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Electrochemical behavior of Ti–Cr alloys in artificial saliva
Journal of Alloys and Compounds 487 (2009) 439–444
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrochemical behavior of Ti–Cr alloys in artificial saliva Hsueh-Chuan Hsu a,b , Shih-Ching Wu a,b , Cheng-Feng Wang b , Wen-Fu Ho c,∗ a
Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taiwan, ROC Institute of Biomedical Engineering and Material Science, Central Taiwan University of Science and Technology, Taiwan, ROC c Department of Materials Science and Engineering, Da-Yeh University, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 19 September 2008 Received in revised form 27 July 2009 Accepted 29 July 2009 Available online 5 August 2009 Keywords: Metals and alloy Corrosion Artificial saliva
a b s t r a c t In this study, the corrosion behavior of commercially pure titanium (c.p. Ti), Ti–6Al–4V and five new experimental Ti–Cr alloys was evaluated through open-circuit potential (OCP) and potentiodynamic polarization measurement in an artificial saliva containing fluoride. Electron spectroscopy for chemical analysis (ESCA) was used to characterize the composition of the passive films on the alloy after potentiodynamic polarization measurement. It was found that in standard artificial saliva the OCP increases with higher Cr content in Ti–Cr alloys. In 0.5% NaF artificial saliva, the OCP decreases with decreasing Cr in Ti–Cr alloys, and all but Ti–5Cr remain consistently higher than those of c.p Ti and Ti–6Al–4V. Linear polarization results show that artificial saliva and artificial saliva containing 0.5% NaF result in different corrosion behavior in Ti–Cr alloys, c.p.Ti and Ti–6Al–4V. The Ti–Cr alloys had greater resistance to corrosion in the fluoride-containing artificial saliva than c.p. Ti and Ti–6Al–4V, respectively. ESCA results verify that after potentiodynamic polarization a passive film consisting of TiO2 and Cr2 O3 forms on the surface of Ti–Cr alloys. These experimental results show that the electrochemical corrosion behavior of Ti–Cr alloys in artificial saliva containing 0.5% NaF can be improved by increasing Cr content. This further indicates that Ti–Cr alloys could successfully be used for crown, bridge, and metal-ceramic restorations. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Commercially pure titanium (c.p. Ti) and its alloys have been widely used for dental restorative applications such as crown/bridge, framework and dentures [1]. The advantages of titanium alloys in these applications are due to their excellent mechanical properties, good corrosion resistance in biological fluids, and biocompatibility [2]. For a metal to be used in an oral environment, it should be biocompatible and have high corrosion resistance. Among the various titanium alloys, the Ti–6Al–4V alloy is the most frequently used in medical implants. It also shows better physical and mechanical properties in comparison to c.p. Ti. However, this alloy might cause some long-term health problems because of the release of Al and V ions [3,4]. Previous studies also showed that the surface of the Ti–6Al–4V alloy was roughened by corrosion in the acidic fluoride-containing saliva [5]. In the present study the corrosion resistance of Ti–6Al–4V alloy will be further investigated. Cr is suitable for alloying with titanium (Ti) for several reasons. For example, Cr is known to control the anodic activity of the alloy and increase the tendency of Ti to passivate [6]. In fact, it has already
been used for many years as a major constituent in dental casting alloys [7]. An additional advantage of alloying Cr to Ti is that the liquidus temperature is gradually reduced from the high melting point of pure Ti (1670 ◦ C) until reaching a minimum of 1410 ◦ C at Cr content of 46% [8]. Thus, a number of Ti–Cr alloys have previously been developed for dental applications [9,10]. Among these, -titanium alloys are the most versatile, and a large number of studies have been performed on them [11]. Ho et al. [9] have studied the structure and properties of a series of binary Ti–Cr alloys with Cr content up to 30 wt%. While the Ti–Cr alloys have been shown to have better mechanical properties than c.p. Ti [9], their corrosion resistance behavior remains unknown. Therefore, in this study the electrochemical behavior of these alloys was specifically investigated. Since Ti alloys are used in the dental field, and the oral environment is exposed to fluoride, the degree of corrosion resistance offered by the Ti alloys in a fluoride-containing medium becomes an important property for their use in dentistry. In addition, some reports have shown the negative influence of fluoride on the corrosion resistance of titanium [5,12]. Therefore, in this research, corrosion behavior of all alloys was studied in simulated oral environments with and without fluoride. 2. Experimental method
∗ Corresponding author at: Department of Materials Science and Engineering, Da-Yeh University, 168 University Road, Dacun, Changhua 51591, Taiwan, ROC. Tel.: +886 4 851 1888x4108 fax: +886 4 851 1224. E-mail addresses: [email protected], [email protected] (W.-F. Ho). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.172
2.1. Specimen preparation Commercially pure titanium (Kobe Steel Co. Ltd., Kobe, Japan), the Ti–6Al–4V (Daido Steel Co. Ltd., Nagoya, Japan) alloy, and the experimental Ti–(5–30) wt% Cr
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Fig. 1. OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in standard artificial saliva.
Fig. 2. OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in saliva solution with 0.5 wt% NaF.
alloys were examined in this study. The Ti–Cr alloys were made from c.p. Ti and Cr (99.95%) using an argon-arc melting furnace (1000 Miller Dimension, USA). For the electrochemical measurement, specimens were cut into 1.0 mm in thickness from a rod. The specimens were grinded with silicon carbide paper until #1500 and then polished with 0.3 m Al2 O3 suspension. After polishing, the samples were cleaned by means of ultrasonic sound in ethanol. 2.2. Electrochemical tests In this study, two artificial saliva solutions were used. The first was a standard artificial saliva (NaCl 400 mg/l, KCl 400 mg/l, CaCl2 ·2H2 O 795 mg/l, NaHPO4 ·H2 O 690 mg/l, KSCN 300 mg/l, Na2 S·9H2 O 5 mg/l, urea 1000 mg/l). The second also contained 0.5 wt% NaF (corresponding to 2500 ppm F). The pH level of both test solutions was adjusted to 4.0 by adding lactic acid in a simulated oral environment [12]. A three-electrode system was used for all the electrochemical measurements. The apparatus for electrochemical measurement consisted of a potentiostat (Versa Stat TM II Potentiostat/Golvanostat Model 263A; Princeton Applied Research, NJ, USA), controlled by a personal computer with dedicated software (PowerSuit-2.56, Princeton Applied Research), a saturated calomel electrode (SCE) as reference electrode, a platinum plate as counter electrode, and the specimen as working electrode. For each test, the open-circuit potential was measured for 2 h. The potentiodynamic polarization of the specimens was recorded in a scanning range from −1.2 to +2.5 V (v.s. SCE) at a scanning rate of 0.1 mV/s. In every test, the medium was maintained at 37 ◦ C. Five specimens were tested for each condition. 2.3. Surface characterization
Fig. 3. Linear polarization diagrams of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the standard artificial saliva.
After electrochemical measurement, surface morphology of specimens was examined using by a scanning electron microscope (S-3000N, HITACHI, Japan) and the surface composition was examined with ESCA 750 (Shimadzu, Japan).
3. Results and discussion 3.1. Electrochemical measurement Fig. 1 shows the change in OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in standard artificial saliva. For all specimens, the potential changed in the negative direction over time until a steady-state potential was reached, and gradually decreased. For c.p. Ti, the initial potential was around −256 mV (SCE), but it gradually decreased after 3500 s, and a value of −586 mV (SCE) was reached. The potential then remained almost constant and after 7200 s its value was −580 mV (SCE). The initial potential for Ti–6Al–4V was approximately −265 mV (SCE), which decreased gradually and after 5000 s remained stable at approximately −496 mV (SCE). After 7200 s the potential for this alloy was −506 mV (SCE). The variation of potential over time for the other four Ti–Cr alloys was similar to those of c.p. Ti and Ti–6Al–4V. The steady potentials of Ti–Cr alloys ranged from −418 to −580 mV (SCE). The corrosion potential of the Ti–Cr alloys increased with
Fig. 4. Linear polarization diagrams for c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the artificial saliva containing 0.5 wt% NaF.
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increasing Cr content, which indicates the noble behavior of these alloys. Fig. 2 shows the change in OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in saliva solution with 0.5 wt% NaF. At all concentrations the potential changed in the positive direction over time until a steady-state potential is reached. For all specimens, the initial potential was around −480 mV (SCE). For Ti–10Cr, Ti–20Cr and Ti–30Cr, it decreased gradually before finally remaining stable around −720 mV (SCE). However, for Ti–6Al–4V, c.p. Ti and Ti–5Cr, initial potentials decreased gradually and when they reached 3000 s, 5500 s and 6500 s, respectively they suddenly dropped to approximately −1250 mV (SCE). After 7200 s the potential for all three alloys was −506 mV (SCE). These results indicate that Ti–10Cr, Ti–20Cr and Ti–30Cr have superior corrosion resistance in artificial saliva containing 0.5 wt% NaF. Fig. 3 shows the linear polarization diagrams of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the standard artificial saliva. All the specimens had distinct, long passive regions. The average corrosion potentials estimated from the linear polarization curves were
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−1109 mV (SCE), −1116 mV (SCE), −1043 mV (SCE), −1127 mV (SCE), −1078 mV (SCE) and −1071 mV (SCE) for c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. These values were significantly lower than those obtained from the opencircuit potential measurements. This is to be expected, as the polarization tests were started at a cathodic potential relatively close to the corrosion potential, so that the passive film at the surface was at least partially removed. An analysis of the results of all electrochemical tests conducted in the standard artificial saliva indicates that all Ti alloys in this study behaved in a very similar way. The differences between the alloys were not found to be significant, which can be ascribed to the fact that the passive film formed on all these alloys is essentially the same. For all alloys, low current densities (passive current density at potential 0.5 V; I0.5 ) (10−6 A/cm2 ) were obtained previous to the establishment of a typical passive behavior at 500 mV (SCE). In fact, an increase in current density with increasing potential can occur if the increase in potential is not accompanied by a corresponding thickening of the oxide film. Nevertheless, it appears that the film
Fig. 5. SEM micrographs of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF; (a)c.p. Ti, (b) Ti–6Al–4 V, (c) Ti–5Cr, (d) Ti–10Cr, (e)Ti–20Cr, and (f) Ti–30Cr.
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Fig. 6. ESCA spectra of specimens after electrochemical measurement in standard artificial saliva. (a) Cr 2p; (b) Ti 2p; (c) O 1s.
formed above 500 mV (SCE) is different from that formed below this potential. Its higher current density value indicates that it is weaker oxide film which cannot provide effective corrosion resistance. Fig. 4 shows the linear polarization diagrams for c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the artificial saliva containing 0.5 wt% NaF. The average corrosion potentials estimated from these curves were −1224 mV (SCE), −1175 mV (SCE), −1156 mV (SCE), −1094 mV (SCE), −1033 mV (SCE) and −1028 mV (SCE) for c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. All the Ti–Cr alloys except Ti–5Cr had higher Ecorr values than c.p. Ti and Ti–6Al–4V. The average corrosion current density estimated from these curves were 62.51 A/cm2 (SCE), 10.68 A/cm2 mV (SCE), 146.51 A/cm2 (SCE), 2.92 A/cm2 (SCE), 2.22 A/cm2 (SCE) and 2.91 A/cm2 (SCE) for c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. All the Ti–Cr alloys except Ti–5Cr had lower Icorr values than c.p. Ti and Ti–6Al–4V. The average I0.5 estimated from these curves were 47.08 A/cm2 (SCE), 274.32 A/cm2 mV (SCE), 21.29 A/cm2 (SCE), 10.03 A/cm2 (SCE), 7.25 A/cm2 (SCE) and 5.50 A/cm2 (SCE) for the c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. I0.5 values for the Ti–Cr alloys were significantly lower than those of c.p. Ti and Ti–6Al–4V. Distinct differences in linear polarization curves were observed between Ti–Cr alloys on one hand, and c.p. Ti and Ti–6Al–4V on the other. Morishita et al. [13] reported that a decrease in the corrosion current density and an increase in corrosion potential of the Ti alloys can promote the transition from an active to a passive state by means of the enhanced cathodic reaction, which would acceler-
ate spontaneous passivation of the Ti surface at the anodic area by forming a TiO2 layer. In contrast, for all Ti–Cr alloys, the polarization curves broke down after reaching a passive region. In fact, a distinctly shorter passive region and lower passive current density were observed for all Ti–Cr alloys as compared to c.p. Ti and Ti–6Al–4V. However, since the potential in a natural oral environment is generally between −0.3 V and +0.3 V [14], the Ti–Cr alloys would most likely not dissolve in such an environment. 3.2. Surface characterization Fig. 5 shows the SEM micrographs of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF. SEM observation revealed rougher surface of c.p. Ti (Fig. 5(a)) and metallographic structures on the surfaces of the Ti–6Al–4 V alloys (Fig. 5(b)) than that of Ti–Cr alloys (Fig. 5(c–f)). As shown in Fig. 3, the linear polarization curves of c.p. Ti and Ti–Cr alloys in standard artificial saliva seem similar. ESCA proved that the Ti–Cr alloys showed a better corrosion resistance to fluoride due to its formation of chromium oxide. The Cr 2p, Ti 2p and O 1s ESCA spectra of specimens after electrochemical measurement in standard artificial saliva are shown in Fig. 6. The ESCA spectrum of Cr 2p showed two peaks (at 573.8 eV and 576.7 eV) which originated from the metallic state (Cr0 ) and the oxide state (Cr3+ ), respectively [15]. The spectrum of Ti–5Cr Ti–10Cr, Ti–20Cr and Ti–30Cr alloys in the Cr 2p region showed Cr3+ peak. The intensity of Cr3+ peak tended to increase as the Cr
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Fig. 7. ESCA spectra of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF. (a) Cr 2p; (b) Ti 2p; (c) O 1s.
component increased. This indicates that more chromium oxides were formed on alloys containing higher Cr. The ESCA spectrum of Ti 2p, the spectrum showed peaks which originated at Ti0 , Ti2+ , Ti3+ , and Ti4+ of Ti 2p3/2 and Ti 2p1/2 . All specimens predominantly showed a Ti4+ state. The ESCA spectrum of O 1s had three peaks, originating from metal oxide, O2− , hydroxide or hydroxyl groups, OH− . Fig. 7 shows Cr 2p, Ti 2p and O 1s ESCA spectra of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF. All Ti–Cr alloys showed Cr3+ peaks in the Cr 2p region. Ti–Cr alloys all showed good corrosion resistance in artificial saliva containing 0.5 wt% NaF. Takemoto et al. has reported that Ti–Cr immersed in a saline solution containing fluoride would form a chromium-rich oxide film which means that specimens may have better corrosion resistance in environment containing fluoride [16]. In agreement with these findings, in artificial saliva a containing NaF, Ti–Cr alloys showed better corrosion resistance than that of c.p. Ti. Thus, the existence of Cr near the surface area seems to play an important role for the improvement of corrosion resistance. 4. Conclusion In this study, the corrosion resistance of Ti–Cr alloys, c.p. Ti and Ti–6Al–4V were examined and compared in terms of their electrochemical corrosion behavior and surface characterization after treatment in two different artificial saliva solutions. This
study specifically compared the effects of a standard artificial saliva with an artificial saliva containing 0.5wt% NaF which more closely resembles the natural oral environment. Under these conditions it was found that, with the exception of Ti–5Cr, all Ti–Cr alloys demonstrated superior corrosion resistance. We, therefore, conclude that these alloys warrant further study as viable dental prostheses. Acknowledgements The authors would like to thank the support of this research by Grant 94-2622-E-166-003-CC3 from the National Science Council, Republic of China. References [1] K. Ida, Y. Tani, S. Tsutsumi, T. Togaya, T. Nambu, K. Suese, T. Kawazoe, M. Nakamura, Dent. Mater. J. 4 (1985) 191–195. [2] K. Elagli, M. Traisnel, H.F. Hildebrand, Electrochim. Acta 38 (1993) 1769. [3] J. Black, J. Bone Joint Surg. 70B (1988) 517–520. [4] K.L. Wapne, Clin. Orthop. 271 (1991) 12–20. [5] M. Nakagawa, S. Matsuya, K. Udoh, Dent. Mater. J. 20 (4) (2001) 305–314. [6] M.J. Donachie Jr., Titanium: A Technical Guide, 2nd ed., ASM International, 2000, p 126. [7] R.G. Craig, Restorative Dental Materials, 9th ed., C.V. Mosby Co, 1993, p. 395. [8] J.L. Murray, Phase Diagrams of Binary Titanium Alloys, ASM International, 1987, p. 69. [9] W.F. Ho, T.Y. Chiang, S.C. Wu, H.C. Hsu, J. Alloys Compd. 468 (2009) 533–538.
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[10] W.F. Ho, T.Y. Chiang, S.C. Wu, H.C. Hsu, J. Alloys Compd. 474 (2009) 505–509. [11] P.J. Bania, in: D. Eylon, R.R. Boyer, D.A. Koss (Eds.), Beta Titanium Industry, Beta Titanium Alloys in the 1990s, TMS, 1993, p. 3. [12] M. Nakagawa, S. Matsuya, T. Shiraishi, M. Ohta, J. Dent. Res. 78 (1999) 1568–1572.
[13] M. Morishita, M. Chikuda, Y. Ashida, M. Morinaga, N. Yukawa, H. Adachi, Mater. Trans. 32 (3) (1991) 264–271. [14] P.P. Corso Jr., R.M. German, H.D. Simmons Jr., J. Dent. Res. (1985) 854–859. [15] K. Asami, K. Hashimoto, Corros. Sci. 17 (1977) 713–723. [16] S. Takemoto, M. Hattori, M. Yoshinari, E. Kawada, Y. Oda, Biomaterials 26 (2005) 829–837.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrochemical behavior of Ti–Cr alloys in artificial saliva Hsueh-Chuan Hsu a,b , Shih-Ching Wu a,b , Cheng-Feng Wang b , Wen-Fu Ho c,∗ a
Department of Dental Laboratory Technology, Central Taiwan University of Science and Technology, Taiwan, ROC Institute of Biomedical Engineering and Material Science, Central Taiwan University of Science and Technology, Taiwan, ROC c Department of Materials Science and Engineering, Da-Yeh University, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 19 September 2008 Received in revised form 27 July 2009 Accepted 29 July 2009 Available online 5 August 2009 Keywords: Metals and alloy Corrosion Artificial saliva
a b s t r a c t In this study, the corrosion behavior of commercially pure titanium (c.p. Ti), Ti–6Al–4V and five new experimental Ti–Cr alloys was evaluated through open-circuit potential (OCP) and potentiodynamic polarization measurement in an artificial saliva containing fluoride. Electron spectroscopy for chemical analysis (ESCA) was used to characterize the composition of the passive films on the alloy after potentiodynamic polarization measurement. It was found that in standard artificial saliva the OCP increases with higher Cr content in Ti–Cr alloys. In 0.5% NaF artificial saliva, the OCP decreases with decreasing Cr in Ti–Cr alloys, and all but Ti–5Cr remain consistently higher than those of c.p Ti and Ti–6Al–4V. Linear polarization results show that artificial saliva and artificial saliva containing 0.5% NaF result in different corrosion behavior in Ti–Cr alloys, c.p.Ti and Ti–6Al–4V. The Ti–Cr alloys had greater resistance to corrosion in the fluoride-containing artificial saliva than c.p. Ti and Ti–6Al–4V, respectively. ESCA results verify that after potentiodynamic polarization a passive film consisting of TiO2 and Cr2 O3 forms on the surface of Ti–Cr alloys. These experimental results show that the electrochemical corrosion behavior of Ti–Cr alloys in artificial saliva containing 0.5% NaF can be improved by increasing Cr content. This further indicates that Ti–Cr alloys could successfully be used for crown, bridge, and metal-ceramic restorations. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Commercially pure titanium (c.p. Ti) and its alloys have been widely used for dental restorative applications such as crown/bridge, framework and dentures [1]. The advantages of titanium alloys in these applications are due to their excellent mechanical properties, good corrosion resistance in biological fluids, and biocompatibility [2]. For a metal to be used in an oral environment, it should be biocompatible and have high corrosion resistance. Among the various titanium alloys, the Ti–6Al–4V alloy is the most frequently used in medical implants. It also shows better physical and mechanical properties in comparison to c.p. Ti. However, this alloy might cause some long-term health problems because of the release of Al and V ions [3,4]. Previous studies also showed that the surface of the Ti–6Al–4V alloy was roughened by corrosion in the acidic fluoride-containing saliva [5]. In the present study the corrosion resistance of Ti–6Al–4V alloy will be further investigated. Cr is suitable for alloying with titanium (Ti) for several reasons. For example, Cr is known to control the anodic activity of the alloy and increase the tendency of Ti to passivate [6]. In fact, it has already
been used for many years as a major constituent in dental casting alloys [7]. An additional advantage of alloying Cr to Ti is that the liquidus temperature is gradually reduced from the high melting point of pure Ti (1670 ◦ C) until reaching a minimum of 1410 ◦ C at Cr content of 46% [8]. Thus, a number of Ti–Cr alloys have previously been developed for dental applications [9,10]. Among these, -titanium alloys are the most versatile, and a large number of studies have been performed on them [11]. Ho et al. [9] have studied the structure and properties of a series of binary Ti–Cr alloys with Cr content up to 30 wt%. While the Ti–Cr alloys have been shown to have better mechanical properties than c.p. Ti [9], their corrosion resistance behavior remains unknown. Therefore, in this study the electrochemical behavior of these alloys was specifically investigated. Since Ti alloys are used in the dental field, and the oral environment is exposed to fluoride, the degree of corrosion resistance offered by the Ti alloys in a fluoride-containing medium becomes an important property for their use in dentistry. In addition, some reports have shown the negative influence of fluoride on the corrosion resistance of titanium [5,12]. Therefore, in this research, corrosion behavior of all alloys was studied in simulated oral environments with and without fluoride. 2. Experimental method
∗ Corresponding author at: Department of Materials Science and Engineering, Da-Yeh University, 168 University Road, Dacun, Changhua 51591, Taiwan, ROC. Tel.: +886 4 851 1888x4108 fax: +886 4 851 1224. E-mail addresses: [email protected], [email protected] (W.-F. Ho). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.172
2.1. Specimen preparation Commercially pure titanium (Kobe Steel Co. Ltd., Kobe, Japan), the Ti–6Al–4V (Daido Steel Co. Ltd., Nagoya, Japan) alloy, and the experimental Ti–(5–30) wt% Cr
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Fig. 1. OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in standard artificial saliva.
Fig. 2. OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in saliva solution with 0.5 wt% NaF.
alloys were examined in this study. The Ti–Cr alloys were made from c.p. Ti and Cr (99.95%) using an argon-arc melting furnace (1000 Miller Dimension, USA). For the electrochemical measurement, specimens were cut into 1.0 mm in thickness from a rod. The specimens were grinded with silicon carbide paper until #1500 and then polished with 0.3 m Al2 O3 suspension. After polishing, the samples were cleaned by means of ultrasonic sound in ethanol. 2.2. Electrochemical tests In this study, two artificial saliva solutions were used. The first was a standard artificial saliva (NaCl 400 mg/l, KCl 400 mg/l, CaCl2 ·2H2 O 795 mg/l, NaHPO4 ·H2 O 690 mg/l, KSCN 300 mg/l, Na2 S·9H2 O 5 mg/l, urea 1000 mg/l). The second also contained 0.5 wt% NaF (corresponding to 2500 ppm F). The pH level of both test solutions was adjusted to 4.0 by adding lactic acid in a simulated oral environment [12]. A three-electrode system was used for all the electrochemical measurements. The apparatus for electrochemical measurement consisted of a potentiostat (Versa Stat TM II Potentiostat/Golvanostat Model 263A; Princeton Applied Research, NJ, USA), controlled by a personal computer with dedicated software (PowerSuit-2.56, Princeton Applied Research), a saturated calomel electrode (SCE) as reference electrode, a platinum plate as counter electrode, and the specimen as working electrode. For each test, the open-circuit potential was measured for 2 h. The potentiodynamic polarization of the specimens was recorded in a scanning range from −1.2 to +2.5 V (v.s. SCE) at a scanning rate of 0.1 mV/s. In every test, the medium was maintained at 37 ◦ C. Five specimens were tested for each condition. 2.3. Surface characterization
Fig. 3. Linear polarization diagrams of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the standard artificial saliva.
After electrochemical measurement, surface morphology of specimens was examined using by a scanning electron microscope (S-3000N, HITACHI, Japan) and the surface composition was examined with ESCA 750 (Shimadzu, Japan).
3. Results and discussion 3.1. Electrochemical measurement Fig. 1 shows the change in OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in standard artificial saliva. For all specimens, the potential changed in the negative direction over time until a steady-state potential was reached, and gradually decreased. For c.p. Ti, the initial potential was around −256 mV (SCE), but it gradually decreased after 3500 s, and a value of −586 mV (SCE) was reached. The potential then remained almost constant and after 7200 s its value was −580 mV (SCE). The initial potential for Ti–6Al–4V was approximately −265 mV (SCE), which decreased gradually and after 5000 s remained stable at approximately −496 mV (SCE). After 7200 s the potential for this alloy was −506 mV (SCE). The variation of potential over time for the other four Ti–Cr alloys was similar to those of c.p. Ti and Ti–6Al–4V. The steady potentials of Ti–Cr alloys ranged from −418 to −580 mV (SCE). The corrosion potential of the Ti–Cr alloys increased with
Fig. 4. Linear polarization diagrams for c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the artificial saliva containing 0.5 wt% NaF.
H.-C. Hsu et al. / Journal of Alloys and Compounds 487 (2009) 439–444
increasing Cr content, which indicates the noble behavior of these alloys. Fig. 2 shows the change in OCP of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys over a period of 2 h in saliva solution with 0.5 wt% NaF. At all concentrations the potential changed in the positive direction over time until a steady-state potential is reached. For all specimens, the initial potential was around −480 mV (SCE). For Ti–10Cr, Ti–20Cr and Ti–30Cr, it decreased gradually before finally remaining stable around −720 mV (SCE). However, for Ti–6Al–4V, c.p. Ti and Ti–5Cr, initial potentials decreased gradually and when they reached 3000 s, 5500 s and 6500 s, respectively they suddenly dropped to approximately −1250 mV (SCE). After 7200 s the potential for all three alloys was −506 mV (SCE). These results indicate that Ti–10Cr, Ti–20Cr and Ti–30Cr have superior corrosion resistance in artificial saliva containing 0.5 wt% NaF. Fig. 3 shows the linear polarization diagrams of c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the standard artificial saliva. All the specimens had distinct, long passive regions. The average corrosion potentials estimated from the linear polarization curves were
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−1109 mV (SCE), −1116 mV (SCE), −1043 mV (SCE), −1127 mV (SCE), −1078 mV (SCE) and −1071 mV (SCE) for c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. These values were significantly lower than those obtained from the opencircuit potential measurements. This is to be expected, as the polarization tests were started at a cathodic potential relatively close to the corrosion potential, so that the passive film at the surface was at least partially removed. An analysis of the results of all electrochemical tests conducted in the standard artificial saliva indicates that all Ti alloys in this study behaved in a very similar way. The differences between the alloys were not found to be significant, which can be ascribed to the fact that the passive film formed on all these alloys is essentially the same. For all alloys, low current densities (passive current density at potential 0.5 V; I0.5 ) (10−6 A/cm2 ) were obtained previous to the establishment of a typical passive behavior at 500 mV (SCE). In fact, an increase in current density with increasing potential can occur if the increase in potential is not accompanied by a corresponding thickening of the oxide film. Nevertheless, it appears that the film
Fig. 5. SEM micrographs of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF; (a)c.p. Ti, (b) Ti–6Al–4 V, (c) Ti–5Cr, (d) Ti–10Cr, (e)Ti–20Cr, and (f) Ti–30Cr.
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Fig. 6. ESCA spectra of specimens after electrochemical measurement in standard artificial saliva. (a) Cr 2p; (b) Ti 2p; (c) O 1s.
formed above 500 mV (SCE) is different from that formed below this potential. Its higher current density value indicates that it is weaker oxide film which cannot provide effective corrosion resistance. Fig. 4 shows the linear polarization diagrams for c.p. Ti, Ti–6Al–4V and Ti–Cr alloys in the artificial saliva containing 0.5 wt% NaF. The average corrosion potentials estimated from these curves were −1224 mV (SCE), −1175 mV (SCE), −1156 mV (SCE), −1094 mV (SCE), −1033 mV (SCE) and −1028 mV (SCE) for c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. All the Ti–Cr alloys except Ti–5Cr had higher Ecorr values than c.p. Ti and Ti–6Al–4V. The average corrosion current density estimated from these curves were 62.51 A/cm2 (SCE), 10.68 A/cm2 mV (SCE), 146.51 A/cm2 (SCE), 2.92 A/cm2 (SCE), 2.22 A/cm2 (SCE) and 2.91 A/cm2 (SCE) for c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. All the Ti–Cr alloys except Ti–5Cr had lower Icorr values than c.p. Ti and Ti–6Al–4V. The average I0.5 estimated from these curves were 47.08 A/cm2 (SCE), 274.32 A/cm2 mV (SCE), 21.29 A/cm2 (SCE), 10.03 A/cm2 (SCE), 7.25 A/cm2 (SCE) and 5.50 A/cm2 (SCE) for the c.p. Ti, Ti–6Al–4V, Ti–5Cr, Ti–10Cr, Ti–20Cr and Ti–30Cr alloys, respectively. I0.5 values for the Ti–Cr alloys were significantly lower than those of c.p. Ti and Ti–6Al–4V. Distinct differences in linear polarization curves were observed between Ti–Cr alloys on one hand, and c.p. Ti and Ti–6Al–4V on the other. Morishita et al. [13] reported that a decrease in the corrosion current density and an increase in corrosion potential of the Ti alloys can promote the transition from an active to a passive state by means of the enhanced cathodic reaction, which would acceler-
ate spontaneous passivation of the Ti surface at the anodic area by forming a TiO2 layer. In contrast, for all Ti–Cr alloys, the polarization curves broke down after reaching a passive region. In fact, a distinctly shorter passive region and lower passive current density were observed for all Ti–Cr alloys as compared to c.p. Ti and Ti–6Al–4V. However, since the potential in a natural oral environment is generally between −0.3 V and +0.3 V [14], the Ti–Cr alloys would most likely not dissolve in such an environment. 3.2. Surface characterization Fig. 5 shows the SEM micrographs of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF. SEM observation revealed rougher surface of c.p. Ti (Fig. 5(a)) and metallographic structures on the surfaces of the Ti–6Al–4 V alloys (Fig. 5(b)) than that of Ti–Cr alloys (Fig. 5(c–f)). As shown in Fig. 3, the linear polarization curves of c.p. Ti and Ti–Cr alloys in standard artificial saliva seem similar. ESCA proved that the Ti–Cr alloys showed a better corrosion resistance to fluoride due to its formation of chromium oxide. The Cr 2p, Ti 2p and O 1s ESCA spectra of specimens after electrochemical measurement in standard artificial saliva are shown in Fig. 6. The ESCA spectrum of Cr 2p showed two peaks (at 573.8 eV and 576.7 eV) which originated from the metallic state (Cr0 ) and the oxide state (Cr3+ ), respectively [15]. The spectrum of Ti–5Cr Ti–10Cr, Ti–20Cr and Ti–30Cr alloys in the Cr 2p region showed Cr3+ peak. The intensity of Cr3+ peak tended to increase as the Cr
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Fig. 7. ESCA spectra of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF. (a) Cr 2p; (b) Ti 2p; (c) O 1s.
component increased. This indicates that more chromium oxides were formed on alloys containing higher Cr. The ESCA spectrum of Ti 2p, the spectrum showed peaks which originated at Ti0 , Ti2+ , Ti3+ , and Ti4+ of Ti 2p3/2 and Ti 2p1/2 . All specimens predominantly showed a Ti4+ state. The ESCA spectrum of O 1s had three peaks, originating from metal oxide, O2− , hydroxide or hydroxyl groups, OH− . Fig. 7 shows Cr 2p, Ti 2p and O 1s ESCA spectra of all titanium specimens after electrochemical measurement in artificial saliva containing 0.5 wt% NaF. All Ti–Cr alloys showed Cr3+ peaks in the Cr 2p region. Ti–Cr alloys all showed good corrosion resistance in artificial saliva containing 0.5 wt% NaF. Takemoto et al. has reported that Ti–Cr immersed in a saline solution containing fluoride would form a chromium-rich oxide film which means that specimens may have better corrosion resistance in environment containing fluoride [16]. In agreement with these findings, in artificial saliva a containing NaF, Ti–Cr alloys showed better corrosion resistance than that of c.p. Ti. Thus, the existence of Cr near the surface area seems to play an important role for the improvement of corrosion resistance. 4. Conclusion In this study, the corrosion resistance of Ti–Cr alloys, c.p. Ti and Ti–6Al–4V were examined and compared in terms of their electrochemical corrosion behavior and surface characterization after treatment in two different artificial saliva solutions. This
study specifically compared the effects of a standard artificial saliva with an artificial saliva containing 0.5wt% NaF which more closely resembles the natural oral environment. Under these conditions it was found that, with the exception of Ti–5Cr, all Ti–Cr alloys demonstrated superior corrosion resistance. We, therefore, conclude that these alloys warrant further study as viable dental prostheses. Acknowledgements The authors would like to thank the support of this research by Grant 94-2622-E-166-003-CC3 from the National Science Council, Republic of China. References [1] K. Ida, Y. Tani, S. Tsutsumi, T. Togaya, T. Nambu, K. Suese, T. Kawazoe, M. Nakamura, Dent. Mater. J. 4 (1985) 191–195. [2] K. Elagli, M. Traisnel, H.F. Hildebrand, Electrochim. Acta 38 (1993) 1769. [3] J. Black, J. Bone Joint Surg. 70B (1988) 517–520. [4] K.L. Wapne, Clin. Orthop. 271 (1991) 12–20. [5] M. Nakagawa, S. Matsuya, K. Udoh, Dent. Mater. J. 20 (4) (2001) 305–314. [6] M.J. Donachie Jr., Titanium: A Technical Guide, 2nd ed., ASM International, 2000, p 126. [7] R.G. Craig, Restorative Dental Materials, 9th ed., C.V. Mosby Co, 1993, p. 395. [8] J.L. Murray, Phase Diagrams of Binary Titanium Alloys, ASM International, 1987, p. 69. [9] W.F. Ho, T.Y. Chiang, S.C. Wu, H.C. Hsu, J. Alloys Compd. 468 (2009) 533–538.
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