ANODIZING AS A METHOD OF CONTROLLING THE CORROSION
RATE OF DEGRADABLE AZ91D ALLOY IN PHYSIOLOGICAL
SOLUTION
Olandir V. Correa1, Mara C. L. de Oliveira2, Stela M. C. Fernandes3, Viviam S. M. Pereira4,
Renato A. Antunes4
1
Centro de Ciência e Tecnologia de Materiais, Instituto de Pesquisas Energéticas e Nucleares, São Paulo (SP),
Brasil
2
Electrocell Ind. Com. De Equip. Elétricos LTDA, Centro de Inovação, Empreendedorismo e Tecnologia
(CIETEC), São Paulo, Brasil
3
Universidade Nove de Julho (UNINOVE), São Paulo, Brasil
4
Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC, Santo André
(SP), Brasil
E-mail: ovcorrea@ipen.br
Abstract. In this work anodizing was investigated as a method of controlling the corrosion
rate of the AZ91D magnesium alloy. The anodizing process was conducted in a 3 M KOH
solution with the addition of 1 M Na2SiO3 at room temperature. The treatment was performed
at 4 V and 6 V for 1 h and 2 h. The corrosion resistance of the anodized specimens was
evaluated by means of electrochemical impedance spectroscopy and potentiodynamic
polarization curves in a 0.9 wt.% NaCl solution at 37 ºC. The anodizing conditions had a
marked influence on the electrochemical response of the AZ91D alloy. The best corrosion
resistance was obtained for the specimens anodized at 6 V for 1 h.
Keywords: AZ91D, Anodizing, Corrosion
1. INTRODUCTION
Corrosion is a highly undesirable process for traditional metallic biomaterials such as
stainless steels, titanium alloys and cobalt-based alloys [Antunes and De Oliveira, 2009]. It
can generate biocompatibility problems due to the leaching of metallic ions and lead to the
structural collapse of prostheses when combined with cyclic stresses if corrosion-fatigue
mechanisms are active [Hosseinalipour et al., 2010; James and Sire, 2010]. However,
corrosion is a necessary phenomenon for magnesium-based biodegradable alloys [Witte et al.,
2008]. These materials are being currently designed for temporary fixation devices [Zhang et
al., 2009]. In this application, the material must sustain the mechanical loads during the period
necessary for the healing of the bone fracture. Furthermore, it must also be gradually absorbed
by the human body, eliminating the need for an additional surgery to remove the fixation
device [Xin et al., 2011]. Corrosion plays an important role in this process. Magnesium-based
alloys are candidate materials for this application due the intrinsic biocompatibility of
magnesium [Jamesh et al., 2011]. The major concern is related to the high corrosion rate of
magnesium within the human body that can lead to the formation of subcutaneous gas cavities
and premature mechanical failure before the necessary healing period of the bone fracture
[Witte et al., 2005]. In this regard, it is imperative to properly control the corrosion rate of
degradable magnesium alloys. Anodizing, though widely employed for the corrosion
protection of these materials in industrial applications [Ardelean et al., 2009; Zuleta et al.,
2011], is hardly considered as a method of controlling the corrosion rate of magnesium-based
biomaterials [Xue et al., 2011]. The aim of this work was to investigate the influence of
different anodizing conditions on the corrosion behavior of the AZ91D magnesium alloy. An
environmentally friendly anodizing solution consisting of KOH and Na2SiO3 was used.
Different anodizing conditions were tested and the corresponding electrochemical response
was evaluated.
2. MATERIAL AND METHODS
2.1 Material
Die-cast alloy AZ91D was used in this work. The nominal chemical composition of
the alloy is given in Table 1.
Table 1. Nominal chemical composition of the AZ91D alloy.
Element
Al
Mn
Zn
Si
Fe
Cu
Mass (%)
8,30 –
0,15
0,35 –
0,10
0,005 0,030 0,002
9,70
mín.
1,00
máx.
máx.
máx.
Ni
Mg
Bal.
máx.
2.2 Preparation of specimens for the anodizing process
The specimens were cut from the as-cast ingot in rectangular parts with approximately
1 cm2. Next, the specimens were electrically connected to a copper wire and then embedded
in room temperature curing epoxy resin. Before anodizing, the electrode surfaces were
prepared by mechanical polishing with progressively finer SiC paper up to 1200 grit size.
2.3 Anodizing process
The anodizing process was conducted in a 3 M KOH solution with addition of 1 M
Na2SiO3 at room temperature. A three-electrode cell configuration was used for the treatment
with the AZ91D as the working electrode, a standard calomel electrode (SCE) as reference
and a platinum wire as the counter-electrode. The anodic films were produced at two different
conditions of constant potentials: 4 V and 6 V. The potential was controlled with a
potentiostat/galvanostat (Autolab PGSTAT 100). The treatment was carried out for 1 h and 2 h
for each condition of potential.
2.4 Electrochemical tests
The corrosion resistance of the anodizing specimens was evaluated by means of
electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. All
the tests were carried out using a potentiostat/galvanostat Autolab PGSTAT 100 with a
frequency response analyzer (FRA) module for the impedance measurements, using the same
configuration described for the anodizing process. The EIS spectra were obtained over the
frequency range of 100 kHz to 10 mHz, with acquisition of 10 points per decade, at the open
circuit potential (OCP), with an amplitude of the perturbation signal of 10 mV.
Potentiodynamic polarization curves were obtained at a scanning rate of 1 mV.s -1 from the
OCP up to 0 V. The potentials mentioned in this work are referred to the standard calomel
electrode. The tests were performed in a 0.9 wt.% NaCl solution at 37 ºC up to 7 days of
immersion.
2.5 Scanning electron microscopy (SEM)
The surface morphology of the anodic films was observed by SEM, using a TM3000
Hitachi microscope.
3. RESULTS AND DISCUSSION
3.1 Anodizing process
The anodizing process was conducted at 4 V and 6 V, during 1 h and 2 h. The
corresponding chronoamperometric curves obtained at each condition are shown in Fig. 1.The
evolution of the resulting electrical current with time can be promptly observed.
6.0E-02
4V 1h
4V 2h
6V 1h
6V 2h
5.0E-02
I (A.cm-2)
4.0E-02
3.0E-02
2.0E-02
1.0E-02
0.0E+00
0
1000
2000
3000
4000
5000
6000
7000
8000
Time (s)
Figure 1. Chronoamperometric curves obtained for each anodizing condition.
The anodic currents can result from both the anodic dissolution of the alloy and the
formation of the anodic film [Fukuda and Matsumoto, 2004]. The presence of Na2SiO3 in the
anodizing solution can give rise to a suppression of the anodic dissolution of the alloy by
forming a denser and more uniform film. From Fig. 1 it is seen that the anodic currents
decreased with increasing the potential from 4 V to 6 V. It is clear that the treatments
conducted at 4 V yielded a very sharp increase of the anodic currents, while for those
performed at 6 V the rate of current increase was much slower as well as the maximum
currents reached during the treatment. In this regard, it is likely that the film growth is more
homogeneous at 6 V which can be favorable to the corrosion resistance of the anodized alloy.
3.2 Electrochemical tests
3.2.1 Electrochemical impedance spectroscopy (EIS)
Nyquist plots of the anodized AZ91D alloy are shown in Fig. 2. Results for the as-cast
(non-anodized) specimen are also displayed for comparison. The specimens were immersed in
a 0.9 wt.% NaCl solution at 37 ºC for 7 days. The plots in Fig. 2a were obtained after 1 day of
immersion in the electrolyte. They consist of one capacitive loop spanning from the high
frequency to the medium frequency domain while an inductive loop is clearly present in the
low frequency region of the spectra. The capacitive loop in the high to medium frequency
domain is frequently ascribed to charge transfer reactions [Wen et al., 2009]. The inductive
loop in the low frequency region is commonly reported for Mg alloys [Chen et al., 2007;
Alvarez-Lopez et al., 2010]. The inductive behavior is often explained by the presence of
adsorbed surface species such as Mg(OH)2ads and Mg(OH)+ads [Song et al., 1998]. This
behavior was observed for both the non-anodized and anodized specimens, independently of
the anodizing condition. The diameter of the Nyquist plot along the real axis is associated
with the charge transfer resistance of the material, thereby indicating its corrosion resistance
[Turhan et al., 2009]. In this regard, the best results were obtained for the specimens anodized
at 6 V for 1 h and at 4 V for 1 h. The specimen anodized at 6 V for 2 h presented a more
flattened capacitive loop and consequently a smaller diameter of the Nyquist plot, but it is still
better than the non-anodized specimen. The specimen anodized at 4 V for 2 h, in turn,
presented the lowest impedance values, even in comparison with the non-treated material.
The evolution of the electrochemical behavior after 7 days of immersion can be
assessed by analyzing the results shown in Fig. 2b. The diameter of the Nyquist plots
decreased for all the specimens, indicating a loss of corrosion resistance with time. This
behavior was more pronounced for the specimen anodized at 4 V for 1 h and for the specimen
anodized at 6 V for 2 h. The first one presented impedance values that are only slightly higher
than those of the non-anodized specimen. The charge transfer resistance of the second one, in
turn, is smaller than that of the non-anodized material as denoted by the comparatively small
radius of its capacitive loop. The specimen anodized at 6 V for 1 h presented the more stable
behavior, maintaining a high diameter of the Nyquist plot even after 7 days of immersion,
being more corrosion resistant than the non-anodized material and the specimens anodized
according to the other experimental conditions. As observed after 1 day of immersion, the
lowest corrosion resistance is associated with the specimen anodized at 4 V for 2 h.
Potentiodynamic polarization curves of the anodized and as-cast specimens of the
AZ91D alloy after immersion in a 0.9 wt.% NaCl solution at 37 ºC for 7 days are shown in
Fig. 3. The corresponding electrochemical parameters are found in Table 2.
1600
Non-anodized
4V 1h
4V 2h
6V 1h
6V 2h
a)
-Z'' (W.cm2)
1200
800
400
0
-400
1200
800
400
0
1600
Z' (W.cm2)
1500
b)
Non-anodized
4V 1h
4V 2h
6V 1h
6V 2h
1200
2
-Z'' (W.cm )
900
600
300
0
0
-300
300
600
900
1200
1500
Z' (W.cm2)
Figure 2. Nyquist plots of the anodized and as-cast specimens of the AZ91D alloy after
immersion in a 0.9 wt.% NaCl solution at 37 ºC for: a) 1 day; b) 7 days.
0.4
Non-anodized
4V 1h
4V 2h
6V 1h
6V 2h
0
E (V)
-0.4
-0.8
-1.2
-1.6
-2
1.0E-09
1.0E-07
1.0E-05
-2
1.0E-03
1.0E-01
I (A.cm )
Figure 3. Potentiodynamic polarization curves of the anodized and as-cast specimens of the
AZ91D alloy after immersion in a 0.9 wt.% NaCl solution at 37 ºC for 7 days.
Table 2. Electrochemical parameters determined from the potentiodynamic polarization
curves shown in Fig. 3.
Specimen
Electrochemical
Non-
parameters
4V 1h
4V 2h
6V 1h
6V 2h
Ecorr (V)
-1.25
-1.38
-1.23
-1.30
-1.24
Icorr( μA/cm²)
0.11
4.67
0.12
0.56
0.18
anodized
The results shown in Table 2 evidenced that the corrosion potential (Ecorr) of the
specimens anodized at 4 V for 1 h and at 6 V for 1 h were only slightly different from that of
the non-anodized material. The corrosion current density (Icorr) values of these specimens, in
turn, are lower than that of the non-anodized material, indicating that the corrosion resistance
was increased after the anodizing process at these conditions. Conversely, the corrosion
potentials of the specimens anodized at 4 V for 2 h and at 6 V for 2 h are less noble than that
of the non-anodized material, suggesting that they are less stable and more active in the
electrolyte. In the same way, the Icorr values of these specimens are higher than that of the
non-anodized material, indicating their relatively low corrosion resistance. These results
support those obtained from the EIS measurements. The best anodizing condition was for the
treatment conducted at 6 V for 1 h, followed by that conducted at 4 V for 1 h. It is also
noteworthy from the polarization curves that the as-cast material and the specimens anodized
at 6 V for 1h and at 4 V for 1h present a self-passivation characteristic while the specimens
anodized at 4 V for 2h and at 6 V for 2h presented active dissolution. The specimen anodized
at 6 V for 1h presented the highest breakdown potential (-0.84 V) and the widest passive
region, confirming the more protective character of the anodic film grown at this condition.
3.3 SEM micrographs
SEM micrographs of the as-anodized specimens are shown in Fig. 4. The anodic films
formed at 6 V were more uniform than those formed at 4 V. The films formed at 4 V displays
several cracks and holes while those formed at 6 V are less defective. This result confirms the
indication of the chronoamperometric curves shown in Fig. 1 where the steep increase of the
anodic currents observed for the films grown at 4 V were hypothesized to favor the formation
of a less homogeneous layer on the surface of the AZ91D alloy. It is noteworthy that the film
formed at 6 V for 1 h is more uniform and less defective than those grown at the other
anodizing conditions. This explains the superior stability of this condition in comparison with
the other films as indicated by the EIS plots shown in Fig. 2.
Figure 4. SEM micrographs of the as-anodized specimens.
4. CONCLUSIONS
Anodizing can be used to control the corrosion rate of the AZ91D alloy in
physiological solution. The effectiveness of the corrosion protection imparted by the anodic
film is strongly related to the experimental conditions of the anodizing process. Both the
potential and time of treatment affect the corrosion behavior of the anodized specimens. The
best performance was achieved for the material anodized at 6 V for 1 h.
ACKNOWLEDGMENTS
The authors are thankful to Rima Industrial S/A for kindly providing the material used
in this work.
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