the ion release and/or corrosion product, especially nickel, is also of concern in terms of its capacity to induce allergic, toxic, and carcinogenic reactions (Peltonen, 1979; Putters et al., 1992; Gawkrodger, 1993). There is no total agreement on the biocompatibility of NiTi alloy. Several authors have evaluated the toxic behavior, in vitro cytocompatibility, and in vivo biological responses of NiTi, and they concluded that NiTi alloy has biocompatibility similar to that of cobalt alloys and stainless steels (Castleman and Motzkin, 1981; Sanders et al., 1993). Another in vivo study, however, indicated that NiTi alloy could possibly induce a cytotoxic reaction (Berger-Gorbet et al., 1996). It is generally believed that the corrosion product would induce a negative effect on biocompatibility, and many researchers have used a surface modification to improve the corrosion resistance of NiTi alloy (Cisse et al., 2002; Tan and Crone, 2002). Except for material properties, corrosion resistance also depends on the environmental constituent, pH values, and loading conditions. Many researchers have used modified environments to evaluate the electrochemical behavior of NiTi in the intra-oral environment (Rondelli and Vicentini, 1999; Widu et al., 1999). Huang investigated the effects of pH values and tensile stress on the corrosion resistance of as-received NiTi wires in artificial saliva (Huang, 2003), but did not simulate the real corrosion conditions to evaluate the corrosion behavior of NiTi wires in orthodontic application. For dental orthodontic applications, NiTi wires are given bending forces rather than tensile forces, which can easily damage passive films. The aim of this study was to investigate the corrosion behavior of NiTi wires in vitro, simulating the intra-oral environment in as realistic a manner as possible. The electrochemical properties of commercial NiTi orthodontic wires under three-point bending were evaluated by potentiodynamic polarization and potentiostatic measurement in artificial saliva with different acidities. We used surface analyses to evaluate the passive film and surface morphology of tested NiTi wires.
Materials Two brands of as-received commercial NiTi dental orthodontic archwires (diameter, 0.016 in) were used in this study: (a) Nitinol (Unitek, Monrovia, CA, USA) and (b) Sentalloy (Tomy, Tokyo, Japan). Each tested specimen was selected from the end side of an archwire, which is free of pre-formed internal stress. A three-point bending force was applied to the specimens during the electrochemical test. The chemical compositions of the two tested materials were analyzed by scanning electron microscopy (JSM 840; JEOL, Tokyo, Japan) equipped with an energy-dispersive x-ray spectrometer (Link AN.1000/85S; Oxford Instruments, High Wycombe, UK). A mixed-metal powder was used as a reference material for identification of elemental composition.
Potentiodynamic Study
Potentiostatic Study
Energy-dispersive x-ray spectrometric analysis revealed the chemical composition (at%) of Nitinol wire to be Ti 50.5, Ni 49.0, and Cr 0.5, and that of Sentalloy to be Ti 48.2 and Ni 51.8. The results of the cyclic potentiodynamic polarization curves for the NiTi wires are shown for pH 5.3 (Fig. 1a  ). Regardless of the bending force, all samples exhibited active-to-passive transition behavior in polarization. Stress significantly influenced the electrochemical properties of the two wires in the passive region, but similar current densities between unstressed and stressed specimens for the two kinds of wires were observed at the same applied voltage in the transpassive region. The reversal potential, where the current density reached 1 mA/cm2, was 1.643 V (Ag/AgCl) for unbent Nitinol, 1.660 V (Ag/AgCl) for stressed Nitinol, 1.616 V (Ag/AgCl) for un-stressed Sentalloy, and 1.636 V (Ag/AgCl) for stressed Sentalloy (Fig. 1a). These results indicated that all specimens showed almost the same value in the pH 5.3 artificial saliva. A summary of the corrosion data obtained from the cyclic potentiodynamic polarization curves is provided in the Table. In comparison with unstressed specimens, the stressed specimens obtained from the same wire manufacturers showed more active corrosion potential, higher corrosion current density, lower polarization resistance, and lower passive current at both 0 V (Ag/AgCl) and 1 V (Ag/AgCl). Statistical analysis indicated that the bending stress had a significant influence on Icorr, Rp, Ipass (0 V), and Ipass (1 V) (p < 0.01) by one-way analysis of variance (ANOVA).
The potentiodynamic scans in artificial saliva at pH 2 are shown (Fig. 1b  ). Similar polarization curves of specimens purchased from the same wire manufacturer were obtained at both pH 2 and pH 5.3. In comparison with the unstressed wires, the stressed specimens showed higher passive current density in the passive region, but similar current density in the transpassive region. The bending stress did not influence the reversal potential of the same commercial wires at pH 2. The corrosion parameters obtained from the cyclic potentiodynamic polarization curves are shown in the Table. The effect of bending stress on the corrosion data for specimens tested at pH 2 had the same tendency as at pH 5.3. The stressed wires also showed that the more active the corrosion potential, the higher the corrosion current density and the lower the polarization resistance, with a lower passive current at both 0 V (Ag/AgCl) and 1 V (Ag/AgCl) compared with unstressed specimens. There are significant differences in Icorr, Rp Ipass (0 V), and Ipass (1 V) between unstressed and stressed wires (p < 0.01) by one-way ANOVA.
Before and after potentiodynamic tests, the surfaces of NiTi wires were observed by scanning electron microscopy (Fig. 2
Although titanium and nickel are highly reactive metals, NiTi alloy forms a dense and coherent passive TiO2-based surface layer, preventing the ingress of corrosion products into the surrounding environment (Wever et al., 1998; Filip, 2001). The factors affecting the corrosion properties of oxidized metal surfaces are structure, composition, thickness, defect structure (including vacancies and interstitial elements), and crystallographic planes of oxide (Browne and Gregson, 1994). In this study, a TiO2-based surface layer formed as an outer film on NiTi wires after 1.0 V (Ag/AgCl) was applied, and the result was confirmed by x-ray photoelectron spectroscopy (APPENDIX Fig. 2). In the passive region, the oxide layer, which reduced corrosion current to a low level, played an important role in corrosion resistance against an aggressive environment. For stressed wires in artificial saliva solution at both pH 2 and pH 5.3, the three-point bending method buckled or cracked the oxide layer of NiTi wires, and the loss of specimen protection allowed active metal to react with the surrounding environment. This explains why the stressed wires showed a significant difference in Icorr, Rp, Ipass (0 V), and Ipass (1 V), compared with unstressed specimens, in the passive region of the electrochemical test. The passive oxide films of both unstressed and stressed specimens, however, lost soluble products (including TiO4–, HTiO3–, etc.) in the transpassive regions (Schenk, 2001). This could explain why bending stress did not influence the electrochemical behavior of specimens in the transpassive regions at both pH 2 and pH 5.3. In the presence of stress and specific ions (in particular, halides, such as chlorides) at pH 2, the passive film on stressed specimens was easily destroyed above the critical pitting potential. Using scanning electron microscopy, we observed localized corrosion at the surfaces of stressed Sentalloy wires after the potentiodynamic test. A localized form of corrosion could result from the increased initial formation of pinholes or pits in the central zones of stressed specimens. The localized corrosion was always observed on the central zone of stressed Sentalloy specimens in the artificial saliva at pH 2. The central zone of the bent wires was the highest stress area, and the highest bending stresses easily damaged the passive film. This suggests that bending stress plays an important role in the corrosion rate in the passive region. Differences in the oxide thickness on specimens subject to 1.0 V (Ag/AgCl) can be qualitatively deduced from the Auger electron spectroscopic depth-profile results. The Guntherschulze-Betz equation for anodic film growth in the passive potential regions is given by:
where i is current density, For dental clinical applications, the oxidation potentials of materials are in the range between –58 and +212 mV (SCE) (Ewers and Greener, 1985). Although the stressed wires also showed passive behavior in this potential range, the experimental results here indicate that the stressed wires showed higher corrosion rates compared with unstressed specimens in the passive regions. The corrosion product is reported to have negative consequences on biocompatibility. It is suggested that bending stress induces damage of the passivated oxide film on NiTi wires in the electrochemical test. For orthodontic applications, a better understanding of the stresses on corrosion behavior and nano-surface properties should be used to evaluate and develop NiTi and related alloy wires.
This study was supported by grant NSC 93-2213-E-006-109 from the National Science Council, Taiwan.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication February 9, 2006. Revision received January 23, 2007. Accepted for publication February 8, 2007.
Journal of Dental Research, Vol. 86, No. 6,
539-543 (2007)
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ABSTRACT
INTRODUCTION
unstressed Nitinol wire; 
stressed Nitinol wire; 
unstressed Sentalloy wire; • stressed Sentalloy wire. At least 3 specimens were used for corrosion testing. Measurements were performed in de-aerated modified Fusayama artificial saliva at 37°C. The stressed specimens showed higher current density in the passive region, but similar current density in the transpassive region. The corrosion parameters for each group are shown in the Table
represents the potential drop across the oxide layer of thickness x, and A and B are constants. In this study, the values of current density and oxide thickness, calculated from Auger electron spectroscopic depth-profile results, obeyed the equation above for the same composition and pH. This suggests that the thickness of the oxide film may also play an important role in current density in the passive region.