Editorial Type:
Article Category: Research Article
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Online Publication Date: 28 Feb 2013

Assessment of nickel titanium and beta titanium corrosion resistance behavior in fluoride and chloride environments

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Page Range: 864 – 869
DOI: 10.2319/091712-740.1
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ABSTRACT

Objective:

To assess the influence of fluoride concentration on the corrosion behavior of nickel titanium (NiTi) superelastic wire and to compare the corrosion resistance of NiTi with that of beta titanium alloy in physiological solution with and without addition of fluoride.

Materials and Methods:

NiTi corrosion resistance was investigated through electrochemical impedance spectroscopy and anodic polarization in sodium chloride (NaCl 0.15 M) with and without addition of 0.02 M sodium fluoride (NaF), and the results were compared with those associated with beta titanium. The influence of fluoride concentration on NiTi corrosion behavior was assessed in NaCl (0.15 M) with and without 0.02, 0.04, 0.05, 0.07, and 0.12 M NaF solution. Galvanic corrosion between NiTi and beta titanium were investigated. All samples were characterized by scanning electron microscopy.

Results:

Polarization resistance decreased when NaF concentration was increased, and, depending on NaF concentration, NiTi can suffer localized or generalized corrosion. In NaCl solution with 0.02 M NaF, NiTi suffer localized corrosion, while beta titanium alloys remained passive. Current values near zero were observed by galvanic coupling of NiTi and beta titanium.

Conclusions:

There is a decrease in NiTi corrosion resistance in the presence of fluoride. The corrosion behavior of NiTi alloy depends on fluoride concentration. When 0.02 and 0.04 M of NaF were added to the NaCl solution, NiTi presented localized corrosion. When NaF concentration increased to 0.05, 0.07, and 0.12 M, the alloy presented general corrosion. NiTi corrosion resistance behavior is lower than that of beta titanium. Galvanic coupling of these alloys does not increase corrosion rates.

Keywords: Nickel titanium wires; Corrosion; Fluoride

INTRODUCTION

Nickel titanium (NiTi) alloys have been widely used in biomedical and dental applications because of their special mechanical properties and good biocompatibility.1 Their mechanical properties—namely, shape memory effect and superelasticity—are based on a martensitic phase transformation, which allows the NiTi alloys to return to a previously defined shape when strained up to 8%.2 For orthodontic application, NiTi equiatomic alloys can provide lighter and more constant force to dental movement.3 Clinically, this controlled force results in longer dental appointment intervals. However, these good mechanical properties are not essentially accompanied by an improvement in corrosion resistance. The poor corrosion resistance might not only affect the treatment effectiveness but may also result in toxic and allergic reactions due to nickel release. In addition, the biocompatibility of NiTi is still under discussion.4 A protection layer is formed on NiTi and the titanium surface when it comes in contact with the aqueous environment. While on the titanium surface the film is constituted by titanium oxide (TiO2), on NiTi the passive film also has the presence of smaller amounts of nickel oxide or metallic Ni,5 which make it more susceptible to chemical attack. Nickel release may be a problem in a biomaterial, once Ni has been described as a toxic6 and allergic ion.7 A recent study8 found a significant increase in Ni content in saliva in patients using NiTi fixed orthodontic appliances. Several surface treatments have been developed in order to decrease nickel release and by favoring the formation of a stable TiO2 layer.9,10 The effect of the addition of a third element (such as copper, iron, or palladium) to the binary NiTi has also been recently researched. The ternary NiTi-based alloys presented a lower localized corrosion resistance than does the binary NiTi.5

The aim of this work is to understand NiTi corrosion behavior in 0.15 M sodium chloride (NaCl) with and without addition of fluoride. The oral cavity is a potential corrosive environment for orthodontic arch wires as a result of its Cl− ion concentration. Moreover, fluoride therapy is well known as an effective method to prevent dental caries.11 It has also been described12 that Ti, which has excellent corrosion resistance behavior, might suffer localized attack in fluoride environments. In order to investigate the corrosion behavior of NiTi alloy, a comparative study with beta titanium alloy was carried out, since beta titanium can be used as reference of good corrosion resistance.

MATERIALS AND METHODS

The materials used for the study were NiTi and beta titanium wires. The NiTi alloy used was a 0.02-inch superelastic wire with a chemical composition comprising 55.94 wt. % Ni, 227 ppm of carbon, 280 ppm of oxygen, and balanced Ti. The NiTi wires were ordered by Memory-Metalle GmbH (Weil am Rhein, BW, Germany; batch No. C7-8814-2-1A). The beta titanium commercial orthodontic wire had a 0.03-inch diameter and was produced by GAC International Inc (Bohemia, New York).

The following solutions were prepared as electrolytes: 0.15 M NaCl pure (Vetec), with additions of 0.02, 0.04, 0.05, 0.07, and 0.12 M sodium fluoride (NaF; Riedel-de Haen). Diluted HCl solution was used to adjust the pH of the solutions containing fluoride to 5.5 in order to obtain the same pH value of pure NaCl solution. The electrochemical experiments were performed in a three-electrode electrochemical cell with a saturated calomel electrode used as a reference electrode and platinum as a counter electrode (Figure 1). This cell was connected to a potentiostat (Metrohm Autolab). Corrosion resistance was studied using anodic potentiodynamic polarization at a potential scanning of 20 mV/min and electrochemical impedance spectroscopy measurement at open-circuit potential, with a sinusoidal signal of 10-mV amplitude and frequencies in the range of 10 kHz to 10 mHz. Prior to the electrochemical experiments the working electrodes were immersed in solution for 1 hour. The NiTi wires were observed using scanning electron microscopy (SEM) for surface analysis. To assess the galvanic corrosion behavior of NiTi in combination with beta titanium, the different wires were immersed in NaCl containing 0.02 M NaF solution and were connected by the built-in zero-resistance ammeter. The current flow resulting when these materials were galvanically coupled was recorded.

RESULTS

NiTi Corrosion Behavior Depending on Fluoride Concentration

Corrosion potential (Ecorr) measured at the end of the 1-hour immersion period is presented in Table 1 and shows that more negative Ecorr values are related to higher fluoride concentrations. This indicates a lower protection capacity of the oxide layer with increasing fluoride concentration. The difference in Ecorr obtained in the lowest F− concentration (0.02 M) and the highest F− concentration (0.12 M) was 113 mV. Figure 2 shows anodic polarization curves in different fluoride concentrations. In solution containing 0.02 and 0.04 M NaF the material remains passive until its breakdown potential, beyond which a breakdown of passivity layer occurs. From this potential, an average of 220 mV for 0.02 M concentration and 240 mV for 0.04 NaF concentrations, the current density increases, and localized corrosion of the material occurs. From 0.05 M fluoride ion concentration and above, the current densities are higher, which indicates active dissolution of the material. Comparing SEM images (Figure 3), the wire tested in 0.04 M NaF shows the presence of pits on the sample surface, while there is no significant surface alteration in 0.07 M NaF. Electrochemical impedance spectroscopy (EIS) measurements (Figure 4a,b) show capacitive arcs in all fluoride concentrations. The Nyquist diagrams reveal that the polarization resistance (Rp) decreases with an increase in fluoride concentration. This justifies the notion that the decrease in corrosion resistance is related to higher fluoride concentrations. The points at which the curves intersect the x-axis were deduced as Rp. The Rp values varied between 600 kΩcm2 in NaCl solution with 0.02 M NaF and 1 kΩcm2 in the electrolyte containing 0.12 M NaF. Figure 4c shows the changes in Rp with fluoride concentration.

Figure 1. . Experimental three-electrode set-up used for corrosion experiments. The arrows indicate the working electrode (WE), the reference electrode (RE), and the counter electrode (CE).Figure 1. . Experimental three-electrode set-up used for corrosion experiments. The arrows indicate the working electrode (WE), the reference electrode (RE), and the counter electrode (CE).Figure 1. . Experimental three-electrode set-up used for corrosion experiments. The arrows indicate the working electrode (WE), the reference electrode (RE), and the counter electrode (CE).
Figure 1.  Experimental three-electrode set-up used for corrosion experiments. The arrows indicate the working electrode (WE), the reference electrode (RE), and the counter electrode (CE).

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

Figure 2. . Anodic polarization measurements of NiTi in 0.15 M NaCl solutions with different fluoride concentrations.Figure 2. . Anodic polarization measurements of NiTi in 0.15 M NaCl solutions with different fluoride concentrations.Figure 2. . Anodic polarization measurements of NiTi in 0.15 M NaCl solutions with different fluoride concentrations.
Figure 2.  Anodic polarization measurements of NiTi in 0.15 M NaCl solutions with different fluoride concentrations.

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

Figure 3. . SEM images at 100× magnification from (a) NiTi wire tested in NaCl solution with 0.04 M NaF and (b) NiTi wire tested in NaCl solution with 0.07 M NaF.Figure 3. . SEM images at 100× magnification from (a) NiTi wire tested in NaCl solution with 0.04 M NaF and (b) NiTi wire tested in NaCl solution with 0.07 M NaF.Figure 3. . SEM images at 100× magnification from (a) NiTi wire tested in NaCl solution with 0.04 M NaF and (b) NiTi wire tested in NaCl solution with 0.07 M NaF.
Figure 3.  SEM images at 100× magnification from (a) NiTi wire tested in NaCl solution with 0.04 M NaF and (b) NiTi wire tested in NaCl solution with 0.07 M NaF.

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

Figure 4. . EIS measurements of NiTi in 0.15 M NaCl with different fluoride concentrations. (a) EIS in 0.12, 0.07, and 0.05 M NaF. (b) EIS in 0.05, 0.04, and 0.02 M NaF. (c) Rp variation with fluoride concentration.Figure 4. . EIS measurements of NiTi in 0.15 M NaCl with different fluoride concentrations. (a) EIS in 0.12, 0.07, and 0.05 M NaF. (b) EIS in 0.05, 0.04, and 0.02 M NaF. (c) Rp variation with fluoride concentration.Figure 4. . EIS measurements of NiTi in 0.15 M NaCl with different fluoride concentrations. (a) EIS in 0.12, 0.07, and 0.05 M NaF. (b) EIS in 0.05, 0.04, and 0.02 M NaF. (c) Rp variation with fluoride concentration.
Figure 4.  EIS measurements of NiTi in 0.15 M NaCl with different fluoride concentrations. (a) EIS in 0.12, 0.07, and 0.05 M NaF. (b) EIS in 0.05, 0.04, and 0.02 M NaF. (c) Rp variation with fluoride concentration.

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

Table 1.  Mean Corrosion Potential Values of Nickel Titanium (NiTi) Wires After 1-Hour Immersion Time in Different Sodium Fluoride (NaF) Concentrationsa
Table 1. 

Comparative Study Between NiTi and Beta Titanium Through Electrochemical Measurements

A comparative study with Ti, which is described as having good corrosion resistance, was carried out in NaCl solution with and without 0.02 M NaF. The mean corrosion potential after 1 hour is shown in Table 2. Beta titanium presented higher Ecorr than does NiTi. Both materials presented more negative Ecorr when fluoride was in the solution, thus indicating a decrease in corrosion resistance of NiTi and beta titanium when in the presence of fluoride. The EIS graphic confirms lower impedance of the passivation film when fluoride is present, since there is a decrease in polarization resistance in this condition (Figure 5). The polarization curve is shown in Figure 6. Beta titanium remains passive in both electrolytes, with low current densities. However, current density increases when the alloy is tested in an environment containing fluoride, indicating a decrease in corrosion resistance in this condition. Passive current density ranged from 0.02 µA/cm2 in NaCl 0.15 M to 0.07 µA/cm2 in NaCl with 0.02 M NaF. NiTi, on the other hand, presented different corrosion behavior in solution with and without fluoride. In NaCl electrolyte, the material remained passive, with passive current density of 0.04 µA/cm2. When fluoride was added into the solution, the current density increased, characterizing localized corrosion, with a mean pitting potential (Epit) of 240 mV and a passivation current density of 0.2 µA/cm2.

Figure 5. . EIS measurements of NiTi and beta titanium.Figure 5. . EIS measurements of NiTi and beta titanium.Figure 5. . EIS measurements of NiTi and beta titanium.
Figure 5.  EIS measurements of NiTi and beta titanium.

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

Figure 6. . Anodic polarization curves of NiTi and beta titanium.Figure 6. . Anodic polarization curves of NiTi and beta titanium.Figure 6. . Anodic polarization curves of NiTi and beta titanium.
Figure 6.  Anodic polarization curves of NiTi and beta titanium.

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

Table 2.  Mean Corrosion Potential (Ecorr) Values of Nickel Titanium (NiTi) and Beta Titanium in Sodium Chloride (NaCl) Either Containing or Not Containing 0.02 M Sodium Fluoride (NaF), Ecorr/mV
Table 2. 

Galvanic Couple

Galvanic corrosion between NiTi and beta titanium was also evaluated because of the electrode potential gap measured between NiTi and beta titanium alloys in NaCl with 0.02 M fluoride solution. In vivo, galvanic corrosion may occur when NiTi wires and titanium brackets are in contact. The study was conducted through current measurement carried out by connecting two electrodes of the materials after a 1-hour immersion period in the solution (Figure 7). After an initial current peak, the current remained stable and the values were around zero.

Figure 7. . Current evaluation with time of NiTi and beta titanium galvanic couple in NaCl containing 0.02 M NaF.Figure 7. . Current evaluation with time of NiTi and beta titanium galvanic couple in NaCl containing 0.02 M NaF.Figure 7. . Current evaluation with time of NiTi and beta titanium galvanic couple in NaCl containing 0.02 M NaF.
Figure 7.  Current evaluation with time of NiTi and beta titanium galvanic couple in NaCl containing 0.02 M NaF.

Citation: The Angle Orthodontist 83, 5; 10.2319/091712-740.1

DISCUSSION

The anodic polarization test with different fluoride concentration showed that for the same pH, there is a fluoride concentration at which the corrosion resistance behavior of the NiTi alloy changes. In solutions containing 0.02 and 0.04 M NaF, an oxide layer is formed, but there is a breakdown in the passivity layer, characterized by localized corrosion. From 0.05 to 0.12 M NaF there is no formation of a passive layer, and the NiTi alloy suffered active dissolution, since current densities start at high values from the open circuit potential, as shown in Figure 2. Localized corrosion can be more prejudicial than generalized corrosion, since the pits can act like preferential locations of crack initiation (Figure 3). Cracks on the wire surface could reduce the force that should be employed in dental movement and prolong orthodontic treatment. Walker et al.13 observed a decrease in unloading mechanical properties of NiTi exposed to topic fluoride.

Previous investigations have shown that fluoride ion can reduce the corrosion resistance of NiTi alloy. There are reports of general corrosion14 and localized corrosion.6 In this work, the results show that there is a critical fluoride concentration at which NiTi can be damaged by localized corrosion. In solutions with 0.02 and 0.04 M of fluoride, NiTi suffered localized corrosion and, from 0.5 M NaF and above, generalized corrosion. EIS measurements show that lower Rp values are related to higher fluoride concentration (Figure 4). The lower Rp from samples tested in 0.04 M NaF compared to 0.02 M indicates a less corrosion-resistant film when fluoride concentration is increased. From 0.05 to 0.12 M of fluoride the lower impedance can be associated with the absence of an oxide film, resulting in active dissolution. It is important to point out that beta titanium did not present localized corrosion susceptibility in the fluoride solution. The current densities were up to 10−6 A/cm2, as shown in Figure 6. However, beta titanium alloy has also presented a lower resistant passivation film in fluoride presence, as can be seen through lower values of impedance in fluoride solution (Figure 5).

The lower corrosion resistance presented by NiTi can likely be justified by the composition of the oxide layer formed on the NiTi surface. Despite the main formation of TiO2, previous reports5,15 indicate that the passive layer also consists of Ni oxides or metallic Ni. The presence of Ni ions and Ni oxides can explain why NiTi suffered localized corrosion in NaCl environments with fluoride, while beta titanium, which forms only TiO2 in its surface, did not exhibit localized corrosion. In our work, NiTi did not presented localized corrosion in chloride solutions, probably because of the good surface conditions of our samples, which influenced the stability of the oxide layer formed on the NiTi surface. Previous research16 has shown that surface treatments, such as polishing, reduce the surface roughness and decrease the corrosion rates of NiTi alloys. In our investigation, we could not find localized corrosion in chloride environments, as described in references.5,17 Another work18 suggests that the manufacturing process of the orthodontic arch wires might also have a strong influence on the corrosion resistance of the NiTi alloys.

The galvanic corrosion test indicates that the NiTi corrosion rate does not change when in contact with beta titanium in NaCl solution. When NiTi and beta titanium were coupled, current values remained low and near zero after a current peak. Similar results have been also reported19 in Hank's solution. Attenuation of the galvanic current occurred as a consequence of the high impedance of the electrodes, as indicated by the Nyquist diagrams presented in Figure 5.

CONCLUSIONS

  • NiTi corrosion resistance behavior is lower than that of beta titanium. In NaCl solution with 0.02 M NaF, NiTi wires are susceptible to localized corrosion, while beta titanium alloy does not show localized corrosion. Localized corrosion of NiTi wires could prolong orthodontic treatment.

  • The corrosion behavior of NiTi alloy depends on fluoride concentration. When 0.02 and 0.04 M of NaF were added to the NaCl solution, NiTi presented localized corrosion. When NaF concentration increased to 0.05, 0.07, and 0.12 M, the alloy presented general corrosion.

  • There is no galvanic corrosion between NiTi and beta titanium in 0.15 M NaCl and 0.02 M NaF electrolytes, since the current values remained around zero.

ACKNOWLEDGMENTS

The authors acknowledge funding by the Coordenação de Aperfeiçoamento de Pessoal de Nível superior (CAPES) and the Fundação Coordenação de Projetos, Pesquisas e Estudos Tecnológicos (Fundação COPPETEC). The authors are also grateful to Lakshman Neelakantan for useful comments.

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Copyright: The EH Angle Education and Research Foundation, Inc.
Figure 1. 
Figure 1. 

Experimental three-electrode set-up used for corrosion experiments. The arrows indicate the working electrode (WE), the reference electrode (RE), and the counter electrode (CE).

Figure 2. 
Figure 2. 

Anodic polarization measurements of NiTi in 0.15 M NaCl solutions with different fluoride concentrations.

Figure 3. 
Figure 3. 

SEM images at 100× magnification from (a) NiTi wire tested in NaCl solution with 0.04 M NaF and (b) NiTi wire tested in NaCl solution with 0.07 M NaF.

Figure 4. 
Figure 4. 

EIS measurements of NiTi in 0.15 M NaCl with different fluoride concentrations. (a) EIS in 0.12, 0.07, and 0.05 M NaF. (b) EIS in 0.05, 0.04, and 0.02 M NaF. (c) Rp variation with fluoride concentration.

Figure 5. 
Figure 5. 

EIS measurements of NiTi and beta titanium.

Figure 6. 
Figure 6. 

Anodic polarization curves of NiTi and beta titanium.

Figure 7. 
Figure 7. 

Current evaluation with time of NiTi and beta titanium galvanic couple in NaCl containing 0.02 M NaF.

Contributor Notes

Corresponding author: Elisa J. Kassab, Department of Metallurgy and Materials Science, Federal University of Rio de Janeiro, Cid. Universita´ria–Centro de Tecnologia-Bloco F, 210, Ilha do Funda˜ o–Postal Box 68505, Rio de Janeiro, RJ 21941- 972, Brazil (e-mail: elisajk@metalmat.ufrj.br)
Received: 01 Sept 2012
Accepted: 01 Jan 2013
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