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Influence of sulfide concentration on the corrosion behavior of titanium in a simulated oral environment
Materials Science and Engineering C 62 (2016) 268–273
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Influence of sulfide concentration on the corrosion behavior of titanium in a simulated oral environment Rino Harada a,b, Shinji Takemoto a,b,⁎, Hideaki Kinoshita a, Masao Yoshinari b, Eiji Kawada a a b
Department of Dental Materials Science, Tokyo Dental College, 2-9-18 Misaki-cho, Chiyoda-ku, 101-0061, Tokyo, Japan Oral Science Center, Tokyo Dental College, 2-9-18 Misaki-cho, Chiyoda-ku, 101-0061, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 20 November 2015 Received in revised form 30 December 2015 Accepted 24 January 2016 Available online 26 January 2016 Keywords: Titanium Corrosion Sulfide X-ray photoelectron spectroscopy Oxide film
a b s t r a c t This study assessed the corrosion behavior of titanium in response to sulfide by determining the effects of sulfide concentration and pH over immersion period. Corrosion was evaluated through changes in color, glossiness, surface characterization, and titanium release. Sulfide solutions were prepared in 3 different concentrations with Na2S, each in pH unadjusted (sulfide-alkaline) and pH adjusted to 7.5 (sulfide-neutral). Titanium discoloration increased and glossiness decreased as sulfide concentration and immersion period increased in sulfide-alkaline solutions. Coral-like complexes were observed on the surface of these specimens, which became more pronounced as concentration increased. Small amounts of titanium release were detected in sulfide-alkaline solutions; however, this was not affected by immersion periods. Corrosion was indicated through considerable surface oxidation suggesting the formation of a thick oxide layer. No significant changes in color and glossiness, or titanium release were indicated for titanium specimens immersed in sulfide-neutral solutions indicating that pH had a significant effect on corrosion. Our findings suggest that a thick oxide layer on the titanium surface was formed in sulfide-alkaline solutions due to excessive oxidation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The use of a titanium dental implant system is common to restore the function of lost teeth. The clinical success of dental implants is dependent on biomechanical factors that determine the integrity of the implant [1–3]. The physical properties of pure titanium or titanium alloys, such as high strength to weight ratio, superior fatigue strength, tensile strength, modulus of elasticity, and cytotoxicity make them very attractive and suitable for the fabrication of any dental prosthesis [4–7]. Titanium is also known for its corrosion resistance, which is a result of the materials' ability to form passive oxide films (TiO2) in normal physiological conditions. However, the nature, composition, and thickness of this protective film depend on environmental conditions [8–10]. Electrochemical corrosion in fluid electrolytes such as saliva can possibly trigger allergic reactions by releasing elements from the alloy [11]. This can also be an issue since titanium ions have a biological effect on the viabilities of osteoblast and osteoclast and on their differentiation which may influence prognosis after implant placement [12]. The dynamicity of the oral cavity makes it essential to determine the causes of material corrosion and subsequent wear. These aspects serve as the foundations to develop materials resistant to corrosion or methods to treat unfavorable conditions. ⁎ Corresponding author at: 2-9-18 Misaki-cho, Chiyoda-ku, 101-0061, Tokyo, Japan. E-mail address: [email protected] (S. Takemoto).
http://dx.doi.org/10.1016/j.msec.2016.01.065 0928-4931/© 2016 Elsevier B.V. All rights reserved.
Literatures have reported that fluoride, peroxide, organic acids, and released-cell products induce titanium corrosion [13–22]. Fluorides found in prophylactic agents, affect the titanium surface by dissolving the passive oxide layer leading to ion dissolution and surface discoloration [19,20]. Peroxides found in denture cleanser affect titanium through surface oxidation in acidic conditions or by inducing dissolution in alkaline solutions [20]. Released-cell products such as lactic acid and formic acid which can be produced by oral bacteria have also shown to affect the corrosion behavior of titanium [14,21]. The biomedical application of titanium is common due to its exceptional properties discussed earlier as well as corrosion resistance, however these papers suggest that titanium may be susceptible to corrosion in the oral environment. Sulfur products are of particular concern in regards to titanium dental implants because periodontopathic bacteria secretes considerable amounts of sulfide byproducts, such as hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl sulfide ((CH3)2S), as a result of their metabolism [23]. Sulfide has been reported to have the potential to cause discoloration in dental alloys. Silver based alloys have shown to tarnish at various degrees in 0.1% sulfide solutions based on chemical composition and microstructure [24]. When silver palladium alloys were placed in cultures containing Fusobacterium nucleatum, discoloration occurred and sulfur was identified on the metal surface [25]. Titanium is known for its corrosion resistance however, Rodrigues et al. retrieved failed implants due to peri-implantitis and reported that titanium dental implants were highly susceptible to pitting attack
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in the oral environment [26]. Yoshida et al. reported discoloration of titanium and titanium alloys in sulfide solutions [27]. These suggest the possibility of sulfides in the oral cavity affecting the corrosion behavior of titanium, however factors which affect the corrosion behavior of titanium in sulfide environments have not yet been studied in detail. The objective of this study is to assess the corrosion behavior of titanium in response to sulfide by determining the effects of sulfide concentration and pH over immersion period. The null hypothesis is that sulfide concentration, pH, and immersion period have no effect on the corrosion behavior of titanium. 2. Materials and methods 2.1. Specimen and solution preparation Commercially available pure titanium disks 1.3 mm thick and 13 mm in diameter were prepared from wrought titanium rods (Grade 2, Tokyo Titanium, Tokyo, Japan). The disks were mirror polished with a polishing machine (Automet 250 and Ecomet, 250 Buehler, Tokyo, Japan) using silicon carbide paper from 320 to 1200 grit, diamond suspension, and colloidal silica suspension according to metallographic procedure. These were ultrasonically washed with acetone and distilled water for 5 min and dried. Ultrapure water (Milli-Q®; Merck Millipore Corporation, Tokyo, Japan: MQ) was used as a representative immersion medium. Sulfide solutions were prepared by adding to 500 mL of 3.12 g, 12.12 g, and 24.00 g of Na2S·9H2O (Wako Pure Chemical Industries, Ltd., Osaka, Japan) to MQ abbreviated as MQS-L, MQS-M, and MQS-H respectively. The pH of sulfide solutions with pH unadjusted (sulfide-alkaline solutions) ranged from 11.6–12.1. Sulfide solutions with pH adjusted to 7.5 using hydrochloric acid (Wako Pure Chemical Industries, Ltd., Osaka, Japan), denoted as -AD (sulfide-neutral solutions), were also prepared. Titanium disks were immersed in 20 mL of solution in a polypropylene bottle and sealed to prevent evaporation and stored at 37 °C for 1, 3, and 7 days. All assays were performed using triplicate samples of each solution in at least 2 different experiments. 2.2. Color and glossiness measurements Initial color and glossiness of the titanium specimens were measured using a colorimeter (MCR-A Luck Office, Tokyo, Japan) and glossmeter (GM-26D, Murakami Color Research Laboratory, Tokyo, Japan). Color difference (ΔE * ab) of titanium specimens were calculated for each condition at 1, 3, and 7 days by measuring CIE L*a*b* color coordinates against reflected light. The colorimeter was calibrated with a white calibration tile before being used. The ΔE * ab was determined using the following equation: h i1=2 2 2 ΔE ab ¼ ðL −L0 Þ þ ða −a0 Þ2 þ ðb −b0 Þ where L0*, a0*, b0* were values before immersion, while L*, a*, b* were values after immersion. For glossiness, incidence light angle of 20° from the surface normal was selected and reflected light was measured using a detector. The glossmeter was calibrated with a black tile with a standard reference value of 86.7% for Gs(20°). 2.3. Surface characterization Surface texture of specimens was observed under a field emission scanning electron microscope (FE-SEM SU6600; Hitachi, Tokyo, Japan) operating at a voltage of 15 kV. The specimens were subsequently coated with carbon using a carbon coater (VC-100S, Vacuum Device, Ibaraki, Japan) before observation.
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Crystal structure on titanium specimens were investigated by X-ray diffraction (XRD) using CuKα radiation operating at a voltage of 40 kV and current of 30 mA. Two theta diffraction angle were set between 20–70° with an incremental angle of 5° per minute using a diffractometer (Ultima IV, Rigaku, Tokyo, Japan) with a semiconductor detector. Thin film XRD (TF-XRD) pattern was also analyzed to determine surface at an incident angle of 1° at a 2θ diffraction angle of 10–50° and incremental angle of 0.5° per minute (Smartlab®, Rigaku, Tokyo, Japan). The chemical composition and binding state of the titanium specimens were analyzed by X-ray photoelectron spectroscopy (XPS); (AxisUltra, Kratos-Shimadzu, Kyoto, Japan). The X-ray resource was monochronized AlKα electrode at 15 kV, 10 mA emission current. The area of measurement was 200 × 600 μm, and take-off angle of photoelectrons was 45°. All XPS spectra were calibrated to the standard carbon C1s hydrocarbon peak at around 285.0 eV. The quantitative analysis and the curve fitting were conducted by the CasaXPS software (Casa Software Ltd., Teignmouth, UK). 2.4. Released element analysis in solution Titanium release into the test solutions was determined using an inductively coupled plasma optical emission spectroscopy (ICP-OES, Vista-MPX, SII, Japan). Release values were calculated by using the concentration (ppm or mg/L) to derive the total amount released titanium in the bottle then dividing the amount by the exposed area (μg/cm2). The detection limit of titanium release from the specimens used in this study was above 0.01 μg/cm2. 2.5. Statistical analysis Data analysis was performed using one-way analysis of variance (ANOVA) and Tukey's HSD multiple comparison tests using software (Excel Tokei, 2010, SSRI, Tokyo, Japan) to compare ΔE * ab, glossiness, and titanium ion release with immersion period. Color difference with respect to immersion period, sulfide concentration, pH, and titanium ion release were statistically analyzed with two-way ANOVA. The influence of pH was compared for each sulfide concentration at constant immersion periods. Statistical significance was set at α = 0.05. Triplicate samples were used in the experiments to ensure the reproducibility of the results.
3. Results 3.1. Color and glossiness change Fig. 1 displays the ΔE * ab of titanium specimens immersed in various sulfide solutions over immersion period. For sulfide-alkaline solutions, Δ E * ab value was significantly higher in order of MQ, MQS-L, MQS-M, and MQS-H at each immersion period. For each sulfidealkaline solution, Δ E * ab value demonstrated significant increase on day 1 and 3, however no further changes were indicated after day 3. In contrast, no significant differences in Δ E * ab were found between titanium specimens immersed in MQ and all sulfide-neutral solutions (− AD), and no significant changes in Δ E * ab were indicated with increasing immersion period. Fig. 2 shows the change in glossiness of titanium specimens immersed in various sulfide solutions over immersion period. Glossiness decrease was greater in order of MQ, MQS-L, MQS-M, and MQS-H at all immersion periods, however no significant differences were found between MQ and MQS-L at all immersion periods. Glossiness decreased with time by day 1 and 3, however no significant changes were indicated after day 3 for all sulfide-alkaline solutions. No significant changes in glossiness were found among MQ and all sulfide-neutral solutions with increasing immersion periods.
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Fig 1. Color change (ΔE * ab) of titanium in sulfide solutions over immersion period (a) and at 7 days (b) (p b 0.05).
3.2. Surface characterization Fig. 3 shows surface morphology of the titanium immersed in various sulfide solutions with different sulfide concentration and pH for 7 days. MQ has smooth morphology while coral-like complexes are observed on the sulfide-alkaline solutions and these complexes become more pronounced as concentration increased. Surface textures were similar among titanium specimens immersed in MQ and sulfide-neutral solutions. There were no signs of any localized corrosion such as pitting or micro cracks on any of the specimens. Typical XRD patterns of the specimens placed in the sulfide-alkaline solutions are shown in Fig. 4(a). The diffraction peaks of mirror polished titanium (MP) had 2θ values at 35.0, 38.4, 40.1, 52.9 and 62.9 which were consistent with the peaks of titanium substrate. Only strict titanium peaks were seen on all patterns indicating no formation of crystalized corrosion byproducts. Also titanium was the only peak seen on the TFXRD pattern (Fig. 4(b)) as well indicating no formation of corrosion products through XRD analysis. Table 1 displays the composition of the outermost surface of titanium specimens after immersion in Na2S solution through XPS analysis. The presence of elements on the titanium surface was deduced to mainly Ti, O, and C with traces of N, F, Na, Ca, and Si. C and N were derived from contaminants and the small amounts of Si, Ca, and F are most likely remain from the polishing procedure and/or traces that were in the test solutions (less than 2 atm%). The surface mostly consisted of titanium and oxygen. Sulfur contained in the immersion solution was not detected on the titanium surface.
In Fig. 5(a), Ti2p spectra consists of Ti2p3/2 and Ti2p1/2 where the doublet originated from. Ti2p3/2 and Ti2p1/2 were decomposed to metal (T0), divalent (Ti2 +), trivalent (Ti3 +), and tetravalent (Ti4 +) forms of Ti. The XPS analyses of titanium after immersion in sulfidealkaline solution revealed the presence of mostly Ti4+. Fig. 5(b) shows the fraction of Ti0/Ti4+ after immersing in sulfide solutions for 7 days. The Ti0/Ti4+ ratio demonstrates an obvious decrease in Ti0/Ti4+ ratio with increasing concentration in sulfide-alkaline solutions. In Fig. 5(c), the fraction of Ti0/Ti4+ after immersing in MQS-L for 1, 3, and 7 days is displayed, and a decrease in ratio with increasing immersion period is observed. The Ti0/Ti4 + ratio did not change as much in sulfideneutral solutions. This indicates the acceleration of oxidation in higher concentrations of sulfide-alkaline solutions, and progression of oxidation as immersion period increases. Titanium immersed in MQ and sulfide-neutral solutions showed no decrease in Ti0/Ti4+ ratio indicating no progression in corrosion. 3.3. Titanium release Table 2 lists the amount of titanium released in the sulfide-alkaline solutions determined through ICP-OES analyses. Titanium release was seen in solutions and released titanium did not significantly differ with immersion period. The maximum amount of released titanium over a span of 7 days was 0.41 μg/cm2. Sulfide concentration had a significant influence on titanium release in that titanium release at 7 days in MQS-H was significantly higher than MQS-L and MQS-M. The amount of titanium release in MQ and sulfide-neutral solutions was under detectable limits at all immersion periods.
Fig 2. Glossiness (Gs(20°)) values of titanium in sulfide solutions over immersion period (a) and at 7 days (b) (p b 0.05).
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Fig 3. SEM observation of titanium surface after immersing in MQ and sulfide solutions for 7 days.
4. Discussion In this study, titanium corrosion occurred in sulfide-alkaline solutions depending on sulfide concentrations, therefore the results of this study reject the null hypothesis that sulfide concentration and pH have no influence on the corrosion behavior of titanium. In dentistry, some studies have reported that titanium was corroded by substances such as fluoride in prophylactic agent, peroxides in denture cleanser, and organic acids, which suggests that titanium could be sensitive to corrosion in the oral environment [14,27]. In-vitro studies have reported that fluoride causes titanium corrosion by dissolving the passive oxide layer causing ion dissolution and surface discoloration due to an increase in surface roughness [13,18,19,20,22]. In acidic conditions, peroxides induce oxidation at the titanium surface by forming insoluble complex compounds leading to considerable change in color [20]. In alkaline solutions, peroxides induce dissolution and the formation of a soluble complex leading to subsequent loss in mass and the formation of a thick oxide film causing a significant change in color [20]. Titanium release in fluoride and peroxide in acidic environments was approximately 70 μg/cm2 in 3 days and 0.3 μg/cm2 in 7 days respectively [13,18]. These studies indicate that corrosion behavior of titanium in sulfide-alkaline solutions is similar to peroxide in acidic environments in that it involves a minor release of titanium and discoloration. Titanium release occurred immediately after immersion in sulfide solutions at
a rate which did not increase with immersion period, whereas discoloration progressed with immersion indicating that the cause of titanium corrosion was most likely not due to titanium release. In this study, the corrosion of titanium immersed in sulfide-alkaline solution was also visually demonstrated through discoloration and glossiness change. Discoloration of titanium in alkaline solutions containing high concentration of sulfide to our study was similar to a study done by Yoshida et al. who reported that color changes for various titanium alloys immersed in solutions containing Na2S, according to dynamic corrosion test in ISO 10271 [27]. Our finding suggests the discoloration of titanium in sulfide alkaline solution depends on sulfide concentration. Although titanium changed in color and dissolved in sulfide-alkaline solutions, no discoloration and release of titanium was detected in sulfide-neutral solutions. Suito et al. performed a titanium immersion test in simulated body fluid of varying pH and demonstrated that eluted titanium ions were detected after 48 h at a pH of 9 however for pH 7.4, even after 4 weeks eluted titanium ions were not detected [28]. These results are similar to ours where titanium release levels in neutral solutions were under detectable values, therefore this raises the possibility of pH having an effect on titanium release. The SEM images as shown in Fig. 2 suggested the formation of corrosion byproducts on the titanium surface however, XRD showed only titanium substrate peaks on all specimens and XPS showed no sulfur
Fig 4. XRD analysis of MP and titanium immersed in sulfide-alkaline solutions (a). Thin film XRD analysis of MP and titanium immersed in 0.1 M sulfide-alkaline solution (MQS-H) for 7 days at 1° incident angle (b).
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Table 1 Composition in outermost surfaces of titanium specimens after immersing in Na2S solution.
MP MQ7 MQS-L1 MQS-L3 MQS-L7 MQS-M1 MQS-M7 MQS-H1 MQS-H7 MQS-L7-AD MQS-M7-AD MQS-H7-AD
Ti
O
C
N
Other
21.2 21.2 19.1 18.0 18.1 17.8 20.5 18.8 21.0 22.8 22.1 16.0
48.1 47.2 47.2 43.4 47.1 44.6 50.2 47.4 51.5 51.3 49.7 48.1
27.7 30.2 30.1 36.0 31.1 34.4 25.7 34.4 24.3 23.6 26.5 34.0
2.2 1.0 1.2 0.8 1.4 2.1 1.5 1.2 1.4 2.1 1.4 0.9
F Ca, F Na, Ca, F, Si Na, Ca, F, Si Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca, F, Si
on the titanium surface. Kim et al. reported that when titanium and titanium alloys are treated with a highly concentrated sodium hydroxide aqueous solution, TF-XRD revealed the formation of an amorphous or microcrystalline sodium titanate hydrogel layer on their surface [29–31]. In this study titanium disks were placed in sulfide solutions that were alkaline condition (approximately pH 12) however, no sodium titanate peaks nor broad peaks of the hydrogel were found in the XRD patterns of the specimens. This indicates titanium surface oxidizes in sulfide-alkaline solutions, however only to a small degree. XPS analysis showed a decrease in Ti0/Ti4+ with increasing concentration and immersion period indicating oxidation of the titanium surface suggesting that discoloration in sulfide-alkaline solutions was caused by the formation of an oxide layer (TiO2) which was much thicker than the passive film. Sulfide forms are dependent on pH and a majority comes in the form of H2S (pH b 7.5), HS− (7.5 b pH b 13), and S2− (pH N 13) [32]. In this experiment, pH of around 7.5 indicates H2S and HS− to be prominent, however in the sulfide-alkaline solutions where the pH is around 12, HS− predominantly exists with some S2− also present. Sulfide gets reduced after it acts as an oxidizing agent. In alkaline solutions, HS− is easily reduced to S2−, therefore titanium oxidation was most likely accelerated in sulfide-alkaline solutions. Mueller reported that consolidated silver was susceptible to sulfide tarnishing and corrosion product on the silver surface was identified to be Ag2S [33]. In a similar manner, sulfide could possibly oxidize titanium and form sulfide corrosion products, however no sulfur was detected on the titanium surface. The explanation for this can be that the standard Gibbs free energy for titanium oxide (TiO2; Anatase) and titanium sulfide (TiS2) is −844.5 and −402.4 kJ/mol respectively. Thus the higher favored TiO2 layer had most likely formed immediately after sulfide attachment. The increase in thickness of the oxide layer causes surface
Table 2 Amounts of titanium release after immersing in sulfide-alkaline solution (μg/cm2). Immersion period (days)
MQS-L MQS-M MQS-H
1
3
7
0.18 ± 0.07 0.28 ± 0.05 0.32 ± 0.02
0.22 ± 0.04 0.19 ± 0.03 0.41 ± 0.05
0.13 ± 0.01 0.16 ± 0.03 0.38 ± 0.04
discoloration of titanium due to light refraction [26]. From this it is likely that the thickness of the oxidized layer affected the degree of titanium discoloration. Corrosion and discoloration of titanium implants removed from the oral environment is observed with failed implants [26]. Sulfide was selected in this study as possible caused of corrosion because oral bacteria that caused protein putrefaction, such as Porphyromonas gingivalis, produced considerable amounts of sulfide byproducts which is the cause of halitosis [22]. Volatile sulfur compounds (VSC) produced by oral bacteria include hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl sulfide ((CH3)2S) and out of these, H2S and CH3SH are produced in the largest amounts [34]. In periodontal pockets of patients with severe periodontal disease, concentrations of VSC may rise locally which could potentially promote titanium corrosion. The concentrations of sulfide solutions used in this study ranged from 400 to 3400 ppm which is much higher but appropriate for an accelerated study to determine whether sulfide triggers titanium corrosion. Methyl mercaptan to hydrogen sulfide ratio increases and methyl mercaptan is more pathogenic in periodontal diseases [34], therefore perhaps future studies should include sulfide in the form of methyl mercaptan or even in vivo experiment involving pathogenic bacteria and their sulfide byproducts. 5. Conclusions Within the limitations addressed, this study demonstrated that sulfide concentration and pH had an influence on the corrosion behavior of titanium at various immersion periods. Corrosion indicators were investigated through visual, micro, and elemental analysis. The conclusions of this study are as follows: 1) Titanium discoloration increased and glossiness decreased as sulfide concentration and immersion period increased in sulfide-alkaline solutions. Corrosion was indicated through considerable surface oxidation suggesting the formation of a thick oxide layer.
Fig 5. Valency peaks of Ti2p spectrum from MP specimens (a). (b) compares fraction of valency of Ti decomposed from Ti2p spectrum at 7 days among various solutions. (c) compares fraction of valency of Ti decomposed from Ti2p spectrum for MQS-L over time.
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2) Titanium release amount in sulfide solutions was not affected by immersion period. 3) The pH of sulfide solutions had a significant effect on titanium corrosion. The degree of titanium corrosion in sulfide-neutral solutions was no different from titanium specimens placed in MQ. Disclosures The authors declare no conflicts of interest. Acknowledgements The authors would like to acknowledge the Cooperative Research and Development Center for Advanced Materials at Tohoku University and the Oral Science Center at Tokyo Dental College for their assistance during XRD and XPS analysis. Financial support was granted by the “Grand-in-Aid for Scientific Research” (contract number: 24792154) from the Ministry of Education, Culture, Sports, Science, and Technology, and also in part by the “Research Achievements of Grant-in-Aid for Encouragement of Scientists” from the Dean at Tokyo Dental College. References [1] R.M. Cibirka, S.K. Nelson, B.R. Lang, F.A. Rueggeberg, Examination of the implant—abutment interface after fatigue testing, J. Prosthet. Dent. 85 (2001) 268–275. [2] C.J. Goodacre, G. Bernal, K. Rungcharassaeng, J.Y.K. Kan, Clinical complications with implants and implant prostheses, J. Prosthet. Dent. 90 (2003) 121–132. [3] C.G. Ribeiro, M.L.C. Maia, S.S. Scherrer, A.C. Cardoso, H.W.A. Wiskott, Resistance of three implant-abutment interfaces to fatigue testing, J. Appl. Oral Sci. 19 (2011) 413–420. [4] N. Wakabayashi, M. Ai, K. Iijima, Y. Takada, O. Okuno, Infrared gold alloy brazing on Titanium and Ti–6Al–4 V Alloy surfaces and its application to removable prosthodontics, J. Prosthodont. 8 (1999) 180–187. [5] R.A. Zavanelli, G.E. Pessanha Henriques, I. Ferreira, J.M. De Almeida Rollo, Corrosionfatigue life of commercially pure titanium and Ti–6Al–4 V alloys in different storage environments, J. Prosthet. Dent. 84 (2000) 274–279. [6] R.R. Wang, Y. Li, In vitro evaluation of biocompatibility of experimental titanium alloys for dental restorations, J. Prosthet. Dent. 80 (1998) 495–500. [7] M. Yoshinari, K. Matsuzaka, T. Inoue, Y. Oda, M. Shimono, Bio-functionalization of titanium surfaces for dental implants, Mater. Trans. 43 (2002) 2494–2501. [8] M.T. Mathew, S. Abbey, N.J. Hallab, D.J. Hall, C. Sukotjo, M.A. Wimmer, Influence of pH on the tribocorrosion behavior of CpTi in the oral environment: synergistic interactions of wear and corrosion, J. Biomed. Mater. Res. B Appl. Biomater. 100 (2012) 1662–1671. [9] M.T. Mathew, E. Ariza, L.A. Rocha, A.C. Fernandes, F. Vaz, TiCxOy thin films for decorative applications: tribocorrosion mechanisms and synergism, Tribol. Int. 41 (2008) 603–615. [10] J. Li, S.J. Li, Y.L. Hao, H.H. Huang, Y. Bai, Y.Q. Hao, et al., Electrochemical and surface analyses of nanostructured Ti–24Nb–4Zr–8Sn alloys in simulated body solution, Acta Biomater. 10 (2014) 2866–2875. [11] A. Sicilia, S. Cuesta, G. Coma, I. Arregui, C. Guisasola, E. Ruiz, et al., Titanium allergy in dental implant patients: a clinical study on 1500 consecutive patients, Clin. Oral Implants Res. 19 (2008) 823–835.
273
[12] Y. Mine, S. Makihira, H. Nikawa, H. Murata, R. Hosokawa, A. Hiyama, et al., Impact of titanium ions on osteoblast-, osteoclast- and gingival epithelial-like cells, J. Prosthodont. Res. 54 (2010) 1–6. [13] T. Noguchi, S. Takemoto, M. Hattori, M. Yoshinari, E. Kawada, Y. Oda, Discoloration and dissolution of titanium and titanium alloys with immersion in peroxide- or fluoride-containing solutions, Dent. Mater. J. 27 (2008) 117–123. [14] G. Mabilleau, S. Bourdon, M.L. Joly-Guillou, R. Filmon, M.F. Baslé, D. Chappard, Influence of fluoride, hydrogen peroxide and lactic acid on the corrosion resistance of commercially pure titanium, Acta Biomater. 2 (2006) 121–129. [15] N. Horasawa, M. Marek, Effect of fluoride from glass ionomer on discoloration and corrosion of titanium, Acta Biomater. 6 (2010) 662–666. [16] X. Li, J. Wang, E.-h. Han, W. Ke, Influence of fluoride and chloride on corrosion behavior of NiTi orthodontic wires, Acta Biomater. 3 (2007) 807–815. [17] M. Cioffi, D. Gilliland, G. Ceccone, R. Chiesa, A. Cigada, Electrochemical release testing of nickel–titanium orthodontic wires in artificial saliva using thin layer activation, Acta Biomater. 1 (2005) 717–724. [18] S. Takemoto, M. Hattori, M. Yoshinari, E. Kawada, Y. Oda, Corrosion behavior and surface characterization of titanium in solution containing fluoride and albumin, Biomaterials 26 (2005) 829–837. [19] M. Nakagawa, S. Matsuya, T. Shiraishi, M. Ohta, Effect of fluoride concentration and pH on corrosion behavior of titanium for dental use, J. Dent. Res. 78 (1999) 1568–1572. [20] M. Hattori, Y. Oda, EQCM analysis of titanium corrosion in peroxide- or fluoridecontaining solutions, Bull. Tokyo. Dent. Coll. 54 (2013) 135–140. [21] M. Koike, H. Fujii, The corrosion resistance of pure titanium in organic acids, Biomaterials 22 (2001) 2931–2936. [22] L. Probster, W. Lin, H. Huttemann, Effect of fluoride prophylactic agents on titanium surfaces, Int. J. Oral. Maxilofac. Implants 7 (1992) 390–394. [23] S. Persson, R. Claesson, J. Carlsson, The capacity of subgingival microbiotas to produce volatile sulfur compounds in human serum, Oral Microbiol. Immunol. 4 (1989) 169–172. [24] Y. Takuma, T. Shiraishi, T. Fujita, K. Hisatsune, Tarnish resistance evaluation of experimental Pd-free Ag–Au–Pt–Cu dental alloys, Dent. Mater. J. 29 (2010) 330–335. [25] Y. Takayama, E. Miura, Y. Hosoi, M. Ishikawa, K. Shibuya, A study on discoloration of Au–Ag–Pd alloy — part 1. Influence of oral microorganism, J. Jpn. Prosthodont. Soc. 46 (2002) 223–232. [26] D. Rodrigues, P. Valderrama, T. Wilson, K. Palmer, A. Thomas, S. Sridhar, et al., Titanium corrosion mechanisms in the oral environment: a retrieval study, Materials 6 (2013) 5258–5274. [27] T. Yoshida, H. Aoki, K. Yoshida, N. Irie, D. Murayama, Color and weight changes of titanium and titanium alloys immersed in various solutions containing sodium sulfide, J. Jpn. Soc. Dent. Prod. 23 (2009) 10–19. [28] H. Suito, Y. Iwawaki, T. Goto, Y. Tomotake, T. Ichikawa, Oral factors affecting titanium elution and corrosion: an in vitro study using simulated body fluid, PLoS One 8 (2013), e66052. [29] H.-M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, Preparation of bioactive Ti and its alloys via simple chemical surface treatment, J. Biomed. Mater. Res. 32 (1996) 409–417. [30] H.-M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, Apatite-forming ability of alkalitreated Ti metal in body environment, J. Ceram. Soc. Jpn. 105 (1997) 111–116. [31] M. Uchida, H.M. Kim, T. Kokubo, S. Fujibayashi, T. Nakamura, Effect of water treatment on the apatite-forming ability of NaOH-treated titanium metal, J. Biomed. Mater. Res. 63 (2002) 522–530. [32] O.I. Radkevych, V.I. Pokhmurs'kyi, Influence of hydrogen sulfide on serviceability of materials of gas field equipment, Mater. Sci. 37 (2001) 319–332. [33] H.J. Mueller, In vitro tarnish and corrosion of a consolidated silver material for direct filling applications, Dent. Mater. 17 (2001) 60–70. [34] K. Yaegaki, K. Sanada, Volatile sulfur compounds in mouth air from clinically healthy subjects and patients with periodontal disease, J. Periodontal Res. 27 (1992) 233–238.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Influence of sulfide concentration on the corrosion behavior of titanium in a simulated oral environment Rino Harada a,b, Shinji Takemoto a,b,⁎, Hideaki Kinoshita a, Masao Yoshinari b, Eiji Kawada a a b
Department of Dental Materials Science, Tokyo Dental College, 2-9-18 Misaki-cho, Chiyoda-ku, 101-0061, Tokyo, Japan Oral Science Center, Tokyo Dental College, 2-9-18 Misaki-cho, Chiyoda-ku, 101-0061, Tokyo, Japan
a r t i c l e
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Article history: Received 20 November 2015 Received in revised form 30 December 2015 Accepted 24 January 2016 Available online 26 January 2016 Keywords: Titanium Corrosion Sulfide X-ray photoelectron spectroscopy Oxide film
a b s t r a c t This study assessed the corrosion behavior of titanium in response to sulfide by determining the effects of sulfide concentration and pH over immersion period. Corrosion was evaluated through changes in color, glossiness, surface characterization, and titanium release. Sulfide solutions were prepared in 3 different concentrations with Na2S, each in pH unadjusted (sulfide-alkaline) and pH adjusted to 7.5 (sulfide-neutral). Titanium discoloration increased and glossiness decreased as sulfide concentration and immersion period increased in sulfide-alkaline solutions. Coral-like complexes were observed on the surface of these specimens, which became more pronounced as concentration increased. Small amounts of titanium release were detected in sulfide-alkaline solutions; however, this was not affected by immersion periods. Corrosion was indicated through considerable surface oxidation suggesting the formation of a thick oxide layer. No significant changes in color and glossiness, or titanium release were indicated for titanium specimens immersed in sulfide-neutral solutions indicating that pH had a significant effect on corrosion. Our findings suggest that a thick oxide layer on the titanium surface was formed in sulfide-alkaline solutions due to excessive oxidation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The use of a titanium dental implant system is common to restore the function of lost teeth. The clinical success of dental implants is dependent on biomechanical factors that determine the integrity of the implant [1–3]. The physical properties of pure titanium or titanium alloys, such as high strength to weight ratio, superior fatigue strength, tensile strength, modulus of elasticity, and cytotoxicity make them very attractive and suitable for the fabrication of any dental prosthesis [4–7]. Titanium is also known for its corrosion resistance, which is a result of the materials' ability to form passive oxide films (TiO2) in normal physiological conditions. However, the nature, composition, and thickness of this protective film depend on environmental conditions [8–10]. Electrochemical corrosion in fluid electrolytes such as saliva can possibly trigger allergic reactions by releasing elements from the alloy [11]. This can also be an issue since titanium ions have a biological effect on the viabilities of osteoblast and osteoclast and on their differentiation which may influence prognosis after implant placement [12]. The dynamicity of the oral cavity makes it essential to determine the causes of material corrosion and subsequent wear. These aspects serve as the foundations to develop materials resistant to corrosion or methods to treat unfavorable conditions. ⁎ Corresponding author at: 2-9-18 Misaki-cho, Chiyoda-ku, 101-0061, Tokyo, Japan. E-mail address: [email protected] (S. Takemoto).
http://dx.doi.org/10.1016/j.msec.2016.01.065 0928-4931/© 2016 Elsevier B.V. All rights reserved.
Literatures have reported that fluoride, peroxide, organic acids, and released-cell products induce titanium corrosion [13–22]. Fluorides found in prophylactic agents, affect the titanium surface by dissolving the passive oxide layer leading to ion dissolution and surface discoloration [19,20]. Peroxides found in denture cleanser affect titanium through surface oxidation in acidic conditions or by inducing dissolution in alkaline solutions [20]. Released-cell products such as lactic acid and formic acid which can be produced by oral bacteria have also shown to affect the corrosion behavior of titanium [14,21]. The biomedical application of titanium is common due to its exceptional properties discussed earlier as well as corrosion resistance, however these papers suggest that titanium may be susceptible to corrosion in the oral environment. Sulfur products are of particular concern in regards to titanium dental implants because periodontopathic bacteria secretes considerable amounts of sulfide byproducts, such as hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl sulfide ((CH3)2S), as a result of their metabolism [23]. Sulfide has been reported to have the potential to cause discoloration in dental alloys. Silver based alloys have shown to tarnish at various degrees in 0.1% sulfide solutions based on chemical composition and microstructure [24]. When silver palladium alloys were placed in cultures containing Fusobacterium nucleatum, discoloration occurred and sulfur was identified on the metal surface [25]. Titanium is known for its corrosion resistance however, Rodrigues et al. retrieved failed implants due to peri-implantitis and reported that titanium dental implants were highly susceptible to pitting attack
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in the oral environment [26]. Yoshida et al. reported discoloration of titanium and titanium alloys in sulfide solutions [27]. These suggest the possibility of sulfides in the oral cavity affecting the corrosion behavior of titanium, however factors which affect the corrosion behavior of titanium in sulfide environments have not yet been studied in detail. The objective of this study is to assess the corrosion behavior of titanium in response to sulfide by determining the effects of sulfide concentration and pH over immersion period. The null hypothesis is that sulfide concentration, pH, and immersion period have no effect on the corrosion behavior of titanium. 2. Materials and methods 2.1. Specimen and solution preparation Commercially available pure titanium disks 1.3 mm thick and 13 mm in diameter were prepared from wrought titanium rods (Grade 2, Tokyo Titanium, Tokyo, Japan). The disks were mirror polished with a polishing machine (Automet 250 and Ecomet, 250 Buehler, Tokyo, Japan) using silicon carbide paper from 320 to 1200 grit, diamond suspension, and colloidal silica suspension according to metallographic procedure. These were ultrasonically washed with acetone and distilled water for 5 min and dried. Ultrapure water (Milli-Q®; Merck Millipore Corporation, Tokyo, Japan: MQ) was used as a representative immersion medium. Sulfide solutions were prepared by adding to 500 mL of 3.12 g, 12.12 g, and 24.00 g of Na2S·9H2O (Wako Pure Chemical Industries, Ltd., Osaka, Japan) to MQ abbreviated as MQS-L, MQS-M, and MQS-H respectively. The pH of sulfide solutions with pH unadjusted (sulfide-alkaline solutions) ranged from 11.6–12.1. Sulfide solutions with pH adjusted to 7.5 using hydrochloric acid (Wako Pure Chemical Industries, Ltd., Osaka, Japan), denoted as -AD (sulfide-neutral solutions), were also prepared. Titanium disks were immersed in 20 mL of solution in a polypropylene bottle and sealed to prevent evaporation and stored at 37 °C for 1, 3, and 7 days. All assays were performed using triplicate samples of each solution in at least 2 different experiments. 2.2. Color and glossiness measurements Initial color and glossiness of the titanium specimens were measured using a colorimeter (MCR-A Luck Office, Tokyo, Japan) and glossmeter (GM-26D, Murakami Color Research Laboratory, Tokyo, Japan). Color difference (ΔE * ab) of titanium specimens were calculated for each condition at 1, 3, and 7 days by measuring CIE L*a*b* color coordinates against reflected light. The colorimeter was calibrated with a white calibration tile before being used. The ΔE * ab was determined using the following equation: h i1=2 2 2 ΔE ab ¼ ðL −L0 Þ þ ða −a0 Þ2 þ ðb −b0 Þ where L0*, a0*, b0* were values before immersion, while L*, a*, b* were values after immersion. For glossiness, incidence light angle of 20° from the surface normal was selected and reflected light was measured using a detector. The glossmeter was calibrated with a black tile with a standard reference value of 86.7% for Gs(20°). 2.3. Surface characterization Surface texture of specimens was observed under a field emission scanning electron microscope (FE-SEM SU6600; Hitachi, Tokyo, Japan) operating at a voltage of 15 kV. The specimens were subsequently coated with carbon using a carbon coater (VC-100S, Vacuum Device, Ibaraki, Japan) before observation.
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Crystal structure on titanium specimens were investigated by X-ray diffraction (XRD) using CuKα radiation operating at a voltage of 40 kV and current of 30 mA. Two theta diffraction angle were set between 20–70° with an incremental angle of 5° per minute using a diffractometer (Ultima IV, Rigaku, Tokyo, Japan) with a semiconductor detector. Thin film XRD (TF-XRD) pattern was also analyzed to determine surface at an incident angle of 1° at a 2θ diffraction angle of 10–50° and incremental angle of 0.5° per minute (Smartlab®, Rigaku, Tokyo, Japan). The chemical composition and binding state of the titanium specimens were analyzed by X-ray photoelectron spectroscopy (XPS); (AxisUltra, Kratos-Shimadzu, Kyoto, Japan). The X-ray resource was monochronized AlKα electrode at 15 kV, 10 mA emission current. The area of measurement was 200 × 600 μm, and take-off angle of photoelectrons was 45°. All XPS spectra were calibrated to the standard carbon C1s hydrocarbon peak at around 285.0 eV. The quantitative analysis and the curve fitting were conducted by the CasaXPS software (Casa Software Ltd., Teignmouth, UK). 2.4. Released element analysis in solution Titanium release into the test solutions was determined using an inductively coupled plasma optical emission spectroscopy (ICP-OES, Vista-MPX, SII, Japan). Release values were calculated by using the concentration (ppm or mg/L) to derive the total amount released titanium in the bottle then dividing the amount by the exposed area (μg/cm2). The detection limit of titanium release from the specimens used in this study was above 0.01 μg/cm2. 2.5. Statistical analysis Data analysis was performed using one-way analysis of variance (ANOVA) and Tukey's HSD multiple comparison tests using software (Excel Tokei, 2010, SSRI, Tokyo, Japan) to compare ΔE * ab, glossiness, and titanium ion release with immersion period. Color difference with respect to immersion period, sulfide concentration, pH, and titanium ion release were statistically analyzed with two-way ANOVA. The influence of pH was compared for each sulfide concentration at constant immersion periods. Statistical significance was set at α = 0.05. Triplicate samples were used in the experiments to ensure the reproducibility of the results.
3. Results 3.1. Color and glossiness change Fig. 1 displays the ΔE * ab of titanium specimens immersed in various sulfide solutions over immersion period. For sulfide-alkaline solutions, Δ E * ab value was significantly higher in order of MQ, MQS-L, MQS-M, and MQS-H at each immersion period. For each sulfidealkaline solution, Δ E * ab value demonstrated significant increase on day 1 and 3, however no further changes were indicated after day 3. In contrast, no significant differences in Δ E * ab were found between titanium specimens immersed in MQ and all sulfide-neutral solutions (− AD), and no significant changes in Δ E * ab were indicated with increasing immersion period. Fig. 2 shows the change in glossiness of titanium specimens immersed in various sulfide solutions over immersion period. Glossiness decrease was greater in order of MQ, MQS-L, MQS-M, and MQS-H at all immersion periods, however no significant differences were found between MQ and MQS-L at all immersion periods. Glossiness decreased with time by day 1 and 3, however no significant changes were indicated after day 3 for all sulfide-alkaline solutions. No significant changes in glossiness were found among MQ and all sulfide-neutral solutions with increasing immersion periods.
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Fig 1. Color change (ΔE * ab) of titanium in sulfide solutions over immersion period (a) and at 7 days (b) (p b 0.05).
3.2. Surface characterization Fig. 3 shows surface morphology of the titanium immersed in various sulfide solutions with different sulfide concentration and pH for 7 days. MQ has smooth morphology while coral-like complexes are observed on the sulfide-alkaline solutions and these complexes become more pronounced as concentration increased. Surface textures were similar among titanium specimens immersed in MQ and sulfide-neutral solutions. There were no signs of any localized corrosion such as pitting or micro cracks on any of the specimens. Typical XRD patterns of the specimens placed in the sulfide-alkaline solutions are shown in Fig. 4(a). The diffraction peaks of mirror polished titanium (MP) had 2θ values at 35.0, 38.4, 40.1, 52.9 and 62.9 which were consistent with the peaks of titanium substrate. Only strict titanium peaks were seen on all patterns indicating no formation of crystalized corrosion byproducts. Also titanium was the only peak seen on the TFXRD pattern (Fig. 4(b)) as well indicating no formation of corrosion products through XRD analysis. Table 1 displays the composition of the outermost surface of titanium specimens after immersion in Na2S solution through XPS analysis. The presence of elements on the titanium surface was deduced to mainly Ti, O, and C with traces of N, F, Na, Ca, and Si. C and N were derived from contaminants and the small amounts of Si, Ca, and F are most likely remain from the polishing procedure and/or traces that were in the test solutions (less than 2 atm%). The surface mostly consisted of titanium and oxygen. Sulfur contained in the immersion solution was not detected on the titanium surface.
In Fig. 5(a), Ti2p spectra consists of Ti2p3/2 and Ti2p1/2 where the doublet originated from. Ti2p3/2 and Ti2p1/2 were decomposed to metal (T0), divalent (Ti2 +), trivalent (Ti3 +), and tetravalent (Ti4 +) forms of Ti. The XPS analyses of titanium after immersion in sulfidealkaline solution revealed the presence of mostly Ti4+. Fig. 5(b) shows the fraction of Ti0/Ti4+ after immersing in sulfide solutions for 7 days. The Ti0/Ti4+ ratio demonstrates an obvious decrease in Ti0/Ti4+ ratio with increasing concentration in sulfide-alkaline solutions. In Fig. 5(c), the fraction of Ti0/Ti4+ after immersing in MQS-L for 1, 3, and 7 days is displayed, and a decrease in ratio with increasing immersion period is observed. The Ti0/Ti4 + ratio did not change as much in sulfideneutral solutions. This indicates the acceleration of oxidation in higher concentrations of sulfide-alkaline solutions, and progression of oxidation as immersion period increases. Titanium immersed in MQ and sulfide-neutral solutions showed no decrease in Ti0/Ti4+ ratio indicating no progression in corrosion. 3.3. Titanium release Table 2 lists the amount of titanium released in the sulfide-alkaline solutions determined through ICP-OES analyses. Titanium release was seen in solutions and released titanium did not significantly differ with immersion period. The maximum amount of released titanium over a span of 7 days was 0.41 μg/cm2. Sulfide concentration had a significant influence on titanium release in that titanium release at 7 days in MQS-H was significantly higher than MQS-L and MQS-M. The amount of titanium release in MQ and sulfide-neutral solutions was under detectable limits at all immersion periods.
Fig 2. Glossiness (Gs(20°)) values of titanium in sulfide solutions over immersion period (a) and at 7 days (b) (p b 0.05).
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Fig 3. SEM observation of titanium surface after immersing in MQ and sulfide solutions for 7 days.
4. Discussion In this study, titanium corrosion occurred in sulfide-alkaline solutions depending on sulfide concentrations, therefore the results of this study reject the null hypothesis that sulfide concentration and pH have no influence on the corrosion behavior of titanium. In dentistry, some studies have reported that titanium was corroded by substances such as fluoride in prophylactic agent, peroxides in denture cleanser, and organic acids, which suggests that titanium could be sensitive to corrosion in the oral environment [14,27]. In-vitro studies have reported that fluoride causes titanium corrosion by dissolving the passive oxide layer causing ion dissolution and surface discoloration due to an increase in surface roughness [13,18,19,20,22]. In acidic conditions, peroxides induce oxidation at the titanium surface by forming insoluble complex compounds leading to considerable change in color [20]. In alkaline solutions, peroxides induce dissolution and the formation of a soluble complex leading to subsequent loss in mass and the formation of a thick oxide film causing a significant change in color [20]. Titanium release in fluoride and peroxide in acidic environments was approximately 70 μg/cm2 in 3 days and 0.3 μg/cm2 in 7 days respectively [13,18]. These studies indicate that corrosion behavior of titanium in sulfide-alkaline solutions is similar to peroxide in acidic environments in that it involves a minor release of titanium and discoloration. Titanium release occurred immediately after immersion in sulfide solutions at
a rate which did not increase with immersion period, whereas discoloration progressed with immersion indicating that the cause of titanium corrosion was most likely not due to titanium release. In this study, the corrosion of titanium immersed in sulfide-alkaline solution was also visually demonstrated through discoloration and glossiness change. Discoloration of titanium in alkaline solutions containing high concentration of sulfide to our study was similar to a study done by Yoshida et al. who reported that color changes for various titanium alloys immersed in solutions containing Na2S, according to dynamic corrosion test in ISO 10271 [27]. Our finding suggests the discoloration of titanium in sulfide alkaline solution depends on sulfide concentration. Although titanium changed in color and dissolved in sulfide-alkaline solutions, no discoloration and release of titanium was detected in sulfide-neutral solutions. Suito et al. performed a titanium immersion test in simulated body fluid of varying pH and demonstrated that eluted titanium ions were detected after 48 h at a pH of 9 however for pH 7.4, even after 4 weeks eluted titanium ions were not detected [28]. These results are similar to ours where titanium release levels in neutral solutions were under detectable values, therefore this raises the possibility of pH having an effect on titanium release. The SEM images as shown in Fig. 2 suggested the formation of corrosion byproducts on the titanium surface however, XRD showed only titanium substrate peaks on all specimens and XPS showed no sulfur
Fig 4. XRD analysis of MP and titanium immersed in sulfide-alkaline solutions (a). Thin film XRD analysis of MP and titanium immersed in 0.1 M sulfide-alkaline solution (MQS-H) for 7 days at 1° incident angle (b).
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Table 1 Composition in outermost surfaces of titanium specimens after immersing in Na2S solution.
MP MQ7 MQS-L1 MQS-L3 MQS-L7 MQS-M1 MQS-M7 MQS-H1 MQS-H7 MQS-L7-AD MQS-M7-AD MQS-H7-AD
Ti
O
C
N
Other
21.2 21.2 19.1 18.0 18.1 17.8 20.5 18.8 21.0 22.8 22.1 16.0
48.1 47.2 47.2 43.4 47.1 44.6 50.2 47.4 51.5 51.3 49.7 48.1
27.7 30.2 30.1 36.0 31.1 34.4 25.7 34.4 24.3 23.6 26.5 34.0
2.2 1.0 1.2 0.8 1.4 2.1 1.5 1.2 1.4 2.1 1.4 0.9
F Ca, F Na, Ca, F, Si Na, Ca, F, Si Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca Na, Ca, F, Si
on the titanium surface. Kim et al. reported that when titanium and titanium alloys are treated with a highly concentrated sodium hydroxide aqueous solution, TF-XRD revealed the formation of an amorphous or microcrystalline sodium titanate hydrogel layer on their surface [29–31]. In this study titanium disks were placed in sulfide solutions that were alkaline condition (approximately pH 12) however, no sodium titanate peaks nor broad peaks of the hydrogel were found in the XRD patterns of the specimens. This indicates titanium surface oxidizes in sulfide-alkaline solutions, however only to a small degree. XPS analysis showed a decrease in Ti0/Ti4+ with increasing concentration and immersion period indicating oxidation of the titanium surface suggesting that discoloration in sulfide-alkaline solutions was caused by the formation of an oxide layer (TiO2) which was much thicker than the passive film. Sulfide forms are dependent on pH and a majority comes in the form of H2S (pH b 7.5), HS− (7.5 b pH b 13), and S2− (pH N 13) [32]. In this experiment, pH of around 7.5 indicates H2S and HS− to be prominent, however in the sulfide-alkaline solutions where the pH is around 12, HS− predominantly exists with some S2− also present. Sulfide gets reduced after it acts as an oxidizing agent. In alkaline solutions, HS− is easily reduced to S2−, therefore titanium oxidation was most likely accelerated in sulfide-alkaline solutions. Mueller reported that consolidated silver was susceptible to sulfide tarnishing and corrosion product on the silver surface was identified to be Ag2S [33]. In a similar manner, sulfide could possibly oxidize titanium and form sulfide corrosion products, however no sulfur was detected on the titanium surface. The explanation for this can be that the standard Gibbs free energy for titanium oxide (TiO2; Anatase) and titanium sulfide (TiS2) is −844.5 and −402.4 kJ/mol respectively. Thus the higher favored TiO2 layer had most likely formed immediately after sulfide attachment. The increase in thickness of the oxide layer causes surface
Table 2 Amounts of titanium release after immersing in sulfide-alkaline solution (μg/cm2). Immersion period (days)
MQS-L MQS-M MQS-H
1
3
7
0.18 ± 0.07 0.28 ± 0.05 0.32 ± 0.02
0.22 ± 0.04 0.19 ± 0.03 0.41 ± 0.05
0.13 ± 0.01 0.16 ± 0.03 0.38 ± 0.04
discoloration of titanium due to light refraction [26]. From this it is likely that the thickness of the oxidized layer affected the degree of titanium discoloration. Corrosion and discoloration of titanium implants removed from the oral environment is observed with failed implants [26]. Sulfide was selected in this study as possible caused of corrosion because oral bacteria that caused protein putrefaction, such as Porphyromonas gingivalis, produced considerable amounts of sulfide byproducts which is the cause of halitosis [22]. Volatile sulfur compounds (VSC) produced by oral bacteria include hydrogen sulfide (H2S), methyl mercaptan (CH3SH), and dimethyl sulfide ((CH3)2S) and out of these, H2S and CH3SH are produced in the largest amounts [34]. In periodontal pockets of patients with severe periodontal disease, concentrations of VSC may rise locally which could potentially promote titanium corrosion. The concentrations of sulfide solutions used in this study ranged from 400 to 3400 ppm which is much higher but appropriate for an accelerated study to determine whether sulfide triggers titanium corrosion. Methyl mercaptan to hydrogen sulfide ratio increases and methyl mercaptan is more pathogenic in periodontal diseases [34], therefore perhaps future studies should include sulfide in the form of methyl mercaptan or even in vivo experiment involving pathogenic bacteria and their sulfide byproducts. 5. Conclusions Within the limitations addressed, this study demonstrated that sulfide concentration and pH had an influence on the corrosion behavior of titanium at various immersion periods. Corrosion indicators were investigated through visual, micro, and elemental analysis. The conclusions of this study are as follows: 1) Titanium discoloration increased and glossiness decreased as sulfide concentration and immersion period increased in sulfide-alkaline solutions. Corrosion was indicated through considerable surface oxidation suggesting the formation of a thick oxide layer.
Fig 5. Valency peaks of Ti2p spectrum from MP specimens (a). (b) compares fraction of valency of Ti decomposed from Ti2p spectrum at 7 days among various solutions. (c) compares fraction of valency of Ti decomposed from Ti2p spectrum for MQS-L over time.
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2) Titanium release amount in sulfide solutions was not affected by immersion period. 3) The pH of sulfide solutions had a significant effect on titanium corrosion. The degree of titanium corrosion in sulfide-neutral solutions was no different from titanium specimens placed in MQ. Disclosures The authors declare no conflicts of interest. Acknowledgements The authors would like to acknowledge the Cooperative Research and Development Center for Advanced Materials at Tohoku University and the Oral Science Center at Tokyo Dental College for their assistance during XRD and XPS analysis. Financial support was granted by the “Grand-in-Aid for Scientific Research” (contract number: 24792154) from the Ministry of Education, Culture, Sports, Science, and Technology, and also in part by the “Research Achievements of Grant-in-Aid for Encouragement of Scientists” from the Dean at Tokyo Dental College. References [1] R.M. Cibirka, S.K. Nelson, B.R. Lang, F.A. Rueggeberg, Examination of the implant—abutment interface after fatigue testing, J. Prosthet. Dent. 85 (2001) 268–275. [2] C.J. Goodacre, G. Bernal, K. Rungcharassaeng, J.Y.K. Kan, Clinical complications with implants and implant prostheses, J. Prosthet. Dent. 90 (2003) 121–132. [3] C.G. Ribeiro, M.L.C. Maia, S.S. Scherrer, A.C. Cardoso, H.W.A. Wiskott, Resistance of three implant-abutment interfaces to fatigue testing, J. Appl. Oral Sci. 19 (2011) 413–420. [4] N. Wakabayashi, M. Ai, K. Iijima, Y. Takada, O. Okuno, Infrared gold alloy brazing on Titanium and Ti–6Al–4 V Alloy surfaces and its application to removable prosthodontics, J. Prosthodont. 8 (1999) 180–187. [5] R.A. Zavanelli, G.E. Pessanha Henriques, I. Ferreira, J.M. De Almeida Rollo, Corrosionfatigue life of commercially pure titanium and Ti–6Al–4 V alloys in different storage environments, J. Prosthet. Dent. 84 (2000) 274–279. [6] R.R. Wang, Y. Li, In vitro evaluation of biocompatibility of experimental titanium alloys for dental restorations, J. Prosthet. Dent. 80 (1998) 495–500. [7] M. Yoshinari, K. Matsuzaka, T. Inoue, Y. Oda, M. Shimono, Bio-functionalization of titanium surfaces for dental implants, Mater. Trans. 43 (2002) 2494–2501. [8] M.T. Mathew, S. Abbey, N.J. Hallab, D.J. Hall, C. Sukotjo, M.A. Wimmer, Influence of pH on the tribocorrosion behavior of CpTi in the oral environment: synergistic interactions of wear and corrosion, J. Biomed. Mater. Res. B Appl. Biomater. 100 (2012) 1662–1671. [9] M.T. Mathew, E. Ariza, L.A. Rocha, A.C. Fernandes, F. Vaz, TiCxOy thin films for decorative applications: tribocorrosion mechanisms and synergism, Tribol. Int. 41 (2008) 603–615. [10] J. Li, S.J. Li, Y.L. Hao, H.H. Huang, Y. Bai, Y.Q. Hao, et al., Electrochemical and surface analyses of nanostructured Ti–24Nb–4Zr–8Sn alloys in simulated body solution, Acta Biomater. 10 (2014) 2866–2875. [11] A. Sicilia, S. Cuesta, G. Coma, I. Arregui, C. Guisasola, E. Ruiz, et al., Titanium allergy in dental implant patients: a clinical study on 1500 consecutive patients, Clin. Oral Implants Res. 19 (2008) 823–835.
273
[12] Y. Mine, S. Makihira, H. Nikawa, H. Murata, R. Hosokawa, A. Hiyama, et al., Impact of titanium ions on osteoblast-, osteoclast- and gingival epithelial-like cells, J. Prosthodont. Res. 54 (2010) 1–6. [13] T. Noguchi, S. Takemoto, M. Hattori, M. Yoshinari, E. Kawada, Y. Oda, Discoloration and dissolution of titanium and titanium alloys with immersion in peroxide- or fluoride-containing solutions, Dent. Mater. J. 27 (2008) 117–123. [14] G. Mabilleau, S. Bourdon, M.L. Joly-Guillou, R. Filmon, M.F. Baslé, D. Chappard, Influence of fluoride, hydrogen peroxide and lactic acid on the corrosion resistance of commercially pure titanium, Acta Biomater. 2 (2006) 121–129. [15] N. Horasawa, M. Marek, Effect of fluoride from glass ionomer on discoloration and corrosion of titanium, Acta Biomater. 6 (2010) 662–666. [16] X. Li, J. Wang, E.-h. Han, W. Ke, Influence of fluoride and chloride on corrosion behavior of NiTi orthodontic wires, Acta Biomater. 3 (2007) 807–815. [17] M. Cioffi, D. Gilliland, G. Ceccone, R. Chiesa, A. Cigada, Electrochemical release testing of nickel–titanium orthodontic wires in artificial saliva using thin layer activation, Acta Biomater. 1 (2005) 717–724. [18] S. Takemoto, M. Hattori, M. Yoshinari, E. Kawada, Y. Oda, Corrosion behavior and surface characterization of titanium in solution containing fluoride and albumin, Biomaterials 26 (2005) 829–837. [19] M. Nakagawa, S. Matsuya, T. Shiraishi, M. Ohta, Effect of fluoride concentration and pH on corrosion behavior of titanium for dental use, J. Dent. Res. 78 (1999) 1568–1572. [20] M. Hattori, Y. Oda, EQCM analysis of titanium corrosion in peroxide- or fluoridecontaining solutions, Bull. Tokyo. Dent. Coll. 54 (2013) 135–140. [21] M. Koike, H. Fujii, The corrosion resistance of pure titanium in organic acids, Biomaterials 22 (2001) 2931–2936. [22] L. Probster, W. Lin, H. Huttemann, Effect of fluoride prophylactic agents on titanium surfaces, Int. J. Oral. Maxilofac. Implants 7 (1992) 390–394. [23] S. Persson, R. Claesson, J. Carlsson, The capacity of subgingival microbiotas to produce volatile sulfur compounds in human serum, Oral Microbiol. Immunol. 4 (1989) 169–172. [24] Y. Takuma, T. Shiraishi, T. Fujita, K. Hisatsune, Tarnish resistance evaluation of experimental Pd-free Ag–Au–Pt–Cu dental alloys, Dent. Mater. J. 29 (2010) 330–335. [25] Y. Takayama, E. Miura, Y. Hosoi, M. Ishikawa, K. Shibuya, A study on discoloration of Au–Ag–Pd alloy — part 1. Influence of oral microorganism, J. Jpn. Prosthodont. Soc. 46 (2002) 223–232. [26] D. Rodrigues, P. Valderrama, T. Wilson, K. Palmer, A. Thomas, S. Sridhar, et al., Titanium corrosion mechanisms in the oral environment: a retrieval study, Materials 6 (2013) 5258–5274. [27] T. Yoshida, H. Aoki, K. Yoshida, N. Irie, D. Murayama, Color and weight changes of titanium and titanium alloys immersed in various solutions containing sodium sulfide, J. Jpn. Soc. Dent. Prod. 23 (2009) 10–19. [28] H. Suito, Y. Iwawaki, T. Goto, Y. Tomotake, T. Ichikawa, Oral factors affecting titanium elution and corrosion: an in vitro study using simulated body fluid, PLoS One 8 (2013), e66052. [29] H.-M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, Preparation of bioactive Ti and its alloys via simple chemical surface treatment, J. Biomed. Mater. Res. 32 (1996) 409–417. [30] H.-M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, Apatite-forming ability of alkalitreated Ti metal in body environment, J. Ceram. Soc. Jpn. 105 (1997) 111–116. [31] M. Uchida, H.M. Kim, T. Kokubo, S. Fujibayashi, T. Nakamura, Effect of water treatment on the apatite-forming ability of NaOH-treated titanium metal, J. Biomed. Mater. Res. 63 (2002) 522–530. [32] O.I. Radkevych, V.I. Pokhmurs'kyi, Influence of hydrogen sulfide on serviceability of materials of gas field equipment, Mater. Sci. 37 (2001) 319–332. [33] H.J. Mueller, In vitro tarnish and corrosion of a consolidated silver material for direct filling applications, Dent. Mater. 17 (2001) 60–70. [34] K. Yaegaki, K. Sanada, Volatile sulfur compounds in mouth air from clinically healthy subjects and patients with periodontal disease, J. Periodontal Res. 27 (1992) 233–238.