Electrochemical corrosion behavior of carbon nanotube-doped hard chromium coatings electrodeposited from Cr(III) baths

Article abstract of DOI:10.1149/1.3073550

Homogeneous chromium-multiwalled carbon nanotube (Cr-MWNT) composite coatings were electrodeposited from trivalent chromium [Cr(III)] electrolyte containing MWNTs under ultrasonic agitation. The microstructure, mechanical properties, and electrochemical c

Full text of DOI:10.1149/1.3073550

Journal of The Electrochemical Society, 156 ͑4͒ C123-C126 ͑2009͒
C123
0013-4651/2009/156͑4͒/C123/4/$23.00 © The Electrochemical Society
Electrochemical Corrosion Behavior of Carbon
Nanotube-Doped Hard Chromium Coatings
Electrodeposited from Cr(III) Baths
Zhixiang Zeng,a,b,z Yuanlie Yu,a,b and Junyan Zhanga,z
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
bGraduate University of Chinese Academy of Sciences, Beijing 100039, China
Homogeneous chromium-multiwalled carbon nanotube ͑Cr-MWNT͒ composite coatings were electrodeposited from trivalent
chromium ͓Cr͑III͔͒ electrolyte containing MWNTs under ultrasonic agitation. The microstructure, mechanical properties, and
electrochemical corrosion behavior of Cr-MWNT composite coatings were investigated with a field-emission scanning electron
microscope, an X-ray photoelectron spectrometer, a Vickers hardness indenter, and an electrochemical workstation. The introduc-
tion of MWNTs obviously improves the hardness and toughness of Cr coatings due to the “fiber reinforcement.’’ Also, the
electrochemical tests show that in H2SO4 solution MWNTs can provide a lot of active sites to accelerate the formation of Cr2O3
passive film, and in KOH solution they can adsorb Crn͑OH͒x polymer film to decrease the corrosion rate of Cr composite coatings.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3073550͔ All rights reserved.
Manuscript submitted August 19, 2008; revised manuscript received December 23, 2008. Published February 4, 2009.
Carbon nanotubes ͑CNTs͒ exhibit excellent properties such as
high mechanical properties, unique electrical and thermal
conductivity,1-5 as well as adsorbability.6 Therefore, CNTs could be
used as a dopant in various matrices to improve electrical, mechani-
cal, or thermal properties.7-9 Indeed, CNT composites such as
polymer-,10-12 ceramic-,13,14 and metal-CNT15,16 composite materi-
als have been extensively studied for their mechanical, thermal,
electrical, and optical properties.17,18 Nevertheless, CNTs may also
play an important role in the electrochemical corrosion process of
anticorrosion composite coatings, due to their adsorbability, nano-
scale effect, and electrical conductivity. Unfortunately, the research
on the electrochemical corrosion behavior of metal-CNT composite
coatings has been overlooked. Electrodeposited Cr coatings are
widely used for antiwear, decoration, and anticorrosion. Hence, in
this work, the hard chromium-multiwalled CNT ͑Cr-MWNT͒ com-
posite coatings are electrodeposited from the “environmentally ac-
ceptable” Cr͑III͒ electrolyte containing MWNTs. The comparison of
the electrochemical corrosion behavior of Cr-MWNT composite
coatings to that of pure Cr coatings is conducted in order to reveal
the role of MWNTs in the electrochemical corrosion process.
ics, USA͒ was used to determine the composition of products during
the electrochemical process, using Al K␣ radiation ͑photon energy
1486.6 eV͒ as the excitation source. The XPS spectra were collected
in constant analyzer energy mode at a chamber pressure of 10−8 Pa,
pass energy of 29.4 eV, and resolution of Ϯ0.2 eV. The corrections
for the background, smoothing, and Gaussian deconvolution of XPS
spectra were accomplished using appropriate software. Using a
CHI760B potentiostat/galvanostat system, the potentiodynamic po-
larization was performed in KOH ͑50 g L−1͒ and H2SO4 ͑mol L−1
͒
solutions, respectively, with an applied potential from −0.8 to 1.2
VSCE and sweep rate of 0.01 V s−1. All electrochemical measure-
ments were carried out in a three-electrode cell, wherein a coated
copper plate was used as working electrode and a platinum plate and
saturated calomel electrode ͑SCE͒ were used as the counter and
reference electrodes, respectively.
Results and Discussion
Microstructure and mechanical properties of Cr-MWNT compos-
ite coatings.— The content of MWNT in the composite coatings is
about 1 mass % determined by the energy-dispersive X-ray spec-
troscopy analysis tool attached to the FE-SEM. Figures 1a and b
show the FE-SEM photos of the top and bottom surfaces of Cr-
MWNT composite coating, respectively. MWNTs are nearly mono-
dispersed in Cr matrix. Figure 1c shows the fracture section micro-
structure of Cr-MWNT composite coating, where MWNTs are also
distributed uniformly in the body of the composite coatings.
Figure 2 demonstrates the indentations on specimens after being
indented by the diamond indenter under different loads for 5 s. The
hardness is about 8 GPa for pure Cr coatings and 10.5 GPa for
Cr-MWNT composite coatings. On the pure Cr coating surface,
some indention-induced cracks and collapses appear around the in-
dentations. In contrast, on the Cr-MWNT composite coating surface,
the areas of indentations are relatively small and no obvious cracks
and collapses are found. Obviously, the introduction of MWNTs
significantly increases the hardness and toughness of Cr coatings,
which may be attributed to the “fiber reinforcement” of MWNTs.
Experimental
Electrodeposition of Cr-MWNTcomposite coatings.— Cr-MWNT
composite coatings ͑20 ␮m thick͒ were deposited from Cr͑III͒ baths
composed of CrCl3·6H2O ͑200 g L−1͒, HCOOH ͑32 mL L−1͒,
CH3COOH ͑10 mL L−1͒, NH4Br ͑30 g L−1͒, KCl ͑60 g L−1͒,
H3BO3 ͑30 g L−1͒, and MWNTs ͑1 g L−1͒ under ultrasonic agita-
tion with a direct current density of 30 A dm−2 and pH value of 2 at
35°C. MWNTs ͑98% purity, commercially obtained from Shenzhen
Nanotechnologies Co., Ltd., China͒ are about 30–60 nm in outer
diameter and 1–2 ␮m in length. Copper plates with an area of 5 cm2
were used as substrates. Dimensionally stable titanium anodes ͑com-
mercially obtained from Xinxiang Future Hydrochemistry Co.,
China͒ with an area of 12 cm2 were used in order to reduce the
anodic oxidation of Cr͑III͒. Before plating, the substrates were pol-
ished and “activated” in HCl solution with a concentration of
1.5 mol L−1
.
Characterizations.— A JSM-6701F field-emission scanning
electron microscope ͑FE-SEM͒ ͑JEOL, Japan͒ was used to investi-
gate the morphology of the Cr-MWNT composite coating surface
and fracture section. Mechanical properties were measured by a
Vickers hardness indenter. A Perkin-Elmer PHI-5702 multifunc-
tional X-ray photoelectron spectroscope ͑XPS͒ ͑Physical Electron-
The role of MWNTs in the electrochemical corrosion process of
Cr coatings.— Figures 3 and 4 express the potentiodynamic polar-
ization curves of the coatings measured in H2SO4 ͑1 mol L−1͒ and
KOH ͑50 g L−1͒ solutions, respectively. As shown in Fig. 3, ca-
thodic segments in the H2SO4 solution correspond to hydrogen
evolution,15 which can be expressed by two reactions as follows
z E-mail: zzx1572000@yahoo.com.cn; junyanzh@yahoo.com
H+ + e → H
͓1͔
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C124
Journal of The Electrochemical Society, 156 ͑4͒ C123-C126 ͑2009͒
(b)
(c)
(a)
Figure 1. FE-SEM photo of the ͑a͒ top
surface, ͑b͒ bottom surface, and ͑c͒ frac-
ture section of Cr-MWNT composite coat-
ings.
respect to the anodic segment of pure Cr coatings, two waves in area
A indicate the dissolution of Cr metal via two steps. Wave 1 is
attributed to the electrochemical oxidation reaction ͑Eq. 3͒ of Cr and
the precipitation reaction ͑Eq. 4͒ of Cr2+ and OH− on the Cr surface
as the result of electromigration
H + H → H2
͓2͔
The cathodic segment of curve 2 for Cr-MWNT composite coatings
shifts to the positive potential direction compared to that of curve 1
for pure Cr coatings, indicating that the introduction of MWNTs into
Cr coatings can accelerate hydrogen evolution.
The anodic segment of the potentiodynamic polarization curves,
following an active-passive-transpassive form, corresponds to active
dissolution and passivation processes of Cr coatings. The segments
in areas A, B, C, and D marked in Fig. 3 are attributed to active
dissolution of Cr metal, the formation of passive film, the stable
passivation state, and the transpassivation stage, respectively. With
Cr − 2e → Cr2+
͓3͔
Cr2+ + 2OH− → Cr͑OH͒
͓4͔
2
Cr2O3 is the primary component of the passive film on the Cr sur-
face generated during the anodic electrochemical polarization.19,20
1.4
1.0
1.2
0.8
D
1.0
0.8
0.6
0.4
(b)
(a)
0.2
C
0.6
2
1
0.0
0.4
-0.2
-0.4
B
A
0.2
E-3
0
1
2
3
0.0
-2
Current density/A cm
-0.2
-0.4
-0.6
-0.8
-1.0
1
2
Single Cr film
Cr-MWNT composite film
-7
-6
-5
-4
-3
-2
-1
log(current density/A cm-2)
Figure 3. Potentiodynamic polarization curves of single Cr films and Cr-
MWNT composite films measured in H2SO4 ͑1 mol L−1͒ solution.
1.0
-2
7.5
Current density/A cm
5.0
2.5
E-5
0.8
0.6
0.2
0.0
0.4
-0.2
-0.4
0.2
0.0
0.0
E-4
10.0
5.0
Current density/A cm
-0.2
-0.4
-0.6
-0.8
-1.0
-2
15µm
3
4
Single Cr film
Cr-MWNT composite film
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
log(Current density/A cm-2)
Figure 2. Indentations on the ͑a͒ single Cr and ͑b͒ Cr-MWNT composite
coatings after being indented by the diamond indenter under different loads
for 5 s.
Figure 4. Potentiodynamic polarization curves of single Cr films and Cr-
MWNT composite films measured in KOH ͑50 g L−1͒ solution.
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Journal of The Electrochemical Society, 156 ͑4͒ C123-C126 ͑2009͒
Thus, wave 2 is ascribed to the reaction as follows
C125
2Cr͑OH͒ − 2e + 2OH− → Cr2O3 + 3H2O
͓5͔
2
With a further potential shift to the positive direction, as shown in
areas B and C, Cr2O3 completely covers the coating surface, as
confirmed by current density decreasing first and then keeping con-
stant in spite of further increase in potential. The critical passive
current density is about 2.95 ϫ 10−3 A cm−2. When the potential is
higher than 1.0 V, the electrochemical behavior enters the transpas-
sivation stage.
Similarly, the anodic polarization of Cr-MWNT composite coat-
ings also undergoes active-passive-transpassive process. But, the
process of passive film formation is different from that of pure Cr
coatings. The critical passive current density for Cr-MWNT com-
posite coatings is 1.1 ϫ 10−3 A cm−2, about two times less than that
of pure Cr coatings, and no current density drop occurs before the
current density remains constant. This indicates that the active dis-
solution of Cr and the formation of passive film occur simulta-
neously in the case of Cr-MWNT composite coatings, which can be
expressed by the following electrochemical reaction
2Cr − 6e + 3H2O → Cr2O3 + 6H+
͓6͔
Obviously, the passive film can be generated more easily on Cr-
MWNT composite coatings than on pure Cr coatings. The reason is
that MWNTs, with nanoscale diameter and good electrical conduc-
tivity, can provide a large number of active sites to accelerate the
formation of passive film.
Figure 4 shows the potentiodynamic polarization curves in KOH
͑50 g L−1͒ solution. The cathodic segment in KOH solution corre-
sponds to the reduction of oxygen, which can be expressed by the
reaction as follows
Figure 5. XPS analysis of the products on ͑a͒ Cr and ͑b͒ Cr-MWNTs after
electrochemical tests.
O2 + 4e + 2H2O → 4OH−
͓7͔
adsorb Crn͑OH͒ polymer film, which can strengthen the adhesion
of Crn͑OH͒x polymer film to the composite coatings to some extent.
x
The cathodic segment of curve 4 for Cr-MWNT composite coatings
shifts to the positive potential direction compared to that of curve 3
for pure Cr coatings. This indicates that oxygen is reduced more
easily on Cr-MWNT composite coatings than on pure Cr coatings.
MWNTs can adsorb oxygen in solutions,21 which is beneficial to the
reduction of oxygen on the coating surface. In addition, the positive
shift of the corrosion potential indicates that Cr-MWNT composite
coatings exhibit better antiself-corrosion ability than pure Cr coat-
ings.
Conclusions
The Cr-MWNT composite coatings were electrodeposited from
trivalent Cr electrolytes containing MWNTs under ultrasonic agita-
tion. The effect of MWNTs on the electrochemical corrosion pro-
cesses of Cr coatings in H2SO4 ͑1 mol L−1͒ and KOH ͑50 g L−1
solutions were investigated.
͒
In H2SO4 solution, MWNTs can provide a large number of ac-
tive sites to accelerate the formation of passive film, due to their
nanoscale diameter and electrical conductivity. Thus, Cr-MWNT
composite coatings exhibit lower critical passive current density
than that of pure Cr coatings. While in KOH solution, MWNTs can
In regard to the anodic polarization process, no obvious passiva-
tion occurs. Figure 5 shows the XPS analysis of the products after
anodic polarization. In the XPS spectra of the Cr 2p3/2 band, the
decomposed peaks located at about 577.0 and 574.5 eV are attrib-
uted to Cr in the form of chromium hydroxide and Cr in metal Cr,
respectively.22-24 In the presence of OH−, the olation reaction25-27
tends to occur between chromium ion and OH−. Thus, the wave at
about −0.4 VSCE is attributed to Reactions 8 and 9
adsorb Crn͑OH͒ polymer film to alleviate the dissolution of Cr
x
coatings. Accordingly, Cr-MWNT composite coatings exhibit a
lower dissolution rate during the anodic polarization process than
pure Cr coatings.
In addition, the introduction of MWNTs can significantly im-
prove the mechanical properties of Cr coatings, which is attributed
to the “fiber reinforcement” of MWNTs.
Therefore, MWNT is an excellent dopant to Cr coating to en-
hance its mechanical and anticorrosion properties significantly.
Cr − 2e → Cr2+
͓8͔
2n−x
x
nCr2+ + xOH− → Crn͑OH͒
͓9͔
And, the wave near −0.17 VSCE is attributed to the following reac-
tion
Acknowledgments
2n−x
2n+y−x
x
Crn͑OH͒
− ye → Crn͑OH͒
͓10͔
x
The authors greatly acknowledge the ‘‘Hundreds Talent Pro-
gram’’ of the Chinese Academy of Sciences and the Innovative
Group Foundation from NSFC ͑grant no. 50721062͒ for financial
support.
The Crn͑OH͒ polymer film with flocculent structure cannot cover
the coating surface compactly and completely as Cr2O3 does. Ac-
cordingly, when Crn͑OH͒ polymer film is formed, the current den-
x
x
sity drops to a low value, but no obvious typical passivation occurs
during the anodic polarization process. At the same potential, the
current density in the anodic polarization process of Cr-MWNT
composite coatings is about 10 times less than that of pure Cr coat-
ings, which indicates that the dissolution rate of Cr-MWNT com-
posite coatings is about 10 times less than that of pure Cr coatings in
KOH ͑50 g L−1͒ solution. The relatively low dissolution rate of Cr-
MWNT composite coatings is attributed to the ability of MWNTs to
Chinese Academy of Sciences assisted in meeting the publication costs of
this article.
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