Density functional treatment and electro analytical measurements of liquid phase interaction of 2-ethylbenzimidazole (EBI) and ethyl (2-ethylbenzimidazolyl) acetate (EEBA) on mild steel in hydrochloric acid

Article abstract of DOI:10.1016/j.molliq.2016.04.113

DFT calculations on the liquid phase interaction and corrosion inhibition properties of 2-ethylbenzimidazole (EBI) and its derivative ethyl (2-ethylbenzimidazolyl)acetate (EEBA) on mild steel in hydrochloric acid (0.5, 5 and 1.5 M) at three different temperatures (303, 308 and 313 K) have been studied using EIS, polarization, adsorption measurements and computational calculations. Results show that EBI and EEBA act as effective inhibitors for the corrosion of mild steel in hydrochloric acid media. Polarization studies showed that both the inhibitors behave as mixed type inhibitors with respect to the electrode reactions. EEBA offers better inhibition efficiency than EBI. The inhibition efficiency found to decrease with increase in temperature in either of the cases. The mechanism involves adsorption phenomenon and in the case of EEBA, the adsorption obeyed Langmuir adsorption isotherm. For EBI, the adsorption obeyed Langmuir adsorption isotherm except for 313 K in 1.5 M HCl, for which the best fit adsorption isotherm was Temkin adsorption isotherm.

Full text of DOI:10.1016/j.molliq.2016.04.113

Journal of Molecular Liquids 220 (2016) 707–717
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Density functional treatment and electro analytical measurements of
liquid phase interaction of 2-ethylbenzimidazole (EBI) and ethyl
(2-ethylbenzimidazolyl) acetate (EEBA) on mild steel in
hydrochloric acid
Revathi Mohan, K. Ramya, K.K. Anupama, Abraham Joseph ⁎
Department of Chemistry, University of Calicut, Calicut University P O, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
DFT calculations on the liquid phase interaction and corrosion inhibition properties of 2-ethylbenzimidazole
Received 23 March 2016
Accepted 28 April 2016
Available online xxxx
(
EBI) and its derivative ethyl (2-ethylbenzimidazolyl)acetate (EEBA) on mild steel in hydrochloric acid (0.5, 5
and 1.5 M) at three different temperatures (303, 308 and 313 K) have been studied using EIS, polarization, ad-
sorption measurements and computational calculations. Results show that EBI and EEBA act as effective inhibi-
tors for the corrosion of mild steel in hydrochloric acid media. Polarization studies showed that both the
inhibitors behave as mixed type inhibitors with respect to the electrode reactions. EEBA offers better inhibition
efficiency than EBI. The inhibition efficiency found to decrease with increase in temperature in either of the
cases. The mechanism involves adsorption phenomenon and in the case of EEBA, the adsorption obeyed Lang-
muir adsorption isotherm. For EBI, the adsorption obeyed Langmuir adsorption isotherm except for 313 K in
1.5 M HCl, for which the best fit adsorption isotherm was Temkin adsorption isotherm.
Keywords:
Acid solutions
EIS
Acid corrosion
Acid inhibition
Polarization
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
medium is very important [14]. In recent years, there is considerable
amount of effort devoted to studying inhibition properties of benzimid-
azole and its derivatives for metallic corrosion. Benzimidazole and its
derivatives have received considerable attention on their inhibition
properties of metallic corrosion over the past years [15–16]. Benzimid-
azole is a heterocyclic organic compound, and the nitrogen atom and
the aromatic ring in the molecular structure are likely to facilitate the
adsorption of the compounds on the metal surface [17]. Some deriva-
tives of benzimidazoles have been demonstrated as excellent inhibitors
for metals and alloys in acidic solution, and exhibit different inhibition
performance with the difference in substituent groups and substituent
positions on the imidazole ring [18–24]. J. Alijourani et al. investigated
the corrosion inhibition of carbon steel in hydrochloric acid and
sulphuric acids in the presence of some benzimidazole derivatives
Mild steel is widely used as structural material in automobiles, pipes
and chemical industries [1]. Acid solutions are generally used for re-
moval of undesirable scale and rust on the metals, cleaning of boilers
and heat exchangers, oil-well acidizing in oil recover and so on [2–6].
However the rate of metal dissolution caused by the acidic media is
quite high. A most efficient solution to the problems caused by corro-
sion is the use of corrosion inhibitors. The use of organic inhibitors is
one of the most practical methods for protection of metals against cor-
rosion, and is becoming increasingly popular according to recent studies
[7]. Most common inhibitors are organic compounds containing func-
tional electronegative groups, π electrons in triple or conjugated double
bonds and hetero atoms like N, S and O [8]. These organic compounds
can adsorb on the metal surface and thereby reduce the corrosion rate
3
such as benzimidazole (BI), 2-methylbenzimidazole (2-CH -BI) and
[
9]. The researches show that these molecules adsorb on the metal sur-
2-mercaptobenzimidazole (2-SH-BI). In both the media 2-SH-BI
showed highest efficiency and 2-BI showed lowest efficiency [25]. X.
Wang et al. had conducted studies on the influences of a benzimidazole
derivative namely 1,8-bis(1-chlorobenzyl-benzimidazolyl)-octane
(CBO) on the corrosion behaviour of mild steel in different concentra-
tions of HCl. The studies showed that the inhibitor is an excellent
mixed type inhibitor [26]. Y. Tang et al. conducted studies on
three novel benzimidazole derivatives, 2-aminomethylbenzimidazole
face by displacing water molecules on the surface and forming a protec-
tive film [10–12]. Especially, the studies show that chloride containing
acidic medium plays an important role because chloride ions easily ad-
sorb on the inner Helmholtz plane [13]. It is very well known that
HCl(aq) is one of the most widely used acid for descaling, degreasing,
pickling, etc. Therefore the protection efficiency of the inhibitor in this
(
2-ABI), bis(2-benzimidazolylmethyl)amine (BBIA) and tris(2-
⁎
Corresponding author.
E-mail address: drabrahamj@gmail.com (A. Joseph).
benzimidazolylmethyl)amine (TBIA) as inhibitors for mild steel in 1 M
http://dx.doi.org/10.1016/j.molliq.2016.04.113
167-7322/© 2016 Elsevier B.V. All rights reserved.
0

708
R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
HCl. The inhibition efficiency was found to depend on the concentration
of the inhibitor, temperature and concentration of the acid solution [27].
A. Dutta et al. tested the different bis-benzimidazole derivatives as po-
tential corrosion inhibitors for mild steel in 1 M HCl and the inhibitors
were proved to be mixed type inhibitors [28]. M. Yadav et al. studied
the inhibitive action of synthesised benzimidazole derivatives namely
conducted using EIS and potentiodynamic polarization techniques.
Quantum chemical parameters were also calculated.
2. Experimental
2.1. Inhibitor
2
2
-((1-morpholinomethyl)1H-benzo[d]imidazole-2-yl)phenol (MBP),
-((1-piperazine-1-yl)methyl-1H-benzo[d]imidazole-2yl)phenol
2.1.1. 2-Ethylbenzimidazole
(
2
PzMBP) and 2-(1-((piperazine-1-yl)methyl)-1H-benzo[d]imidazole-
-yl)phenol (PMBP) on corrosion of N80 steel in 15% HCl solution. It
A mixture of o-phenylenediamine (10 g, 0.092 mol) and propionic
aid (0.11 mol) was dissolved in 4 M HCl (10 ml) and refluxed at
100 °C for 12 h. Completion of the reaction was monitored using TLC.
The contents were cooled to room temperature and neutralized with
was found that the inhibition efficiency of all the inhibitors increased
with increase in concentration of inhibitors and decreased with increase
in temperature. The studies also found out that the inhibitors were
mixed type [29]. All the studies revealed that the benzimidazole deriv-
atives inhibited the corrosion by getting adsorbed on to the metal sur-
face. Another remarkable feature is that during the adsorption on to
the metal surface, benzimidazole molecules show two anchoring sites
suitable for surface bonding: the nitrogen atom with its lonely sp elec-
tron pair and the aromatic rings [30].
The aim of the present investigation is to examine the inhibitive
properties of 2-ethylbenzimidazol (EBI) (Fig. 1a) and its derivative
ethyl(2-ethylbenzimidazolyl)acetate (EEBA) (Fig. 1b) on mild steel in
different concentrations of HCl (0.5, 1 and 1.5 M). The study has been
3
saturated solution of NaHCO [31].
2.1.2. Ethyl (2-ethylbenzimidazolyl)acetate
The solution of 2-ethylbenzimidazol (0.062 mol) in acetone (20 ml)
was mixed with ethylchloroacetate (7.9 ml, 0.074 mol) and potassium
carbonate (16.5 g, 0.12 mol) and refluxed for 6 h. Completion of the re-
action was monitored by TLC. The reaction mixture was filtered. From
the clear filtrate, excess acetone was removed by distillation and then
was added to water. The solid product separated was collected by filtra-
tion and dried. Further purification was done by crystallization from
ethyl acetate to give ethyl(2-methylbenzimidazolyl)acetate [31].
2
2.2. Medium
The medium of the study was made from reagent grade HCl (E.
Merck) and doubly distilled water. All tests were performed in aerated
medium under different temperatures (303, 308 and 313 K) and atmo-
spheric pressure.
2.3. Materials
The mild steel specimen used was of the following composition
(
wt.): C (0.20%), Mn (1%), P (0.03%), S (0.02%) and Fe (98.75%). The
2
mild steel specimens used were cut in 4.8 × 1.9 cm coupons and
polished as recommended by ASTM (0–4 grit of 1200 mesh). During
2
the electrochemical measurements only 1 cm area was exposed. Before
measurements, the samples were polished using buffing machine and
different grades of emery paper (6–1200 grade) followed by washing
with ethanol, acetone and finally with distilled water for achieving mir-
ror bright finish.
2.4. Electrochemical measurements
Electrochemical tests were carried out in a conventional three elec-
2
trode corrosion cell with platinum sheet (1 cm surface area) as auxilia-
ry electrode and saturated calomel electrode (SCE) as the reference
electrode. The working electrode was first immersed in the test solution
and after establishing a steady state open circuit potential (OCP), the
electrochemical measurements were carried out in a Gill AC computer
controlled electrochemical work station (ACM, UK, Model No: 1475).
Electrochemical impedance spectroscopy (EIS) measurements were
carried out with amplitude of 10 mV (RMS) AC Sine wave with frequen-
cy range of 10 kHz–0.1 Hz. The polarization curves were obtained in the
potential range from −250 mV to +250 mV (vs. SCE) with a sweep rate
of 60 mV/min.
2.5. Computational study
The theoretical calculations were carried out using B3LYP. It is a ver-
sion of DFT method which uses Beck's three parameter functional (B3)
and it includes a mixture of HF with DFT exchange terms associated
with the gradient corrected correlation functional of Lee, Yang and
Parr (LYP) [32]. It has much less convergence problems than those
found for pure DFT methods [33]. The full geometry optimization of
Fig. 1. Optimized geometries of (a)EBI (b) EEBA.

R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
709
the inhibitor was carried out at the B3LYP/6-31G* level using Gaussian
3 program package. The quantum chemical parameters calculated
0
were EHOMO, ELUMO, χ (electro negativity), η (global hardness), etc.
Fukui indices were also determined.
3
3
3
. Results and discussion
.1. Electrochemical measurements
.1.1. Electrochemical impedance spectroscopy (EIS)
The EIS study was done for mild steel in various concentrations of
Fig. 2. Randle's circuit.
HCl ranging from 0.5 M to 1.5 M in the presence and absence of the in-
hibitors EBI and EEBA. The study was done in the absence and presence
Fig. 3. Nyquist plots for mild steel corrosion in (a) 0.5 M HCl(b) 1 M HCl and (c) 1.5 M HCl
Fig. 4. Nyquist plots for mild steel corrosion in (a) 0.5 M HCl (b) 1 M HCl and (c) 1.5 M HCl
in the absence and presence of EBI at 303 K.
in the absence and presence of EEBA at 303 K.

710
R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
Table 1a
Charge transfer resistance (Rct), corrosion rate (C.R) and percentage inhibition efficiency of EBI obtained from EIS study.
Conc. Of
HCl (M)
Conc. Of
EBI (ppm)
Temperature (K)
03
3
308
ct (Ω cm2)
86.20
313
ct (Ω cm2)
67.42
114.00
226.80
345.40
441.80
7.79
12.84
16.22
26.32
34.06
4.65
R
ct (Ω cm2)
C.R (mm/yr)
%IE
R
C.R (mm/yr)
%IE
R
C.R (mm/yr)
%IE
0
1
1
.5
Blank
8.24
36.67
7.97
3.10
1.52
0.71
49.96
20.37
8.48
7.67
2.56
–
3.51
1.77
1.33
0.56
0.48
23.26
13.64
7.54
6.52
4.69
46.29
31.46
31.02
29.33
25.22
–
4.49
2.65
1.33
0.88
0.68
38.33
23.55
18.64
11.49
8.88
65.02
58.82
50.62
46.34
44.23
–
5
1
1
2
0
37.92
97.35
199.3
426.3
6.05
14.84
35.65
39.41
118.10
9.79
78.38
91.58
95.89
98.08
–
171.20
227.50
539.00
633.00
13.00
22.16
40.11
46.38
64.47
6.53
49.65
62.11
84.01
86.41
–
40.86
70.27
80.48
84.74
–
00
50
00
Blank
5
1
1
2
0
59.2
83.02
84.64
94.88
–
41.34
67.59
71.97
79.8
–
39.06
51.9
70.36
77.09
–
00
50
00
.5
Blank
30.90
13.45
11.73
5.34
5
1
1
2
0
22.48
25.77
56.59
175.60
56.44
62.01
82.69
94.42
9.61
9.75
10.31
11.99
32.00
33.03
36.66
45.54
5.14
5.97
6.52
6.84
9.5
00
50
00
21.19
28.68
32.01
1.72
of various concentrations of inhibitors ranging from 50 to 200 ppm
Thus the parallel combination of double layer capacitance and charge
transfer resistance which are in series with solution resistance, particu-
larly in the presence of an efficient inhibitor, is found to be an inade-
quate approach for modelling the metal-acid solution interface [39].
Thus to account for the non-ideal behaviour, it is necessary to use a con-
stant phase element, CPE instead of double layer capacitance [40]. The
CPE can be modelled as follows:
(
ppm by weight) at temperatures 303, 308 and 313 K.
The equivalent circuit proposed to fit and interpret the EIS data was
Randle's circuit (Fig. 2) in which there is charge transfer resistance (Rct
parallel with double layer capacitance (Cdl) in series with solution resis-
tance (R ). Nyquist plots obtained from the study were shown in Figs. 3
)
s
and 4 (only the representative figures are given).
Nyquist plots are electrochemical impedance spectra in a complex
plane. The existence of a single semi-circle showed the single charge
transfer process during dissolution which is unaffected by the presence
of inhibitor molecules. The addition of inhibitor molecules causes an in-
crease in Rct values [34]. The increase in Rct values is attributed to the
formation of a protective film on the metal-solution interface [35,36].
Thus it can be concluded that the inhibitor molecules function by ad-
sorption on to the metal surface and thereby causing an increase in Rct
value [37].
All the Nyquist plots had a depressed semi-circle shape in the com-
plex plane with centre under the real axis. This behaviour is a character-
istic of solid electrodes. The depressed semi-circle shape is due to the
non-ideal behaviour of double layer as a capacitor. This in turn is due
to the surface heterogeneity resulting from surface roughness, impuri-
ties, dislocations, grain boundaries, adsorption of inhibitors, etc. The
high frequency part of the impedance and phase angle reflects the be-
haviour of heterogeneous surface layer whereas the low frequency
part shows the kinetic response for the charge transfer reaction [38].
ZCPE ¼ ðjωcÞ−α
ð1Þ
where ZCPE is the impedance, j is the square root of −1, c is the capaci-
tance and α is the measure of non-ideality of the capacitor and has a
value in the range of 0 b α b 1 [41].
The charge transfer resistance values were calculated from the dif-
ference in impedance at lower and higher frequencies. The double
layer capacitance, Cdl and the frequency at which the imaginary compo-
nent of the impedance maximum were obtained from the equation
given below:
1
Cdl
¼
ð2Þ
ωXRct
where ω = 2πfmax and fmax is the frequency at which the imaginary
component of impedance is maximum.
Table 1b
Charge transfer resistance (Rct), Corrosion rate (C.R) and percentage inhibition efficiency of EEBA obtained from EIS study.
Conc. Of
HCl (M)
Conc. Of EEBA
(ppm)
Temperature (K)
03
3
308
ct (Ω cm2)
21.34
160.60
215.60
270.00
438.10
9.22
65.46
96.50
146.10
160.00
8.24
66.18
73.33
77.16
83.92
313
ct (Ω cm2)
19.03
90.23
135.10
228.70
328.10
5.49
33.28
49.20
53.02
70.88
5.19
22.58
25.25
30.87
45.35
R
ct (Ω cm2)
C.R (mm/yr)
%IE
R
C.R (mm/yr)
%IE
R
C.R (mm/yr)
%IE
0
1
1
.5
Blank
36.54
8.27
0.38
0.36
0.24
0.22
8.65
0.63
0.45
0.36
0.34
16.13
1.43
0.68
0.53
0.45
–
14.17
1.88
1.40
1.12
0.69
32.78
4.62
3.13
2.07
1.89
36.68
4.57
4.12
3.92
3.60
–
15.89
3.35
2.24
1.32
0.92
54.98
9.09
6.15
5.70
4.27
58.2
13.39
11.97
9.79
6.67
–
5
1
1
2
0
787.60
841.90
1285.0
1388.0
34.96
477.10
673.10
846.60
898.10
18.74
95.4
95.7
97.2
97.4
–
86.7
90.1
92.1
95.1
–
78.9
85.9
91.7
94.2
–
00
50
00
Blank
5
1
1
2
0
92.7
94.8
95.9
96.1
–
85.9
90.4
93.7
94.2
–
83.5
88.8
89.6
92.2
–
00
50
00
.5
Blank
5
1
1
2
0
211.1
443.8
573.3
665.1
91.1
95.8
96.7
97.2
87.5
88.8
89.3
90.2
76.9
79.4
83.2
88.5
00
50
00

R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
711
Percentage inhibition efficiencies were calculated from charge trans-
fer resistance values as follows:
the inhibitor. The inhibition efficiency decreased with acid concentra-
tion. Another observation made from EIS study was that the efficiency
of the inhibitors decreased with increase in temperature. This leads to
the conclusion that inhibition was achieved by the adsorption of the in-
hibitors on the mild steel surface. As the temperature was increased, the
adsorption of the inhibitor on to the mild steel surface became difficult
which caused the inhibition efficiency to decrease. Thus maximum effi-
ciency was obtained for 200 ppm concentration of the inhibitor in 0.5 M
HCl at 303 K for both the inhibitors. It was also found that EEBA is a more
efficient inhibitor than EBI.
ꢀ
R −Rct
ct
IE ð%Þ ¼
ꢁ 100
ð3Þ
ꢀ
R
ct
⁎
where Rct and Rct denote charge transfer resistance in the presence and
absence of the inhibitors.
Result obtained from EIS study for the inhibitor, EBI has been sum-
marized in Table 1a and that for EEBA in Table 1b. From the thorough
analysis of the result, it can be seen that for both the inhibitors, at all
the three temperatures, the charge transfer resistance values as well
as the inhibition efficiency increased with increase in concentration of
Fig. 5. Polarization curves for mild steel corrosion in (a) 0.5 M HCl (b) 1 M HCl and
Fig. 6. Polarization curves for mild steel corrosion in (a) 0.5 M HCl (b) 1 M HCl and
(c) 1.5 M HCl in the absence and presence of EBI at 303 K.
(c) 1.5 M HCl in the absence and presence of EEBA at 303 K.

712
R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
Table 2a
Corrosion current density (icorr), corrosion rate (C.R) and percentage inhibition efficiency (%IE) of EBI in 0.5 M HCl obtained from potentiodynamic polarization study.
Conc. Of EBI Temperature (K)
(ppm)
303
308
313
−
E
corr (mV)
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
470.49
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
432.85
i
corr (mA/cm2) C.R (mm/yr) %IE
Blank
445.23
475.09
474.36
478.35
485.00
3.99
0.79
0.31
0.16
0.08
46.33
9.21
3.62
1.87
0.98
–
0.46
0.24
0.18
0.07
0.065
5.37
2.83
2.05
0.88
0.76
–
0.36
4.21
2.49
1.29
0.80
0.067
–
5
1
1
2
0
80.1 442.84
92.5 442.91
95.9 457.02
97.9 455.99
47.3 439.85
61.8 462.65
83.6 445.56
85.8 458.4
0.22
0.11
0.069
0.057
40.6
69.3
80.9
84.2
00
50
00
Table 2b
Corrosion current density (icorr), corrosion rate (C.R) and percentage inhibition efficiency (%IE) of EBI in 1 M HCl obtained from potentiodynamic polarization study.
Conc. Of EBI Temperature (K)
(ppm)
303
308
313
−
E
corr (mV)
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
498.88
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
547.29
i
corr (mA/cm2) C.R (mm/yr) %IE
Blank
484.00
447.37
457.32
452.87
472.16
1.55
0.67
0.25
0.23
0.10
18.01
7.77
2.92
2.70
1.22
–
1.03
0.59
0.36
0.29
0.22
11.99
6.83
4.15
3.33
2.59
–
4.29
49.78
30.49
25.24
14.13
12.13
–
5
1
1
2
0
56.9 422.48
83.8 530
85.2 414.34
93.2 413.96
43.0 406.79
65.4 489.39
72.3 477.75
78.4 420.85
2.63
2.18
1.22
1.05
38.8
49.3
71.6
75.6
00
50
00
Table 2c
Corrosion current density (icorr), corrosion rate (C.R) and percentage inhibition efficiency (%IE) of EBI in 1.5 M HCl obtained from potentiodynamic polarization study.
Conc. Of EBI Temperature (K)
(ppm)
303
308
313
−
E
corr (mV)
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
401.93
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
399.21
i
corr (mA/cm2) C.R (mm/yr) %IE
Blank
448.8
2.32
1.00
0.86
0.40
0.17
26.85
11.68
10.00
4.69
–
3.19
2.21
2.14
2.04
1.83
37.04
25.56
24.80
23.67
21.16
–
4.63
53.63
48.78
41.69
38.93
35.80
–
5
1
1
2
0
457.65
457.56
466.24
469.05
56.5 399.82
62.7 479.7
82.5 399.96
92.6 398.72
30.9 387.98
33.1 398.5
36.1 395.98
42.9 399.86
4.21
3.59
3.36
3.09
9.0
00
50
00
22.3
27.4
33.2
1.99
Table 3a
Corrosion current density (icorr), corrosion rate (C.R) and percentage inhibition efficiency (%IE) of EEBA in 0.5 M HCl obtained from potentiodynamic polarization study.
Conc. Of EEBA
ppm)
Temperature (K)
03
(
3
308
313
−
E
mV)
corr
i
corr
C.R
(mm/yr)
%IE
−Ecorr
(mV)
i
corr
C.R
(mm/yr)
%IE
−Ecorr
(mV)
i
corr
C.R
(mm/yr)
%IE
2
2
2
(
(mA/cm )
(mA/cm )
(mA/cm )
Blank
510.83
457.54
497.40
476.45
479.74
0.657
0.041
0.032
0.031
0.027
7.61
0.48
0.38
0.36
0.31
–
455.13
467.43
481.61
478.64
490.85
1.163
0.162
0.113
0.091
0.055
13.47
1.87
1.31
1.05
0.64
–
420.31
480.11
489.56
491.25
485.98
1.253
0.269
0.180
0.107
0.076
14.53
3.12
2.09
1.24
0.88
–
5
1
1
2
0
93.7
95.1
95.2
95.9
86.1
90.3
92.2
95.2
78.5
85.6
91.5
93.9
00
50
00
Table 3b
Corrosion current density (icorr), corrosion rate (C.R) and percentage inhibition efficiency (%IE) of EEBA in 1 M HCl obtained from potentiodynamic polarization study.
Conc. Of EEBA Temperature (K)
(ppm)
303
308
313
−
E
corr (mV)
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
494.7
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
490.25
i
corr (mA/cm2) C.R (mm/yr) %IE
Blank
489.12
485.25
456.33
435.14
463.86
0.728
0.057
0.043
0.039
0.029
8.44
0.65
0.49
0.44
0.34
–
2.534
0.394
0.240
0.167
0.155
29.37
4.57
2.79
1.94
1.81
–
3.884
45.01
7.50
5.49
4.72
3.50
–
5
1
1
2
0
92.2 466.6
94.1 469.7
94.7 484.2
95.9 476.2
84.4 466.56
90.5 484.33
93.4 502.47
93.9 472.14
0.647
0.473
0.407
0.302
83.3
87.8
89.5
92.2
00
50
00

R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
713
Table 3c
Corrosion current density (icorr), corrosion rate (C.R) and percentage inhibition efficiency (%IE) of EEBA in 1.5 M HCl obtained from potentiodynamic polarization study.
Conc. Of EEBA Temperature (K)
(ppm)
303
308
313
−
E
corr (mV)
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
438.38
i
corr (mA/cm2) C.R (mm/yr) %IE
−Ecorr (mV)
442.43
i
corr (mA/cm2) C.R (mm/yr) %IE
Blank
486.41
479.84
474.3
448.65
469.62
1.269
0.114
0.058
0.041
0.038
14.72
1.32
0.67
0.47
0.44
–
3.032
0.367
0.351
0.317
0.281
35.14
4.25
4.06
3.67
3.25
–
4.987
1.142
1.031
0.839
0.573
57.79
13.24
11.95
9.73
–
5
1
1
2
0
91.0 444.50
95.4 459.08
96.8 458.90
97.0 452.27
87.9 431.19
88.4 447.33
89.5 444.55
90.7 443.65
77.1
79.3
83.2
88.5
00
50
00
6.65
3
.1.2. Potentiodynamic polarization study
This method involves the monitoring of the current that is produced
and 313 K are shown in Figs. 5 and 6. The result obtained from
the study for EBI is shown in Tables 2a–2c and that for EEBA in
Tables 3a–3c.The percentage inhibition efficiency was calculated in
each case using the following equation:
as a function of time or potential by varying the potential of the working
electrode [42]. Both anodic and cathodic polarization curves for mild
steel in various concentrations of HCl ranging from 0.5 to 1.5 M HCl in
the presence and absence of different concentrations of the inhibitors,
EBI and EEBA, ranging from 50 to 200 ppm at temperatures 303, 308
ꢀ
icorr−i
corr
IE ð%Þ ¼
ꢁ 100
ð4Þ
icorr
Fig. 8. Langmuir adsorption isotherms for the interaction of 2-EBI on mild steel in (a) 0.5 M (b) 1 M (c) 1.5 M HCl and (d) Temkin adsorption isotherm for 2-EBI-mild steel interaction in
.5 M HCl at 313 K.
1

714
R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
where icorr and i*corr denote corrosion current density in the absence and
presence of the inhibitor. The corrosion current densities were deter-
mined by the extrapolation of anodic and cathodic tafel lines to the cor-
where Cinh is the concentration of inhibitor in ppm, Kads is the equilibri-
um constant for adsorption-desorption process which is obtained from
the following equation:
rosion potential, Ecorr
.
From the result obtained from the study, it is clearly evident that
for both the inhibitors, at all the three temperatures 303, 308 and
θ
Kads
¼
:
ð7Þ
Cinhð1−θÞ
3
13 K, the corrosion current density decreased with increase in concen-
tration of the inhibitor. Therefore, the percentage inhibition efficiency
also increased with increase in concentration of the inhibitor. Increase
in temperature as well as increase in acid concentration caused
an increase in corrosion current density thereby decreasing the
inhibition efficiency. The maximum efficiency was obtained for
The free energy of adsorption ΔGads was calculated using the
equation:
ΔGads ¼ −RTlnð55:5 KadsÞ:
ð8Þ
2
00 ppm concentration of the inhibitors in 0.5 M HCl at 303 K. From po-
tentiodynamic polarization study also, it was seen that EEBA was more
efficient inhibitor than EBI. The result obtained from the potentiody-
namic polarization study was in perfect agreement with that obtained
from EIS study.
On analyzing the tafel plots it could be understood that the addition
of the inhibitors, EBI and EEBA, didn't cause the corrosion potential to
shift towards anodic or cathodic region. Also there was change on
both anodic and cathodic polarization curves after inhibitor addition.
This observation leads to the conclusion that the inhibitors, EBI and
EEBA act as mixed type inhibitors. A comparison of percentage inhibi-
tion efficiencies of the two inhibitors were given in Fig. 7 in the online
version at http://dx.doi.org/10.1016/j.molliq.2016.04.113. (Please refer
in the supplementary materials).
3
.2. Adsorption study
As known, the adsorption isotherms provide important information
on interaction between the inhibitor and metal surface [43]. The inter-
action of the inhibitor and steel iron surface is described on the model
followed by adsorption isotherms. Adsorption of inhibitor molecules
on surface is a substitution process where an exchange of adsorbed
water molecules with organic molecules occurred. The degree of surface
covered (θ) by inhibitor on steel is dependent on inhibitor concentra-
tion at constant temperature, this way adsorption isotherm is evaluated
at equilibrium conditions [44]. The inhibition efficiencies of organic
molecules mainly depend on their adsorption ability on the metal
surface. Therefore, it is of utmost importance to determine the relation
between adsorption and corrosion inhibition. Several adsorption
isotherms were attempted to fit the degree of surface coverage values,
θ (%IE/100) to adsorption isotherms including Frumkin, Temkin,
Freundlich and Langmuir isotherms. The values for various concentra-
tions of inhibitors in acidic media have been evaluated from EIS
measurements. In the case of EBI, when Cinh/ vs. Cinh was plotted, a
2
straight line with R value close to unity was obtained in all the cases
and thus obeyed Langmuir adsorption isotherm (Fig. 8a–c), except
value at 313 K in 1.5 M HCl which gave straight line with R2 value
close to unity when vs. Log Cinh was plotted and in this case
Temkin adsorption isotherm was obeyed as represented in Fig. 8d. The
θ values for EEBA obeyed Langmuir adsorption isotherm at all the
three temperatures in different acid concentrations which are repre-
sented in Fig. 9.
Langmuir adsorption isotherm can be represented using the follow-
ing equation:
Cinh
θ
1
¼ K þ Cinh
:
ð5Þ
ads
Temkin adsorption isotherm can be represented using the following
equation:
Fig. 9. Langmuir adsorption isotherms for EEBA-mild steel interaction in (a) 0.5 M (b) 1 M
and (c) 1.5 M HCl.
expðfθÞ ¼ KadsCinh
ð6Þ

R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
715
Table 4a
Adsorption parameters obtained for different concentrations of EBI at different acid concentration and temperatures.
Conc. of
HCl (M)
Conc. Of
EBI (ppm)
Temperature (K)
03
ads (104 M−1
3
308
313
K
)
−ΔGads (kJ/mol)
K
ads (104 M−1
)
−ΔGads (kJ/mol)
K
ads (104 M−1
)
−ΔGads (kJ/mol)
0
1
1
.5
50
1.06
1.59
2.28
3.77
0.42
0.71
0.53
1.35
0.38
0.238
0.46
1.24
33.47
34.49
35.39
36.66
31.16
32.47
31.75
34.08
30.88
29.71
31.39
33.85
0.28
0.24
0.51
0.46
0.21
0.30
0.25
0.29
0.14
0.07
0.05
0.06
30.68
30.21
32.15
31.90
29.82
30.82
30.32
30.69
28.79
27.13
26.50
26.71
0.20
0.35
0.40
0.41
0.18
0.15
0.23
0.245
0.03
30.25
31.65
32.04
32.07
30.06
29.61
30.61
30.77
25.35
25.99
25.98
25.65
1
1
2
00
50
00
50
00
50
00
50
00
50
00
1
1
2
.5
1
1
2
0.0392
0.0391
0.03
From Tables 4a and 4b, it can be seen that Kads values are relatively
inhibition [14]. The HOMO and LUMO of the two inhibitor molecules
were given in Fig. 10 in the online version at http://dx.doi.org/10.
1016/j.molliq.2016.04.113. (please refer in the supplementary mate-
rials). The ΔE value for EEBA is smaller than that of EBI which shows
that EEBA is more efficient inhibitor than EBI. Thus the theoretical
study is in perfect agreement with the electrochemical studies.
high. This shows the high adsorption ability of inhibitor on mild steel
surface [45]. It can also be seen from the table that ΔGads values are neg-
ative. This indicates the spontaneity of adsorption and the stability of
adsorbed layer on mild steel surface [46,47]. It is said that the values
of ΔGads obtained around −20 kJ/mol or lower indicate physisorption
whereas the values around −40 kJ/mol indicate chemisorption
48–50]. The adsorption phenomena of an organic molecule cannot be
considered as purely physical or chemical adsorption phenomena [51,
2]. The adsorption of inhibitor on to the mild steel surface may involve
both physisorption and chemisorption. From electrochemical studies it
was found that %IE decreased with increase in temperature. This can
be explained by considering the point that the adsorption of inhibitor
on mild steel surface is not effective at higher temperatures.
The optimized molecular geometries of EBI and EEBA were deter-
mined and shown in Fig. 1. Condensed Fukui indices of EBI and EEBA
were calculated and the results are given in Tables 5 in the online ver-
sion at http://dx.doi.org/10.1016/j.molliq.2016.04.113.a and b (please
refer in the supplementary materials). The local reactivity of the inhibi-
tor molecule had been investigated using the condensed Fukui indices
which give an indication about the reactive centres of molecules (nucle-
ophilic and electrophilic centres). The molecular regions where the
Fukui function is large are chemically softer than the regions where
the Fukui function is small. The site of nucleophilic attack will be the
[
5
+
3
.3. Computational study
one with maximum f value and the site of electrophilic attack will be
−
the one with maximum f value. From the table we can see that in
+
To investigate the effect of molecular structure on inhibition efficien-
the case of EBI, f value is the highest for C (3). Hence this is the site
−
cy, some quantum chemical calculations were performed. Various
quantum chemical parameters of the two inhibitors were calculated
using Gaussian 03 program package. In the case of EBI, the energy of
highest occupied molecular orbital (EHOMO) is −5.8762 eV. For EEBA,
the value of EHOMO is −5.798 eV. The energy of the lowest unoccupied
molecular orbital (ELUMO) obtained for EBI is −0.2313 eV. For EEBA,
the value of ELUMO is −0.205 eV. In the case of EBI, the energy gap
for nucleophilic attack. The f value is the highest for C (5). Hence this
+
is the site for electrophilic attack. In the case of EEBA, f value is the
highest for C (7) and hence this is the site for nucleophilic attack. The
−
f
value is highest for C (5). Hence this is the site for electrophilic attack.
4. Conclusions
(
5
ΔE), ELUMO–EHOMO is 5.6449 eV and the energy gap for EEBA is
.593 eV. An idea about the inhibition efficiency of the molecule will
Electrochemical measurements such as EIS and potentiodynamic
be obtained from ΔE value. A small ΔE value indicates more efficient
polarization were used to study the corrosion inhibition efficiency of
Table 4b
Adsorption parameters obtained for different concentrations of EEBA in different acid concentration and temperatures.
Conc. of
HCl (M)
Conc. Of
EEBA (ppm)
Temperature (K)
03
ads (104 M−1
3
308
313
K
)
−ΔGads (kJ/mol)
K
ads (104 M−1
)
−ΔGads (kJ/mol)
K
ads (104 M−1
)
−ΔGads (kJ/mol)
0
1
1
.5
50
8.63
4.63
4.81
3.89
5.28
3.79
3.24
2.56
4.26
4.74
4.06
3.61
38.75
37.18
37.28
36.74
37.51
36.68
36.28
35.69
36.97
37.24
36.85
36.55
2.71
1.89
1.62
2.02
2.25
1.95
1.96
1.60
2.91
1.65
1.16
0.95
36.42
35.49
35.11
35.67
35.95
35.58
35.59
35.07
36.61
35.15
34.25
33.74
1.56
1.27
1.53
1.69
2.11
1.65
1.19
1.23
1.39
0.802
0.69
0.80
35.58
35.04
35.53
35.79
36.36
35.72
34.87
34.96
35.28
33.85
33.45
33.84
1
1
2
00
50
00
50
00
50
00
50
00
50
00
1
1
2
.5
1
1
2

716
R. Mohan et al. / Journal of Molecular Liquids 220 (2016) 707–717
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