Benzimidazole derivatives as novel inhibitors for the corrosion of mild steel in acidic solution: Experimental and theoretical studies

Article abstract of DOI:10.1016/j.molliq.2019.01.060

The inhibition effects of three kinds of benzimidazole derivatives (2-mercaptobenzimidazole (MBI), 2-thiobenzylbenzimidazole (TBBI) and 1-butyl-2-thiobenzylbenzimidazole (BTBBI)) on the corrosion of mild steel in 1 M HCl solution were studied by weight loss and electrochemical measurements, and theoretical calculations. It is found that these compounds act as mixed type inhibitors with predominant cathodic effectiveness and exhibit high inhibition efficiencies with the order: BTBBI > TBBI > MBI. Molecular dynamics simulations reveal that these benzimidazole derivatives adsorb on Fe (1 1 0) surface in flat orientation. The theoretical calculations are in good accordance with the weight loss and electrochemical measurements.

Full text of DOI:10.1016/j.molliq.2019.01.060

Accepted Manuscript
Benzimidazole derivatives as novel inhibitors for the corrosion of
mild steel in acidic solution: Experimental and theoretical studies
G.A. Zhang, X.M. Hou, B.S. Hou, H.F. Liu
PII:
S0167-7322(18)33908-4
DOI:
https://doi.org/10.1016/j.molliq.2019.01.060
MOLLIQ 10291
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
30 July 2018
22 December 2018
10 January 2019
Please cite this article as: G.A. Zhang, X.M. Hou, B.S. Hou, H.F. Liu , Benzimidazole
derivatives as novel inhibitors for the corrosion of mild steel in acidic solution:
Experimental and theoretical studies. Molliq (2019), https://doi.org/10.1016/
j.molliq.2019.01.060
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ACCEPTED MANUSCRIPT
Benzimidazole derivatives as novel inhibitors for the corrosion of
mild steel in acidic solution: experimental and theoretical studies
a,
a
a
a, b
G. A. Zhang
X. M. Hou
B. S. Hou
H. F. Liu
a
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of
Education, Hubei Key Laboratory of Materials Chemistry and Service Failure, School of
Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan
430074, P.R. China
b
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,
Wuhan 410074, P.R. China
Abstract: The inhibition effects of three kinds of benzimidazole derivatives
(2-mercaptobenzimidazole (MBI), 2-thiobenzylbenzimidazole (TBBI) and
1
-butyl-2-thiobenzylbenzimidazole (BTBBI)) on the corrosion of mild steel in 1 M
HCl solution were studied by weight loss and electrochemical measurements, and
theoretical calculations. It is found that these compounds act as mixed type inhibitors
with predominant cathodic effectiveness and exhibit high inhibition efficiencies with
the order: BTBBI > TBBI > MBI. Molecular dynamics simulations reveal that these
benzimidazole derivatives adsorb on Fe (1 1 0) surface in flat orientation. The
theoretical calculations are in good accordance with the weight loss and
electrochemical measurements.
Keywords: Mild steel; Inhibitors; Weight loss measurements; Electrochemical
measurements; Theoretical calculations

Corresponding author; Tel.: +86-27-87559068; Fax: +86-27-87543632
E-mail address: zhangguoan@gmail.com (G.A. Zhang)
1

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1
. Introduction
Mild steels are extensively used in industry because of their good mechanical
performances and relatively low cost [1-3]. However, their poor corrosion resistances
in aggressive environments hinder their practical applications in industrial production.
During the oil well acidification and industrial pickling process, severe corrosion of
mild steels will occur in the acidic solutions. Adding corrosion inhibitors is an
effective method to inhibit the corrosion of mild steels [4-11].
It is suggested [12-15] that the organic compounds containing heteroatoms (O, N,
S) possess lone pairs of electrons, which makes them particularly effective as
corrosion inhibitors. The adsorptions of these organic compounds via heteroatoms
block the active sites and then inhibit the corrosion of metals. The inhibition effects of
the inhibitors with different heteroatoms increase in the order of O < N < S.
Furthermore, the multiple bonds or aromatic rings in the organic compounds can
provide excellent inhibition performance. The inhibition efficiencies of inhibitors vary
with their molecular structures and sizes, heteroatoms, adsorptive tendencies, and the
surface charges of metals [5, 16-20].
It has been found that many organic compounds are effective on the inhibition of
metal corrosion. Unfortunately, most of these organic inhibitors are toxic and their use
is restricted due to the environmental protection. It has been demonstrated that
benzimidazole and its derivatives have low toxicity and show the good inhibition
effects on the corrosion of metals in acidic media [4-6, 16, 21-28]. For benzimidazole
molecules, the N atom and aromatic ring are the suitable sites for bonding to metal
2

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surface [21, 29-33]. The inhibition effects of inhibitors on metal corrosion are
dependent upon their chemical structures. Consequently, great efforts have been
devoted to studying the inhibition performances of the inhibitors with different
substituent groups and substituent positions. Zhang et al. [34] evaluated the inhibition
performances of five kinds of imidazole inhibitors with different lengths of alkyl
chains. They pointed out that the long alkyl chain, with good hydrophobic property,
can form a compact hydrophobic film on the metal surface with a high coverage,
which could effectively prevent the mild steel corrosion. Moreover, it has been
revealed that an inhibitor containing both N and S atoms could promote its adsorption
on the metal surface compared with the inhibitor only containing N or S atom [35, 36].
Although the great achievement has been made in the studies about inhibitors,
developing a highly effective inhibitor is still a great challenge due to the diversity of
organic compounds.
In this work, three kinds of benzimidazole derivatives, i.e.,
2
1
-mercaptobenzimidazole
(MBI),
2-thiobenzylbenzimidazole
(TBBI),
-butyl-2-thiobenzylbenzimidazole (BTBBI), were used as the inhibitors for mild
steel in HCl solution. The inhibition effects of benzimidazole derivatives were
evaluated by weight loss and electrochemical measurements. The relationship
between the molecular structures and inhibition performances of these three
benzimidazole derivatives was also elucidated by quantum chemical calculations. The
adsorbed configurations of these three benzimidazole derivative molecules were
predicted by molecular dynamics (MD) simulations.
3

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2
. Experimental
2
.1. Synthesis of inhibitors
In this work, three kinds of benzimidazole derivatives, i.e.,
2
1
-mercaptobenzimidazole
(MBI),
2-thiobenzylbenzimidazole
(TBBI),
-butyl-2-thiobenzylbenzimidazole (BTBBI), were used as the inhibitors. Scheme 1
displays the molecular structures of these three benzimidazole derivatives. Among
them, MBI was purchased from a chemical reagent company. TBBI was synthesized
from MBI referring to a previously described experimental procedure [37]. Typically,
5
0 mL acetone, 0.02 mol MBI and 0.02 mol KOH were added to a round-bottomed
flask. The mixture was heated to the reflux temperature under stirring condition. Then
.02 mol benzyl chloride was added dropwise. After reaction for 3 h, a large amount
0
of distilled water was added, and then colorless acicular crystals of TBBI precipitated.
The obtained product was washed with distilled water, and then filtered and dried to
obtain colorless pure acicular TBBI.
BTBBI was synthesized from TBBI. Typically, 150 mL tetrahydrofuran (THF),
0
.33 mol KOH, and 0.3 mol TBBI were added to a 500 mL round-bottomed flask. The
mixture was heated to reflux temperature under stirring condition. Then 0.33 mol
-bromobutane was added dropwise. After reaction for 15 h, the mixture was filtrated
1
and the filtrate was rotary evaporated. The products were further purified by column
chromatography. Then, a bright yellow liquid (BTBBI) was obtained. Scheme 2
shows the synthetic route for TBBI and BTBBI.
4

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The chemical structures of TBBI and BTBBI were analyzed by nuclear magnetic
resonance (NMR) spectroscopy (Fig. 1). The NMR data of the synthesized
1
benzimidazole derivatives: (1) TBBI: H NMR (400 MHz, DMSO-d ): δ (ppm) =
6
4
7
.577 (s, 2H, -SCH ), 7.125 (m, 2H, Ar-H), 7.247 (t, 1H, Ar-H), 7.310 (t, 2H, Ar-H),
2
.371 (m, 1H, Ar-H), 7.451 (d, 2H, Ar-H), 7.549 (m, 1H, Ar-H), 12.600 (s, 1H, NH).
1
(
2) BTBBI: H NMR (400 MHz, CDCl ): δ (ppm) = 0.906 (t, 3H, CH (CH ) N-),
3
3
2 3
1
2
.309 (m, 2H (CH )CH (CH ) N-), 1.704 (m, 2H, (CH CH )CH (CH )N-), 4.021 (t,
3 2 2 2 3 2 2 2
H, (CH CH CH )CH N-), 4.624 (s, 2H, CH S-), 7.250 (m, 6H, Ar-H), 7.409 (d, 2H,
3
2
2
2
2
Ar-H), 7.716 (m, 1H, Ar-H).
2
.2. Material and solution
In this work, mild steel was used as the material, with a chemical composition
wt. %) of 0.02% C, 0.3% Mn, 0.017% P, 0.007% S, 0.30% Si, 0.072% Al, and bal. Fe.
(
The sizes of the specimens for weight loss and electrochemical measurements were 50
mm  10 mm  3 mm, 10 mm  10 mm  3 mm, respectively. For electrochemical
measurements, the specimens were embedded into epoxy resin with an exposed area
2
of 1 cm . Before testing, the specimens surface were abraded with 800 grit SiC paper.
The test solution was 1 M HCl solution, which was prepared with reagent grade HCl
(37%) and distilled water.
2
.3. Weight loss measurements
The corrosion rate of mild steel in the solution without or with inhibitors was
measured by weight loss method. Three parallel specimens were immersed into the
solution for 24 h for each condition. After immersion, the specimens were removed,
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and the average weight loss was measured to calculate the average weight loss
corrosion rate. Then, the inhibition efficiencies (η %) of inhibitors were calculated:
w
V0 V
V0
w (%) 
100%
(1)
where V and V were the weight loss corrosion rates without and with inhibitors,
0
respectively.
2
.4. Electrochemical measurements
For electrochemical measurements, a conventional three-electrode cell was used,
with the steel specimen as working electrode (WE), a platinum plate as counter
electrode (CE) and a saturated calomel electrode (SCE) as reference electrode (RE).
After the WE was immersed in the solution for 1 h and a steady potential reached,
electrochemical impedance spectroscopy (EIS) was measured at open circuit potential
(OCP) with a amplitude of 5 mV and the frequency from 100,000 to 0.01 Hz. The
inhibition efficiencies (EIS %) of inhibitors were determined by EIS measurements:
ꢃ ꢅꢆꢃ-ꢃ ꢇ
ꢅꢆ
η ꢂꢃꢄꢃ
ꢃꢈꢃꢉꢇꢇ
(2)
ꢀ
ꢁ
ꢅꢆ
0
ct
where
R
and Rct were the charge transfer resistances without and with inhibitors,
respectively.
Polarization curves measurements were conducted from -0.2 V to 0.2 V vs. OCP
with a sweep rate of 0.5 mV/s. The inhibition efficiencies (η %) of inhibitors were
p
also calculated according to the polarization curves:
ꢇ
ꢀ
ꢃ-ꢃꢀꢅꢋꢌꢌ
ꢅꢋꢌꢌ
ꢅꢋꢌꢌ
ηꢊꢂꢃꢄꢃ
ꢃꢈꢃꢉꢇꢇ
(3)
ꢇ
ꢀ
0
where i
and icorr are the corrosion current densities in the solutions without and
corr
6

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with inhibitors, respectively.
2
.5. Surface characterization of specimens after corrosion
After being immersed in the solution without or with inhibitors, the specimens
were removed from the solution, and cleaned with deionized water. Then, the surface
morphologies of the corroded specimens were observed and captured by scanning
electron microscope (SEM).
2
.6. Quantum chemical calculations
Quantum chemical calculations were conducted with Materials Studio software
based on density function theory (DFT). Geometrical optimizations were performed
with the generalized gradient approximation (GGA) functional of Becke exchange
plus Lee–Yang–Parr correlation (BLYP) method with a double numerical plus
polarization (DNP) basis set without any symmetry and spin constraints. In the
calculations, solvent effects were considered with water as the solvent. Furthermore,
frequency analysis was performed to make sure that there was no imaginary
frequency and the obtained structure had the minimum potential energy. The
convergence criteria for energy, maximum force, maximum displacement,
-5
-3
self-consistent field (SCF) and k-point set were 1.0 × 10 Ha, 2.0× 10 Ha/Å, 5.0×
-3
-6
1
0 Å, 1.0 × 10 and 1×1×1, respectively.
2
.7. Molecular dynamics simulations
7

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The adsorption process of three kinds of benzimidazole derivative inhibitors on
Fe surface was studied by molecular dynamics (MD) simulations. First, 11 layers Fe
(1 1 0) surface were cleaved from bcc Fe crystal, and then enlarged to a (13 × 13)
supercell. The size of supercell was 3.2 nm × 3.2 nm × 2.0 nm. Then the second layer
containing 8 inhibitor molecules and 1000 H O molecules in aqueous solution
2
3
(
density 0.997 g/cm ) was built above the Fe (1 1 0) surface. Above the solution layer,
a vacuum slab (a thickness of 20 Å) was built. All the Fe atoms were fixed, and all the
inhibitor and H O molecules could move freely.
2
Dynamics simulations were conducted with NVT constant canonical ensemble at
3
03.15 K. The temperature control method was Berendsen thermostat. The total
simulation time was 500 ps with the time step of 1.0 fs.
The average adsorption energy (E ) of one inhibitor molecule on Fe (1 1 0)
ads
surface was calculated:
Eads = (Einhibitor + surface - (Einhibitor + Esurface))/8
(4)
where Einhibitor + surface is the total energy of the Fe (1 1 0) surface together with the 8
adsorbed inhibitor molecules; Einhibitor is the total energy of the 8 free inhibitor
molecules; Esurface is the energy of Fe (1 1 0) surface.
3
. Results and discussion
3
.1. Weight loss measurements
Fig. 2 shows the weight loss corrosion rate of mild steel in 1 M HCl solution
without or with different concentrations of three kinds of benzimidazole derivative
8

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o
inhibitors at 30 C for 24 h. The corresponding inhibition efficiencies of these three
inhibitors are also shown in Fig. 2. It is seen that the corrosion rate of mild steel
decreases significantly in the solutions with inhibitors. Furthermore, the corrosion rate
decreases with the increasing inhibitor concentration. The inhibition efficiencies of
these three kinds of benzimidazole derivatives follow the order: BTBBI > TBBI >
MBI. Particularly, the inhibition efficiency of BTBBI reaches 92.75% even in a low
concentration of 10 mg/L, and increases to 98.88% with a concentration of 100 mg/L.
Therefore, the introduction of benzyl and alkyl chain to 2-mercaptobenzimidazole
significantly promotes the inhibition performance for the corrosion of mild steel in
acidic solution.
3
.2. Electrochemical impedance measurements
Fig. 3 shows the EIS of mild steel in 1 M HCl solution without or with different
o
concentrations of benzimidazole derivatives inhibitors at 30 C for 1 h. The Nyquist
plots in the solutions without or with inhibitors are similar, i.e., only a single
capacitive loop corresponding to the charge transfer resistance and double layer
capacitance at the electrode/solution interface during the corrosion process [38]. The
similar capacitive loops may indicate that the electrode reactions are controlled by the
charge transfer process in the absence or presence of inhibitors [39]. However, the
capacitive loop enlarges obviously in the inhibited solution, indicating the great
inhibition effect of inhibitors. The impedance increases with inhibitor concentration
because of the increasing surface coverage of inhibitor molecules. In the Bode plots,
9

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only one phase angle peak is observed, suggesting one time constant. The frequency
range of the maximum phase angle becomes larger in the presence of inhibitors,
which suggests the effective adsorption of inhibitors on the mild steel surface [40]. To
determine the thermodynamic parameters of the adsorption of inhibitor, the EIS of
mild steel in 1 M HCl solution without or with different concentrations of BTBBI at
different temperatures for 1 h were also measured, as shown in Fig. 4. It is seen that
the Nyquist plots are also characterized by a single capacitive loop, and the
impedance decreases with the increasing temperature in the both blank and inhibited
solutions.
To obtain the electrochemical parameters, the EIS data were fitted with an
equivalent circuit shown in Fig. 5. Table 1 and Table 2 list the corresponding
electrochemical parameters. In the equivalent circuit, R is the solution resistance; Qdl
s
is the constant phase element (CPE) representing the double layer capacitance; R is
ct
the charge transfer resistance. The Nyquist plots show a depressed capacitive loop
because of the inhomogeneous electrode surface after corrosion [41, 42]. Accordingly,
CPE was the used to represent the double layer capacitance (C ). The value of C can
dl
dl
be determined from CPE [43, 44]:
n1
C Y (2 f )
(5)
dl
0
max
where Y , n is the magnitude and phase shift constant of CPE, fmax is the frequency
0
corresponding to the maximum imaginary value.
It is seen from Table 1 and Table 2 that the R increases and the C decreases
ct
dl
with the increasing inhibitor concentration. The decrease in C may be due to the
dl
1
0

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decreasing dielectric constant and/or the increasing electrical double layer thickness
[
44]. The increase in R suggests an increase in the adsorbed inhibitor molecules.
ct
Furthermore, the value of R in the inhibited solution increases in the order: MBI <
ct
TBBI < BTBBI, i.e., the inhibition efficiency increases in the order: MBI < TBBI <
BTBBI. With the increase of temperature, the value of R in the solution with BTBBI
ct
decreases.
3
.3. Polarization curves measurements
Fig. 6 shows the polarization curves of mild steel in 1 M HCl solution without or
o
with various concentrations of inhibitors at 30 C for 1 h. Both the anodic and
cathodic current densities decrease after adding inhibitors, which suggests that the
presence of inhibitors not only inhibits the anodic dissolution of steel but also
suppresses the cathodic hydrogen evolution [14, 45]. Furthermore, the inhibition of
cathodic reaction is more significant than that of anodic reaction, which leads to a
negative shift of corrosion potential (E ) after adding inhibitors. The negative shift
corr
of Ecorr is less than 85 mV, which indicates that these benzimidazole derivatives act as
mixed-type inhibitors with predominant cathodic effectiveness [2, 46, 47]. Fig. 7
shows the polarization curves of mild steel in 1 M HCl solution without or with
different concentrations of BTBBI at different temperatures for 1 h. Both the anodic
and cathodic current densities are reduced in the presence of BTBBI inhibitor at
various temperatures.
The values of corresponding electrochemical parameters, including corrosion
11

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potential (E ), corrosion current density (i ), anodic and cathodic Tafel slopes (b ,
corr
corr
a
bc), which were determined by extrapolation, are presented in Table 3 and Table 4. It
is clear that the corrosion current densities after adding inhibitors are much less than
those without inhibitors. Furthermore, the corrosion current densities in the solutions
with BTBBI are much less than those in the solutions with TBBI or MBI, indicating
the higher inhibition efficiency of BTBBI. Additionally, the corrosion current
densities in the solutions without or with BTBBI increase with the increase of
temperature.
3
.4. Adsorption isotherms
To determine the adsorption behavior of benzimidazole derivatives inhibitors, the
corresponding absorption isotherms were evaluated. It is found that the absorption
behavior of these three benzimidazole derivatives follows the Langmuir adsorption
isotherm [47, 48]:
Cinh
1

Cinh
(6)

Kads
where Cinh is the inhibitor concentration, mol/L;  is the surface coverage of inhibitors,
which is equal to the inhibition efficiency determined by EIS ( EIS ); Kads is the
adsorption equilibrium constant, which is obtained by the reciprocal of the intercept
of C/ axis. The plots of C/ vs. C and the corresponding linear fitted straight lines for
o
MBI, TBBI, and BTBBI at 30 C are shown in Fig. 8. It can be seen that all these
straight lines have the correlation coefficient more than 0.99, which confirms that the
adsorption of these inhibitors follows the Langmuir adsorption isotherm. After the
1
2

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value of Kads is determined by the adsorption isotherm, the standard free energy of
ο
ads
adsorption ( G ) can be calculated [48]:
ο
ads

G  RT ln(55.5K )
(7)
ads
where R is the gas constant (8.314 J/(mol K) ), and T is the absolute temperature (K).
ο
ads
Table 5 lists the values of Kads and G for MBI, TBBI and BTBBI. The large
ο
ads
negative value of G indicates the spontaneous adsorption of inhibitors by a strong
ο
interaction with the steel surface. It is acknowledged [13, 49] that, when G > -20
ads
kJ/mol, inhibitors can adsorb on metal surface by electrostatic interaction
ο
ads
(
physisorption). When G < -40 kJ/mol, coordinate bonds (chemisorption) are
formed by charge sharing or transferring from inhibitors to metal surface. When -40
ο
ads
kJ/mol < G
< -20 kJ/mol, the adsorption of inhibitors involves both
physisorption and chemisorption [50]. It is clear from Table 5 that the adsorption of
MBI and TBBI is a mixed type including both physisorption and chemisorption, while
the absorption of BTBBI is typical chemisorption. Generally, the higher values of Kads
ο
and G , the higher adsorption ability and then higher inhibitive effect of inhibitor.
ads
Therefore, there is higher adsorption ability and inhibitive effect of BTBBI with
ο
ads
higher values of Kads and G
.
ο
ο
To determine the enthalpy ( H ) and entropy ( S ) of the adsorption of
ads
ads
inhibitor, EIS and polarization curves measurements for mild steel in 1 M HCl
solution without or with different concentrations of BTBBI inhibitor were performed
at different temperatures for 1 h. The corresponding plots of C/ vs. C and the linear
fitted straight lines are shown in Fig. 9. After the values of Kads at different
1
3

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temperatures were determined by the adsorption isotherms, the thermodynamic
ο
ο
parameters, H and S , could be calculated according to following equation
ads
ads
[
51]:
ο
ads
ο
ads

H
S
ln K  

ln(55.5)
(8)
ads
RT
R
Fig. 10 shows the linear relationship between lnKads and 1/T for mild steel in 1 M
ο
ads
HCl solution with different concentrations of BTBBI inhibitor. The values of H
ο
and S of the adsorption of BTBBI inhibitor were determined by the slope and
ads
ο
intercept of the lnKads ~1/T plots. Table 6 lists the standard free energy ( G ),
ads
ο
ο
enthalpy ( H ), and entropy ( S ) of the adsorption of BTBBI inhibitor at
ads
ads
ο
different temperatures. It is seen that the value of H is negative, indicating an
ads
exothermic process of the adsorption of BTBBI on the mild steel surface. The positive
ο
sign of S indicates that the adsorption process of BTBBI accompanies an
ads
increase in entropy, which serves as the driving force for the adsorption of inhibitor
on the mild steel surface.
3
.5. Corrosion kinetic analysis
To determine the kinetic parameters of the corrosion process of mild steel in the
solutions without or with inhibitors, polarization curves measurements were
conducted at various temperatures. Then the activation energy (E ), enꢆhalpyꢃ(ΔH )
a
a
andꢃenꢆꢌꢋpyꢃ(ΔS ) of activation were determined by following equations [52]:
a
Ea
icorr  Aexp(
)
(9)
RT
1
4

ACCEPTED MANUSCRIPT
icorr
R
Sa Ha
ln
 ln


(10)
T
Nh
R
RT
where A is the pre-exponential constant. h is the Plank's constant and N is the
Avogadro's number.
Fig. 11 and Fig. 12 show the Arrhenius plots of lnicorr vs. 1/T and the transition
state plots of ln(i /T) vs. 1/T in the solutions without or with 50 mg/L MBI, TBBI
corr
and BTBBI. Based on the equations (9) and (10), the values of E ,ꢃΔH andꢃΔS can be
a
a
a
obtained by the slopes and intercepts of the straight lines, and their values are
presented in Table 7. It is seen that the value of E in the solution without inhibitor is
a
5
5
1.73 kJ/mol, while in the solutions with MBI or TBBI, the values of E are 56.20 and
a
4.12 kJ/mol, respectively, which are a little higher than that in the blank solution.
The E of BTBBI is 27.27 kJ/mol, which is much lower than that in the solution
a
without inhibitor. The low E of BTBBI may be ascribed to the formation of strong
a
chemisorption between the BTBBI molecules and steel [36]. TheꢃvalueꢃꢋfꢃΔH is
a
relatively close and follow the same pattern as the value of E , which is in line with
a
the concept of transition-sꢆaꢆeꢃꢆheꢋꢌy.ꢃFuꢌꢆheꢌmꢋꢌe,ꢃꢆheꢃvalueꢃꢋfꢃΔH is positive,
a
indicating an endothermic process of the mild steel dissolution. This has also been
reported to be indicative of the retarded dissolution of metal in the solution with
inhibitor [53]. TheꢃnegaꢆiveꢃvalueꢃꢋfꢃΔS suggests that the activated complex is
a
associative rather than dissociative in the rate determining step, i.e., transition from
reactants to activated complex is accompanied by increase in orderliness [54].
3
.6. SEM surface morphology after corrosion
1
5

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Fig. 13 shows the SEM surface morphologies of the specimens after exposed in 1
o
M HCl solution without or with 50 mg/L MBI, TBBI, BTBBI inhibitors at 30 C for
2
4 h. It is seen that severe corrosion occurs on the specimens in the solution without
inhibitor. A large amount of corrosion products are observed on the whole specimen
surface. After adding inhibitors, the specimen surface is relatively smooth and the
scratches produced during the abrasion pretreatment process are still observed. This
indicates that the adsorption of inhibitor to some extent prevents the contact between
the steel and aggressive species. Furthermore, the corrosion of the specimens in the
solution with BTBBI is slighter than those in the solutions with TBBI or MBI,
suggesting a higher inhibition performance of BTBBI.
3
.7. Stability of the inhibition effect
To determine the long-term inhibition effects of inhibitors, the specimens were
immersed in the solution containing 50 mg/L each inhibitor for 72 h, and the time
dependence of the inhibitor efficiency of each inhibitor was determined by EIS
measurements, as shown in Fig. 14. The inhibition efficiency of MBI drops quickly
with immersion time and loses its inhibition effect after 24 h. For TBBI, the decrease
in inhibition efficiency is slow and the inhibition efficiency drops to 90% after 72 h.
However, BTBBI always shows an excellent inhibition performance with an
inhibition efficiency more than 97.4% during the whole immersion time. This
indicates that benzene ring and alkyl chain contribute to strong absorption of BTBBI
on the specimen surface with long-term stable inhibition effect.
1
6

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3
.8. Quantum chemical calculations
It has been demonstrated that quantum chemical calculation is the useful way to
theoretically elucidate the correlation between the inhibitor molecule structure and its
inhibition mechanism at a molecular level. According to the frontier molecular
orbitals (FMO) theory, the highest occupied molecular orbital (HOMO) is associated
with the ability of inhibitor to donate electrons, while the lowest unoccupied
molecular orbital (LUMO) is related to the ability of inhibitor to accept electrons [55].
The energy band gap (ΔE = ELUMO - EHOMO) indicates the activity of inhibitor to
adsorb on metal surface.
Fig. 15 shows that the optimized molecular structures of the three benzimidazole
derivatives (MBI, TBBI, BTBBI). It is seen that MBI presents a rigid planar structure
of the benzimidazole segment and sulfydryl group, while the benzene ring and alkyl
chain in TBBI and BTBBI are not in the plane with benzimidazole ring. Fig. 16 shows
the HOMO and LUMO distributions of these three benzimidazole derivatives. The
electron densities of HOMO for these three benzimidazole derivatives mostly locate
at the benzimidazole ring and sulfydryl group. These Benzimidazole derivatives could
donate electrons to the steel to form coordinate bonds at these positions. For MBI, the
electron densities of LUMO also locate at the benzimidazole ring and sulfydryl group,
while for TBBI and BTBBI, the electron densities of LUMO locate at the benzene
ring and sulfydryl group. These positions could be acceptor of electrons from steel to
form feedback bonds.
1
7

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Additionally, the eleꢅꢆꢌꢋnegaꢆiviꢆyꢃ(χ)ꢃandꢃ glꢋbalꢃhaꢌdness (γ)ꢃꢋfꢃinhibiꢆꢋꢌꢃ
molecules can be calculated with EHOMO and ELUMO [24]:
EHOMO  ELUMO

 
(9)
2
ELUMO  EHOMO


(10)
2
Then the fraction of electron transferred (ΔN) can be determined with the
electronegativities and global hardnesses of inhibitor molecules (χ , γ ) and Fe
inh
inh
atom (χ , γ ) [56]:
Fe Fe
Fe  inh
N 
(11)
2
(  )
Fe
inh
The theoretical values of χ and γ are 7 eV and 0 eV, respectively [57]. The
Fe
Fe
parameter of ΔNꢃreflects the inhibition performance from electrons donation. IfꢃΔN<
.6, the chemisorption and inhibition efficiency increase with the electron-donating
ability [58].
Table 8 lists the quantum parameters of these three benzimidazole derivatives. It
3
is suggested that the values of EHOMO, ELUMO and ΔE are associated with the inhibition
efficiencies of inhibitors. A higher EHOMO and lower ΔE corresponds to a stronger
chemisorption of inhibitor molecules, and then higher inhibition efficiency. From
Table 8, the ΔE of these three inhibitors follows the order: MBI > TBBI > BTBBI,
while the ΔN of these three inhibitors is in the order of MBI < TBBI < BTBBI.
Therefore, the inhibition performances of these three benzimidazole derivatives
follow the order: MBI < TBBI < BTBBI.
+
-
To elucidate the local reactive sites in inhibitor molecules, Fukui indices ( fk
, fk )
1
8

ACCEPTED MANUSCRIPT
+
-
were determined. It is well known that the atoms with higher values of fk or fk
have higher abilities to accept or donate electrons, i.e., susceptible to be attacked by a
nucleophilic reagent or an electrophilic reagent, respectively. Table 9 lists the Fukui
+
indices (mulliken) of the atoms of inhibitors. For MBI, the high value of fk is
present at C2, C5, C8, and S10, indicating the high susceptibility of nucleophilic
-
attack at these sites, while high value of fk is present at C1, C3, N7, and S10, i.e.,
these sites are susceptible to electrophilic attack. For TBBI and BTBBI, the high
+
value of fk is present at the benzene ring and sulfydryl group, while the high value
-
of fk is present at the benzimidazole ring and sulfydryl group. Therefore, the
benzene ring and sulfydryl group are susceptible to nucleophilic attack, and the
benzimidazole ring and sulfydryl group are susceptible to electrophilic attack.
3
.9. Molecular dynamics (MD) simulations
MD simulations were conducted to study the adsorption behavior of the
benzimidazole derivatives on Fe (1 1 0) surface. As shown in Fig. 17, both MBI and
TBBI molecules adsorb on the Fe surface with a flat orientation. The adsorption of
MBI molecules is through forming coordinate bonds between the benzimidazole ring
and sulfydryl group and the Fe surface. For TBBI, apart from the adsorptions of
benzimidazole ring and sulfydryl group, benzene ring also adsorb on the Fe surface by
forming feedback bonds. During the adsorption, a twirling of benzene ring occurs so
that benzimidazole ring, sulfydryl group, and benzene ring adsorb on the Fe surface in
a flat orientation. The adsorption of BTBBI on Fe surface is also by the adsorptions of
1
9

ACCEPTED MANUSCRIPT
benzimidazole ring, sulfydryl group, and benzene ring in a flat orientation. Meanwhile,
the alkyl chain arranges far from the Fe surface to form a hydrophobic layer. The
calculated adsorption energies (E ) of these three benzimidazole derivatives are
ads
listed in Table 10. The adsorption energies (E ) of these three benzimidazole
ads
derivative inhibitors are in the order: BTBBI < TBBI < MBI. Generally, a more
negative adsorption energy means a higher inhibition efficiency of inhibitor.
Therefore, it is expected that the inhibition efficiencies of these three benzimidazole
derivatives follow the order: BTBBI > TBBI > MBI. The MD simulations are
consistent with the weight loss and electrochemical measurements.
3
.10. Inhibition mechanism of benzimidazole derivative inhibitors
The adsorption behavior of inhibitor molecules is dependent upon their
physicochemical properties (e.g. functional groups, electron density) and the charge
of metal surface. Based on the experimental measurement results and theoretical
calculations, a possible inhibition mechanism of these three benzimidazole derivatives
on Fe surface is elucidated in Fig.18. A lot of research have indicated that the mild
steel surface is positively charged in HCl solution, i.e., there are excess positive
charges on the steel surface [14, 31, 59, 60]. The positively charged specimen surface
-
favors the adsorption of Cl to create a negative charge surface, which facilitates the
adsorption of the cations in the solution. These benzimidazole derivatives molecules
may be protonated in the solution because of the unshared electron pair of the N and S
atoms. The protonated molecules could adsorb on specimen surface via electrostatic
2
0

ACCEPTED MANUSCRIPT
interaction, i.e., physisorption (Fig. 18(a)). Meanwhile, further adsorption of these
inhibitor molecules could be realized by forming covalent bonds (chemisorption), as
shown in Fig. 18(b). Quantum chemical calculations and MD simulations demonstrate
that both the HOMO and LUMO of MBI molecules locate at the benzimidazole ring
and sulfydryl group, where the MBI molecules could adsorb on the specimen surface
in a flat orientation by forming coordinate bonds and/or feedback bonds. The
absorption isotherms confirm the presence of both physisorption and chemisorption.
For TBBI, apart from the adsorption of the benzimidazole ring and sulfydryl group,
the introduction of benzyl ring increases the electron density in the TBBI molecules.
TBBI molecules could also adsorb on Fe surface by forming feedback bonds at the
benzene ring. Therefore, a higher inhibition efficiency of TBBI is present in both
weight loss and electrochemical measurements. In the case of BTBBI, besides the
adsorptions of benzimidazole ring, sulfydryl group, and benzene ring, the introduced
alkyl chain arranges far from the Fe surface to form a hydrophobic layer, which
further improves the inhibition performance of BTBBI. Therefore, the inhibition
efficiency of BTBBI is higher than those of MBI and TBBI. Both weight loss and
electrochemical measurements, quantum chemical calculations and molecular
dynamics (MD) simulations, indicate that the inhibition efficiencies of three
benzimidazole derivatives follow the order: BTBBI > TBBI > MBI.
4
. Conclusions
1
. The inhibition performances of three kinds of benzimidazole derivatives (MBI,
2
1

ACCEPTED MANUSCRIPT
TBBI, BTBBI) for the corrosion of mild steel in 1 M HCl solution were studied.
Both the weight loss and electrochemical measurements indicate that these three
inhibitors exhibit good inhibition performances and their efficiencies increase
with the increasing concentration in the order: BTBBI > TBBI > MBI. The
introduction of benzyl and alkyl chain to 2-mercaptobenzimidazole (MBI)
prominently improves the inhibition performance.
2
. These three benzimidazole derivatives act as mixed type inhibitors with
predominant cathodic effectiveness. Their adsorptions follow the Langmuir
adsorption isotherm. The adsorption of MBI and TBBI includes physisorption and
chemisorption while adsorption of BTBBI is typical chemisorption.
3
4
. Quantum chemical calculations reveal the order of the inhibition efficiencies of
these three benzimidazole derivatives: BTBBI > TBBI > MBI, which is in
accordance with the experimental results.
. Molecular dynamics simulation indicates that the three benzimidazole derivatives
adsorb on Fe (1 1 0) surface in flat orientation. For BTBBI, the alkyl chain
arranges far from the Fe surface to form a hydrophobic layer, which further
improves the inhibition efficiency of BTBBI.
Acknowledgements
The authors thank the support of National Natural Science Foundation of China
(Nos. 51571097, 51371086). The authors also thank the support of analytical and
testing center of Huazhong University of Science and Technology.
2
2

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Table 1 Fitted electrochemical parameters of the EIS of mild steel in 1 M HCl
solution without or with various concentrations of benzimdazole derivative inhibitors
o
at 30 C for 1 h
Concentration
2
2
2
Inhibitor
R (Ωcm )
C (μF/cm )
R (Ωcm )
ηEIS (%)
θ
s
dl
ct
(mg/L)
Without
inhibitor
0
0.74
172.2
9.33
-
-
MBI
10
0.52
0.62
0.71
0.73
0.71
148.4
84.2
43.3
36.4
32.1
19.49
51.72
55.6
82.0
92.2
93.9
95.0
0.556
0.820
0.922
0.939
0.950
2
5
7
5
0
5
119.78
152.10
187.90
100
TBBI
10
1.16
0.78
0.91
1.07
1.16
84.0
69.2
33.4
21.1
18.5
21.1
61.6
55.8
84.9
94.7
96.4
97.4
0.558
0.849
0.947
0.964
0.974
2
5
0
5
5
7
177.1
257.5
362.9
100
BTBBI
10
0.65
0.53
0.69
0.46
0.47
23.9
19.2
18.3
17.8
17.2
414.8
550.6
655.8
807.8
1052.1
97.7
98.3
98.6
98.8
99.1
0.977
0.983
0.986
0.988
0.991
2
5
0
5
5
7
100
2
9