Materials Chemistry and Physics 115 (2009) 439–443
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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Corrosion behavior of ZnO nanosheets on brass substrate in NaCl solutions
Quanzhang An a,b , Yunchang Xin b,c , Kaifu Huo b,d,∗∗ , Xun Cai a , Paul K. Chu b,∗
a
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
c
Advanced Materials Institute, Tsinghua University, Shenzhen Graduate School, Shenzhen 518055, China
d
Hubei Province Key Laboratory of Refractories and Ceramics, College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
b
a r t i c l e
i n f o
Article history:
Received 25 September 2008
Received in revised form
22 December 2008
Accepted 2 January 2009
Keywords:
Brass
ZnO nanosheets
Corrosion
NaCl solution
a b s t r a c t
The in situ growth of ZnO nanosheets has been demonstrated by our research group and the excellent
electrical contact between the ZnO nanosheets and brass substrate enables many potential applications
in gas sensing and photocatalytic degradation. However, problems arising from corrosion, especially that
arising from chloride ions, are inevitable in the field. In this work, the corrosion behavior of these ZnO
nanosheets is investigated in chloride solutions. Our results show that in a NaCl solution, the chloride ions
can react with ZnO to form ZnCl2 . When the NaCl concentration is relatively low (1 wt%), the structure
exhibits a strong passivation behavior but a higher concentration of chloride ions can accelerate the
transformation from ZnO to ZnCl2 . The results also disclose that a high concentration of NaCl weakens
the passivation performance and when the concentration reaches 3 wt%, the passivation ability vanishes
completely.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Zinc oxide (ZnO) possesses many unique properties including
photoelectric conversion, high piezoelectricity, and good biocompatibility and are potentially useful in applications involving
solar cells [1,2], piezoelectric nanogenerators [3,4], gas sensors,
and biosensors [5–7]. Nanostructured ZnO which has attracted
increasing attention due to their high aspect ratio and surfaceto-volume ratio [8,9] exhibits high sensitivity and high efficiency
when used as gas sensors or photoelectrochemical devices. In
our previous work [10], ZnO nanosheets (ZnO NSs) were fabricated in situ on Cu0.66 Zn0.34 brass substrate using a convenient
and economical method. The conducting brass substrate provides
excellent electrical contact serving as the electrode in a ZnObased gas sensor. Compared to ZnO powders, these ZnO NSs
can protect the solutions from secondary pollution in photocatalytic degradation processes. However, corrosion is encountered
in these applications and it is important to investigate the corrosion behavior of the nanostructure in an aggressive environment
such as an aqueous medium containing chloride ions. In the work
reported here, the corrosion behavior of ZnO NSs on brass in
solutions containing different concentrations of NaCl is investigated and the corroded surfaces are characterized to elucidate the
factors affecting the anti-corrosion characteristics of the nanostructures.
2. Experimental details
A commercial available Cu0.66 Zn0.34 sheet was cut into 10 mm × 10 mm × 2 mm
coupons. The samples were grinded and polished followed by ultrasonical cleaning
in alcohol. Uniform ZnO NSs were then fabricated in situ by the method described in
Ref. [10]. The phase constituents of the ZnO NSs were determined by X-ray diffraction (XRD). The morphologies of the samples before and after electrochemical tests
were monitored by SEM (SEM, Hitachi S4700) and energy-dispersive X-ray spectrometry (EDS) was employed to determine the elemental compositions in selected
regions.
The corrosion behaviors of brass with and without ZnO NSs were investigated
by potentiodynamic polarization and open circuit potential evolution (OCP) in NaCl
solutions. 1, 2 and 3 wt% NaCl solutions, were adopted to disclose the influence of
NaCl concentrations on the corrosion behavior of ZnO NSs coated brass.
The electrochemical tests were carried out using a model 363 potentiostat/galvanostat (EG&G Princeton Applied Research). A three-electrode cell
consisting of the sample as the working electrode, graphite rod as counter electrode,
and calomel electrode (SCE) as the reference electrode was adopted. A 2 mV s−1 scanning rate and a potential range from −1 to 2 V were used in the potentiodynamic
polarization test. The corrosion potential (Ecorr ) and corrosion current density (Icorr )
were determined using the Tafel fit. In the open circuit potential (OCP) measurement,
changes in the open circuit potential (Ecorr ) were monitored as a function of immersion time for about 20 ks. The data were recorded every 2 s. All the electrochemical
tests were performed at room temperature.
3. Results and discussion
∗ Corresponding author. Tel.: +852 27887724; fax: +852 27889549.
∗∗ Corresponding author. Tel.: +852 27844257.
E-mail addresses: [email protected] (K. Huo),
[email protected] (P.K. Chu).
0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2009.01.001
The XRD results acquired from the ZnO NSs on brass in Fig. 1
show the presence of ZnO with good crystallinity. Fig. 2 shows
the surface morphology of ZnO NSs. The ZnO has a fine nanosheet
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Q. An et al. / Materials Chemistry and Physics 115 (2009) 439–443
Fig. 2. Surface morphology of ZnO NSs fabricated in situ on brass under 650 ◦ C.
◦
Fig. 1. XRD patterns obtained from the Cu0.66 Zn0.34 plate when heated to 650 C in
air at a heating rate of 15 ◦ C min−1 .
structure with a thickness of tens of nanometers and width of
hundreds of nanometers.
The potentiodynamic polarization curves of the brass substrate
with and without the ZnO NSs in 1, 2 and 3 wt% NaCl solutions are
shown in Fig. 3. The corrosion current density (Icorr ) and corrosion
potential (Ecorr ) values determined from the polarization curves are
listed in Table 1. No obvious enhancement in the corrosion potentials is observed from the ZnO NSs coated brass in all three solutions
can be observed compared to pure brass. The corrosion current densities of the two sets of samples in the three solutions are similar
as well. However, it should be noted that in the anodic polarization stage, when the polarization potential reaches a certain value,
the current densities of the ZnO NSs coated brass are much lower
than those of the uncoated brass. As soon as the ZnO NSs coated
brass experiences anodic polarization, a much lower corrosion rate
is observed than pure brass. With increasing NaCl concentrations,
the corrosion potential and corrosion current density of the ZnO NSs
coated brass change insignificantly. In the 1 wt% NaCl solution, the
ZnO NSs coated brass exhibits a strong passivation behavior. In contrast, in the 2 wt% NaCl solution, the passivation behavior weakens
and when the concentration reaches 3 wt%, the passivation behavior disappears almost entirely.
The OCP results acquired from the samples in 1, 2, and 3 wt%
NaCl solutions are presented in Fig. 4. The corrosion potentials
of the ZnO NSs coated brass in the three solutions are all much
more positive than that of pure brass during the entire exposure
duration. No intense fluctuations in the potentials can be observed
from both sets of samples in the three solutions can be observed
Fig. 3. Potentiodynamic polarization (vs. SCE) curves of uncoated and ZnO NSs coated brass in: (a) 1 wt%, (b) 2 wt% and (c) 3 wt% NaCl solutions at a scanning rate of 2 mV s−1 .
Q. An et al. / Materials Chemistry and Physics 115 (2009) 439–443
441
Table 1
. Electrochemical response of uncoated and ZnO NSs coated brass in aqueous NaCl solutions.
Samples
Brass
ZnO NSs coated brass
1%
2%
−2
Ecorr (mV)
Icorr (␮A cm
−254.8
−292.62
22.68
11.41
)
during immersion. Fluctuations would be an indication that it is
difficult for electrode at the electrode/solution interface to achieve
equilibrium [11]. Although the smooth curves suggest a general
corrosion process for all the samples in the three solutions, the
two sets of samples exhibit different behaviors in the solutions.
During the early stage, the potential of the ZnO NSs coated brass
moves to a negative direction gradually and then remains nearly
constant during subsequent immersion. The initial decrease in
the corrosion potential is probably related to the penetration of
aggressive ions. After some time, ion absorption from the solution by the ZnO NSs is saturated leading to a stable corrosion
potential. The behavior of brass in the three solutions is similar.
However, during the immersion period, a gradual cathodic polarization process is observed on brass in the three solutions. This
may be related to the Zn dealloying process during corrosion of
brass.
Dealloying, a common phenomenon in corrosion of alloys, is a
process during which an element selectively segregates from an
alloy. Generally, the more active element preferentially departs
from the surface [12,13]. In brass, Zn possesses a more negative
standard potential compared to Cu and preferentially dissolves in
the solution. The gradual dissolution of Zn enlarges the area of pure
Cu on the surface. Compared to pure brass, the complex potentials
of Cu and brass are more negative. Thus, with the dissolution of Zn,
the potential moves to a negative direction gradually.
3%
−2
Ecorr (mV)
Icorr (␮A cm
−289.84
−288.12
19.50
10.45
)
Ecorr (mV)
Icorr (␮A cm−2 )
−260.98
−249.73
17.20
8.76
Fig. 5 shows the surface morphologies of the brass substrate as
well as ZnO NSs covered brass after the potentiodynamic polarization tests in the three NaCl solutions. The morphologies of pure
brass after immersion in the three solutions are similar and show
a typical porous structure. As aforementioned, corrosion of brass
is usually in the form of dezincification. Formation of a porous
morphology is common in the dealloying process. In the 1 wt%
NaCl solution, the anodic polarization process creates the black
regions. The EDS results reveal that some ZnCl2 forms in these
regions but no Cu can be observed indicating that there are no
corrosion pits. High magnification views of the white area show
that the original ZnO NSs are retained quite well. In the 2 wt%
NaCl solution, the number of black area increases dramatically
but in the other regions, the nanosheet pattern is well retained.
In the 3 wt% NaCl solution, the nanosheet vanishes totally leaving behind a rough surface. EDS results indicate that these regions
also contain high contents of ZnCl2 . The results show that chloride ions in an aqueous solution attack the ZnO NSs forming ZnCl2
and the degree of corrosion depends on the NaCl concentration.
A higher chloride ion concentration accelerates the reaction with
ZnO. Hence, during applications in the field, it is imperative that
these ZnO nanostructures are shielded from a high concentration
of chloride ions of 3% or higher, e.g., 3.5 wt% in seawater, otherwise
extensive corrosion results rendering complete dissolution of the
nanosheets.
Fig. 4. Open circuit potentials (vs. SCE) vs. time determined from the uncoated and ZnO NSs coated brass in: (a) 1 wt%, (b) 2 wt%, and (c) 3 wt% NaCl solutions.
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Q. An et al. / Materials Chemistry and Physics 115 (2009) 439–443
Fig. 5. SEM images of brass after potentiodynamic polarization experiments in NaCl solutions with different concentrations: (a), (c) and (e) are brass in 1, 2 and 3 wt% NaCl
solutions, respectively; (b), (d) and (f) are ZnO NSs coated brass in 1, 2 and 3 wt% NaCl solutions, respectively; (b1), (d1) and (f1) are EDS results obtained from the corresponding
areas in (b), (d), and (f), respectively.
Q. An et al. / Materials Chemistry and Physics 115 (2009) 439–443
4. Conclusion
The corrosion characteristics of ZnO NSs fabricated on brass are
evaluated in aqueous solutions with different concentrations of
NaCl. Compared to the corrosion behavior of pure brass, the corrosion resistance of the ZnO NSs coated brass is enhanced. The ZnO
coated brass does not exhibit pitting corrosion in the NaCl solutions.
When the NaCl concentration is low, a strong passivation behavior
is observed from the ZnO NSs coated brass. However, when the
chloride concentrations increases, the passivation effects diminish
and when it reaches 3%, complete dissolution of the ZnO NSs is
observed. Hence, it is important that these ZnO nanostructures are
not subject to a concentration of chloride ions of 3% or higher.
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The work was financially supported by Hong Kong Research
Grants Council (RGC) General Research Funds (GRF) No. CityU
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