Corrosion Resistance of Different Nickel-Titanium Archwires in Acidic Fluoride-containing Artificial Saliva

When using nickel-titanium (NiTi) archwire for dental orthodontic treatment, the possible danger associated with archwire corrosion derives from the biologically harmful effects of the released Ni ion.1–3 Therefore, NiTi archwire with a good corrosion resistance is crucial to its biocompatibility. On the other hand, the surface corrosion of NiTi archwires may increase the friction that appears at the interface between the archwire and bracket, reducing the free sliding action during orthodontic treatment.

In dental applications, fluoride-containing commercial mouthwashes, toothpastes, and prophylactic gels are generally used to avoid dental caries or to reduce dental sensitivity. The fluoride ions degrade the protective TiO₂ film formed on Ti and Ti alloys.4–8 Since the outermost surface of NiTi archwire contains mainly TiO₂ film with small amounts of NiO,9 fluoride-enhanced corrosion of the NiTi archwire may occur. The corrosion behavior of a NiTi archwire in fluoride-containing mouthwashes (fluoride ion: a few hundred parts per million)10,11 or prophylactic gels (fluoride ion: close to or more than 10,000 ppm)12 has been investigated. However, information on the influence of fluoride concentration, contained in commercial fluorinated toothpastes (fluoride ion: a few hundred to a few thousand parts per million), on the corrosion resistance of commercial NiTi orthodontic archwires is still limited. Furthermore, NiTi archwires produced by various manufacturers may have different corrosion resistances in the oral environment. In the previous study,13 the corrosion resistance of different Ni-Ti archwires in fluoride-free acidic artificial saliva is different. However, related knowledge concerning the effect of fluoride concentration on the comprehensive difference in corrosion resistance of commercial NiTi archwires from various manufacturers still requires further investigation.

The hypothesis is that different brands of NiTi archwires may have dissimilar corrosion resistance in oral environments with different fluoride concentrations. In the present investigation, we studied the dissimilarity of the corrosion resistance of commercial NiTi archwires produced by different manufacturers. Experiments were performed in acidic artificial saliva with different NaF concentrations, simulating commercial fluoride-containing toothpastes. The surface characterizations of different NiTi archwires were compared and correlated with the corrosion resistance.

Four different as-received commercial round (Ø: 0.016-inch) NiTi dental orthodontic archwires containing approximately equi-atomic Ni and Ti elements were used in this study. These four types of archwires were designated as follows: Dent (Rematitan® Lite, Dentaurum, Ispringen, Germany), Ormco (Ni-Ti®, Ormco, Glendora, Calif), RMO (Orthonol® Nickel-Titanium Wire, RMO Inc, Denver, Colo), and SY (NiTi Wire, Shin-Ya Co, Taipei, Taiwan).

A scanning electron microscope (ABT-150S, Topcon, Tokyo, Japan) was used to observe the surface morphologies of the archwires. The three-dimensional surface roughness (R_a) of the archwires was evaluated using atomic force microscopy (Nanoscope III, Digital Instruments Inc, Santa Barbara, Calif). The R_a represents the arithmetical mean of the absolute values of the scanned surface profile (50 µm × 50 µm). The outermost surface chemical structures of the archwires were assessed using x-ray photoelectron spectroscopy (XPS) (ESCALAB 210, VG Scientific Ltd, East Grinstead, UK). For the surface characterization analysis of each NiTi archwire group, the sample size was three. These three samples had identical material batch numbers.

A potentiostat (AUTOLAB PGSTAT 30, Eco Chemie BV, Utrecht, The Netherlands) was used to perform the linear polarization test,14 a fast and nearly nondestructive electrochemical technique. NiTi archwires were used as working electrodes. A saturated calomel electrode and platinum sheet were used as the reference electrode and counter electrode, respectively. Modified Fusayama artificial saliva with a pH of 6.5 at 37°C was used as the corrosion test electrolyte13; the artificial saliva consisted of NaCl (400 mg/L), KCl (400 mg/L), CaCl₂·2H₂O (795 mg/L), NaH₂PO4·H₂O (690 mg/L), KSCN (300 mg/L), Na₂S·9H₂O (5 mg/L), and urea (1000 mg/L). To evaluate the effect of fluoride concentration on the corrosion resistance of commercial NiTi archwires, different amounts of NaF were added to the artificial saliva to prepare 0.01%, 0.1%, 0.25%, and 0.5% NaF concentrations, simulating the fluoride concentrations contained in commercial fluoridated toothpastes. The fluoride ion concentrations in the artificial saliva containing 0.01%, 0.1%, 0.25%, and 0.5% NaF were measured using a fluoride electrode (No. 27502-19, Cole-Parmer Instrument Co, Vernon Hills, NY), which revealed concentrations of about 46 ppm, 461 ppm, 1130 ppm, and 2250 ppm, respectively.

The linear polarization tests, in terms of linear polarization curve measurements, of the NiTi archwires were carried out after dipping the archwires into the test electrolyte for 2 hours. Note that the open circuit potential of the NiTi archwires in all test electrolytes could reach stable value in about 1 hour. The linear polarization curves were measured from −10 mV to +10 mV (vs corrosion potential), with a scan rate of 0.1 mV/s. The polarization resistance (R_p [Ω cm²]), which is inversely proportional to the corrosion rate, is defined as the slope of the potential (V) vs the current density (A cm⁻²) near corrosion potential in the linear polarization curves.14 The R_p value was statistically analyzed using two-way analysis of variance for analyzing the variable factors of the archwire manufacturer and NaF concentration (α  =  0.05). For the corrosion test of each NiTi archwire group, the sample size was five. These five samples had identical material batch numbers.

Figure 1 shows the scanning electron microscope images of the test as-received commercial NiTi archwires. Surface defects produced during the manufacturing process were noticeable on Ormco and SY archwires, while some small surface scratches created during the manufacturing process were observed on Dent and RMO archwires. The scanning electron microscope images in Figure 1 actually represented the surface textures of the test NiTi archwires. Figure 2 and Table 1, respectively, show the atomic force microscopy images and the corresponding surface roughness (R_a: µm) of the test as-received commercial NiTi archwires. A rougher surface was visible on Ormco (R_a: 1.27 µm) and SY (R_a: 0.90 µm) archwires, whereas RMO archwire exhibited the least surface roughness (R_a: 0.19 µm). Figure 3 shows the XPS surface analysis results of the commercial NiTi archwire (Ormco) before (as-received condition) and after the corrosion test in 0.5% NaF–containing artificial saliva. Results show that the outermost surface chemical structure of the passive film on the as-received Ormco archwire contained mainly TiO₂, with trace amounts of NiO. After the corrosion test in 0.5% NaF–containing solution, the Ti-F complex compound (Na₂TiF₆) could be detected on the Ormco archwire along with the those oxides detected on the as-received archwires. Similar XPS analysis results mentioned above were also obtained for the other three archwires (Dent, RMO, and SY).

Table 2 shows the R_p values, obtained from the linear polarization tests, of the NiTi archwires from different manufacturers. Results of two-way analysis of variance showed that the archwire manufacturer and NaF concentration had a statistically significant influence on the R_p value (archwire manufacture: P < .01; NaF concentration: P < .001). Increasing the NaF concentration resulted in a decrease in the R_p values of NiTi archwires, regardless of archwire manufacturer. The R_p values of NiTi archwires decreased from 45.5–91.2 × 10³ to 14.2–31.6 × 10³ Ω cm² when the NaF concentration changed from 0% to 0.25% (fluoride ion: from 0 to 1130 ppm): the Ormco archwire had the highest R_p value and for the most part the SY archwire had the lowest R_p value under the same NaF concentration. In artificial saliva with a high 0.5% NaF concentration (fluoride ion: 2250 ppm), the R_p values of all test NiTi archwires significantly decreased to 1.2–2.2 × 10³ Ω cm², and no significant difference in R_p was present among the four types of archwires at this high NaF concentration of 0.5%.

In this study, the R_p value could be rapidly calculated from the linear polarization test (selected as a nondestructive and rapid electrochemical technique) and used as a parameter for corrosion resistance evaluation of NiTi archwires. Although the XPS analysis results revealed that the surface chemical structure of the passive film on all test NiTi archwires was similar, a statistically significant difference in R_p, or corrosion resistance, was found among the NiTi archwires from different manufacturers (P < .01), regardless of the NaF concentration (0%–0.25%) in artificial saliva. The significant difference in R_p could be due to the variation in surface characterizations.

A previous study15 showed that the surface roughness of orthodontic archwires ought to be taken as an important indicator of the trend toward archwires' corrosion resistance. The surface defects on NiTi orthodontic archwire produced during the manufacturing procedure can be the probable locations for corrosion occurrence.16 However, in this study, the Ormco archwire (with the notable surface defects and highest surface roughness shown in Figure 1 and Table 1) did not exhibit a lower R_p value or a lower corrosion resistance, regardless of the NaF concentration in artificial saliva (Table 2). This was similar to the results reported by the previous studies, which showed that a NiTi archwire with a rougher surface does not exhibit greater metal ion release17 or lower corrosion resistance13 in fluoride-free acidic artificial saliva. In this study, the difference in R_p among the four types of NiTi archwires with the same surface passive film was believed to be related to the varying surface residual stress produced during the different manufacturing procedures rather than to the notable surface defects and/or surface roughness.

It is well recognized that a metal's biocompatibility is related to the protectiveness of its surface passive film.18,19 NiTi archwire with a good long-term corrosion resistance, namely with a lastingly protective TiO₂-based passive film, in an acidic oral environment is necessary to its biocompatibility. Among the test NiTi archwires in this study, Ormco archwire showed the highest R_p (31.6–91.2 × 10³ Ω cm²) in the acidic artificial saliva with lower NaF (≤0.25%) concentration (fluoride ion ≤ 1130 ppm). Therefore, in terms of corrosion resistance, this Ormco archwire was suggested as a potential candidate among the test NiTi archwires for dental orthodontic treatment in low–fluoride-containing environment. However, regardless of the archwire manufacturer, all test NiTi archwires showed a significantly decreased R_p value of about 1.2–2.2 × 10³ Ω cm² in high–0.5% NaF-containing environment (fluoride ion: 2250 ppm). This indicated that the surface passive film on all test NiTi archwires was severely damaged by fluoride ions when the NaF concentration in artificial saliva increased to 0.5%.

It has been reported6,7 that the protectiveness of TiO₂ formed on Ti and Ti-6Al-4V alloy is degraded by fluoride ions when the NaF concentration exceeds 0.1% (fluoride ion: close to 500 ppm) via the formation of a Ti-F complex compound. Previous studies10,11 have shown that a NiTi orthodontic archwire in commercially available fluoride mouthwashes (fluoride ion: close to 250 ppm) can corrode. In this study, the corrosion resistance, in terms of R_p, of the test NiTi archwires decreased as the NaF concentration increased (Table 2), regardless of archwire manufacturer. In other words, the protectiveness of the TiO₂-based passive film on the four types of NiTi archwires was harmfully influenced by the presence of NaF (fluoride ions: 46 ppm, 461 ppm, 1130 ppm, and 2250 ppm) in the acidic artificial saliva, especially at a higher fluoride concentration of 2250 ppm, through the formation of Ti-F complex compound Na₂TiF₆ (Figure 3).

In acidic fluoride-free artificial saliva, the R_p value for Ti with a surface chemical structure composed of TiO₂ is approximately 10⁶ Ω cm²,20 whereas the R_p value for Ti-6Al-4V with a surface structure composed primarily of TiO₂ is around 4.5 × 10⁴ Ω cm².7 In this study, the R_p values of the test NiTi archwires in low–NaF (≤0.1%)-containing artificial saliva ranged between 3.1 × 10⁴ Ω cm² and 9.1 × 10⁴ Ω cm². These values were lower than that of Ti but close to or greater than that of Ti-6Al-4V alloy in fluoride-free artificial saliva, as mentioned above. Based on the above-mentioned results, the corrosion resistance of the four types of NiTi archwires investigated in the low–NaF (≤0.01%)-containing acidic oral environment could be acceptable, despite variance in the corrosion resistance among the test NiTi archwires. Note that the R_p values for the test NiTi archwires decreased to about 1.2–2.2 × 10³ Ω cm² when tested in a high–0.5% NaF-containing environment. This was expected to result in greater metal ion release and enlarged friction between the archwire and bracket as a result of increased archwire surface roughness.

In this study, the R_p values were measured after exposing the archwires to the fluoride-containing test electrolytes for 2 hours. This simulated the in vivo corrosion resistance of the archwires when the residual fluorinated toothpastes interacted with the archwires for 2 hours after tooth brushing. It was expected that the in vivo corrosion resistance of the archwires in oral environments might decrease if the highly fluorinated toothpastes were used and/or the exposure time of archwires to residual fluorinated toothpastes was prolonged. Therefore, complete removal of residual high-fluorinated toothpastes from the crevice between archwire and bracket during tooth brushing is required. Furthermore, the repair of the protectiveness of surface oxide film on the NiTi archwires might occur after full removal of residual fluorinated toothpastes by mechanical tooth brushing. However, further investigation is required to determine if there is a critical concentration of residual fluoride ions.

It should be noted that in this study the 0.5% NaF concentration (fluoride ions: 2250 ppm) offered the worst situation in terms of destroying the protectiveness of passive film on the test NiTi archwires. Nevertheless, certain dental fluoride gels may contain fluoride ions at a concentration of greater than 10⁴ ppm, which is expected to damage the TiO₂-based passive film on NiTi archwires. Thus, careful and precise post–tooth brushing is strongly suggested. However, in high–fluoride (0.5% NaF)-containing artificial saliva, the corrosion resistance of all test NiTi archwires notably decreased, and no significant difference in polarization resistance was present among the archwires. Therefore, it is suggested that cleaning away the residual high-fluorinated toothpastes from the crevice between the NiTi archwire and bracket during tooth brushing is critical.

• The archwire manufacturer and NaF concentration had a statistically significant influence on the corrosion resistance, in terms of polarization resistance, of the four different kinds of commercial NiTi orthodontic archwires in acidic fluoride-containing artificial saliva.

• The surface topography of the commercial NiTi archwires with identical surface chemical structures, TiO₂, and small amounts of NiO, did not correspond to the variation in corrosion resistance in fluoride-containing artificial saliva.

The authors would like to thank the National Science Council, Taiwan (grant NSC 96-2628-B-010-032-MY3), and Taipei City Hospital (grant 95004-62-169) for financial support.