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A preliminary study of TiO2 deposition on NiTi by a hydrothermal method
Surface & Coatings Technology 187 (2004) 26 – 32 www.elsevier.com/locate/surfcoat
A preliminary study of TiO2 deposition on NiTi by a hydrothermal method F.T. Cheng a,*, P. Shi a,b, H.C. Man c a
Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR China b Department of Materials and Chemical Engineering, Liaoning Institute of Technology, PR China c Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China Received 30 June 2003; accepted in revised form 16 January 2004 Available online 27 March 2004
Abstract A simple low-temperature method was employed in depositing an oxide coating on NiTi for enhancing corrosion resistance. Mechanically polished NiTi samples were pretreated in a solution containing Ti4+ ions at 60 jC for 8 h, followed by hydrothermal treatment at 140 jC for 10 h. After hydrothermal treatment, scanning electron microscopy (SEM) indicated the presence of an oxide film of approximately 200 nm on the NiTi substrate. Composition depth profiling of the oxide film by X-ray photoelectron spectroscopy (XPS) revealed that the oxide was TiO2. Thin-film X-ray diffractometry (TF-XRD) confirmed that anatase was the only crystalline phase present in the oxide film. Atomic force microscopy (AFM) recorded a mean surface roughness of 7.3 nm for the coated surface, which was of the same order as that of the polished surface (10.3 nm). Characterization of the corrosion behaviors in Hank’s solution at 37 jC using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique showed that the coated samples had a significantly higher corrosion resistance than samples prepared by mechanical polishing and chemical cleaning. The deposition method employed in the present study meets the important requirements in the surface modification of NiTi implants: (a) the substrate material should not become part of the treated layer in order to minimize the Ni content in the surface; (b) the treatment temperature should be low, preferably not exceeding 300 – 400 jC, so as not to disrupt the thermomechanical properties of the NiTi implants; (c) the process should not be a line-of-sight one to ensure a uniformly modified surface layer for implants of complex geometry. D 2004 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; TiO2; Coating; NiTi; Corrosion
1. Introduction In the past two decades, NiTi, a near-equiatomic intermetallic compound, has emerged as a new type of implant material by virtue of its distinctive thermomechanical and mechanical properties, namely, shape memory effect, superelasticity and high damping capacity [1]. The material of a medical implant, irrespective of its function, must be corrosion-resistant and biocompatible so as to ensure long service life and to minimize its adverse biological effect on the human body. Conflicting views regarding the corrosion resistance of NiTi, however, have been reported in the literature [2,3]. The controversy over the corrosion resis-
* Corresponding author. Tel.: +852-2766-5691; fax: +852-2333-7629. E-mail address: [email protected] (F.T. Cheng). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.023
tance of NiTi may be attributed partly to the surface conditions of the samples under test [3], and partly to the difference in performance requirements in different fields. In non-medical applications, the corrosion resistance of NiTi is usually regarded as superior [4]. In fact, attempts to employ NiTi as a cladding material to enhance cavitation erosion and/or corrosion resistance have been reported [[5,6] and references therein]. However, views regarding the corrosion resistance of NiTi in medical applications are more conservative [2,3], mainly because NiTi has a high content of Ni, an element which is known to be allergenic and toxic when present at a sufficiently high level. Since the naturally formed oxide film is rather thin (of the order of a few nm) and usually defective, passivation treatment of NiTi implants is necessary to thicken and to improve the quality of the oxide film so as to reduce the corrosion rate to a very low value. Among the studies reporting various types of surface modification for NiTi implants to enhance the
F.T. Cheng et al. / Surface & Coatings Technology 187 (2004) 26–32
surface properties, a significant portion has been devoted to treatment techniques for improving corrosion resistance. These include chemical passivation, anodic oxidation, electropolishing, thermal oxidation [7 –10], laser surface melting [11,12], nitriding [13], and plasma ion implantation [14 – 16], and different degrees of improvement in corrosion resistance have been achieved. In view of the high Ni content in NiTi, the sensitivity of NiTi to heat treatment, and the possible irregular shapes of implants, treatment processes for NiTi should ideally meet certain requirements. The most important ones include: (a) the substrate material (i.e. NiTi) should not become part of the treated layer in order to minimize the Ni content in the surface; (b) the treatment temperature should be low, preferably not exceeding 300 –400 jC, so as not to disrupt the thermomechanical properties of the implant; (c) the process should not be a line-of-sight one to ensure a uniformly modified surface layer on implants of irregular shapes. It is clear that most of the treatment methods listed above do not satisfy, one way or the other, these requirements. For example, anodization inevitably incorporates Ni in the surface layer since both Ti and Ni dissolve simultaneously during anodization, while other processes such as thermal oxidation and nitriding involves a temperature which is well above 400 jC. Laser surface treatment, though versatile and efficient, is both a high-temperature and a line-of-sight process. The present study is a preliminary attempt to deposit a titanium oxide coating on NiTi samples by hydrothermal treatment, a process which meets the three requirements stated above. The oxide coating was characterized by various techniques and the corrosion resistance of the coated samples in Hank’s solution was assessed.
2. Experimental details
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to lie in the range 6 – 7. NiTi samples were fixed at the bottom of a Teflon container, which was then filled with the Ti4+ aqueous solution until it was 70% full. The Teflon container was placed in a stainless steel cell for the hydrothermal process. The cell was kept at a temperature of 60 jC for 8 h and then at 140 jC for 10 h [19]. After the coating process, the samples were ultrasonically cleaned with deionized water and dried in an air stream before film characterization. For comparison in corrosion behaviors, NiTi samples polished in a similar manner were chemically cleaned by immersion in an acid solution containing 1 part of HF acid and 10 parts of HNO3 acid at 49 jC according to ASTM Standard B600-91 [20]. 2.2. Morphological study The surface and the cross-section of the coated samples were studied by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6335F) and the coating thickness was determined. The surface roughness of the coating was determined by atomic force microscopy (AFM) (NanoScope IV, Digital Instruments). 2.3. Composition and phase analysis Composition depth profiling of the coated NiTi samples was achieved using X-ray photoelectron spectroscopy (XPS) (Quantum 2000, Physical Electronics) with 1 keV Ar+ beam for sputtering, the radiation source being monochromated Al Ka (1486.6 eV). The crystalline phase in the surface was identified by thin-film X-ray diffractometry (TF-XRD) (Bruker D8 Discover), with Cu Ka radiation (40 kV, 40 mA) at a glancing angle h of 3j (for the surface oxide layer) and 15j (for a thicker layer to include the substrate material beneath), while the angle 2h was scanned from 20 to 90j.
2.1. Sample preparation 2.4. Assessment of corrosion resistance NiTi (50.8 at.% Ni) samples of 1313 mm were spark cut from NiTi plates of thickness 2 mm. The samples were mechanically polished successively with SiC papers of grit 200, 400, 800, 1200 and 1-Am diamond paste. The samples were then ultrasonically cleaned with acetone and deionized water, and dried in air before coating. The coating process consisted of two steps: (1) preparation of a solution containing Ti ions by dissolving Ti powder in an ammoniacal solution of H2O2; and (2) hydrothermal treatment of the samples in the solution prepared in step (1) [17 – 19]. For step (1), Ti powder of particle size f40 Am was added to a 30% H2O2 solution, and NH3 gas was slowly passed into the solution. After all the Ti powder had dissolved, the solution was filtered. The excess H2O2 and NH3 in the filtrate were driven off over a steam cone. The solution was diluted with deionized water to a concentration of 0.6 M Ti4+, and the pH value was adjusted
Samples for corrosion study were mounted in epoxy to expose a surface area of 1 cm2. Corrosion studies using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique were both performed in Hank’s solution (composition given in Table 1) at 37 jC in a three-electrode cell with a pair of graphite rods as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The open-circuit potential Eoc was monitored until a steady value was reached. AC impedance measurements were made with an amplitude of 7 mV about Eoc from 100 kHz down to 3 mHz using an impedance analyzer (EG & G Model 6310). Potentiodynamic polarization measurements were performed conforming to ASTM Standard G5-94 [21] with a potentiostat (EG&G PARC 273). The polarization scan started at 200 mV below Eoc and continued in the anodic
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Table 1 Chemical composition of Hank’s solution
follows. Titanium powder was first oxidized by H2O2 to form Ti4+ ions, the most stable oxidation state of titanium,
Component
Concentration (g/l)
NaCl CaCl2 KCl NaHCO3 Glucose MgCl2 6H2O Na2HPO4 2H2O KH2PO4 MgSO4 7H2O
8.0 0.14 0.4 0.35 1.0 0.1 0.06 0.06 0.06
direction with a potential sweep rate of 0.6 V/h (0.167 mV/s) until 1600 mV SCE.
TiðsÞ þ 2H2 O2 ðlÞ! Ti4þ ðaqÞ þ 4OH ðaqÞ
ð1Þ
According to Chen et al. [19], pretreatment of the samples in a solution containing Ti ions at 60 jC resulted in the nucleation of titania followed by formation of an amorphous film on the substrate, while subsequent treatment at 140 jC crystallized the film. The reactions for the formation of titania, excluding the route involving TiO42 ions, might probably proceed as follows. Under basic conditions, a hydroxo – aquo complex [Ti2(OH)2(OH2)x]6+ was formed by hydrolysis and condensation [24,25], 2Ti4þ ðaqÞ þ 2OH ðaqÞ þ xH2 OðlÞ
3. Results and discussion
!½Ti2 ðOHÞ2 ðOH2 Þx 6þ ðaqÞ
3.1. Coating characteristics After the hydrothermal treatment process, the surface of the NiTi samples appeared pale yellow in contrast to the grayish color of the untreated ones. Fig. 1 shows the TFXRD patterns of a coated sample at glancing angles of 3 and 15j, which reveal the phases present in the surface layer and those in a thicker layer. The patterns indicate that the substrate was composed of the NiTi B2 (austenite) phase [22] while the coating consisted of TiO2 (anatase) [23]. It was proposed [18] that TiO42 ions were formed by dissolving Ti powder in an ammoniacal solution of H2O2, and titania was formed via TiO42. However, the existence of this species is doubtful as it would mean an oxidation state of +6 for Ti. A more probable route for the formation of titania is as
ð2Þ
The dimeric species then underwent further condensation to form titania: ½Ti2 ðOHÞ2 ðOH2 Þx 6þ ðaqÞ þ 6OH ðaqÞ! TiO2 nH2 OðgelÞ þ ð4 þ xnÞH2 OðlÞ TiO2 nH2 OðgelÞ! TiO2 ðsÞ þ nH2 OðlÞ
ð3Þ ð4Þ
The SEM micrographs showing the surface and the crosssection of a coated sample are shown in Fig. 2a,b, respectively. It can be observed from Fig. 2b that a uniform coating of approximately 200 nm and without cracks or pores was deposited on the substrate (a dirt particle appear-
Fig. 1. Thin-film XRD spectra of coated NiTi at glancing angles of 3 and 15j.
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Fig. 2. SEM micrographs of oxide-coated NiTi sample, (a) surface (b) cross-section.
ing on the surface was deliberately included to show sharp boundaries for thickness measurement). The coating was thicker than that reported by Chen et al. [19] in depositing titania film on Si. According to Chen et al. [19], the thickness of the coating was determined by the concentration of Ti ions and the pretreatment time, and the thicker film in the present case probably resulted from a longer pretreatment time. The AFM images of a mechanically polished and a coated sample are shown in Fig. 3a,b, respectively. The mean surface roughness Ra for the coating was 7.3 nm, which was comparable to that for the mechanically polished sample (10.3 nm). The relatively low temperature involved in hydrothermal treatments (140 jC in the present case) is especially important for NiTi as the built-in thermomechanical properties would not be disrupted. Ceramic coating fabricated at a low temperature has an additional advantage compared with other processes with respect to film integrity. For example, fabrication of sol –gel derived films usually requires heat
treatment at a much higher temperature for crystallization, typically above 500 jC [26], and cracks are not uncommon [27]. The XPS depth profile of coated NiTi is shown in Fig. 4. Except for the presence of some carbon, a common contaminant, at the surface, the film was essentially TiO2. Ni was almost absent in the film except near the interface between the film and the substrate. Peaks corresponding to elemental Ni or its oxides (peaks in the range 850 –860 eV) [9,28] were absent in the XPS surface survey scan of the coated sample (Fig. 5) because the oxide film in the present case was an add-on layer without involving the substrate material. On the contrary, Ni is commonly reported to be present on the surface of NiTi, with an amount depending on the methods of surface treatment [3,9], since the Ni in the substrate takes part in the treatment, such as chemical etching. The low at.% of Ti in the substrate in the XPS profile was indeed an artifact arising from the preferential sputtering of Ti [29]. The gradual decrease of O across the
Fig. 3. AFM images of (a) mechanically polished NiTi sample, (b) coated NiTi sample.
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F.T. Cheng et al. / Surface & Coatings Technology 187 (2004) 26–32
Fig. 4. XPS composition depth profile of coated NiTi sample. Fig. 6. Bode plots of uncoated and coated NiTi samples in Hank’s solution at 37 jC.
interface into the substrate is typical of oxide layers on Ti alloys owing to the fairly high solubility of O in Ti, and may be considered as an advantage in terms of reduced residual stress at the interface by virtue of the gradual change in properties [30]. 3.2. EIS measurements The corrosion resistance of an oxide-covered sample in an electrolyte is closely related to the barrier properties of the oxide film to the transport of ions/electrons across it. Owing to high resistance value, the electrical resistance of oxide films is conveniently studied by electrochemical impedance spectroscopy (EIS) in which AC voltages are used. The EIS data for an uncoated (polished and chemically cleaned with acids) and a coated NiTi sample in Hank’s solution at 37 jC are displayed in the Bode plots shown in Fig. 6. Bode plots (magnitude and phase of impedance Z vs. log of frequency) are used because it has been argued that they are more informative than the conventionally popular Nyquist plots (imaginary part of Z vs. real part of Z) [31].
Fig. 5. XPS surface survey scan of coated NiTi sample.
It is common to study the oxide film on a passive metal using a two-layer model consisting of an inner layer which is compact and of the barrier type, and an outer layer which is relatively porous [32,33]. The absence of more than one sloping segments in the Bode-jZj plot in the present study (Fig. 6) indicates that the time constants that might be present were close together and unresolvable. As the aim of EIS measurements in the present study was to assess the improvement in electrical resistance due to oxide coating, detailed analysis of the spectra and the corresponding equivalent circuits is not dealt with here. Instead, for the sake of simplicity, the Randle’s model (Fig. 7) consisting of a resistance R and a capacitance C in parallel is adequate for assessing the electrical resistance of the oxide film [34], though the model is commonly used to simulate a corroding system under charge transfer control [33]. The resistance R (representing the electrical resistance of the oxide film), which was estimated from the Bode plots, was found to be increased by a factor of approximately 5.8 times (from 300 kV for uncoated NiTi to 1740 kV for coated NiTi). A certain degree of deviation from ideal capacitive behavior can be observed from the Bode-phase plots in Fig. 6 in which the maximum phase angle reached was approximately 10j below 90j. Such a deviation has also been reported by others for TiO2 films [34,35].
Fig. 7. Randle’s model. R=coating resistance, C=coating capacitance, Re=electrolyte resistance between sample and reference electrode.
F.T. Cheng et al. / Surface & Coatings Technology 187 (2004) 26–32
3.3. Potentiodynamic polarization tests While the EIS measurements above yielded the electrical resistance and also provided some information on the equivalent circuit of the NiTi samples at open-circuit potential, the corrosion behavior at different anodic potentials was probed by potentiodynamic polarization. The polarization curves for an uncoated and a coated NiTi sample in Hank’s solution at 37 jC are displayed in Fig. 8. The corrosion potential Ecorr, corrosion current density icorr, mean passive current density hipassi, and pitting potential Epit, were extracted from the curves and tabulated in Table 2. A significant improvement in corrosion resistance due to the oxide coating is evidenced by a shift of the whole polarization curve towards the region of lower current density. In particular, the corrosion current density was decreased by a factor of approximately five times, and the mean passive current density was reduced by a factor of approximately 3.3 times. The passive current density of the coated NiTi sample was of the order of 0.1 AA/cm2, which is comparable to that reported by Trepanier et al. [7] for NiTi samples which had been subjected to electropolishing and chemical passivation. However, it is interesting to note that both the uncoated and coated samples had similar Epit values (approx. 825 mV SCE), though the coated samples had an oxide thickness of approximately 200 nm, which was much thicker than the natural oxide film (generally less than 10 nm) on the uncoated samples. The independence of Epit on oxide film thickness (within a certain thickness range) has also been reported in the literature [36 –38]. A simplified explanation might be formulated as follows, assuming similar oxide film quality. The electrode potential E might be regarded as the sum of two parts, the potential drop across the oxide film (E1), and the potential drop across the oxide/electrolyte interface (E2). The pitting potential is the electrode potential at which pit initiation takes place. The initiation event is
31
Table 2 Corrosion parameters of uncoated and coated NiTi in Hank’s solution at 37 jC
Uncoated NiTi Coated NiTi
Ecorr (mV)
icorr (AA/cm2)
hipassi (AA/cm2)
Epit (mV)
445 523
0.60 0.12
2.2 0.67
824 826
closely related to the adsorption of aggressive ions, chloride ions in the present case, on the oxide surface, which in turn depends on the potential drop across the oxide/electrolyte interface E2. The value of E1 is almost independent of the oxide thickness since the higher resistance of a thicker film is compensated by a smaller current as shown in Fig. 8. As the value of E2 for initiation of pitting is also unrelated to oxide thickness, the pitting potential is thickness independent. It has been pointed that the corrosion resistance of NiTi reported in the literature varies appreciably among different studies owing to different test methodologies, material quality, and in particular, surface conditions [3,7]. The pitting potential Epit in solutions containing similar amounts of chloride ions, for example, could vary from approximately 200 mV [7] to approximately 600 mV [3] in potentiodynamic tests, possibly due to different surface conditions. The value of Epit for the uncoated NiTi samples (polished and chemically cleaned with acids) in the present study is exceptionally high, reaching approximately 800 mV, which is a value attained by passivation reported by Trepanier et al. [7]. The high Epit might result from the fine polishing followed by chemical cleaning to remove inclusions. The deposition of an oxide film (approx. 200 nm thick) by the hydrothermal process in the present study resulted in a higher electrical resistance, which led to lower corrosion and passive current densities, while the high value of Epit remained unchanged.
4. Conclusions A preliminary attempt to deposit a titanium oxide coating on NiTi implants by hydrothermal method has been undertaken. The oxide coating has been characterized by various methods and the corrosion resistance in Hank’s solution at 37 jC has been investigated. The following conclusions are drawn.
Fig. 8. Potentiodynamic polarization curves for uncoated and coated NiTi samples in Hank’s solution at 37 jC.
1. With pretreatment in a 0.6 M Ti4+ solution at 60 jC for 8 h, followed by hydrothermal treatment at 140 jC for 10 h, an oxide film of approximately 200 nm thick was formed on NiTi substrate. The coating was smooth and without cracks. 2. XPS and XRD analyses indicated that the coating was composed of TiO2 with anatase as the only crystalline phase. XPS surface survey scan showed that Ni was not present in the surface of the coated samples.
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3. EIS measurements recorded an increase in electrical resistance of approximately 5.8 times in Hank’s solution at open-circuit potential relative to the uncoated NiTi samples. 4. The corrosion resistance of the coated samples was significantly improved as evidenced by a reduction in the corrosion current density (by approx. five times) and in the mean passive current density (by approx. 3.3 times). 5. The coating method employed in the present study meets the three important requirements for surface modification of NiTi implants, namely, (a) Ni is not present in the coating since the process, being an addon process, does not involve the substrate; (b) it is a lowtemperature process; (c) it is not a line-of-sight process. The present paper only reports a preliminary study; it is expected that further investigations on the pretreatment parameters and the crystallization process would result in oxide coatings of higher quality.
Acknowledgements The authors would like to acknowledge the Research Committee of the Hong Kong Polytechnic University for the provision of a research grant (No. G-YD30).
References [1] S. Miyazaki, Medical and dental applications of shape memory alloys, in: K. Otsuka, C.M. Wayman (Eds.), Shape Memory Materials, Cambridge University Press, 1998, pp. 267 – 281. [2] G. Rondelli, Biomaterials 17 (1996) 2003 – 2008. [3] S.A. Shabalovskaya, Bio-Med. Mater. Eng. 12 (2002) 69 – 109. [4] D.Y. Li, Smart Mater. Struct. 9 (2000) 717 – 726. [5] F.T. Cheng, K.H. Lo, H.C. Man, Surf. Coat. Technol. 172 (2003) 308 – 315. [6] F.T. Cheng, K.H. Lo, H.C. Man, Surf. Coat. Technol. 172 (2003) 316 – 321. [7] C. Trepanier, M. Tabrizian, L’H. Yahia, L. Bilodeau, D.L. Piron, J. Biomed. Mater. Res. 43 (1998) 433 – 440. [8] B. Thierry, M. Tabrizian, C. Trepanier, O. Savadogo, L’H. Yahia, J. Biomed. Mater. Res. 51 (2000) 685 – 693. [9] S. Trigwell, R.D. Hayden, K.F. Nelson, G. Selvaduray, Surf. Interface Anal. 26 (1998) 483 – 489. [10] O. Cisse, O. Savadogo, M. Wu, L’H. Yahia, J. Biomed. Mater. Res. 61 (2002) 339 – 345. [11] F. Villermaux, M. Tabrizian, L’H Yahia, M. Meunier, D.L. Piron, Appl. Surf. Sci. 109 – 110 (1997) 62 – 66.
[12] H.C. Man, Z.D. Cui, T.M. Yue, Scripta Mater. 45 (2001) 1447 – 1453. [13] D. Starosvetsky, I. Gotman, Surf. Coat. Technol. 148 (2001) 268 – 276. [14] S. Mandl, D. Krause, G. Thorwarth, R. Sader, F. Zeilhofer, H.H. Horch, et al., Surf. Coat. Technol. 142 – 144 (2001) 1046 – 1050. [15] X. Zhao, W. Cai, L. Zhao, Surf. Coat. Technol. 155 (2002) 236 – 238. [16] Y. Fu, X. Wu, Y. Wang, B. Li, S. Yang, Appl. Surf. Sci. 157 (2000) 167 – 177. [17] Y. Qian, Q. Chen, Z. Chen, C. Fan, G. Zhou, J. Mater. Chem. 3 (1993) 203 – 205. [18] A.W. Czanderna, A.F. Clifford, J.M. Honig, J. Am. Chem. Soc. 79 (1957) 5407 – 5409. [19] Q. Chen, Y. Qian, Z. Chen, W. Wu, Z. Chen, G. Zhou, et al, Appl. Phys. Lett. 66 (1995) 1608 – 1610. [20] ASTM Standard B600-91 (Reapproved 2002), Descaling and Cleaning Titanium and Titanium Alloy Surfaces, ASTM Standards, Philadelphia, PA, USA. [21] ASTM Standard G5-94, Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements, ASTM Standards, Philadelphia, PA, USA. [22] S. Miyazaki, A. Ishida, Mater. Sci. Eng. A 273 – 275 (1999) 106 – 133. [23] E. Vigil, J.A. Ayllon, A.M. Peiro, R. Rodriguez-Clemente, X. Domenech, J. Peral, Langmuir 17 (2001) 891 – 896. [24] L. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259 – 341. [25] C.J. Brinker, G.W. Scherer, Sol – Gel Science, Academic Press, 1990, Chapter 2. [26] L.F. Francis, Sol – gel methods for oxide coatings, in: N.D. Dahotre, T.S. Sudarshan, M. Dekker (Eds.), Intermetallic and Ceramic Coatings, 1999, pp. 31 – 82. [27] U. Selvaraj, A.V. Prasadarao, S. Komaneni, R. Roy, J. Am. Ceram. Soc. 75 (1992) 1167 – 1170. [28] P. Filip, J. Lausmaa, J. Musialek, K. Mazanec, Biomaterials 22 (2001) 2131 – 2138. [29] G.S. Firstov, R.G. Vitchev, H. Kumar, B. Blanpain, J. Van Humbeeck, Biomaterials 23 (2002) 4863 – 4871. [30] M. Textor, C. Sittig, V. Frauchiger, S. Tosatti, D.M. Brunette, Properties and biological significance of natural oxide films on titanium and its alloys, in: D.M. Brunette, P. Tengvall, M. Textor, P. Thomen (Eds.), Titanium in Medicine, Springer, 2001, p. 181. [31] F. Mansfeld, Corrosion 44 (1987) 558 – 559. [32] J. Pan, C. Leygraf, D. Thierry, A.M. Ektessabi, J. Biomed. Mater. Res. 35 (1997) 309 – 318. [33] F. Mansfeld, ‘Analysis and interpretation of EIS data for metals and alloys’, Technical Report 26, Solartron-Schlumberger, Hamshire, England, 1993. [34] W.A. Badawy, S.S. Elegamy, Kh.M. Ismail, Brit. Corros. J. 28 (1993) 133 – 136. [35] N. Ibris, J. Claudia, M. Rosca, J. Electroanal. Chem. 526 (2002) 53 – 62. [36] Z. Szklarska-Smialowska, Pitting Corrosion of Metals, National Association of Corrosion Engineers, Houston, TX, USA, 1986, Ch. 8. [37] H. Boehni, H.H. Uhlig, J. Electrochem. Soc. 116 (1969) 906 – 910. [38] Z.A. Foroulis, M.J. Thubrikar, J. Electrochem. Soc. 122 (1975) 1296 – 1301.
A preliminary study of TiO2 deposition on NiTi by a hydrothermal method F.T. Cheng a,*, P. Shi a,b, H.C. Man c a
Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR China b Department of Materials and Chemical Engineering, Liaoning Institute of Technology, PR China c Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China Received 30 June 2003; accepted in revised form 16 January 2004 Available online 27 March 2004
Abstract A simple low-temperature method was employed in depositing an oxide coating on NiTi for enhancing corrosion resistance. Mechanically polished NiTi samples were pretreated in a solution containing Ti4+ ions at 60 jC for 8 h, followed by hydrothermal treatment at 140 jC for 10 h. After hydrothermal treatment, scanning electron microscopy (SEM) indicated the presence of an oxide film of approximately 200 nm on the NiTi substrate. Composition depth profiling of the oxide film by X-ray photoelectron spectroscopy (XPS) revealed that the oxide was TiO2. Thin-film X-ray diffractometry (TF-XRD) confirmed that anatase was the only crystalline phase present in the oxide film. Atomic force microscopy (AFM) recorded a mean surface roughness of 7.3 nm for the coated surface, which was of the same order as that of the polished surface (10.3 nm). Characterization of the corrosion behaviors in Hank’s solution at 37 jC using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique showed that the coated samples had a significantly higher corrosion resistance than samples prepared by mechanical polishing and chemical cleaning. The deposition method employed in the present study meets the important requirements in the surface modification of NiTi implants: (a) the substrate material should not become part of the treated layer in order to minimize the Ni content in the surface; (b) the treatment temperature should be low, preferably not exceeding 300 – 400 jC, so as not to disrupt the thermomechanical properties of the NiTi implants; (c) the process should not be a line-of-sight one to ensure a uniformly modified surface layer for implants of complex geometry. D 2004 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; TiO2; Coating; NiTi; Corrosion
1. Introduction In the past two decades, NiTi, a near-equiatomic intermetallic compound, has emerged as a new type of implant material by virtue of its distinctive thermomechanical and mechanical properties, namely, shape memory effect, superelasticity and high damping capacity [1]. The material of a medical implant, irrespective of its function, must be corrosion-resistant and biocompatible so as to ensure long service life and to minimize its adverse biological effect on the human body. Conflicting views regarding the corrosion resistance of NiTi, however, have been reported in the literature [2,3]. The controversy over the corrosion resis-
* Corresponding author. Tel.: +852-2766-5691; fax: +852-2333-7629. E-mail address: [email protected] (F.T. Cheng). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.023
tance of NiTi may be attributed partly to the surface conditions of the samples under test [3], and partly to the difference in performance requirements in different fields. In non-medical applications, the corrosion resistance of NiTi is usually regarded as superior [4]. In fact, attempts to employ NiTi as a cladding material to enhance cavitation erosion and/or corrosion resistance have been reported [[5,6] and references therein]. However, views regarding the corrosion resistance of NiTi in medical applications are more conservative [2,3], mainly because NiTi has a high content of Ni, an element which is known to be allergenic and toxic when present at a sufficiently high level. Since the naturally formed oxide film is rather thin (of the order of a few nm) and usually defective, passivation treatment of NiTi implants is necessary to thicken and to improve the quality of the oxide film so as to reduce the corrosion rate to a very low value. Among the studies reporting various types of surface modification for NiTi implants to enhance the
F.T. Cheng et al. / Surface & Coatings Technology 187 (2004) 26–32
surface properties, a significant portion has been devoted to treatment techniques for improving corrosion resistance. These include chemical passivation, anodic oxidation, electropolishing, thermal oxidation [7 –10], laser surface melting [11,12], nitriding [13], and plasma ion implantation [14 – 16], and different degrees of improvement in corrosion resistance have been achieved. In view of the high Ni content in NiTi, the sensitivity of NiTi to heat treatment, and the possible irregular shapes of implants, treatment processes for NiTi should ideally meet certain requirements. The most important ones include: (a) the substrate material (i.e. NiTi) should not become part of the treated layer in order to minimize the Ni content in the surface; (b) the treatment temperature should be low, preferably not exceeding 300 –400 jC, so as not to disrupt the thermomechanical properties of the implant; (c) the process should not be a line-of-sight one to ensure a uniformly modified surface layer on implants of irregular shapes. It is clear that most of the treatment methods listed above do not satisfy, one way or the other, these requirements. For example, anodization inevitably incorporates Ni in the surface layer since both Ti and Ni dissolve simultaneously during anodization, while other processes such as thermal oxidation and nitriding involves a temperature which is well above 400 jC. Laser surface treatment, though versatile and efficient, is both a high-temperature and a line-of-sight process. The present study is a preliminary attempt to deposit a titanium oxide coating on NiTi samples by hydrothermal treatment, a process which meets the three requirements stated above. The oxide coating was characterized by various techniques and the corrosion resistance of the coated samples in Hank’s solution was assessed.
2. Experimental details
27
to lie in the range 6 – 7. NiTi samples were fixed at the bottom of a Teflon container, which was then filled with the Ti4+ aqueous solution until it was 70% full. The Teflon container was placed in a stainless steel cell for the hydrothermal process. The cell was kept at a temperature of 60 jC for 8 h and then at 140 jC for 10 h [19]. After the coating process, the samples were ultrasonically cleaned with deionized water and dried in an air stream before film characterization. For comparison in corrosion behaviors, NiTi samples polished in a similar manner were chemically cleaned by immersion in an acid solution containing 1 part of HF acid and 10 parts of HNO3 acid at 49 jC according to ASTM Standard B600-91 [20]. 2.2. Morphological study The surface and the cross-section of the coated samples were studied by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6335F) and the coating thickness was determined. The surface roughness of the coating was determined by atomic force microscopy (AFM) (NanoScope IV, Digital Instruments). 2.3. Composition and phase analysis Composition depth profiling of the coated NiTi samples was achieved using X-ray photoelectron spectroscopy (XPS) (Quantum 2000, Physical Electronics) with 1 keV Ar+ beam for sputtering, the radiation source being monochromated Al Ka (1486.6 eV). The crystalline phase in the surface was identified by thin-film X-ray diffractometry (TF-XRD) (Bruker D8 Discover), with Cu Ka radiation (40 kV, 40 mA) at a glancing angle h of 3j (for the surface oxide layer) and 15j (for a thicker layer to include the substrate material beneath), while the angle 2h was scanned from 20 to 90j.
2.1. Sample preparation 2.4. Assessment of corrosion resistance NiTi (50.8 at.% Ni) samples of 1313 mm were spark cut from NiTi plates of thickness 2 mm. The samples were mechanically polished successively with SiC papers of grit 200, 400, 800, 1200 and 1-Am diamond paste. The samples were then ultrasonically cleaned with acetone and deionized water, and dried in air before coating. The coating process consisted of two steps: (1) preparation of a solution containing Ti ions by dissolving Ti powder in an ammoniacal solution of H2O2; and (2) hydrothermal treatment of the samples in the solution prepared in step (1) [17 – 19]. For step (1), Ti powder of particle size f40 Am was added to a 30% H2O2 solution, and NH3 gas was slowly passed into the solution. After all the Ti powder had dissolved, the solution was filtered. The excess H2O2 and NH3 in the filtrate were driven off over a steam cone. The solution was diluted with deionized water to a concentration of 0.6 M Ti4+, and the pH value was adjusted
Samples for corrosion study were mounted in epoxy to expose a surface area of 1 cm2. Corrosion studies using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique were both performed in Hank’s solution (composition given in Table 1) at 37 jC in a three-electrode cell with a pair of graphite rods as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The open-circuit potential Eoc was monitored until a steady value was reached. AC impedance measurements were made with an amplitude of 7 mV about Eoc from 100 kHz down to 3 mHz using an impedance analyzer (EG & G Model 6310). Potentiodynamic polarization measurements were performed conforming to ASTM Standard G5-94 [21] with a potentiostat (EG&G PARC 273). The polarization scan started at 200 mV below Eoc and continued in the anodic
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Table 1 Chemical composition of Hank’s solution
follows. Titanium powder was first oxidized by H2O2 to form Ti4+ ions, the most stable oxidation state of titanium,
Component
Concentration (g/l)
NaCl CaCl2 KCl NaHCO3 Glucose MgCl2 6H2O Na2HPO4 2H2O KH2PO4 MgSO4 7H2O
8.0 0.14 0.4 0.35 1.0 0.1 0.06 0.06 0.06
direction with a potential sweep rate of 0.6 V/h (0.167 mV/s) until 1600 mV SCE.
TiðsÞ þ 2H2 O2 ðlÞ! Ti4þ ðaqÞ þ 4OH ðaqÞ
ð1Þ
According to Chen et al. [19], pretreatment of the samples in a solution containing Ti ions at 60 jC resulted in the nucleation of titania followed by formation of an amorphous film on the substrate, while subsequent treatment at 140 jC crystallized the film. The reactions for the formation of titania, excluding the route involving TiO42 ions, might probably proceed as follows. Under basic conditions, a hydroxo – aquo complex [Ti2(OH)2(OH2)x]6+ was formed by hydrolysis and condensation [24,25], 2Ti4þ ðaqÞ þ 2OH ðaqÞ þ xH2 OðlÞ
3. Results and discussion
!½Ti2 ðOHÞ2 ðOH2 Þx 6þ ðaqÞ
3.1. Coating characteristics After the hydrothermal treatment process, the surface of the NiTi samples appeared pale yellow in contrast to the grayish color of the untreated ones. Fig. 1 shows the TFXRD patterns of a coated sample at glancing angles of 3 and 15j, which reveal the phases present in the surface layer and those in a thicker layer. The patterns indicate that the substrate was composed of the NiTi B2 (austenite) phase [22] while the coating consisted of TiO2 (anatase) [23]. It was proposed [18] that TiO42 ions were formed by dissolving Ti powder in an ammoniacal solution of H2O2, and titania was formed via TiO42. However, the existence of this species is doubtful as it would mean an oxidation state of +6 for Ti. A more probable route for the formation of titania is as
ð2Þ
The dimeric species then underwent further condensation to form titania: ½Ti2 ðOHÞ2 ðOH2 Þx 6þ ðaqÞ þ 6OH ðaqÞ! TiO2 nH2 OðgelÞ þ ð4 þ xnÞH2 OðlÞ TiO2 nH2 OðgelÞ! TiO2 ðsÞ þ nH2 OðlÞ
ð3Þ ð4Þ
The SEM micrographs showing the surface and the crosssection of a coated sample are shown in Fig. 2a,b, respectively. It can be observed from Fig. 2b that a uniform coating of approximately 200 nm and without cracks or pores was deposited on the substrate (a dirt particle appear-
Fig. 1. Thin-film XRD spectra of coated NiTi at glancing angles of 3 and 15j.
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29
Fig. 2. SEM micrographs of oxide-coated NiTi sample, (a) surface (b) cross-section.
ing on the surface was deliberately included to show sharp boundaries for thickness measurement). The coating was thicker than that reported by Chen et al. [19] in depositing titania film on Si. According to Chen et al. [19], the thickness of the coating was determined by the concentration of Ti ions and the pretreatment time, and the thicker film in the present case probably resulted from a longer pretreatment time. The AFM images of a mechanically polished and a coated sample are shown in Fig. 3a,b, respectively. The mean surface roughness Ra for the coating was 7.3 nm, which was comparable to that for the mechanically polished sample (10.3 nm). The relatively low temperature involved in hydrothermal treatments (140 jC in the present case) is especially important for NiTi as the built-in thermomechanical properties would not be disrupted. Ceramic coating fabricated at a low temperature has an additional advantage compared with other processes with respect to film integrity. For example, fabrication of sol –gel derived films usually requires heat
treatment at a much higher temperature for crystallization, typically above 500 jC [26], and cracks are not uncommon [27]. The XPS depth profile of coated NiTi is shown in Fig. 4. Except for the presence of some carbon, a common contaminant, at the surface, the film was essentially TiO2. Ni was almost absent in the film except near the interface between the film and the substrate. Peaks corresponding to elemental Ni or its oxides (peaks in the range 850 –860 eV) [9,28] were absent in the XPS surface survey scan of the coated sample (Fig. 5) because the oxide film in the present case was an add-on layer without involving the substrate material. On the contrary, Ni is commonly reported to be present on the surface of NiTi, with an amount depending on the methods of surface treatment [3,9], since the Ni in the substrate takes part in the treatment, such as chemical etching. The low at.% of Ti in the substrate in the XPS profile was indeed an artifact arising from the preferential sputtering of Ti [29]. The gradual decrease of O across the
Fig. 3. AFM images of (a) mechanically polished NiTi sample, (b) coated NiTi sample.
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Fig. 4. XPS composition depth profile of coated NiTi sample. Fig. 6. Bode plots of uncoated and coated NiTi samples in Hank’s solution at 37 jC.
interface into the substrate is typical of oxide layers on Ti alloys owing to the fairly high solubility of O in Ti, and may be considered as an advantage in terms of reduced residual stress at the interface by virtue of the gradual change in properties [30]. 3.2. EIS measurements The corrosion resistance of an oxide-covered sample in an electrolyte is closely related to the barrier properties of the oxide film to the transport of ions/electrons across it. Owing to high resistance value, the electrical resistance of oxide films is conveniently studied by electrochemical impedance spectroscopy (EIS) in which AC voltages are used. The EIS data for an uncoated (polished and chemically cleaned with acids) and a coated NiTi sample in Hank’s solution at 37 jC are displayed in the Bode plots shown in Fig. 6. Bode plots (magnitude and phase of impedance Z vs. log of frequency) are used because it has been argued that they are more informative than the conventionally popular Nyquist plots (imaginary part of Z vs. real part of Z) [31].
Fig. 5. XPS surface survey scan of coated NiTi sample.
It is common to study the oxide film on a passive metal using a two-layer model consisting of an inner layer which is compact and of the barrier type, and an outer layer which is relatively porous [32,33]. The absence of more than one sloping segments in the Bode-jZj plot in the present study (Fig. 6) indicates that the time constants that might be present were close together and unresolvable. As the aim of EIS measurements in the present study was to assess the improvement in electrical resistance due to oxide coating, detailed analysis of the spectra and the corresponding equivalent circuits is not dealt with here. Instead, for the sake of simplicity, the Randle’s model (Fig. 7) consisting of a resistance R and a capacitance C in parallel is adequate for assessing the electrical resistance of the oxide film [34], though the model is commonly used to simulate a corroding system under charge transfer control [33]. The resistance R (representing the electrical resistance of the oxide film), which was estimated from the Bode plots, was found to be increased by a factor of approximately 5.8 times (from 300 kV for uncoated NiTi to 1740 kV for coated NiTi). A certain degree of deviation from ideal capacitive behavior can be observed from the Bode-phase plots in Fig. 6 in which the maximum phase angle reached was approximately 10j below 90j. Such a deviation has also been reported by others for TiO2 films [34,35].
Fig. 7. Randle’s model. R=coating resistance, C=coating capacitance, Re=electrolyte resistance between sample and reference electrode.
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3.3. Potentiodynamic polarization tests While the EIS measurements above yielded the electrical resistance and also provided some information on the equivalent circuit of the NiTi samples at open-circuit potential, the corrosion behavior at different anodic potentials was probed by potentiodynamic polarization. The polarization curves for an uncoated and a coated NiTi sample in Hank’s solution at 37 jC are displayed in Fig. 8. The corrosion potential Ecorr, corrosion current density icorr, mean passive current density hipassi, and pitting potential Epit, were extracted from the curves and tabulated in Table 2. A significant improvement in corrosion resistance due to the oxide coating is evidenced by a shift of the whole polarization curve towards the region of lower current density. In particular, the corrosion current density was decreased by a factor of approximately five times, and the mean passive current density was reduced by a factor of approximately 3.3 times. The passive current density of the coated NiTi sample was of the order of 0.1 AA/cm2, which is comparable to that reported by Trepanier et al. [7] for NiTi samples which had been subjected to electropolishing and chemical passivation. However, it is interesting to note that both the uncoated and coated samples had similar Epit values (approx. 825 mV SCE), though the coated samples had an oxide thickness of approximately 200 nm, which was much thicker than the natural oxide film (generally less than 10 nm) on the uncoated samples. The independence of Epit on oxide film thickness (within a certain thickness range) has also been reported in the literature [36 –38]. A simplified explanation might be formulated as follows, assuming similar oxide film quality. The electrode potential E might be regarded as the sum of two parts, the potential drop across the oxide film (E1), and the potential drop across the oxide/electrolyte interface (E2). The pitting potential is the electrode potential at which pit initiation takes place. The initiation event is
31
Table 2 Corrosion parameters of uncoated and coated NiTi in Hank’s solution at 37 jC
Uncoated NiTi Coated NiTi
Ecorr (mV)
icorr (AA/cm2)
hipassi (AA/cm2)
Epit (mV)
445 523
0.60 0.12
2.2 0.67
824 826
closely related to the adsorption of aggressive ions, chloride ions in the present case, on the oxide surface, which in turn depends on the potential drop across the oxide/electrolyte interface E2. The value of E1 is almost independent of the oxide thickness since the higher resistance of a thicker film is compensated by a smaller current as shown in Fig. 8. As the value of E2 for initiation of pitting is also unrelated to oxide thickness, the pitting potential is thickness independent. It has been pointed that the corrosion resistance of NiTi reported in the literature varies appreciably among different studies owing to different test methodologies, material quality, and in particular, surface conditions [3,7]. The pitting potential Epit in solutions containing similar amounts of chloride ions, for example, could vary from approximately 200 mV [7] to approximately 600 mV [3] in potentiodynamic tests, possibly due to different surface conditions. The value of Epit for the uncoated NiTi samples (polished and chemically cleaned with acids) in the present study is exceptionally high, reaching approximately 800 mV, which is a value attained by passivation reported by Trepanier et al. [7]. The high Epit might result from the fine polishing followed by chemical cleaning to remove inclusions. The deposition of an oxide film (approx. 200 nm thick) by the hydrothermal process in the present study resulted in a higher electrical resistance, which led to lower corrosion and passive current densities, while the high value of Epit remained unchanged.
4. Conclusions A preliminary attempt to deposit a titanium oxide coating on NiTi implants by hydrothermal method has been undertaken. The oxide coating has been characterized by various methods and the corrosion resistance in Hank’s solution at 37 jC has been investigated. The following conclusions are drawn.
Fig. 8. Potentiodynamic polarization curves for uncoated and coated NiTi samples in Hank’s solution at 37 jC.
1. With pretreatment in a 0.6 M Ti4+ solution at 60 jC for 8 h, followed by hydrothermal treatment at 140 jC for 10 h, an oxide film of approximately 200 nm thick was formed on NiTi substrate. The coating was smooth and without cracks. 2. XPS and XRD analyses indicated that the coating was composed of TiO2 with anatase as the only crystalline phase. XPS surface survey scan showed that Ni was not present in the surface of the coated samples.
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3. EIS measurements recorded an increase in electrical resistance of approximately 5.8 times in Hank’s solution at open-circuit potential relative to the uncoated NiTi samples. 4. The corrosion resistance of the coated samples was significantly improved as evidenced by a reduction in the corrosion current density (by approx. five times) and in the mean passive current density (by approx. 3.3 times). 5. The coating method employed in the present study meets the three important requirements for surface modification of NiTi implants, namely, (a) Ni is not present in the coating since the process, being an addon process, does not involve the substrate; (b) it is a lowtemperature process; (c) it is not a line-of-sight process. The present paper only reports a preliminary study; it is expected that further investigations on the pretreatment parameters and the crystallization process would result in oxide coatings of higher quality.
Acknowledgements The authors would like to acknowledge the Research Committee of the Hong Kong Polytechnic University for the provision of a research grant (No. G-YD30).
References [1] S. Miyazaki, Medical and dental applications of shape memory alloys, in: K. Otsuka, C.M. Wayman (Eds.), Shape Memory Materials, Cambridge University Press, 1998, pp. 267 – 281. [2] G. Rondelli, Biomaterials 17 (1996) 2003 – 2008. [3] S.A. Shabalovskaya, Bio-Med. Mater. Eng. 12 (2002) 69 – 109. [4] D.Y. Li, Smart Mater. Struct. 9 (2000) 717 – 726. [5] F.T. Cheng, K.H. Lo, H.C. Man, Surf. Coat. Technol. 172 (2003) 308 – 315. [6] F.T. Cheng, K.H. Lo, H.C. Man, Surf. Coat. Technol. 172 (2003) 316 – 321. [7] C. Trepanier, M. Tabrizian, L’H. Yahia, L. Bilodeau, D.L. Piron, J. Biomed. Mater. Res. 43 (1998) 433 – 440. [8] B. Thierry, M. Tabrizian, C. Trepanier, O. Savadogo, L’H. Yahia, J. Biomed. Mater. Res. 51 (2000) 685 – 693. [9] S. Trigwell, R.D. Hayden, K.F. Nelson, G. Selvaduray, Surf. Interface Anal. 26 (1998) 483 – 489. [10] O. Cisse, O. Savadogo, M. Wu, L’H. Yahia, J. Biomed. Mater. Res. 61 (2002) 339 – 345. [11] F. Villermaux, M. Tabrizian, L’H Yahia, M. Meunier, D.L. Piron, Appl. Surf. Sci. 109 – 110 (1997) 62 – 66.
[12] H.C. Man, Z.D. Cui, T.M. Yue, Scripta Mater. 45 (2001) 1447 – 1453. [13] D. Starosvetsky, I. Gotman, Surf. Coat. Technol. 148 (2001) 268 – 276. [14] S. Mandl, D. Krause, G. Thorwarth, R. Sader, F. Zeilhofer, H.H. Horch, et al., Surf. Coat. Technol. 142 – 144 (2001) 1046 – 1050. [15] X. Zhao, W. Cai, L. Zhao, Surf. Coat. Technol. 155 (2002) 236 – 238. [16] Y. Fu, X. Wu, Y. Wang, B. Li, S. Yang, Appl. Surf. Sci. 157 (2000) 167 – 177. [17] Y. Qian, Q. Chen, Z. Chen, C. Fan, G. Zhou, J. Mater. Chem. 3 (1993) 203 – 205. [18] A.W. Czanderna, A.F. Clifford, J.M. Honig, J. Am. Chem. Soc. 79 (1957) 5407 – 5409. [19] Q. Chen, Y. Qian, Z. Chen, W. Wu, Z. Chen, G. Zhou, et al, Appl. Phys. Lett. 66 (1995) 1608 – 1610. [20] ASTM Standard B600-91 (Reapproved 2002), Descaling and Cleaning Titanium and Titanium Alloy Surfaces, ASTM Standards, Philadelphia, PA, USA. [21] ASTM Standard G5-94, Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements, ASTM Standards, Philadelphia, PA, USA. [22] S. Miyazaki, A. Ishida, Mater. Sci. Eng. A 273 – 275 (1999) 106 – 133. [23] E. Vigil, J.A. Ayllon, A.M. Peiro, R. Rodriguez-Clemente, X. Domenech, J. Peral, Langmuir 17 (2001) 891 – 896. [24] L. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259 – 341. [25] C.J. Brinker, G.W. Scherer, Sol – Gel Science, Academic Press, 1990, Chapter 2. [26] L.F. Francis, Sol – gel methods for oxide coatings, in: N.D. Dahotre, T.S. Sudarshan, M. Dekker (Eds.), Intermetallic and Ceramic Coatings, 1999, pp. 31 – 82. [27] U. Selvaraj, A.V. Prasadarao, S. Komaneni, R. Roy, J. Am. Ceram. Soc. 75 (1992) 1167 – 1170. [28] P. Filip, J. Lausmaa, J. Musialek, K. Mazanec, Biomaterials 22 (2001) 2131 – 2138. [29] G.S. Firstov, R.G. Vitchev, H. Kumar, B. Blanpain, J. Van Humbeeck, Biomaterials 23 (2002) 4863 – 4871. [30] M. Textor, C. Sittig, V. Frauchiger, S. Tosatti, D.M. Brunette, Properties and biological significance of natural oxide films on titanium and its alloys, in: D.M. Brunette, P. Tengvall, M. Textor, P. Thomen (Eds.), Titanium in Medicine, Springer, 2001, p. 181. [31] F. Mansfeld, Corrosion 44 (1987) 558 – 559. [32] J. Pan, C. Leygraf, D. Thierry, A.M. Ektessabi, J. Biomed. Mater. Res. 35 (1997) 309 – 318. [33] F. Mansfeld, ‘Analysis and interpretation of EIS data for metals and alloys’, Technical Report 26, Solartron-Schlumberger, Hamshire, England, 1993. [34] W.A. Badawy, S.S. Elegamy, Kh.M. Ismail, Brit. Corros. J. 28 (1993) 133 – 136. [35] N. Ibris, J. Claudia, M. Rosca, J. Electroanal. Chem. 526 (2002) 53 – 62. [36] Z. Szklarska-Smialowska, Pitting Corrosion of Metals, National Association of Corrosion Engineers, Houston, TX, USA, 1986, Ch. 8. [37] H. Boehni, H.H. Uhlig, J. Electrochem. Soc. 116 (1969) 906 – 910. [38] Z.A. Foroulis, M.J. Thubrikar, J. Electrochem. Soc. 122 (1975) 1296 – 1301.