Effect of optimized structure and electronic properties of some benzimidazole derivatives on corrosion inhibition of mild steel in hydrochloric acid medium: Electrochemical and theoretical studies

Article abstract of DOI:10.1007/s12039-015-0850-x

Abstract The corrosion inhibitive action of a few benzimidazole derivatives namely 2-(benzamido) ethylbenzimidazole (BAEBI), 2-(β-benzenesulphonamido) ethylbenzimidazole (BSAEBI), 2-(benzamido) methylbenzimidazole (BAMBI) and 2-(β-benzenesulphonamido) met

Full text of DOI:10.1007/s12039-015-0850-x

c
J. Chem. Sci. Vol. 127, No. 5, May 2015, pp. 921–929. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0850-x
Effect of optimized structure and electronic properties of some
benzimidazole derivatives on corrosion inhibition of mild steel in
hydrochloric acid medium: Electrochemical and theoretical studies
ALOKDUT DUTTA, SUJIT SANKAR PANJA, M M NANDI and DIPANKAR SUKUL∗
Department of Chemistry, National Institute of Technology Durgapur, Durgapur 713 209, India
e-mail: dipankar.sukul@gmail.com
MS received 30 July 2014; revised 26 November 2014; accepted 6 January 2015
Abstract. The corrosion inhibitive action of a few benzimidazole derivatives namely 2-(benzamido) ethyl-
benzimidazole (BAEBI), 2-(β-benzenesulphonamido) ethylbenzimidazole (BSAEBI), 2-(benzamido) methyl-
benzimidazole (BAMBI) and 2-(β-benzenesulphonamido) methylbenzimidazole (BSAMBI), towards mild
steel in hydrochloric acid has been studied using potentiodynamic polarization and electrochemical impedance
spectroscopic (EIS) methods. The results show that these compounds get adsorbed on the mild steel surface
following Temkin adsorption isotherm, and act as mixed-type inhibitors. The inhibition efficiencies are found
to follow the order, BAEBI > BSAEBI > BAMBI > BSAMBI. This observation is explained in terms of
chain length, relative effects of amido and sulphonamido groups, possible structural factors, spatial orienta-
tions, energy gap between the frontier molecular orbitals, different intrinsic molecular parameters, like, global
hardness and softness, and number of electrons transferred.
Keywords. Adsorption; corrosion inhibition; electrochemical techniques; quantum mechanical calculations.
1. Introduction
In the present study, we have used four different
benzenesulphonamido and benzamido derivatives of BI
(table 1) to investigate their corrosion inhibitory
properties for mild steel in 1 M HCl. Sulphonamides
and benzamides are also known for their antimicrobial
properties.18–20 Our main endeavour is to correlate the
corrosion inhibition potential with the structure and
molecular parameters of BI derivatives where benzene-
sulphonamido and benzamido groups are separated
from the BI moiety by one and two methylene groups.
Corrosion inhibition efficiency of these compounds is
determined using different electrochemical techniques,
like potentiodynamic polarization and electrochemi-
cal AC impedance. Structure effect of the inhibitors
towards corrosion inhibition of mild steel is evaluated
by quantum chemical calculations and comparing var-
ious molecular parameters, such as energy optimized
geometry, HOMO and LUMO energy levels and other
molecular parameters, which include global hardness
and softness, and number of electron transferred.
Organic inhibitors including aromatic heterocycles con-
taining sulfur, oxygen and nitrogen are effectively used
to minimize the aggressiveness of acids towards corro-
sion of metals. Replacing the surface adsorbed water
molecules, these inhibitors get adsorbed on the metal
surface, and thereby diminish the rate of either cathodic
reduction reaction, or anodic metal dissolution reaction,
or both. Various approaches have been made to corre-
late the extent of adsorption, and subsequent inhibitory
action with molecular parameters of the inhibitors.
These include the presence of lone pair of electrons
on heterocyclic atoms, chain length (i.e., molecular
volume), presence of unsaturated bond, aromatic substi-
tuent group, planarity of heterocycles, steric factor,
HOMO-LUMO energy gap of the molecules, and other
molecular parameters.1–12
Several classes of N-heterocyclic compounds includ-
ing benzimidazole (BI) and its derivatives have drawn
considerable attention as acid corrosion inhibitors,
mainly due to their significant corrosion inhibition effi-
ciency, good chelating ability, relatively higher solubil-
ity in acid media and also their environment-friendly
properties.13–17
2. Experimental
2.1 Synthesis of inhibitors
All the inhibitors were prepared in simple two steps
reactions as per reported procedure.21–23 In short, for
∗For correspondence
921

922
Alokdut Dutta et al.
Table 1. Structures and characteristics of benzimidazole derivatives.
Structure and
abbreviation
Characteristic
IR peaks(cm−1
)
Molecules
Melting point
N
CH2CH2NHCO
N
H
2-(benzamido)
187◦C
132◦C
1586(C=N) 1637 (C=O) 3300 (N-H)
1541 (C=N) 1339 (S=O) 3388 (N-H)
BAEBI
ethylbenzimidazole
N
CH2CH2NHSO2
N
H
2-(β-benzenesulphonamido)
ethylbenzimidazole
BSAEBI
N
CH2NHCO
N
H
2-(benzamido)
methylbenzimidazole
152◦C
142◦C
1578 (C=N) 1623 (C=O) 3335 (N-H)
1587 (C=N) 1366 (S=O) 3303 (N-H)
BAMBI
N
CH2NHSO2
N
H
2-(β-benzenesulphonamido)
methylbenzimidazole
BSAMBI
preparing BSAEBI and BSAMBI, benzenesulfonyl (1 M) solution was prepared using 35% HCl (GR grade,
chloride was added drop wise to the vigorously stirred Merck India).
aqueous solution of glycine and alanine, respectively
(1:1 mole ratio), maintaining pH of the solution at 9.
2.3 Electrochemical studies
After complete addition, the mixture was stirred further
for 15 min followed by the addition of 4.5M H2SO4 to
adjust pH at 2. The precipitate was filtered out, washed
and recrystallized from aqueous ethanol mixed solvent.
It was then refluxed with o-phenylenediamine (1:1 mole
ratio) for 3 h in 2M H2SO4. Mixture was cooled at
room temperature and pH was adjusted to 9 by adding
10% NaOH and 7M NH3 (aq). The final compound
which precipitated, was filtered, washed and recrystal-
lized from aqueous ethanol mixed solvent. For prepar-
ing BAEBI and BAMBI, same procedure was adopted
with the use of benzoyl chloride, and the final pH was
adjusted to 6–7. These compounds were characterized
by FT-IR (Thermo-Nicolet iS10 IR spectrometer in the
range 4000–400 cm−1) and melting point measurement
(tabulated in table 1).21,22
Potentiodynamic polarization and electrochemical
impedance measurement were done by a conventional
three-electrode system (model: Gill AC, ACM Instru-
ments, UK) consist of mild steel as a working electrode
(WE) with exposed area of 0.25 cm2, large area plat-
inum foil as counter electrode and saturated calomel
electrode (SCE) as the reference. Before measure-
ments, the WE was kept in contact with test solution
(200 mL) for 45 min to achieve a steady state (by
observing variation of potential and current with time).
The potential sweep rate for potentiodynamic polariza-
tion curves was 0.5 mV/sec. Corrosion current density
(icorr) was determined from the intercept of the extrap-
olated cathodic and anodic Tafel lines at the corrosion
potential (Ecorr). Electrochemical impedance measure-
ments were performed in the frequency range 10 mHz
to 100 kHz with a.c. amplitude of 10 mV (r.m.s.) at
the open circuit potential (OCP). All the experiments
were done at around the room temperature of 30◦C.
2.2 Specimens and solution
Cylindrical specimens were prepared by cutting com-
mercially available mild steel rod (wt% composition:
0.22 C, 0.31 Si, 0.60 Mn, 0.04 P, 0.06 S and the remain-
der iron). Prior to each experiment, the surfaces were
pre-treated by grinding on a belt grinding polishing
2.4 Theoretical calculations
machine followed by metallurgical grade emery papers All the quantum chemical ground electronic state calcu-
of 400–1600 grit. Then the samples were degreased lations in gas phase were performed using Gaussian09
with ethanol and rinsed with bi-distilled water. HCl program.24 Geometry optimization and determination

Benzimidazole derivatives as corrosion inhibitors
923
of energy of frontier molecular orbitals for all the 3. Results and Discussion
molecules have been carried out considering density
3.1 Polarization studies
functional theory (DFT) at the B3LPY level through 6-
31++g(d,p) basis set. The frequency calculations are
done with the same basis set. All the frequencies are
found to be positive, which reflects that the ground state
optimized structures correspond to global minima. In
addition to the energies of the highest occupied molec-
ular orbital (EHOMO), and lowest unoccupied molecu-
lar orbital (ELUMO), other molecular parameters of the
inhibitors, like electronegativity (χ), global hardness
(η) and softness (σ), electron affinity (A), ionization
potential (I), and number of electrons transferred (ꢀN)
have been calculated.
Potentiodynamic polarization curves for mild steel in
1M HCl and in presence of the inhibitors used having
1 mM concentration are shown in figure 1 (a–e). Vari-
ations of cathodic and anodic currents with potential
for all the four inhibitors with 0.1 mM and 1 mM con-
centrations are shown in figure S1 (a–d). Values of the
electrochemical corrosion parameters, such as corro-
sion potential (Ecorr), cathodic Tafel slope (bc), anodic
Tafel slope (ba), and corrosion current density (icorr) for
all the inhibitors having concentration range of 0.1 to 1
mM are given in table 2. The percentage inhibition effi-
ciency ηP(%) is calculated from the following equation:
icorr − icorr(inh)
ηP(%) =
× 100
(1)
icorr
where, icorr and icorr(inh) are the values of corrosion cur-
rent density of uninhibited and inhibited specimens,
respectively. A gradual increment in corrosion inhibi-
tion efficiency is seen with increase in inhibitor concen-
tration. Figure S1 (a–d) clearly shows that with increase
in inhibitor concentration, both the cathodic and anodic
current decreases. Furthermore, no regular variation of
Ecorr values with concentration of the inhibitors can
be observed (table 2). These observations suggest that
the compounds behave as mixed type inhibitors which
reduce both the rate of cathodic and anodic reactions
by blocking the respective reaction sites on the metal
surface.
Figure 1. Potentiodynamic polarization curves for mild
steel in 1 M HCl in the presence of BI derivatives having 1
mM concentration.
Table 2. Data from potentiodynamic polarization studies for mild steel in 1M HCl in various
inhibitor systems.
System Conc. (mM) −Ecorr (mV/ SCE) icorr (μA cm−2) ba (mVdec−1) −bc (mVdec−1) ηp(%)
BLANK
BAEBI
491
489
486
500
494
495
471
483
478
497
471
486
501
496
476
470
481
1200
525
148
123
66
685
231
156
102
787
273
179
110
835
362
240
207
65
77
80
91
80
81
75
85
71
70
86
82
92
85
82
90
77
77
107
104
129
119
106
119
112
105
100
112
116
136
109
105
124
121
0.10
0.75
0.90
1.00
0.10
0.75
0.90
1.00
0.10
0.75
0.90
1.00
0.10
0.75
0.90
1.00
56
88
90
95
43
81
87
92
34
77
85
90
30
70
80
83
BSAEBI
BAMBI
BSAMBI

924
Alokdut Dutta et al.
It is observed that when benzenesulphonamido or an indicative parameter proportional to the capacitance
benzamido groups are attached to the BI moiety through of the double layer formed at the metal surface for 0>
two methylene groups, inhibition efficiencies are higher n>1. For whole numbers of n = 1, 0, −1, CPE is
than the corresponding compounds where one methy- reduced to the classical lumped elements capacitor (C),
lene group acts as a separator. This is in agreement resistance (R), and inductance (L), respectively. The
with the general observation that inhibition efficiency fitted parameters using the above model are presented
increases with increase in chain length (i.e., molecu- in table 3.
lar volume).5 Between benzenesulphonamido and ben-
zamido substituted compounds, latter compounds show of Rct gradually increases with concentration, while,
It is seen in table 3 that for all the inhibitors, the value
higher corrosion inhibition efficiency.
the value of Q decreases. This confirms the adsorp-
tion of inhibitors on the metal surface which provide a
corrosion inhibitive layer.
The percentage inhibition efficiencies ηZ(%) in terms
of Rct are calculated using eq. 3.
3.2 Electrochemical impedance measurements
The corrosion behaviour of mild steel in HCl in pres-
ence of inhibitors was investigated by EIS. Correspond-
ing Nyquist plots for different inhibitor systems in 1mM
concentration are given in figure 2. Bode plots are
shown in figure S2 (a–d). From Bode magnitude and
phase plots, it is observed that the metal-electrolyte
interface in the present study is characterized by a sin-
gle time constant system. Comparing the plots for all
the inhibitor systems having concentration in the range
of 0.1 mM to 1 mM, it is seen that diameter of the capac-
itive loop increases with increase in concentration of
all the inhibitors suggesting the formation of protective
surface film. All the spectra have depressed semi-circle
with centre under the real axis, which may be attributed
to the roughness and inhomogeneities of solid surface.
To analyse those, a model consisting of parallel combi-
nation of charge transfer resistance-constant phase ele-
ment, whichisinserieswithsolutionresistance, Rs[Rct−
CPE] (figure S3), is used in the present study.25–27 The
impedance of CPE is given by
Rct − Rc0t
ηZ(%) =
× 100
(3)
Rct
where, Rct and Rc0t are the values of charge transfer
resistance observed in the presence and absence of
inhibitor, respectively. Following Rct values, ηZ(%) are
seen to follow the order BAEBI > BSAEBI > BAMBI
> BSAMBI, i.e. the same trend as observed in the
polarisation measurements.
3.3 Adsorption isotherm
To demonstrate the adsorption characteristics of the
BI derivatives on mild steel surface, and to determine
the free energy of adsorption, we have assessed sev-
eral types of adsorption isotherms. Temkin adsorp-
tion isotherm, which is based on the consideration of
Table 3. Impedance parameters for the corrosion of mild
steel in 1M HCl in various inhibitor systems.
ZCPE = Q−1(iω)−n
(2)
Conc.
Rct
(mM) (ꢁ cm2)
Q (μꢁ−1 sn
where, Q is a proportionality coefficient, ω is the angu-
lar frequency, n is a measure of surface irregularity. Q is
System
cm−2
)
n
ηz(%)
BLANK
BAEBI
6.3
593
457
130
122
119
816
405
282
172
885
751
428
114
798
725
558
304
0.85
0.84
0.83
0.83
0.82
0.75
0.77
0.80
0.75
0.75
0.70
0.72
0.85
0.78
0.72
0.74
0.81
0.1
0.75
0.9
1.0
0.1
0.75
0.9
1.0
0.1
0.75
0.9
1.0
0.1
0.75
0.9
1.0
23
143
164
237
20
72
95
96
97
67
91
94
96
56
88
93
95
40
85
90
93
6
BAEBI
BSAEBI
BAMBI
BSAMBI
1M HCl
4
100
13.2 HZ
2
0
80
2
4
6
8
10
25.4 Hz
BSAEBI
BAMBI
BSAMBI
8.56 Hz
60.8 Hz
80
60
40
20
0
2.88 Hz
128
185
15
5.36 Hz
2.32 Hz
13.2 Hz
4.45 Hz
2.36 Hz
52
0.326 Hz
200
0.504 Hz
250
107
149
11
45
66
0.78 Hz
50
0
100
150
2
Z' / . cm
Figure 2. Nyquist plots for mild steel in 1M HCl in the
104
presence of BI derivatives (1mM).

Benzimidazole derivatives as corrosion inhibitors
925
spontaneous adsorption of the inhibitor molecules on
mild steel surface in acid medium.30–33 BAEBI is seen
to have the maximum potentiality of adsorption, while
BSAMBI having the least.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
It is proposed in literature that BI derivatives may
interact with metal surface in acid medium in two
ways.15 Either the neutral form of the derivatives may
directly interact with metal surface (i.e., electron trans-
fer from the inhibitors to the positively charges metal
surface), or protonated form of the derivatives can be
adsorbed on the metal surface through pre-adsorbed Cl−
ions. But, it is well-known that for 3d metal ions (parti-
cularly for Fe2+, Cu2+, Ni2+, Co2+), the formation con-
stant of chloro-aqua complex is very low corresponding
to hexa-aqua ions.34 In 1M HCl, the predominant chloro
complex for Fe3+ is [FeCl(H2O)5]2+. Anionic chloro
complex of Fe3+ (i.e. FeCl−4 ) is only stable in concen-
trated HCl.34 On the other hand, BI ligands in 1M HCl
mostly present in protonated form (pKa value of the
protonated BI and its derivatives being within 4–5).35,36
However, in presence of metal ions, BI ligands should
prefer to bind with more positively charged metal ions
over singly positively charged protons. This is sup-
ported from the observation that formation constant for
BI derivatives-Metal (II) complexes is much higher than
that for protonated BI derivatives.35,36 Hence, it may be
presumed that during adsorption, H+ from the proto-
nated inhibitor and Cl− from the metal surface will be
displaced as HCl and complexes involving the neutral
inhibitor molecules and metal ions will be formed at the
metal surface.
BAEBI
BSAEBI
BAMBI
BSAMBI
-9.5
-9.0
-8.5
-8.0
ln (C/M)
-7.5
-7.0
-6.5
Figure 3. Temkin adsorption plots for mild steel in 1M
HCl in presence of benzimidazole derivatives.
the heterogeneity of the surface, is found to be best
suited for fitting of the experimental obtained results
(figure 3, table 4) as evident from the minimum value
of correlation coefficient, r2 = 0.995. According to
ꢀ
ꢁ
ηZ(%)
this model, degree of surface coverage θ θ =
is
related to the concentration of the inhibitor (C)1b00y the
eq. 4:28,29
exp(f θ) = KadsC
(4)
where, Kads is the constant of adsorption and f is the
interaction parameter. Plotting θ versus ln C and from
its slope and intercept, the values of f and Kads have
been obtained (figure 3, table 4). The interaction param-
eter, f is found to be positive and greater than 1 for all
the inhibitors, which signifies the existence of molecu-
lar interaction in the adsorbed layer as well as energetic
heterogeneity of the surface.28,29
3.4 Quantum chemical study and mechanism of
inhibition action
From the values of the adsorption constant, Kads, the
standard free energy of adsorption, ꢀG0ads for all the
inhibitors are determined using the following equation:
In general, electron donating ability of a molecule
increases with increase in EHOMO, whereas, a lower
value of ELUMO indicates the molecule to be more sus-
ceptible towards accepting electrons.37,38 Thus, adsorp-
tion of an inhibitor and its corrosion inhibition effi-
ciency is characterized by increased EHOMO, decreased
ELUMO and a low value of the energy gap ꢀE =
ELUMO − EHOMO. Electron distribution in the optimized
HOMO and LUMO energy levels of the inhibitors are
shown in figure 4 and the corresponding energy values
are tabulated in table 5. It is seen that EHOMO values
increase linearly with increase in their corresponding IE
values (with a correlation factor of 0.987) (figure 5). For
ELUMO, such a nice linear variation is absent. Though
ꢀE decreases with increase in inhibition efficiency, it
lacks sufficient linearity (correlation factor of 0.925)
(figure 5). All these observation indicate that adsorp-
tion of the inhibitors studied is mostly governed by the
ꢀG0ads = −RT ln(55.55Kads)
(5)
where, 55.55 is the molar concentration of water. ꢀGa0ds
at 303 K for all the four inhibitors are shown in table 4.
Negative values of ꢀG0ads for all the inhibitors suggest
Table 4. Adsorption parameters for benzimidazole deriva-
tives on mild steel in 1M HCl.
Sample
r2
f
Kads (dm3mol−1) −ꢀG0ads(kJ mol−1
)
BAEBI 0.997 9.09
BSAEBI 0.997 8.10
BAMBI 0.995 5.99
BSAMBI 0.999 4.39
7.01×106
2.26×106
2.85×105
5.76×104
49.8
46.9
41.7
37.7

926
Alokdut Dutta et al.
NAME Optimized structure
e- distribution in HOMO
e- distribution in LUMO
Figure 4. Optimized molecular structures of benzimidazole derivatives with HOMO and LUMO orbitals.
electron transfer from the HOMO of the inhibitors to inhibitors is not the same for all the inhibitor, which
the vacant metal d orbitals. Extent of any possible back is reflected in the observed variation of inhibition effi-
donation of metal 4s electrons to the LUMO of the ciency with ELUMO and ꢀE values. Above findings may
Table 5. The calculated quantum chemical parameters for benzimidazole derivatives with DFT method.
System
EHOMO (eV)
ELUMO (eV)
ꢀE (eV)
I (eV)
A (eV)
χ (eV)
η (eV)
σ (eV−1
)
ꢀN
BAEBI
−5.90
−6.00
−6.18
−6.41
−1.78
−1.83
−1.68
−1.64
4.12
4.17
4.50
4.77
5.90
6.00
6.18
6.41
1.78
1.83
1.68
1.64
3.84
3.915
3.93
2.06
2.085
2.25
0.485
0.48
0.44
0.42
0.24
0.22
0.20
0.17
BSAEBI
BAMBI
BSAMBI
4.02
2.38

Benzimidazole derivatives as corrosion inhibitors
927
These quantities are related to electron affinity (A) and
ionization potential (I) as follows:
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
-5.9
-6.0
-6.1
-6.2
-6.3
-6.4
-6.5
χ = (I + A)/2
(6)
η = (I − A)/2
(7)
σ = 1/η = 2/(I − A)
I = −EHOMO
(8)
(9)
92
94
96
98
(%) (1mM Conc.)
A = −ELUMO
(10)
Figure 5. Correlation of (a) EHOMO
(
) and (b) energy
gap (ꢀE = ELUMO − EHOMO) ( ) with percent inhibition
From these values, the number of electrons transferred
(ꢀN), from inhibitor molecules having lower elec-
tronegativity to the more electronegative iron surface
until the chemical potential becomes equalized, are
computed using the equation (table 5):
efficiency of benzimidazole derivatives (1 mM).
be interpreted in terms of the optimized geometry of
the inhibitor molecules. It is seen that HOMO energy
level of all the molecules is mostly distributed over
planar BI group, while LUMO energy level is on the
benzenesulphonamido or benzamido substituents. For
(χFe − χinh)
ꢀN =
(11)
2(ηFe + ηinh)
BAEBI, BSAEBI and BAMBI, strong H bond is seen where, χFe is taken as 4.82 eV/mol, equivalent to
to exist (1.9 Å to 2.12 Å) between the H atom attached the work function for Fe (110) surface and ηFe is 0
to N atom of BI moiety and O atom of carbonyl or eV/mol.43 Electronegativity values of all the inhibitor
sulfonyl group. This provides sufficient rigidity to the molecules are found to be lower than that of iron, sug-
molecules that BI plane and the aromatic plane of ben- gesting electron flow from the HOMO of the inhibitors
zenesulphonamido or benzamido groups are oriented towards the vacant 3d orbitals of Fe to be more signifi-
in such a fashion which is not conducive for simulta- cant than that from the filled 4s orbital of Fe to LUMO
neous two ways bonding, i.e., electron donation from of the inhibitors. Among benzenesulphonamido and
HOMO of inhibitors to the vacant iron 3d orbitals, benzamido substituted compounds, latter compounds
and the back electron transfer from metal 4s orbital to show lower χ, which may be attributed towards higher
LUMO energy levels of the inhibitors, with compara- electronegativity of S atom over C atom, as well as
ble extent. From the optimized geometry of BSAMBI, presence of more number of highly electronegative
which exhibits least corrosion inhibition efficiency, it O atoms in sulphonamido group. As a result, elec-
is observed that benzene of benzenesulphonamido moi- tron flow from benzamido substituted compounds is
ety is almost poised over the BI group. Such spatial potentially more favourable than the others, resulting
orientation is ideal for non-bonded aromatic π − π in higher inhibition efficiency. Also, it is seen from
interaction which reduces the π-electron density of the table 5 that inhibition efficiency increases when
BI moiety and subsequent reduction in the extent of global hardness decreases and global softness increases.
adsorption.39
Thus, with decrease in energy gap between the fron-
Following Koopmans’ theorem, different intrinsic tier molecular orbitals of the inhibitors, reactivity of
molecular quantities like electronegativity (χ, a mea- molecules towards charge transfer with metal atoms
sure of the power of a group of atoms to attract elec- increases. This is manifested in increasing number of
trons towards itself), global hardness (η, a parameter electron transferred (ꢀN). According to Lukovits’s
related to the resistance of an atom to a charge trans- study,44 if the value of ꢀN<3.6, the inhibition effi-
fer) and global softness (σ, which shows the reactivity ciency is increased with increasing electron donating
of the inhibitor molecules in terms of charge transfer), ability of inhibitor at the metal surface. For BSAMBI,
and number of electrons transferred (ꢀN), which are which has least inhibition efficiency, ꢀN is seen to
supposed to influence the overall reaction between two be 0.17, whereas for BAEBI having highest inhibition
interacting systems, have been calculated (table 5).40–42 efficiency, ꢀN is 0.24.

928
Alokdut Dutta et al.
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It is observed that different structural aspects like spa-
tial orientation, chain length, presence of hetero atoms
and energies of frontier molecular orbitals provide pro-
found influence on the adsorption and subsequent cor-
rosion inhibition by the BI derivatives used in this
present study. Inhibitors having higher chain length
(i.e., molecular volume) provide higher inhibition effi-
ciency. Between benzenesulphonamido and benzamido
substituted compounds, latter compounds show higher
corrosion inhibition efficiency, which may be attributed
to higher electronegativity of S atom over C atom, as
well as presence of higher number of highly electroneg-
ative O atoms in sulphonamido group. Inhibition effi-
ciency is also found to depend on the spatial orienta-
tion of the substituents relative to the BI moiety. All
the molecules are found to act as mixed type inhibitors.
From adsorption isotherm studies (Temkin adsorption
isotherm), it may be argued that inhibitors adsorb spon-
taneously on the mild steel surface. Quantum mechan-
ical calculations suggest that during chemical adsorp-
tion, electrons are transferred from HOMO energy level
of the molecules to the vacant d orbitals of iron. As
the electron density in the HOMO level is mostly dis-
tributed over the planar BI moiety, it is proposed that
inhibitors are adsorbed parallel to the metal surface
through the BI moiety. A linear correlation is found
to exist between the inhibition efficiencies and EHOMO
of the inhibitors. Other intrinsic molecular properties
based on the energies of frontier molecular orbitals also
explain quite satisfactorily the observed anti-corrosive
characteristics of the inhibitors.
21. Nandi M M and Ray R 1987 Ind. J. Chem. 26A 345
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Electrochim. Acta 51 1473
Supplementary Information
Supplementary material includes potentiodynamic
polarizarions curves for all the inhibitors with 1M and
0.1M concentrations (figures S1 a–d), Bode plots for all
the inhibitors with 1M concentration (figures S2 a–d),
and diagram of the equivalent circuit used to fit the EIS
data (figure S3) and the SEM images of the corroded
metal sample (figure S4 a–b).
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122 325
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Bentiss F 2013 Mater. Chem. Phys. 141 240
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Acknowledgements
DS thanks Department of Science and Technology, Govt.
of India for supporting a research project under Fast
Track Scheme for Young Scientists (no. SR/FT/CS-
110/2010).
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