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Biodegradability of β-Mg17Al12 phase in simulated body fluid
Materials Letters 82 (2012) 54–56
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Biodegradability of β-Mg17Al12 phase in simulated body fluid M. Bobby Kannan a,⁎, Erkan Koc b, Mehmet Unal b a b
Discipline of Chemical Engineering, School of Engineering and Physical Sciences, James Cook University, Townsville, Queensland 4811, Australia Department of Metals, Karabuk University, Karabuk 78200, Turkey
a r t i c l e
i n f o
Article history: Received 22 March 2012 Accepted 12 May 2012 Available online 18 May 2012 Keywords: Magnesium alloy β phase Biomaterials Corrosion
a b s t r a c t In vitro degradation behaviour of β phase (Mg17Al12), commonly present in AZ series magnesium alloys, was studied using electrochemical techniques in simulated body fluid. The experimental results suggested that the degradation rate of β phase was significantly lower than that of pure magnesium. Scanning electron microscopy (SEM) analysis of the polarized β phase revealed localized corrosion. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been a significant amount of work on the in vitro and in vivo degradation behaviour of AZ series magnesium alloys e.g. AZ31 and AZ91, for potential biodegradable implant applications [1–5]. Typically, AZ series magnesium alloys contain Mg17Al12 intermetallic particles, otherwise known as β phase. The volume fraction of β phase in AZ series magnesium alloys primarily depends on the wt.% of aluminium in the alloy [6]. Heat-treatment also alters the volume fraction of β phase in Mg–Al alloys [7]. Due to the difference in chemistry, β phase is expected to degrade at a different rate from that of the α phase when exposed to body fluid. Literature suggests that in chloride-containing solution β phase in AZ series alloys corrodes slower than that of the α phase, which was attributed to the electrochemical potential difference [8,9]. For example, in 5% NaCl the electrochemical potential of β phase was 490 mV noble to pure magnesium and the potential difference decreased to 420 mV when compared with AZ91 alloy [9]. Notably, the volume fraction of β phase plays a crucial role in the alloy corrosion resistance [10,11]. A low volume fraction of β phase induces micro-galvanic coupling and accelerates the dissolution of α phase, whereas a high volume fraction of β phase reduces the alloy corrosion rate by acting as a stable barrier against corrosion. However, the biodegradation behaviour of β phase has not been studied in detail. A recent study by one of the authors on the in vitro electrochemical degradation of AZ91magnesium alloys suggested that α phase dissolved preferentially leaving behind the relatively stable β phase [12]. It is critical to know the degradation rate of β phase in body
⁎ Corresponding author. Tel.: + 61 7 4781 5080; fax: + 61 7 4781 6788. E-mail address: [email protected] (M.B. Kannan). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.047
fluid since a complete dissolution of the material is desired for biodegradable implant applications. In this study, the in vitro degradation behaviour of β phase was studied using electrochemical techniques in simulated body fluid. 2. Experimental procedure In this study, β phase was prepared by melting pure magnesium and aluminium (mixing ratio: Mg 58 wt.% and Al 42%) in a graphite crucible under argon gas atmosphere at 750 °C. The molten sample was then casted in a preheated (250 °C) cast iron mould under protective SF6 gas. The composition of β phase is given in Table 1. In vitro degradation behaviour of β phase was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. For comparison, pure magnesium (chemical composition shown in Table 1) was also used in this study. A potentiostat and frequency response analyser (Model VersaSTAT 3) driven by VersaStudio software were used for the electrochemical experiments. In vitro degradation studies were carried out in simulated body fluid (SBF) maintained at a body temperature of 36.5 ± 0.5 °C. The chemical composition of the SBF can be found elsewhere [13]. The solution was buffered with TRIS buffer at a physiological pH of 7.4. The samples were ground with SiC paper up to 2500 grit and later polished with 1 μm alumina powder, and washed with distilled water and ultrasonically cleaned in acetone prior to the electrochemical experiments. Electrochemical impedance spectroscopy (EIS) experiments were performed over the frequency range of 10 5 Hz to 10 − 2 Hz at an AC amplitude of 5 mV. Potentiodynamic polarization experiments were done at a scan rate of 0.5 mV/s. A typical three electrode system consisting of graphite as a counter electrode, Ag/AgCl electrode as a reference electrode and the sample as a working electrode was used. Scanning electron microscopy
M.B. Kannan et al. / Materials Letters 82 (2012) 54–56 Table 1 Chemical composition (wt.%) of pure magnesium and β phase. Sample
Al
Zn
Mn
Si
Fe
Mg
Pure Mg β phase
0.02 41.43
0.01 –
0.01 0.004
0.01 0.007
0.003 0.003
Bal. Bal.
55
Table 2 Electrochemical data obtained from potentiodynamic polarization curves of pure magnesium and β phase. Sample
Ecorr (VAg/AgCl)
Ebd (VAg/AgCl)
icorr (μA/cm2)
Degradation rate (mm/yr)
Pure Mg β phase
− 1.80 − 1.12
− 1.40 − 0.99
190 70
5.50 1.18
(SEM) analysis of the polarized samples was carried out to understand the mode of corrosion. 3. Results and discussion Fig. 1 shows the Nyquist plots for pure magnesium and β phase in SBF and the equivalent circuit (Rs corresponds to solution resistance, CPEdl the double layer capacitance, Rt the charge transfer resistance, and Rf and CPEf represent the film effect [14]) used for modelling the plots. Both the samples showed two capacitive loops i.e. one large capacitive loop at a high frequency range and a small capacitive loop at a mid-frequency range. The occurrence of two capacitive loops in both the samples suggests that a partially protective film was formed on the samples, since the second mid-frequency range capacitive loop corresponds to the relaxation of mass transport through the corrosion product layer [15]. At a low frequency range, both the samples showed inductive loop which indicates that pure magnesium and β phase underwent localized corrosion [16]. Although the Nyquist plots of pure magnesium and β phase showed a similar
electrochemical phenomenon, the polarization resistance (RP) calculated by adding Rt and Rf suggested that β phase exhibited a higher degradation resistance than that of pure magnesium i.e. pure magnesium and β phase showed a RP of 230 Ω.cm 2 and 740 Ω.cm 2, respectively. The potentiodynamic polarization curves for pure magnesium and β phase are shown in Fig. 2. The electrochemical data obtained from the polarization curves are given in Table 2. The corrosion potential of β phase was noble to that of pure magnesium with a potential difference of 680 mV. It was noted that the potential difference in SBF was 190 mV higher than that reported for the NaCl solution [9]. This suggests that the galvanic corrosion in β phase containing magnesium alloy would be higher in SBF than in NaCl solution. Both pure magnesium and β phase showed a breakdown potential above Ecorr, which indicates the presence of a passive film. However, the low passive current of β phase as compared to that of pure magnesium suggests that the protection is higher in β phase than in pure magnesium, which is consistent with the EIS results. Further, the corrosion current indicates that β phase dissolves slower than pure magnesium in SBF. Pure magnesium and β phase exhibited an icorr of 190 μA/cm 2 and 70 μA/cm 2, respectively. The calculated degradation rate of pure magnesium was 5.50 mm/yr, whereas for β phase it was significantly lower i.e. 1.18 mm/yr. Fig. 3 shows the SEM micrographs of pure magnesium and β phase following the potentiodynamic polarization in SBF. Fig. 3a shows that pure magnesium was heavily corroded. Pitting and mud-cracking can be seen in Fig. 3b. It is well known that magnesium undergoes pitting in a chloride-containing solution. Interestingly, β phase also underwent localized corrosion (Fig. 3d). However, unlike pure magnesium, β phase exhibited significant unattacked regions (Fig. 3c). This is in agreement with the polarization curves (Fig. 2) showing lower corrosion current for β phase than that of pure magnesium. 4. Conclusions
Fig. 1. Nyquist plots of pure magnesium and β phase in SBF at 37 °C.
In vitro degradation behaviour of the β phase was studied and compared with pure magnesium. The degradation rate of β phase was 80% lower as compared to pure magnesium in SBF. Postdegradation analysis revealed localized corrosion in β phase. Acknowledgment The authors would like to thank Mr. Rhys Walter for the technical assistance. References
Fig. 2. Potentiodynamic polarization curves of pure magnesium and β phase in simulated body fluid at 37 °C.
[1] Witte F, Kaese V, Haferkamp H, Switzer E, Meyer-Lindenberg A, Wirth CJ, et al. Biomaterials 2005;17:3557. [2] Song Y, Shan D, Han E. Mater Lett 2008;62:3276. [3] Wen Z, Wu C, Dai C, Yang F. J Alloy Comp 2009;488:392. [4] Kannan MB, Singh Raman RK. J Biomed Mater Res A 2010;93:1050. [5] Alvarez-Lopez M, Pereda MD, del Valle J, Fernandez-Lorenzo M, Garcia-Alonso MC, Ruano OA, et al. Acta Biomater 2010;6:1763. [6] Pardo A, Merino MC, Coy AE, Arrabal R, Viejo F, Matykina E. Corros Sci 2008; 50:823. [7] Zhao MC, Liu M, Song G, Atrens A. Corros Sci 2008;50:1939. [8] Song GL, Atrens A. Adv Eng Mater 1999;1:11. [9] Lunder O, Lein JE, Aune TK, Nisancioglu K. Corrosion 1998;45:741. [10] Song G, Atrens A, Wu X, Zhang B. Corros Sci 1998;40:1769.
56
M.B. Kannan et al. / Materials Letters 82 (2012) 54–56
Fig. 3. Post-degradation SEM micrographs of (a) pure magnesium — low magnification shows substantial corrosion, (b) pure magnesium — high magnification shows pits and mud-cracking, (c) β phase — low magnification shows significant unattacked regions, and (d) β phase — high magnification shows localized attack.
[11] Ambat R, Aung NN, Zhou W. Corros Sci 2000;42:1433. [12] Kannan MB. Mater Lett 2010;64:739. [13] Oyane A, Kim HM, Furuya T, Kokubo T, Miyazaki T, Nakamura T. J Biomed Mater Res A 2003;651:88.
[14] Walter R, Kannan MB. Mater Lett 2010;65:748. [15] Zucchi F, Grassi V, Frignani A, Monticelli C, Trabanelli G. J Appl Electrochem 2006;36:195. [16] Jin S, Amira S, Ghali E. Adv Eng Mater 2007;9:75.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Biodegradability of β-Mg17Al12 phase in simulated body fluid M. Bobby Kannan a,⁎, Erkan Koc b, Mehmet Unal b a b
Discipline of Chemical Engineering, School of Engineering and Physical Sciences, James Cook University, Townsville, Queensland 4811, Australia Department of Metals, Karabuk University, Karabuk 78200, Turkey
a r t i c l e
i n f o
Article history: Received 22 March 2012 Accepted 12 May 2012 Available online 18 May 2012 Keywords: Magnesium alloy β phase Biomaterials Corrosion
a b s t r a c t In vitro degradation behaviour of β phase (Mg17Al12), commonly present in AZ series magnesium alloys, was studied using electrochemical techniques in simulated body fluid. The experimental results suggested that the degradation rate of β phase was significantly lower than that of pure magnesium. Scanning electron microscopy (SEM) analysis of the polarized β phase revealed localized corrosion. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been a significant amount of work on the in vitro and in vivo degradation behaviour of AZ series magnesium alloys e.g. AZ31 and AZ91, for potential biodegradable implant applications [1–5]. Typically, AZ series magnesium alloys contain Mg17Al12 intermetallic particles, otherwise known as β phase. The volume fraction of β phase in AZ series magnesium alloys primarily depends on the wt.% of aluminium in the alloy [6]. Heat-treatment also alters the volume fraction of β phase in Mg–Al alloys [7]. Due to the difference in chemistry, β phase is expected to degrade at a different rate from that of the α phase when exposed to body fluid. Literature suggests that in chloride-containing solution β phase in AZ series alloys corrodes slower than that of the α phase, which was attributed to the electrochemical potential difference [8,9]. For example, in 5% NaCl the electrochemical potential of β phase was 490 mV noble to pure magnesium and the potential difference decreased to 420 mV when compared with AZ91 alloy [9]. Notably, the volume fraction of β phase plays a crucial role in the alloy corrosion resistance [10,11]. A low volume fraction of β phase induces micro-galvanic coupling and accelerates the dissolution of α phase, whereas a high volume fraction of β phase reduces the alloy corrosion rate by acting as a stable barrier against corrosion. However, the biodegradation behaviour of β phase has not been studied in detail. A recent study by one of the authors on the in vitro electrochemical degradation of AZ91magnesium alloys suggested that α phase dissolved preferentially leaving behind the relatively stable β phase [12]. It is critical to know the degradation rate of β phase in body
⁎ Corresponding author. Tel.: + 61 7 4781 5080; fax: + 61 7 4781 6788. E-mail address: [email protected] (M.B. Kannan). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.047
fluid since a complete dissolution of the material is desired for biodegradable implant applications. In this study, the in vitro degradation behaviour of β phase was studied using electrochemical techniques in simulated body fluid. 2. Experimental procedure In this study, β phase was prepared by melting pure magnesium and aluminium (mixing ratio: Mg 58 wt.% and Al 42%) in a graphite crucible under argon gas atmosphere at 750 °C. The molten sample was then casted in a preheated (250 °C) cast iron mould under protective SF6 gas. The composition of β phase is given in Table 1. In vitro degradation behaviour of β phase was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. For comparison, pure magnesium (chemical composition shown in Table 1) was also used in this study. A potentiostat and frequency response analyser (Model VersaSTAT 3) driven by VersaStudio software were used for the electrochemical experiments. In vitro degradation studies were carried out in simulated body fluid (SBF) maintained at a body temperature of 36.5 ± 0.5 °C. The chemical composition of the SBF can be found elsewhere [13]. The solution was buffered with TRIS buffer at a physiological pH of 7.4. The samples were ground with SiC paper up to 2500 grit and later polished with 1 μm alumina powder, and washed with distilled water and ultrasonically cleaned in acetone prior to the electrochemical experiments. Electrochemical impedance spectroscopy (EIS) experiments were performed over the frequency range of 10 5 Hz to 10 − 2 Hz at an AC amplitude of 5 mV. Potentiodynamic polarization experiments were done at a scan rate of 0.5 mV/s. A typical three electrode system consisting of graphite as a counter electrode, Ag/AgCl electrode as a reference electrode and the sample as a working electrode was used. Scanning electron microscopy
M.B. Kannan et al. / Materials Letters 82 (2012) 54–56 Table 1 Chemical composition (wt.%) of pure magnesium and β phase. Sample
Al
Zn
Mn
Si
Fe
Mg
Pure Mg β phase
0.02 41.43
0.01 –
0.01 0.004
0.01 0.007
0.003 0.003
Bal. Bal.
55
Table 2 Electrochemical data obtained from potentiodynamic polarization curves of pure magnesium and β phase. Sample
Ecorr (VAg/AgCl)
Ebd (VAg/AgCl)
icorr (μA/cm2)
Degradation rate (mm/yr)
Pure Mg β phase
− 1.80 − 1.12
− 1.40 − 0.99
190 70
5.50 1.18
(SEM) analysis of the polarized samples was carried out to understand the mode of corrosion. 3. Results and discussion Fig. 1 shows the Nyquist plots for pure magnesium and β phase in SBF and the equivalent circuit (Rs corresponds to solution resistance, CPEdl the double layer capacitance, Rt the charge transfer resistance, and Rf and CPEf represent the film effect [14]) used for modelling the plots. Both the samples showed two capacitive loops i.e. one large capacitive loop at a high frequency range and a small capacitive loop at a mid-frequency range. The occurrence of two capacitive loops in both the samples suggests that a partially protective film was formed on the samples, since the second mid-frequency range capacitive loop corresponds to the relaxation of mass transport through the corrosion product layer [15]. At a low frequency range, both the samples showed inductive loop which indicates that pure magnesium and β phase underwent localized corrosion [16]. Although the Nyquist plots of pure magnesium and β phase showed a similar
electrochemical phenomenon, the polarization resistance (RP) calculated by adding Rt and Rf suggested that β phase exhibited a higher degradation resistance than that of pure magnesium i.e. pure magnesium and β phase showed a RP of 230 Ω.cm 2 and 740 Ω.cm 2, respectively. The potentiodynamic polarization curves for pure magnesium and β phase are shown in Fig. 2. The electrochemical data obtained from the polarization curves are given in Table 2. The corrosion potential of β phase was noble to that of pure magnesium with a potential difference of 680 mV. It was noted that the potential difference in SBF was 190 mV higher than that reported for the NaCl solution [9]. This suggests that the galvanic corrosion in β phase containing magnesium alloy would be higher in SBF than in NaCl solution. Both pure magnesium and β phase showed a breakdown potential above Ecorr, which indicates the presence of a passive film. However, the low passive current of β phase as compared to that of pure magnesium suggests that the protection is higher in β phase than in pure magnesium, which is consistent with the EIS results. Further, the corrosion current indicates that β phase dissolves slower than pure magnesium in SBF. Pure magnesium and β phase exhibited an icorr of 190 μA/cm 2 and 70 μA/cm 2, respectively. The calculated degradation rate of pure magnesium was 5.50 mm/yr, whereas for β phase it was significantly lower i.e. 1.18 mm/yr. Fig. 3 shows the SEM micrographs of pure magnesium and β phase following the potentiodynamic polarization in SBF. Fig. 3a shows that pure magnesium was heavily corroded. Pitting and mud-cracking can be seen in Fig. 3b. It is well known that magnesium undergoes pitting in a chloride-containing solution. Interestingly, β phase also underwent localized corrosion (Fig. 3d). However, unlike pure magnesium, β phase exhibited significant unattacked regions (Fig. 3c). This is in agreement with the polarization curves (Fig. 2) showing lower corrosion current for β phase than that of pure magnesium. 4. Conclusions
Fig. 1. Nyquist plots of pure magnesium and β phase in SBF at 37 °C.
In vitro degradation behaviour of the β phase was studied and compared with pure magnesium. The degradation rate of β phase was 80% lower as compared to pure magnesium in SBF. Postdegradation analysis revealed localized corrosion in β phase. Acknowledgment The authors would like to thank Mr. Rhys Walter for the technical assistance. References
Fig. 2. Potentiodynamic polarization curves of pure magnesium and β phase in simulated body fluid at 37 °C.
[1] Witte F, Kaese V, Haferkamp H, Switzer E, Meyer-Lindenberg A, Wirth CJ, et al. Biomaterials 2005;17:3557. [2] Song Y, Shan D, Han E. Mater Lett 2008;62:3276. [3] Wen Z, Wu C, Dai C, Yang F. J Alloy Comp 2009;488:392. [4] Kannan MB, Singh Raman RK. J Biomed Mater Res A 2010;93:1050. [5] Alvarez-Lopez M, Pereda MD, del Valle J, Fernandez-Lorenzo M, Garcia-Alonso MC, Ruano OA, et al. Acta Biomater 2010;6:1763. [6] Pardo A, Merino MC, Coy AE, Arrabal R, Viejo F, Matykina E. Corros Sci 2008; 50:823. [7] Zhao MC, Liu M, Song G, Atrens A. Corros Sci 2008;50:1939. [8] Song GL, Atrens A. Adv Eng Mater 1999;1:11. [9] Lunder O, Lein JE, Aune TK, Nisancioglu K. Corrosion 1998;45:741. [10] Song G, Atrens A, Wu X, Zhang B. Corros Sci 1998;40:1769.
56
M.B. Kannan et al. / Materials Letters 82 (2012) 54–56
Fig. 3. Post-degradation SEM micrographs of (a) pure magnesium — low magnification shows substantial corrosion, (b) pure magnesium — high magnification shows pits and mud-cracking, (c) β phase — low magnification shows significant unattacked regions, and (d) β phase — high magnification shows localized attack.
[11] Ambat R, Aung NN, Zhou W. Corros Sci 2000;42:1433. [12] Kannan MB. Mater Lett 2010;64:739. [13] Oyane A, Kim HM, Furuya T, Kokubo T, Miyazaki T, Nakamura T. J Biomed Mater Res A 2003;651:88.
[14] Walter R, Kannan MB. Mater Lett 2010;65:748. [15] Zucchi F, Grassi V, Frignani A, Monticelli C, Trabanelli G. J Appl Electrochem 2006;36:195. [16] Jin S, Amira S, Ghali E. Adv Eng Mater 2007;9:75.