Corrosion Science 52 (2010) 3917–3923

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Corrosion Sciencejournal homepage: www.elsevier.com/locate/corsci

The inﬂuence of carbon nanotubes on the corrosion behaviour of AZ31B
magnesium alloy
Hiroyuki Fukuda a,b,⇑, Jerzy A. Szpunar c, Katsuyoshi Kondoh d, Richard Chromik aa

Department of Mining Metals and Materials Engineering, McGill University, The Wong Building, 3610 University Ave., Montreal, Canada H3A 2B2
Graduate School of Engineering, Osaka University, 1 Yamadaoka, Suita, Osaka 565-0871, Japanc
Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada S7N 5A9dJoining and Welding Research Institution, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japanba r t i c l e

i n f oArticle history:
Received 25 June 2010
Accepted 7 August 2010
Available online 11 August 2010
Keywords:
A. Magnesium
A. Metal matrix composites
B. Polarization
B. Weight loss
C. Anodic dissolution
C. Interfacesa b s t r a c t
Carbon nanotubes (CNTs) are an effective reinforcement for magnesium (Mg) and its alloys due to their
excellent mechanical properties. However, due to their quite different electrical properties compared toother carbon allotropes, the inﬂuence of CNTs on the corrosion of Mg is expected to be different. For thisreason, the corrosion of AZ31B Mg alloy based composite with CNTs (AZ31B/CNT composite) was investigated with immersion tests, polarization tests and surface potential measurements. The galvanic corrosion between the Mg matrix and CNTs played an important role in the corrosion behaviour of the AZ31B/
CNT composite.
Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction
Magnesium (Mg) alloys, due to their low density of 1.738 g/cm3
ﬁnd applications in various structural and mechanical components.
It is widely recognized that replacing the conventional structural
materials such as steels or aluminium alloys with Mg alloys can signiﬁcantly reduce the fuel consumption of the transportation vehicles [1]. However, some difﬁculties such as poor corrosion
resistance, low mechanical properties, and low formability havebeen limited the application of Mg and its alloys. In terms of the corrosion resistance, quite low standard electrode potential (SEP) of
2.36 V [2] highly accelerates the corrosion of Mg and its alloys.
When Mg contacts with other nobler elements, ignoble Mg must
be anode in the galvanic cell formed between Mg and nobler elements [3]. For example, iron (Fe), nickel (Ni), and copper (Cu), onlywith a few hundreds ppm, rapidly enhance the corrosion rate of
Mg matrix [4]. Since these elements are usually introduced as contaminations, with careful avoidance of these elements in fabricating
steps, at present, more or less anti-corrosive high purity Mg and its
alloys are produced.
On the other hand, for wider use of Mg and its alloys, mechanical properties of the conventional Mg alloys must be improved. At
the moment, the ultimate tensile strength of the mould casting⇑ Corresponding author. Address: Graduate School of Engineering, Osaka
University, 1 Yamadaoka, Suita, Osaka 565-0871, Japan. Fax.: +81 06 68798669.
E-mail address: fukkun-fukuda@jwri.osaka-u.ac.jp (H. Fukuda).
0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2010.08.009

AZ91C Mg alloy is only 275 MPa [5] while that of the casting
AZ6N4 aluminium (Al) alloy is 540 MPa [6]. One strategy to improve the mechanical properties of Mg alloys is the fabrication of
Mg alloy based metal matrix composites (Mg-MMCs). Though lotsof kinds of reinforcements have been tested so far, carbon nanotubes (CNTs) which is one of carbon allotropes having cylindricalshape of scrolled graphen sheets [7], and with sizes ranging from
10–100 nm diameter and up to a few micrometer length, havebeen considered as one of the superior reinforcement materials
in Mg-MMC because of their exceptional mechanical properties.
For instance, their Young’s modulus is around 4 TPa [8] compared
to only 44 GPa for pure Mg [5]. Thus, many attempts have been
made to produce Mg-MMC with CNTs to improve mechanicalproperties of various Mg alloys [9–14]. In spite of the difﬁculty inthe segregation of CNT bundles caused by its own van der Waals
force, some recent studies succeeded to improve mechanical properties of Mg alloys with CNTs [9,10,14].
Meanwhile, elemental carbon, which is a much nobler element
than elemental Mg [2], harms the corrosion resistance of Mg. Hall
reported that PAN-based carbon ﬁbre deteriorated Mg corrosion
resistance due to the formation of galvanic cell between the Mg
matrix and carbon ﬁbre [15]. However, CNTs show quite different
electrical properties from other carbon allotropes [16]. Thus, theeffect of CNTs on Mg corrosion phenomenon is expected to bedifferent from that of other carbon allotropes. In the previous study,
Endo et al. reported that the corrosion resistance of AZ91D Mg-MMC
with CNTs (AZ91D/CNT composite) was improved compared to3918

H. Fukuda et al. / Corrosion Science 52 (2010) 3917–3923

that of pristine AZ91D [17]. In their report, it was claimed that CNTs
act as a water repellent and reinforce the surface protective layers.
Though both phenomena should enhance the corrosion resistance,
no speciﬁc effort was made to explore the role of galvanic corrosion
between Mg matrix and CNTs. On the other hand, Aung et al.
reported that the corrosion resistance of pure Mg composite with
CNTs was decreased compared to that of pristine pure Mg by measuring hydrogen evolution and mass loss during immersion tests,
and polarization curves in 3.5 wt.% NaCl solution [18]. They concluded that the corrosion resistance of pure Mg composite with
CNTs was deteriorated due to the galvanic corrosion between the
Mg matrix and CNTs. However, they did not investigate the role of
CNTs on the local corrosion behaviour of the Mg matrix, neither.
The inﬂuence of CNTs on the corrosion behaviour of Mg matrix
remains unclear. Hence, in this study, ﬁrst overall corrosion behaviour of AZ31B Mg alloy based composite with CNT (AZ31B/CNT composite) was evaluated with immersion tests, and then, the role of
CNTs on the corrosion behaviour of the Mg matrix was investigatedwith polarization tests and surface potential measurements.

Powder surface

AZ31B raw powders
CNTs
Surfactant solution
with CNTs

Removing H2O
at 80 °C for 2 hours
in air, and surfactant
residues at 500 °C
for 10 minutes in
H2:Ar=1:1 gas
atmosphere.

2. Experimental

Surfactant
residues
+
water

CNTs

2.1. Fabrication of AZ31B/CNT composites
AZ31B Mg alloy powders with 97 lm mean diameter and multiwalled CNTs with 13–16 nm outer diameter and 1–10 lm length,
as shown in Fig. 1, were prepared as raw materials. The chemicalcompositions of the AZ31B Mg alloy powders are Al, 2.95; Zn,
1.03; Mn, 0.21; Si, 0.012; Fe, 0.0048 mass%. With these materials,
AZ31B/CNT composite was fabricated through two steps: theCNT-coating step with the solution containing zwitterionic surfactant, which is the surface active agent having both negative andpositive charges at the tip of its hydrophilic groups, and the consolidation step with spark plasma sintering (SPS) technique [19]. In
the CNT-coating steps, CNT coating on the powder surface wasconducted as the schematic illustration shown in Fig. 2. First,
AZ31B raw powders were dipped into and taken out from theaqueous solution containing the zwitterionic surfactant, 3-(N,N-dimethyl stearyl ammonio) propane sulfonate, with 1 and 3 mass%
CNTs. The details about how CNT bundles were disassembled in
the solution are given in another reference [20]. Then, these powders were kept in argon (Ar) gas atmosphere at 80 °C for 2 h to dry
the surface. At this point, these powders were coated with not only

AZ31B powders
coated with CNTs
Fig. 2. The schematic illustration of the preparation steps of the AZ31B powders
coated with CNTs. As-received powders were dipped into and taken out from thesurfactant solution with CNTs, dried at 80 °C for 2 h in air, and then, heat treated at
500 °C for 10 min in H2:Ar = 1:1 gas atmosphere.

CNTs but also surfactant residues which obstruct powders frombonding together in the consolidation steps. Therefore, heat treatment was employed at 500 °C for 10 min in hydrogen (H2):Ar = 1:1
gas atmosphere to thermally remove the surfactant residues. Afterthe heat treatment, the content of carbon in the CNT-coated powders were measured with Inert gas fusion instruments-carbon–sulphur determination (LECO Co., CS-200) technique. This carbon
content was regarded as the content of CNTs in each sample as
the previous study [21]. Subsequently, the CNT-coated powders
were consolidated in carbon die (diameter; 43 mm) at 550 °C for
30 min under 30 MPa pressure in vacuum with SPS equipment(Syntech Co., SPS-1030S). Due to the nature of the fabrication process, CNTs were distributed only at the primary particle boundaries
in the specimens. Thus, at a very local level, the distribution of
CNTs might be considered heterogeneous. However, since themean particle diameter of Mg, 97 lm, is much less than the entire
specimen area exposed to the corrosion media, we expect that the
distribution of CNTs is homogenous at the scale studied for theoverall corrosion analysis. For convenience, fabricated samples
were coded depending on their content of CNTs as indicated inTable 1. Their bulk densities, which were calculated from theirweight and volume, were also listed in Table 1. As the corrosion
resistance of the composites should be discussed around the conTable 1
The sample code, the CNT content and the bulk density of each sample.

Fig. 1. The scanning electron microscopy (SEM) observation on the as-received
CNTs.

*

Code

CNT content (vol.%)

Bulk density (%)

a*
b
c

0
0.89
2.66

98.24
97.69
96.92

AZ31B pristine material.

H. Fukuda et al. / Corrosion Science 52 (2010) 3917–3923

tent of CNTs at which the mechanical properties are expected to be
improved, the content of CNTs in this study, 0.89 vol.% and2.66 vol.%, was determined corresponding to that of the previous
research which reported that yield stress of pure Mg/CNT composite and AZ31B/CNT composite were, with approximately 1 vol.% of
CNTs, improved about 34–42% and 24–27%, respectively [9].
2.2. Evaluation of the corrosion behaviour of AZ31B/CNT composite
Immersion tests were performed to investigate the overall corrosion resistance of each sample. All samples were cut into cylindrical specimens having 10 mm height and 12 mm diameter.
Specimens were immersed into 1200 ml of 0.51 M sodium chloride(NaCl) solution at pH = 6.2. The solution was deaerated and agitated with 1000 ml/min Ar gas ﬂow. Throughout the immersion
tests, the pH changes of the test solutions were collected in theintervals of 1 h with pH meter. To increase the credibility of thetests, the tests were repeated twice under the same condition.
The Mg reaction with deaerated aqueous solution is expressed in
the equation below [22].
Mg(s) ? Mg2+(aq) + 2e (anodic reaction)
2H2O + 2e ? H2(g) + 2OH(aq) (cathodic reaction)
Mg2+(aq) + 2OH(aq) ? Mg(OH)2(s) (corrosion product)
When the Mg matrix dissolves, hydroxide ions (OH) are released into the test solution. The more Mg dissolves into the solution, the higher solution pH value can be expected. Mass loss was
also calculated by weighing specimens before and after the test.
When Mg matrix dissolves into the test solution, specimens shouldlose its mass corresponding to the mass of dissolved Mg. Therefore,
the specimen experienced severer corrosion loses greater mass
after the immersion test.
The polarization test was carried out to investigate whether thegalvanic corrosion progressed in AZ31B/CNT composite. The samples were cut into square specimens (15 mm  15 mm  3 mm),and ground in ethanol with #600 and #1200 silicon carbide (SiC)
abrasive papers. After the grinding processes, the specimens werecleaned in ethanol using ultrasonic vibration. Since Mg easily reacts with water and creates magnesium hydroxide (Mg(OH)2),
the contact of specimens with water was avoided at all specimen
preparation steps. The specimens were immersed into 75 ml of
0.017 M NaCl solution at pH = 6.47 for 1 h to stabilize the sample
surface. Since, with the solution for the immersion tests containing
0.51 M Cl ions, all specimen surface was too corroded to understand where CNTs existed, the less corrosive solution containing
0.017 M Cl ions was used in this test. The solution pH value wasadjusted at 12 by using sodium hydroxide (NaOH) to emphasize
the galvanic corrosion progressed in samples by reducing the uniform dissolution of Mg matrix. While Cl ions breaks down the surface protective ﬁlm, the ﬁlm could be more or less effective in thealkaline solution used in this study since the concentration of Cl
in the test solution was low, 0.017 M. Although, as an alloying element, the AZ31B contains Al, whose passivation ﬁlm of Al(OH)3
was unstable at alkaline pH value [23,24], all Al was solutionized
into Mg matrix in AZ31B used in this test. Thus, the specimen surface exposed to the test solution did not contain Al phase, so entire
specimen surface was protected with stable passivation ﬁlm during the tests. The polarization curves were measured at the scanning ratio of 1 mV/s with a laboratory potentiostat (AUTOLABO
model PGSTAT 302). A saturated calomel electrode (SCE) as a reference electrode and a graphite rod as a counter electrode, were submerged into the test solution. The specimen was exposed to thetest solution by ﬁxing it to a window of the plastic container; noother material was in the test solution except a specimen, SCE,
and a graphite rod throughout the tests. The cathode/anode area3919

ratio (7.66) and the distance between cathode and anode(45 mm) were kept constant during this test. The polarization test
was repeated twice to validate the test results. The surface potential difference (SPD) between Mg and CNTs was calculated by measuring the surface potential between specimen and the needle atthe tip of the cantilever attached to the Scanning Kelvin Probe
Force Microscope (SKPFM) [25]. The needle was coated with platinum iridium5 (PtIr5) to conduct electrons. The surface potential
was deﬁned by the expression below.

Surface potential ¼ ðUPtIr5  Uspecimen Þ=ewhere e is an electrical charge, and UPtIr5 and Uspecimen are the work
function of PtIr5 and specimen, respectively. In the previous research, Schmutz and Frankel reported the proportional relationshipbetween surface potential and SEP in NaCl aqueous solution [26].
Therefore, the surface potential measurement provides the basic
knowledge whether the driving force is enough to provoke the local
corrosion. Meanwhile, SKPFM measures the surface potential between the cantilever needle and specimen as described above. Thatis, the measured surface potential becomes low when the SEP of themeasured surface is high. The SPD between two phases on the specimen surface is deﬁned as an expression below.

SPD ¼ V SHE;2  V SHE;1 ¼ ðUPtIr5  U2 Þ=e  ðUPtIr5  U1 Þ=e ¼ ðU1  U2 Þ=e
SPD = VSHE,2 – VSHE,1 = (UPtIr5 – U2)/e  (UPtIr5 – U1)/e = (U1 – U2)/e,
where VSHE,1, VSHE,2, U1 andU2 is the surface potential of phase 1,
that of phase 2, the work function of phase 1 and that of phase 2,
respectively. For the SKPFM scanning, the specimen (10  10 mm
square) was ground with #2000 and #4000 SiC abrasive papers,
and polished with 0.25 lm diamond pastes. The specimen surface
was rinsed in ethanol using ultrasonic vibration after the polishing.
After the SPD measurement, specimens were corroded in 0.017 M
NaCl solution at pH = 6.02 for 5 min to investigate the surface morphology around CNTs after corrosion. Throughout this study, thesurface conditions were characterized with ﬁeld emission scanningelectron microscopy (FE-SEM, JOEL, JSM-6500F) equipped with energy dispersive X-ray spectroscopy (EDS, JEOL, JED-2300) system.
X-ray diffraction (XRD) system with copper (Cu) tube was also
operated to identify the structure of compounds formed after the
immersion tests.
3. Results and discussion
3.1. Overall corrosion behaviour of AZ31B/CNT composite
Fig. 3 shows the appearance of sample a and c at 1, 6 and 12 hfrom the beginning of the immersion. Sample c was severely corroded after 6 h-immersion while sample a kept its original shapeeven after 12 h-immersion. Sediments around sample c after
12 h-immersion were scanned with XRD as indicated in Fig. 4.
The XRD proﬁle identiﬁed the sediments as the mixture ofMg(OH)2 and few amount of aluminium hydroxide (Al(OH)3). As
widely recognized, Mg produces Mg(OH)2 when it reacts withwater or moisture. Thus, the sediments were considered as the detached corrosion products detached from specimen surface due tothe harsh H2 gas evolution of the cathodic reaction. Though, in the
XRD proﬁle, Al(OH)3 was identiﬁed as well, it was usually includedin the surface corrosion products of AZ31 as Wang et al. reported in
the previous report [27].
The pH changes during the immersion test were shown in Fig. 5.
The pH values of sample b and c increased faster and reached higher
values than that of sample a. This means that the Mg matrix of sample b and c was corroded faster and greater into the test solution. The
pH value of sample c with CNTs stabilized around pH = 11 after
approximately 3 h from the beginning of the immersion. This

H. Fukuda et al. / Corrosion Science 52 (2010) 3917–3923

1 hour; pH=10.85

6 hours; pH=11.08

12 hours; pH=11.13

1 hour; pH=9.22

6 hours; pH=10.30

12 hours; pH=10.69

Sample aSample c

3920

10 mm
Fig. 3. The photos of sample c and a during the immersion test in 0.51 M NaCl solution with Ar gas ﬂow. The sample c was severely corroded after 6 h although the sample a
did not show such serious corrosion even after 12 h.

12

c

11

b

pH

10
9

a

8
7
6
0

10000

20000
30000
Time, t / s

40000

50000

Fig. 5. The pH changes of each sample in 0.51 M NaCl solution with the Ar gas ﬂow.
In the ﬁgure, a, b, and c mean the sample without, with 0.89 and 2.66 vol.% CNTs,
respectively. The pH values of sample b and c increased faster and to higher values
at any time than that of samples a, the sintered pristine Mg alloy.

Fig. 4. The XRD proﬁle of the sediments accumulated around sample c after 12 himmersion in 0.51 M NaCl solution. The sediments mainly consisted of Mg(OH)2,
the main corrosion product of Mg in aqueous solution.

120.72Mass loss, mg·cm-2

phenomenon was also observed in sample b after approximately
6 h-immersion. These results suggests that, the test solution of sample b and c was almost saturated with Mg(OH)2 at the beginning of
the tests. Additionally, the sediments appeared around the corroded
specimen after 6 h-immersion previously shown in Fig. 3. These sediments, which were mainly composed with Mg(OH)2, also justiﬁed
the Mg(OH)2 saturation of the test solution at the beginning of thetests. On the other hand, the test solution pH value of sample a increased and few sediments were observed around the sample a asshown in Fig. 3 even after 12 h-immersion, so the test solution ofsample a was not or just saturated with Mg(OH)2 after 12 h-immersion. Hence, though the pH value of sample a approached to that ofsample b and c at the end of the test, this is because the test solutionof sample b and c was saturated with Mg(OH)2 at the beginning of thetests. The earlier saturation of the test solution of sample b and c also
indicated the fast Mg matrix dissolution. Moreover, the signiﬁcantmass loss was calculated in sample b and c as shown in Fig. 6. Sample
b and c lost their mass of 19.56 and 120.72 mg/cm2 after immersion

150

100

50
19.56
0.16

0
a

b

c

Fig. 6. The mass loss of each sample after 2 h-immersion in 0.51 M NaCl solution. In
the ﬁgure, a, b, and c mean the sample without, with 0.89 and 2.66 vol.% CNTs,
respectively. The mass loss of sample b and c was much greater than that of sample
a, the sintered pristine Mg alloy.

H. Fukuda et al. / Corrosion Science 52 (2010) 3917–3923

1

c

0.5

b

E, VSHE

0
-0.5

a

-1
-1.5
-2
-7

-6

-5

Log10(i), µA cm

-4

-3

-2

Fig. 7. The polarization curves of each sample at pH = 12 in 0.017 M NaCl solution.
In the ﬁgure, a, b, and c means the sample without, with 0.89 and 2.66 vol.% CNTs,
respectively. In sample b and c, the electropositive shift was approximately 0.7 V.

test, respectively despite that sample a lost only 0.16 mg/cm2. Fasterand greater Mg matrix dissolution of sample b and c caused greatermass loss. Surely, corrosion products, which increase the sample
mass after the immersion test, might be stuck on the sample surface.
However, the corrosion of these samples was quite sever, and thedetachment of corrosion products was often observed everywhereon the specimen surface. Additionally, the corrosion product layer
was not much thick compared to the amount of the deposited corrosion product. The metallic surface soon appeared when the corroded
specimens were ground with SiC paper. Thus, the mass gain due tothe surface corrosion products after the immersion test was insignificant compared to the mass loss caused by the mass of Mg matrix dissolved into the solution.
3.2. Investigations of the role of CNTs on the corrosion behaviour of Mg
matirix
The polarization curves at pH = 12 are shown in Fig. 7. The corrosion potential of sample b and c positively shifted about 0.7 V
from that of sample a. This electropositive shift indicates that thegalvanic corrosion progressed in sample b and c. Besides, polarization curves of sample b and c did not exhibit any passivation
behaviour. When the Mg reacts with the aqueous solution, usually,
the corrosion products were left on the surface, and more or less,behaves as a protective ﬁlm. However, sample b and c did not showsuch behaviour. The corrosion products on the surface were noteffective in protecting Mg matrix from dissolution. The surface
morphology of sample c after the polarization test was shown in
Fig. 8 (a). The specimen surface was locally damaged at primary

3921

particle boundary, where lots of CNTs were present as shown in
Fig. 8 (b). In the vicinity of CNTs, harsh corrosion progressed, andthe surface protective ﬁlm lost its effectiveness. The 2D and 3D surface potential distribution of sample c at the primary particle
boundary was presented in Fig. 9 (a) and (b), respectively. The primary particle boundary, the dark area, showed low surface potential; that means the high absolute potential. SPDs between Mg
matrix and the dark area, A–B, C–D, and E–F, were calculated
1.06, 1.09, and 1.14 V, respectively although SPDs between tow
points at matrix, G–H, was less than 0.01 V. On the other hand,
SEM observation revealed the presence of CNTs at the primary particle boundary, as displayed in Fig. 9 (c). The high absolute potential at the primary particle boundary was due to the presence of
CNTs. As a result, SPD between Mg matrix and CNTs were around
1.1 V. In the previous study, Takei et al. measured the SPD between
AZ91D Mg matrix and Fe [28], which is the well-known harmful
element for the corrosion resistance of Mg matrix due to the formation of the galvanic cell with Mg matrix [29]. They reported that
the SPD between Mg matrix and Fe was 0.5  0.7 V. Furthermore,in some earlier studies, the SPD between Mg matrix and intermetallic compounds was measured [30–32]. For example, Andreatta
et al. reported that the average SPD of AZ80 Mg matrix with
Al8Mn5, Mg2Si, and Mg17Al12 (b phase) were 257.3, 97, and152.3 mV, respectively [30]. It is widely recognized that Al–Mn
intermetallics and b phase behave like a cathode to the Mg matrix,
and form the galvanic cell with the Mg matrix [32]. Compared tothese prior studies, SPD between the Mg matrix and CNTs obtainedin this study (1.1 V), was enough high to form the galvanic cell between them. Surface conditions before and after immersion into
0.017 M NaCl solution for 5 min were shown in Fig. 10 (a) and(b), respectively. Same areas were observed to easily compare
the surface morphology before and after the immersion test. In
Fig. 10 (b), white debris were concentrated along the primary particle boundaries, where CNTs were located. Since the morphologyof white debris was quite similar to the corrosion product,
Mg(OH)2, observed after the immersion test in 0.51 M NaCl solution identiﬁed with XRD in Fig. 4, the white debris can be regarded
as the corrosion products piled up around CNTs. Obviously, due tothe strong galvanic corrosion at the interfaces between the Mg matrix and CNTs, large amount of corrosion products were accumulated in the vicinity of CNTs.
4. Conclusion
The formation of the galvanic cell between the Mg matrix and
CNTs due to the high potential difference (1.1 V), decreased thecorrosion resistance of the AZ31B Mg alloy. Throughout theimmersion tests, the pH values of the test solution for AZ31B/
CNT composite increased rapidly and reached higher value thanFig. 8. The corroded surface of sample a after the polarization test at pH = 12 in 0.017 M NaCl solution. The severely corroded areas were locally observed along the primaryparticle boundary (a), where individual CNTs were present (b).

3922

H. Fukuda et al. / Corrosion Science 52 (2010) 3917–3923

Fig. 9. The 2D and 3D surface potential distribution of sample c, (a) and (b), respectively, and CNTs at the primary particle boundary, (c). Large SPD was observed between Mg
matrix and CNTs.

Fig. 10. Surface conditions of sample c before (a) and after (b) immersion test in 0.51 M NaCl solution for 5 min. The corrosion products were accumulated around CNTs after
immersion test as a result of the formation of the strong galvanic cell between Mg matrix and CNTs.

that of the pristine sintered AZ31B Mg alloy. The signiﬁcant massloss of AZ31B/CNT composites was measured after immersion in
0.51 M NaCl as well. Corrosion potential of AZ31B/CNT composites
sifted 0.7 V positively compared to that of AZ31B pristine material;that gave the information of progression of the galvanic corrosion
inside of AZ31B/CNT composite. Surface potential measurementwith SKPFM revealed 1.1 V SPD between Mg matrix and CNTs.
The corrosion products were observed around CNTs after the
immersion test in 0.017 M NaCl aqueous solution. Due to thestrong driving force (1.1 V), the galvanic cell was constructed between Mg matrix and CNTs, and large amount of corrosion products were piled up in the vicinity of CNTs.

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