Experimental/computational assessments of API steel in 6 M H2SO4 medium containing novel pyridine derivatives as corrosion inhibitors

Article abstract of DOI:10.1016/j.molliq.2021.115705

In this work, two pyridine derivatives were synthesized and explored their inhibitive influence on the API carbon steel corrosion in a 6 M H₂SO₄ solution. The compound was examined at different concentrations to optimized the best co

Full text of DOI:10.1016/j.molliq.2021.115705

Journal of Molecular Liquids 330 (2021) 115705
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Experimental/computational assessments of API steel in 6 M H2SO4
medium containing novel pyridine derivatives as corrosion inhibitors
a
b
b
Ahmed A. Farag a, , Eslam A. Mohamed * , Galal H. Sayed , Kurls E. Anwer
⁎
a
Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt
b
Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, 11566, Cairo, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, two pyridine derivatives were synthesized and explored their inhibitive influence on the API carbon
steel corrosion in a 6 M H2SO4 solution. The compound was examined at different concentrations to optimized
the best concentration for the corrosion inhibition effect. Gravimetric measurements, potentiodynamic polariza-
tion measurement (PDP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM)
and theoretical methods were used to evaluating the inhibition efficiency. The obtained adsorption data achieved
the model of Langmuir isotherm. The PDP tests proposed that pyridine derivatives are mixed-type inhibitors. The
EIS parameters demonstrate that the double-layer capacitance (Cdl) decreases with the increase of inhibitor con-
centration, which implies a reduces dielectric constant. The parameters derived from the theoretical quantum
chemical calculations were found adequately correlated with the obtained experimental results.
© 2021 Elsevier B.V. All rights reserved.
Received 27 December 2020
Received in revised form 31 January 2021
Accepted 16 February 2021
Available online 21 February 2021
Keywords:
API carbon steel
Acid corrosion
SEM
Theoretical study
Molecular dynamics simulations
1. Introduction
corrosion inhibitors usually containing the heteroatoms as nitrogen,
sulfur, and/or oxygen atoms in their molecular structures [8–10]. The
The acidization process is a widely used technique for a depleted oil
well to enhancing oil recovery. In the acidizing process acidic solution,
as sulfuric and hydrochloric acids are generally used in the concentra-
tion range of (5–28%) to easily dissolve carbonate and other calcareous
materials [1]. Unfortunately, these acidic solutions have aggressive cor-
rosion influences on the metallic surfaces. To protect the metallic sur-
faces from corrosion damage, corrosion inhibitors are added to the
acidic solutions [2,3]. For example, to effectively clean the steel surfaces
from scales and rusts, the acid solution with high concentration
(40–60%vol) containing corrosion inhibitors with concentration 1–2%.
However, the suitable choice of organic and inorganic corrosion inhibi-
tors should be considered, such as low cost and nontoxic compounds to
be appropriate for various industrial applications [4]. The organic com-
pounds with heteroatoms consider a potential type of corrosion inhibi-
tor. The organic corrosion inhibitors are reducing the metallic corrosion
process through adsorption on the surfaces of metals and block the ac-
tive sites [5,6]. The inhibition effect of the organic inhibitors strongly re-
lated to their molecular chemical structure, functional groups, bonding
ability of the chain length, aromatic behavior, and the strength of bond-
ing between inhibitor and metal surface [7]. The most effective organic
adsorption process of the organic corrosion inhibitors occurred at the
metal surfaces by the three main mechanisms: (1) interactions between
the lone pair electrons of heteroatoms (i.e. N, S, O) of the organic inhib-
itors and the vacant d-orbital atoms at the metal surface, (2) interactions
between the π-electrons on the aromatic rings and the vacant d-orbital
atoms at the metal surface, (3) electrostatic attractions between the
charged inhibitor molecules and the charged metal surface [11]. So,
the nature and charge of the metal surface strongly affect the inhibitor's
adsorption. Therefore, if the metal surface charges positive, the anionic
inhibitors easily adsorbed, while if the metal surface charges negative,
the cationic inhibitors easily adsorbed. The prepared pyridine deriva-
tives widely used in pharmaceutical and biological activities as antitu-
mor, anti-metallurgical, and antibacterial [12]. Also, some derivatives
of pyridine compounds have been reported as corrosion inhibitors for
different steel in acidic solutions [13–18]. Based on what has been
paved, the present work aims to synthesis two compounds of pyridine
derivatives contained aromatic rings, amino group, methoxy group as
well as heterocyclic with O and N atoms and evaluate their inhibition
performance for steel corrosion. Gravimetric measurements, potentio-
dynamic polarization measurement (PDP), electrochemical impedance
spectroscopy (EIS), and scanning electron microscopy (SEM) tech-
niques were used to study the inhibitive performance of the synthesized
compounds for carbon steel corrosion in 6 M H2SO4 solution. Theoretical
studies were conducted using Density Functional Theory (DFT) and
Monte Carlo (MC) simulation to corroborate the experimental results.
⁎
Corrsponding author.
E-mail addresses: ahmedafm@yahoo.com (A.A. Farag), eslamazmy60@yahoo.com
(E.A. Mohamed).
https://doi.org/10.1016/j.molliq.2021.115705
0167-7322/© 2021 Elsevier B.V. All rights reserved.

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
2. Experimental details
Journal of Molecular Liquids 330 (2021) 115705
Table 2
Elemental analysis data of the synthesized pyridine derivatives.
2.1. Synthesis of inhibitors
Compound
I
II
Elements analysis
Parameters
C
H
N
Calc.
72.51
5.14
12.69
C20H17N3O2
331
85
210–212
Found
72.46
5.19
Calc.
72.51
5.14
12.69
C19H16N2O2
331
Found
72.49
5.16
Table 1 gives nomenclature, chemical structures, and code (I, and
II) of the synthesized pyridine derivatives. A mixture of benzaldehyde
(0.01 mol, 1.06 mL), 3,4-dimethoxyacetophenone (0.01 mol, 1.8 g),
malononitrile (0.01 mol, 0.66 g), and ammonium acetate (0.015 mol,
1.16 g) was fused for 5 h at 373 K to give compound (I). To synthesis
compound (II) a mixture of 3,4-dimethoxy benzaldehyde (0.01 mol,
1.66 g), acetophenone (0.01 mol, 1.2 mL), malononitrile (0.01 mol,
0.66 g), and ammonium acetate (0.015 mol, 1.16 g) was fused for
5 h at 373 K. After cooling, the solid product of compounds (I & II)
was washed with water and recrystallized from ethanol. The elemen-
tal analysis data of the synthesized pyridine derivatives were listed in
Table 2. As can be cleared from the elemental analysis data, the exper-
imental and the calculated values are in harmony with each other,
which confirms the purity of the synthesized compounds. The IR,
1HNMR, and Mass spectra analysis of the synthesized pyridine deriva-
tives as the following: Compound (I): IR (KBr) showed absorption
bands at 3512, 3405 cm−1 (NH2), 3078 cm−1 (CHar), 2200 cm−1
(CN), 1604 cm−1 (C=N). 1H NMR (DMSO‑d6) exhibited signals at δ
(ppm): 3.80 (s, 3H, OCH3), 3.83 (s, 3H, OCH3), 6.90 (s, 2H, NH2, D2O
exchangeable), 7.02–7.80 (m, 8H, Ar\\H and olefinic-H). Mass spec-
trum showed the molecular ion peak at m/z 331 (99.6%). Compound
(II): IR (KBr) spectrum showed absorption bands at 3304,
3171 cm−1 (NH2), 3085 cm−1 (CHar), 2203 cm−1 (CN), 1642 cm−1
(C=N). 1H NMR (DMSO‑d6) exhibited signals at δ (ppm): 3.82
(s, 3H, OCH3), 3.84 (s, 3H, OCH3), 6.92 (s, 2H, NH2, D2O exchange-
able), 7.02–8.12 (m, 8H, Ar\\H and olefinic-H). The mass spectrum
showed the molecular ion peak at m/z 331 (99.9%). The charts of
IR, 1H NMR, and mass spectrum were represented in Figs. (1–3),
respectively.
12.67
12.68
M. Formula
M.w. (g/mol)
Yield (%)
74
> 300
m.p. (°C)
2.3. Preparation of test solutions
The blank corrosive solution of 6 M H2SO4 was prepared from sulfu-
ric acid Fisher Scientific grade (H2SO4, 98%) using distilled water. Vari-
ous molar concentrations (8.30 × 10−5–2.66 × 10−3) of inhibitors
were prepared using the acidic solutions of 6 M H2SO4. The volume of
corrosive medium used for the gravimetric measurements and the elec-
trochemical testes was 200 ml and 100 ml, respectively.
2.4. Methods for evaluation of corrosion inhibition
2.4.1. Gravimetric measurements
The gravimetric measurements for carbon steel coupons were per-
formed according to the ASTM G 31–72. The specimens were immersed
in the acidic solution of 6 M H2SO4 for 8 h without and with various con-
centrations studied inhibitors (I, and II) at 298 K. After immersion time,
the specimens were firstly rinsed several times with distilled water, and
then dried in an oven. The carbon steel coupons were weighed before
(W1) and after (W2) immersion and the weight loss (ΔW) calculated
(ΔW = W1 – W2). The corrosion rate (ν, mg cm−2 h−1), the surface cov-
erage (θ), and the inhibition efficiency (ηWL%) were evaluated by
Eq. (1)–(3), respectively [19].
ΔW
At
ν ¼
θ ¼
ð1Þ
ð2Þ
2.2. Steel sample composition and surface preparation
νo−νi
νo
Carbon steel coupons of the dimensions 2 × 2 × 0.3 cm (exposed sur-
face area-10.4 cm2) were used for gravimetric measurements. The com-
position of the steel by weight is; C = 0.07, Mn = 0.19, P = 0.02, Si =
0.03, Cr = 0.05, Al = 0.02, Cu = 0.12 and balance Fe. Circular coupons
with exposed surface area 1-cm2 were used for the electrochemical
measurements. The carbon steel samples were mechanically abraded
with various wet silicon carbide (SiC) paper (grades 220–1200). Finally,
the steel specimens rinsed with double-distilled water, degreased ultra-
sonically with AR grade acetone/ethanol mixture to degreased and then
dried before use.
νo−νi
νo
ηWL
¼
ꢀ 100
ð3Þ
where A is the surface area of the carbon steel specimen (cm2); and t is
the immersion time (h), νo and νi are the corrosion rates in the absence
and presence of the studied inhibitor, respectively. All weight-loss tests
were repeated three times at the same conditions to give creditable
values.
Table 1
Nomenclature, chemical structures and code of the pyridine derivatives.
Nomenclature
Molecular structure
Code
I
2-amino-6-(3,4-dimethoxyphenyl)-4-phenylnicotinonitrile
2-amino-4-(3,4-dimethoxyphenyl)-6-phenylnicotinonitrile
II
2

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Fig. 1. FTIR charts of the synthesized pyridine derivatives.
2.4.2. Electrochemical measurements
by the Gaussian 09 W program package [21]. The electronic structural
descriptors namely high occupied molecular orbital energies (EHOMO
and low unoccupied molecular orbital energies (ELUMO), an energy gap
(ΔE) between LUMO and HOMO, dipole moment (μ) and the number
of the transferred electron (ΔN) were calculated [22].
Electrochemical measurements were performed in a conventional
three-electrode cylindrical Pyrex glass cell using a Potentiostat PGZ
402 controlled by Volta-Master 4 programming. A platinum wire was
used as a counter electrode and a saturated calomel electrode (SCE)
was used as reference electrodes. Carbon steel specimens were used
as the working electrodes. The experiments were carried out in aerated
solutions at the end of 1800 s of immersion, which allowed the OCP
values to acquire a steady state. The temperature was fixed at 298
2 K using a water thermostat. The anodic and cathodic polarization
curves were attained at a scan rate of 5 mV s−1 in the potential range
)
2.7. Molecular dynamic simulation details
Monte Carlo simulations (MCs) using Materials studio 9 software
packages were used to study the interaction phenomena carried out be-
tween the inhibitor molecules and the metal surface (Fe110) [23]. The
simulation box of Fe consisted of 5 floors of iron atoms split along the
(110) plane in the simulation cell (29.78 Å/29.78 Å/30.13 Å) with peri-
odic boundary conditions with a supercell of (10 × 10) to increase the
coverage area of the Fe (110) was created and a vacuum of 19.30 Å
was mounted above the surface considered.
from E = Ecorr
250 mV. The EIS measurements were performed
over a frequency range of 10 kHz to 10 mHz with the AC voltage ampli-
tude of 5 mV peak to peak. The impedance data were analyzed and
fitted with the simulation ZSimpWin 3.60, equivalent circuit software.
All electrochemical tests were repeated three times at the same condi-
tions to produce creditable values.
3. Results and discussion
2.5. Surface morphological study
3.1. Electrochemical impedance spectroscopy measurements
The carbon steel coupons with dimensions 1 × 1 × 0.1 cm were used
for the surface analysis. After preparing the steel specimens according to
the above-reported procedure, immersed in the 6 M H2SO4 and 6 M
H2SO4 containing without and with an optimum concentration of the
selected inhibitor for 8 h. After that, the specimens subjected to SEM
analysis. SEM analysis was carried out using a JEOL JSM 6390electron
microscope.
The Nyquist plots of carbon steel electrode in 6 M H2SO4 solutions
without and with various concentrations of the prepared compounds
are presented in Fig. 4. Nyquist curves showed large capacitive loop at
high frequencies followed by small inductive loop at low frequency
values in the presence and absence of the studied inhibitors. This sug-
gested that the dissolution of the carbon steel in 6 M H2SO4 solution is
mostly restricted by the charge transfer at the high frequency capacitive
loop. On the other hand, the low frequency inductive loop may be re-
lated to the relaxation process resulted from adsorption species like
FeSO4 or inhibitor species on the carbon steel surface electrode [24].
Furthermore, the Nyquist plots for all the tested concentrations of
2.6. DFT theoretical details
The theoretical calculations for the studied inhibitor molecules were
performed using the DFT (B3LYP) [20] methods with 6-311G basis set
Fig. 2. 1H NMR charts of the synthesized pyridine derivatives.
3

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Fig. 3. Mass spectra charts of the synthesized pyridine derivatives.
pyridine compounds are semi-circles with identical shapes. Moreover,
the diameter of the semi-circle increases as the concentration of pyri-
dine compounds increased. This proposed that no valuable modification
in the corrosion mechanism, as a consequence of the presence of inhib-
itor molecules [25]. ZSimpWin 3.60 software was used to fit the Nyquist
plots obtained for the carbon steel electrode in the absence and pres-
ence of pyridine inhibitors using the equivalent circuit represented in
Fig. 5. The equivalent circuit included, Rs, Rct, and CPE which represent
the solution resistance, charge transfer resistance, and the constant
phase element, respectively. EIS parameters deduced from the fitting
process are given in Table 3. The chi-squared (χ2) value was used to
judge the quality of the fitted data. Table 3 showed that the χ2 values
are low, which suggested that the fitted data in good agreement with
the experimental data. Generally, the double-layer capacitance (Cdl)
Fig. 4. Nyquist and Bode plots of the carbon steel in 6 M H2SO4 solution without and with different inhibitors concentrations at 298 K.
4

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
of Table 3 reveals that the Cdl values are reduced and the Rct values are
rise with increasing the inhibitor concentrations. The phenomenon at-
tributes to the adsorption of pyridine molecules onto the metal surface
and desorption of water molecules from the metal surface [29]. The Rct
values increase to reach a maximum at 2.66 × 10−3 mol dm−3 of inhib-
itor concentrations which give inhibition efficiency of 86.6%, and 75.0%
for I, and II, respectively.
The Bode and phase angle plots of impedance were shown in Fig. 4. It
can be found that the absolute impedance |Z| at low frequencies is grad-
ually rising by concentrations of the pyridine derivatives, which is re-
lated to the protection of the carbon steel through enhancement of
the adsorption of the inhibitor's molecules with concentration. The ap-
erture of phase angles increases with the inhibitor concentration, pro-
posed better surface coverage of the carbon steel by the inhibitor
molecule [30]. The reduced values of capacitance with the inhibitor con-
centration (Table 3), could be related to reduces the dielectric constant.
The protective layer formed by the interactions between inhibitor
molecules and the metal surface can be interpreted by the Helmholtz
model [31]:
Fig. 5. Randles equivalent circuit applied for fitting the impedance spectra.
value is influenced by the roughness of the metal surface, so it simulated
using CPE [26]. The impedance of CPE and the Cdl values were calculated
by the following Eqs. [27]:
εoε
d
Cdl
¼
A
ð7Þ
1
ZCPE
¼
ð4Þ
n
where εo is the vacuum dielectric constant, ε is the dielectric constant, A
is the surface area of the electrode, and d is the thickness of the double
layer at the metal surface. The results demonstrate that the inhibitor
molecules adsorbed on the metal surface by dispelling water molecules
to forming a protective film, that impedes the charge transfer and hence
inhibits the corrosion process [32]. These results confirm the obtained
from EIS measurements and show the same trend.
QðjωÞ
n−1
Cdl ¼ Qðωmax
Þ
ð5Þ
where Q (Ω−1 sn cm−2) is a coefficient represent the combination of dif-
ferent physical phenomena like surface heterogeneity and electro-
active species in the solution, j is an imaginary number (j2 = −1), ω is
the angular frequency (ω = 2πf) and the exponent n represents the
phase shift (its value between −1 and 1). A value of −1 is corresponded
to an inductance, a value 1 characteristic to a capacitor, and a value of
0.5 can be indicated to the diffusion phenomenon. Therefore, the expo-
nent n considered as a measure of roughness and inhomogeneity of the
surface. It can be noted from the obtained impedance data that the CPE
values are decreased by increasing the inhibitor concentration. This may
be related to the adsorption of inhibitor molecules on the metal surface
and displacing of the original adsorbed of ions or water molecules. The
inhibition efficiency extracted from fitting impedance data (ηIMP) were
calculated using the following Eq. [28]:
3.2. Potentiodynamic polarization (PDP) measurement
Fig. 6 give the PDP curves of carbon steel in 6 M H2SO4 without and
with various concentrations of pyridine derivatives at 298 K. It was
found that as the concentration of pyridine molecules increases, the cor-
rosion potentials (Ecorr) move toward the positive anodic direction. This
suggested that the pyridine derivatives molecules inhibit the anodic dis-
solution of the carbon steel electrode more than that of hydrogen ions
reduction. Furthermore, in the presence of pyridine derivatives, the po-
larization curves of both anodic and cathodic branches showed a signif-
icant downward trend, suggesting that they behave as mixed-type
inhibitors. Fig. 6, showed that the cathodic branches of polarization
curves in parallel trend to each other, which suggested that the pres-
ence of pyridine derivatives molecule in the acidic solution has no
change on the cathodic reaction mechanism of carbon steel surface
[33]. The PDP parameters recorded in Table 4 showed that the slope of
Rict −Roct
Rict
ηIMP
¼
ꢀ 100
ð6Þ
where Roct and Rci t is the charge transfer resistance of the carbon steel in
the acidic solution without and with inhibitor, respectively. Inspection
Table 3
Fitting of electrochemical parameters calculated from EIS measurements for the carbon steel electrode in 6 M H2SO4 without and with different inhibitors molar concentrations at 298 K.
Inhibitor
C (M)
Rs (Ω cm2)
Q (Ω−1sn cm−2) × 104
n
Rct (Ωcm2)
Cdl (μFcm−2
)
χ2 × 103
θ
ηIMP (%)
Blank
I
0
2.7
2.5
2.6
2.5
2.4
2.6
2.6
2.5
2.2
2.6
2.7
2.4
2.6
2.7
2.5
9.27
0.87
0.88
0.87
0.85
0.86
0.83
0.85
0.87
0.80
0.81
0.82
0.93
0.77
0.96
0.96
8.7
758.6
360.3
299.6
226.8
179.0
151.7
120.6
101.7
458.2
345.2
293.6
273.1
243.5
228.3
189.7
1.2
1.9
0.7
0.8
0.5
0.4
0.6
0.7
0.5
0.5
0.6
0.4
0.6
0.6
0.5
–
–
8.30 × 10−5
1.66 × 10−4
3.32 × 10−4
6.64 × 10−4
1.33 × 10−3
1.99 × 10−3
2.66 × 10−3
8.30 × 10−5
1.66 × 10−4
3.32 × 10−4
6.64 × 10−4
1.33 × 10−3
1.99 × 10−3
2.66 × 10−3
27.19
11.71
11.29
10.25
11.48
8.53
29.33
10.28
11.64
11.54
8.76
18.3
22.0
29.1
36.8
43.5
54.6
65.1
14.4
19.1
22.5
24.2
27.1
28.9
34.8
0.525
0.605
0.701
0.764
0.800
0.841
0.866
0.396
0.545
0.613
0.640
0.679
0.699
0.750
52.5
60.5
70.1
76.4
80.0
84.1
86.6
39.6
54.5
61.3
64.0
67.9
69.9
75.0
II
39.33
7.14
8.28
5

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Fig. 6. Polarization curves of the carbon steel in 6 M H2SO4 solution without and with various inhibitors concentrations at 298 K.
the anodic curves changes significantly, which suggests that the adsorp-
tion of pyridine derivatives molecules on the carbon steel surface leads
to inhibit the precipitation of iron ions [34]. The dissolution mechanism
for steel in H2SO4 medium proposed by Bockris and Drazic as the follow-
ing [35]:
the active site of hydrogen ions reduction is decreased. The corrosion in-
hibition efficiency can be extracted from PDP data by applying the fol-
lowing Eq. [37–39]:
iocorr−iicorr
ηPDP
¼
ꢀ 100
ð13Þ
Fe þ H2O↔FeOHads þ Hþ þ e−
FeOHads ! FeOHþ þ e−
ð8Þ
ð9Þ
iocorr
where iocorr and iciorr is the corrosion current density of carbon steel in the
acidic solution of 6 M H2SO4 without and with inhibitor, respectively. It
can be noted that the corrosion current density (icorr) of carbon steel is
significantly reduced after the addition of the pyridine derivatives mol-
ecules as shown in Table 4, that means excellent inhibition ability for
the steel corrosion even at low concentrations. In addition, the corrosion
potential (Ecorr) values shifted to an anodic direction less than 85 mV af-
ter the addition of the pyridine derivatives, which means that the pyri-
dine derivatives behave as mixed-type inhibitors [40]. Table 4 show that
the values of corrosion current density (icorr) decrease from 8.65 mA for
the blank solution to reach a minimum at 2.66 × 10−3 mol dm−3 of in-
hibitors concentrations which give inhibition efficiency of 1.08, and
2.02 mA for inhibitors I, and II, respectively. This suggested that the pyr-
idine derivatives inhibitors molecules adsorb on the carbon steel surface
and create a barrier layer and impede the electron transfer of the corro-
sion current. These results confirm the gravimetric measurements and
impedance results and show the same trend.
FeOHþ þ Hþ↔Fe2þ þ H2O
ð10Þ
The cathodic process of hydrogen reduction reaction is as fol-
lows [36]:
À
Á
Fe þ Hþ↔ FeHþ
ð11Þ
ads
The mechanism of the pyridine derivatives molecules adsorption on
the iron surface can be represented by the following formula:
Fe þ Inh↔ðFeInhÞads
ð12Þ
From the above reaction can be concluded that the adsorption of
pyridine derivatives molecules on the surface of carbon steel can im-
pede the dissolution of anodic ferrous ions (Fe2+). With respect to the
cathodic reaction, the exposed area of the carbon steel reduced by in-
creasing the adsorbed molecules of pyridine derivatives, thus where
3.3. Gravimetric measurements
Table 4
Potentiodynamic polarization parameters of the carbon steel in 6 M H2SO4 without and
with different inhibitors molar concentrations at 298 K.
The gravimetric measurements technique is usually used to measure
the corrosion rate as well as determine the inhibition behavior of corro-
sion inhibitor due to its simplicity. The variation of the corrosion rate (ν)
of carbon steel in 6 M H2SO4 solution as a function of the inhibitor con-
centrations at 298 K is recorded in Table 5 and displayed in Fig. 7. It was
found that the corrosion rate of carbon steel gradually decreases with
the concentration while the inhibition efficiency increased. The mini-
mum corrosion rate values of the compounds I, and II at the concentra-
Medium C (M)
–Ecorr icorr
(mV) (mA)
β
a (mV
–βc (mV
θ
ηPDP%
decade−1
)
decade−1
)
Blank
I
0
499
8.6521 359
3.8588 141
479
281
282
276
276
279
276
266
301
311
305
236
245
236
240
–
–
8.30 × 10−5 488
1.66 × 10−4 486
3.32 × 10−4 483
6.64 × 10−4 489
1.33 × 10−3 492
1.99 × 10−3 468
2.66 × 10−3 454
8.30 × 10−5 466
1.66 × 10−4 490
3.32 × 10−4 480
6.64 × 10−4 470
1.33 × 10−3 463
1.99 × 10−3 478
2.66 × 10−3 474
0.554 55.4
0.641 64.1
0.735 73.5
0.776 77.6
0.829 82.9
0.848 84.8
0.875 87.5
0.436 43.6
0.558 55.8
0.636 63.6
0.658 65.8
0.685 68.5
0.724 72.4
0.767 76.7
3.1061 141
2.2928 146
1.9381 144
1.4795 144
1.3151 141
1.0815 133
4.8798 184
3.8242 225
3.1494 223
2.9590 155
2.7254 168
2.3880 166
2.0159 167
tion of 2.66
×
10−3 mol/dm3 are approximately 1.4335, and
2.2780 mg cm−2 h−1, respectively. This information indicates the suc-
cessful inhibitory action for the pyridine derivatives is obtained. This in-
hibition attributed to the formation of a protective layer from the
inhibitor molecules at the steel/solution interface [41]. Interestingly,
the corrosion rate of inhibitor I is less than inhibitor II. It means that
the inhibitor I has higher inhibition efficiency than the inhibitor II.
This may be related to the steric hindrance effect resulted from the
nearest two (–OCH3) groups and the (–CN) group in inhibitor II.
II
6

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Table 5
3.4. Adsorption isotherm study
The weight loss parameters of the carbon steel in 6 M H2SO4 without and with different
inhibitors molar concentrations at 298 K.
The adsorption of the studied pyridine inhibitor molecules on the
carbon steel surface takes place by displacement of the water molecules
originally adsorbed. This displacement process can be represented by
the following Eq. [42]:
Medium
C (M)
ν (mg/cm2 h)
θ
ηWL%
Blank
I
0
11.1124
6.0340
4.3005
3.1337
2.5336
1.7891
1.5446
1.4335
7.1675
5.9229
5.2784
4.0671
2.9337
2.5336
2.2780
–
–
8.30 × 10−5
1.66 × 10−4
3.32 × 10−4
6.64 × 10−4
1.33 × 10−3
1.99 × 10−3
2.66 × 10−3
8.30 × 10−5
1.66 × 10−4
3.32 × 10−4
6.64 × 10−4
1.33 × 10−3
1.99 × 10−3
2.66 × 10−3
0.457
0.613
0.718
0.772
0.839
0.861
0.871
0.355
0.467
0.525
0.634
0.736
0.772
0.795
45.7
61.3
71.8
77.2
83.9
86.1
87.1
35.5
46.7
52.5
63.4
73.6
77.2
79.5
InhðsolÞ þ xH2O
↔InhðadsÞ þ xH2O
ð14Þ
ðadsÞ
ðsolÞ
where Inh(sol) is the inhibitor molecules in solution, H2O(ads) is the water
molecules originally adsorbed on the carbon steel surface, the x is the
water molecules replaced by the inhibitor's molecules, Inh(ads) is the in-
hibitor molecules adsorbed on the carbon steel surface, and H2O(sol) is
the water molecules migrate in bulk solution. In order to further inves-
tigate the mechanism of inhibitor adsorption on the metal surface, var-
ious isotherms of adsorption were used to fitting the gravimetric
measurements data. The fitted linear regression coefficients (R2) pre-
sented in Fig. 8 indicate that the adsorption of pyridine derivatives
(I, and II) on the carbon steel surface obeyed the Langmuir adsorption
isotherm model with the following formula [43]:
II
C
θ
1
Kads
¼
þ C
ð15Þ
The adsorption equilibrium constant (Kads) of pyridine inhibitors on
the carbon steel surface can be calculated from Langmuir adsorption
isotherm, which used to obtain the value of ΔGoads via the following for-
mula [44]:
ꢀ
ꢁ
1
55:5
ΔGoads
RT
Kads
¼
exp
−
ð16Þ
The large values of Kads and small values ΔGoads given in Fig. 8 were
corresponding to the high inhibition performance. The negative values
of ΔGoads, indicating spontaneous adsorption of the pyridine inhibitors
molecules on the carbon steel surface. Besides, the ΔGoads values were
between −20 kJ/mol and − 40 kJ/mol, indicating that the pyridine in-
hibitors molecules adsorbed on the carbon steel surface by a combina-
tion of physical and chemical adsorption [45,46].
Fig. 7. The decay of corrosion rate of the carbon steel with different inhibitors
concentrations in 6 M H2SO4 solutions at 298 K.
Fig. 8. Langmuir adsorption plots of the carbon steel in 6 M H2SO4 with different inhibitors concentrations at 298 K.
7

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Fig. 9. SEM images of (a) polished carbon steel surface, (b) after 8 h immersion in 6 M H2SO4 and (c) after 8 h immersion in 6 M H2SO4 + 2.66 × 10−3 M of inhibitor II at 298 K.
3.5. SEM analysis
depending on an electron affinity (A) and ionization potential (I) as
given in the following Eqs.:
Fig. 9 represents the surface morphology of carbon steel at diverse
conditions. Fig. 9(a) shows the freshly polished carbon steel surface
which has scratches left from abrading. Fig. 9(b) gives the SEM of carbon
steel immersed in the blank solution of 6 M H2SO4 for 8 h without inhib-
itor. It can be seen that the carbon steel surface has been highly dam-
aged. After the addition of 2.66 × 10−3 mol dm−3 of inhibitor I, a
significant enhancement in steel surface smoothness was observed
as shown in Fig. 9(c). This finding confirmed that the inhibitor mole-
cules of pyridine derivatives formed a protective layer on the carbon
steel surface which protects from the corrosive attack of acid 6 M
H2SO4 solution [47].
I þ A
χ ¼
γ ¼
ð18Þ
ð19Þ
2
I−A
2
According to Koopman's theorem, the I and A are depending to the
frontier orbital energies according to the following Eqs. [50]:
I ¼ −EHOMO
A ¼ −ELUMO
ð20Þ
ð21Þ
3.6. Computational studies
For iron, the theoretical values of χ and γ are 7.0 and 0 eV mol−1, re-
spectively [51]. Generally, the maximum fraction of ΔN indicating that
the most absorb effectively on the metal surface. The quantum chemical
calculations in the present study suggested that the corrosion inhibition
occurred through the electron transfer from the high electron density of
the inhibitor's molecules of the pyridine derivatives to the surface atoms
of the carbon steel. The results recorded in Table 6 indicated that the ΔN
values for studied inhibitors I, and II were 47.1, and 44.5, respectively.
This finding of theoretical data confirms and supports the obtained
data from gravimetric measurements, PDP, EIS, and SEM analysis.
The electrostatic potential (ESP) is also represented in Fig. 10. The
positive red areas of the molecular electrostatic potential (MEP) are re-
lated to the nucleophilic region (i.e., electron-rich area), while the neg-
ative blue areas are associated with the electrophilic region (i.e., low
electron density area). The order of electron density as the follows:
red > orange > yellow > green > blue. Generally, the metal surface be-
haves as electrophilic material which draws the negatively charged cen-
ters of the inhibitor's molecules (which behave as the nucleophilic part)
via lone electron pairs and π -electrons of conjugated double bonds,
which tend to form firm covalent bonds [52].
3.6.1. Quantum chemical study
The quantum chemical calculations were conducted to study the re-
lation between the molecular structure of the pyridine derivatives and
their inhibition influence. Table 6 recorded the calculated energy levels
of the frontier molecular orbitals (Fig. 10), and the experimental aver-
age inhibition efficiencies of the studied pyridine derivatives. The higher
values of the HOMO indicating the less the value of the ionization poten-
tial; which leads to electrons donated easily. The lower values of energy
gap (ΔE) usually lead to easier polarization of the molecule and hence
greater adsorption on the surface [48]. The number of transferred elec-
trons (ΔN) can be calculated using the following Eq. [49]:
ðχFe−χInh
Þ
ΔN ¼
ð17Þ
2ðγFe þ γInh
Þ
where χFe, χInh, γFe, and γInh are the absolute electronegativity of iron,
the absolute electronegativity inhibitor, the absolute hardness of iron
and the absolute hardness inhibitor, respectively. These terms are
3.6.2. Molecular dynamics simulations (MCs)
Table 6
Calculated quantum chemical parameters of studied inhibitors.
Molecular dynamics simulations consider a well-established tool in
computational chemistry that gives information about the most opti-
mized configuration of adsorbed inhibitor molecule on the metal sur-
face [53]. In the present study, the MD simulations were conducted to
study the adsorption performance of the two pyridine derivatives mol-
ecules on the Fe (110) surface. The equilibrium configurations of the
studied compounds are represented in Fig. 11. Inspection of Fig. 11 ob-
vious that during the MD simulation process, the molecules of inhibitors
moved gradually near the Fe (110) surface with a complete horizontal
flat orientation. The complete horizontal flat orientation molecules of
inhibitors lead to more surface coverage of the metal surface and
hence high inhibition efficiency. The interaction energy (Eint) of mole-
cules with Fe surface was obtained using the following equation:
Parameters
I
II
E(HOMO)
E(LUMO)
ΔE(L-H)
I
A
χ(inh)
γ(inh)
χ(Fe)
−0.2251
−0.0780
0.1471
0.2251
0.0780
0.1515
0.0735
7.0000
0.0000
47.0964
3.7100
87.1000
−0.2342
−0.0787
0.1555
0.2342
0.0787
0.1565
0.0778
7.0000
0.0000
44.5074
3.2200
77.1000
γ(Fe)
ΔN
μ
Average η (%)
8

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Fig. 10. The frontier molecular orbital and electrostatic potential of the studied inhibitors.
equilibrium. It was found that the Ebin values for I, and II were
46.2, and 34.3, respectively. Therefore, the interaction energy (−Ebin
Eint ¼ Etotal−ðEFe surface þ Emolecule
Þ
ð22Þ
)
is negative for the studied inhibitors which confirmed that the adsorp-
tion of the pyridine derivatives on the Fe (110) surface occurred
spontaneously [54].
where Etotal is the total energy of the molecules and the metal surface
system; Esurface is defined as the energy of metal surface without adsorp-
tion of molecules and Emolecule is the energy of isolated molecules. The
binding energy is the negative of the interaction energy and is given as:
3.7. Corrosion prevention study
Ebin ¼ −Eint
ð23Þ
Heterocyclic inhibitors exhibit well through their heteroatoms like
nitrogen, sulfur, oxygen, selenium, phosphorous and aromatic rings as
well as double or triple bonds. Inhibitors act by adsorption to form a
The values of Eint and binding energy (Ebin) between studied inhibi-
tors and Fe (110) surface were evaluated when systems reach
9

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
Fig. 11. Equilibrium adsorption configurations of studied inhibitors on the Fe (110) surface obtained by molecular dynamic simulations.
barrier between metal and acid ions. This kind of organic molecules can
adsorb on the metal surface because it can form a bond between the N
electron pair and/or the electron cloud and the metal thereby reducing
the corrosive attack on metals in acidic media. The pyridine derivatives
are adsorbed on the carbon steel surface physically or chemically and
impede the direct contact between the metal surface and the corrosive
acidic solution. The studied two pyridine derivatives contain chemically
active two benzene rings, pyridine ring with nitrogen heteroatoms, and
the functional groups like –NH2, and OCH3. These active centers react
chemically with the metal through electron transfer to the vacant d-
orbital at the metal surface to form a thin film that prevents further cor-
rosion. Furthermore, the pyridine derivatives also adsorb onto the base
metal physically and restrict the passage of corrosive ions toward the in-
terior that inhibit the corrosion process [55–57]. The suggested corro-
sion inhibition mechanism possessed by the pyridine derivatives
regarding the carbon steel surface is schematically explained in
Fig. 12. The pyridine molecules may adsorb on the carbon steel/electro-
lyte interface surface by (1) electrostatic interaction between the pro-
tonated form of pyridine molecules and already adsorbed counter
sulfate ions (physisorption), (2) donor-acceptor interaction between
the unshared electron pairs of heteroatoms and the vacant d-orbital of
iron atoms (chemisorption), (3) donor-acceptor interaction between
the π-electrons of aromatic rings and the vacant d-orbital of iron
atoms (chemisorption), and (4) donor-acceptor interaction of
d-electrons of iron atoms to the empty antibonding molecular orbital
Fig. 12. Schematic representation of suggested adsorption behavior of pyridine derivative
on the steel in 6 M H2SO4 solution.
10

A.A. Farag, E.A. Mohamed, G.H. Sayed et al.
Journal of Molecular Liquids 330 (2021) 115705
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of the pyridine molecules (retro-donation). During the first stage of ad-
sorption, the cationic form of pyridine molecules begins to compete
with the hydrogen ions to get the electrons on the steel surface. How-
ever, after hydrogen gas evolved, the neutral pyridine molecules were
adsorbed chemically on the steel surface by donation of non-bonding
electrons of heteroatoms in the d-orbital [31]. From the above investiga-
tion it is cleared that compound II is less efficient than compound I, due
to the steric hindrance effect resulted from the nearest two methoxide
(–OCH3) groups and the (–CN) group in inhibitor II. This steric hin-
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4. Conclusion
In the present study, two compounds of pyridine derivatives (I, and II)
were synthesized and evaluated as corrosion inhibitors for carbon steel
in 6 M H2SO4 by combining experimental and theoretical methods. The
obtained adsorption data achieved the model of Langmuir isotherm. The
PDP tests proposed that pyridine derivatives are mixed-type inhibitors.
The EIS parameters demonstrate that the double-layer capacitance (Cdl)
decreases with the increasing of inhibitor concentration, which implies
a reduces dielectric constant. The Rct values increase to reach a maxi-
mum at 2.66 × 10−3 mol dm−3 of inhibitor concentrations which give
inhibition efficiency of 86.6%, and 75.0% for I, and II, respectively. The pa-
rameters derived from the theoretical quantum chemical calculations
were found adequately correlated with the obtained experimental re-
sults. The values of ΔGoads were–33.3, and − 31.6 kJ/mol for I, and II, re-
spectively, indicate that the adsorption of these compounds on the
carbon steel surface occurs by a combination of physical and chemical
adsorption. The film formation of the inhibitor molecules on the carbon
steel surface was confirmed by the SEM analysis. DFT and MCs results
were found compatible with the obtained experimental results.
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Author statement
All authors have participated in (a) conception and design, or analy-
sis and interpretation of the data; (b) drafting the article or revising it
critically for important intellectual content; and (c) approval of the
final version. This manuscript has been submitted, is under review, an-
other journal or other publishing venue. The authors have no affiliation
with any organization with a direct or indirect financial interest in the
subject matter discussed in the manuscript.
[18] V.S. Sastri, J.R. Perumareddi, Molecular orbital theoretical studies of some organic
corrosion inhibitors, Corros. 53 (1997) 617–622, https://doi.org/10.5006/1.
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Declaration of Competing Interest
[19] A.S. Ismail, A.A. Farag, Experimental , theoretical and simulation studies of extracted
crab waste protein as a green polymer inhibitor for carbon steel corrosion in 2 M H 3
PO 4, Surf. Interf. 19 (2020), 100483, https://doi.org/10.1016/j.surfin.2020.100483.
[20] R. Nabah, F. Benhiba, Y. Ramli, M. Ouakki, M. Cherkaoui, H. Oudda, R. Touir, I. Warad,
A. Zarrouk, Corrosion inhibition study of 5, 5-diphenylimidazolidine-2, 4-dione for
mild steel corrosion in 1 M Hcl solution: experimental, theoretical computational
and Monte Carlo simulations studies, Anal. Bioanal. Electrochem. 10 (2018)
1375–1398 https://www.scopus.com/inward/record.uri?eid=2-s2.0-85055147771&
partnerID=40&md5=71467bee4208c459a2fa6ccc59a5c016.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
[21] M. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G. Petersson, Gaussian 09, Revision d. 01, 201,
Gaussian, Inc., Wallingford CT, 2009.
The authors are greatly thankful for funding and support from
Egyptian Petroleum Research Institute, and Chemistry Depart., Faculty
of Science, Ain Shams Uni., Egypt.
[22] A. Ramazani, M. Sheikhi, H. Yahyaei, Molecular structure, NMR, FMO, MEP and NBO
analysis of ethyl-(Z)-3-phenyl-2-(5-phenyl-2H-1, 2, 3, 4-tetraazol-2-yl)-2-
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8-hydroxyquinoline derivatives as an efficient corrosion inhibitors for mild steel in
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