Evaluating two new Schiff bases synthesized on the inhibition of corrosion of copper in NaCl solutions

Article abstract of DOI:10.1039/c4ra14449j

Two kinds of new Schiff base derivatives, 2-((4-(4-(dimethylamino)styryl)phenylimino)methyl) (DSM) and its intermediate 4-(4-aminostyryl)-N,N-dimethylaniline (AND), form self-assembled monolayers (SAMs) on copper surfaces. The SAMs have been examined by a series of techniques, including contact angle (CA), atomic force microscope (AFM), scanning electronic microscope (SEM), and electrochemical measurements. The results suggest that the two derivatives adsorb onto the copper surface and generate corresponding hydrophobic films, which play an important role in the anticorrosion of copper in 3% NaCl solution. Quantum chemical calculations and molecular dynamics (MD) simulation are also used for further insight into the adsorption mechanism.

Full text of DOI:10.1039/c4ra14449j

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Evaluating two new Schiff bases synthesized on the
inhibition of corrosion of copper in NaCl solutions
Cite this: RSC Adv., 2015, 5, 14804
*
Yang Zhou,† Shenying Xu,† Lei Guo, Shengtao Zhang, Hao Lu, Yulong Gong
and Fang Gao
Two kinds of new Schiff base derivatives, 2-((4-(4-(dimethylamino)styryl)phenylimino)methyl) (DSM) and its
intermediate 4-(4-aminostyryl)-N,N-dimethylaniline (AND), form self-assembled monolayers (SAMs) on
copper surfaces. The SAMs have been examined by a series of techniques, including contact angle (CA),
atomic force microscope (AFM), scanning electronic microscope (SEM), and electrochemical
measurements. The results suggest that the two derivatives adsorb onto the copper surface and
generate corresponding hydrophobic films, which play an important role in the anticorrosion of copper
in 3% NaCl solution. Quantum chemical calculations and molecular dynamics (MD) simulation are also
used for further insight into the adsorption mechanism.
Received 14th November 2014
Accepted 12th January 2015
DOI: 10.1039/c4ra14449j
www.rsc.org/advances
Quantum chemical calculations have proved to be a very
powerful tool for studying the inhibition mechanism.
1. Introduction
Adsorption-related electronic properties, adsorption geome-
tries and energies, charge transferred from or toward the
surface or a full description of the bonding electronic struc-
ture are useful and can be achieved by means of density
functional theory (DFT) methodology.13–15 Besides, molecular
dynamics simulation has also been used to analyze the
characteristics of the inhibitor/surface mechanism and to
describe the structural nature of the inhibitor in the corro-
sion process.16–18 Many results indicated that inhibitor
molecules can adsorb onto the copper surface through p-type
bonding of the aromatic ring as well as of the –C]N–
double bond. Therefore, we inferred that if there are more
aromatic rings, then the strength of adsorption will be
stronger. To examine this assumption, AND and DSM were
synthesized.
The anticorrosion performances of SAMs formed by AND
and DSM have been examined by complementary techniques,
such as electrochemical methods, scanning electron micro-
scope (SEM), atomic force microscopy (AFM), and contact angle
measurement (CAM). Additionally, the relationship between the
theoretical calculations and experimental results of the SAMs
are discussed.
Copper and its alloys are extensively investigated and used in
industry, owing to high their electrical and thermal conductiv-
ities as well as excellent workability into mechanics.1,2 In
particular, due to its high strength at low temperature, Cu is
used in condenser pipes of ships, coastal power plant heat
exchangers and so on. Although its resistance is quite good in
nearly neutral or slightly alkaline aqueous environments,
copper is still vulnerable to corrosion in harsh conditions. As
corrosion of Cu in seawater, with high concentration of chlorine
ions, is even more serious, copper corrosion in aqueous salt
conditions is a problem, which needs to be faced.
There are several methods of copper anticorrosion, e.g., metal
coating protective technologies (electroplating, chemical, hot
dipping),3 metal surface transformation (metal oxide, metal
phosphating, passivation),4 and non-metallic coating.5 However,
the disadvantages of these approaches, such as low inhibition
efficiency, inconvenient operation and environmental pollution,
are hard to avoid. The adjusting of surfaces for copper protection
with self-assembled monolayers (SAMs) has gained considerable
attention in recent years.6–8 Because of the dense and stable
structure of the SAM, it works as an insulated layer against the
penetration of corrosive substances. Up to now, several types of
organic compounds have been used for generating SAMs on
copper surfaces for anticorrosion in aggressive environments.9–11
Generally, compounds having oxygen, nitrogen, sulfur and
aromatic rings possess greater stability due to stronger chemical
interaction between these atoms and the copper surface.12
2. Experimental
2.1 Materials and sample preparation
AND and DSM were synthesized in our laboratory. Pure DSM is a
yellow solid and AND is deep yellow. The compounds' synthetic
routes and molecular structures are presented in Fig. 1. The
chemical structures of the organic compounds were character-
ized using nuclear magnetic resonance (NMR) spectroscopy.
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing
400044, China. E-mail: 15023289608@163.com; Fax: +86 023 65106756
† Both authors contributed equally to this work.
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Fig. 1 Synthetic route and molecular structure of AND and DSM.
Tetramethylsilane (TMS) was used as the internal standard with electrodes, a platinum electrode as the counter one, and a
a Bruker 500 MHz apparatus at room temperature. saturated calomel electrode (SCE) as the reference electrode.
1
AND. H-NMR (400 MHz, DMSO-d6) d (ppm): 13.206 (s, 1H, The reference electrode was connected to a Luggin capillary to
Ar-OH), 8.999 (s, 1H, –N]CH–), 7.629–7.640 (d, J ¼ 4.4 Hz, 1H, minimize the IR drop and chloride contamination. All experi-
Ar-H), 7.600–7.614 (d, J ¼ 5.6 Hz, 2H, Ar-H), 7.405–7.439 (m, 5H, ments were performed at 298 K, using a thermostatic water bath
Ar-H), 7.150–7.178 (d, J ¼ 11.2 Hz, 1H, Ar-H), 7.013 (s, 1H, Ar-H), to keep the temperature constant. The polarization curves were
6.941–6.986 (m, 2H, –CH]CH–), 6.707–7.722 (d, J ¼ 6.0 Hz, 2H, obtained from ꢀ250 to +250 mV (versus open circuit potential
Ar-H), 2.924 (s, 6H, –N(CH3)2), 13C-NMR (100 MHz, DMSO-d6) d (OCP)) with 0.1 mV sꢀ1 scan rate, and the data were collected
(ppm): 823.60, 964.41, 1178.51, 1357.89, 1523.76, 1610.56, and analyzed. Inhibition efficiency was calculated by
3369.94, 3462.22.
Ic0orr ꢀ Icorr
DSM. 1H-NMR (400 MHz, DMSO-d6) d (ppm): 7.315–7.330 (d,
J ¼ 6.0 Hz, 2H, Ar-H), 7.194–7.208 (d, J ¼ 5.6 Hz, 2H, Ar-H), 6.781
(s, 2H, –CH]CH–), 6.678–6.3993 (d, J ¼ 6.0 Hz, 2H, Ar-H),
6.533–6.647 (d, J ¼ 5.6 Hz, 2H, Ar-H), 2.894 (s, 6H, –N(CH3)2),
13C-NMR (100 MHz, DMSO-d6) d (ppm): 756.10, 828.53, 947.05,
964.41, 1193.94, 1219.01, 1354.03, 1446.61, 1521.84, 1604.77,
3014.74.
h% ¼
ꢁ 100%
(1)
Ic0orr
where I0corr and Icorr indicates the current density in 3% NaCl
solution with the bare copper electrode and with the different
modied-lms-electrodes, respectively.
EIS measurements were carried out at the OCP. The ac
frequency range extended from 100 kHz to 10 MHz with a 10 mV
peak-to-peak sine wave as the excitation signal. Then, the
impedance data were analyzed and tted. The inhibition effi-
ciency obtained from the EIS measurements was calculated by
2.2 Preparation of monolayers on copper
The Schiff base DSM and its intermediate AND were dissolved
in tetrahydrofuran in the desired concentrations. A 3% NaCl
solution was prepared with NaCl dissolved in double-distilled
water. Pure copper (Cu > 99.5%) was used. Emery papers of
grading 800, 1200, 1500, and 2000 were used one by one for
preparing the specimens' surfaces. Then, the specimens were
rinsed by ethanol and immersed into different tetrahydrofuran
solutions with different concentrations of DSM or AND for 6 h at
room temperature. When the lming process nished, the
copper electrodes were taken out of the solution and rinsed by
copious quantities of ethanol and double-distilled water.
Rct ꢀ R0ct
h% ¼
ꢁ 100%
(2)
Rct
where R0ct and Rct are the resistance of charge transfer in the
absence and presence of inhibitors, respectively.
2.4 Surface analysis
The morphologies of the copper surface with and without SAMs
were examined by SEM and AFM. The SEM and AFM images
were taken by a KYKY2800B SEM instrument at 25.0 kV and
Seiko SPIN 3800N using non-contact mode, respectively. The
2.3 Electrochemical tests
All the electrochemical experiments were carried out in a typical contact angles (CA) on the bare copper, AND- and DSM-
three-compartment glass cell, while CHI660B was used. Copper modied copper were measured by the sessile water drop
electrodes with and without modied lm were used as working method by a contact angle goniometer (Dataphysics OCA20,
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Germany). The average CA value was obtained by more than ve interaction was carried out in a simulation box (2.3 ꢁ 2.3 ꢁ 3.84
valid measurements on different spots of the same sample.
nm) with periodic boundary conditions. The cutoff distance was
1.25 nm. Six layers of copper atoms were used to ensure that the
depth of the surface was greater than the non-bond cutoff used
in the calculation. COMPASS forceeld was chosen to optimize
the structures of all components of the system. The molecular
dynamics simulation was carried out at 298 K with a NVT
ensemble with a time step of 1 fs and simulation time of
1000 ps.
2.5 Spectroscopic ellipsometry measurement
Spectroscopic ellipsometry is an optical method for surface
analysis, which is based on measuring the change of the
polarization state of a light beam during reection.19 The
complex-reectivity ratio is dened by r ¼ rr/rs ¼ tan j exp(jD),
where rr and rs are the complex-amplitude reection coeffi-
cients. The angles j and D are the conventional ellipsometric
parameters. The spectroscopic ellipsometry of the thickness of
lm was measured by a M22000U Automatic Ellipsometer
(Woollam Corporation, USA). To avoid the strong absorption of
light, we selected the wavelength range of incident light as 400–
800 nm with the incident angle of 70ꢂ. The optical data was
analyzed by the soware WVASE32.
3. Results and discussion
3.1 Molecular designs
Schiff bases possess photochemical properties, catalytic activi-
ties, structural and electronic properties, ligand effects, and
protective anticorrosion properties for metals.24,25 There are
many reports about various Schiff base inhibitors, which form
complexes with copper ions and generate highly protective lms
chemisorbed onto the copper surface.26–28 The use of nitrogen
containing organic compounds as a corrosion inhibitor for
copper has been widely investigated. Phenyl rings containing
conjugated bonds (p electrons) positively affect the interactions
between copper and the inhibitor compound.
We designed a new molecule modied with three aromatic
rings and the p-electrons of –C]N–, because molecules con-
taining nitrogen, sulfur, and aromatic rings are known as good
corrosion inhibitors for copper in 3% NaCl solution.12,29 The
SAM formed by DSM is quite stable, and the good adsorption of
its molecules to the metal surface may be attributed to inter-
action between p electrons of the aromatic rings and lone pair
electrons of the azomethine (–C]N–) group. Compared to its
intermediate (AND), the adsorption mechanism of DSM can be
better understood.
2.6 Computational details
All quantum calculations were performed by DFT with Becke's
three parameter exchange functional along with the Lee–Yang–
Parr nonlocal correlation functional (B3LYP)20 with the 6-
311G++(d,p) basis set as implemented in Gaussian 03.
For an N-electron system with total energy E, qualitative
chemical concepts electronegativity (c) and hardness (x) are
dened as the following rst-order and second-order
derivatives,21
ꢀ
ꢁ
vE
vN
c ¼ ꢀm ¼ ꢀ
(3)
nðrÞ
ꢀ
ꢁ
v2E
vN2
x ¼
(4)
nðrÞ
where n(r) and m are the external and electronic chemical
potentials, respectively. From the values of the total electronic
energy, the ionization potential (I) and electron affinity (A) of the
inhibitors are calculated using the following equations,
3.2 Polarization test
The corrosion behavior of copper in NaCl solution has received
considerable attention in literature.30,31 The cathodic chemical
process occurs as the following:
I ¼ E(Nꢀ1) ꢀ EN
A ¼ EN ꢀ E(N+1)
(5)
(6)
O2 + 4eꢀ + 2H2O / 4OHꢀ
(8)
In contrast, the anodic process undergoes a series of
complicated and continuous chemical reactions, as demon-
strated in Fig. 2: (i) oxidization of Cu(0) to Cu(I), (ii) immediate
formation of insoluble CuCl, due to the presence of Clꢀ, (iii)
formation of soluble cuprous chloride complex, because CuCl
cannot be xed due to its weak adhesion to the copper surface
and is further attacked by Clꢀ, forming the soluble complex
where E(Nꢀ1), E(N), and E(N+1) are the ground state energies of the
system with N ꢀ 1, N, and N + 1 electrons respectively. Hence, c
and x are calculated as follows: c ¼ (I + A)/2, x ¼ (I ꢀ A)/2.
The number of transferred electrons (DN) was calculated
using the following equation:22
cCu ꢀ cinh
FCu ꢀ cinh
2xinh
CuCl2 rapidly, (iv) formation of Cu2+, because the unstable
complex diffused from the copper surface rapidly undergoes an
ꢀ
DN ¼
¼
(7)
2ðxCu þ xinh
Þ
where the work function (F) is used for the electronegativity of oxidation reaction, and (v) corrosion of Cu, because once Cu2+
the copper surface, and the global hardness is neglected by appears, a disproportionation reaction between Cu(0) and Cu(II)
assuming that of a metallic bulk I ¼ A, because they are soer occurs: Cu2+ + Cu + 2Clꢀ / 2CuCl.
than the neutral metallic atoms.23
In Fig. 3, both anodic and cathodic reactions of the copper
The adsorption behavior was simulated by the Forcite electrode were inhibited aer modication, and this became
module from Accelrys Inc. The surface Cu (111) was chosen to more distinct as concentration increased. This indicates that
simulate the adsorption process. The simulation of the aer the modied AND or DSM reduce copper anodic
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Fig. 2 Corrosion process of copper in 3% NaCl solutions.
Fig. 4 Tafel extrapolation of the anodic polarization curve for copper
with 0.15 mmol Lꢀ1 AND-modified in 3% NaCl solution.
dissolution, they also retard the oxygen reduction. Moreover,
the shi in corrosion potential towards a more positive value
may suggest that the inhibitor molecule mainly functions by
inhibiting the anodic dissolution of copper. Moreover, it can be
seen from Fig. 3 that the copper electrode displays a Tafel
behavior of the anodic region. The Tafel line of the anodic
polarization curve is extended to the electrode potential below
the corrosion potential, and then a vertical line is made parallel
to the Y-axis at the corrosion potential. The electrochemical
parameters can be obtained from the point of intersection. In
addition, the Tafel slope can be obtained by the Tafel extrapo-
lation method in accordance with literature,32 as shown in
Fig. 4. The corresponding electrochemical kinetic parameters,
such as corrosion potential (Ecorr), anodic Tafel slope (ba), and
corrosion current density (Icorr) are listed in Table 1.
The effect on ba values may be due to many factors, such as
the composition of the copper electrode, concentration of the
medium, scan rate, and charge transfer coefficient. Table 1
shows that ba changes obviously with the modied of AND or
DSM, indicating that the adsorbed inhibitor molecules affect
the mechanism of copper dissolution. Due to the adsorption of
AND or DSM on the copper surface, the activation energy of
anodic dissolution at the electrode surface is increased. It is
observed that the inhibition efficiency increases with increasing
concentration. The inhibition efficiency reaches a maximum
value (AND: 84.69% at 0.15 mmol Lꢀ1, DSM: 94.50% at 0.15
mmol Lꢀ1).
Table 1 Polarization curve results obtained in 3% NaCl for the bare
copper and the modified copper electrodes after 6 h of assembly in
different concentrations of AND and DSM solutions
C
Icorr
Ecorr
(V)
ba
(mV decꢀ1
(mmol Lꢀ1
)
(mA cmꢀ2
)
)
SDa h (%)
0.01
0.05 77.58
0.03 78.88
0.05 83.62
0.02 84.69
0.05 88.50
0.03 89.78
0.06 93.59
0.03 94.50
Bare
AND 0.03
10.174
2.281
2.149
1.667
1.558
1.170
1.040
0.651
0.559
ꢀ0.268 500.0
ꢀ0.272 173.0
—
0.05
0.10
0.15
ꢀ0.268
ꢀ0.257
ꢀ0.256
ꢀ0.229
ꢀ0.226
ꢀ0.219
ꢀ0.211
83.3
62.5
55.6
59.0
41.7
40.0
37.8
DSM 0.03
0.05
0.10
0.15
a
SD: standard deviation.
3.3 Electrochemical impedance spectroscopy (EIS)
Fig. 5 shows the Nyquist plots for the bare copper, AND-
modied and DSM-modied copper electrodes aer 6 h of
assembly in different concentrations. From the inset in Fig. 4,
the Nyquist plots of the bare copper display a depressed semi-
circle at the high frequency and follow a straight line at low
frequency. The high frequency semicircle may be caused by the
Fig. 3 Potentiodynamic polarization curves obtained in 3% NaCl solution for the bare copper and modified copper electrodes after 6 h of
assembly in different concentrations of (a) AND and (b) DSM.
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Fig. 5 EIS obtained in 3% NaCl solution for the bare copper, AND-modified (a) and DSM-modified (b) copper electrodes after 6 h of assembly in
different concentrations.
charge transfer process, which is related to the relaxation time the working electrode and the reference electrode. Rct reects
constant of the charge-transfer resistance (Rct) and the double- the charge-transfer resistance. Rf is the resistance of the lm
layer capacitance (Cdl) at the copper/electrolyte interface.33 The formed on the copper surface. Constant phase element Q1 is
high frequency loop is not perfect semicircles, which can be composed of the membrane capacitance CPEf and the deviation
attributed to the frequency dispersion as a result of the rough- parameter n1, and Q2 is composed of the double-layer capaci-
ness and inhomogeneity of the electrode surface.34 The low tance CPEdl and the deviation parameter n2.
frequency tail generally is known as Warburg impedance, which
On the basis of the results in Table 2, the modication of the
is attributed to the anodic diffusion process of CuCl2 from the copper increases the Rct values. It seems to be enhanced with
surface of the electrode to the bulk solution and/or to the increasing thickness of SAM. The h values reach the maximum
cathodic diffusion process of dissolved oxygen from the bulk (AND: 89.20%, DSM: 92.49%) at the concentration of 0.15 mmol
solution to the surface of the electrode. For the AND- and DSM- Lꢀ1. The charge transfer process is greatly inhibited by SAM,
modied copper electrodes, the Warburg impedance disap- resulting in the prevention of copper from corrosion in Clꢀ
pears at low frequencies. Only some large capacitive loops are solution. Compared with the blank, the values of the corrosive
observed in the Nyquist plots. The disappearance of Warburg medium increase obviously with increasing AND or DSM
impedance indicates that the SAM is sufficiently densely packed concentration in tetrahydrofuran. This suggests that the
to prevent the diffusion process of the corrosion reaction and strength of the SAM has a positive correlation with the
the copper corrosion is controlled by the charge transfer concentration of AND or DSM in tetrahydrofuran.
process.
Table 2 shows that the CPEdl values decrease with the
In Fig. 5, several convex arcs exist and each diameter of the increasing of inhibitor concentrations, because water molecules
arcs increases with the increase in concentration. This indicates are gradually replaced by the adsorption of the inhibitor
that the impedance values have increased and the copper has molecules at the metal/solution interface, which leads to a
gotten more protection. When the concentration was 0.15 mmol protective lm adsorbing on the copper surface. CPEdl is
Lꢀ1, the capacitive reactance arc radius of both reached the calculated by equation,35
largest value.
303
Cdl
¼
S
(9)
The equivalent circuit model employed for this system is
shown in Fig. 6. Rs shows the resistance of the solution between
d
where, 30 is the permittivity constant of the air, 3 is the local
dielectric constant, d is the thickness of the lm, and S is the
surface of the electrode. There may be three reasons for the
decrease of CPEdl values: the increase in the adsorption lm
area (which decreases the electrode surface area), the decrease
in the local dielectric constant and/or the increase in the double
layer thickness. In a word, the diffusion of ions from the
interface to the solution may be delayed and the dissolution
reactions of copper may be inhibited to a great extent.
3.4 Contact angle measurements
The wettability of AND and DSM were examined by measuring
the contact angle. Fig. 7 shows the images of the sessile water
drop on the bare copper, the AND- and DSM-modied copper
surface, respectively. It was found that the bare copper has a
Fig. 6 Equivalent circuit models for the bare copper electrode (a), and
the AND- or DSM-modified copper electrode (b) in a 3% NaCl solution.
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Table 2 Impedance parameters of copper in 3% NaCl solution with different concentrations of AND and DSM
Qf
Qdl
C
Y0
Y0
(mmol Lꢀ1
)
Rs (U cm2)
(ꢁ10ꢀ5 S sn cmꢀ2
)
n1
Rf (U cm2)
(ꢁ10ꢀ4 S sn cmꢀ2
)
n2
Rct (U cm2)
W
h (%)
Bare
0.03
0.05
0.10
0.15
0.03
0.05
0.10
0.15
4.253
3.863
3.679
2.556
0.395
3.680
2.515
0.488
0.151
0.592
0.553
0.494
0.436
0.591
0.494
0.436
0.755
0.540
938
4498
5654
7980
8690
5650
8391
9967
11020
8.889
—
AND
DSM
1.616
1.507
1.644
1.154
1.507
1.444
3.544
3.878
8.949
7.571
7.368
2.209
7.571
7.968
14.05
3.634
1.000
1.000
1.000
1.000
1.000
1.000
0.548
0.989
32.69
33.01
34.80
35.77
33.01
34.80
37.98
39.09
79.14
83.41
88.25
89.20
83.40
88.82
90.59
92.49
Fig. 7 Sessile water drop images on: (a) a bare copper surface, (b) a copper surface covered with 0.15 mmol Lꢀ1 AND, and (c) a copper surface
covered with 0.15 mmol Lꢀ1 DSM.
hydrophilic surface for the contact angle, which is 72.5ꢂ DSM had better surface hydrophobicity than AND. Generally,
(Fig. 7a), whereas the angle on the AND-modied copper surface the value of contact angle strongly and mainly depends on the
is 93.2ꢂ (Fig. 7b) and for DSM-modied it is 102.0ꢂ (Fig. 7c). It is structure of the adsorption molecule and the properties of the
evident that the contact angles signicantly increase aer substrate surface. In the process of the adsorption of AND or
modication of the copper by AND or DSM, indicating forma- DSM, the polar head group of these two compounds x to the
tion of a hydrophobic lm. Moreover, the results showed that copper surface and all of the nonpolar group is in contact with
Fig. 8 SEM micrographs of freshly polished copper specimen (a), bare copper (b), 0.15 mmol Lꢀ1 AND-modified copper (c) and 0.15 mmol Lꢀ1
DSM-modified copper (d) immersed in 3% NaCl solutions for 3 days.
Fig. 9 EDS images of bare copper and (a) AND-modified (b) and DSM-modified (c) copper.
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Fig. 10 2D/3D AFM images of freshly polished copper and (a and d) AND-modified (b and e) and DSM-modified (c and f) copper immersed in 3%
NaCl solutions for 3 h.
the solution, resulting in a homogeneous and hydrophobic shown in Fig. 9b and c, the composition of the surface changed
interface, which gives rise to a larger contact angle.
aer the specimen was immersed in AND or DSM solution. In
the images, it can be concluded that the surface is covered by
some kind of lm, which consists of carbon. However, nitrogen
atoms cannot be observed due to the high carbon–nitrogen
mass ratio (AND: 6.86, DSM: 9.86) and imprecise instrument.
Even so, it is fair enough to suggest the formation of the SAMs
on the copper surface. It is also shown that the proportion of
copper surface is smaller than that of AND, meaning the
3.5 SEM and EDS analysis
Fig. 8 shows what happens on bare copper and modied copper
electrodes aer being immersed in 3% NaCl solutions for 3
days. Before corrosion, the surface morphology of the sample
was a freshly polished copper surface, as seen in Fig. 8a. Aer
corrosion in 3% NaCl solution for 3 days, the surface
morphology becomes porous and rough, as shown in Fig. 8b,
which means copper was severely corroded. Compared with the
bare copper, the AND-modied copper surface was smooth, but
for a few moderate notches. Moreover, the DSM-modied
copper was smooth with a few small notches. This fact is
indicative of a good inhibition performance for the AND and
DSM against copper corrosion in 3% NaCl solution.
Fig. 9a shows the EDS test results of bare copper. The bare
Fig. 12 Cauchy model used for spectroscopic ellipsometry interpre-
copper mainly consists of copper and some nickel. However, as tation corresponding to AND or DSM on Cu substrate.
Fig. 11 Fitting results of thickness of AND (a) and DSM (b) modified film.
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lms were obtained through the Cauchy model36 (see Fig. 12) by
tting experimental reectance spectra. The results are presented
in Table 3. The thickness of the lms formed in AND and DSM
solution are 49.51 nm and 101.56 nm, respectively. The larger the
value, the better is the anticorrosion ability. Therefore, DSM has
better performance, same as in the other tests.
Table 3 Fit results of j and D by Cauchy model
Thickness (nm)
An
Bn
Cn
AND
DSM
49.5
101.6
1.6043
1.4398
0.00788
0.08190
ꢀ0.0027
ꢀ0.0084
coverage of DSM on the copper surface is higher. This conclu-
sion agrees with the results of electrochemical and SEM tests.
3.8 Theoretical calculations
The full optimized minimum energy geometrical congurations
of AND and DSM are shown in Fig. 13. The computed molecular
properties, including EHOMO, ELUMO, DELUMO–HOMO, dipole
moment (m), surface area (SA), molecular volume (V), projected
area (PA), ionization potential (I), electron affinity (A) and frac-
tion of transferred electrons (DN) are listed in the Table 4. As
seen from the table, the positive DN value indicates that elec-
tron transfer from the inhibitor molecule to copper surface is
available.
In frontier molecular orbital theory, the inhibition efficiency of
the inhibitor is closely related to the HOMO and LUMO.15 It is
known that EHOMO is oen associated with the electron donating
ability of the inhibitor molecule and the higher the value of
EHOMO, the greater is the ease of donating electrons to the unoc-
cupied d orbital of the metal, whereas, ELUMO indicates the ability
of the molecule to accept electrons. Therefore, the lower the value
of ELUMO, the more probable it is that the molecule would accept
electrons. Thus, higher EHOMO and lower ELUMO values generally
enhance inhibition efficiency. Moreover, the smaller the value of
the LUMO–HOMO energy gap (DE) for an inhibitor, the higher is
the inhibition efficiency of that inhibitor.
3.6 AFM analysis
The surface topographies of freshly polished copper and
modied copper electrodes aer 3 h of immersion in 3% NaCl
solution are shown in Fig. 10. It can be seen that aer modi-
cation with AND or DSM, signicant differences are found on
the surface morphology of the corroded specimens. The AFM
image of the freshly polished sample surface looks mostly
uniform with only some tiny scratches. When the copper is
modied with AND or DSM, the sample surface becomes rela-
tively at aer NaCl treatment in general, and the DSM-
modied copper surface looks smoother than the AND-
modied copper surface. The mean roughness of the copper
surface before NaCl treatment and aer AND and DSA protec-
tion and NaCl treatment are 9.293 nm, 37.60 nm and 12.26 nm,
respectively. This suggests that AND and DSM in 3% NaCl
solution form a protective lm on the copper surface, which
prevents the surface from corroding.
3.7 Ellipsometer measurements
The ellipsometer, a typical tool to test the thickness of the lm,
was used. Good values were obtained by the Cauchy model. The
points in Fig. 11 show the results of j and D of AND- and DSM-
modied copper. Because of the good permeability of the lms
between the visible spectrum and infrared, the thicknesses of the
It can be seen from Fig. 14 that the MOs of the studied
inhibitors are very similar. The MOs, which cover most of the
molecular structure, are made up of the p-type bonding in the
benzene ring p-bonding and along the –C]N– double bond (for
DSM). This also can be proved by the contribution of some
groups to molecular orbitals, which are shown in Table 5. The
EHOMO of two studied inhibitors are ꢀ4.70 eV and ꢀ4.98 eV,
respectively. The trend of EHOMO is different from the one for
efficiency, but the difference of the two inhibitors is not
signicant. In addition, the ELUMO are ꢀ1.09 eV, ꢀ1.99 eV,
respectively, so the values of DE are huge, 3.60 eV and 2.99 eV,
which follows the trend for efficiency.
The total electron density, as well as the electrostatic
potential (ESP), of AND and DSM are shown in Fig. 15. From an
overall perspective, both inhibitors are strong electron donors,
for which evidence can be found in the total electron density
(Fig. 15a). This has also been conrmed by the electrostatic
potential map, in which the negatively charged red regions are
located at the upper/lower surface of the molecular skeleton.
Fig. 13 Optimized structures of AND and DSM.
Table 4 Quantum chemical descriptors for the studied inhibitor calculated at B3LYP/6-311G** level
2
3
2
˚
˚
˚
Inhibitor
m (Debye)
SA (A )
V (A )
PA (A )
I
A
c
x
DN
AND
DSM
1.15
3.28
309.5
480.11
334.42
450.2
104.52
131.64
4.7
4.98
1.09
1.99
2.89
3.49
1.8
1.5
0.53
0.44
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Fig. 16 Representative snapshots of AND and DSM on Cu (111)
surface. Inset images show the top-down views.
could be noticed that the two inhibitors adsorbed nearly
parallel to the copper surface through donation of p electrons of
the benzene rings and the lone pairs of the hetero-atoms to the
metal. The average centroid distance (d) between the inhibitors
and the Cu (111) surface is also shown in Fig. 16. The distance of
DSM is shorter; this is due to the –C]N– double bond and
additional benzene ring.
Fig. 14 Frontier molecular orbits of AND and DSM as the HOMO and
LUMO.
Quantitative appraisal of the interaction was achieved by
calculating the adsorption energy (Eads) using eqn (10):37
Table 5 Contributions of some groups to the HOMO and LUMO
orbitals
Eads ¼ Ecomplex ꢀ (ECu + Einh
)
(10)
AND
DSM
LUMO
HOMO
LUMO
HOMO
where Ecomplex is the total energy of the surface and inhibitor
and ECu and Einh are the total energy of the copper crystal and of
the free inhibitor molecule, respectively. The calculated Eads
values of the adsorption systems were ꢀ332.0 and ꢀ438.7 kJ
molꢀ1 for AND and DSM, respectively. The larger negative
values of interaction energy can be attributed to the strong
adsorption between the inhibitor molecules and the copper
surface. It is obvious that DSM gives a more negative adsorption
Benzene-A
Benzene-B
Benzene-C
–C]C–
28.78
29.91
31.82
24.56
5.78
19.67
31.19
8.95
35.41
17.30
2.34
19.69
4.51
34.17
21.06
–C]N–
32.29
These electron-rich areas would be preferred sites for adsorp- energy, which means that adsorption of DSM on the Cu (111)
tion to metal surfaces.
surface is stronger than that of AND. This accords with their
Finally, molecular dynamics simulations were performed to performance in experiments.
study the adsorption behaviour of specic inhibitor molecules
on the Cu (111) surface. The close contact between the inhibitor
molecules and copper surface, as well as the best adsorption
4. Conclusions
The self-assembled monolayer formed by AND and DSM can
congurations for the compounds, are depicted in Fig. 16. It
protect copper from corrosion in 3% NaCl solution. The inhi-
bition efficiency obtained from the polarization test is coinci-
dent with ones obtained from EIS, and both of the inhibition
efficiencies increase with the concentration of impregnation
liquid. SAMs formed by AND or DSM on the copper surface were
characterized by surface analyses. The contact angle, thickness
of layers, and corrosion morphology obtained from SEM and
AFM indicate that the corrosion inhibition efficiency of DSM is
better than that of AND. At the same time, theoretical param-
eters obtained from DFT calculation and MD simulation, such
as energy of the gap, dipole moment, adsorption energy, display
the same trend. The results of the theoretical calculation are in
good agreement with those of the experiments.
Acknowledgements
Fig. 15 Electronic properties of the inhibitor molecules: (a) total
electron density, (b) isosurface and (c) contour representation of
electrostatic potential, respectively. Negative (positive) regions are
colored red (blue).
This research was supported by Natural Science Foundation of
China (no. 21376282), and Chongqing Innovation Fund for
Graduate Students (no. CYB14019).
14812 | RSC Adv., 2015, 5, 14804–14813
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19 G. E. Jellison and F. A. Modine, Appl. Phys. Lett., 1996, 69,
371–373.
References
20 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
21 S. B. Liu, Acta Phys. Chim. Sin., 2009, 25, 590–600.
22 R. G. Parr and R. G. Pearson, J. Am. Chem. Soc., 1983, 105,
7512–7516.
23 A. Kokalj, Electrochim. Acta, 2010, 56, 745–755.
24 R. Noyori and S. Hashiguchi, Acc. Chem. Res., 1997, 30, 97–
102.
25 S. V. Eliseeva and J. C. G. Bunzli, Chem. Soc. Rev., 2010, 39,
189–227.
26 H. Ma, S. Chen, L. Niu, S. Zhao, S. Li and D. Li, J. Appl.
Electrochem., 2002, 32, 65–72.
1 R. L. Powell, H. M. Roder and W. J. Hall, Phys. Rev., 1959, 115,
314–323.
2 A. Bazzaoui, J. I. Martins, E. A. Bazzaoui, L. Martins and
E. Machnikova, Electrochim. Acta, 2007, 52, 3568–3581.
3 R. Subasri and T. Shinohara, Electrochemistry, 2004, 72, 880–
884.
4 W. H. Leng, D. P. Liu, X. F. Cheng, W. C. Zhu, J. Q. Zhang and
C. A. Cao, Acta Metall. Sin., 2007, 43, 764–768.
5 A. V. Rao, S. S. Latthe, S. A. Mahadik and C. Kappenstein,
Appl. Surf. Sci., 2011, 257, 5772–5776.
6 A. T. Lusk and G. K. Jennings, Langmuir, 2001, 17, 7830–7836.
7 C. X. Li, L. Li and C. Wang, Electrochim. Acta, 2014, 115, 531–
536.
27 C. R. Bhattacharjee, G. Das and P. Mondal, Liq. Cryst., 2011,
38, 441–449.
28 Z. L. Quan, S. H. Chen, L. Li and X. G. Cui, Corros. Sci., 2002,
44, 703–715.
8 F. Caprioli, A. Martinelli, D. Gazzoli, V. Di Castro and
F. Decker, J. Phys. Chem. C, 2012, 116, 4628–4636.
9 S. M. Song, C. E. Park, H. K. Yun, C. S. Hwang, S. Y. Oh and
J. M. Park, J. Adhes. Sci. Technol., 1998, 12, 541–561.
10 W. Chen, S. Hong, H. Q. Luo and N. B. Li, J. Mater. Eng.
Perform., 2014, 23, 527–537.
11 F. Caprioli, A. Martinelli, V. Di Castro and F. Decker, J.
Electroanal. Chem., 2013, 693, 86–94.
12 M. M. Antonijevic and M. B. Petrovic, Int. J. Electrochem. Sci.,
2008, 3, 1–28.
29 B. E. A. Rani and B. B. J. Basu, Int. J. Corros., 2012, 2012, 1–15.
30 K. F. Khaled, Mater. Chem. Phys., 2008, 112, 104–111.
31 A. Dafali, B. Hammouti, R. Touzani, S. Kertit, A. Ramdani
and K. El Kacemi, Anti-Corros. Methods Mater., 2002, 49,
96–104.
32 M. A. Amin, S. S. A. El Rehim and H. T. M. Abdel-Fatah,
Corros. Sci., 2009, 51, 882–894.
33 J. M. Steigerwald, S. P. Murarka, R. J. Gutmann and
D. J. Duquette, Mater. Chem. Phys., 1995, 41, 217–228.
34 Q. Qu, L. Li, S. Jiang, W. Bai and Z. T. Ding, J. Appl.
Electrochem., 2009, 39, 569–576.
13 G. Gece, Corros. Sci., 2008, 50, 2981–2992.
14 L. Guo, W. P. Dong and S. T. Zhang, RSC Adv., 2014, 4,
41956–41967.
35 S. John and A. Joseph, RSC Adv., 2012, 2, 9944–9951.
36 M. Rebien, W. Henrion, M. Hong, J. P. Mannaerts and
M. Fleischer, Appl. Phys. Lett., 2002, 81, 250–252.
37 E. E. Oguzie, Y. Li, S. G. Wang and F. Wang, RSC Adv., 2011,
1, 866–873.
15 I. Lukovits, E. Kalman and F. Zucchi, Corrosion, 2001, 57, 3–
8.
16 K. F. Khaled, J. Solid State Electrochem., 2009, 13, 1743–1756.
17 A. L. Guo, G. C. Duan, K. He, B. Sun, C. C. Fan and S. Q. Hu,
Comput. Theor. Chem., 2013, 1015, 21–26.
18 J. Zhang, G. Qiao, S. Hu, Y. Yan, Z. Ren and L. Yu, Corros. Sci.,
2011, 53, 147–152.
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