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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5568_Библиотеки_им_академика_М_И_Перельмана
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138 Surface Modification of Magnesium and its Alloys for Biomedical Applications
6 M KOH solution for anodic oxidation of Mg alloy, the formation of MgO coating
was anticipated. On the other hand, when the potential was controlled in the
passivation region, Mg(OH)
coating was produced.
2
Based on the anodic oxidation process of magnesium alloy in 6 and 10 M KOH
alkaline solution, MgO or Mg(OH)
coating on Mg allo y is prepared by anodic
2
electrodeposition at a constant potential of 1.0 V (vs. saturated calomel electrode)
for 2 h. The Mg(OH)
treatment in air at 400e450
coating is then converted to MgO by subsequent calcination
2
C(Hahn, Brunner, Kunze, Schmuki, & Virtanen,
2008; Latha, Li, Charles, Roger, & Wang, 2009)
6.2.2 Characterization of oxide coatings on Mg alloy
X-ray diffraction was employed to identify and investigate the phase composition of
the oxide coatings formed by anodic oxidation of magnesium in different KOH
solutions. Figure 6.2 shows the X-ray diffraction spectra of the as-grown and
annealed samples. The XRD patterns, as shown in Figure 6.2a and b,indicate
that the oxide coating formed in 6 M KOH solution, either at room temperature
or after annea ling treatment, consists mainly of MgO, which is confirmed by the specific peak at 2q ¼ 42.9
86e0441) besides the peaks of a-Mg. This fact mean s that Mg O coating films
can be obtained by simple anodic oxidation of magnesium allo y in 6 M KOH
(Joint Committee on Powder Diffraction Standards card
10MKOH
annealed
(d)
10MKOH
(c)
6MKOH
Intensity [a.u.]
annealed
6MKOH
(b)
(a)
10 20 30 40 50 60 70 80
2-theta / degree
Figure 6.2 X-ray diffraction spectra of as-grown and annealed samples (at 450C) for
(a) anodizing in 6 M KOH at room temperature and (b) after annealing, as well as (c) anodizing
in 10 M KOH at room temperature and (d) after annealing.
Anodic electrodeposition of MgO coatings to improve corrosion resistance in vivo 139
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solution. In contrast , the XRD patterns of the magnesium oxidized in 10 M KOH
solution, as shown in Figure 6.2c, reveal two peaks at 2q ¼ 18.3
tively, corresponding to Mg (OH)
crystal structure. After annealing treatment, these
2
and 38,respec-
two peaks disappeared; instead, a peak corresponding to MgO at 2q ¼ 42.9
emerged, as shown in Figure 6.2d, indicative of crystalline MgO formation.
Figure 6.3 shows the SEM surface morphologies of the oxide coatings formed at
room temperature and 50
C. Anodic oxidation of magnesium alloy in 6 M KOH
(a)
(b)
Figure 6.3 Surface morphology of MgO coatings produced by anodic oxidation in 6 M KOH
(a) at room temperature (the inset is EDX analysis corresponding to the assigned area), (b) at
50
C, and (c) annealed at 450C for 2 h in air (the inset is a cross-section micrograph).
140 Surface Modification of Magnesium and its Alloys for Biomedical Applications
(c)
Figure 6.3 (Continued)
solution at 1.0 V (vs. SCE) for 2 h at room temperature resulted in lamellar-like oxide
coatings, as observed in Figure 6.3a. EDX analysis indicated that the oxide coating
consists mainly of Mg and O, and the atomic ratio between Mg and O is approximately
1:1, implying that the oxide coatings consist of MgO, as evident by the inset in
Figure 6.3a. When the anodic oxidation temperature was raised up to 50
C while
keeping other conditions unchanged, platelet-like MgO coatings were produced, as
shown by Figure 6.3b. Figure 6.3c depicts the SEM micrograph of the as-formed
MgO coatings after annealing treatment at 450
C for 6 h in air. It was found that
the platelet-like morphology remains unchanged without any loss of structural
morphology, indicating the as-prepared platelet-like MgO coatings were thermally
stable. The cross-section micrograph, shown in the inset image in Figure 6.3c,
disclosed that the thickness of adherent MgO coating on the Mg alloy surface is
approximately 9.0 mm.
Anodic oxidation of magnesium alloy in 10 M KOH solution at 1.0 V (vs. SCE) for
2 h at room temperature results in compact oxide coatings, as observed in Figure 6.4a.
EDX analysis indicates the oxide coating consists mainly of Mg and O, and the atomic
ratio between Mg and O is approximately 1:2; evidence of Mg(OH)
formation is
2
shown in the inset of Figure 6.4a. Dense oxide coatings were formed by anneal ing
treatment at 450
C for 6 h in air, as shown by Figure 6.4b. The top left inset, showing
the EDX investigation for the assigned area in Figure 6.4b, revealed that the atomic
ratio between Mg and O is approximately 1:1, indicating a change in chemistry of
the oxide coatings from Mg(OH)
to MgO due to dehydration of Mg(OH)2. The
2
EDX analyses are in good agreement with the XRD results in Figure 6 2. The lower
left inset image shows the cross-section micrograph, indicative of a thickness of
approximately 7.5 mm MgO coating on the surface of the Mg alloy.
Anodic electrodeposition of MgO coatings to improve corrosion resistance in vivo 141
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(a)
(b)
Figure 6.4 Surface morphology of oxide coatings produced by anodic oxidation in 10 M KOH
(a) at room temperature (the inset is EDX analysis corresponding to the assigned area), and (b)
annealed at 450
area and the lower left inset is a cross-section micrograph).
C for 2 h in air (the top left inset is EDX analysis corresponding to the assigned
6.2.3 Corrosion resistance of the as-grown MgO coatings
The corrosion resistance of the MgO coating-modified Mg alloy was determined in a
3.5-wt% NaCl solution after 45 min of immersion time using a potentiodynamic
polarization test, as shown in Figure 6.5. To avoid the presence of some degree of
nonlinearity in the Tafel slope region of the obtained polarization curves, the Tafel
142 Surface Modification of Magnesium and its Alloys for Biomedical Applications
0
a bare Mg alloy
b MgO (6MKOH)
–1
c MgO (6MKOH, annealed)
d MgO (10MKOH)
)
–2
2
–3
–4
–5
–6
Log current density (A / cm
–7
–8
a
c
b
d
–1.7
–1.6 –1.5 –1.4
Potential E / V vs. SCE
Figure 6.5 Potentiodynamic polarization curves of as-grown and annealed samples in 3.5-wt%
NaCl solution.
constants were calculated as the slope of the points after the corrosion potential (E
by 60 mV. The E
slopes (b
and bc), were derived directly from the polarization curves by Tafel region
a
and corrosion current density (i
corr
), and anodic/cathodic Tafel
corr
corr
extrapolation. In general, the cathodic polarization curve is attributed to hydrogen
evolution reaction due to the reduction of water, while the anodic polarization curve
is associated with the dissolution of Mg, leading to the formation of Mg
2þ
(Ambat,
Aung, & Zhou, 2000). The polarization resistance (Rp) values were determined
from the SterneGeary equation (Stern & Geary, 1957):
bab
R
¼
p
2:303 i
c
corrðba
þ bcÞ
(6.4)
These results of potentiodynamic polarization are summ arized in Table 6.1. It was
found that the polarization curve for annealed MgO coating formed in 6 M KOH
showed a more positive shift than untreated MgO film, likely due to the consolidation
and higher crystallization of MgO coating after thermal treatment. Moreover,
compared with the bare Mg alloy, the E
of annealed MgO coatings, formed in either
corr
6 or 10 M KOH, positively shifted approximately 100 and 260 mV, respectively, and
the i
lowered more than 10 to 100 times, while the corrosion resistances increased
corr
by three and four orders. The positive shift of corrosion potentials accompanied by
a decrease of corrosion currents and increase of polarization resistances indicate
a decreased tendency to corrosion of MgO-coated samples. The polarization test
)
Anodic electrodeposition of MgO coatings to improve corrosion resistance in vivo 143
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Table 6.1 Corrosion parameters of bare and MgO coatings modified
Mg alloy in 3.5-wt% NaCl
Samples E
Bare Mg alloy 1.62 1.25 10
MgO (6 M KOH) 1.53 2.16 10
MgO (6 M KOH,
(V) i
corr
(A/cm2)R
corr
1.51 6.30 10
4
5
6
(U cm2)
p
4.27 10
1.68 10
1.35 10
2
4
5
annealed)
MgO (10 M KOH) 1.46 6.02 10
6
2.84 10
5
results lead directly to the conclusion that MgO coating fi lm could significantly
improve overall corrosion protection of Mg alloy in NaCl solution.
The different polarization behaviors of the MgO coatings on magnesium alloy are
mainly du e to their different structure and compact state. When the bare Mg alloy
substrate was immersed in NaCl solution, loose and porous Mg(OH)
on Mg alloy surface. With increasing anodic potential during polarization, corrosive
intermediate (Cl
) was rapidly transferred through the outer porous layer and reached
formed quickly
2
the inner Mg alloy matrix, which then resulted in increasing polarization current. At
the same time, the chloride ions can transform Mg(OH)
High Cl
concentration will accelerate the transform reaction of Mg(OH)2to MgCl
to more soluble MgCl2.
2
2
and promote the dissolution of magnesium alloy (Altun & Sen, 2004). Therefore,
the bare Mg alloy showed rapid corrosion rate. As evidenced by SEM, EDX, and
XRD analysis, lamellar-like MgO coatings were produced on Mg alloy surface in
6 M KOH. After annealing treatment, the surface coating film was fused and consolidated to a thick layer, while the MgO coatings formed in 10 M KOH following
annealing treatment were densely compact with a thickness of 7.5 mm. Thus, the
as-grown MgO coating on Mg alloy surface may serve as a barrier, which could
prevent the Mg matrix from direct contact with the corrosive medium as well as block
the transportation of water and Cl
needed for the corrosion reaction into the under-
lying Mg alloy matrix.
It was reported that the protection afforded by the coating is proportional to the
coating thickness (Gray & Luan, 2002). Accordingly, the MgO coating film could
protect substrate from corrosion attack by acting as a physical shield between metal
andmedium,andthusthecorrosionprocessslowsdownbyadoubleeffectof
adecreaseofchargetransferrateandthedecreaseofdiffusionflux across the surface layer (Barchiche, Rocca, Juers, Hazan, & Steinmetz, 2007). Moreover,
annealedMgOcoatingformedin10MKOHshowedsuperiorcorrosionresistance
to that produced in 6 M KOH without performing thermal treatm ent, likely because
there existed more micropores in the lamellar-like MgO film than in densely
compact MgO film. App are nt ly, the corrosion properties of t he MgO coating are
strongly dependent on their morphology, thickness, agglomeration state, and preparation process.
)
144 Surface Modification of Magnesium and its Alloys for Biomedical Applications
6.2.4 Immersion behavior of MgO-coated Mg alloy in SBF
Regarding the immersion test, annealed MgO coating formed in 10 M KOH is
discussed. Figure 6.6 shows the variation of the pH value of SBF as a function of
immersion time. It can be seen that the pH values of the solutions corresponding to
MgO-coated and bare Mg alloy specimens increased rapidly, from 7.2 to 8.6 in the
initial 8 h of immersion. After that, the pH value of the solution for MgO-coated
Mg alloy increased slowly with immersion time and became placid at 8.8, whereas
the pH value of the solution for bare Mg specimens kept increasing and reached to
9.5 after 72 h of immersion.
It is well known that magnesium is an active metal element with a low potential of
2.34 V against normal hydrogen electrode. When a magnesium alloy is exposed to
corrosive electrolyte, the main reaction is the anodic Mg dissolution and cathodic
hydrogen evolution, which can be described as follows (Song, Andrej, & St. John, 2001):
Mg/Mg
2H
2þ
O þ 2ee/2OHeþ H2[ (6.6)
2
þ 2e
e
(6.5)
In the case of MgO-coated Mg alloy, the MgO coatings function as a barrier, whi ch
could prevent the Mg matrix from direct contact wi th corrosive media as well as block
the penetration of electrolytes. Accordingly, at the initial immersion stage, the
corrosion attack is limited at the MgO coating layer. Thermodynamic calculations
indicate that MgO and Mg(OH)
could stably exist in pH domains higher than
2
13.83 and 11.46, respectively (Zhang, Yan, Wang, & Li, 2005). As a result, MgO
coating film will be subjected to progressive chemical dissolution in neutral aqueous
solutions, according to following equations (Ambat et al., 2000):
9.5
9.0
8.5
pH value
8.0
7.5
(a)
(b)
7.0
Figure 6.6 The pH value of SBF as a function of immersion time for (a) the bare and (b)
MgO-coated Mg alloy specimens.
10 20 30 40 50 60 70 80
0
Time(h
Anodic electrodeposition of MgO coatings to improve corrosion resistance in vivo 145
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MgO þ H2O/Mg(OH)
Mg(OH)
/Mg2þþ 2OH
2
As a result of the OH
2
e
ion formation, the pH at the specimen surface rises quickly
(6.7)
(6.8)
in the initial immersion stage. The slow increase of pH value of the solution for
a specimen with MgO coating indicates a relatively slow chemical dissolution and
an improvement of the corrosion resistance of MgO coatings.
The corrosion behavior of MgO-coated Mg alloy specimens was also evaluated by
immersion tests in SBF. At the beginning of immersion, large numbers of hydrogen
bubbles are evidently observed arising from the surface of the bare Mg alloy substrate due to the reaction of substrate with the corrosive electrolyte, while almost
no hydrogen evolution is found on the surface of MgO-coated Mg alloy in the
72-h immersion period, which indicates that the MgO coatings effectively retard
the hydrogen gas evolution. After a few hours, the bare Mg samples were covered
with white corrosion products. As the immersion time increased, the amount of
corrosion products formed on the untreated specimen increased and detached corrosion products were observed on the bottom of the beaker. On the other hand, the
appearance of MgO-coated Mg alloy specimens had no significant changes.
Surface morphologies after immersion in SBF for 72 h were examined to compare
the corrosion resistance of Mg alloy with and without MgO coatings. The surface
appearance of the samples after the immersion tests are shown in Figure 6.7, which
reveals large differences in the degree of corrosion attack during the immersion
period. The micrograph depicted in Figure 6.7a shows that the bare Mg alloy surface
was totally covered with corrosion products with a big concave pit after 72 h of immersion in SBF, implying that locally severe corrosion occurred on the surface and
the corrosion products in pits were detached away into solution. On the other hand,
after the total immersion tests, a small amount of corrosion products were observed
on the surface of the MgO-coated specimens, as shown by Figure 6.7b, but there
were no detached corrosion products throughout the immersion test, indicating a
much milder corrosion.
To get better insight on the corroded surface appearance of Mg alloy with and
without MgO coatings after immersion tests, the corrosion products and MgO coatings were removed by chromic acid. Figure 6.8 shows the SEM surface morphol-
ogies of the as-cleaned Mg alloy specimens. As can be seen in Figure 6.8a,thereare
deep pits (indicated by arrows ) on the bare Mg all oy surface, implying that the untreatedMgalloysufferedfrom localized severe corrosion attack. It is well known
that the presence of pits is detrimental to the overall corrosion resistance, and the
destruction of Mg alloy will proceed by means of pitting corrosion from one or
more of them (Zhang et al., 2005). On the other hand, there is no obvious corrosion
site area visible on the MgO-coated Mg alloy surface after the removal of MgO
coatings, as shown in Figure 6.8b. Zooming in, only slightly corrosion-attacked
spots exist on the as-cleaned Mg alloy substrate, as shown by the inset in
Figure 6.8b. This is likely due to the penetration of water and electrolytes into
the underlying Mg alloy matrix through the micropores existing in the MgO
146 Surface Modification of Magnesium and its Alloys for Biomedical Applications
Figure 6.7 Surface morphologies after immersion in SBF for 72 h for (a) the bare and (b)
MgO-coated Mg alloy specimens.
coatings, resulting in the corrosion attack. However, this decreased corrosion reaction reveals that the MgO coating film on Mg alloy could effectively protect the
substrate from corrosion attacks during the immersion tests by acting as a barrier
against corrosive electrolyte ingress into the underlying Mg alloy matrix. Consequently, the immersion tests indicate a much milder and more uniform corrosion
attack for the MgO-coated Mg alloy specimens, consistent with the electrochemical
Anodic electrodeposition of MgO coatings to improve corrosion resistance in vivo 147
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Figure 6.8 After 72 h of immersion in SBF. (a) The locally severe attacked surface morphology
of the bare Mg alloy after the removal of corrosion products. (b) Slightly corroded surface
morphology of Mg alloy after removal of the corrosion products and MgO coating layer. Inset is
an enlarged image.
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