Materials Science and Engineering C 29 (2009) 1039–1045
Contents lists available at ScienceDirect
Materials Science and Engineering C
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Biodegradable behaviors of AZ31 magnesium alloy in simulated body fluid
Yingwei Song ⁎, Dayong Shan, Rongshi Chen, Fan Zhang, En-Hou Han
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang, 110016, China
a r t i c l e
i n f o
Article history:
Received 21 April 2008
Received in revised form 26 June 2008
Accepted 26 August 2008
Available online 6 September 2008
Keywords:
Magnesium alloys
Biodegradable behaviors
Electrochemical test
Corrosion
a b s t r a c t
Magnesium alloys have unique advantages to act as biodegradable implants for clinical application. The
biodegradable behaviors of AZ31 in simulated body fluid (SBF) for various immersion time intervals were
investigated by electrochemical impedance spectroscopy (EIS) tests and scanning electron microscope (SEM)
observation, and then the biodegradable mechanisms were discussed. It was found that a protective film
layer was formed on the surface of AZ31 in SBF. With increasing of immersion time, the film layer became
more compact. If the immersion time was more than 24 h, the film layer began to degenerate and emerge
corrosion pits. In the meantime, there was hydroxyapatite particles deposited on the film layer. The
hydroxyapatite is the essential component of human bone, which indicates the perfect biocompatibility of
AZ31 magnesium alloy.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Biodegradable implants show unique advantages for repairing the
damaged bone tissues. They can be gradually dissolved, absorbed,
consumed or excreted in human body environment, and then
disappear spontaneously after the bone tissues heal. The available
biodegradable implants in the current market are made of polymeric
or ceramic materials. However, these implants have an unsatisfactory
mechanical strength [1]. Magnesium and its alloys are potential
biodegradable metallic implants based on the following reasons [2–5]:
(1) good biocompatibility: the presence of magnesium element in the
bone system is beneficial to the strength and growth of bone tissues;
(2) non-toxicity: Mg2+ is an essential element to the human body (the
daily intake of Mg2+ for a normal adult is about 300–400 mg).
Redundant Mg2+ is harmless and can be excreted in the urine;
(3) biodegradation: metal magnesium can be biodegraded in human
body fluid by corrosion; (4) good mechanical properties: the density,
elastic modulus and yield strength of magnesium are closer to the bone
tissue than other implants, which can minimize or avoid the stress
shielding effect and (5) other characteristics: such as non-magnetic,
available for roentgenoscopy, good machinability and so on. Thus, the
development of biodegradable magnesium alloys implants is a
promising research subject.
Magnesium alloys as biodegradable implants had already been
introduced to the orthopedic and trauma surgery at the first half of last
century [6]. Though good biocompatibility was found during the clinical
application, a large amount of hydrogen gas accumulated around the
implants due to the rapid corrosion [7], restricting the widespread use of
magnesium alloys as biomaterials. With the improvement of melting
⁎ Corresponding author. Tel.: +86 24 23915897; fax: +86 24 23894149.
E-mail address: [email protected] (Y. Song).
0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.msec.2008.08.026
and machining technologies, various high-qualify magnesium alloys
were developed. Magnesium alloys were paid more attention in the field
of biomaterials again. Recently, many researches about the biodegradable magnesium alloys had been carried out [8–14]. Actually, some
magnesium alloys containing Zr, Cd, rare earth elements and heavy
metals are not suitable for biomaterials application from the medical
aspect [15]. But the mechanical and corrosion properties of pure
magnesium are unsatisfactory. In comparison with other magnesium
alloys, AZ31 magnesium alloy, with low Al content, good mechanical
properties and corrosion resistance, is suitable to act as biodegradable
materials. Thus, the aim of this paper is to study on the biodegradation
process of AZ31 magnesium alloy in simulated body fluid, and then
reveal the biodegradation mechanisms.
2. Experimental
The experimental material used for the investigation is AZ31
magnesium alloy. Its chemical composition (wt.%) is 2.89Al, 0.92Zn,
0.05Mn, 0.01Si, 0.002Cu, 0.001Ni, 0.004Fe and balance Mg. The
surface of substrate materials was ground with 4000 grit waterproof
abrasive paper to ensure the same surface roughness, and then
cleaned ultrasonically in acetone.
A stimulated body fluid (SBF) is composed of 8.8 g/l NaCl, 0.4 g/l KCl,
0.14 g/l CaCl2, 0.35 g/l NaHCO3, 1.0 g/l C6H6O6 (glucose), 0.2 g/l
MgSO4·7H2O, 0.1 g/l KH2PO4·H2O, 0.06 g/l Na2HPO4·7H2O, pH 7.4, and
temperature 37 °C [15].
For the immersion tests, the ratio of sample surface (cm2) to the
volume of the SBF (ml) was set to 1/100. In the meantime, the SBF was
renewed every 8 h. Thus, the pH value of bulk SBF can almost keep
constant. The magnesium alloy samples were immersed in the SBF for
different time intervals. Then the electrochemical impedance spectroscopy (EIS) was measured using a model 5210 lock in amplifier
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Y. Song et al. / Materials Science and Engineering C 29 (2009) 1039–1045
Table 1
The fitting results of AZ31 immersed in SBF for 1 h.
Time Rs
Cdl
Rct
Y0
n
(h) (Ω cm2) (F cm− 2) (Ω cm2) (Ω−1 cm−2 s−n)
1 h 36.72
0.92E−04 2930
7.2E− 06
Rf
L
RL
(Ω cm2) (H cm−2) (Ω cm2)
0.4995 3364
8.546E+ 1801
03
than 10%. The fitting results are listed in Table 1. Rs was the solution
resistance. Rct referred to the charge transfer resistance and Cdl
represented the electric double layer capacity at the interface of
magnesium alloy substrate and SBF (the high frequency capacitance
Fig. 1. EIS of AZ31 immersed in SBF for different time.
coupled with potentiostat model 273. A classical three electrodes cell
was used with platinum as counter electrode, saturated calomel
electrode SCE (+0.242 V vs SHE) as reference electrode and the
samples as working electrode. The scan frequency ranged from
100 kHz to 10 mHz, and the perturbation amplitude was 5 mV.
The surface morphologies were observed with Philips XL30
scanning electron microscope (SEM) equipped with energy dispersive
X-ray spectroscopy (EDX).
The chemical composition was probed using ESCALAB 250 X-ray
photoelectron spectroscopy (XPS). The X-ray source was the Kα peak
of magnesium. All energy values were corrected according to the
adventitious C 1 s signal, which was set at 284.60 eV. The data were
analyzed with Xp speak 4.1 software.
3. Results and discussion
Fig. 1 shows the EIS of AZ31 immersed in SBF for different time
intervals. After immersion for 1 h, the plot consisted of one high
frequency capacitance loop, one medium frequency capacitance loop
and one low frequency inductance loop. When the immersion time
reached 2 h, the low frequency inductance loop disappeared and the
diameter of high frequency capacitance loop increased. There was not
great change in the EIS for 6 h immersion in comparison with 2 h
immersion. At immersion for 12 h, the plot still included two
capacitance loops, but the dimension of capacitance loops increased
markedly. With increasing the immersion time to 24 h, the plot began
to shrink slightly. When AZ31 was immersed in SBF for 48 h, the
dimension of the capacitance loops almost degenerated into an half as
big as that of 24 h immersion. The most significant change happened
at 72 h immersion. The low frequency capacitance loop was replaced
by an inductance loop.
The degradation process of AZ31 in SBF was analyzed based on the
EIS. The Nyquist plot for 1 h immersion displayed three time constants,
which corresponded to the characteristics of electric double layer,
surface film and the existence of the metastable Mg+ respectively [16].
The plot can be explained by the equivalent circuit as shown in Fig. 2. The
data were fitted with ZSimpWin 3.20 software and the errors were less
Fig. 2. Equivalent circuit of AZ31 immersed in SBF for 1 h.
Fig. 3. Surface morphology of AZ31 immersed in SBF for 1 h. (a) Low magnification
morphology; (b) the magnified morphology of Fig. 3(a) in the white circle; (c) the
magnified morphology of Fig. 3(a) at the smooth sites.
Y. Song et al. / Materials Science and Engineering C 29 (2009) 1039–1045
1041
Fig. 4. XPS analysis of AZ31 immersed in SBF for 1 h.
loop). The Rf represented the film resistance. A constant phase element
(CPEf) was used to describe the film capacity. The CPEf was defined by
two values, Y0 and n. If n was equal to 1, CPEf was identical to a capacitor.
Often a constant phase element was used in a model in place of a
capacitor to compensate for the non-homogeneity in the system. The
characteristics of surface products film were described with Rf and CPEf
(the medium frequency capacitance loop). RL and L indicated the
existence of metastable Mg+ during the dissolution of magnesium alloy
substrate (the low frequency inductance loop). The EIS implied that
there was film layer formed on the AZ31 surface after immersion for 1 h.
But AZ31 was not covered with surface film layer completely, and the
dissolution of magnesium alloy was still carried out.
Fig. 3 shows the surface morphology of AZ31 immersed in SBF for 1 h.
Some sites of the substrate surface were covered with irregular pits, and
others were smooth. Especially, plenty of white products were adhered
onto the pits. In order to observe the white products clearly, Fig. 3(b)
shows the amplified morphology of Fig. 3(a) in the white circle. It can be
seen that the substrate surface appeared network structure due to the
presence of plenty of cracks. In the meantime, a large number of round
white particles conglomerating to clusters were deposited on the
network structure. The surface morphology of the smooth sites in Fig. 3
(a) was magnified in Fig. 3(c). The substrate surface was uniform and
smooth, and only a small quantity of cracks and white round particles
were visible. It was clear that the magnesium alloy substrate was already
covered with a film layer. But the film layer was thin. Some of
magnesium alloy substrate was possible to be exposed, and AZ31 can
continue to dissolve in SBF.
The chemical composition of AZ31 after 1 h immersion was
analyzed by XPS as shown in Fig. 4. According to the XPS survey
scanning spectrum, the chemical composition included O, P, Ca, Mg,
Al, Zn and C elements. The high concentration of carbon in the film
surface is common in XPS survey scanning due to the adventitious
hydrocarbons from the environment. The elements of Mg, Al, and Zn
originated from the AZ31 substrate, and Ca and P originated from the
new products deposited on the AZ31 surface. The high-resolution
spectrum for calcium element split into two peaks of Ca 2p3/2 and Ca
2P1/2 as a result of spin orbit splitting, corresponding to Ca10 (PO4)6
(OH)2. The high-resolution spectrum for magnesium element
Table 2
The fitting results of AZ31 immersed in SBF for 2–48 h.
Fig. 5. Equivalent circuit of AZ31 immersed in SBF for 2–48 h.
Time (h) Rs (Ω cm2) Cdl (F cm− 2) Rct (Ω cm2) Y0 (Ω−1 cm−2 s−n) n
Rf (Ω cm2)
2h
6h
12 h
24 h
48 h
3239
3408
3833
3765
2712
38.46
36.23
25.62
28.12
28.43
1.02E− 04
1.21E− 04
1.28E− 04
1.33E− 04
1.25E− 04
5345
5408
7980
7688
4921
7.9E− 06
8.08E− 06
8.32E− 06
8.5E− 06
8.38E− 06
0.5301
0.5873
0.6021
0.5644
0.5528
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Y. Song et al. / Materials Science and Engineering C 29 (2009) 1039–1045
increased with increasing of immersion time, and reached the
maximum value at 12 h immersion, and then the values began to
reduce with longer immersion time. The electric double layer capacity
Cdl and film layer capacity CPEf kept stable. The above EIS results
showed that there was an integral film layer covering the AZ31 surface
for 2–48 h immersion. The integrity of the film layer was improved at
the early stage of immersion, but it became worse with longer
immersion time.
The surface morphology of AZ31 in SBF for 2 h immersion is shown
in Fig. 6. In the case of low magnification morphology in Fig. 6(a),
plenty of round corrosion pits and some white particles were
Fig. 6. Surface morphology of AZ31 immersed in SBF for 2 h. (a) Low magnification
morphology; (b) the magnified morphology of Fig. 6(a) in the white circle; (c) the
magnified morphology of Fig. 6(a) at the smooth sites.
displayed two peaks, corresponding to Mg3 (PO4)2 and Mg (OH)2
respectively.
After immersion for 2–48 h, the EIS only contained one high
frequency capacitance loop and one low frequency capacitance loop.
The high frequency capacitance loop was related to the characteristics
of electric double layer. The low frequency capacitance loop showed
the surface film effect. The disappearance of inductance loop implied
that the substrate was covered with film layer completely. The
equivalent circuit for the EIS is shown in Fig. 5. The charge transfer
resistance Rct was parallel with the electric double layer capacity Cdl.
The film layer capacity CPEf was parallel with the film layer resistance
Rf. The fitting results are listed in Table 2. The values of Rf and Rct
Fig. 7. Surface morphology of AZ31 immersed in SBF for 12 h. (a) Low magnification
morphology; (b) the magnified morphology of the cobweb-like structure in Fig. 7(a);
(c) the magnified morphology of Fig. 7(a) at the smooth sites.
Y. Song et al. / Materials Science and Engineering C 29 (2009) 1039–1045
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Table 3
The fitting results of AZ31 immersed in SBF for 72 h.
Time (h)
Rs (Ω cm2)
Cdl (F cm− 2)
Rct (Ω cm2)
L (H cm−2)
RL (Ω cm2)
72 h
26.5
1.48E− 4
3586
3.341E+ 04
6957
immersion for 6 h. After immersion for 24 h, the morphology of AZ31
surface was similar to that of immersion for 12 h. When the
immersion time was 48 h, the white particles increased obviously
according to Fig. 8(a). But the surface film layer began to delaminate
and flake off according to the magnified morphology of Fig. 8(b).
Based on the surface morphology of immersion for 2–48 h, intact
film layer was formed at 2 h immersion. With increasing the
immersion time, the film layer was improved gradually. The film
layer displayed the optimum structure at 12 h immersion, which can
provide the best protection to the AZ31 substrate. However, the film
layer did not continue to grow with increasing of immersion time.
Contrarily, the film layer suffered attack by the corrosive mediums in
SBF. The degradation of the film layer was not notable at 24 h. When
the immersion time reached 48 h, the film layer worsened markedly.
But the film layer still kept integrity and can go on to protect the
substrate from corroding.
At immersion for 72 h, the low frequency capacitance loop was
replaced by the inductance loop in the Nyqusit plot. The equivalent
circuit is shown in Fig. 9. RL and L were used to describe the
characteristic of inductance loop, implying the initiation of pitting
corrosion [17]. Table 3 lists the fitting results. The surface morphology
of immersion for 72 h is shown in Fig. 10. It was found that the film
layer at the centre regions of the cobweb-like structure ruptured and
Fig. 8. Surface morphology of AZ31 immersed in SBF for 48 h. (a) Low magnification
morphology; (b) high magnification morphology.
observed. The morphology in the white circle in Fig. 6(a) was
magnified as shown in Fig. 6(b). The cracks were deep. There were
white particles congregating on these special locations. Except for
these special regions, the film layer was uniform as shown in Fig. 6(c).
Plenty of cracks divided the surface into network structure, and some
white particles uniformly deposited on the network structure. The
SEM morphologies indicated that an intact surface film was already
formed on the AZ31 surface at 2 h immersion. Increasing the
immersion time to 6 h, there was not obvious change to the surface
morphology. When the immersion time reached 12 h, the film layer
became more compact as shown in Fig. 7. The corrosion pits was not so
clear and some white nodules protruded from the substrate surface
according to Fig. 7(a). The magnified morphology of the white nodules
displayed the cobweb-like structure in Fig. 7(b). A large number of
white products were deposited in the centre regions of the cobweblike structure. The film layer in the centre regions was thicker than
that of at the verge. Except for these white nodules, the surface still
exhibited network structure as shown in Fig. 7(c). But it was obvious
that the film layer was more compact and harder than that of
Fig. 9. Equivalent circuit of AZ31 immersed in SBF for 72 h.
Fig. 10. Surface morphology of AZ31 immersed in SBF for 72 h. (a) Fracture of surface
film; (b) corrosion pits on the surface film.
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Y. Song et al. / Materials Science and Engineering C 29 (2009) 1039–1045
flaked away, resulting in the exposure of AZ31substrate as shown in
Fig. 10(a). The nude substrate was the weak sites of the whole surface.
The corrosive mediums such as Cl− preferentially aggregated at these
sites to attack the substrate material then to produce pitting corrosion
as shown in Fig. 10(b).
Based on the results of EIS, SEM and XPS, the biodegradable
mechanisms of AZ31 in SBF were discussed as follows:
Magnesium is very active metal. Corrosion reactions can happen in
pH 7.4 SBF. When the immersion time was less than 1 h, the
electrochemical corrosion mechanisms of magnesium in the neutral
corrosive mediums were proposed according to G. Song [18].
At the anode regions:
þ
Mg→Mg þ e−
ð1Þ
þ
2þ
Mg þ H2 O→Mg
−
þ OH þ 1=2H2 :
ð2Þ
Mg+ was a metastable ion. The low frequency inductance loop for
1 h immersion was attributed to the existence of Mg+. Mg+ was easily
oxidized to form Mg2+, accompanying with the hydrogen evolution.
The total anodic reaction:
2þ
Mg þ H2 O→Mg
−
þ OH þ 1=2H2 þ e− :
ð3Þ
The phenomenon of anodic hydrogen evolution was only special
for magnesium alloys.
At the cathode regions:
−
H2 O þ e− →OH þ 1=2H2 ↑:
ð4Þ
The total reaction:
Mg þ 2H2 O→MgðOHÞ2 þ H2 ↑:
ð5Þ
The metal Mg was transferred into Mg (OH)2 film. In the SBF, the
Mg (OH)2 connected with some H2O molecule to form the hydrate of
Mg (OH)2·nH2O. When the samples were dried in the air, the film
shrank due to dehydration. Then lots of cracks were formed on the film
surface. Thus, the surface layer with many cracks is mainly composed
of Mg (OH)2.
The dissolution of metal magnesium was accompanied with the
hydrogen evolution reaction, resulting in the increase of SBF pH value.
Especially, the OH− concentration near to the substrate surface was
promoted greatly. Then, some accompanying reactions occurred.
−
−
3−
H2 PO4 þ 2OH →PO4 þ 2H2 O
2
−
3−
HPO4 − þ OH →PO4 þ H2 O:
ð6Þ
ð7Þ
The solubility product constant Ksp for Ca10(PO4)6(OH)2 and Mg3
(PO4)2 were 1.6 × 10− 58 and 1.04 × 10− 24 respectively, which were much
lower than the Ksp of other products. Thus, PO3−
will preferentially
4
bond with Ca2+ and Mg2+ to form the white compound of Ca10(PO4)6
(OH)2 and Mg3(PO4)2. Then the white compounds were adhered onto
the surface of Mg (OH)2 film. H. Kuwahara [19] reported that the
magnesium apatite of (Ca0.86Mg0.14)10(PO4)6(OH)2 was precipitated on
the pure magnesium surface in SBF, which was not in contradiction with
our experiment result. Thus, the white particles were mainly composed
of the compounds of hydroxyapatite and magnesium phosphate.
At the beginning of immersion, the metal magnesium in the special
locations of boundaries and defects was preferentially dissolved to
form Mg (OH)2 film, resulting in the increase of pH value.
Correspondingly, phosphate particles were also deposited onto the
Mg (OH)2 film. Thus, from the observation of SEM morphology in Fig. 3
for 1 h immersion, some corrosion pits can be found, and plenty of
white particles mainly located in the pits. At other regions, metal
magnesium was dissolved slowly. Thus, the Mg (OH)2 film was thinner
and the quantities of white particles were less.
At immersion for 2 h, the substrate was covered with surface film
completely. At this stage, the film layer will grow according to the
following three models [20]. (1) The Mg2+ at the interface of substrate
and film layer will penetrate the film to form Mg (OH)2 at the exterior
surface of the film layer; (2) electrolytes in SBF penetrate the film
layer to form Mg (OH)2 at the interior surface of the film layer and (3)
The Mg2+ diffuses toward the exterior surface of the film layer. In the
meantime, the electrolytes diffuse toward the interior surface of the
film layer. Mg2+ and electrolytes meet inside the film layer to form Mg
(OH)2. No matter what the models are, the pH value near to the film
layer surface will be promoted, resulting in more and more white
particles deposited. At this stage, the growth of film layer was slow
due to the slow diffusion. Thus, there were not obvious changes for the
EIS and SEM at immersion for 2 h and 6 h.
With increasing of immersion time, the film layer became more
compact. The diffusion of ions inside the film was restricted. The film
cannot continue to grow again. The dissolution and formation of surface
film kept dynamic balance at this period. With longer immersion time,
more and more corrosive mediums such as Cl− adsorbed on the film layer
surface to destroy the dynamic balance. The possible reaction was Mg
(OH)2 + 2Cl− → MgCl2 +2OH−. The film dissolution rate was faster than
the film formation rate. The thickness of the film layer reduced gradually.
Thus, the capacitance loops in the EIS shrank for longer immersion time.
At the early stage, plenty of phosphate particles were mainly
deposited at the locations where magnesium was preferentially
dissolved. Increasing the immersion time, more and more white
particles were adsorbed at these locations to form cobweb-like
structure. The film layer at the center regions of the cobweb-like
structure was thicker than that of at the edge regions. Residual stress can
exist in the film layer. The film layer at the centre regions was easy to
rupture and flake away as shown in Fig. 10(a). The corrosive mediums
preferentially attacked the exposed magnesium alloy substrate to
produce corrosion pits. Finally, magnesium alloy substrate was
biodegraded gradually with increasing of immersion time.
As it is well known that hydroxyapatite is the essential component
of human bone. The deposition of hydroxyapatite particles on AZ31
substrate surface in SBF can accelerate the bone tissue to heal, which
indicates that AZ31 magnesium alloy has perfect biocompatibility.
4. Conclusions
AZ31 magnesium alloy was immersed in SBF for various time
intervals to investigate its biodegradable behaviors. At the early stage
of immersion, the substrate was covered with Mg(OH)2 film gradually,
and the film layer became more compact with increasing of immersion
time. In the meantime, plenty of white particles consisting of
hydroxyapatite and magnesium phosphate were deposited on the
Mg (OH)2 film due to the promotion of SBF pH value by the evolution
hydrogen reaction. The hydroxyapatite is the essential component of
human bone, which indicates the perfect biocompatibility of AZ31
magnesium alloy. When the immersion time reached 24 h, the film
layer began to degenerate. But the film layer was intact, and can
provide protection to the AZ31 in SBF. After immersion for 72 h, the
presence of pitting corrosion indicated the complete failure of the film
layer.
Acknowledgement
This work was supported by the National Key Basic Research
Program (No.2007CB613705) and National Key Technology R&D
Program (2006BAE04B05-2).
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