Electrochemical corrosion behavior of Ti–24Nb–4Zr–8Sn alloy in a simulated physiological environment

Applied Surface Science 258 (2012) 4035–4040

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Electrochemical corrosion behavior of Ti–24Nb–4Zr–8Sn alloy in a simulated physiological environment Y. Bai a , S.J. Li a , F. Prima b , Y.L. Hao a,∗ , R. Yang a a b

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Laboratoire de Physico-Chimie des Surfaces, (CNRS-UMR 7045) Group de Métallurgie Structurale, ENSCP Paris, France

a r t i c l e

i n f o

Article history: Received 24 August 2011 Received in revised form 10 November 2011 Accepted 20 December 2011 Available online 26 December 2011 Keywords: Titanium alloy Polarization EIS Passive film

a b s t r a c t Electrochemical corrosion behavior of a biomedical titanium alloy Ti–24Nb–4Zr–8Sn in weight percent was investigated in a phosphate buffered saline solution at 37 ◦ C utilizing open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Both commercially pure titanium and Ti–6Al–4V alloy were also investigated to make a comparison. The results show that all the samples were spontaneously passivated once immersion into the electrolyte. Ti–24Nb–4Zr–8Sn alloy exhibited a much wider passive region compared with pure titanium and Ti–6Al–4V and also relatively low corrosion current density which is comparable to that of pure titanium in the buffered saline solution, which was attributed to a stable passive film mainly consisted of titanium oxide and niobium oxide on its surface. The EIS results indicated the presence of a single passive layer with thickness ∼2 nm for Ti–24Nb–4Zr–8Sn and pure titanium but a duplex film consisting an inner barrier layer and an outer porous layer on Ti–6Al–4V alloy with thickness of ∼3 nm and ∼2.5 nm, respectively. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Titanium and its alloy have been widely used in medical applications owing to their light weight, low elastic modulus, corrosion resistance and biocompatibility [1,2]. Ti–6Al–4V alloy is still the most widely used titanium alloy for medical implants. However, it have been reported that element V and Al can be released into the cell tissue by the way of passive film dissolution and induce to senile dementia, neurological disorders and allergic reactions [3–5], so the recently developed biomedical titanium alloys contains mainly the non-toxic and non-allergic elements such as Nb, Ta, Zr, Mo and Sn [6–9]. A new multifunctional ␤-type titanium alloy Ti–24Nb–4Zr–8Sn in weight percent (abbreviated as Ti2448) has been developed for biomedical application recently [10–12]. It possesses better balance between high strength and low elastic modulus as compared with the previously reported alloys. As a kind of biomaterials, its corrosion behavior in human body is crucial important. Since the environment of human body is extremely complicated electrolyte which contains many erosive species and specially facilitated the electrochemical mechanisms of corrosion and hydrolysis. The corrosion behavior of titanium and titanium alloys has been investigated extensively [13–20]. These studies have confirmed that

∗ Corresponding author. Tel.: +86 24 83978841; fax: +86 24 23972021. E-mail address: [email protected] (Y.L. Hao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.12.096

corrosion and surface oxide film dissolution are the two key mechanisms for ions release into the body environment. Extensive release of ions from prosthesis can result in adverse biological reactions and lead to mechanical failure of the implant device. This study focuses on the corrosion performance of Ti2448 alloy in a simulated physiological environment. Utilizing the open circuit potential, the potentiodynamic polarization and the electrochemical impedance spectroscopy (EIS) techniques, the electrochemical behavior of Ti2448 alloy in phosphate buffered saline (PBS) solution at 37 ◦ C was investigated. To make a comparison, both commercially pure titanium (CP-Ti) and Ti–6Al–4V alloy were also investigated.

2. Experimental An ingot with a nominal chemical composition of Ti–24Nb–4Zr–8Sn (wt.%) was fabricated by the vacuum arcmelting method using a TiSn master alloy and pure Ti, pure Nb and pure Zr as raw materials. The ingot was hot-forged at 850 ◦ C to a slab with a thickness of 50 mm and then heat-treated at 750 ◦ C for an hour followed by air cooling. Rectangular plates with a typical dimension of 10 mm × 10 mm × 5 mm were cut by electrical spark. To avoid undesirable ohmic effect, a copper wire was connected to the back surface of samples with the use of a brass nut. All samples were then mounted in a plastic tube and sealed with epoxy resin, leaving cross-sectional area of 1.0 cm2 . Surfaces of the samples were mechanically polished with SiC papers from 57 to 10 ␮m,

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Fig. 1. Optical micrographs of (a) Ti2448 alloy, (b) CP-Ti and (c) Ti–6Al–4V alloy.

and then cleaned ultrasonically in acetone, ethanol and distilled water for 10 min respectively, and dried under a cold air stream finally. Measurements were carried out in aerated PBS solution consisting of (g l−1 ) 8 NaCl, 0.2 KCl, 2.9 Na2 HPO4 ·12H2 O and 0.2 KH2 PO4 with pH value 7.4. The fresh PBS solution was used for study with constant temperature 37 ◦ C. All electrochemical tests were conducted using a potentiostat (EG & G Princeton Applied Research, model 2273) controlled by a personal computer for data acquisition. The cell used was a typical three-electrode one fitted with a large platinum sheet of size 15 mm × 20 mm × 1 mm as the counter electrode (CE), saturated calomel (SCE) as the reference electrode (RE). All potentials were measured and given with respect to SCE (E = 0.241 V/SHE). The tip of the RE was kept at ∼4 mm from the sample in order to minimize the ohmic drop. The open circuit potential (OCP) measurement was carried out for 5.0 × 104 s starting from the electrode immersing into the electrolyte. The potentiodynamic polarization curves were obtained after 1 h immersion in PBS, in the range from −500 mV with respect to OCP to 2500 mV (vs. SCE) using a scan rate of 0.667 mV/s. The EIS studies were performed by applying a sinusoidal potential perturbation of 10 mV at the OCP, the impedance spectra were acquired with frequency sweep from 100 kHz to 10 mHz in logarithmic increment. Nyquist and Bote plots were reported after the samples immersed in the solution for 1 h. The impedance data were analyzed using ZSimpWin 3.0 software (EG & G, USA) and fitted to appropriate equivalent electrical circuit using a complex non-linear least-squares fitting routine, using both the real and imaginary components of the data [21]. The microscopy of the three Ti alloys were characterized using optical microscopy (Axiovert 200MAT), the phases constitution were verified by X-ray diffractometry (XRD) using Cu K␣ irradiation with an accelerating voltage of 40 kV and a current of 250 mA. The surface passive film composition of the tested samples was examined using X-ray photoelectron spectroscopy (XPS) which conducted in a vacuum chamber at a base pressure at

∼3.5 × 10−8 Pa with beam spots ∼500 ␮m (Escalab250, VG Thermo, England).

3. Results and discussion 3.1. Microstructures and phase constitutions Optical microstructures of Ti2448, CP-Ti and Ti–6Al–4V were shown in Fig. 1. It is clear that Ti2448 alloy has equiaxed ␤ grains with the averaged size ∼150 ␮m (Fig. 1(a)). CP-Ti contains single ␣ phase which was elongated significantly along the rolling direction (Fig. 1(b)). Ti–6Al–4V alloy has a typical ␣+␤ microstructure with needle like ␣ phase (acicular alpha) in ␤ phase matrix (Fig. 1(c)). The above phase constitutions were identified from XRD spectra shown in Fig. 2.

Fig. 2. X-ray diffraction profiles of (a) Ti2448, (b) CP-Ti and (c) Ti–6Al–4V alloy.

Y. Bai et al. / Applied Surface Science 258 (2012) 4035–4040

-0.1

Ti2448

2.5

Ti2448

CP-Ti

2.0

CP-Ti

Potential vs.SCE(V)

Potential vs. SCE (V)

0.0

4037

Ti-6Al-4V

-0.2 -0.3 -0.4

1.5

Ti-6Al-4V

1.0 0.5 0.0 -0.5 -1.0

-0.5

0

1x10

4

2x10

4

3x10

4

4x10

4

5x10

-11

4

-10

-9

-8

-7

-6

-5

-4

2

Log(I/A)(A/cm )

Immersion time (s) Fig. 4. Potentiodynamic polarization curves in the PBS solution at 37 ◦ C. Fig. 3. Open circuit potential as a function of time in the PBS solution at 37 ◦ C.

3.2. Open circuit potential The variations of OCP for Ti2448, CP-Ti and Ti–6Al–4V samples with immersion time in PBS solution are presented in Fig. 3. It can be seen that the potentials of the studied materials shift in the positive direction, indicating the formation of protective passive films on their surfaces. After an initial increase in the potentials, the OCP of Ti2448 decreases gradually and reaches to a stable valve about -0.18 V after 1 × 104 s, suggesting the stabilization of the passive film. The OCP curve of CP-Ti shows a similar tendency with Ti2448 alloy except the potential peak at the initial stage while that of Ti–6Al–4V alloy exhibits the fluctuations at the initial stage up to 6 × 103 s and then keeps stable at a value about −0.20 V. The above results suggest that the three materials have similar stable values of OCP. The high corrosion resistance of Ti and its alloys is due to the spontaneous formation of a protective oxide film on their surface. XPS analysis has shown that this amorphous oxide film consists of three layers: TiO, Ti2 O3 and TiO2 [22]. Once the material is contact with the electrolyte, the transformation from TiO or Ti2 O3 to TiO2 (more stale) will appear on the electrode/electrolyte interface [23], therefore the corrosion resistance of the electrode was increased and eventually reached to a relatively stable state. The increase of corrosion resistance caused the decrease of the anodic dissolution current of titanium alloy. As the OCP is a non-equilibrium electrode potential, which is determined by both the anodic and cathodic reaction, according to the electro-neutrality theory, the decrease of the anodic current will move the OCP gradually in the positive direction so that the cathodic current can be low enough to balance the decreased anodic current. Once the corrosion resistance of the titanium alloy oxide film reached a relatively stable state, the anodic current, and therefore the OCP, also reach a stable value. 3.3. Potentiodynamic polarization Fig. 4 shows the potentiodynamic polarization curves of Ti2448, CP-Ti and Ti–6Al–4V samples in the PBS solution at 37 ◦ C. This procedure was performed in order to analyze the continuity, stability and intensity of the passive Ti oxide film formation. It is clear all the samples exhibit a typical active–passive characterization, translating directly into the passive region from the Tafel region. The corrosion potentials (Ecorr ) can be estimated from these curves as −0.53, −0.39 and −0.69 V (vs. SCE) for the Ti2448, CP-Ti and Ti–6Al–4V, respectively. The corrosion current densities (Icorr ) were obtained by Tafel analysis using both anodic and cathodic branches of the polarization curves. The results show that Ti2448

and CP-Ti have similar Icorr values 0.035 and 0.033 ␮A/cm2 respectively, which is slight higher than Ti–6Al–4V about 0.018 ␮A/cm2 . From −0.53 V up to −0.12 V (vs. SCE), the Ti2448 exhibits typical activation polarization, showing a well defined linear relationship between the potential and the current density. Then the Ti2448 alloy shows a wider passivation region (−0.12–1.2 V (vs. SCE)) than CP-Ti and Ti–6Al–4V, as evidenced by the current remaining constant with the increase of potential, which indicated that the passive film formed on the surface of Ti2448 alloy is very integral and protective to prevent corrosion. The difference can also be judged from passive current density (Ipp ) in the potential region: Ti2448 has the Ipp value ∼3.05 ␮A/cm2 , which is higher than CP-Ti and Ti–6Al–4V being ∼2.22 and ∼1.87 ␮A/cm2 , respectively. As the potentials higher than ∼1.2 V (vs. SCE), a slight increase in the current density was observed for Ti2448. The increase may be related to the formation of TiO2 from TiO and Ti2 O3 [24]. With the further increase of the potential up to 1.4 V (vs. SCE), Ti2448 shows repassivation behavior. Since no breakdown potential was observed with the potential up to 2.5 V (vs. SCE), Ti2448 shows a relatively high resistance to localized corrosion. This is different with CP-Ti and Ti–6Al–4V alloy: the current density increases remarkably with the potential up to 0.5 V (vs. SCE) and then turns into the repassivation region after an activation peak appearing. The above electrochemical parameters for the studied materials are listed in Table 1. From the polarization curves and the active dissolution parameters listed in Table 1, it can be judged that Ti2448 has better passivation properties than CP-Ti and Ti–6Al–4V and comparable corrosion rate with CP-Ti. This is probably due to the constitution of the protective oxide film formed on the surface of Ti2448 alloy. To characterize this protective oxide film, they were examined by XPS. It is clear from Fig. 5(a)–(d) that the passive film of Ti2448 is mainly constituted of TiO2 , Nb2 O5 and few ZrO2 and SnO2 , as evidenced by the binding energies of Ti 2p signal at 458.4 eV, Nb 3d signal at 206.5 eV, Zr 3d signal at 183.2 eV and Sn 3d signal at 486.6 eV. Several authors have discussed the influence of alloying elements on the electrochemical behavior of titanium alloy. Compared with Ti–6Al–4V alloy, Ti–6Al–7Nb alloy is less susceptible to corrosion [25], suggesting Nb is benefit for improving the passivation properties of titanium alloy. Yu et al. [26] concluded that alloying addition of Nb results in improved resistance to active Table 1 Corrosion parameters of Ti2448, CP-Ti and Ti–6Al–4V in the PBS solution at 37 ◦ C. Alloy

Icorr (␮A/cm2 )

Ti–6Al–4V CP-Ti Ti2448

0.018 0.033 0.035

Ecorr (V) −0.69 −0.39 −0.53

Ipp (␮A/cm2 ) 1.87 2.22 3.05

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Y. Bai et al. / Applied Surface Science 258 (2012) 4035–4040

Nb 3d

(a)

(b)

Intensity, a.u.

Intensity, a.u.

Ti 2p

471 468 465 462 459 456 453

216

Binging energy(eV)

213

210

207

204

201

Binging energy(eV)

Zr 3d

(d)

Sn 3d

Intensity (a.u.)

Intensity (a.u.)

(c)

189

186

183

180

177

501 498 495 492 489 486 483 480

Binging energy(eV)

Binging energy(eV)

Fig. 5. XPS spectra of Ti2448 alloy in the PBS solution at 37 ◦ C: (a) Ti 2p, (b) Nb 3d, (c) Zr 3d (d) Sn 3d.

-3x10

Z′ (Ω.cm2)

dissolution in acidic solution and enhanced passivation relative to pure titanium. The previous studies also noted the combinational contribution of alloying elements, for example, both Nb and Zr additions favor the formation of strong covalent bands between near neighbors Ti, Nb and Zr by the sharing of unpaired d-level electron [27]. In fact, the effects of alloying elements on the passivation properties of materials are decided by the structural changes of the TiO2 oxide caused by alloying addition. According to Point Defect Model developed by Macdonald [28], the properties of the surface film can be described in terms of the presence of various types of point defects, the less of the points defects existed in the surface film, the more stable of this layer film, so the materials are more resistant to local corrosion. Since the formation of Nb5+ cations that locate in the crystal lattice of titanium oxide, can cause a decrease in the concentration of anion vacancies generated by the presence of lower titanium oxidation states, Ti3+ and Ti2+ [29], Nb makes oxide film on Ti2448 alloy more stoichiometric and corrosion resistant. As a result, Ti2448 exhibit better passivation properties in PBS solution than CP-Ti and Ti–6Al–4V alloy.

5

-2x10

5

-1x10

5

Ti2448 CP-Ti Ti-6Al-4V

0 0.0

4

5

5

1.0x10

5.0x10

1.5x10

5

2.0x10

Z′′ (Ω.cm2) Fig. 6. Nyquist plots in the PBS solution at 37 ◦ C.

10 6 -80

3.4. Electrochemical impedance spectroscopy

-60 10 4 -40

10 3 10 2

-20

Ti2448 CP-Ti

10 1 10 0 -2 10

0

Ti-6Al-4V simulated -1

10

0

10

θ(deg.)

2

|Z|(Ω. cm )

The impedance spectra of Ti2448 alloy, CP-Ti and Ti–6Al–4V are presented in Figs. 6 and 7 using both the Nyquist and the Bode plots respectively. It is clear from Fig. 6 that all the Nyquist plots can be characterized by the incomplete semicircle, a behavior of near capacitive response. The diameter of the semicircle for Ti2448 is almost equal to that of CP-Ti but less than that of Ti–6Al–4V. For the Bode magnitude plots shown in Fig. 7, two distinct regions for these three materials are observed. In the high frequency (103 –105 Hz) range, there exists a flat portion (slope ≈ 0) due to the response of electrolyte resistance. In the region with frequency less than 103 Hz, the spectra displayed a linear slope of about −1, which is the characteristic response of a capacitive behavior of passive film [30].

10 5

1

10

2

10

3

10

4

10

20 5 10

Frequency(Hz) Fig. 7. Bode plots in the PBS solution at 37 ◦ C, in which the open symbols are the experimental results and the solid curves are the modeling results.

Y. Bai et al. / Applied Surface Science 258 (2012) 4035–4040

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Table 2 Electrical parameters of the equivalent circuits obtained by fitting the experimental results of EIS data together with the chi-square values. Materials

RS ( cm2 )

R1 (M cm2 )

Q1 (␮F/cm2 )

n1

R2 (M cm2 )

Q2 (␮F/cm2 )

n2

Chi-square

Ti2448 CP-Ti Ti–6Al–4V

11.4 11.0 7.2

0.22 0.26 0.0003

58.6 51.9 39.7

0.916 0.922 0.850

– – 0.54

– – 28.9

– – 0.928

0.04 0.03 0.02

For Ti2448 and CP-Ti, the Bode phase plots in Fig. 7 reveal a wide frequency range (10−1 to 102 Hz) in which the phase angle being −85◦ . Such phenomenon can be explained by the formation of single passive oxide film on the surface [31]. Fig. 7 also shows that the phase angle drops to 0◦ in the high frequency range (103 to 105 Hz) and decreases to −40◦ in the low frequency range (10−2 to 10−1 Hz). The former is due to the response of electrolyte resistance while the latter is due to the contribution of passive film resistance. The variation of phase angle with frequency of Ti–6Al–4V alloy is different with Ti2448 alloy and CP-Ti: two phase angle maximum locate in the high and the low frequency ranges were observed respectively. Such difference would be due to the formation of the duplex passive film structure on Ti–6Al–4V alloy: a compact inner layer and a porous outer layer. The proposed duplex structure is consistent with the previous investigations [32–34]. The formation of porous layer can be explained by the dissolution of vanadium oxide in the electrolyte through the mechanism of vacancy generation and diffusion across the outer passive layer [32]. The variation of the impedance with frequency can be described by equivalent circuits with one or more time constants. According to the EIS data presented in Figs. 6 and 7, a Rs (Q1 R1 ) model with only one time constant, as shown in Fig. 8(a), was used to fit the interface behavior of Ti2448 alloy and CP-Ti, on which form a single passive film. Rs is the electrolyte resistance, R1 is the resistance of passive film and Q1 is the constant phase element (CPE) of passive film. Here the CPE was used in the fitting to instead of capacitance C because the former considers the fact that the barrier film never exhibits the theoretically expected phase shift of −90◦ and a slope of −1 for an ideal dielectric. In fact, the impedance of a CPE is defined as [35]: n −1

ZCPE = [C(jw) ]

(1)

where C is the capacitance associated to an ideal capacitor, w is the angular frequency and n is a factor accounting for the deviation from the ideal capacitive behavior due to surface inhomogeneity, roughness factors and adsorption effects. When the value of n is close to 1, the behavior of the surface layer approached that of an ideal capacitor. For Ti–6Al–4V alloy, a Rs (Q1 R1 )(Q2 R2 ) model with two time constants (Fig. 8(b)) was used to fit the EIS data, in which Rs is the electrolyte resistance and the footnote 1 and 2 represents the

Q1

(a) Rs

R1

(b)

Rs

Q1

Q2

R1

R2

Fig. 8. Equivalent circuit fitted for three samples in the PBS solution at 37 ◦ C (a) for Ti2448 and CP-Ti (b) for Ti–6Al–4V alloy.

porous outer and compact inner layers, respectively. The equivalent circuit used in the present work has already been proposed for Ti–6Al–4V alloy, resulting in similar fitted values [14,33,36]. The modeling curves are also plotted in Fig. 7 and show good agreement with the experimental results. The results of the numerical analysis are listed in Table 2 together with the errors expressed by the chi-square value. The obtained values of chi-square vary from 0.02 to 0.04, indicating a good agreement. As shown in Table 2, the capacitance (Q1 ) and the resistance (R1 ) of the passive film for Ti2448 alloy sample was 58.6 ␮F/cm2 and 0.22 M cm2 respectively, which is close to those of CP-Ti. The low Q1 and high R1 indicate the formation of a highly stable film on Ti2448 alloy in the studied electrolyte while the n value of 0.916 shows a near capacitive behavior of this passive film. So Ti2448 alloy possesses a noble electrochemical corrosion behavior, being consistent with the relatively low corrosion rates determined in polarization test. The passive film thickness can be estimated by assuming that the passive film behaves as parallel plate capacitor, using the wellknown equation [37]:

d=

εε0 A C

(2)

where d represents the thickness of passive layer, ε is the dielectric constant for the passive film, ε0 is the dielectric permittivity of vacuum (ε0 = 8.85 × 10−12 F/m), A is the effective area of passive area, and C is the capacitance obtained from fitting the experimental results to the model proposed (which is equivalent to the capacitance Q). The value of the dielectric constant, ε, for thin films depends on the experimental conditions, sample preparation and oxide film growth rate. In this work, by assuming the dielectric constant of titanium oxide as 100, which is the typical dielectric value for TiO2 , the thickness of the passive layer formed on Ti2448 alloy and CP-Ti is estimated as ∼2 nm. The value may be smaller than the actual one, because the actual surface area are much larger than the geometric area considered in the calculations, and a high value for the surface area would result in higher thickness. According to the proposed model, Rs (Q1 R1 )(Q2 R2 ), the passive film formed on Ti–6Al–4V alloy in PBS solution consists of two layers. The resistance value of the inner barrier layer is significantly larger than that associated to the outer porous layer (Table 2). This suggests that the corrosion protection would origin predominantly from the inner barrier layer. In addition, the exponent of Q2 , n2 is 0.9282, which is indicative of near-capacitive behavior, and coupled with high R2 resistance tends to suggest a compact inner layer. This is slight different from those impedance parameters result obtained for Ti–35Nb alloy [38], which has concluded that both inner and outer layers can provide corrosion protection. When comparing the EIS results of the three samples in PBS solution at 37 ◦ C, it can be seen that the resistance of the barrier layer of Ti–6Al–4V alloy is much higher than those of the other two materials. So it maybe permit to conclude that both inner barrier and outer porous layers can provide a better corrosion protection when compared to the samples with a single film, and which is consistent with the corrosion rates results determined in the polarization test. Based on Eq. (2), the thickness of barrier and porous layer for Ti–6Al–4V alloy in PBS solution are estimated as ∼3 nm and ∼2.5 nm respectively, and

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the former is thicker than those of Ti2448 and CP-Ti in the same electrolyte tested. 4. Conclusions The electrochemical corrosion behavior of Ti2448 alloy, CP-Ti and Ti–6Al–4V alloy in the PBS at 37 ◦ C have been evaluated and given the following conclusions: (1) The protective passive films forms spontaneously on the surface of Ti2448, CP-Ti and Ti–6Al–4V once they immersed into the PBS solution at 37 ◦ C. (2) Ti2448 presented a good passivation performance and its corrosion rate is comparable to that of CP-Ti. The formation of Nb5+ cations decreases the concentration of defects in the passive film of Ti2448 and makes it more stable to improve the corrosion resistance. (3) A single passive layer with thickness ∼2 nm forms on the surface of Ti2448 and CP-Ti whereas a duplex film with an inner barrier layer and an outer porous layer forms on the surface of Ti–6Al–4V alloy. Acknowledgement This work was partially supported by NSFC grants 51071152 and 50901080, Liaoning S&T Project grant 20092075, as well as 863 Project grant 2011AA030106. References [1] L.S. Assis, S. Wolynec, I. Costa, Corrosion characterization of titanium alloys by electrochemical techniques, Electrochim. Acta 51 (2006) 1815–1819. [2] M. Niinomi, Recent metallic materials for biomedical applications, Metall. Mater. Trans. A 33 (2002) 477–486. [3] Y. Okazaki, Effect of friction on anodic polarization properties of metallic biomaterials, Biomaterials 23 (2002) 2071–2077. [4] M.A. Khan, R.L. Williams, D.F. Williams, The corrosion behaviour of Ti–6Al–4V, Ti–6Al–7Nb and Ti–13Nb–13Zr in protein solutions, Biomaterials 20 (1999) 631–637. [5] L.J. Xu, Y.Y. Chen, Z.G. Liu, F.T. Kong, The microstructure and properties of Ti–Mo–Nb alloys for biomedical application, J. Alloy Compd. 453 (2008) 320–324. [6] Y. Okazaki, Y. Ito, K. Kyo, T. Tateishi, Corrosion resistance and corrosion fatigue strength of new titanium alloys for medical implants without V and Al, Mater. Sci. Eng. A 213 (1996) 138–147. [7] M. Niinomi, Mechanical properties of biomedical titanium alloys, Mater. Sci. Eng. A 243 (1998) 231–236. [8] A. Choubey, R. Balasubramaniam, B. Basui, Effect of replacement of V by Nb and Fe on the electrochemical and corrosion behavior of Ti–6Al–4V in simulated physiological environment, J. Alloy Compd. 381 (2004) 288–294. [9] Y. Mantani, M. Tajima, Phase transformation of quenched alpha martensite by aging in Ti–Nb alloys, Mater. Sci. Eng. A 438 (2006) 315–319. [10] Y.L. Hao, S.J. Li, S.Y. Sun, C.Y. Zheng, Q.M. Hu, R. Yang, Super-elastic titanium alloy with unstable plastic deformation, Appl. Phys. Lett. 87 (2005) 091906. [11] Y.L. Hao, S.J. Li, S.Y. Sun, Elastic deformation behaviour of Ti–24Nb–4Zr–7.9Sn for biomedical applications, Acta Biomater. 3 (2007) 277–286. [12] Y.L. Hao, S.J. Li, S.Y. Sun, Effect of Zr and Sn on Young’s modulus and superelasticity of Ti–Nb-based alloys, Mater. Sci. Eng. A 441 (2006) 112–118. [13] C. Kuphasuk, Y. Oshida, C.J. Andres, S.T. Hovijitra, Electrochemical corrosion of titanium and titanium-based alloys, J. Prosthet. Dent. 85 (2001) 195–202.

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