Materials Science, Vol. 47, No. 6, May, 2012 (Ukrainian Original Vol. 47, No. 6, November December, 2011) INFLUENCE OF ARTIFICIAL SALIVA ON NiTi ORTHODONTIC WIRES: A STUDY ON THE SURFACE CHARACTERIZATION İ. Ünal, 1 S. Zor, 2 and H. Atapek 1 Nickel- and titanium-based alloys are promising materials for dental orthodontic wires due to their superior mechanical properties and corrosion resistance. The studies of the corrosion resistance of these materials according to their surface characterization in artificial salivas are limited. We study the changes of the surfaces of NiTi-alloy-based orthodontic wires in artificial salivas with (or without) additions of F 3 and PO 4 after a period of time by the SEM and EDS methods. Keywords: orthodontic materials, corrosion, scanning-electron microscopy, energy-dispersive spectroscopy. There are several metals and metal alloys used in dentistry and orthodontic applications. The traces of corrosion on the surfaces of metals used in any application can be formed after a period of time depending on the environment of mouth [1]. The corrosion process occurs as a result either of the loss of metal ions directly into the solution or of the progressive dissolution of the surface films, as a rule oxide or sulfide. The stainless-steel, cobalt-chromium, and titanium alloys used in orthodontic appliances rely on the formation of passive surface oxide films to resist corrosion. These protective layers are not reliable. They are susceptible to both mechanical and chemical degradation. Even without destruction, oxide films often slowly dissolve only to reform as the metal surface is exposed to oxygen from the air or the from surrounding medium [1]. The acidic drinks and foods containing sodium chloride are corrosive materials. The aggressive media, such as chloride ions and acidic conditions, accelerate the process of corrosion. Moreover, the fluoride ions in toothpaste and in the products used as mouthwash play the role of an important factor accelerating corrosion. Several studies reported that fluoride-based acidic solutions increase the corrosion of titanium [1 5]. Therefore, clinically, the role of fluoride in the corrosion of orthodontic appliances might be not as important as suggested by the in vitro studies. Schiff, et al. compared the corrosion resistances of three types of orthodontic brackets (stainless steel, cobalt-chromium, and titanium) placed in a reference solution of artificial saliva and in three commercially available fluoride mouthwashes. According to the electrochemical results, all three mouthwashes exhibited a weak effect on the cobalt-chromium brackets but the presence of stannous fluoride in one mouthwash caused considerable corrosion of the stainless-steel and titanium brackets [2]. Some nickel and titanium alloys were developed as orthodontic materials. The studies on the corrosion resistance of these materials in artificial salivas are very popular. In the present work, the changes of the surfaces of NiTi-alloy-based orthodontic wires in artificial salivas with (or without) additions of F and PO 4 3 after a period of time are investigated by the SEM (Scanning Electron Microscopy) and EDS (Energy-Dispersive Spectroscopy) methods. 1 Kocaeli University, Metallurgical & Materials Engineering, Umuttepe Campus, Turkey. 2 Kocaeli University, Department of Chemistry, Umuttepe Campus, Turkey; e-mail: szor2001@yahoo.com (corresponding author). Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 47, No. 6, pp. 101 106, November December, 2011. Original article submitted December 9, 2009. 830 1068-820X/12/4706 0830 2012 Springer Science+Business Media, Inc.
INFLUENCE OF ARTIFICIAL SALIVA ON NITI ORTHODONTIC WIRES: A STUDY ON THE SURFACE CHARACTERIZATION 831 Experimental Study Material and Method. A nickel-titanium (NiTi) alloy was used as orthodontic wire in our experimental investigations. Its composition was chosen among the materials commonly used and frequently mentioned in [6]. The chemical composition of the alloy studied in the present work is as follows: Ni/55Ti/45 NiTi alloy. The orthodontic wires were cut into pieces 1.5 cm in length. Then all wires were immersed in artificial salivas within the temperature range 22 37 C. All solutions listed in Table 1 were numbered as I, II, and III to simplify notation. The changes on the surfaces of the metal specimens were examined after 10 days by the SEM and EDS methods. Table 1. Compositions of the Artificial Salivas Used in the Experimental Studies Modified Fusuyama solution (I) (normal oral environment) KCl (0.4 g/liter) NaCl (0.4 g/liter) NaH 2 PO 4 2H 2 O (0.69 g/liter) CaCl 2 2H 2 O (0.906 g/liter) Na 2 S 9H 2 O (0.005 g/liter) Modified Fusuyama solution with an addition of 1 g/liter NaF (II) (toothpaste effective oral environment) KCl (0.4 g/liter) NaCl (0.4 g/liter) NaH 2 PO 4 2H 2 O (0.69 g/liter) CaCl 2 2H 2 O (0.906 g/liter) Na 2 S 9H 2 O (0.005 g/liter)+ NaF (1 g/liter) ph 4.8 Modified Fusuyama solution with an addition of 1.7% H 3 PO 4 (III) (acidic nutrient effective oral environment) KCl (0.4 g/liter) NaCl (0.4 g/liter) NaH 2 PO 4 2H 2 O (0.69 g/liter) CaCl 2 2H 2 O (0.906 g/liter) Na 2 S 9H 2 O (0.005 g/liter)+ 1.7% H 3 PO 4 ph 2.5 The NiTi alloy is commonly used as a promising material for dental orthodontic wires due to its superior mechanical properties and high corrosion resistance. It is possible that protective passive films exist on the NiTi alloy due to electrochemical reactions. The Ni or Ti ions may be released from the metal surface into the oral environment in the course of corrosion processes [7]. On the other hand, NiTi alloys contain certain amounts of dispersed Ni x Ti y -type intermetallic precipitations in the Ni-rich matrix depending on the chemical composition of the alloy and the applied heat treatment. The coherence between precipitations and the matrix is very important for the mechanical properties and corrosion resistance. Microcracks or microvoids occur due to the incoherent interface and, moreover, corrosion is aggressive around these precipitations with incoherent interface in the matrix [8]. Results and Discussions Effect of the Fusuyama Artificial Saliva and Surface Examinations. The artificial-saliva-based solutions have a corrosive effect due to the presence of chloride ions. If the environment contains certain amounts of chloride ions, then they lead to the formation of pitting corrosion. Pitting corrosion caused by the existence of chloride ions in the saliva solutions is well visible in the experimental studies after SEM examinations of the surface of NiTi alloy.
832 İ. ÜNAL, S. ZOR, AND H. ATAPEK Fig. 1. General view of the NiTi alloy immersed in the artificial saliva for 10 days (a), Ti-based oxide in the crystalline form (b). Fig. 2. General view of the surface of NiTi alloy in artificial saliva (a), the results of EDS analysis for the numbers marked on the surface (b, c). In Fig. 1, we show the surfaces of NiTi orthodontic wires immersed in Solution I for 10 days. There are many voids formed as a result of corrosion and many particles in dark-gray contrast. These are the corrosion products formed on the surface as several oxides (Fig. 1a). The formation of voids is inevitable due to the chemical interaction between the metal and the solution, which results in the dissolution of the matrix in the course of corrosion. In the artificial saliva, Ti ions are released from the metal surface due to the lower affinity of nickel to oxygen, and the Ti-based oxide may appear in the crystalline form as a result of corrosion (Fig. 1b). The oxide formations on the surface commonly appear in the crystalline form due to the nucleation [9]. The EDS studies prove to be very useful in determining the components formed on the surface of the metal as a result of corrosion in the artificial saliva after a period of time. A general view of the surface of NiTi alloy is presented in Fig. 2a and two points in the image are marked to show where the EDS analysis is performed. Thus, No. 1 corresponds to the matrix and No. 2 corresponds to the formed corrosion product.
INFLUENCE OF ARTIFICIAL SALIVA ON NITI ORTHODONTIC WIRES: A STUDY ON THE SURFACE CHARACTERIZATION 833 Fig. 3. EDS mapping of the selected region of NiTi alloy immersed in Solution I for 10 days. Nickel and titanium are the base metals in the matrix and the concentrations of these metals are very high and close to the original composition of the alloy mentioned above. The EDS results shown in Fig. 2b indicate that there is a clear decrease in the amounts of Ni and Ti due to the dissolution of the metals. From the clinical point of view, the final result of the orthodontic treatment may be compromised by corrosion, and the metal ions appearing as corrosion products (Ni 2+ ) may result in symptoms of toxicity and allergic reactions [10]. The concentration of oxygen in the region marked by No. 1 is very low and its existence is a proof of a thin oxide film on the metal surface. The oxide film behaves as a protective layer during corrosion in aggressive environments [11]. On the other hand, there are several impurities, such as C, Al, and Si due to the composition of the alloy and Na, P, K, and Ca existing in the artificial saliva. Under the SEM examination, the formed oxide-based particles appear in the dark-gray contrast (Fig. 2c). It is clearly seen that the concentration of oxygen is higher than in the matrix. This means that the Ti-based oxide is formed as a result of strong interaction between the metal and the solution. The dissolution of nickel from the matrix into the solution is very high. All effects can be traced by considering the concentration differences in the EDS analyses presented in Figs. 2b and c. The EDS mapping proves to be very useful in understanding the elemental distribution for given materials. In Fig. 3, we present the EDS mapping of the studied surface of NiTi alloy. Nickel and titanium are the predominant elements on the surface and the oxide-based particles can be easily separated from the matrix due to their oxygen level. Effect of the F -Added Fusuyama Solution. For the purposes of tooth protection, fluorides are extensively used to provide oral health by means of toothpastes, mouth rinses, orthodontic gels, and other therapeutic dental products. Additionally, systemic fluorides may be ingested orally through tea, dietary supplements, and fluoridated bottled water. Therefore, the NiTi orthodontic wires are readily exposed to fluoride media [12]. Fluoride ions have an abrasive effect. The corrosion resistance of NiTi and β -Ti alloys decreases due to the increase in the hydrogen embrittlement in the environments containing fluorine ions. Walker, et al. reported that fluoride ions in tooth gels increase the corrosion behavior of orthodontic wires [13].
834 İ. ÜNAL, S. ZOR, AND H. ATAPEK Fig. 4. General view of the surface of NiTi alloy immersed in Solution II for 10 days. Fig. 5. Regions where the EDS analyses were performed on the surface of NiTi alloy immersed in the F -added artificial saliva (a); the elemental peaks of the matrix with No. 3 in the microstructure (b); the results of the EDS analysis for the Nos. 4 and 5 on the surface (c and d, respectively). It is clearly observed that the degradation of the surface of NiTi alloy is more pronounced than the degradation of the metal affected by the artificial saliva (Fig. 4). This is a consequence of the effect of fluoride on the surface. Several EDS analyses marked by Nos. 3 5 were performed on the metal surface to understand the effect of fluoride and also of the formed corrosion products (Fig. 5a). First, the matrix is characterized and the EDS results reveal a similar elemental distribution on the surface as compared with the other analyses made earlier (Fig. 5b). The region with No. 3 corresponds to the matrix, and the EDS results show that the amounts of Ni and Ti are very high, while the concentration of oxygen is very low. Therefore, nickel and titanium exist in the marked region as the fine oxide phase. These oxide phases occur due to the interaction between the metal and water molecules. The particles in dark/gray contrasts are obtained in the course of the SEM examinations.
INFLUENCE OF ARTIFICIAL SALIVA ON NITI ORTHODONTIC WIRES: A STUDY ON THE SURFACE CHARACTERIZATION 835 Fig. 6. SEM image showing the effect of PO 3 4 -added artificial saliva on the NiTi wire. Fig. 7. Regions where the EDS analyses were performed on the surface of NiTi alloy immersed in the PO 3 4 -added artificial saliva (a); the elemental peaks of the matrix with No. 6 in the microstructure (b); the results of the EDS analysis for Nos. 7 and 8 on the surface (c and d, respectively). Moreover, Figs. 5c d show that the particles formed on the surface of the metal are corrosion products characterized by high levels of the concentration of oxygen. The formation of the intense oxide layer can be mentioned for the particle marked by No. 4. The EDS results (Fig. 5c) show that the particle contains both impurities existing in the composition of the metal and also elements existing in the composition of the artificial saliva. In Fig. 5d, according to the EDS analysis, there is a peak referred to a strong Ti-based oxide particle. Titanium exists in the solution as TiO 2 having a more stable form. 3 Effect of the PO 4 -Added Fusuyama Solution. The phosphate-added Fusuyama solution has a more acidic characteristic as compared with the other artificial salivas. Huang, et al. concluded that the manufacturer, the ph value, and also the immersion period have a significant statistical influence on the amount of released Ni
836 İ. ÜNAL, S. ZOR, AND H. ATAPEK and Ti ions. In their experimental study, the NiTi orthodontic wires were examined in artificial salivas with various acidities. The amounts of Ti ions released for certain ph values were almost undetectable. It was indicated that the TiO 2 films on NiTi wires exhibit good protective properties against corrosion. The NiTi wire with the highest release of metal ions exhibited the maximum increase in the surface roughness after the immersion test, while a rougher surface did not correspond to a higher release of metal ions [14]. The effect of Solution III on the NiTi wire is illustrated by the SEM image in Fig. 6. As seen, there are much more dimples and particles as corrosion products on the surface of the wire made of experimental alloy. The EDS analysis is necessary to understand the effect of the solution on the metal surface similar to the studies performed earlier in the present work (Fig. 7). In Fig. 7a, the points where the analysis was performed are marked by Nos. 6 8. The peaks belonging to the elemental distribution on the matrix surface are shown in Fig. 7b and it is obvious that there is an oxide film formed on the surface. On the other hand, the results obtained for particles on the surface are presented in Figs. 7c d, where the corrosion products formed due to high levels of oxygen can be clearly observed. There is a certain amount of phosphorus indicating the environment. CONCLUSIONS In the present work, we investigate the influence of artificial-saliva solutions on the NiTi orthodontic wires. Several artificial salivas were prepared to determine the effect of different environments depending on the composition of saliva solutions. The conclusions are as follows: the presence of corrosion is determined by examining the surfaces immersed in all artificial-saliva solutions (Solutions I II III) for a period of 10 days. The SEM examinations reveal the matrix with a fine oxide-film layer and particles formed as corrosion products on the surface. The EDS analysis is useful to support the formation of thin oxide films on the surface and to determine the type of particles. The elemental distribution differs depending on the dissimilarities in the corrosive environments. Pitting corrosion due to the presence of chloride ions is observed on the metal surfaces. The fluorideand phosphate-added Fusuyama solutions are more effective in the formation of corrosion products than the Fusuyama solution without additives. REFERENCES 1. K. House, F. Sernetz, D. Dymock, et al., Corrosion of orthodontic appliances Should we care?, Amer. J. Orthodontics Dentofacial Orthopedics, 133, No. 4, 584 592 (2008). 2. N. Schiff, F. Dalard, M. Lissac, et al., Corrosion resistance on three orthodontic brackets: a comparative study of three fluoride mouthwashes, Europ. J. Orthod., 27, 541 549 (2005). 3. F. Toumelin Chemla, F. Rouelle, and G. Burdairon, Corrosive properties of fluoride-containing odontologic gels against titanium, J. Dent., 24, 109 115 (1996). 4. T. Büyükyılmaz, V. Tangursan, B. Ogoard, et al., The effect of titanium tetrafluoride (TiF 4 ) application around orthodontic brackets, Amer. J. Orthodontics Dentofacial Orthopedics, 105, 293 296 (1994). 5. Q. Y. Wang and Y. F. Zheng, The electrochemical behavior and surface analysis of Ti 50 Ni 47.2 Co 2.8 alloy for orthodontic use, Dental Mater., 24, 1207 1211 (2008). 6. N. Schiff, B. Grosgogeat, M. Lissac, and F. Dalard, Titanium alloys orthodontics wires: electrochemical study in fluoride dental rinses, Europ. Cells Mater., 9, No. 1, 45 47 (2005). 7. H. Huang, Variation in corrosion resistance of nickel-titanium wires from different manufacturers, Angle Orthodontist, 75, No. 4, 661 665 (2005). 8. D. Holec, O. Bojda, and A. Dlouhý, Ni 4 Ti 3 precipitate structures in Ni-rich NiTi shape memory alloys, Mater. Sci. Eng., A, 481 482, 462 465 (2008). 9. Y. Oshida, R. C. L. Sachdeva, and S. Miyazaki, Microanalytical characterization and surface modification of TiNi orthodontic archwires, Biomed. Mater. Eng., 2, 51 69 (1992).
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