Highly effective inhibition of mild steel corrosion in HCl solution by using pyrido[1,2-a]benzimidazoles

Article abstract of DOI:10.1039/C8NJ04028A

The inhibition performance of three pyrido[1,2-a]benzimidazoles, namely, benzo[4,5]imidazo[1,2-a]pyridine (BIP), 2-methylbenzo[4,5]imidazo[1,2-a]pyridine (MBIP), and 2-(trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine (TBIP), for steel corrosion in hydrochloric acid solution were researched through a weight loss test, electrochemical techniques, surface morphology (SEM, XPS) studies, and theoretical methods. The experimental results revealed that TBIP, BIP, and MBIP had excellent inhibition performance, and their maximum inhibition efficiencies at 0.25 mmol L^?1 were 95.48%, 97.65%, and 98.96%, respectively. The properties of these inhibitors are considered to be mixed-type, and their adsorption mode belongs to the Langmuir isothermal type. The formation and properties of an adsorbed film on a steel surface were investigated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Quantum chemical calculations provided insightful quantitative information to conclude the correlation between the molecular structures and inhibition performance. Molecular dynamics (MD) simulations were carried out to explore the configurationally adsorption behavior of these pyrido[1,2-a]benzimidazoles on Fe(110) surface, and the binding energy of these three inhibitors followed the order MBIP > BIP > TBIP, which is consistent with the experimental evaluation results.

Full text of DOI:10.1039/C8NJ04028A

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Highly effective inhibition of mild steel corrosion
in HCl solution by using pyrido[1,2-a]benzimidazoles
Cite this: New J. Chem., 2019,
43, 413
Weiwei Zhang,a Hui-Jing Li,*a Meirong Wang,b Li-Juan Wang,*b Ai-Han Zhanga
a
and Yan-Chao Wu
*
The inhibition performance of three pyrido[1,2-a]benzimidazoles, namely, benzo[4,5]imidazo[1,2-a]pyridine
(BIP), 2-methylbenzo[4,5]imidazo[1,2-a]pyridine (MBIP), and 2-(trifluoromethyl)benzo[4,5]imidazo[1,2-a]-
pyridine (TBIP), for steel corrosion in hydrochloric acid solution were researched through a weight loss
test, electrochemical techniques, surface morphology (SEM, XPS) studies, and theoretical methods.
The experimental results revealed that TBIP, BIP, and MBIP had excellent inhibition performance, and
their maximum inhibition efficiencies at 0.25 mmol Lꢀ1 were 95.48%, 97.65%, and 98.96%, respectively.
The properties of these inhibitors are considered to be mixed-type, and their adsorption mode belongs
to the Langmuir isothermal type. The formation and properties of an adsorbed film on a steel surface
were investigated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy
(XPS). Quantum chemical calculations provided insightful quantitative information to conclude the
correlation between the molecular structures and inhibition performance. Molecular dynamics (MD)
simulations were carried out to explore the configurationally adsorption behavior of these pyrido[1,2-a]-
benzimidazoles on Fe(110) surface, and the binding energy of these three inhibitors followed the order
MBIP 4 BIP 4 TBIP, which is consistent with the experimental evaluation results.
Received 9th August 2018,
Accepted 19th November 2018
DOI: 10.1039/c8nj04028a
rsc.li/njc
the gap between corrosion protection tests and the actual steel
protection. Nevertheless, the pursuit of practical corrosion
Introduction
As an important structural material, mild steel has been inhibitors is still in its early phases and further investigation
extensively applied in construction and other industrial fields is needed to adequately understand the mechanism of inhibitor
owing to its remarkable mechanic performance. However, mild action. Different inhibitor platforms are needed to prevent mild
steel is an active metal material, which is extremely sensitive to steel corrosion in a hydrochloric acid medium, which makes
corrosion in HCl solution, and this can result in huge economic the exploitation of the late-model chemical class of corrosion
losses and potential environmental problems under acidic inhibitors for mild steel protection a high priority. Considering
conditions.1–3 A practical and effective solution to this problem that benzimidazole and pyridine show good inhibition effi-
is to use synthetic organic corrosion inhibitors due to their easy ciency in hydrochloric acid solution,8,12 we envisioned that the
synthesis, remarkable inhibition effect, and economic advantages. combination of benzimidazole and pyridine might provide a
Generally, most effective inhibitors are heterocyclic compounds new corrosion inhibitor platform. In this context, we report
bearing larger electronegative atoms (i.e., N, O, S, and P), polar here our endeavors toward the development of pyrido[1,2-a]-
functional groups, and conjugated double bonds.4–7 Several benzimidazoles as a highly effective inhibitor platform for mild
organic heterocyclic compounds, such as Schiff bases,8 steel corrosion in HCl solution.
quinolones,9 thioureas,10 pyridines,11 and benzimidazoles,12
In this paper, three pyrido[1,2-a]benzimidazoles, namely,
have been reported as some of the most effective acidic corrosion benzo[4,5]imidazo[1,2-a]pyridine (BIP), 2-(trifluoromethyl)-benzo-
inhibitors. These molecules replace the water molecules [4,5]imidazo[1,2-a]pyridine (TBIP), and 2-methyl-benzo[4,5]imidazo-
adsorbed on the steel surface, and form an adsorption film [1,2-a]pyridine (MBIP), were designed and smoothly synthesized
with physical or chemical properties on a metallic surface. by using a modified synthetic procedure for pyrido[1,2-a]benz-
Various corrosion inhibitors have been developed to reduce imidazole synthesis. For the first time pyrido[1,2-a]benzimid-
azoles were found to be potential inhibitors for mild steel corrosion
in acid solution, and they were selected as corrosion inhibitors
mainly based on the following factors: (1) able to be easily
synthesized at low cost; (2) contain multiple active centers and
aromatic rings system; (3) have a special affinity to inhibit
a School of Marine Science and Technology, Harbin Institute of Technology,
Weihai 264209, P. R. China. E-mail: lihuijing@iccas.ac.cn, ycwu@iccas.ac.cn
b School of Materials Science and Engineering, Harbin Institute of Technology,
Weihai 264209, P. R. China. E-mail: lijuanwanghit@163.com
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corrosion metals in acidic solutions; (4) have bulky steric IR (film): nmax = 2985, 2359, 1736, 1462, 1373, 1319, 1240,
hindrance. Here, the inhibition action of the studied inhibitors 1128, 1046, 847, 753, 679 cmꢀ1
for mild steel in 1 mol Lꢀ1 HCl solution was studied using a
.
Materials and electrolytes
weight loss method, potentiodynamic polarization, and electro-
chemical impedance spectroscopy (EIS) techniques. Scanning
electron microscopy (SEM) and X-ray photoelectron spectro-
scopy (XPS) were used to test and verify the formation and
properties of an adsorbed film on the steel surface. By calculat-
ing the electronic structure property of the inhibitor molecule,
the inhibition mechanism of the inhibitors with mild steel was
considered using a quantum chemical calculation method.
Moreover, molecular dynamics simulations were employed to
ascertain the adsorption behaviour of the inhibitors on Fe(110)
surface. Our efforts should lead to a better comprehension of
the elements controlled by their structural units, and might
shed new light on the design of efficient corrosion inhibitors.
The elemental composition of the mild steel materials used
for the experiments was as follows (wt%): 0.076 C, 0.025 Si,
0.174 Mn, 0.012 P, 0.005 S, and the balance Fe. The steel
materials used for the electrochemical experiments and surface
analysis were sized 1.8 ꢁ 0.6 ꢁ 0.4 cm and 1.0 ꢁ 1.0 ꢁ 0.1 cm,
respectively. For the electrochemical experiments, the electrodes
retained a working area of 0.2 cm2, and then the non-working
surface was encapsulated with epoxy resin. Prior to the measure-
ments, SiC abrasive papers with different grades were used to
polish the electrode working surface, which was then washed
with ethanol and dried with cold air. Analytical grade 37%
hydrochloric acid and ultrapure water were used to formulate
1 mol Lꢀ1 HCl test solutions. The concentration of pyrido[1,2-a]-
benzimidazoles ranged from 0.10 mmol Lꢀ1 to 0.25 mmol Lꢀ1
for the test solutions.
Experimental
Inhibitor synthesis and characterization
Weight loss experiments
The pyrido[1,2-a]benzimidazoles were synthesized in the laboratory
following the reported procedure in the literature,13 and the
synthetic procedure is shown in Fig. 1. The pyrido[1,2-a]benz-
imidazoles synthetic procedure possesses many advantages,
such as using molecular oxygen as a green oxidant, readily
available starting materials, and high regioselectivity. The
molecular structures of the inhibitors were confirmed by their
spectroscopic and spectrometric data (1H NMR, 13C NMR, IR),
as shown below.
Weight loss tests were performed under the condition of different
concentrations of BIP, MBIP, and TBIP inhibitors in 1 mol Lꢀ1
HCl medium at room temperature [(30 ꢂ 0.1) 1C] for 72 h. After
the corrosion test was completed, the specimen’s surface was
carefully rinsed with ultrapure water, dried and weighed. In order
to obtain the average weight loss reduction error, three parallel
experiments were carried out in each group with a standard
deviation of less than 7% (Fig. 2), indicating that the experiment
was reproducible. The corrosion rate (v) was obtained from the
average of the three parallel experimental data.5,14 Besides, the
effect of temperature (30 1C to 60 1C) on the corrosion inhibition
performance of BIP, MBIP, and TBIP was also considered by the
weight loss method.
1
Benzo[4,5]imidazo[1,2-a]pyridine (BIP). H NMR (400 MHz,
CDCl3): d/ppm = 8.47 (d, J = 6.8 Hz, 1H), 7.94 (d, J = 8.4 Hz, 1H),
7.90 (d, J = 9.2 Hz, 1H), 7.54 (t, J = 7.6 Hz, 1H), 7.46–7.37
(m, 2H), 6.85 (t, J = 6.6 Hz, 1H); 13C NMR (100 MHz, CDCl3):
d/ppm = 148.4, 144.3, 129.2, 128.7, 125.5, 125.0, 120.9, 119.8,
117.7, 110.3, 110.1; IR (film): nmax = 2983, 2359, 1737, 1503,
1464, 1373, 1238, 938, 847, 763, 730 cmꢀ1
.
Electrochemical measurements
2-Methylbenzo[4,5]imidazo[1,2-a]pyridine (MBIP). 1H NMR
(400 MHz, CDCl3): d/ppm = 8.22 (s, 1H), 7.93 (d, J = 8.0 Hz, 1H),
7.84 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 9.2 Hz, 1H), 7.51 (t, J = 7.8 Hz,
1H), 7.37–7.29 (m, 2H), 2.41 (s, 3H); 13C NMR (100 MHz, CDCl3):
d/ppm = 147.6, 144.4, 132.5, 128.3, 125.1, 122.5, 120.6, 119.8, 119.6,
117.1, 110.2, 17.9; IR (film): nmax = 3036, 2981, 2358, 1737, 1511,
A CHI 760E electrochemical analyzer was used to carry out the
experiments, with a standard three-electrode system: mild steel
as the working electrode, a large platinum plate as the auxiliary
1460, 1417, 1373, 1306, 1236, 1044, 938, 847, 814, 749, 729 cmꢀ1
.
2-(Trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine (TBIP).
1H NMR (400 MHz, CDCl3): d/ppm = 8.82 (s, 1H), 8.00–7.96
(m, 2H), 7.81 (d, J = 9.6 Hz, 1H), 7.64–7.55 (m, 2H), 7.47 (t, J =
7.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): d/ppm = 147.4, 144.8,
128.6, 126.5, 124.6 (q, J = 2.5 Hz), 124.4 (q, J = 5.7 Hz), 123.4
(q, J = 269.1 Hz), 122.1, 120.4, 118.6, 114.3 (q, J = 34.2 Hz);
Fig. 2 Variation of corrosion rate and inhibition efficiency with different
concentrations of TBIP, BIP, and MBIP by weight loss test. Error bars (
represent the standard deviations (%).
)
Fig. 1 Synthetic route of the studied pyrido[1,2-a]benzimidazoles.
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electrode, and a saturated calomel electrode (SCE) as the where Etotal is the total energy of the entire system, Esurface+solution
reference electrode. Polarization curves were measured in the represents the total energy of the Fe(110) surface and the
potential range from ꢀ760 to ꢀ210 mV (vs. SCE) with a scan solution without the inhibitor, and Einhibitor is the energy of
rate of 1 mV sꢀ1, and EIS measurements were performed at the free inhibitor molecule.
EOCP in the frequency range from 100 kHz to 100 mHz with a
sinusoidal potential perturbation of 5 mV. The EIS data were
fitted with Zsimpwin software to obtain the corresponding
Results and discussion
Weight loss study
impedance parameters and equivalent circuit. All the electro-
chemical tests were measured at room temperature.
Weight loss is one of the simpler monitoring methods for
evaluating the inhibition performance of corrosion inhibitors,
and has high reliability. Fig. 2 displays the corrosion rate (v)
and inhibition efficiency (Zw) of the steel in 1 mol Lꢀ1 HCl with
different concentrations of BIP, MBIP, and TBIP. The values of v
and Zw were calculated by the following equations:
Surface morphology investigation: SEM and XPS
The surface morphologies of the steel specimens after immer-
sion in 1 mol Lꢀ1 HCl solution without and with 0.25 mmol Lꢀ1
pyrido[1,2-a]benzimidazoles at 30 1C were observed by SEM and
XPS. The SEM study was conducted on a SUPRt55 instrument
(Zeiss, Germany) at 2kꢁ magnification. The X-ray photo-
electron spectroscopy (XPS) spectra tests were recorded using
a Thermo VG Scientific ESCALAB 250 spectrometer with a
monochromatic Mg Ka X-ray source. The XPS spectra were
fitted by XPS PEAK 4.1 software with a reference value of C 1s
peak at 284.6 eV.15
W1 ꢀ W2
v ¼
(3)
(4)
s ꢁ t
v0 ꢀ v
Zw
¼
ꢁ 100%
v0
where v is corrosion rate, W1 and W2 are the weight of steel before
and after immersion, s is the exposed area of steel, t is the
immersion time, and v0 and v represent the corrosion rate without
and with inhibitors, respectively. As seen in Fig. 2, the values of v are
significantly reduced and Zw increases after adding any of the
inhibitors under study. At 0.25 mmol Lꢀ1, the maximum Zw values
of TBIP, BIP, and MBIP were 94.39%, 96.86%, and 98.11%, respec-
tively, and the v value follows the order of v(MBIP) o v(BIP) o
v(TBIP). The results indicated that the presence of pyrido[1,2-
a]benzimidazoles compounds could efficiently restrain the steel
corrosion in HCl solution, and the highest Zw of MBIP in the given
studied inhibitors was attributed to the existence of an electron-
donating –CH3 group. The adsorption mechanism between the
pyrido[1,2-a]benzimidazoles inhibitors and steel surface will be
further clarified in the quantum chemical calculations section.
Quantum chemical calculations
Quantum chemical calculations were performed using density
function theory (DFT) at the B3LYP/6-31G(d,p) level within the
Gaussian 09 program package.16 Generally, the calculated quan-
tum chemical parameters, such as the energy of the lowest
unoccupied molecular orbital (ELUMO), energy of the highest
occupied molecular orbital (EHOMO), energy gap (DE = ELUMO
ꢀ
EHOMO), fraction of electrons transferred (DN), Mu¨lliken charges,
and natural bond orbital (NBO) charges, were used to analyze the
relationship between the adsorption activity of the corrosion
inhibitors and the corrosion resistance of the steel.
Molecular dynamics simulation
Effect of temperature
The interaction of the studied molecules with the Fe(110)
surface was investigated by MD simulations using Materials
Studio5.0 software. The iron crystal was imported and cleaved
along the (110) plane and a slab of 5 Å was employed. Here, the
Fe(110) surface was chosen due to its packed surface and better
stabilization. The Fe(110) surface was then amplified to a (10 ꢁ 10)
supercell to provide a large surface for the interaction inhibitors,
and a zero thickness vacuum slab was established. A supercell
with a = b = 24.82 Å, c = 25.14 Å, containing 500H2O, 5H3O+,
5Clꢀ, and a tested inhibitor molecule was built. The simulation
process was conducted in a simulation box (24.82 Å ꢁ 24.82 Å ꢁ
43.21 Å) by COMPASS force field with 1.0 fs time steps and
500 ps of simulation time at ambient temperature (B25 1C)
under a macro canonical ensemble (NVT). The interaction
energy and binding energy of the inhibitor molecules with
the Fe(110) surface can be expressed by eqn (1) and (2),
respectively:14
In order to understand the inhibition mechanism of tempera-
ture on corrosion inhibition, weight loss measurements were
applied for the steel in 1.0 mol Lꢀ1 HCl without and with
0.25 mmol Lꢀ1 pyrido[1,2-a]benzimidazoles inhibitor at a tem-
perature range of 30 1C to 60 1C (Table 1). As illustrated in
Table 1, the v value increased significantly with increasing
temperature in the uninhibited and inhibited solutions, which
is attributed to the desorption of adsorbed inhibitor molecules
at elevated temperatures, resulting in exposure to corrosive
Table 1 Weight loss results of mild steel in 1 mol Lꢀ1 HCl with 0.25 mmol Lꢀ1
inhibitors at different temperatures
TBIP
BIP
MBIP
v
Zw
v
Zw
v
Zw
T (1C) (mg cmꢀ2 hꢀ1) (%) (mg cmꢀ2 hꢀ1) (%) (mg cmꢀ2 hꢀ1) (%)
30
40
50
60
0.403
0.999
2.150
4.069
94.39 0.226
90.37 0.696
84.21 1.616
77.35 3.402
96.86 0.136
93.29 0.459
88.13 1.030
81.06 2.567
98.11
95.57
92.43
85.71
Einteraction = Etotal ꢀ (Esurface+solution + Einhibitor
Ebinding = ꢀEinteraction
)
(1)
(2)
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Table 2 Activation parameters for the steel dissolution in 1 mol Lꢀ1 HCl
without and with inhibitors
media with a large metal surface area, leading to the efficiency
of the inhibitor decreasing. That means TBIP, BIP, and MBIP
are temperature-dependent inhibitors. Furthermore, it was find
that MBIP had a higher inhibition efficiency than TBIP and BIP
at any temperature tested.
Inhibitors R2
Ea (kJ molꢀ1
)
DH* (kJ molꢀ1
)
DS* (J molꢀ1 Kꢀ1
)
Blank
TBIP
0.996 25.36
0.997 64.72
0.994 75.46
0.993 80.83
22.72
62.08
72.82
78.12
ꢀ153.99
ꢀ47.82
ꢀ16.93
ꢀ3.69
In addition, kinetic analysis was performed to better under- BIP
MBIP
stand the effect of temperature on the inhibition mechanism. The
activation energy (Ea) can be obtained from the Arrhenius eqn (5):
Ea
Adsorption isotherms
ln vcorr ¼ ln A ꢀ
(5)
RT
Generally, adsorption isotherms can give basic information
about the adsorption behavior of an inhibitor by fitting the
relation function between the surface coverage and concen-
tration. Although there are many isotherms models in the
literature, the most widely employed in corrosion inhibition
studies are the Langmuir, Temkin, and Frumkin isotherms,
which can be calculated by the following equations:
where vcorr is the corrosion rate, R is gas constant, T is the absolute
temperature, and A represents pre-exponential factor. The Arrhe-
nius plots of the steel in the test solutions are shown in Fig. 3. The
value of Ea can be obtained from the Arrhenius plots line slope, as
shown in Table 2. As depicted in Table 2, all the regression
coefficients are almost close to unity, indicating that the corrosion
of mild steel in 1 mol Lꢀ1 HCl solution can follow the kinetic
model. It is noteworthy that the values of Ea in the presence of
studied inhibitors were obviously higher than those of the unin-
hibited values, which may be related to the phenomenon of
physical adsorption occurring on the steel surface formed by
the electrostatic characteristics, which has been reported in the
literature.17,18 The activation enthalpy and entropy (DH*, DS*) of
the corrosion process were calculated by the transition state
eqn (6), and the results are shown in Table 2:
c
1
Langmuir:
¼
þ c
(7)
y
Kads
Temkin: exp(fy) = Kads
c
(8)
(9)
y
Frumkin:
expðꢀ2f yÞ ¼ Kadsc
1 ꢀ y
where c is the inhibitor concentration, y represents the surface
coverage, which was calculated according to eqn (4), f is the
factor of energetic inhomogeneity, and Kads is the adsorption
equilibrium constant. Fig. 5 shows the plots obtained from
eqn (7)–(9), where the correlation coefficient R2 of the Langmuir
isotherm (Fig. 5a) is above 0.99, which is better than for the
Temkin isotherm (Fig. 5b) and Frumkin isotherm (Fig. 5c).
From this observation, it was found that the Langmuir adsorp-
tion calculated by eqn (7) best fits the data from the weight loss
test, with a linear regression coefficient (R2) close to 1 (Table 3),
which suggests that the experimental data are well described by
the Langmuir isotherm. The standard adsorption free energy
(DG0ads) values were calculated by eqn (10):
ꢀ
ꢁ
ꢀ
ꢁ
vcorr RT
¼
DSꢃ
R
ꢀDHꢃ
exp
exp
(6)
T
Nh
RT
where h is Planck’s constant and N is Avogadro’s number. A plot
of ln(vcorr/T) vs. 1/T for mild steel in the test solutions at the
experimental temperatures is given in Fig. 4. Observing the data
in Table 2, it can be found that the DH* values are positive,
which indicates that the activation process is a corrosive
endothermic process, and the hindered dissolution of metal in
the inhibitor solution occurs.18 The higher values of DS* in the
presence of pyrido[1,2-a]benzimidazoles may be caused by the
adsorption of inhibitor molecules, which could be considered as
a quasi-substitution process between the inhibitor molecules
and the water molecules at the solution and metal interface.17,18
DG0ads = ꢀRT ln(55.5Kads
)
(10)
Fig. 3 Arrhenius plots of ln vcorr vs. 1/T for steel in 1 mol Lꢀ1 HCl in the
Fig. 4 Arrhenius plots of ln(vcorr/T) vs. 1/T for steel in 1 mol Lꢀ1 HCl in the
absence and presence of inhibitors.
absence and presence of inhibitors.
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electrostatic interaction of positively charged inhibitor mole-
cules and negatively charged metal surfaces, belonging to
physisorption. On the contrary, values around ꢀ40 kJ molꢀ1
may involve the formation of chemical bonds between the
inhibitor molecules and metal surface through electron sharing
or transfer, thus belonging to chemisorption.4,21 According
to the actual value of DGa0ds for the inhibitors in this work
(ꢀ36.33 to ꢀ38.51 kJ molꢀ1), the adsorption of corrosion
inhibitors on the steel surface consists of physical adsorption
and chemical adsorption interaction rather than a single
adsorption action. Besides, the values of Kads follows the order
of MBIP 4 BIP 4 TBIP, and MBIP shows the maximum
negative value of DG0ads, further illustrating that MBIP to BIP
and MBIP are more capable of adsorption on a metal surface.
Also, the strong interaction between inhibitors and mild steel can
be ascribed to the existence of nitrogen heteroatoms, electron-
donating groups, and p-electrons in the MBIP molecules.
Potentiodynamic polarization curves
The polarization curves of mild steel in 1 mol Lꢀ1 HCl solution
with and without pyrido[1,2-a]benzimidazoles inhibitors are
shown in Fig. 6. Table 4 summarizes the calculated inhibition
efficiency (Z) and related electrochemical parameters obtained
by Tafel extrapolation method. The inhibition efficiency was
calculated by eqn (11) based on the measured icorr
ic0orr ꢀ icorr
:
Zð%Þ ¼
ꢁ 100
(11)
ic0orr
where, i0corr and icorr are the current density values of unprotected
and protected steel in acidic solution, respectively. As illustrated
in Fig. 6 and Table 4, all the polarization curves show a trend to
the low current density direction with the use of TBIP, BIP, and
MBIP inhibitors. In addition, the icorr values decreased rapidly
and the inhibition efficiency increased with the increase in
concentration. These results indicated that the three studied
inhibitors have an inhibitive effect on the anodic dissolution and
cathodic hydrogen evolution of the electrode. Besides, the cathodic
polarization curves of the test were nearly parallel and did not
change noticeably, indicating that the mechanism of the hydrogen
evolution reaction was not affected by the pyrido[1,2-a]benz-
imidazoles inhibitors. The reduction of H+ ions available on the
steel surface was due to the active sites of the hydrogen
evolution reaction being inhibited by the inhibitor molecules
attached to the metal surface. In the anodic region, the anode
curves of TBIP and BIP became steep, before overlapping and
intersecting with the blank hydrochloric curve at a value of
ꢀ0.22 V vs. SCE. According to Zhang,4 this phenomenon at the
anodic region means the dissolution rate of steel is faster than
the formation of the protective film, causing the inhibitor
molecules desorption on the steel surface. For MBIP at a
concentration above 0.2 mmol Lꢀ1, the anodic curves also
became steep, overlapping but not intersecting with the blank
hydrochloric curve, demonstrating that MBIP adsorption is
more stable on mild steel surfaces than TBIP and BIP.
Fig. 5 Plots of (a) Langmuir, (b) Temkin, and (c) Frumkin isotherms for the
adsorption of inhibitors on the surface of mild steel in 1 mol Lꢀ1 HCl.
where T and R represent the thermodynamic temperature and
gas constant, respectively. The DG0ads values of the three inhibitors
were calculated and listed in Table 3 as ꢀ36.33, ꢀ37.64, and
ꢀ38.51 kJ molꢀ1, respectively. As already reported previously,19–21
DG0ads values around ꢀ20 kJ molꢀ1 or less are related to the
Table 3 Thermodynamic parameters of the inhibitors on mild steel surface
in 1 mol Lꢀ1 HCl solution
Inhibitors
R2
Slope
Kads (104)
DG0ads (kJ molꢀ1
)
TBIP
BIP
MBIP
0.9995
0.9988
0.9995
0.933
0.945
0.965
3.30
4.79
7.84
ꢀ36.33
ꢀ37.64
ꢀ38.51
Moreover, it is noteworthy that the Z of the three pyrido[1,2-a]-
benzimidazoles followed the order of Z(MBIP) 4 Z(BIP) 4 Z(TBIP),
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Table 4 Polarization curve parameters for the corrosion of mild steel in
1 mol Lꢀ1 HCl solution without and with different concentrations of
inhibitors
c
Ecorr icorr
ba
bc
Inhibitors (mmol Lꢀ1) (mV) (mA cmꢀ2) (mV decꢀ1) (mV decꢀ1) Z (%)
—
Blank
ꢀ474 649.57
117.2
ꢀ97.6
—
TBIP
0.10
0.15
0.20
0.25
ꢀ472 151.62
ꢀ483 65.47
ꢀ485 38.12
ꢀ481 29.36
111.0
120.9
117.2
119.8
ꢀ84.1
ꢀ78.2
ꢀ75.9
ꢀ78.9
76.66
89.92
94.13
95.48
BIP
0.10
0.15
0.20
0.25
ꢀ482 97.40
ꢀ477 46.31
ꢀ484 25.27
ꢀ499 15.26
113.5
113.1
116.3
121.9
ꢀ89.4
ꢀ90.6
ꢀ93.9
ꢀ107.8
85.01
92.87
96.11
97.65
MBIP
0.10
0.15
0.20
0.25
ꢀ490 72.55
ꢀ488 31.95
ꢀ484 11.74
ꢀ493 6.72
118.1
118.3
125.4
121.8
ꢀ113.6
ꢀ90.0
88.83
95.08
98.19
98.96
ꢀ93.6
ꢀ106.4
inhibitors can simply block the active reaction sites on the steel
surface without effecting the corrosion mechanism of mild steel.
In this way, it can be expected that the inhibition efficiency will
increase as the inhibitor concentration increases.
Electrochemical impedance spectroscopy (EIS)
EIS measurements were performed to investigate the corrosion
behavior of the uninhibited and inhibited steel. The Nyquist plots
of steel with varying concentrations of TBIP, BIP, and MBIP in
1 mol Lꢀ1 HCl are shown in Fig. 7. As shown in Fig. 7, the
diameters of the capacitive loop with either studied inhibitor were
much larger than with the blank hydrochloric acid, and the
inhibition strength was enhanced with increasing the concen-
tration, indicating that the studied pyrido[1,2-a]benzimidazoles
have a high inhibition ability. Besides, the Nyquist plots exhibit an
approximately compressed semi-circle capacitive loop, which is
usually considered to be caused by a ‘‘dispersion effect’’.24–26 In
general, this phenomenon may be caused by the surface roughness
and other non-uniformities of metal electrodes. Also, according to
the Bode plots in Fig. 8, the phase angles of the studied inhibitors
show two peaks, indicating that there are two phase constants of
these compounds and that the double-layer impedance does not
show a perfect capacitance behavior. Besides, the absolute impe-
dance increases significantly with the constant addition of the
inhibitors, which is ascribed to the fact that the steel surface
Fig. 6 Potentiodynamic polarization curves of mild steel in 1 mol Lꢀ1 HCl
solution without and with different concentrations of inhibitors: (a) TBIP,
(b) BIP, and (c) MBIP.
with the maximum inhibition efficiencies of 98.96%, 97.65%, and adsorbs more molecules at high concentrations, making the
96.48%, respectively (Table 4), and the experimental results were in adsorption film thicker and causing the charge transport process
accordance with those obtained by the weight loss method. to become more difficult. Considering the influence of the adsorp-
Simultaneously, the potential of the studied inhibitors at any tion properties of the inhibitor on the charge transfer process,
concentration was negatively shifted and less than 25 mV constant phase elements (CPEs) were applied to optimize the
compared with the blank hydrochloric acid, showing that the double-layer capacitor to obtain the ideal electrochemical
three inhibitors studied mainly restrained the cathodic reaction process.10,27 The impedance of a CPE(ZCPE) is defined as follows:
and belonged to the mixed-type class of inhibitors. Other
ZCPE = [Y0(jw)n]ꢀ1
(12)
N/S-heterocyclic compounds with similar results for the corro-
sion of mild steel in acidic solution have been reported,22,23 where Y0 is a proportionality coefficient, j is an imaginary
Moreover, the ba and bc values were slightly changed after number, and o and n represent the angular frequency and
adding the inhibitor, which revealed that these adsorptive phase shift, respectively.
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Fig. 7 Nyquist plots for mild steel in 1 mol Lꢀ1 HCl solution without and
Fig. 8 Bode plots for mild steel in 1 mol Lꢀ1 HCl solution without and with
with different concentrations of inhibitors: (a) TBIP, (b) BIP, and (c) MBIP.
different concentrations of inhibitors: (a) TBIP, (b) BIP, and (c) MBIP.
The corresponding two equivalent circuits in Fig. 9(a) and (b) data based on the EIS measurements are listed in Table 5, where
were used to fit the impedance data. The fitted solid lines in Rs is electrolyte resistance, Rct is charge transfer resistance, Rf is
Fig. 7 were fitted by the electric circuit in Fig. 9, which is film resistance, and n is an exponent relevant to the phase shift.
consistent with the EIS experimental data, indicating that the The CPEdl element can be substituted for the double-layer
equivalent circuits of the selection conform to the experimental capacitance (Cd), which contains H2O and other ions adsorbed
requirements. The accuracy of the fitted data was evaluated on steel surface, and CPEf reflects the film capacitance (Cf).
using a chi-squared (w2). Table 5 shows that the w2 values were The inhibition efficiency (Zz) can be expressed by eqn (13):
of the order of 10ꢀ3, indicating that the fitted data agree well
ꢂ
ꢃ
Rp ꢀ R0p
with the experimental data. For example, the EIS result of TBIP
was simulated by an equivalent circuit, as revealed in Fig. 9(b),
with the obtained fitting curve illustrated in Fig. 10. The small
values of w2 (5.33 ꢁ 10ꢀ3) indicate a better fit.
Zzð%Þ ¼
ꢁ 100
(13)
Rp
where R0p and Rp represent the sum of Rf and Rct of the mild steel
Based on the fitted lines and w2 analysis, the equivalent electrode without and with pyrido[1,2-a]benzimidazoles inhibitors,
circuits given in Fig. 9(a) and (b) were found to be sufficient and respectively. According to Table 5, the Rf and Rct values increased
gave excellent fitting to the experimental EIS date. The fitted dramatically after adding the studied inhibitors, indicating that
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Fig. 9 Equivalent circuit used to fit the EIS experiment data: (a) without an
inhibitor and (b) in the presence of an inhibitor.
Fig. 10 Equivalent circuit fitting for mild steel in the presence of TBIP at
0.25 mmol Lꢀ1: (a) Nyquist plot and (b) Bode plot.
these compounds were adsorbed on the metal surface and thus
effectively inhibited the charge transfer behavior. While, the values
of Cf and Cd were gradually reduced with the constant addition of
inhibitors, which is usually caused by a decrease in the local respectively, which is consistent with the results from the weight
dielectric constant or the increase in the thickness of the electric loss test and the polarization curves.
double layer capacitor.28–30 It may be that the inhibitor molecules
Compared with the earlier studied imidazole derivative
gradually replaced the water molecules on the steel surface, inhibitors by EIS method in 1 M HCl, as shown in Table 6,
forming a layer of dense protective film to prevent contact between TBIP, BIP, and MBIP exhibited better corrosion inhibition
the metal and acidic medium, thus preventing the occurrence of behaviors. This phenomenon is related to the aromatic rings
corrosion. In addition, another interesting finding in Table 5 was system and steric hindrance effect in their structures.
that the nd and nf values are near unity, pointing to a capacitive
Surface characterization: SEM and XPS
interface. However, after the addition of the inhibitors, the nd
values decreased compared to the blank solution, which indicates SEM was implemented to investigate the correlation between the
an increase in heterogeneity resulting from the inhibitor’s adsorp- surface properties of metals and the electrochemical corrosion
tion. However, the values of nf showed no significant change, behavior. Fig. 11 presents the SEM images of mild steel
revealing that the corrosion reaction of the electrode surface was samples immersion in 1 mol Lꢀ1 HCl solution without and
suppressed by the absorbed inhibitor film. Finally, it should be with 0.25 mmol Lꢀ1 pyrido[1,2-a]benzimidazoles for 36 h.
pointed out that with the addition of the inhibitors, the calculated Before soaking, the steel samples surface (Fig. 11a) was flat
Zz of EIS increased, and the maximum Zw reached 96.22% for TBIP, and smooth with only a few scratches produced by the polish-
97.23% for BIP, and 98.34% for MBIP at 0.25 mmol Lꢀ1
,
ing process. However, the uninhibited steel sample (Fig. 11b)
Table 5 EIS parameters for the corrosion of mild steel in 1 mol Lꢀ1 HCl solution without and with different concentrations of inhibitors
Inhibitors c (mmol Lꢀ1
)
Rs (O cm2) Cd (mF cmꢀ2
0.91 ꢂ 0.04 161.0 ꢂ 3.52 0.89 ꢂ 0.04
)
nd
Rct (O cm2)
55.6 ꢂ 1.32
Cf (mF cmꢀ2
)
nf
Rf (O cm2) w2 (ꢁ10ꢀ3
—
)
Zz (%)
—
Blank
—
—
—
TBIP
0.10
0.15
0.20
0.25
0.21 ꢂ 0.03 115.1 ꢂ 2.61 0.66 ꢂ 0.06 257.5 ꢂ 4.54 14.1 ꢂ 0.71
1.0 ꢂ 0.02 5.0 ꢂ 0.43 1.48
5.9 ꢂ 0.41 0.99 ꢂ 0.03 13.5 ꢂ 0.78 3.25
4.0 ꢂ 0.32 1.0 ꢂ 0.02 19.5 ꢂ 1.02 4.57
2.5 ꢂ 0.18 0.99 ꢂ 0.04 28.9 ꢂ 1.42 5.33
78.82
91.03
94.33
96.22
0.32 ꢂ 0.05 92.9 ꢂ 1.95 0.67 ꢂ 0.03 606.1 ꢂ 6.57
0.86 ꢂ 0.04 46.1 ꢂ 1.07 0.70 ꢂ 0.02 961.1 ꢂ 8.23
0.72 ꢂ 0.03 28.2 ꢂ 0.95 0.65 ꢂ 0.03 1441.0 ꢂ 9.46
BIP
0.10
0.15
0.20
0.25
0.39 ꢂ 0.04 103.9 ꢂ 2.11 0.75 ꢂ 0.04 442.2 ꢂ 4.96
0.58 ꢂ 0.02 72.1 ꢂ 1.43 0.66 ꢂ 0.05 784.7 ꢂ 6.62
1.09 ꢂ 0.04 32.9 ꢂ 1.01 0.73 ꢂ 0.03 1506.0 ꢂ 9.22
0.67 ꢂ 0.05 22.7 ꢂ 0.86 0.72 ꢂ 0.02 1982.3 ꢂ 9.87
8.4 ꢂ 0.53
4.7 ꢂ 0.29
1.0 ꢂ 0.06 7.1 ꢂ 0.51 2.53
1.0 ꢂ 0.02 21.8 ꢂ 1.04 1.77
87.63
93.11
96.36
97.23
2.8 ꢂ 0.11 0.99 ꢂ 0.03 23.5 ꢂ 1.08 3.62
2.1 ꢂ 0.08 0.99 ꢂ 0.03 28.1 ꢂ 1.13 5.37
MBIP
0.10
0.15
0.20
0.25
0.27 ꢂ 0.03 82.4 ꢂ 1.21 0.65 ꢂ 0.04 544.3 ꢂ 5.02
0.76 ꢂ 0.02 49.1 ꢂ 1.09 0.61 ꢂ 0.03 1615.2 ꢂ 8.34
6.8 ꢂ 0.52
3.1 ꢂ 0.12
1.0 ꢂ 0.04 15.4 ꢂ 0.62 2.23
1.0 ꢂ 0.02 15.7 ꢂ 0.56 3.82
90.07
96.59
98.03
98.34
1.27 ꢂ 0.03 24.9 ꢂ 0.84 0.71 ꢂ 0.02 2804.1 ꢂ 10.03 1.6 ꢂ 0.06 0.98 ꢂ 0.03 24.3 ꢂ 1.12 2.54
0.71 ꢂ 0.04 19.7 ꢂ 0.81 0.68 ꢂ 0.03 3299.0 ꢂ 12.45 1.8 ꢂ 0.07 0.99 ꢂ 0.02 40.9 ꢂ 1.96 3.91
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Table 6 Comparison of the inhibition efficiency of TBIP, BIP, and MBIP with the literature data as corrosion inhibitors for mild steel in 1 mol Lꢀ1 HCl
solution
Inhibitor
c (mmolꢀ1
)
Z (%)
Ref.
2-(Trifluoromethyl)benzo[4,5]imidazo[1,2-a]pyridine
Benzo[4,5]imidazo[1,2-a]pyridine
2-Methylbenzo[4,5]imidazo[1,2-a]pyridine
2-(Imidazol-2-yl)-pyridine
2-Aminobenzimidazole
2-Mercaptobenzimidazole
0.2
0.2
0.2
0.2
0.5
0.5
0.25
0.5
0.5
1.0
0.75
0.75
1.0
1.0
1.0
94.3
96.4
98.0
90.3
47.1
92.0
71.5
56.8
85.7
73.5
90.0
86.0
56.3
45.5
84.0
This paper
This paper
This paper
1
4
12
14
19
19
22
25
25
26
26
28
2-(4-Pyridyl)-benzimidazole
2-Aminomethyl benzimidazole
Bis(2-benzimidazolylmethyl)amine
2-(Ethyllthio)-1,4,5-triphenyl-1H-imidazole
Bis-(2-benzimidazolylmethyl)oxide
1,2-Bis-(2-benzimidazolyl)ethylene
2-Aminomethylbenzimidazole
2-Hydroxybenzimidazole
2-(Benzylthio)-1,4,5-triphenyl-1H-imidazole
was seriously corroded with large pits and cracks due to acid the thermodynamic study confirmed. In addition, the Fe 2p3/2
erosion. Moreover, in contrast, the surface of specimen (Fig. 11c–e) analysis showed that the formation of a stable and insoluble
was relatively smooth and only a few shallow pitting corrosions layer (Fe2O3, FeOOH) makes the protected layer become more
occurred after adding the three inhibitors. In particular, the complete and it possesses excellent anti-corrosion ability.
surface of steel protected by MBIP was the flattest and close to
Quantum chemical calculations
the sample polished surface. It can be deduced from these results
that the three studied inhibitors can be stably adsorbed on steel
surface to form a dense protective film, and the corrosion resis-
tance of MBIP is stronger than that of the other two inhibitors.
With regard to the chemical composition of the adsorption
layer, XPS tests were performed to study this in the presence
of the best inhibitor (MBIP). Fig. 12(a–c) illustrate the high-
resolution XPS spectra of specific elements (C, N, and Fe)
protected layers. According to Fig. 12a, the C 1s spectra were
fitted to three peaks. The small peak at 284.6 eV was ascribed to
C–C, CQC, and C–H aromatic bonds, and another peak located at
286.7 eV was related to C–N and CQN bonds in pyrido[1,2-a]-
benzimidazole ring. The peak at 288.9 eV, which has a higher
binding energy, was attributed to CQN+ structures and may be
caused by the proton of the QN– structure in the pyrido[1,2-a]-
benzimidazole ring.31,32 N 1s peaks (Fig. 12b) at 399.7 eV, 400.8 eV,
and 401.5 eV could be observed, with the first peak assigned to
C–N and the unprotonated N atoms (QN– structure),10,15 while
the peak at 400.8 eV represents N–Fe groups due to the nitrogen
atoms bonded to iron atoms. Actually, the coordination of a
nitrogen atom with an iron atom (N–Fe) can produce a higher
binding energy than an uncoordinated QN– structure.15 The
third peak at 401.5 eV was ascribed to the protonated nitrogen,
which gives rise to positive polarization nitrogen atoms, leading
to a larger binding energy.10 The Fe 2p3/2 (Fig. 12c) showed three
peaks around 710.7 eV, 713.4 eV, and 715.2 eV. The first peak at
710.7 eV was attributed to ferric compounds, which came from
Fe2O3 (i.e., Fe3+ oxide) and FeOOH (i.e., oxyhydroxide). Another
peak appearing at 713.4 eV was due to the presence of FeCl3 in
the hydrochloric acid solution, while the last peak at 715.2 eV
was assigned to the satellite of Fe(III).32
Quantum chemical calculations were performed to better study
the structure–activity relationship between the molecular struc-
ture of the inhibitor and its inhibitory effect in this work.
Fig. 13 displays an optimized molecular structure of TBIP,
BIP, and MBIP. According to ref. 4, Mu¨lliken charges and Fukui
function are commonly used to analyze the adsorption-
activated sites and the donor–acceptor electron interaction
between inhibitor molecules and iron atoms. In that research,
C and N atomic charges obtained by Mu¨lliken charges were
reported and these are listed in Table 7. It can be seen that the
atoms with a larger negative change are C12, N19, N20, F22, F23
and F24 for TBIP, C12, C14, N20 and N21 for BIP, and C12, N19,
N20 and C21 for MBIP, which are the main active atoms
capable of forming coordinate bonds through the lone pair
electrons and the empty d-orbital of iron atoms. Therefore,
TBIP, BIP, and MBIP molecules can be efficiently adsorbed on
steel surface by these active sites (C, F, and N) to restrain
metallic corrosion. Whereas, Neodo et al. reported that N21 was
protonated (pKa ca. 4) and usually the affinity of the inhibitor
was significantly reduced.33 Actually, the protonated inhibitor
can be electrostatically adsorbed on the surface of negatively
charged mild steel due to the specific adsorption of Clꢀ anions
(physical adsorption), namely, protonated organic compounds
can produce a synergistic effect with Clꢀ, and then the anodic
dissolution reaction of iron can be prevented. Moreover, Fukui
function f (r)+ and f (r)ꢀ were used to predict the selectivity of
local sites, which are given by eqn (14) and (15):
fi(r)+ = qi(N + 1) ꢀ qi(N)
fi(r)ꢀ = qi(N) ꢀ qi(N ꢀ 1)
(14)
(15)
The XPS results showing the existence of nitrogen species
illustrated that the chemical adsorption of MBIP inhibitor on where qi(N + 1), qi(N), and qi(N ꢀ 1) are the charges of atom i in
steel surface was accompanied by physical adsorption, which the cationic, neutral, and anionic forms, respectively. The calculated
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Fig. 12 High-resolution XPS spectra of (a) C 1s, (b) N 1s, and (c) Fe 2p3/2
for MBIP treated mild steel.
N20 for TBIP, C1, C3, and N21 for BIP, and C1, C3, and N20 for
MBIP, implying the better tendency of an atom in an inhibitor
molecule to donate an electron to the metal surface. Compared with
the other two compounds, the N20 atom in MBIP had a larger f(r)ꢀ
Fig. 11 SEM images of mild steel surfaces: (a) polished metal, (b) 1 mol Lꢀ1 value, which indicates that the N20 atom of TBIP is more prone to
HCl, (c) TBIP, (d) BIP, (e) MBIP.
electrophilic attack and to form a coordination bond with iron
atoms, leading to a higher inhibition efficiency.
The electron distributions of the HOMO and LUMO of the
results based on DFT for the three compounds are shown in frontier molecular orbitals are favorable for investigating the
Table 7. As a widely accepted view, the larger f(r)+ and f(r)ꢀ values adsorption activity of inhibitor molecules, which is related to
imply higher acceptance and donation electronic capabilities, the molecular donor and accept electrons ability, respectively.4
respectively. In all three inhibitor molecules, the larger f(r)+ values The HOMO and LUMO diagrams of the three compounds
were found on C12, C13, and C15 atoms, which were capable of presented in Fig. 14 all showed that the electron densities were
forming a back-donating bond due to accepting electrons from the distributed over the entire ring contained in the molecule. In
steel surface. Whereas, the larger f(r)ꢀ were found on C1, C3, and MBIP, the –CH3 group at the 21-position makes little contribution
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Fig. 14 The frontier orbital density distributions and 3D MEP plot of
pyrido[1,2-a]benzimidazoles molecules.
Additionally, in the ESP map (Fig. 14), the electrostatic potential
negative regions (shades of red/brownish yellow) associated with
electrophilic attack are mainly located in the N atoms, –CF3 group,
and phenyl ring. However, positive potential regions (shades of
blue) are located around hydrogen atoms and the –CH3 group,
which are possible sites for nucleophilic attack. Thus, it can be
rationally concluded that these compounds have two or three
major adsorption activity sites, bearing N atoms, a –CF3 group,
and phenyl ring. The values of the quantum chemical parameters
(EHOMO, ELUMO, and DE) are given in Table 8. Previous studies34,35
have reported that molecules with high EHOMO values give greater
electron donation capabilities, and conversely, the lower the ELUMO
values, the stronger the acceptance of electronic ability. As shown in
Fig. 13 Optimized structures of TBIP (a), BIP (b), and MBIP (c).
to the HOMO, while it does not contribute to the LUMO, revealing Table 8, the EHOMO values obey the order: EHOMO,MBIP 4 EHOMO,BIP
that the –CH3 group tends to donate charges to the vacancy orbital 4 EHOMO,TBIP, which is in the same order as the inhibition efficiency
of Fe through s-type HOMOs. The –CF3 group at the 21-position of of Z(MBIP) 4 Z(BIP) 4 Z(TBIP). This shows that there is a good
TBIP does not seem to service the HOMO and LUMO, meaning correlation between Z and EHOMO. While, the values of ELUMO
that the –CF3 group is not involved in the role of donor–acceptor. obey the order: ELUMO,TBIP o ELUMO,MBIP o ELUMO,BIP, which is
Table 7 Mulliken charge, f (r)+ and f (r)ꢀ for TBIP, BIP, and MBIP
Compounds
TBIP
Atoms
Mulliken charge charge
f (r)+
0.0380
0.0470
0.0723
ꢀ0.0030
ꢀ0.0087
0.0708
f (r)ꢀ
0.1064
Atoms
Mulliken charge
f (r)+
0.0193
0.1555
0.0279
0.0709
0.0043
0.0188
0.0192
0.0147
f (r)ꢀ
0.0595
0.0516
0.0241
0.1677
ꢀ0.0005
0.0197
0.0201
0.0145
C1
C2
C3
C4
C5
C6
C11
C12
C13
ꢀ0.1112
ꢀ0.0904
ꢀ0.1203
0.2098
C14
C15
N19
N20
C21
F22
F23
F24
ꢀ0.1205
0.1385
ꢀ0.6372
ꢀ0.5726
0.7927
ꢀ0.2651
ꢀ0.2634
ꢀ0.2723
0.0056
0.0996
ꢀ0.0085
0.0225
0.0538
0.0049
0.0593
0.0746
0.3009
ꢀ0.0853
0.5224
ꢀ0.0150
ꢀ0.1311
ꢀ0.0658
0.1334
0.0911
BIP
C1
C2
C3
C4
C5
C6
C11
ꢀ0.1127
ꢀ0.0916
ꢀ0.1225
0.2136
0.0408
0.0457
0.0801
ꢀ0.0040
ꢀ0.0080
0.0773
ꢀ0.0071
0.1052
0.0084
0.0960
C12
C13
C14
C15
N20
N21
ꢀ0.1261
ꢀ0.0675
ꢀ0.1509
0.1384
0.1239
0.1056
0.0089
0.1434
0.0269
0.0714
0.0674
0.0651
0.0614
0.0647
0.0224
0.1777
ꢀ0.0125
0.2938
0.0264
ꢀ0.6192
ꢀ0.0869
0.0483
ꢀ0.0014
ꢀ0.5791
0.5136
MBIP
C1
C2
C3
C4
C5
C6
C11
ꢀ0.1131
ꢀ0.0923
ꢀ0.1227
0.2120
0.0419
0.0427
0.0807
ꢀ0.0046
ꢀ0.0085
0.0764
ꢀ0.0045
0.0981
0.0105
0.0937
C12
C13
C14
C15
N19
N20
C21
ꢀ0.1298
ꢀ0.087
0.0562
0.1205
0.1146
0.0109
0.1299
0.0288
0.0707
ꢀ0.0188
0.0655
0.0557
0.0693
0.0675
0.0259
0.1801
ꢀ0.0220
ꢀ0.0165
0.1007
0.2941
0.0232
ꢀ0.6241
ꢀ0.5808
ꢀ0.3804
ꢀ0.0873
0.0486
ꢀ0.0064
0.5139
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Table 8 The calculated quantum chemical parameters for TBIP, BIP, and
MBIP
pyrido[1,2-a]benzimidazoles is derived from intermolecular
electrostatic force to some extent.
Compounds
EHOMO (eV)
ELUMO (eV)
DE (eV)
DN
m (D)
Molecular dynamics simulation
TBIP
BIP
MBIP
ꢀ5.86
ꢀ5.47
ꢀ5.39
ꢀ1.43
ꢀ1.36
ꢀ1.39
4.43
4.11
4.00
0.26
0.34
0.36
1.43
3.36
3.77
The adsorption behavior of the studied inhibitors on mild steel
surface was also investigated using molecular dynamics simu-
lation, which has emerged as a powerful tool to study the
adsorption nature of inhibitor molecules on a metallic surface.
In the geometric optimization process, atomic coordinates were
adjusted based on the COMPASS force field38,39 until the total
energy of a single structure was minimized. Also, a system
containing 500 water molecules, 5H3O+, 5Clꢀ, and one mole-
cule of the tested inhibitors was constructed to simulate the
actual aggressive medium. After 500 000 steps, the system
reached equilibrium, which explains that both the temperature
and energy reach balance. Fig. 15 represents the top and side
views of the inhibitor molecules adsorbed on the Fe(110)
surface. It can be clearly seen from Fig. 15 that the studied
inhibitor was adsorbed on the Fe(110) surface almost parallel
or flat when the system reached equilibrium, indicating that
the inhibitor molecules has a strong interaction between active
sites and iron atoms. Moreover, the interaction and binding
energies of the adsorption of inhibitors on the Fe(110) surface
were calculated and are listed in Table 8. Additionally, the
binding energy values can also be used to explain the adsorption
capabilities of the molecules. These displayed a higher value of
binding energy, and the more stable adsorption of inhibitor
molecules on the Fe surface. As shown in Table 9, it is quite clear
that the Ebinding of the tested inhibitors and Fe(110) surface
follows the order: MBIP 4 BIP 4 TBIP. This reflects that MBIP
has a better capability for adsorption than the others, and these
outcomes agree well with the experimental evaluation results as
well as the inference of the quantum chemical calculations.
Fig. 16 shows the radial distribution functions of C, N, and F
of the inhibitors and Fe atoms. It is generally believed that the
inconsistent with the results obtained by experiment, and may
be affected by the complex interaction of the adsorption
process.4 The other fundamental parameter in Table 8 is the
separation energy (DE = ELUMO ꢀ EHOMO), which determines the
adsorption activity of inhibitor molecules on a metal surface.
The lower the DE value, the stronger the reactivity of the
corrosion inhibitor is. According to Table 8, the DE values of
different inhibitors decreased in the order: DEMBIP o DEBIP
o
DETBIP, which illustrated that the adsorption capacity of MBIP
among the studied inhibitors is the most stable and possesses a
higher anti-corrosion ability.
The relationship between corrosion inhibition and the
molecular structure can also be analyzed and predicted using
the fraction of transfer electrons (DN). A perusal of the literature36
revealed that the work function (F) of a metal surface is an
appropriate measure of its electronegativity, and the fraction of
electrons (DN) can be calculated by eqn (16):
F ꢀ winh
DN ¼
(16)
2ðZFe þ Zinh
Þ
where w and Z are the absolute electronegativity and global
hardness, respectively, as indicated by eqn (17) and (18):
ꢀEHOMO ꢀ ELUMO
w ¼
Z ¼
(17)
(18)
2
ELUMO ꢀ EHOMO
2
In eqn (16), the value of ZFe is 0 eV and F obtained from DFT
calculations was 4.82 eV for the Fe(110) plane. As reported
previously,36 if DN 4 0, the electrons transfer from the inhibitor
to the metal and vice versa if DN o 0. As shown in Table 8, the
three inhibitors have positive DN values, which mean the
pyrido[1,2-a]benzimidazoles inhibitor molecule generates coor-
dination bonds that donate electrons to the empty d-orbital of
the Fe atom. The ability of the inhibitor molecule to donate
electrons follows the order: DNMBIP 4 DNBIP 4 DNTBIP, which is
consistent with the results obtained by the weight loss tests and
electrochemical measurements. This result suggests that the
ability of pyrido[1,2-a]benzimidazoles to inhibit metal corrosion
depends on its high proportion of electrons donated to the
metal. In addition, m, which is regarded as an important index
related to the inhibition efficiency, is another meaningful para-
meter in Table 8. The adsorption capacity of inhibitor molecules
may be enhanced by a large value of m though the electronic
force.37 As shown in Table 8, the values of m follow the order:
m(MBIP)
4 m(BIP) 4 m(TBIP), which is in agreement
with the inhibition efficiency of the experimental chapters,
and implies that the better inhibitive performance of
Fig. 15 Equilibrium adsorption configuration of the inhibitors: (a) side
view and (b) top view.
424 | New J. Chem., 2019, 43, 413--426
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Table 9 Interaction energy and binding energy values between the
inhibitors and Fe(110) surface
be seen that the peak distances of C, N, and F are within the
range of 2.9 to 3.3 Å from the Fe surface. Following the
literature value,38 one can conclude that the obtained range
of peak distance represents the formation of a covalent bond,
which occurs mostly due to chemisorption.
EInteraction
EBinding
System
(kJ molꢀ1
)
(kJ molꢀ1
)
Fe + TBIP + 500H2O + 5H3O+ + 5Clꢀ
Fe + BIP + 500H2O + 5H3O+ + 5Clꢀ
Fe + MBIP + 500H2O + 5H3O+ + 5Clꢀ
ꢀ800.01
ꢀ889.58
ꢀ915.76
800.01
889.58
915.76
Conclusions
For the first time, pyrido[1,2-a]benzimidazoles were found to be
effective inhibitors for mild steel corrosion in HCl solution.
MBIP, BIP, and TBIP exhibited excellent inhibition perfor-
mance, and their inhibition efficiency followed the order of
Z(MBIP) 4 Z(BIP) 4 Z(TBIP). Potentiodynamic polarization
curve results showed that three compounds belonged to
mixed-type inhibitors, which primarily inhibited cathodic reac-
tion, and the adsorption on the steel surface followed a
Langmuir adsorption isotherm with both physical adsorption
and chemical adsorption. Surface morphology by SEM and XPS
indicated that the adsorption film of pyrido[1,2-a]benzimida-
zoles on the steel surface effectively inhibited the corrosion
process. Besides, the quantum chemical parameters proved
that the adsorption activity centers were mainly phenyl ring
and N atoms, which play an important role in the corrosion
resistance of steel. MD simulations also revealed that the
interaction and binding energies of the three studied inhibitors
followed the order: MBIP 4 BIP 4 TBIP, which firmly supports
the experimentally obtained results. This research gives an
inspiration to design reasonable routes to modify the studied
inhibitors to improve their inhibitive ability.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was funded by the National Natural Science Foundation
of China (21672046, 21372054, 21503056), and the Fundamental
Research Funds for the Central Universities (HIT.NSRIF.201701).
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