| Abstract|| |
Context: Teeth and restorations are subjected continuously to degradation in hostile physical and chemical environments, resulting in corrosion. Component of liquid or solid is an important factor influencing the corrosion of metallic appliances placed in the oral cavity.
Aims: To study in vitro corrosion of titanium wires immersed in solutions of toothpaste and chocolate in artificial saliva.
Materials and Methods: Immersion test and electrochemical studies using different parameters, including corrosion potential (E corr ), cathodic Tafel constant (βc), anodic Tafel constant (βa), corrosion current (I corr ), polarization resistance (R p ), and corrosion rate of titanium wires, were done in solutions of artificial saliva containing Colgate™ toothpaste and Amul™ chocolate. Photomicrographs were also taken.
Results: The results showed degradation of titanium wires by electrochemical attack when they were placed in the hostile electrolytic environments provided in the experiments. Surface analysis of titanium wires showed pitting and localized attacks on the surface. Pitting corrosion was found in the titanium wires.
Keywords: Chocolate, corrosion, dental alloy, orthodontic wires, pitting corrosion, titanium alloy, toothpaste
|How to cite this article:|
Chaturvedi TP, Dubey RS. Corrosion behavior of titanium wires: An in vitro study. Indian J Dent Res 2012;23:479-83
Artificial and natural teeth, metallic dental implants, and restorative materials within the mouth interact continually with physiological fluids and the constituents of drinks and food. They function in one of the most inhospitable environments in the human body. Over the course of a 24-hour day, restorations and teeth are exposed to a variety of eatables, from tea or coffee, causing increase in temperature, to ice or cold drinks, causing decrease in temperature; from fruit juices, causing reduction of pH, to milk or other products, causing increase in pH in the oral cavity. Therefore, dental materials need to be selected very carefully. Before selecting a material for dental application, it is necessary to remember that the choice of the material depends on a number of factors, including:
Thus, for the application of presently available materials and new dental materials it is essential to have comprehensive knowledge of these attributes.
- Corrosion behavior
- Mechanical properties
- Esthetic appearance
Different types of corrosion that occur in dental materials have been described in dental literature. , Corrosion shortens the life expectancy of some restorations/appliances, necessitating their early replacement, or even failure of dental treatment. Many corrosive reactions occur inside the mouth and cause deterioration of metallic dental appliances. Microimplants, prosthetic implants, and orthodontic wires used in dental practice are made up of titanium alloy. Titanium is attractive in dentistry due to its low weight-to-volume ratio, high strength-to-weight ratio, high fatigue resistance, and high corrosion resistance. The formation of an oxide film on titanium provides corrosion resistance under static conditions, but the oxide film is not sufficiently stable to prevent galling and seizing under loading conditions. The galvanic corrosion of titanium in contact with amalgam and cast prosthodontic alloys has been studied. 
Diet is an important factor influencing the corrosion of metallic appliances placed in the oral cavity.  Toothpaste is usually used for cleaning of teeth, and one of the components of food of children is chocolate. Therefore, the objective of present paper was to study the in vitro corrosion of titanium wires/alloy in solutions of toothpaste and chocolate in artificial saliva using electrochemical techniques and surface analysis tests.
| Materials and Methods|| |
The composition of the titanium alloy used in this study is shown [Table 1]. Chemicals used for preparation of artificial saliva  were potassium dihydrogen orthophosphate (K 2 HPO 4 ), sodium hydrogen orthophosphate (NaH 2 PO 4 .2H 2 O), potassium hydrogen carbonate (KHCO 3 ), sodium chloride (NaCl), magnesium chloride hexahydrate (MgCl 2 .6H 2 O), citric acid (3-carboxy-3-hydroxypentanedioic acid, C 6 H 8 O 7 ), and calcium chloride (CaCl 2 ); these chemicals were obtained from Merck India Ltd. and Qualigens India Ltd. Colgate™ Ice Blue Gel toothpaste (a nonfluoridated toothpaste containing silica and sorbital) and Amul™ chocolate [composition: Total fat (29 g) and total carbohydrates (62 g)] were employed at different concentrations.
Apparatus for electrochemical and surface morphology studies
For titanium alloy, CMS-100 (Gamry, USA) corrosion systems were used. The surface morphology of the material before and after corrosion was examined under a Quanta 200F field-emission gun scanning electron microscope.
Methods of corrosion testing
Total-immersion test (classical weight-loss method)
Artificial saliva was used for the immersion test. Fifty-mm-long sample specimens were cut from the titanium alloy. The specimens were rinsed with alcohol before the immersion test and weighed on an electronic balance before and after the test. The specimen was immersed in a conical flask (250 ml) containing 100 ml of the test solution along with 20 mg of Amul™ chocolate and 5 mg Colgate™ toothpaste mixed ultrasonically. The specimen was cleaned and reweighed after 30 days of immersion. The surface of the wire was examined using a scanning electron microscope (SEM).
Electrical measurements in the laboratory are used to assess the corrosion behavior of metals and alloys in service, so as to avoid the more tedious and prolonged immersion testing. The susceptibility of a metal to bimetallic corrosion, pitting, intergranular corrosion, stress corrosion cracking, etc., are examples of corrosion phenomena that are studied in the laboratory by means of electrochemical methods. For electrochemical polarization studies, samples of the wire were cut into 3-cm-long pieces. One cm of wire was uncovered and rest of the specimen was painted with lacquer. The uncovered end was attached with the corrosion system (CMS-100; Gamry, USA). The electrochemical cell containing 200 ml of the test medium at different concentration of toothpaste (2, 4, 6, 8, 10, 12 mg) and chocolate (5, 10, 15, 20, 25, 30 mg), as well as control (without toothpaste and chocolate), were kept on the stand and the three electrodes were dipped into the test medium and positioned properly. The temperature of the medium was maintained at room temperature.
| Results and Discussion|| |
After 30 days exposure, specimens were removed from the test solution, cleaned, and weighed. The weight losses of the specimens are shown in [Table 2]. It can be seen that weight loss increases with increase in the concentration of toothpaste and increase in exposure period, while weight loss was lower than control in the presence of chocolate.
|Table 2: Weight loss of samples in test solution containing 5 mg Colgate™ toothpaste and 20 mg Amul™ chocolate after various exposure periods|
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In the weight-loss method, corrosion is allowed to occur by the natural process. It is a slow method of determining the destruction of materials in solution and, sometimes, passive corrosion is common in the weight-loss method. In [Table 2], weight loss of wire in chocolate solution is less than control because passive corrosion is more in chocolate solution. This passivity is caused by the formation a thin, protective, hydrated oxide, corrosion-product surface film that acts as the barrier to the anodic dissolution reaction. In certain areas the passive film is thin and fragile and breakdown at these points can result in unpredictable localized severe pitting corrosion. On the surface of the specimen, fouling was observed. [Figure 1], [Figure 2] and [Figure 3] show the SEM photomicrographs of titanium alloy after immersion tests in artificial saliva containing various concentrations of chocolate and toothpaste. After a 30-day immersion period, significant differences in surface morphology were observed.
|Figure 2: SEM photomicrograph of titanium alloy exposed to Colgate™ toothpaste|
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[Figure 4] and [Figure 5] show the variation of initial corrosion potential with various concentrations of toothpaste and chocolate. It was observed that the initial corrosion potential increased with increasing concentrations of toothpaste and chocolate in the test solution. The various electrochemical parameters obtained from polarization curves are listed in [Table 3] and [Table 4]; these include corrosion potential (E corr ), cathodic Tafel constant (βc), anodic Tafel constant (βa), corrosion current (I corr ), polarization resistance (R p ), and corrosion rate (expressed in cm/year).
|Figure 4: Potentiodynamic polarization of titanium-based alloy exposed to the test solution containing various concentrations of Colgate™ gel [12 mg (1), 10 mg (2), 8 mg (3), 6 mg (4), 4 mg (5), 2 mg (6)] and control (7)|
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|Figure 5: Potentiodynamic polarization of titanium alloy exposed to test solutions containing various concentrations of chocolate [30 mg (1), 25 mg (2), 20 mg (3), 15 mg (4), 10 mg (5), and 5 mg (6)] and control (7)|
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|Table 3: Electrochemical parameters for the corrosion of titanium-based alloy in test solution containing various concentrations of Colgate™ toothpaste at 25°C|
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|Table 4: Electrochemical parameters for the corrosion of titanium-based alloy in test solution containing various concentrations of Amul™ chocolate at 25°C|
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The overlay plots of the potentiodynamic polarization curves of titanium alloy in toothpaste and chocolate are shown in [Figure 4] and [Figure 5]. Increasing the concentrations of toothpaste and chocolate increases the concentrations of current density, which ultimate causes more corrosive effects on wires. The results from [Table 3] and [Table 4] show that E corr , I corr , and corrosion rate increase with increasing concentrations of toothpaste and chocolate. All corrosion reactions involved here are anodic aqueous reactions. The surface morphology of titanium shows pitting on the surface of the wires and evidence of localized attack [Figure 6] and [Figure 7]. Chocolate produced more pits than toothpaste. The inference drawn on the basis of polarization curves and corresponding values of corrosion rate, E corr, and I corr also support these observations.
|Figure 6: SEM photomicrograph of titanium alloy exposed to Colgate™ toothpaste|
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|Figure 7: SEM photomicrograph of titanium alloy exposed to Amul™ chocolate|
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Pitting is a form of localized corrosion in which pits form on the metal surface. It usually occurs on base metals, which are protected by a naturally forming thin film of an oxide. In the presence of chlorides in the environment, the film breaks down locally and rapid dissolution of the underlying metal occurs in the forming pits. Pitting is a form of extreme attack that results in holes on the metal surface.  Surfaces of stainless steel, nickel-titanium (NiTi), and implant may exhibit crevices and pores, which may be sites of attack since they represent sites susceptible to corrosion. Liu et al. studied the mechanical characteristics and corrosion behavior of titanium-aluminum nitride (TiAlN) coating on dental alloys, especially in nickel-based and chromium-based dental materials. They tested corrosion behaviors in 0.9% NaCl solution and observed a higher positive corrosion potential and a lower corrosion current density in alloy coated with TiAlN compared to the uncoated dental alloy. In addition, the pitting corrosion was substantially reduced by the employment of TiAlN coating. Stress-corrosion cracking, torsional smooth fatigue, and notched corrosion fatigue are properties of titanium materials. Es-Souni et al. found that Cr-Co alloy (Elgiloy) showed high pitting corrosion compared with NiTi alloy, with lower repassivation potentials and increase in current density once pitting had occurred. Kim et al. showed that NiTi and stainless steel wires were susceptible to pitting and localized corrosion. Diet chocolate (critical pH=5.7) was found to have no acidogenic effect on dental plaque.  Amul™ chocolate, which was used in this study, is milk chocolate and has a pH of 6.74. 
Many studies , have shown that components of diet and fluoride ions can destroy the protectiveness of the surface TiO 2 passive film on titanium or titanium alloy, leading to attacked corrosion morphology, decrease in polarization resistance, and increase in anodic current density or metal ion release. The electrochemical results are directly correlated to the coating of titanium alloy and concentration of materials. Fouling of dissolved materials on the dental metallic restorations may invite problems such as microbial growth, tarnish, etc. Ennoblement of the potential increases the propagation rate of pitting attaches and make susceptible passive alloys.
| Conclusion|| |
Weight loss of test samples in toothpaste solution increased with increase in concentration of test solution and exposure period. Corrosion, the graded degradation of materials by electrochemical attack, is of particular concern when metallic materials are placed in the hostile electrolytic environment provided by the human mouth. Photomicrographs of the specimen show formation of various shapes of pits covered with the remnants of passive films. The formation of oxide film on titanium provides corrosion resistance under static conditions, but the oxide film is not sufficiently stable in corrosive environments under loading conditions.
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Thakur Prasad Chaturvedi
Department of Orthodontics, IMS, BHU, Varanasi, UP
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4]