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Corrosion behavior of Zr-based bulk metallic glasses in different artificial body fluids
Journal of Alloys and Compounds 425 (2006) 268–273
Corrosion behavior of Zr-based bulk metallic glasses in different artificial body fluids L. Liu a,∗ , C.L. Qiu a , Q. Chen a , S.M. Zhang b a
Department of Materials Science and Engineering, The State Key Lab of Die & Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, PR China b Advanced Biomaterials and Tissue Engineering Center, Huazhong University of Science and Technology, 430074 Wuhan, PR China Received 28 October 2005; received in revised form 2 January 2006; accepted 4 January 2006 Available online 2 March 2006
Abstract Bulk metallic glasses (BMGs) of Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 were prepared by copper mold casting. The corrosion behavior of the BMGs in different types of artificial body fluids, including artificial salvia solution (ASS), phosphate buffered solution (PBS) and artificial blood plasma solution (ABP), was investigated by electrochemical polarization and galvanostatic-step measurements at a constant temperature of 37 ◦ C. It was found that all three BMGs exhibited excellent corrosion resistance in various artificial body fluids. Compared with Zr65 Cu17.5 Ni10 Al7.5 base system, the BMGs containing Nb, or Nb and Pd, exhibited superior corrosion resistance against pitting, indicating that the addition of Nb enhanced the corrosion resistance. © 2006 Elsevier B.V. All rights reserved. Keywords: Zr-based bulk metallic glass; Electrochemical polarization; Galvanostatic-step measurement; Artificial body fluids
1. Introduction Bulk metallic glasses (BMGs), especially Zr-based alloys, have attracted an increasing attention in the last decade due to their unique properties including superior strength (∼2 Gpa), high elastic strain limit (∼2%), relatively low Young’s modulus (50–100 Gpa), excellent corrosion resistance and improved wear resistance [1–7]. The properties together with easy forming ability in viscous state make them extremely promising for biomedical applications. Hiromoto et al. [8–10] investigated firstly the effect of chloride-ion concentration, pH value, surface finishing and dissolved oxygen pressure on the polarization behavior of Zr65 Cu17.5 Ni10 Al7.5 bulk metallic glass in phosphate buffered solution, and found that the BMG exhibited a similar polarization resistance to pure titanium and is thus expected to have high corrosion resistance in vivo. Subsequently, Horton and Parsell performed [11] a series of tests on the corrosion behavior and biocompatibility of the BAM-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni) and found that BAM-
∗
Corresponding author. Tel.: +86 27 87556894; fax: +86 27 87554405. E-mail address: [email protected] (L. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.048
11 displayed as good biocompatibility as the titanium and polyethylene specimens. Its corrosion resistance in artificial body fluids can meet the requirement for biomedical use. In our previous works, we have studied the effect of a small amount of Nb on the mechanical properties and corrosion behavior of a few of Zr-based bulk metallic glasses in artificial body fluid. It was found that the addition of 2–5 at% of Nb in Zr65 Cu17.5 Ni10 Al7.5 could not only improve the roomtemperature plasticity but also enhance the corrosion resistance and suppress ion release of the base alloy [12]. The enhanced corrosion resistance in different electrolytes have also been reported in a few of other systems with the addition of Nb, including Zr59 Cu20 Al10 Ni8 Nb3 [13], (Cu60 Zr30 Ti10 )95 Nb5 [14] and Zr55 Al20−x Co25 Nbx (x = 0 to 5 at.%) [15] BMGs. However, the drawback of Zr65 Cu17.5 Ni10 Al7.5 -based system for biomedical use may result from the inclusion of 10 at% Ni in the alloy, which is usually considered harmful to body tissues. Therefore, it is anticipant to prepare Ni-free or, at least, Ni-reduced Zr-based BMGs in order to further improve their biocompatibility. In this paper, we have prepared three BMGs: i.e., Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 , and examined comparatively their electrochemical behavior in different types of artificial body fluids (ABFs). We will evaluate
L. Liu et al. / Journal of Alloys and Compounds 425 (2006) 268–273
269
Thermal response of the BMGs was investigated using a differential scanning calorimetry (DSC, Perkin-Elmer DSC-7) at a heating rate of 20 ◦ C/min under a constant flow of argon. Electrochemical polarization was conducted in a three-electrode cell using a platinum counter electrode and a saturated calomel reference electrode (SCE). The whole cell was kept at 37 ◦ C and the distance between reference and work-
Fig. 1. X-ray diffraction patterns of as-cast Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) BMGs.
preliminarily the biocompatibility of the three BMGs through electrochemical measurement and galvanostatic-step investigation in different artificial body fluids. 2. Experimental Three alloy ingots with nominal compositions of Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (named as sample A, B and C, respectively) were prepared by arc-melting the mixture of pure Zr, Nb, Cu, Ni, Pd and Al metals (purity > 99.5%) under an argon atmosphere. From the master alloys, sample rods with a diameter of 3 mm and a length of 50 mm were produced by copper mould casting. The rods were then cut into a few thin slices for structural and thermal investigations. The amorphous feature of the three alloys obtained was verified by X-ray diffraction (XRD, ’Pert PRO).
Fig. 2. DSC curves of as-cast Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 ) Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys at a heating rate of 20 K/min. The insert shows a electron diffraction pattern for the sample B or C annealed upto the end of the first DSC peak. The pattern exhibits a five-fold symmetry, which is typical pattern for icosahedral quasicrystal.
Fig. 3. Potentiodynamic polarization curves of Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 ) Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys in ASS (a), PBS (b), and ABP (c).
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Table 1 Compositions (g/L) of artificial salvia solution (ASS pH 6.2), phosphate buffered solution (PBS, pH 7.4) and artificial blood plasma solution (ABP, pH 7.4) Solution
NaCl
NaHCO3
KCl
NaH2 PO4
KH2 PO4
KSCN
Lactic acid
Na2 HPO4 ·3H2 O
MgCl2 ·6H2 O
CaCl2
Na2 SO4
ASS PBS ABP
1.5 8 8.036
1.5 – 0.352
– 0.2 0.225
0.5 0.14 –
– 0.2 –
0.5 – –
0.9 – –
– – 0.238
– – 0.311
– – 0.293
– – 0.072
ing electrodes was fixed throughout the test. The electrolytes used in the present study are artificial salvia solution (ASS, pH 6.2), phosphate buffered solution (PBS, pH 7.4) and artificial blood plasma solution (ABP, pH 7.4) whose compositions are listed in Table 1. Specimens for corrosion test were closely sealed with epoxy resin and only leave an end-surface (with a cross-section area of about 7 mm2 ) exposed for testing. Prior to the test, the testing surface of each specimen was mechanically polished to mirror finish, then degreased in acetone, washed in distilled water and dried in air. The potentiodynamic polarization curves of the specimens were recorded at a potential sweep rate of 1 mV/s when the opencircuit potential became almost steady after immersion in artificial body fluids for at least 20 min. In order to clarify the morphology of the alloys after corrosion, some specimens were anodically polarized to the point where pitting corrosion just started, and were then immediately taken out for morphological examinations by scanning electron microscopy (SEM, Philips Quanta 200). In addition, galvanostatic-step technique (GIT) was applied to investigate the formation process of the passive film under a constant electrode current density of 2 mA/cm2 . After testing, some specimens were taken out for morphological observations using optical microscopy (OM).
3. Results Fig. 1 shows the X-ray diffraction (XRD) patterns of the three Zr-based alloys with different compositions. A broad diffraction hump without any trace of crystalline peaks in their XRD patterns indicates that all as-cast samples are basically amorphous. Fig. 2 shows the DSC curves of the three BMGs at heating rate of 20 ◦ C/min. They all exhibit a distinct glass transition and wide supercooled liquid region before crystallization. The glass transition temperature (Tg ), the onset temperature of crystallization (Tx ) and the resulting temperature interval of the supercooled liquid region (Tx = Tx − Tg ), as well as the enthalpy for crystallization (Hx ) are summarized in Table 2. It can be seen that sample B shows a higher Tg but a lower Tx than sample A, leading to a significant decrease in Tx from 103 ◦ C for sample A to 68 ◦ C for sample B. However, further substitution of Ni with 5 at% Pd yields a great decrease in Tg but only a slight decrease
Table 2 The data of Tg , Tx , Tx and Hx for as-cast Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys at a heating rate of 20 ◦ C/min Alloy samples Sample A Sample B Sample C
Tg (◦ C)
Tx (◦ C)
366 389 343
469 457 451
Tx (◦ C) 103 68 108
H (J/g) −53 −48 −52
in Tx , causing the increase of Tx back to 108 ◦ C This indicates that elements Nb and Pd play a contrary function on the thermal stability of the supercooled liquid of the base BMG. On the other hand, the addition of Nb and Pd also have a significant effect on the crystallization of the base alloy, as indicated by the change from a single crystallization event for Nb-free BMG (sample A) to a multi-crystallization process for the Nb- and/or Pd-bearing BMGs (samples B and C). XRD and TEM revealed that the first DSC peaks for samples B and C correspond to the formation of quasicrystals, as demonstrated by the inserted electron diffraction pattern with a typical five-fold symmetry. The promoting effect of Nb and Pd, as well as other elements of Ta, V, and Ag, on the formation of quasicrystals has been previously reported in a few Zr-based BMG systems [16,17]. It is generally believed that the formation of a strong short-range ordering with icosahedral structure promoted by these additives would account for the preferential precipitation of quasicrystals in the process of annealing. Fig. 3(a–c) show the potentiodynamic polarization curves of the three BMGs in ASS, PBS, and ABP solutions open to air at 37 ◦ C, respectively. 316L stainless steel and Ti–6Al–4V alloy were also tested under the same condition for comparison. It can be seen that all BMGs showed quite similar polarization behaviors in the three solutions, i.e., they were spontaneously passivated with similar passive current densities and showed
Fig. 4. SEM images of the corroded surface of the BMGs after potentiodynamic polarization. (a) Zr65 Cu17.5 Ni10 Al7.5 (sample A), (b) (Zr60 Nb5 ) Cu17.5 Ni10 Al7.5 (sample B).
L. Liu et al. / Journal of Alloys and Compounds 425 (2006) 268–273
wide passive regions before pitting occurred. However, as compared with sample A, sample B (containing 5 at% Nb) exhibited a much higher pitting potential, especially in ASS and PBS solutions. This indicates that the addition of Nb enhanced pitting resistance of the base BMG in the artificial body fluids. For sample C, which involves additional 5 at% Pd in the base system, the passive current density kept almost the same level as sample B, implying that the passive film formed on the sample was the also highly protective although the pitting potential was little bit lower than sample B. However, in comparison of 316L stainless steel and Ti–6Al–4V alloy, the Nb-bearing BMGs (samples B and C) showed evidently much lower passive current density, suggesting that the passive films formed on the two BMGs are more protective in the artificial body fluids although Ti–6Al–4V alloy did not show significant pitting. To better understand the corrosion behavior of the Zr-based BMGs, SEM was employed to investigate the morphologies of the samples after corrosion. For this purpose, samples that were tested in artificial salvia solution (ASS) were chosen as an example. Fig. 4 shows the SEM micrographs of samples A and B, which were anodically polarized to the stage where pitting corrosion had just started (i.e., a stage corresponding to a rapid increase in the anodic current density in the polarization curve). Distinct pits were clearly observed on the surface of the Nb-free sample (see Fig. 4(a)) while no obvious pits were found on the Nb-bearing sample (see Fig. 4(b)). The result confirms again that the addition of Nb enhanced the pitting corrosion resistance of the base BMG. Finally, galvanostatic-step measurements were performed under a constant current of 2 mA/cm2 on anode at 37 ◦ C to understand the formation process and status of passive films for different BMGs in ASS, PBS, and ABP solutions. The corresponding potential–time (E–t) curves of the BMGs with different compositions are shown in Fig. 5. It can be seen that all of the BMGs show a very similar potential–time response in ASS, i.e., the potential initially increased linearly with time (referring to IR drop and the charging of electric double layer) until a steady state with a constant potential (referring to the protection of passive film) was achieved. As compared with 316L stainless steel and Ti–6Al–4V alloy, all BMGs exhibited evidently much higher equilibrium potentials although initial increasing rate were almost identical. This implies that the passive film formed on the BMGs are more protective than those on the stainless steal and Ti–6Al–4V alloy. This is well consistent with the polarization results as shown in Fig. 3(a), in which the BMGs usually exhibited lower passive current densities. Similar relationship between potential and time for the BMGs were also found in PBS and ABP. The only exception for sample A is that the potential primarily increases up to the maximum values of 2.38 V in PBS and 1.78 V in ABP, and then drastically drops down to around 0 V. The potential drop should be associated with the formation of pitting during galvanostatic-step testing. This result indicates that the passive films formed on sample A in PBS and ABP solutions are possibly less protective against pitting than on samples B and C if considering that the distance between the reference and working electrodes was kept almost the same in each galvanostatic-step measurement.
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In fact, optical microscopy revealed that a severe corrosion with the formation of a large quantity of pits occurred in sample A after galvanostatic-step testing, while only a few slight pits were observed on the surface of sample B (see Fig. 6).
Fig. 5. Potential–time (E–t) curves of the Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 ) Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys under a constant current density of 2 mA/cm2 in ASS (a), PBS (b), and ABP (c).
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Fig. 6. OM images of the corroded surface of sample A (a), and sample B (b) after galvanostatic-step testing.
4. Discussion The experimental results demonstrate that the Zr-based BMGs show excellent corrosion resistance in artificial body fluids, as indicated by the low passive current density. This is probably due to the formation of highly uniform and protective passive films on the BMGs owing to their homogeneous structure of amorphous phase. It is worthy of mention that the addition of a small amount of Nb can further enhance the corrosion resistance of the base alloy. Both electrochemical polarization and microscopic observations indicate that the Nb-bearing BMGs (e.g. samples B and C) have a much higher pitting potential with respect to the Nb-free BMG (i.e., sample A) (see Figs. 3 and 4). It has previously been reported that, for the corrosion of (Cu60 Zr30 Ti10 )95−x Nbx BMG (where x = 0, and 5 at%) in NaCl solution, the addition of Nb caused a depletion in the corrosion-active element of Cu, but enrichment in the corrosion-resistant elements of Zr, Ti and Nb in the passive film formed [14]. It is believed that the same function by Nb may be taken in the present Zr-based BMGs i.e., the addition of Nb may cause the enrichment of corrosion-resistant elements of Zr, Al and Nb (in the form of oxides) on the surface and thus make the passive films more protective. This hypothesis has actually been sustained by SEM and OM observations, which clearly showed that the surface of Nb-bearing BMGs remained in a good form after corrosion in both electrochemical polarization and galvanostatic-step testing, while serious pitting occurred in the Nb-free BMG (see Figs. 4 and 6). It is also worth mentioning that the Nb-free BMG exhibits better pitting resistance in ASS than in ABP and PBS, as indicated by the higher pitting potential in the polarization measurement (see Fig. 3) and higher equilibrium potential in the galvanostatic-step test (see Fig. 5). This may be attributed to the lower concentration of chloride ion in ASS than that in PBS and ABP solutions, because chloride ions are usually
considered to induce the initiation of pitting. In contrast, the Nb-bearing BMGs seem to be insensitive to the concentration of chloride ions and variation of compositions. This demonstrates further that the addition of Nb made the passive film more protective. On the other hand, it is also noted that, although Pd shows a great effect on the glass transition of the base BMG (see Fig. 2), it does not affect so much the corrosion behavior of the BMGs in the three artificial body fluids. Especially, the formation of passive film in samples B and C follows almost the same process based on galvanostatic-step measurements. This is probably due to that Ni and Pd belong to the same group in the elemental periodic table, and the substitution of Ni by Pd will not change much the overall chemical properties of the alloy. Therefore, it is expected to prepare a Ni-free BMG by the substitution of Ni with Pd in Zr-based BMG without compromising its corrosion properties for better biomedical use. Additionally, the large Tx of 108 ◦ C in sample C can facilitate net-shape forming in supercooled liquid region. 5. Conclusion Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 bulk metallic glasses were prepared by copper mould casting. All the BMGs were spontaneously passivated and showed a quite low passive current densities and relatively wide passive region in artificial salvia, phosphate buffered, and artificial blood plasma solutions, implying that the BMGs have a good corrosion resistance in artificial body fluids. The addition of Nb enhanced significantly the pitting resistance of the Zr-based BMGs, which was possibly caused by the enrichment of highly corrosion-resistant Zr-, Al-, Nb-oxides in the passive film. In addition, using 5 at% Pd to substitute part of Ni did not affect much the corrosion behavior, but extends the supercooled liquid region of the amorphous alloy. The reduction
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of Ni content should further improve the biocompatibility of the Zr-based BMGs. Acknowledgement This work was financially supported by the Natural Science Foundation of China under grant No. 50571039 and by the Innovation Foundation of Science and Technology for Graduate Students in HUST. References [1] H.A. Bruck, T. Christman, A.J. Rosakis, W.L. Johnson, Scr. Mater. 30 (1994) 429–434. [2] A. Inoue, Mater. Trans. JIM 36 (1995) 866–875. [3] C.J. Gilbert, R.O. Ritchie, W.L. Johnson, Appl. Phys. Lett. 71 (1997) 476–478. [4] W.L. Johnson, MRS Bull. 24 (1999) 42–56. [5] S. Pang, T. Zhang, H. Kimura, K. Asami, A. Inoue, Mater. Trans. JIM 41 (2000) 1490–1494.
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[6] L. Liu, C.L. Qiu, K.C. Chan, J. Alloys. Compd. 399 (2005) 144–148. [7] J.G. Wang, B.W. Choi, T.G. Nieh, C.T. Liu, J. Mater. Res. 15 (2000) 913–922. [8] S. Hiromoto, A.P. Tsai, M. Sumita, T. Hanawa, Corr. Sci. 42 (2000) 651–1660. [9] S. Hiromoto, A.P. Tsai, M. Sumita, T. Hanawa, Corr. Sci. 42 (2000) 2193–2200. [10] S. Hiromoto, A.P. Tsai, M. Sumita, T. Hanawa, Corr. Sci. 42 (2000) 2167–2185. [11] J.A. Horton, D.E. Parsell, Mater. Res. Soc. Symp. Proc. 754 (2003) CC1.5.1–CC1.5.16. [12] C.L. Qiu, L. Liu, M. Sun, S.M. Zhang, J. Biomed. Mater. Res. Part A 75 (2005) 950–956. [13] V.R. Raju, U. K¨uhn, U. Wolff, F. Schneider, J. Eckert, R. Reiche, A. Gebert, Mater. Lett. 57 (2002) 173–177. [14] C.L. Qin, K. Asami, T. Zhang, W. Zhang, A. Inoue, Mater. Trans. 44 (2003) 749–753. [15] S.J. Pang, T. Zhang, K. Asami, A. Inoue, J. Mater. Res. 18 (2003) 1652–1658. [16] A. Inoue, T. Zhang, J. Saida, M. Matsushita, M.W. Chen, T. Sakurai, Mater. Trans. JIM 40 (1999) 1181–1184. [17] L. Liu, K.C. Chan, G.K.H. Pang, Appl. Phys. Lett. 85 (2004) 2788–2790.
Corrosion behavior of Zr-based bulk metallic glasses in different artificial body fluids L. Liu a,∗ , C.L. Qiu a , Q. Chen a , S.M. Zhang b a
Department of Materials Science and Engineering, The State Key Lab of Die & Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, PR China b Advanced Biomaterials and Tissue Engineering Center, Huazhong University of Science and Technology, 430074 Wuhan, PR China Received 28 October 2005; received in revised form 2 January 2006; accepted 4 January 2006 Available online 2 March 2006
Abstract Bulk metallic glasses (BMGs) of Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 were prepared by copper mold casting. The corrosion behavior of the BMGs in different types of artificial body fluids, including artificial salvia solution (ASS), phosphate buffered solution (PBS) and artificial blood plasma solution (ABP), was investigated by electrochemical polarization and galvanostatic-step measurements at a constant temperature of 37 ◦ C. It was found that all three BMGs exhibited excellent corrosion resistance in various artificial body fluids. Compared with Zr65 Cu17.5 Ni10 Al7.5 base system, the BMGs containing Nb, or Nb and Pd, exhibited superior corrosion resistance against pitting, indicating that the addition of Nb enhanced the corrosion resistance. © 2006 Elsevier B.V. All rights reserved. Keywords: Zr-based bulk metallic glass; Electrochemical polarization; Galvanostatic-step measurement; Artificial body fluids
1. Introduction Bulk metallic glasses (BMGs), especially Zr-based alloys, have attracted an increasing attention in the last decade due to their unique properties including superior strength (∼2 Gpa), high elastic strain limit (∼2%), relatively low Young’s modulus (50–100 Gpa), excellent corrosion resistance and improved wear resistance [1–7]. The properties together with easy forming ability in viscous state make them extremely promising for biomedical applications. Hiromoto et al. [8–10] investigated firstly the effect of chloride-ion concentration, pH value, surface finishing and dissolved oxygen pressure on the polarization behavior of Zr65 Cu17.5 Ni10 Al7.5 bulk metallic glass in phosphate buffered solution, and found that the BMG exhibited a similar polarization resistance to pure titanium and is thus expected to have high corrosion resistance in vivo. Subsequently, Horton and Parsell performed [11] a series of tests on the corrosion behavior and biocompatibility of the BAM-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni) and found that BAM-
∗
Corresponding author. Tel.: +86 27 87556894; fax: +86 27 87554405. E-mail address: [email protected] (L. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.048
11 displayed as good biocompatibility as the titanium and polyethylene specimens. Its corrosion resistance in artificial body fluids can meet the requirement for biomedical use. In our previous works, we have studied the effect of a small amount of Nb on the mechanical properties and corrosion behavior of a few of Zr-based bulk metallic glasses in artificial body fluid. It was found that the addition of 2–5 at% of Nb in Zr65 Cu17.5 Ni10 Al7.5 could not only improve the roomtemperature plasticity but also enhance the corrosion resistance and suppress ion release of the base alloy [12]. The enhanced corrosion resistance in different electrolytes have also been reported in a few of other systems with the addition of Nb, including Zr59 Cu20 Al10 Ni8 Nb3 [13], (Cu60 Zr30 Ti10 )95 Nb5 [14] and Zr55 Al20−x Co25 Nbx (x = 0 to 5 at.%) [15] BMGs. However, the drawback of Zr65 Cu17.5 Ni10 Al7.5 -based system for biomedical use may result from the inclusion of 10 at% Ni in the alloy, which is usually considered harmful to body tissues. Therefore, it is anticipant to prepare Ni-free or, at least, Ni-reduced Zr-based BMGs in order to further improve their biocompatibility. In this paper, we have prepared three BMGs: i.e., Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 , and examined comparatively their electrochemical behavior in different types of artificial body fluids (ABFs). We will evaluate
L. Liu et al. / Journal of Alloys and Compounds 425 (2006) 268–273
269
Thermal response of the BMGs was investigated using a differential scanning calorimetry (DSC, Perkin-Elmer DSC-7) at a heating rate of 20 ◦ C/min under a constant flow of argon. Electrochemical polarization was conducted in a three-electrode cell using a platinum counter electrode and a saturated calomel reference electrode (SCE). The whole cell was kept at 37 ◦ C and the distance between reference and work-
Fig. 1. X-ray diffraction patterns of as-cast Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) BMGs.
preliminarily the biocompatibility of the three BMGs through electrochemical measurement and galvanostatic-step investigation in different artificial body fluids. 2. Experimental Three alloy ingots with nominal compositions of Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (named as sample A, B and C, respectively) were prepared by arc-melting the mixture of pure Zr, Nb, Cu, Ni, Pd and Al metals (purity > 99.5%) under an argon atmosphere. From the master alloys, sample rods with a diameter of 3 mm and a length of 50 mm were produced by copper mould casting. The rods were then cut into a few thin slices for structural and thermal investigations. The amorphous feature of the three alloys obtained was verified by X-ray diffraction (XRD, ’Pert PRO).
Fig. 2. DSC curves of as-cast Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 ) Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys at a heating rate of 20 K/min. The insert shows a electron diffraction pattern for the sample B or C annealed upto the end of the first DSC peak. The pattern exhibits a five-fold symmetry, which is typical pattern for icosahedral quasicrystal.
Fig. 3. Potentiodynamic polarization curves of Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 ) Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys in ASS (a), PBS (b), and ABP (c).
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Table 1 Compositions (g/L) of artificial salvia solution (ASS pH 6.2), phosphate buffered solution (PBS, pH 7.4) and artificial blood plasma solution (ABP, pH 7.4) Solution
NaCl
NaHCO3
KCl
NaH2 PO4
KH2 PO4
KSCN
Lactic acid
Na2 HPO4 ·3H2 O
MgCl2 ·6H2 O
CaCl2
Na2 SO4
ASS PBS ABP
1.5 8 8.036
1.5 – 0.352
– 0.2 0.225
0.5 0.14 –
– 0.2 –
0.5 – –
0.9 – –
– – 0.238
– – 0.311
– – 0.293
– – 0.072
ing electrodes was fixed throughout the test. The electrolytes used in the present study are artificial salvia solution (ASS, pH 6.2), phosphate buffered solution (PBS, pH 7.4) and artificial blood plasma solution (ABP, pH 7.4) whose compositions are listed in Table 1. Specimens for corrosion test were closely sealed with epoxy resin and only leave an end-surface (with a cross-section area of about 7 mm2 ) exposed for testing. Prior to the test, the testing surface of each specimen was mechanically polished to mirror finish, then degreased in acetone, washed in distilled water and dried in air. The potentiodynamic polarization curves of the specimens were recorded at a potential sweep rate of 1 mV/s when the opencircuit potential became almost steady after immersion in artificial body fluids for at least 20 min. In order to clarify the morphology of the alloys after corrosion, some specimens were anodically polarized to the point where pitting corrosion just started, and were then immediately taken out for morphological examinations by scanning electron microscopy (SEM, Philips Quanta 200). In addition, galvanostatic-step technique (GIT) was applied to investigate the formation process of the passive film under a constant electrode current density of 2 mA/cm2 . After testing, some specimens were taken out for morphological observations using optical microscopy (OM).
3. Results Fig. 1 shows the X-ray diffraction (XRD) patterns of the three Zr-based alloys with different compositions. A broad diffraction hump without any trace of crystalline peaks in their XRD patterns indicates that all as-cast samples are basically amorphous. Fig. 2 shows the DSC curves of the three BMGs at heating rate of 20 ◦ C/min. They all exhibit a distinct glass transition and wide supercooled liquid region before crystallization. The glass transition temperature (Tg ), the onset temperature of crystallization (Tx ) and the resulting temperature interval of the supercooled liquid region (Tx = Tx − Tg ), as well as the enthalpy for crystallization (Hx ) are summarized in Table 2. It can be seen that sample B shows a higher Tg but a lower Tx than sample A, leading to a significant decrease in Tx from 103 ◦ C for sample A to 68 ◦ C for sample B. However, further substitution of Ni with 5 at% Pd yields a great decrease in Tg but only a slight decrease
Table 2 The data of Tg , Tx , Tx and Hx for as-cast Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys at a heating rate of 20 ◦ C/min Alloy samples Sample A Sample B Sample C
Tg (◦ C)
Tx (◦ C)
366 389 343
469 457 451
Tx (◦ C) 103 68 108
H (J/g) −53 −48 −52
in Tx , causing the increase of Tx back to 108 ◦ C This indicates that elements Nb and Pd play a contrary function on the thermal stability of the supercooled liquid of the base BMG. On the other hand, the addition of Nb and Pd also have a significant effect on the crystallization of the base alloy, as indicated by the change from a single crystallization event for Nb-free BMG (sample A) to a multi-crystallization process for the Nb- and/or Pd-bearing BMGs (samples B and C). XRD and TEM revealed that the first DSC peaks for samples B and C correspond to the formation of quasicrystals, as demonstrated by the inserted electron diffraction pattern with a typical five-fold symmetry. The promoting effect of Nb and Pd, as well as other elements of Ta, V, and Ag, on the formation of quasicrystals has been previously reported in a few Zr-based BMG systems [16,17]. It is generally believed that the formation of a strong short-range ordering with icosahedral structure promoted by these additives would account for the preferential precipitation of quasicrystals in the process of annealing. Fig. 3(a–c) show the potentiodynamic polarization curves of the three BMGs in ASS, PBS, and ABP solutions open to air at 37 ◦ C, respectively. 316L stainless steel and Ti–6Al–4V alloy were also tested under the same condition for comparison. It can be seen that all BMGs showed quite similar polarization behaviors in the three solutions, i.e., they were spontaneously passivated with similar passive current densities and showed
Fig. 4. SEM images of the corroded surface of the BMGs after potentiodynamic polarization. (a) Zr65 Cu17.5 Ni10 Al7.5 (sample A), (b) (Zr60 Nb5 ) Cu17.5 Ni10 Al7.5 (sample B).
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wide passive regions before pitting occurred. However, as compared with sample A, sample B (containing 5 at% Nb) exhibited a much higher pitting potential, especially in ASS and PBS solutions. This indicates that the addition of Nb enhanced pitting resistance of the base BMG in the artificial body fluids. For sample C, which involves additional 5 at% Pd in the base system, the passive current density kept almost the same level as sample B, implying that the passive film formed on the sample was the also highly protective although the pitting potential was little bit lower than sample B. However, in comparison of 316L stainless steel and Ti–6Al–4V alloy, the Nb-bearing BMGs (samples B and C) showed evidently much lower passive current density, suggesting that the passive films formed on the two BMGs are more protective in the artificial body fluids although Ti–6Al–4V alloy did not show significant pitting. To better understand the corrosion behavior of the Zr-based BMGs, SEM was employed to investigate the morphologies of the samples after corrosion. For this purpose, samples that were tested in artificial salvia solution (ASS) were chosen as an example. Fig. 4 shows the SEM micrographs of samples A and B, which were anodically polarized to the stage where pitting corrosion had just started (i.e., a stage corresponding to a rapid increase in the anodic current density in the polarization curve). Distinct pits were clearly observed on the surface of the Nb-free sample (see Fig. 4(a)) while no obvious pits were found on the Nb-bearing sample (see Fig. 4(b)). The result confirms again that the addition of Nb enhanced the pitting corrosion resistance of the base BMG. Finally, galvanostatic-step measurements were performed under a constant current of 2 mA/cm2 on anode at 37 ◦ C to understand the formation process and status of passive films for different BMGs in ASS, PBS, and ABP solutions. The corresponding potential–time (E–t) curves of the BMGs with different compositions are shown in Fig. 5. It can be seen that all of the BMGs show a very similar potential–time response in ASS, i.e., the potential initially increased linearly with time (referring to IR drop and the charging of electric double layer) until a steady state with a constant potential (referring to the protection of passive film) was achieved. As compared with 316L stainless steel and Ti–6Al–4V alloy, all BMGs exhibited evidently much higher equilibrium potentials although initial increasing rate were almost identical. This implies that the passive film formed on the BMGs are more protective than those on the stainless steal and Ti–6Al–4V alloy. This is well consistent with the polarization results as shown in Fig. 3(a), in which the BMGs usually exhibited lower passive current densities. Similar relationship between potential and time for the BMGs were also found in PBS and ABP. The only exception for sample A is that the potential primarily increases up to the maximum values of 2.38 V in PBS and 1.78 V in ABP, and then drastically drops down to around 0 V. The potential drop should be associated with the formation of pitting during galvanostatic-step testing. This result indicates that the passive films formed on sample A in PBS and ABP solutions are possibly less protective against pitting than on samples B and C if considering that the distance between the reference and working electrodes was kept almost the same in each galvanostatic-step measurement.
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In fact, optical microscopy revealed that a severe corrosion with the formation of a large quantity of pits occurred in sample A after galvanostatic-step testing, while only a few slight pits were observed on the surface of sample B (see Fig. 6).
Fig. 5. Potential–time (E–t) curves of the Zr65 Cu17.5 Ni10 Al7.5 (sample A), (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 (sample B) and (Zr60 Nb5 ) Cu17.5 (Ni5 Pd5 )Al7.5 (sample C) alloys under a constant current density of 2 mA/cm2 in ASS (a), PBS (b), and ABP (c).
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Fig. 6. OM images of the corroded surface of sample A (a), and sample B (b) after galvanostatic-step testing.
4. Discussion The experimental results demonstrate that the Zr-based BMGs show excellent corrosion resistance in artificial body fluids, as indicated by the low passive current density. This is probably due to the formation of highly uniform and protective passive films on the BMGs owing to their homogeneous structure of amorphous phase. It is worthy of mention that the addition of a small amount of Nb can further enhance the corrosion resistance of the base alloy. Both electrochemical polarization and microscopic observations indicate that the Nb-bearing BMGs (e.g. samples B and C) have a much higher pitting potential with respect to the Nb-free BMG (i.e., sample A) (see Figs. 3 and 4). It has previously been reported that, for the corrosion of (Cu60 Zr30 Ti10 )95−x Nbx BMG (where x = 0, and 5 at%) in NaCl solution, the addition of Nb caused a depletion in the corrosion-active element of Cu, but enrichment in the corrosion-resistant elements of Zr, Ti and Nb in the passive film formed [14]. It is believed that the same function by Nb may be taken in the present Zr-based BMGs i.e., the addition of Nb may cause the enrichment of corrosion-resistant elements of Zr, Al and Nb (in the form of oxides) on the surface and thus make the passive films more protective. This hypothesis has actually been sustained by SEM and OM observations, which clearly showed that the surface of Nb-bearing BMGs remained in a good form after corrosion in both electrochemical polarization and galvanostatic-step testing, while serious pitting occurred in the Nb-free BMG (see Figs. 4 and 6). It is also worth mentioning that the Nb-free BMG exhibits better pitting resistance in ASS than in ABP and PBS, as indicated by the higher pitting potential in the polarization measurement (see Fig. 3) and higher equilibrium potential in the galvanostatic-step test (see Fig. 5). This may be attributed to the lower concentration of chloride ion in ASS than that in PBS and ABP solutions, because chloride ions are usually
considered to induce the initiation of pitting. In contrast, the Nb-bearing BMGs seem to be insensitive to the concentration of chloride ions and variation of compositions. This demonstrates further that the addition of Nb made the passive film more protective. On the other hand, it is also noted that, although Pd shows a great effect on the glass transition of the base BMG (see Fig. 2), it does not affect so much the corrosion behavior of the BMGs in the three artificial body fluids. Especially, the formation of passive film in samples B and C follows almost the same process based on galvanostatic-step measurements. This is probably due to that Ni and Pd belong to the same group in the elemental periodic table, and the substitution of Ni by Pd will not change much the overall chemical properties of the alloy. Therefore, it is expected to prepare a Ni-free BMG by the substitution of Ni with Pd in Zr-based BMG without compromising its corrosion properties for better biomedical use. Additionally, the large Tx of 108 ◦ C in sample C can facilitate net-shape forming in supercooled liquid region. 5. Conclusion Zr65 Cu17.5 Ni10 Al7.5 , (Zr60 Nb5 )Cu17.5 Ni10 Al7.5 and (Zr60 Nb5 )Cu17.5 (Ni5 Pd5 )Al7.5 bulk metallic glasses were prepared by copper mould casting. All the BMGs were spontaneously passivated and showed a quite low passive current densities and relatively wide passive region in artificial salvia, phosphate buffered, and artificial blood plasma solutions, implying that the BMGs have a good corrosion resistance in artificial body fluids. The addition of Nb enhanced significantly the pitting resistance of the Zr-based BMGs, which was possibly caused by the enrichment of highly corrosion-resistant Zr-, Al-, Nb-oxides in the passive film. In addition, using 5 at% Pd to substitute part of Ni did not affect much the corrosion behavior, but extends the supercooled liquid region of the amorphous alloy. The reduction
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