Electroanalytical and computational studies on the corrosion inhibition behavior of ethyl (2-methylbenzimidazolyl) acetate (EMBA) on mild steel in hydrochloric acid

Article abstract of DOI:10.1007/s11164-014-1568-y

The interaction and corrosion protection properties of ethyl (2-methylbenzimidazolyl) acetate (EMBA) on mild steel in hydrochloric acid (0.5, 1 and 1.5 M) at different temperatures have been studied by polarization, EIS, adsorption, surface studies, and computational calculations. Polarization studies showed that this molecule act as mixed-type inhibitor. EMBA acts as an effective inhibitor for mild steel in hydrochloric acid at different temperatures (303, 308, and 313 K). At room temperature, EMBA was found to be a more effective inhibitor and its efficiency decreases with increasing temperature. The mechanism involves adsorption of inhibitor molecules on the metal surface and this process obeys the Langmuir isotherm.

Full text of DOI:10.1007/s11164-014-1568-y

Res Chem Intermed
DOI 10.1007/s11164-014-1568-y
Electroanalytical and computational studies
on the corrosion inhibition behavior of ethyl
(2-methylbenzimidazolyl) acetate (EMBA) on mild steel
in hydrochloric acid
•
Abraham Joseph Revathi Mohan
Received: 18 November 2013 / Accepted: 15 February 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract The interaction and corrosion protection properties of ethyl (2-methyl-
benzimidazolyl) acetate (EMBA) on mild steel in hydrochloric acid (0.5, 1 and 1.5
M) at different temperatures have been studied by polarization, EIS, adsorption,
surface studies, and computational calculations. Polarization studies showed that
this molecule act as mixed-type inhibitor. EMBA acts as an effective inhibitor for
mild steel in hydrochloric acid at different temperatures (303, 308, and 313 K). At
room temperature, EMBA was found to be a more effective inhibitor and its effi-
ciency decreases with increasing temperature. The mechanism involves adsorption
of inhibitor molecules on the metal surface and this process obeys the Langmuir
isotherm.
Keywords Mild steel ꢀ Acid solutions ꢀ EIS ꢀ Polarization ꢀ Acid
inhibition
Introduction
A study of the mechanism of action of corrosion inhibitors has relevance not only from
the point of view of a search for new inhibitors but also for their effective application
[1]. Hydrochloric acid solutions are widely used for industrial cleaning, oil well
acidification, and pickling. In such environments, corrosion of steel and its inhibition
constitute a complex problem of the processes. The main problem in using mild steel in
acidic solution is that of uniform corrosion [2]. The use of organic inhibitors for the
inhibition of corrosion of mild steel in acidic media is highly cost-effective [3]. The
development of an efficient corrosion inhibitor based on organic compounds
containing hetero atoms (nitrogen, sulfur, and oxygen) is a challenging objective in
A. Joseph (&) ꢀ R. Mohan
University of Calicut, Kozhikode, Kerala, India
e-mail: drabrahamj@gmail.com
123

A. Joseph, R. Mohan
the field of corrosion chemistry [4]. Benzimidazole is a heterocyclic aromatic
compound with a bicyclic structure consisting of the fusion of benzene and imidazole
rings, and hydrogen atoms on the rings can be substituted by other groups or atoms [5].
In recent years, benzimidazole and its derivatives have received considerable attention
regarding their inhibition properties for metallic corrosion and it has been demon-
strated that they are excellent inhibitors for metals and alloys in acidic solutions since
the nitrogen atom and the aromatic ring in their molecular structure are likely to
facilitate the adsorption of compounds on the metallic surface [6]. Studies show that
benzimidazole and its derivatives such as 2-methylbenzimidazole and 2-mercapto-
benzimidazole can act as efficient inhibitors for corrosion of steel in hydrochloric acid
[7]. Compounds containing benzimidazole groups can also act as effective corrosion
inhibitors in acidic media and thus they can be used in the petroleum industry, mostly
in boiler cleaning and heat exchangers [8]. Reports show that imidazole-type organic
compounds are good corrosion inhibitors for many metals and alloys in aggressive
media and they are also environment friendly [9]. It had been suggested that the most
effective factors for the inhibiting effects are the electronegative atoms (such as N, S,
O, P, etc.) [10, 11], the unsaturated bonds (such as double bonds, triple bonds, etc.) [12,
13], and the plane conjugated systems including all kinds of aromatic cycles which can
offer special active electrons or vacant orbitals to donate or accept electrons,
respectively [14, 15]. Most of the heterocyclic compounds possess the above-
mentioned features within their structures and hence they act as effective inhibitors.
Studies report that adsorption plays a very important role in corrosion protection using
organic inhibitors which depend on some physico-chemical properties of the
molecule, its functional groups, steric effects, and electron density of donor atoms
in the molecule. Another assumption is that adsorption depends on the possible
interaction of p orbitals of the inhibitor with d orbitals of surface atoms. This will lead
to greater adsorption of inhibitor molecules onto the metal surface and finally a
corrosion-protecting film is formed [16]. The details of the molecular structure
including the electronic parameters can be obtained by means of theoretical
calculations using computational quantum chemistry [17, 18].
The objective of this work is to investigate the corrosion behavior of mild steel in
different concentrations of HCl (0.5, 1, 1.5 M) in the presence and absence of the
inhibitor ethyl(2-methylbenzimidazolyl)acetate (Fig. 1) using electrochemical
impedance spectroscopy (EIS) and potentiodynamic polarization study. The
quantum chemical parameters of the inhibitor were also calculated.
Experimental
Synthesis of inhibitor
A mixture of o-phenylenediamine (10 g, 0.092 mol) and acetic acid (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 by TLC. The contents were cooled to room temperature and
neutralized with saturated solution of NaHCO3. The separated solid was 2-meth-
ylbenzimidazole. It was filtered and dried. The solution of 2-methylbenzimidazole
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Electroanalytical and computational studies of EMBA
Fig. 1 Optimized geometry of EMBA
(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 reaction was monitored by TLC. The reaction mixture was
filtered. From the clear filtrate, excess acetone was removed by distillation and was
then added to water. The solid product separated was collected by filtration and
dried. Further purification was done by crystallization from ethyl acetate to give
ethyl(2-methylbenzimidazolyl)acetate [19].
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. The tests were done at
303, 308, and 313 K, at atmospheric pressure.
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 mild steel specimens used were
cut into 4.8 9 1.9 cm2 coupons and polished as recommended by ASTM (0–4 grit of
1,200 mesh). During the electrochemical measurements, only 1 cm2 area was
exposed. Before measurements, the samples were abraded using a buffing machine
and different grades of emery paper (6–1,200 grade) followed by washing with
ethanol, acetone, and, finally, with distilled water for achieving a mirror bright finish.
Electrochemical measurements
Electrochemical tests were carried out in a conventional three-electrode corrosion cell
with platinum sheet (1 cm2 surface area) as auxiliary 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),
123

A. Joseph, R. Mohan
the electrochemical measurements were carried out in a Gill AC computer-controlled
electrochemical work station (Model No: 1475; ACM, UK). EIS measurements were
carried out with amplitude of 10 mV (RMS) AC sine wave with frequency range of
10 kHz–0.1 Hz. The polarization curves were obtained in the potential range from
-250 to ?250 mV (vs. SCE) with a sweep rate of 60 mV/min.
Computational study
The theoretical calculations were carried out using B3LYP, which is a version of the
DFT method which uses Beck’s three-parameter functional (B3), and includes a
mixture of HF with DFT exchange terms associated with the gradient corrected
correlation functional of Lee, Yang and Parr (LYP) [20]. It has many fewer
convergence problems than those found for pure DFT methods [21]. The full
geometry optimization of the inhibitor was carried out at the B3LYP/6-31G* level
using the Gaussian 03 program package. The quantum chemical parameters
calculated were EHOMO, ELUMO, v (electro negativity), g (global hardness), etc.
Fukui indices were also determined.
Scanning electron microscopy
The metal samples were immersed in HCl solutions of concentrations 0.5, 1, and
1.5 M, both in the presence and absence of the inhibitor, for 6 h at 30 °C. The inhibitor
concentration taken was 200 ppm. The samples were cleaned using distilled water,
dried, and used for surface morphological study. The study of the surface morphology
of the metal samples in both the absence and presence of the inhibitor was carried out
using a Digital Microscope Imaging Scanning Electron Microscope model SU6600
with an accelerating voltage of 20.0 kV at a scan speed = slow 5. Samples were
attached on the top of an Al stopper by means of carbon conductive adhesive tapes. All
micrographs of the samples were taken at the magnification of 9500.
Atomic force microscopy
The mild steel samples were immersed in 0.5, 1, and 1.5 M concentrations of HCl,
both in the presence and absence of the inhibitor. The inhibitor was taken at
200 ppm concentration. The samples were immersed at 30 °C for 6 h. The samples
were cleaned with distilled water, dried, and then used for AFM study.
Results and discussion
Electrochemical measurements
Electrochemical impedance spectroscopy
The EIS study was carried out for mild steel in various concentrations of HCl
ranging from 0.5 to 1.5 M in the presence and absence of the inhibitor, ethyl(2-
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Electroanalytical and computational studies of EMBA
Fig. 2 Randles circuit
methylbenzimidazolyl)acetate. The study was carried out for various concentrations
of inhibitors ranging from 50 to 200 ppm (ppm by weight) at temperatures 303, 308,
and 313 K.
The semi-circle Nyquist plots can be modeled by a simple Randles circuit
(Fig. 2), including the charge transfer resistance (Rct) parallel with double layer
capacitance (Cdl) in series with solution resistance (Rs).
The Nyquist plots obtained in the present study were depressed semi-circles. This
difference can be explained by the non-ideal behavior of the double layer as a
capacitor. Therefore, it is necessary to use a constant phase element (CPE) instead
of double layer capacitance to account for non-ideal behavior. The appearance of
CPE is often related to the electrode roughness or to the inhomogeneity in the
conductance or dielectric constant [22–24]. The CPE can be modeled as follows:
ZCPE ¼ ðjxcÞꢁa
;
ð1Þ
where ZCPE is the impedance, j is the square root of -1, c is the capacitance, and a is
the measure of non-ideality of the capacitor and has a value in the range of
0 \ a \ 1 [25].
As compared to a parallel combination of resistor and capacitor, the CPE is able
to provide a much better fit to most impedance data. The double layer between the
charged metal surface and the solution is considered as an electrical double
capacitor. The adsorption of the inhibitor on the copper metal decreases its electrical
capacity because they displace the water molecules and other ions originally
adsorbed on the metal surface. The decrease in this capacity with increase in
concentration of the inhibitor may be attributed to the formation of a protective film
on the electrode surface [26, 27].
The charge transfer resistance values were calculated from the difference in
impedance at lower and higher frequencies. The double layer capacitance, Cdl, and
the frequency at which the imaginary component of impedance is maximum were
obtained from the equation given below:
1
Cdl
¼
;
ð2Þ
x ꢂ Rct
where x = 2pfmax and fmax is the frequency at which the imaginary component of
impedance is maximum [28].
The result of impedance study is given in Table 1. The Nyquist plots are given in
Figs. 3, 4, and 5.
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A. Joseph, R. Mohan
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Electroanalytical and computational studies of EMBA
Fig. 3 Nyquist plots for EMBA in 0.5 M HCl in mild steel at a 303 K, b 308 K, and c 313 K
123

A. Joseph, R. Mohan
Fig. 4 Nyquist plots for EMBA in 1 M HCl in mild steel at a 303 K, b 308 K, and c 313 K
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Electroanalytical and computational studies of EMBA
Fig. 5 Nyquist plots for EMBA in 1.5 M HCl in mild steel at a 303 K, b 308 K, and c 313 K
123

A. Joseph, R. Mohan
From EIS data for mild steel in 0.5, 1, and 1.5 HCl in the presence of various
concentrations of the inhibitor, ethyl(2-methylbenzimidazolyl)acetate (ranging from
50 to 200 ppm), at 303 K, it can be seen that the charge transfer resistance, Rct,
increases with increasing concentration. Percentage inhibition efficiencies were
calculated from charge transfer resistance values as follows:
Rꢃct ꢁ Rct
%IE ¼
ꢂ 100;
ð3Þ
Rꢃct
where Rꢃct and Rct denote charge transfer resistance in the presence and absence of
inhibitors. At 303 K, the maximum efficiency is obtained for 200 ppm concentration
of the inhibitor. Analysis of EIS data for 308 and 313 K showed that the Rct values
increased only up to 100 ppm concentration for all the three acid concentrations.
When the concentration was increased beyond 100 ppm, the Rct values began to
decrease. Thus, the maximum efficiency at 308 and 313 K was obtained for 100 ppm
concentration of the inhibitor. EIS study showed that, at all the temperatures, the
efficiency of the inhibitor was maximum in 0.5 M concentration of HCl. The effi-
ciency of the inhibitor was found to decrease with increase in the acid concentration.
Another observation made from the EIS study was that the efficiency of the inhibitor
decreased with increasing temperature. This leads to the conclusion that inhibition was
achieved by the adsorption of the inhibitor on the mild steel surface. As the temper-
ature was increased, the adsorption of the inhibitor on to the mild steel surface became
difficult which caused the inhibition efficiency to decrease.
Potentiodynamic polarization study
Measurements of current–potential values under carefully controlled conditions can
yield information on corrosion rates (CR), coatings, films, passivity, pitting
tendency and other important phenomena. The generation of polarization curves
continues to be important in aqueous corrosion research. In addition, polarization
curves are useful in identifying the nature of inhibition process. Thus changes
observed in the polarization curves after inhibitor addition are usually used as
criteria to classify the inhibitor as cathodic, anodic or mixed [29].
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 inhibitor ranging from 50 to 200 ppm at
temperatures 303, 308, and 313 K are shown in Figs. 6, 7, and 8. The result
obtained from the study is shown in Tables 2, 3, and 4.
At room temperature, a decrease in CR with increasing concentration of the
inhibitor was observed. The %IE was calculated in each case using the following
equation:
i0corr ꢁ icorr
%IE ¼
ꢂ 100;
ð4Þ
i0corr
where icorr and i0corr denote corrosion current density in the presence and absence of
inhibitor.
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Electroanalytical and computational studies of EMBA
Fig. 6 Plot of change in charge
(a)
transfer resistance (Rct) with
respect to concentration of
inhibitor (EMBA) for 0.5 N HCl
at a 303 K, b 308 K, and
c 313 K
700
600
500
400
300
200
100
0
0
50
100
150
200
Cinh (ppm)
(b)1000
800
600
400
200
0
0
50
100
150
200
Cinh (ppm)
(c)
250
200
150
100
50
0
0
50
100
150
200
Cinh (ppm)
123

A. Joseph, R. Mohan
Fig. 7 Potentiodynamic polarization plots for EMBA in 0.5 M HCl in mild steel at a 303 K, b 308 K,
and c 313 K
123

Electroanalytical and computational studies of EMBA
Fig. 8 Potentiodynamic polarization plots for EMBA in 1 M HCl in mild steel at a 303 K, b 308 K, and
c 313 K
123

A. Joseph, R. Mohan
123

Electroanalytical and computational studies of EMBA
123

A. Joseph, R. Mohan
123

Electroanalytical and computational studies of EMBA
On analyzing the data for 303 K, it can be seen that, as the concentration of the
inhibitor was increased, the inhibition efficiency also increased. This indicated that
the inhibitors resist corrosion by getting adsorbed onto the metal surface. Thus, the
metal surface will remain protected from the action of the corrodent. At 303 K,
maximum efficiency was obtained for 200 ppm in all the three concentrations of
HCl. At higher temperatures, the inhibition efficiency began to decrease beyond a
certain concentration of the inhibitor. This is because of the difficulty in the
adsorption of higher concentrations of inhibitors at higher temperatures. At 308 and
313 K, the CR decreased up to 100 ppm. Further increase in concentration of the
inhibitor resulted in increase in CR. Thus, at 308 and 313 K, maximum efficiency
was obtained for 100 ppm in all the three concentrations of HCl. Potentiodynamic
polarization study showed that the efficiency of the inhibitor decreased with
increasing concentration of HCl and with increasing temperature. The results
obtained from the potentiodynamic study were in agreement with those obtained
from the EIS study.
On analyzing the potentiodynamic polarization plots, it can also be seen that the
corrosion potential did not follow any particular shift towards the anodic or cathodic
region as a result of the inhibitor addition. Moreover, there was change in both the
anodic and cathodic polarization curves on inhibitor addition. From these findings, it
can be concluded that the inhibitor under investigation acts as mixed-type inhibitors.
Adsorption study
The adsorption isotherm gives a clear picture about the adsorption behavior of
organic inhibitors and the mechanism of corrosion inhibition. It also yields
information about the interaction between the inhibitor molecules and the metal
surface. Several adsorption isotherms were attempted to fit the degree of surface
coverage values, h (%IE/100) to adsorption isotherms including Frumkin, Temkin,
Freundlich, and Langmuir isotherms. The h values for various concentrations of
inhibitors in acidic media have been evaluated from EIS measurements. The best fit
was obtained in case of the Langmuir isotherm which assumes that the solid surface
contains a fixed number of adsorption sites and each site holds one adsorbed species
[30, 31]. The plot of Cinh/h vs. Cinh (Fig. 9) yields a straight line with R2 value close
to unity. Thus, it can be said that the adsorption of EMBA from HCl solution (0.5–
1.5 M) on the mild steel surface obeys the Langmuir adsorption isotherm which can
be represented by the equation:
Cinh
1
¼
þ Cinh
;
ð5Þ
h
Kads
where Cinh is the concentration of the inhibitor and Kads is the equilibrium
constant for the adsorption–desorption process. The free energy of adsorption DGads
was calculated using the equation:
DGads ¼ ꢁRT lnð55:5 Kads
Þ
ð6Þ
Kads obtained from Eq. (5) was substituted in Eq. (6) to determine DGads. From
Tables 5 and 6, it can be seen that the Kads values are relatively high. This shows the
123

A. Joseph, R. Mohan
Fig. 9 Potentiodynamic polarization plots for EMBA in 1.5 M HCl in mild steel at a 303 K, b 308 K,
and c 313 K
123

Electroanalytical and computational studies of EMBA
Table 5 Adsorption parameters for different concentrations of EMBA in different concentrations of HCl
at different temperatures
Conc. of Conc. of
HCl (M) EMBA (ppm)
Temperature (K)
303
308
313
Kads
(104 M-1
-DGads Kads
(kJ/mol) (104 M-1
-DGads Kads
(kJ/mol) (104 M-1
-DGads
(kJ/mol)
)
)
)
0.5
50
100
150
200
50
11.05
13.40
11.97
9.80
0.47
1.58
2.49
4.85
1.06
1.23
2.45
2.76
39.37
39.86
39.57
39.07
31.40
34.48
35.60
37.30
33.46
33.84
35.58
35.88
0.68
2.20
0.72
0.53
0.18
1.14
0.11
0.08
1.07
1.06
0.46
0.01
32.89
35.89
33.03
32.24
29.50
34.21
28.31
27.34
34.04
34.03
31.90
22.32
0.63
1.38
0.71
0.18
0.21
0.59
0.08
0.05
0.78
0.44
0.22
0.01
33.20
35.27
33.50
29.96
30.30
33.04
27.80
26.80
33.76
32.26
30.53
22.90
1
100
150
200
50
1.5
100
150
200
Table 6 Slope and R2 value of
Langmuir adsorption plots
Conc. of HCl (M)
Slope
R2
obtained for EMBA in different
concentrations of HCl at 30 °C
0.5
1
1.00
0.73
0.91
0.99
0.96
0.99
1.5
high adsorption ability of the inhibitor on the mild steel surface [32]. It can also be
seen from the table that DGads values are negative. This indicates the spontaneity of
adsorption and the stability of adsorbed layer on the mild steel surface [33, 34]. It is
said that the values of DGads obtained around -20 kJ/mol or lower indicate
physisorption whereas the values around -40 kJ/mol indicate chemisorption [35–
37]. The adsorption phenomena of an organic molecule cannot be considered as
purely physical or chemical adsorption phenomena [38, 39]. The adsorption of the
inhibitor on to the mild steel surface may involve both physisorption and
chemisorption. From electrochemical studies, it has been found that %IE decreased
with increasing temperature. This can be explained by considering the point that the
adsorption of the inhibitor on the mild steel surface is not effective at higher
temperatures.
Computational study
Various quantum chemical parameters of the inhibitor were calculated using the
Gaussian 03 program package. The energy of the highest occupied molecular orbital
123

A. Joseph, R. Mohan
Fig. 10 Plot of change in
corrosion current (icorr) with
respect to concentration of
inhibitor (EMBA) for 0.5 N HCl
at a 303 K, b 308 K, and
c 313 K
(a)
200
150
100
50
0
0.0
0.5
0.05
0.1
1.0
1.5
2.0
2.5
3.0
icorr (mA/cm2)
(b)
200
150
100
50
0
0.00
0.10
0.15
0.20
0.25
0.30
icorr (mA/cm2)
(c)
200
150
100
50
0
0.0
0.2
0.3
0.4
0.5
0.6
0.7
i
corr (mA/cm2)
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Electroanalytical and computational studies of EMBA
(EHOMO) is -5.813 eV, the energy of the lowest unoccupied molecular orbital
(ELUMO) is -0.1706 eV,the energy gap (DE) which is nothing but ELUMO–EHOMO is
5.642 eV. From the literature, it was seen that the other benzimidazole derivatives
such as 2-mercaptobenzimidazole and 2-methylbenzimidazole had DE values 5.16
and 5.47 eV, respectively [40]. The dipole moment of the inhibitor was also
calculated and the value is 5.3819 D. The optimized geometry of the inhibitor
molecule was also obtained (refer Fig. 1). HOMO and LUMO of the inhibitor
molecule are given in Fig. 10a, b.
It is generally said that the efficiency of inhibitors increases with decreases in the
energy gap, DE, increases in EHOMO, and decreases in ELUMO [41]. Higher EHOMO
values enhance the adsorption of inhibitors thereby increasing the inhibition
Table 7 Fukui functions for
EMBA
Atoms
f-
f?
C(1)
0.1326
0.0861
0.0766
0.2370
0.0117
0.1434
0.1453
0.0000
0.0000
0.0000
0.0000
0.0165
0.1231
0.0050
0.0097
0.0095
0.0000
0.0005
0.0005
0.0002
0.0012
0.0007
0.0002
0.0001
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0073
0.0064
0.2031
0.1475
0.0204
0.2226
0.1957
0.0000
0.0000
0.0000
0.0000
0.0251
0.0445
0.0075
0.0153
0.0176
0.0000
0.0017
0.0000
0.0049
0.0102
0.0048
0.0015
0.0004
0.0001
0.0002
0.0000
0.0000
0.0000
0.0000
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
H(8)
H(9)
H(10)
H(11)
N(12)
N(13)
C(14)
H(15)
H(16)
H(17)
C(18)
H(19)
H(20)
C(21)
O(22)
O(23)
C(24)
H(25)
H(26)
C(27)
H(28)
H(29)
H(30)
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A. Joseph, R. Mohan
Fig. 11 Langmuir adsorption
isotherms for EMBA in mild
steel exposed to a 0.5 M, b 1 M,
and c 1.5 M concentrations of
HCl
(a)
340
320
300
280
260
240
220
200
180
160
140
120
100
80
303 K
308 K
313 K
60
40
40
60
80
100 120 140 160 180 200 220
Cinh (ppm)
(b) 650
303 K
308 K
313 K
600
550
500
450
400
350
300
250
200
150
100
40
60
80
100 120 140 160 180 200 220
Cinh (ppm)
(c)
2500
303 K
308 K
313 K
2000
1500
1000
500
0
40
60
80
100 120 140 160 180 200 220
Cinh (ppm)
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Electroanalytical and computational studies of EMBA
efficiency. Smaller ELUMO values will increase the probability of molecules to
accept electrons. When DE values are small, only small energy is required to
remove electrons from HOMO.
Condensed Fukui indices of EMBA were calculated and the results are given in
Table 7. The condensed Fukui functions allow us to distinguish each part of the
molecule on the basis of its distinct chemical behavior due to the different
substituent functional groups [42]. The site of nucleophilic attack will be the place
where the value of f? is maximum. The electrophilic attack will be maximum at a
site where f- value is maximum. From the table, we can see that, in the case of
EMBA, the f? value is maximum for C(6). Hence, this is the site for nucleophilic
attack. The f- value is maximum for C(4). Hence, this is the site for electrophilic
attack (Figs. 11, 12).
Scanning electron microscopy
The metal samples were dipped in different concentrations of HCl, for 6 h, both in
the presence and absence of the inhibitor EMBA. From analyzing the SEM images
Fig. 12 a HOMO and b LUMO of EMBA
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A. Joseph, R. Mohan
Fig. 13 SEM images of a plain mild steel sample, b mild steel sample in 0.5 M HCl, c mild steel sample
in 0.5 M HCl in presence of 200 ppm concentration of EMBA, d mild steel sample in 1 M HCl, e mild
steel sample in 1 M HCl in the presence of 200 ppm concentration of EMBA, f mild steel sample in
1.5 M HCl, and g mild steel sample in 1.5 M HCl in the presence of 200 ppm concentration of EMBA
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Electroanalytical and computational studies of EMBA
Fig. 14 AFM images of a plain mild steel sample, b mild steel sample in 0.5 M HCl, c mild steel sample
in 0.5 M HCl in presence of 200 ppm concentration of EMBA, d mild steel sample in 1 M HCl, e mild
steel sample in 1 M HCl in the presence of 200 ppm concentration of EMBA, f mild steel sample in
1.5 M HCl, and g mild steel sample in 1.5 M HCl in the presence of 200 ppm concentration of EMBA
123

A. Joseph, R. Mohan
Fig. 14 continued
(Fig. 13), it can be seen that the inhibitor has been adsorbed onto the mild steel
surface forming a protective coating. Thus, it can be concluded that the inhibitor
protects the mild steel from corrosion by getting adsorbed onto the mild steel
surface.
Atomic force microscopy
The three dimensional AFM images are given in Fig. 14a–g. Figure 14a displays the
surface topography of a plain mild steel surface for which the average roughness
value (Ra) was obtained as 7.9 nm. Figure 14b displays the surface topography of a
mild steel surface immersed in 0.5 M HCl for which the Ra value was obtained as
57.9 nm. Figure 14c displays the surface topography of a mild steel surface
immersed in 0.5 M HCl in the presence of 200 ppm EMBA for which the Ra value
was obtained as 10.6 nm. Figure 14d displays the surface topography of a mild steel
surface immersed in 1 M HCl for which the Ra value was obtained as 77 nm.
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Electroanalytical and computational studies of EMBA
Fig. 14 continued
Figure 14e displays the surface topography of a mild steel surface immersed in 1 M
HCl in the presence of 200 ppm EMBA for which the Ra value was obtained as
18 nm. Figure 14f displays the surface topography of a mild steel surface immersed
in 1.5 M HCl for which the Ra value was obtained as 87 nm. Figure 14g displays
the surface topography of a mild steel surface immersed in 1.5 M HCl in the
presence of 200 ppm EMBA for which the Ra value was obtained as 19 nm. Thus,
from the AFM study, it can be clearly understood that the surface roughness of the
mild steel surface has been decreased in the presence of the inhibitor. This shows
that EMBA acts as an effective inhibitor for the corrosion of mild steel in HCl.
Conclusion
Electrochemical measurements such as EIS and potentiodynamic polarization were
used to study the corrosion inhibition efficiency of different concentrations of
123

A. Joseph, R. Mohan
EMBA in various concentrations of HCl ranging from 0.5 to 1.5 M at temperatures
303, 308, and 313 K. Based on this study, it can be concluded that
(i) In the presence of EMBA, there is effective reduction in corrosion of mild steel
exposed to HCl solution.
(ii) The inhibition efficiency of EMBA increases with increasing concentration at
room temperature. The efficiency was found to decrease with increasing
concentration at higher temperatures. For 308 and 313 K, the maximum
efficiency was obtained for 100 ppm concentration of EMBA. The inhibition
efficiency of EMBA was found to decrease with increasing concentration of
HCl solution. EMBA showed maximum efficiency in 0.5 M HCl solution at
room temperature.
(iii) The mode of inhibition of EMBA was through adsorption onto the mild steel
surface. The mode of adsorption was believed to be both physisorption and
chemisorption. The adsorption obeyed the Langmuir adsorption isotherm.
(iv) Various quantum chemical parameters as well as optimized geometry of the
inhibitor were calculated. The Fukui indices were also calculated.
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