INTRODUCTION

Four major orthodontic alloy wires, namely, nickel-titanium, beta titanium, stainless steel, and cobalt-chromium- nickel wires, exhibit good corrosion resistance, owing to thin protective films on their surface. Films of nickel-titanium or beta titanium alloy are mainly composed of titanium oxide, whereas those of stainless steel or cobalt-chromium-nickel alloy are mainly composed of chromium oxide. These orthodontic wires shows high corrosion resistance in various solutions1–8 such as Ringer's solution,18 artificial saliva,3 and NaCl solution.6 In these solutions, the corrosion resistance of titanium alloys is higher than that of stainless steels or cobalt-based alloys from the viewpoint of film breakdown. In the oral cavity, however, even titanium alloys do not always exhibit high corrosion resistance. Actually, corrosion of orthodontic wires in the oral cavity has been observed.9–13 One reason for this is that the corrosion resistance of titanium alloys decreases in solutions that contain fluoride.14–28 In acid fluoride solutions such as prophylactic agents, the titanium protective film reacts with hydrofluoric acid to form sodium titanium fluoride.2526 The breakdown of the film leads to a decrease in corrosion resistance14–28 because titanium shows intrinsically high activities.

As another serious effect of fluoride on titanium alloys, significant degradation of the mechanical properties of Ni- Ti superelastic and beta titanium alloys caused by hydrogen absorption, ie, hydrogen embrittlement, occurs in fluoride solutions.2930 On the other hand, stress corrosion cracking of stainless steel has been reported in fluoride solutions.31–35 Stress corrosion cracking and hydrogen embrittlement are closely related. Hydrogen embrittlement of stainless steel36–40 and cobalt-chromium alloys41 were investigated by cathodic hydrogen charging. However, the correlation between hydrogen absorption and degradation of stainless steel in fluoride solutions has not yet been clarified. In addition, it is necessary to confirm whether degradation in performance of orthodontic wires occurs during short-term contact with fluoride solution. The degradation of such wires in the oral cavity leads to a decrease in orthodontic force.

The purpose of the present study was to examine the degradation in performance of four major orthodontic wires after short-term immersion in acid fluoride solution from the viewpoint of hydrogen embrittlement. For the evaluation of the degradation in performance, tensile tests, surface observation by scanning electron microscope, and hydrogen thermal desorption analysis (TDA) were conducted.

MATERIALS AND METHODS

Four major orthodontic wires, namely, nickel-titanium, beta titanium, stainless steel, and cobalt-chromium-nickel wires, were tested in this study. The wire abbreviations, brand names, and manufacturers are listed in Table 1. These 0.40-mm (0.016-inch)–diameter wires were cut as 50-mm- long specimens. The specimens were immersed separately in 10 mL of an aqueous solution of 2.0% acidulated phosphate fluoride (APF; 2.0% NaF + 1.7% H₃PO₄) with pH 5.0 at 37°C for 60 minutes. Percent in solution means mass percent unless otherwise stated.

TABLE 1. Orthodontic Wires Used in the Study

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Tensile tests on the nonimmersed and immersed specimens were carried out at room temperature (25 ± 2°C) using a Shimadzu Autograph AG-100A machine at a strain rate of 8.33 × 10⁻⁴/second. The gauge length of the specimens was 10 mm. All the immersed specimens were tested within a few minutes after being taken out of the solution. The mass change of the immersed specimens was measured. The standard deviation of tensile strengths and mass changes was calculated from the results obtained from more than five specimens. The fracture surface and side surface of the specimens were examined using a scanning electron microscope (SEM).

The amount of desorbed hydrogen was measured by hydrogen TDA for the nonimmersed and immersed specimens. TDA was started 30 minutes after removal of specimens from the solution. A quadrupole mass spectrometer (ULVAC, Kanagawa, Japan) was used for hydrogen detection. Sampling was conducted at 30-second intervals, and the heating rate was 100°C/hour up to a terminal temperature of 800°C.

RESULTS

The tensile test results for the nonimmersed and immersed specimens are shown in Figure 1. The tensile strength of the immersed nickel-titanium wire decreased from 1380 to 1160 MPa. The reduction in tensile strength was 16.2%. It should be noted that the immersed nickel- titanium wire fractured before yielding. The critical stress for the stress-induced martensite transformation of the immersed nickel-titanium wire increased from 430 to 480 MPa compared with that of the nonimmersed wire. For the beta titanium wire, the tensile strength slightly decreased after immersion, and the reduction in tensile strength was 5.6%. On the other hand, the tensile strength of the immersed stainless steel wire only slightly decreased (reduction in tensile strength, 2.6%), and that of the immersed cobalt-chromium-nickel wire hardly decreased (reduction in tensile strength, 0.9%).

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Figure 2 shows the fractographs of the nonimmersed (Figure 2a) and immersed nickel-titanium wires (Figure 2b). The fracture surface of the nonimmersed wire was ductile and was characterized macroscopically with a cup-cone morphology. In contrast, the fracture surface of the immersed wire was fairly flat and exhibited no reduction in area. For the other wires, the fracture surface was not significantly different for the nonimmersed and immersed wires and was characterized macroscopically with a cup- cone morphology.

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The SEM micrographs of the side surface of the nonimmersed and immersed nickel-titanium wires are shown in Figure 3. The side surface of the nonimmersed wires indicates that surface defects related to the wire drawing, pickling, or electropolishing procedures performed during the manufacturing process contribute to corrosion. The surface of the immersed nickel-titanium wire became rough because of corrosion, as shown in Figure 3c,d. The side surface of the nonimmersed beta titanium wire is shown in Figure 4a,b. Surface defects and corrosion pits produced by the manufacturing process were observed on the entire surface. For the immersed beta titanium wire, corrosion products uniformly formed on its surface, as shown in Figure 4c,d. The side surface of the nonimmersed stainless steel wire exhibited scratches and pits on its surface, as shown in Figure 5a,b. Upon immersion, corrosion products were distributed inhomogeneously on its surface (Figure 5c,d). The side surface wire was not significantly different for the nonimmersed and immersed cobalt-chromium-nickel wires, as shown in Figure 6a through d.

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The results of the mass change after immersion are shown in Figure 7. The mass of the nickel-titanium wire decreased, whereas that of the beta titanium wire increased. The mass loss of the nickel-titanium wire is attributable to corrosion during immersion in the solution. For the beta titanium wire, the mass increase resulting from the corrosion products deposited on its surface seems to be larger than the mass loss arising from dissolution. The mass of the stainless steel wire increased slightly because of the deposition of corrosion products on its surface. On the other hand, the mass change of the cobalt-chromium-nickel wire was hardly confirmed.

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Figure 8a through d shows the TDA curves for the nonimmersed and immersed wires. The progress of hydrogen entry into the wires was denoted by an increase in the total desorbed hydrogen, defined as the integrated peak intensity. The amount of hydrogen that desorbed from the nonimmersed wires gives the concentration of predissolved hydrogen. For the immersed nickel-titanium and beta titanium wires, thermal hydrogen desorption appeared with a desorption peak at approximately 400°C and 650°C, respectively. This TDA result clearly indicates that the nickel- titanium and beta titanium wires absorbed a large amount of hydrogen in the solution even with a 60-minute immersion. The amounts of absorbed hydrogen during the immersion of the nickel-titanium and beta titanium wires were calculated by subtracting the predissolved hydrogen content in each wire and were found to be 227 and 94 mass ppm, respectively. On the other hand, for the immersed stainless steel and cobalt-chromium-nickel wires, no increase in the amount of desorbed hydrogen was confirmed.

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DISCUSSION

Orthodontic wires with a titanium protective film degenerated by immersion in 2.0% APF solution for 60 minutes. The nickel-titanium wire absorbed a substantial amount of hydrogen and the fracture mode changed from ductile to brittle, as shown in Figure 2. Furthermore, the increase in critical stress for martensite transformation by immersion might obstruct the generation of an appropriate orthodontic force. The increase in critical stress for martensite transformation is likely ascribable to the effect of hydrogen of preventing martensite transformation because martensite transformation is sensitive to the presence of interstitial atoms.42 These results are in good agreement with that obtained by immersion in 0.2% APF solution in our previous study.29 In acid fluoride solutions, the breakdown of a titanium protective film occurs readily, and a nickel-titanium alloy absorbs hydrogen because of the high affinity of titanium to hydrogen. When the amount of absorbed hydrogen exceeds 50–200 mass ppm, a pronounced degradation in the performance of nickel-titanium superelastic alloys occurs.2943 As with the findings of previous studies, fracture stress presumably decreases to critical stress for martensite transformation, and a wire fractures before martensite transformation with longer immersion time.

Fracture of nickel-titanium orthodontic wires is often experienced during clinical use in the oral cavity.4445 Its main cause has been considered to be surface defects generated during wire manufacture, fatigue, or corrosion-related phenomena.946 However, our results clearly indicate that one of the reasons for the fracture is degradation in performance caused by hydrogen absorption in the presence of fluoride in the oral cavity. Various mechanisms have been proposed for hydrogen embrittlement, eg, the stress-induced hydride formation and cleavage mechanism,47 hydrogen-induced decohesion theory,48 and hydrogen-enhanced localized plasticity model,4950 but a detailed discussion is beyond the scope of the present study.

The beta titanium wire absorbed substantial amounts of hydrogen like the nickel-titanium wire, although its tensile strength was slightly decreased by immersion in 2.0% APF solution for 60 minutes. In general, the tensile strength of beta titanium alloys decreases considerably with increasing hydrogen content.51–57 When hydrogen content is larger than several thousand mass ppm, pronounced degradation, such as ductile-to-brittle transition, occurs. Moreover, the delayed fracture of beta titanium alloys takes place because of hydrogen absorption in APF solutions.30 Therefore, a marked degradation in performance of beta titanium orthodontic wires will occur with longer immersion times.

The absorbed hydrogen in titanium alloys diffuses from the surface inward even at room temperature, and diffusion distance depends on the coefficient of hydrogen diffusion in materials. Hence, for thinner nickel-titanium and beta titanium wires, degradation in performance caused by hydrogen absorption probably occurs for a short immersion time in comparison with the results of the present study.

In the current study, hydrogen contents in the nonimmersed nickel-titanium and beta titanium wires were relatively large, as shown in Figure 8a,b, suggesting that hydrogen was introduced by manufacturing processes such as electropolishing. Wu and Wayman58 have reported that nickel-titanium alloys absorb hydrogen during electropolishing in the preparation of transmission electron microscopy specimens. Diminution of the hydrogen content by the manufacturer will help improve the clinical performance of wires.

The tensile strength of the orthodontic wire with a chromium protective film was only slightly reduced by immersion in 2.0% APF solution for 60 minutes. For the stainless steel wire, the effect of immersion was recognized on the basis of the results of SEM observation and mass change measurement. However, the increase in the amount of desorbed hydrogen was not confirmed. It should be noted that local hydrogen density was not measured in the present experiment of TDA because the amount of absorbed hydrogen was taken as the mean value over the immersed wire.

Hydrogen embrittlement of stainless steels36–40 and cobalt-based alloys41 has been examined by cathodic hydrogen charging. When stainless steels and cobalt-based alloys absorb hydrogen, a degradation of their mechanical properties results, ie, there is reduced tensile strength and ductility. The stress corrosion cracking of stainless steels in the presence of fluoride has been reported by several authors,31– 35 although the involvement of hydrogen absorption has not been mentioned. In the case of stress corrosion cracking in acid solutions, crack growth is accompanied by anodic dissolution and cathodic hydrogen generation at the crack tip. The breakdown of the protective film at the crack tip often leads to hydrogen absorption. Even an extremely small amount of hydrogen in the vicinity of the crack can sometimes play an important role in stress corrosion cracking.5960 Therefore, that the slight degradation of stainless steel is attributable to a small amount of hydrogen should not be excluded, although hydrogen desorption was not detected in the present study. Nonetheless, for a short immersion time without loading, we provisionally consider that the serious degradation of orthodontic wires with a chromium protective film caused by hydrogen absorption does not occur in a 2.0% APF solution. Local hydrogen absorption and long-term immersion tests should be investigated in the future.

CONCLUSIONS

The degradation in performance of four major orthodontic wires caused by hydrogen absorption was investigated by immersion in a 2.0% APF solution for 60 minutes. Upon immersion, the nickel-titanium and beta titanium wires absorbed substantial amounts of hydrogen and degenerated, whereas the performance of the stainless steel and cobalt- chromium-nickel wires was only slightly affected. In APF solutions, the degradation resistance of wires with a titanium protective film is considered to be lower than that of wires with a chromium protective film. Clinically, the present results suggest that orthodontists should, as much as possible, avoid placing wires with a titanium protective film in contact with prophylactic agents, toothpastes, or dental rinses that contain fluoride.