Surface protection of copper by self-assembly of novel poly(5- methylenebenzotriazol-N-yl)

Article abstract of DOI:10.1039/c3ta01250f

The novel polymer poly(5-methylenebenzotriazol-N-yl), denoted BTA-poly, was investigated for the protection of copper by electrochemical studies and weight loss measurements. The electrochemical measurements were compared to weight loss measurements and satisfactory results were obtained. Compared with benzotriazole, which is one of the best corrosion inhibitors for copper, BTA-poly achieves good protection under the same test conditions. Due to the rapid adsorption of BTA-poly onto copper, its corrosion-inhibition efficiency can be controlled by varying the concentration. The protecting effects of the polymers were studied in a sodium chloride solution using electrochemical measurements and an immersion test. The surface characteristics of the polymer-modified copper were analyzed via X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). These results show that BTA-poly can be easily adsorbed onto the copper surface via self-assembly to achieve excellent anticorrosion/antioxidation effects.

Full text of DOI:10.1039/c3ta01250f

Journal of
Materials Chemistry A
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PAPER
Surface protection of copper by self-assembly of novel
poly(5-methylenebenzotriazol-N-yl)†
Cite this: J. Mater. Chem. A, 2013, 1,
3629
a
c
ab
Yu Ming Lee, Ying Hsueh Chang Chien, Man-kit Leung,
and Chi-Chao Wanc
Chi-Chang Huc
*
The novel polymer poly(5-methylenebenzotriazol-N-yl), denoted BTA-poly, was investigated for the
protection of copper by electrochemical studies and weight loss measurements. The electrochemical
measurements were compared to weight loss measurements and satisfactory results were obtained.
Compared with benzotriazole, which is one of the best corrosion inhibitors for copper, BTA-poly
achieves good protection under the same test conditions. Due to the rapid adsorption of BTA-poly onto
copper, its corrosion-inhibition efficiency can be controlled by varying the concentration. The protecting
effects of the polymers were studied in a sodium chloride solution using electrochemical measurements
and an immersion test. The surface characteristics of the polymer-modified copper were analyzed via
X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). These results show
that BTA-poly can be easily adsorbed onto the copper surface via self-assembly to achieve excellent
anticorrosion/antioxidation effects.
Received 26th November 2012
Accepted 9th January 2013
DOI: 10.1039/c3ta01250f
www.rsc.org/MaterialsA
water systems, etc.).1 These inhibitors will form a protective lm
on the surface, such that the rate of either anodic oxidation,
Introduction
Metal corrosion, the electrochemical oxidation of metals
coupled with the electrochemical reduction of oxidants (e.g.,
oxygen molecules), exists in many industries as well as domestic
applications. The cost of corrosion in industrialized countries
has been estimated to be about 3–4% of the gross national
product. It has been further estimated that about 20% of this
loss could have been avoided by using suitable inhibitors in
corrosion protection. It has also been estimated that the cost of
protection against atmospheric corrosion is approximately 50%
of the total cost of all corrosion protection methods.1 The
protection of metals and alloys is thus of particular interest.2
The main purpose of studying the processes of corrosion is to
nd methods of minimizing or preventing the problem. One
approach is the use of corrosion inhibitors. Many efficient
inhibitors are heterocyclic organic compounds consisting of p-
systems and/or containing O, N, or S heteroatoms. In general,
inhibitors can be classied in three ways, according to (1)
chemical nature (organic or inorganic substances), (2) charac-
teristics (oxidizing or nonoxidizing compounds), and (3) tech-
nical eld of application (acid cleaning, descaling, cooling
cathodic reduction, or both, could be reduced. It has been
postulated that the inhibitors are adsorbed onto the metal
surface through physisorption or chemisorption. Physisorption
usually arises from the electrostatic attraction between organic
ions and the electrically charged metal surface. Chemisorption
is the transfer or sharing of the inhibitor molecule's charge with
the metal surface, forming a coordinate-type bond. The adsor-
bed inhibitor reduces the corrosion rate of the metal surface by
retarding either the anodic dissolution reaction of the metal,
the cathodic evolution of hydrogen, or both.
Since the pioneering work of Cotton and co-workers,3 ben-
zotriazole (BTAH) has become one of the most effective inhib-
itors for copper and its alloys in many electrolytes.4–7 In
addition, BTAH has been found to increase copper passivity by
forming a Cu(I)–BTA polymeric complex, and the formation of a
multilayer structure on the surface lm, Cu/Cu2O/Cu(I)-BTA,
was proposed.6
Self-assembled monolayers (SAMs) can provide a useful tool
to develop inhibitors. SAMs on reactive substrates like copper
can produce effective barriers to the penetration of corrosive
species like water, oxygen, and aggressive ions into the metal
surface.8–10 Thus, the SAMs approach has several advantages: (1)
the lms can form spontaneously through simple chemisorp-
tion to the metal surface, (2) the thickness of the lm can be
controlled by controlling the reaction conditions, and (3) SAMs
do not signicantly change the appearance or other character-
istics of a metal substrate, except for its hydrophobicity.9,11
However, the formation of a monolayer is susceptible to many
aDepartment of Chemistry, National Taiwan University, Taipei 106, Taiwan. E-mail:
mkleung@ntu.edu.tw
bInstitute of Polymer Science and Engineering, National Taiwan University, Taipei 106,
Taiwan
cDepartment of Chemical Engineering, National Tsing-Hua University, Hsinchu 300,
Taiwan
† Electronic supplementary information (ESI) available: detailed information on
contact angle experiments. See DOI: 10.1039/c3ta01250f
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sulfuric acid (5.0 g) at room temperature. The mixture was
reuxed at 80 ꢀC for 2 h under a nitrogen atmosphere and
quenched by pouring into water. The aqueous layer was
extracted with CH2Cl2. The combined extracts were dried
(Na2SO4), and concentrated under reduced pressure. The crude
product was puried by liquid chromatography (meth-
anol : ethyl acetate ¼ 5%) on silica gel to give 2 (5.0 g, 85%) as a
1
ꢀ
light pink solid. m.p.: 110 C, H-NMR (400 MHz, DMSO-d6) d
8.53 (s, 1H), 8.00–7.93 (m, 2H), 4.34 (q, J ¼ 8.0 Hz, 2H), 1.34 (t,
J ¼ 8.0 Hz, 3H); 13C-NMR (100 MHz, DMSO-d6) d 165.47, 140.27,
139.03, 126.57, 126.10, 118.50, 113.97, 61.02, 14.13; HRMS calcd
for C9H9N3O2 191.0695 (M+ + H), obsd 191.0649.
Scheme 1 An illustration of the self-assembly of BTA-poly on a copper surface.
Synthesis of 5-hydroxymethylbenzotriazole (3).14 To
a
suspension of LiAlH4 (2.98 g, 78.5 mmol) in dry THF (110 mL),
ethyl benzotriazole-5-carboxylate (5.01 g, 26.2 mmol) was added
dropwise in THF (10.0 mL) at 0 C. The mixture was reuxed
overnight, poured into ice water, neutralized with aqueous HCl
and extracted with ether to give 5-hydroxymethylbenzotriazole
(3) (3.21 g, 82%) as a white solid. m.p.: 155 ꢀC, 1H-NMR
(400 MHz, DMSO-d6) d 7.86 (d, J ¼ 8.0 Hz, 1H), 7.78 (s, 1H), 7.37
(d, J ¼ 8.0 Hz, 1H), 4.63 (s, 2H); 13C-NMR (100 MHz, DMSO-d6) d
140.80, 139.06, 138.17, 124.63, 115.21, 111.04, 62.93; HRMS
calcd for C7H7N3O 149.0589 (M+ + H), obsd 149.0557.
factors, such as surface pretreatment and the adsorption of
contaminants during preparation of the monolayer
samples.8,12,13 For example, the self-assembly of alkanethiol
monolayers on a copper substrate is relatively challenging due
to the spontaneous oxidation of the copper surface, which
weakens the interaction between the mercapto (–SH) group and
the metal surface.8,12,13 Furthermore, the toxicity of alkanthiols
and their offensive smell limit their application in industry.
Hence, developing new kinds of SAMs with lower toxicity has
attracted intensive research interest.
ꢀ
Synthesis of 5-chloromethylbenzotriazole (4).14 A mixture of
3 (0.50 g, 3.4 mmol) and SOCl2 (3.4 mL) in toluene (4.2 mL) was
stirred at room temperature for 3.5 h. The resulting precipitate
was collected by ltration to give 5-chloromethybenzotriazole
(4) (0.52 g, 93%) as a white solid. m.p.: 124 ꢀC, 1H-NMR
(400 MHz, DMSO-d6) d 8.00 (s, 1H), 7.92 (d, J ¼ 8.0 Hz, 1H), 7.50
(d, J ¼ 8.0 Hz, 1H), 4.95 (s, 2H); 13C-NMR (100 MHz, DMSO-d6) d
138.73, 138.50, 135.21, 124.50, 115.39, 115.05, 46.29; HRMS
calcd for C7H6ClN3 167.0250 (M+ + H), obsd 167.0208.
In this paper, we report the synthesis of a new inhibitor,
poly(5-methylenebenzotriazol-N-yl) (denoted BTA-poly, see
Scheme 1). The unique BTA-poly SAMs formed on copper
surfaces were characterized by X-ray photoelectron spectroscopy
(XPS) and electrochemical techniques.
Experimental section
1
Synthesis of BTA-poly
Synthesis of poly(5-methylenebenzotriazol-N-yl) (5).
A
Materials. Ethyl benzotriazole-5-carboxylate was purchased
mixture of 4 (0.20 g, 1.2 mmol), K2CO3 (0.66 g, 4.8 mmol) and
18-crown-6 ether (0.13 g, 0.48 mmol) was dissolved in dry DMF
at room temperature in an argon atmosphere. The mixture was
reuxed at 90 C for 1 day and the reaction was quenched by
pouring into water. The mixture was reprecipitated by deionized
from Aldrich. Lithium aluminium hydride (LAH) and dime-
thylacetamide (DMAc) were purchased from Acros. Dime-
thylformamide (DMF) and thionyl chloride (SOCl2) were
obtained from Merck. All other chemicals used in this study
were of AR grade. The synthetic route for BTA-poly used in our
experiments is shown in Scheme 2.
Synthesis of ethyl benzotriazole-5-carboxylate (2).14 To a
mixture of benzotriazole-5-carboxylic acid (5.0 g, 31 mmol),
ethanol (15 mL) and toluene (7.0 mL) was added concentrated
ꢀ
(DI) water and the resulting precipitate was collected by ltra-
1
tion to give polymer 5 (0.15 g, 75%) as a white solid. H-NMR
(400 MHz, DMSO-d6) d 7.94–7.82 (br, 2H), 7.24 (br, 1H), 6.06 (br,
2H). The Mw, Mn, and PDI data are listed in Table 1.
Scheme 2 The synthetic route for BTA-poly.
3630 | J. Mater. Chem. A, 2013, 1, 3629–3638
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Table 1 Synthesis results of BTA-poly under different conditions
(3.5 wt%) under nitrogen at room temperature. A conventional
three-electrode glass cell with a capacity of 80 mL was used. A
copper rod with a circular area of 0.07 cm2 was embedded in a
Teon holder and served as the working electrode. A saturated
calomel electrode (SCE) was used as the reference electrode, and
a platinum plate as the counter electrode. CV measurements
were performed through a linear potential sweep from a nega-
tive initial potential of ꢂ1.2 V (SCE) to the designated positive
potential of 0.4 V (SCE) at a scan rate of 20 mV sꢂ1. Prior to each
measurement the bare copper electrodes were held at a poten-
tial of ꢂ1.3 V (SCE) for 90 s to remove spontaneously formed
oxides. The scan rate in the potential dynamic polarization
measurements was 1 mV sꢂ1, and the scan range was ꢃ 600 mV
vs. open-circuit potential (OCP). Before each potential dynamic
polarization measurement, the copper electrode was immersed
in NaCl solution (3.5 wt%) for several minutes.
Temperature
Entry Solvent Rxn time (ꢀC)
Mw
Mn
PDI Yield (%)
a
a
a
a
1
2
3
Acetone 1 day
70
90
80
—
—
—
—
DMF
1 day
1 day
5685 4737 1.20 65
4820 4048 1.19 45
DMAc
a
Data could not be obtained.
2
Copper surface preparation
The copper surface was rst polished with SiC abrasive paper
(progressively from no. 400, 800, 2500 to 4000) to get a smooth
surface. The newly polished copper was then etched in
concentrated HNO3 (7.0 M) solution for 30 s to obtain a fresh,
oxide-free copper surface. Finally, the copper plate was rinsed
with DI water, and then with absolute ethanol.12,15
7
Weight loss measurements in immersion tests
Copper samples were cut to a regular size of 10 ꢁ 25 ꢁ 0.1 mm
for each immersion test. Two types of immersion test were
performed. Before exposure to the test medium, each specimen
was weighed. The specimens were rinsed with DI water, care-
fully wiped with a paper towel to remove attached corrosion
products, rinsed with DI water again, dried with a stream of
nitrogen, and weighed aer exposure. The following immersion
test procedures were carried out according to a related study.16
In the rst immersion test, the loss of mass was measured at
2, 5 and 8 days on the same specimens under stationary
conditions. Tests were performed in 3.5 wt% aqueous NaCl
solution (120 mL). The specimens were weightd on a Mettler
Toledo precision microbalance, model AG245.
3
Preparation of SAMs on copper
Aer the above pretreatments, the copper was rapidly immersed
into an analytical grade DMF solution containing the BTA-poly
for different time periods, from 1 h to 24 h. At the end of the
designated period, the electrodes were taken out, rinsed with DI
water and absolute ethanol, and dried in an argon atmosphere.
To ensure the SAMs process proceeded under deoxygenation
conditions, the solutions were degassed under a ow of Ar for
2 h before the experiment, through and above the solution.7
4
Surface characterization
The surface composition of the functionalized copper plates
was characterized by X-ray photoelectron spectroscopy (XPS).
The electrode used for XPS analysis was a copper plate with
dimensions of 7.5 ꢁ 12.5 ꢁ 0.1 mm. The XPS measurements
were performed using a Thermo Scientic XPS spectrometer
with an Al Ka X-ray source (1486.6 eV photons).
In the second immersion test, the test was performed under
dynamic conditions. The specimens were put into a 1 L glass
ask containing 3.5 wt% NaCl aqueous solution (350 mL). The
solution was stirred with a mechanical stirrer at 600 rpm for
15 days.
5
Contact angle measurements
8
Scanning electron microscopy (SEM)
The contact angle measurements were performed on a FTA-125
(America) using the sessile drop method with a DI water droplet
(1 mL). Aer dropping the DI water droplet onto the sample
surface, the value of the contact angle was automatically
measured by the analysis system. The uncertainty in the value of
contact angle is less than 0.1ꢀ. The average value of contact
angle was obtained from measurements at ten locations on the
same samples. For the experimental results, see the ESI.†
The surface morphology was investigated with a eld emission
scanning electron microscope (FEI, Nova NanoSEM 230), whose
the beam landing energy is 50 V–30 kV, or a eld emission
scanning electron microscope (JOEL, JSM-6700F), whose the
beam landing energy is 0.5–30 kV.
Results and discussion
1
Preparation of BTA-poly
6
Electrochemical measurements
By taking advantage of its unique chemical reactivity, 5-chlor-
A copper rod (99.999%) with a diameter of 3 mm was embedded omethylbenzotriazole could be self-condensed to form BTA-poly
in a Teon holder and used for electrochemical measurements. under mild conditions. The optimized conditions for the
Cyclic voltammetry (CV) was carried out using a CHI 4052a condensation polymerization are summarized in Table 1.
instrument. The potential dynamic polarization studies were Among the solvents we explored, DMF was found to be the most
carried using a potentiostat/galvanostat (model PGSTAT 302N) effective for the promotion of the condensation. Changing the
instrument, and the data obtained were analyzed using NOVA solvent from DMF to other polar solvents such as acetone or
soware, version 1.7. In all electrochemical studies, the corro- DMAc led to lower chemical yields. This might be due to the fact
sion experiments were carried out in aqueous NaCl solution that the growth of the polymer is limited by the solubility of the
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oligomers under the reaction conditions; the solubility of the
oligomers in DMF is relatively high, so it would prevent
the oligomers from precipitating, allowing the polymerization
continue.
2
XPS analysis
XPS analysis was used to elucidate whether BTA-poly could
really be adsorbed onto the surface of the copper plate through
a self-assembly mechanism. Fig. 1 shows the XPS survey spectra
of the BTA-poly assembled on a copper plate. The plate was
immersed in a BTA-poly solution (c ¼ 1 ꢁ 10ꢂ3 M) in DMF for
1 h before the XPS analysis, and then dried. The presence of XPS
signals of N 1s and C 1s at 399 and 285 (eV) indicates the
presence of C and N on the copper surface, providing evidence
to support the self-assembly hypothesis. Although one might
suspect that adsorption of DMF on the surface might occur,
leading to a misinterpretation of the spectra, we also checked
the XPS of a sample from a bare copper plate in DMF as a
background (see Fig. 1). The absence of an XPS signal for C 1s
and N 1s indicated that the DMF solvent was not adsorbed onto
the copper plate. The strong intensity of the Cu LMM peak also
suggests that DMF was not adsorbed onto the copper surface.
From these results, we concluded that BTA-poly can be adsor-
bed onto copper surface selectively over DMF.
The second question we are going to address is whether the
BTA-poly was adsorbed through physisorption or chemisorp-
tion. In order to differentiate between these two mechanisms,
the self-assembled layer was subjected to etching experiments,
in which the etching time would provide us an important clue to
the assembly mechanism. In the experiments, we focused on
the XPS intensity of the C 1s signal. With increasing etching
time, the signal intensity would decay. Usually, if the assembled
layer was physisorbed, the signal intensity would completely
decay aer several seconds of etching. However, for our sample,
the C 1s signal lasted for several minutes during the etching
experiments. The data are summarized in Fig. 2. About 34% of
the signal intensity remains aer the sample had been etched
for 10 min. This result suggests that BTA-poly was likely to be
adsorbed through chemisorption on the copper surface. The
Fig. 2 The C 1s etching spectra of the BTA-poly SAMs formed on the copper
surface.
XPS spectra of Cu(II) ions (e.g., in CuO) exhibit some charac-
teristic signals: (1) multiple line broadening of both Cu 2p3/2
and Cu 2p1/2 peaks, and (2) intense shake-up satellite peaks at
ca. 10 eV higher binding energy (BE) than the main Cu 2p3/2 and
Cu 2p1/2 peaks.17 Therefore, the analysis of the Cu 2p peaks by
XPS allows the distinction between copper and cuprous or
cupric species to be made. Fig. 3 shows the Cu 2p spectra of
copper modied by BTA-poly SAMs. No satellite peaks appear at
around 943 and 963 eV, nor is Cu 2p peak broadening observed.
This result indicates that no Cu(II) species formed on the copper
surface. The curve of BTA-poly shows a slight shi in BE. We
suggest that the BTA-poly SAMs might have highly polar groups
and these polar groups attached to the central atom, resulting
in a slight chemical shi.19
Cu LMM can be used to discriminate between the lines for
metallic Cu(0) and Cu(I), due to the differences in electron
relaxation energies in these species.18 In order to distinguish
between Cu and Cu(I) species, the Cu LMM spectrum was
obtained (Fig. 4). We also checked bare copper and the copper
immersed in DMF for comparison. The spectrum shows four
distinct peaks (labelled 1–4) with a very sharp main peak located
Fig. 1 XPS survey spectrum of BTA-poly SAMs covered copper. The survey
Fig. 3 The Cu 2p spectrum of BTA-poly SAMs covered copper. The Cu 2p spectra
spectra of bare copper and copper immersed in DMF are given for comparison.
of bare copper and copper immersed in DMF are given for comparison.
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copper electrode; (2) a copper electrode pre-treated with BTAH
solution; (3) a copper electrode pre-treated with BTA-poly. In the
CV of the bare copper electrode, two oxidation waves peaking at
0.16 V and 0.31 V (SCE) were recorded in the forward scan, while
a large reduction wave peaking at ꢂ0.36 V (SCE) was recorded in
the reverse scan. The oxidation waves at 0.16 V and 0.31 V (SCE)
were assigned to the oxidation of Cu(0) to the CuCl salt layer
and the oxidation of Cu(I) to soluble Cu(II) species, respec-
tively.21,22 The reduction wave corresponded to the reduction of
the CuCl layer formed on the copper surface. Anodic dissolution
of copper in aqueous Clꢂ solution has been extensively studied
by many research teams.23,24
Cu + Clꢂ # CuCl + eꢂ
(1)
(2)
ꢂ
CuCl + Clꢂ # CuCl2
Fig. 4 X-ray induced Auger spectrum of BTA-poly SAMs covered copper. The Cu
LMM spectra of bare copper and copper immersed in DMF are given for
comparison.
In the reverse sweep, the corrosion product of CuCl would be
partially reduced back to the metallic state.
CuCl + eꢂ # Cu + Clꢂ
(3)
at 569.6 eV BE (labelled 2). This peak is the most intense of the
four and represents pure copper. Peak 3 has a higher BE than
peak 2, and represents the Cu(I) chemical state. It is sometimes
possible to determine the content of metallic copper or Cu2O on
It is noteworthy that a characteristic anodic current hump
appears in the CV plot in the reverse sweep at about
+0.04 V.16,25–27 This was attributed to the competition between
the dissolution and precipitation of corrosion products on the
copper surface, in which the rate of dissolution is hardly
compensated by the precipitation process.
For the second electrode which was pretreated with BTAH
(c ¼ 1 ꢁ 10ꢂ3 M) for 30 min, the CV shows a similar redox
pattern, indicating that a similar corrosion mechanism by Clꢂ
would still proceed in this case. Prolonging the pre-treatment
time period to 5.5 h only led to a small change in the oxidation
peaks. These results are shown in Fig. 6.
the copper surface using the ratio of Ipeak2 : Ipeak3.
19,20 If the ratio
of Ipeak2 : Ipeak3 is high, the content of metallic Cu is higher than
that of Cu2O. In our studies, the ratio of Ipeak2 : Ipeak3 for bare
copper is 2.04, for DMF/Cu it is 1.86, and for BTA-poly/Cu it is
1.45. It is concluded that BTA-poly might form BTA–poly Cu(I)
complexes, and this result was in agreement with observations
of the Cu 2p spectra and XPS etching experiments.
3
Cyclic voltammetry measurements
In order to gure out the best conditions for our CV studies, we
attempted to search for the optimized scan rate. Among the
scan rates we tested, the scan rate of 20 mV sꢂ1 provides the best
CVs, with the oxidation waves being well resolved. Fig. 5 shows
the CVs of the copper electrodes in aqueous NaCl (3.5 wt%)
under different pre-treatment conditions as follows: (1) a bare
When the electrode was pre-treated with BTA-poly (c ¼ 1 ꢁ
10ꢂ3 M), the oxidation wave intensity at 0.16 V was clearly
suppressed. Increasing the concentration of BTA-poly to 1 ꢁ
10ꢂ2 M led to further suppression, which indicated that this
phenomenon is governed by the self-assembly of BTA-poly on
Fig. 6 Immersion time effect in BTAH: CVs of copper electrodes in aqueous NaCl
(3.5 wt%) solution. (1) Bare copper electrode, (2) a copper electrode in the same
solution containing BTAH (1 ꢁ 10ꢂ3 M) for 30 min, and (3) BTAH (1 ꢁ 10ꢂ3 M) for
5.5 h.
Fig. 5 BTA-poly SAMs effect: CVs of copper electrodes in aqueous NaCl (3.5 wt%)
solution. (1) Bare copper electrode, (2) a copper electrode in the same solution
containing BTAH (1 ꢁ 10ꢂ3 M), and (3) copper covered with the BTA-poly SAMs.
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curves are summarized in Table 2. The values of corrosion
potentials (Ecorr, vs. SCE), corrosion current densities (icorr), and
Tafel slopes (ba and bc) were obtained by extrapolating the
linear parts of the anodic and cathodic branches to their
intersection.28,29 The corrosion rates (r) of copper in a 3.5 wt%
aqueous NaCl solution in the absence and presence of inhibi-
tors were calculated using eqn (4) as shown below:30
0:129 ꢁ icorr ꢁ EW
r ¼
ðin mpyÞ
(4)
D
where icorr is the corrosion current density (mA cmꢂ2), EW is the
equivalent weight, and D is the density of the specimen
(g cmꢂ3). From Table 2, it is obvious that the corrosion potential
of BTAH and BTA-poly have shied slightly in the positive
direction, resulting in a decrease of corrosion current density
compared with bare copper. Literature research15a,16c,29a,31–33
revealed that the corrosion current density of bare copper spans
the range 10 to 40 mA cmꢂ2, which is equivalent to a corrosion
rate of 5 to 18 mpy, according to eqn (4). The corrosion rate of
8 mpy for bare copper obtained from our electrochemical
analysis falls into this region, suggesting that our measurement
is reliable. In the presence of inhibitors containing nitrogen
atoms,16c,32–35 the corrosion current density of copper is usually
suppressed to about 0.9 to 7 mA cmꢂ2, corresponding to a
corrosion rate of 0.4 to 3 mpy. The corrosion rate of BTA-poly
was found to be 1.4 mpy in our study, indicating that BTA-poly
also has good inhibitory efficiency. We reason that BTA-poly
may have formed densely packed and hydrophobic monolayers
on the copper surface. The monolayers can not only retard
electron transfer across the electrode interface, but also prevent
the contact of water with the underlying copper surface. The
barrier behavior of the coatings is known to be mainly depen-
dent on their structures and thickness, as well as the chemical
properties of the inhibitor molecules.36,37
Fig. 7 BTA-poly SAMs concentration effect: CVs of copper electrodes in aqueous
NaCl (3.5 wt%) solution. (1) Copper covered with BTA-poly SAMs (1 ꢁ 10ꢂ2 M), (2)
copper covered with BTA-poly SAMs (1 ꢁ 10ꢂ3 M), and (3) a bare copper
electrode.
the copper surface (see Fig. 7). Similar suppression effects have
been reported in the study of SAMs of C18H37SH.8 By comparing
the CV against that of the bare copper electrode, one may easily
perceive that the oxidation process from Cu(0) to Cu(I) is
effectively inhibited by the BTA-poly self-assembled layer. The
presence of the residual anodic wave might arise from oxidation
at defect sites. Increasing the applied potential led to further
oxidation of the electrode surface. At this stage, the BTA-poly
SAMs would be stripped from the copper substrate. Removal of
the SAMs during the rst forward sweep would lead to a bare
copper surface, leading to a similar reduction pattern in the
reverse sweep in all three cases.
4
Tafel polarization curves measurements
The anticorrosion behavior of the self-assembled layer could be
determined by the measurement of Tafel polarization curves.
Fig. 8 shows the Tafel polarization curves of bare copper, BTAH,
and BTA-poly SAMs covered copper in aqueous NaCl (3.5 wt%)
solution. The parameters associated with Tafel polarization
5
Immersion tests
Immersion tests were carried out on copper specimens in the
corrosion medium with and without protection by BTA-poly
aer various exposure periods. We also compared our results
with BTAH, one of the best corrosion inhibitors. When bare
copper was immersed in a 3.5 wt% aqueous NaCl solution
without inhibitors, the copper specimen continually dissolved
and the change of mass was ꢂ408 mg cmꢂ2 and ꢂ760 mg cmꢂ2
aer 2 and 8 days of immersion, respectively (Fig. 9). In the
presence of BTA-poly only a slight decrease in mass was
observed during the rst 5 days of immersion. The mass change
of the BTA-poly covered sample was ꢂ132 mg cmꢂ2 and ꢂ276 mg
cmꢂ2 aer 2 and 5 days of immersion, respectively (Fig. 9). It
can be concluded that the copper specimen modied with BTA-
poly can form a very stable layer, because rinsing and wiping of
the electrode did not strip the layer off the sample. This layer
can protect the copper plate against corrosion in the corrosive
medium and reduced the copper corrosion rate in the immer-
sion test. In the presence of BTAH, the mass change of the
copper electrode was ꢂ196 mg cmꢂ2 and ꢂ468 mg cmꢂ2 aer
Fig. 8 Tafel polarization curves of copper electrodes in aqueous NaCl (3.5 wt%)
solution. (1) Bare copper electrode, (2) a copper electrode in the same solution
containing BTAH (1 ꢁ 10ꢂ3 M), and (3) copper covered with the BTA-poly SAMs. 2 and 8 days of immersion respectively. It is obvious that
3634 | J. Mater. Chem. A, 2013, 1, 3629–3638
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Table 2 Analysis of the results of Tafel polarization curves in a 3.5 wt% aqueous NaCl solution for different samples
Ecorr
(mV, vs. SCE)
icorr
Sample
bad (mV decꢂ1
)
bce (mV decꢂ1
)
(mA cmꢂ2
)
r (mpy)
Bare Cua
331
426
185
260
124
136
ꢂ388
ꢂ287
ꢂ341
17.6
3.20
2.97
8.05
1.46
1.36
Cu–BTAb
Cu–BTA-polyc
a
Bare Cu is a newly polished copper sample. b Cu–BTA is a copper electrode in the 3.5 wt% aqueous NaCl solution containing BTAH (1 ꢁ 10ꢂ3 M).
c
Cu–BTA-poly is copper covered with the BTA-poly SAMs. d ba is the Tafel plot slope of the anodic polarization curve. e bc is the Tafel plot slope of the
cathodic polarization curve.
Fig. 9 Change in mass over 8 days of immersion in 3.5 wt% aqueous NaCl
Fig. 10 Variation of the corrosion rate (r, mpy) with time for different copper
solution.
specimens in NaCl solution (3.5 wt%).
BTA-poly has better anticorrosive properties than BTAH during
the immersion periods.
surface to prevent the attack of chloride ions, which is why the
corrosion rate of copper is high in spite of the increasing
immersion time. However, when the copper was pre-treated
with BTAH or BTA-poly, the corrosion rate is lower than that of
bare copper. It is supposed that BTAH and BTA-poly can be
We also calculated the corrosion rate of copper in the
immersion tests to compare with the results of the electro-
chemical study. The corrosion rates (r) of copper in 3.5 wt%
NaCl solution with and without inhibitors were calculated
according to eqn (5):30
534W
r ¼
ðin mpyÞ
(5)
DAT
where W is the weight loss in milligrams, D is the density of the
specimen (g cmꢂ3), A is the area of the specimen (square
inches), and T is the time in hours. Fig. 10 presents the variation
of the corrosion rate of the specimens in the immersion tests. It
is obvious that the corrosion rate of bare copper in NaCl solu-
tion (3.5 wt%) is the highest, and the corrosion rate decreases
with increasing immersion time. The decrease of corrosion rate
with time is due to the formation of Cu2O and the accumulation
of other corrosion products on the copper surface. It has been
reported that increasing the immersion of copper in the chlo-
ride solution alone leads to the hydrolyzes of the formed oxide
layer in the presence of Clꢂ forming a top layer of atacamite
(Cu2(OH)3Cl).16c The reaction mechanism is shown below:
Fig. 11 SEM micrographs of the morphology of Cu specimens in 3.5 wt%
aqueous NaCl solution under stationary conditions. (a) Bare copper before the
experiment, (b) bare copper after 7 days of corrosion, (c) a BTAH pre-treated
copper sample after 7 days of corrosion, and (d) a BTA-poly pre-treated copper
Cu2O + Clꢂ + 2H2O # Cu2(OH)3Cl + H+ + 2eꢂ
(6)
The formed atacamite on the copper surface is not fully
protective and not sufficiently tightly adsorbed onto the copper sample after 7 days of corrosion.
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have a lower corrosion rate than BTAH, which is probably due to
the hydrophobic character effectively preventing the corrosion.
Comparing with the corrosion rates obtained from the
electrochemical studies, it can be seen that the trends of the
corrosion rates from the electrochemical studies and immer-
sion tests are similar, and can prove that BTA-poly shows good
corrosion inhibition in the corrosive medium. Using electro-
chemical measurements is a straightforward and fast method to
obtain the corrosion rate of a specimen. Although the corrosion
rates obtained from the electrochemical studies and immersion
tests are slightly different, it is probably due to the different
exposure times leading to the different corrosion rates.23,27b,33,38
However, we can use the trends in the corrosion rates from the
electrochemical studies and immersion tests to explain the
results and prove that BTA-poly has good inhibitory efficiency in
a corrosive medium.
6
SEM measurements
To gain more insight into the morphological changes before
and aer the corrosion experiments, SEM was employed to
examine the lm morphology. Copper specimens were treated
in 3.5 wt% aqueous NaCl solution in the presence and absence
of 10 mM of BTAH or BTA-poly under stationary and dynamic
conditions. Under stationary conditions, the bare copper
formed a lot of particles on the surface aer the corrosion
process (Fig. 11(b)). However, if the copper surfaces were pre-
treated with BTAH or BTA-poly, it is obvious that the copper
Fig. 12 SEM micrographs of the morphology of Cu specimens in 3.5 wt%
aqueous NaCl solution under dynamic conditions. (a) Bare copper after 15 days of
corrosion, (b) a BTAH pre-treated copper sample after 15 days of corrosion, and (c)
a BTA-poly pre-treated copper sample after 15 days of corrosion.
adsorbed onto the copper surface and can protect the copper
surface from the attack of chloride ions to form atacamite and
cuprous chloride complexes. The BTA-poly treated samples
Fig. 13 (a) A SEM cross section micrograph of BTA-poly on the Cu surface, (b) diagram of the molecular length of the 5-BTA-CH2NH2 monomer as a model, and (c) a
schematic diagram of the positional arrangement of BTA-poly on the Cu surface.
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Journal of Materials Chemistry A
surface was relatively smooth in comparison to the bare copper micrograph provide evidence for the BTA-poly formed on the Cu
plate (see Fig. 11(c) and (d)). We conclude that BTAH and BTA- surface, and these data can conrm our hypotheses.
poly have anticorrosive properties in the chloride medium.
The corrosion of the copper specimens under dynamic
conditions was more vigorous (see Fig. 12). The rough surface of
Acknowledgements
the bare copper was clearly observed aer 15 days of immersion. The present work was supported by the Ministry of Education
Granular particles as large as 500 nm of Cu2O and CuO and National Taiwan University, Academia Sinica Thematic
(Fig. 12(a)), determined by XPS analysis, can be observed. Project, and the National Science Council of Taiwan (NSC-98-
However, the BTAH protected specimens show a relatively 2119-M-002-006-MY3, NSC-98-2221-E-002-038-MY3, NSC-99-
smooth surface aer corrosion under similar conditions 2218-E-155-003, NSC-99-2221-E-155-092, and NSC-99-2622-E-
(Fig. 12(b)). As we discussed before, the surface roughness is 155-010-CC3).
related to the degree of corrosion. Prolonging immersion time
will lead to a rougher surface with bigger granular size. The
present observations suggest that the presence of a BTAH layer
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