Electrochimica Acta 49 (2004) 485–495
Ortho-substituted anilines to inhibit copper corrosion
in aerated 0.5 M hydrochloric acid
K.F. Khaled a,b,∗ , N. Hackerman b
b
a Chemistry Department, Ain Shams University, Roxy, Cairo, Egypt
Chemistry Department, Rice University, 6100 Main street, Houston, TX 77005, USA
Received 14 July 2003; received in revised form 14 July 2003; accepted 6 September 2003
Abstract
Aniline derivatives, namely 2-chloroaniline, 2-fluoroaniline, 2-aminophenetole, 2-ethylaniline, o-aminoanisole and o-toluidine were studied
for their possible use as copper corrosion inhibitors in 0.5 M HCl. These compounds were studied in concentrations from 10−3 to 10−4 M
at temperature 298 K. Effectiveness of these compounds was assessed through potentiodynamic polarization and electrochemical impedance
spectroscopy (EIS) measurements. These compounds inhibit the corrosion of copper in HCl solution to some extent. In each case, inhibition
efficiencies increase with increasing concentration. A suggested model for the interface as well as some kinetic data is presented. These
inhibitors obey the Temkin adsorption isotherm. A correlation between structure and inhibition efficiencies is suggested.
© 2003 Elsevier Ltd. All rights reserved.
Keywords: Copper; Potentiodynamic polarization; EIS; Acid corrosion inhibition
1. Introduction
Due to its excellent thermal conductivity and good mechanical workability, copper is a material commonly used
in heating and cooling systems. Scale and corrosion products have a negative effect on heat transfer, and they cause
a decrease in heating efficiency of the equipment, which is
why periodic descaling and cleaning in hydrochloric acid
pickling solutions are necessary.
Most corrosion inhibitors can eliminate the undesirable
destructive effect and prevent metal dissolution. Copper normally does not displace hydrogen from acid solutions and,
therefore, is virtually unattacked in non-oxidizing conditions. In fact, if hydrogen is bubbled through a solution of
copper salts, it reduces as fast as the process occurs [1]. Yet,
most solutions to be handled contain dissolved air, which
causes cathodic depolarization and enables some corrosion
to take place.
Copper dissolution in acidic solutions has been studied by
several researchers [2–7]. Corrosion inhibitors can be used
to prevent copper dissolution, benzotriazole, for instances,
was studied and found to have excellent inhibition proper∗
Corresponding author. Fax: +202-25-81-243.
E-mail address: [email protected] (K.F. Khaled).
0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2003.09.005
ties in several corrosive environments [8–11]. This molecule
contains nitrogen atoms and it is also useful in preventing
copper staining and tarnishing [12–17].
In the present study, efficiency of a series of ortho-aniline
derivatives for inhibition of copper corrosion in 0.5 M
hydrochloric acid was investigated. The series consisted
of 2-chloroaniline, 2-fluoroaniline, 2-aminophenetole,
2-ethylaniline, o-aminoanisole and o-toluidine.
Corrosion inhibition was investigated using electrochemical impedance spectroscopy (EIS) measurements and potentiodynamic polarization (dc) measurements. The inhibition
efficiency obtained from (EIS) measurements was compared
with those obtained from potentiodynamic polarization (dc).
Impedance spectra obtained from this study were analyzed
to show the equivalent circuit that fits the corrosion data.
The adsorption behavior of this series is examined. Correlation between the inhibition efficiency and the structure of
these compounds is suggested.
2. Experimental
Chemical structures of the studied compounds are presented in Fig. 1. Copper specimens from Johnson Mattey
(Puratronic, 99.999%) were mounted in Teflon. An epoxy
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K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
Fig. 1. 2D structure of the o-substituted anilines.
resin was used to fill the space between Teflon and copper electrode. The electrochemical measurements were performed in a typical three-compartment glass cell consisted
of the copper specimen as working electrode (WE), platinum
counter electrode (CE), and a saturated calomel electrode
(SCE) as the reference electrode. The counter electrode was
separated from the working electrode compartment by fritted glass. The reference electrode was connected to a Luggin capillary to minimize IR drop. Solutions were prepared
from bidistilled water of the resistivity 13 M cm, The copper electrode was polished with different grit emery papers
up to 4/0 grade, cleaned with acetone, washed with bidistilled water and finally dried.
The electrode potential was allowed to stabilize 30 min
before starting the measurements. All experiments were
conducted at 298 K. The electrolyte solution was made from
HCl (Fisher Scientific) and bidistilled water. The inhibitors
(Aldrich Chemical Co.) used without any pretreatment.
Electrochemical measurements were recorded with EG&G
Princeton Applied Research Potentiostat/Galvanostat (PAR
model 273) in combination with a Solarton 1250 frequency
response analyzer.
Fig. 2. Anodic and cathodic Tafel lines for copper in 0.5 M HCl without and with 10−3 M of o-substituted anilines.
K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
487
Fig. 3. Anodic and cathodic Tafel lines for copper in 0.5 M HCl without and with 5 × 10−3 M of o-substituted anilines.
The potentiodynamic current-potential curves were obtained by changing the electrode potential automatically
from (−250 to +250 mVSCE ) versus open circuit potential with scan rate of 0.166 mV/s. EIS measurements were
carried out in frequency range from 100 kHz to 30 mHz
with an amplitude of 5 mV peak-to-peak using ac signals at
open circuit potential. EIS spectra were analyzed by Zview
impedance analysis software (Scribner Associates, Inc.,
Southern Pines, NC).
3. Results and discussion
Polarization curves for copper electrode in aerated 0.5 M
HCl in the absence and the presence of various concentrations of o-substituted anilines are shown in Figs. 2–4 (representative examples). Values of associated electrochemical
parameters and inhibition efficiencies (I %) of the studied
compounds are given in Table 1. The inhibition efficiencies
are defined as:
Fig. 4. Anodic and cathodic Tafel lines for copper in 0.5 M HCl without and with 9 × 10−3 M of o-substituted anilines.
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K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
Table 1
Electrochemical parameters of copper in 0.5 M HCl without and with different concentrations of o-substituted anilines
Conc. (M)
Blank
icorr (␮A cm−2 )
−Ecorr (mV)
−bc (mV dec−1 )
ba (mV dec−1 )
CR (mpy)
11.17
I (%)
253.9
209.0
51.47
5.12
2-Ethylaniline
10−3
5 × 10−3
9 × 10−3
10−2
1.796
1.659
1.635
1.522
254.8
256.1
259.8
266.1
227.7
228.7
207.1
203.9
51.64
52.34
52.62
55.1
0.82
0.76
0.75
0.69
83.92
85.147
85.36
86.37
2-Chloroaniline
10−3
5 × 10−3
9 × 10−3
10−2
1.856
1.766
1.674
1.593
255.1
258.0
269.7
269.9
251.7
221.3
175.0
191.2
52.58
54.04
55.7
55.83
0.85
0.81
0.76
0.73
83.33
84.8
85.01
85.73
2-Fluoroaniline
10−3
5 × 10−3
9 × 10−3
10−2
2.234
2.09
1.71
1.7
257.1
269.2
272.8
284.1
303.1
190.3
203
174.8
52.89
53.54
54.13
55.97
1.02
0.95
0.78
0.77
80.0
81.28
84.69
84.78
o-Toluidine
10−3
5 × 10−3
9 × 10−3
10−2
2.324
2.142
2.055
1.878
259.1
261.6
261.2
273.9
264.8
200.0
204.1
296.7
53.84
54.12
54.7
54.9
1.06
0.98
0.94
0.86
79.19
80.08
81.6
83.18
o-Aminoanisole
10−3
5 × 10−3
9 × 10−3
10−2
2.455
1.998
2.014
2.034
254.4
254.8
255.3
259.2
224.6
269.9
245.3
281.0
51.45
51.36
54.43
52.84
1.12
0.92
0.92
0.93
78.02
82.11
82.09
81.82
2-Aminophenetole
10−3
5 × 10−3
9 × 10−3
10−2
2.608
2.353
2.146
2.134
259.1
255.8
260.0
269.8
257.0
280.0
205.4
270.2
53.4
54.28
55.56
54.80
1.19
1.07
0.98
0.97
76.65
78.93
80.7
80.89
I(%) =
i0corr − icorr
× 100
i0corr
(1)
where i0corr and icorr are the uninhibited and inhibited
corrosion current densities, respectively, determined by
–
extrapolation of Tafel lines to corrosion potential. These
results reveal that the best inhibition in the short time test
was at the highest concentration (10−2 M). By increasing
inhibitor concentration, inhibition efficiency increases and
the corrosion rate decreases. The corrosion potential Ecorr
Fig. 5. Complex-plane impedance for copper in 0.5 M HCl without and with 10−3 M of o-substituted aniline.
K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
489
Fig. 6. Complex-plane impedance for copper in 0.5 M HCl without and with 5 × 10−3 M of o-substituted anilines.
shifted towards more cathodic values in the presence of
o-substituted anilines. The shift of Ecorr with increasing
concentration of the inhibitor could be due to the fact that
the inhibitor had a strong influence on the oxygen reduction
than the copper dissolution [18]. The corrosion potential
Ecorr , anodic Tafel slopes ba and cathodic Tafel slopes bc obtained in uninhibited 0.5 M HCl solution agreed with those
reported in the literature [19]. At concentrations <1 M HCl,
the accepted copper dissolution mechanism [18] follows
two steps:
fast :
slow :
−
Cu(S) → Cu+
(ads) + e
2+
−
Cu+
(ads) → Cu(sol) + e
The slope variation in the inhibited solution especially at
high inhibitor concentrations indicated a change in the copper dissolution. Unlike two electrons’ electro-dissolution
mechanism, copper in the presence of o-substituted anilines
Fig. 7. Complex-plane impedance for copper in 0.5 M HCl without and with 9 × 10−3 M of o-substituted anilines.
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K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
Fig. 8. Complex-plane impedance for copper in 0.5 M HCl without and with 10−2 M of o-substituted anilines.
could as reported for other inhibitors [20] electro-oxidize
primarily to Cu+ and is able to form slightly soluble
[Cu(o-anilines)n ]+
ads complex as the main electro-deposition
products in the presence of a “clean” surface (i.e. in
the case of (Cu/Cu2 O-system) [21–23]. For example, the
cuprous organometallic complex with the participation of
indole, a molecule similar to the anilines, was proposed by
Balogh–Hergovic and Speier to explain the dehydrogenation
Table 2
Impedance data of copper in 0.5 M HCl without and with different concentration of o-substituted anilines
Conc. (M)
Rct (Ohm)
10.22 ×
Blank
10−3
5 × 10−3
9 × 10−3
10−2
27.56
31.35
33.94
36.4
2-Chloroaniline
10−3
5 × 10−3
9 × 10−3
10−2
2-Fluoroaniline
103
1/Rct (Ohm−1 )
9.78 ×
Cdl (␮F)
10−5
9.83
10−5
I (%)
Coverage (θ)
–
103
×
× 103
× 103
× 103
3.62
3.18
2.94
2.74
×
× 10−5
× 10−5
× 10−5
3.64
3.20
2.96
2.76
62.9
67.40
69.88
71.92
0.62
0.67
0.69
0.72
24.72
27.39
31.07
34.4
×
×
×
×
103
103
103
103
4.04
3.65
3.21
2.91
×
×
×
×
10−5
10−5
10−5
10−5
4.06
3.66
3.23
2.92
58.65
62.68
67.1
70.29
0.58
0.63
0.671
0.71
10−3
5 × 10−3
9 × 10−3
10−2
23.98
26.98
30.75
32.85
×
×
×
×
103
103
103
103
4.17
3.71
3.2
3.04
×
×
×
×
10−5
10−5
10−5
10−5
4.18
3.72
3.26
3.05
57.38
62.12
66.76
68.88
0.57
0.62
0.66
0.68
o-Toluidine
10−3
5 × 10−3
9 × 10−3
10−2
22.61
26.38
30.01
31.82
×
×
×
×
103
103
103
103
4.42
3.79
3.33
3.14
×
×
×
×
10−5
10−5
10−5
10−5
4.44
3.81
3.34
3.15
54.79
61.25
65.94
67.88
0.54
0.61
0.65
0.67
o-Aminoanisole
10−3
5 × 10−3
9 × 10−3
10−2
20.7
24.63
29.02
30.88
×
×
×
×
103
103
103
103
4.83
4.1
3.44
3.23
×
×
×
×
10−5
10−5
10−5
10−5
4.85
4.07
3.46
3.25
50.62
58.50
64.78
66.91
0.51
0.58
0.64
0.66
2-Aminophenetole
10−3
5 × 10−3
9 × 10−3
10−2
19.30
22.77
26.1
27.26
×
×
×
×
103
103
103
103
5.183
4.39
3.83
3.66
×
×
×
×
10−5
10−5
10−5
10−5
5.2
4.41
3.85
3.68
47.04
55.11
60.84
62.51
0.47
0.55
0.61
0.62
2-Ethylaniline
K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
491
case of tryptophan as inhibitor for copper in aerated sulfuric
acid [18].
A typical set of Nyquist plots for copper in 0.5 M HCl
solutions in the presence of various concentrations of
o-anilines are shown at Figs. 5–8. Curves obtained from
impedance data response of copper changes significantly
on addition of inhibitors. Generally, the impedance of the
inhibited substrate increased with increasing concentration
of inhibitor.
The impedance spectra for copper were recorded at 2 h
immersion time. The shape of the Nyquist plot for copper
in the absence and the presence of the studied compounds
are incomplete semi-circle in the frequency range. Viewing the impedance results in the format of the Bode plots,
CPE
R1
R2
Fig. 9. Equivalent circuit model for the studied inhibitors.
of indoline derivatives to indoles catalyzed by Cu complexes
[22]. The same behavior was hypothesized in the case of
indole and 5-chloroindole mild steel inhibitors, through
the formation of [Cu(indole)n ]+ or [Cu(5-chloroindole)n ]+
complexes [23]. This assumption was suggested also in
-10000
FitResult
Z''
-7500
-5000
-2500
0
0
2500
5000
7500
10000
Z'
(a)
10 4
-100
10 3
-75
10 2
-50
10 1
-25
10 0
10-1
(b)
(b)
100
10 1
10 2
10 3
10 4
Phase angle
|Z |
FitResult
0
10 5
Frequency (Hz)
Fig. 10. (a) Nyquist plots of copper corrosion in 0.5 M HCl solution at Ecorr . (b) Bode-phase plots of copper corrosion in 0.5 M HCl solution at Ecorr .
492
K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
-40000
FitResult
-30000
Z''
-20000
-10000
0
0
100
00
20000
30000
40000
Z'
(a)
5
10
-100
104
-75
103
-50
102
-25
101
10 -2
(b)
10 -1
100
101
102
103
Phase angle
|Z|
FitResult
0
104
Frequency (Hz)
Fig. 11. (a) Nyquist plots of copper corrosion in 0.5 M HCl solution with 10−2 M of 2-ethylaniline at Ecorr . (b) Bode-phase plots of copper corrosion in
0.5 M HCl solution with 10−2 M of 2-ethylaniline at Ecorr .
Figs. 10(b) and 11(b) showed that there is one phase angle
maximum. Addition of various concentrations from the studied compounds increases the diameter of the semi-circles.
The impedance loops measured are often depressed semicircles with their center below the real axis. This kind of
phenomenon is known as the “dispersing effect” [24,25].
The charge transfer resistance Rct , double layer capacitance Cdl are calculated from Eq. (2):
)=
f(−Zmax
1
2πCdl Rct
(2)
) is the maximum imaginary component of
where (−Zmax
the impedance, and inhibition efficiency is calculated from
Rct data by Eq. (3)
I% =
1/R0ct − 1/Rct
1/R0ct
× 100
(3)
here R0ct and Rct are the charge transfer resistance values
without and with inhibitor, respectively. Data from Table 2
reveals that by increasing the concentration of o-substituted
anilines the corrosion rate (1/Rct ) decreases.
K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
493
Fig. 12. Temkin adsorption plots of copper in 0.5 M HCl containing 10−2 M of o-substituted anilines.
Results from EIS and potentiodynamic polarization measurements are in good agreement.
Analysis of impedance data by EQUIVCRT.PAS &
Zview impedance analysis computer programs (Scribner
Associates, Inc., Southern Pines, NC) yielded different
parameters, such as Ohmic resistance (R) and constant
phase element (CPE) represented as Q. These elements
are necessary for the description of the equivalent circuit of the electrode process. The impedance of CPE is
given by:
ZCPE = [Q(iω)n ]−1
(4)
where ω is the angular frequency. Depending on the value
of the exponent n, Q may be a resistance, R (n = 0); a
capacitance, C (n = 1); Warburg impedance, W (n = 0.5)
or an inductance, L (n = −1) [26]. The equivalent circuit
is suggested as in Fig. 9, where R1 represents the solution resistance, R2 the charge transfer resistance, and CPE,
Fig. 13. Correlation of log P with inhibition efficiency of o-substituted anilines.
494
K.F. Khaled, N. Hackerman / Electrochimica Acta 49 (2004) 485–495
the constant phase element. Considering the impedance of
a double layer does not behave as an ideal capacitor in the
presence of dispersing effect, a constant phase element CPE
is used as a substitute for capacitor in Fig. 9 to fit more accurately the impedance behavior of the electric double-layer.
CPE has widely been used to account for deviations
brought about by surface roughness. The value of n was
found to increase from 0.79 to 0.92. Values of charge transfer resistance Rct are in good agreement with R2 values.
Figs. 10 and 11 shows some impedance diagrams fitted
for copper in 0.5 M HCl in the presence and the absence
of additives (representative examples) in both Nyquist and
Bode format. Both simulated and measured data are fitted
very well. The high frequency part in Nyquist and Bode
plots describes the behavior of an inhomogeneous surface
layer of the electrode surface, while the low frequency contribution shows the kinetic response for the charge transfer
reaction [27].
The values of θ have been inserted into Table 2.
Basic information on the interaction between the
inhibitors and the iron surface can be provided by the adsorption isotherm. The degree of surface coverage (θ) of
different concentrations of inhibitor in acidic media have
been evaluated from electrochemical impedance measurements according to [28,29], Eq. (5):
θ=
Cdl(θ=0) − Cdlθ
Cdl(θ=0) − Cdl(θ=1)
(5)
where Cdl(θ=0) and Cdl(θ=1) are the double layer capacitances (per unit area) of the inhibitor-free and entirely
inhibitor-covered surfaces, respectively, Cdlθ is the composite total double layer capacitance for any intermediate
coverage θ. Attempts were made to fit these θ values to
various isotherms including Frumkin, Langmuir, Temkin,
etc. The simple Langmuir adsorption model could not
fully explain the inhibition of o-anilines in the present
case.
The surface coverage θ is tested graphically for fitting a
suitable adsorption isotherm as indicated in Fig. 12. The plot
of θ versus ln Cinh yields a straight line with correlation coefficient equals more than 0.92, showing that the adsorption of
these inhibitors can obeys Temkin adsorption isotherm. For
a reliable and linear plot, surface coverage should fall within
the linear window (monolayer) for inhibitor adsorption, i.e.
θ =0.2–0.8 [30]. The strong correlation for the Temkin adsorption isotherm plot for o-substituted anilines confirms the
validity of this approach.
Molecular orbital theoretical calculations based on the
semi-empirical self-consistent field method (SCF). Molecular modeling of these corrosion inhibitors was carried out
using Hyperchem version 7, a quantum mechanical program
marketed by Hypercube, Inc. A full geometry optimization
was performed using PM3 semi-empirical molecular orbital
method. The correlations generated were, in general, poor
and of little use for optimizing inhibitor structure, however,
the values of log P (P is the partition coefficient) [31,32]
shows a degree of correlation with the inhibitor performance
Fig. 13.
o-Substituted anilines present similar inhibition efficiency. Ethyl aniline gives more protection due to the
repelling power of ethyl group than other ortho groups.
Chloro and fluoro substitute give also, good inhibition because of increase the delocalization of electron density in the
molecule, which make the molecule more stable, i.e. better
inhibition. Methyl substituted do the same action like ethyl
substituted but with lower inhibition. In case of methoxy
aniline and ethoxy aniline, the inhibition decrease owing to
the presence of −I (inductive) and +M (mesomeric) effects
which decrease the electron clouds on those substitutes.
The previous discussion indicate that o-substituted anilines
inhibit the corrosion of Cu in 0.5 M HCl according to
the order:
−C2 H5 > −Cl > −F > −CH3 > −OCH3 > −OC2 H5
4. Conclusions
1. All investigated o-substituted anilines show good inhibiting properties for copper corrosion in 0.5 M HCl.
2. The structure of o-substituted aniline influences their inhibiting efficiency
3. The inhibiting efficiency of those o-substituted aniline
increases in the following order, –C2 H5 > –Cl > –F >
–CH3 > –OCH3 > –OC2 H5 .
4. Results obtained from potentiodynamic polarization indicated that the o-substituted aniline are cathodic-type
inhibitors.
Acknowledgements
The authors’ wish gratefully acknowledges the financial
support provided by Robert A. Welch Foundation of Houston, TX.
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