Experimental and theoretical research on a new corrosion inhibitor for effective oil and gas acidification

Article abstract of DOI:10.1039/c9ra04638k

A new dibenzylamine-quinoline derivative (DEEQ) was synthesized and investigated as a corrosion inhibitor for mild steel in 15% HCl solution in various ways, including via weight loss measurements, contact angle measurements, electrochemical measurements

Full text of DOI:10.1039/c9ra04638k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Experimental and theoretical research on a new
corrosion inhibitor for effective oil and gas
acidification
Cite this: RSC Adv., 2019, 9, 26464
a
Yongming Li,a Dingli Wang
and Lei Zhangb
*
A new dibenzylamine-quinoline derivative (DEEQ) was synthesized and investigated as a corrosion inhibitor
for mild steel in 15% HCl solution in various ways, including via weight loss measurements, contact angle
measurements, electrochemical measurements (EIS), scanning electron microscopy (SEM), scanning
Kelvin probe (SKP) and theoretical calculations. The experimental results revealed that DEEQ is an
effective corrosion inhibitor for oil and gas acidification. In an oil–water two-phase system, the
wettability of mild steel can be changed by adsorption, while obeying the Langmuir adsorption isotherm.
Finally, quantum chemical calculations and molecular dynamic simulation parameters further show
a definite correlation between the theoretical and experimental results.
Received 20th June 2019
Accepted 1st August 2019
DOI: 10.1039/c9ra04638k
rsc.li/rsc-advances
corrosion inhibitors represented by quaternary ammonium salt
ionic liquids has received more and more attention. At the same
1. Introduction
time, the hydrophobic chains can align on the metal surface to
form a hydrophobic lm, which hinders the diffusion of water
molecules into the cathode region and inhibits the cathode
reaction.17,18 In addition, because of the molecular structure
itself, the cations and anions can form a wide range of struc-
tures.8,11,12,20 In previous literature, imidazolium-based and
benzimidazole ionic liquids were researched as inhibitors for
mild steel in acid solution, and the results provide guidance or
direction to design and develop new ionic liquids as inhibitors.
In particular, the long alkyl chain induced poor water solubility
and foaming ability are detrimental for corrosion inhibitors.
Thus, short chain and special functional groups became the
direction for design. A dibenzylamine-quinoline derivative has
not been reported as a corrosion inhibitor, and the properties of
good water solubility and stability and the functional groups
determine its excellent corrosion inhibition performance.
Therefore, in this paper, we synthesized a new corrosion inhib-
itor, and the mechanism of corrosion between the inhibitor
molecule and the mild steel surface was studied. The test
methods include weight loss, potentiodynamic polarization,
electrochemical impedance spectroscopy (EIS), scanning Kelvin
probe (SKP), and scanning electron microscopy (SEM). In addi-
tion, the corrosion mechanism was researched further via
theoretical quantum chemical calculations and molecular
dynamic simulations.
Metal acid corrosion not only wastes precious resources and
energy but also seriously pollutes the environment. Under the
action of corrosion, 10% of the world’s steel output is corroded
and consumed each year, especially in the corrosion and rust
cleaning of mechanical equipment, pipelines of natural gas and
long-distance transmission systems.13 A hydrochloric acid
solution not only corrodes equipment, but the acid mist also
causes harm to people. Therefore, adding a corrosion inhibitor
is an economical and effective method that has received
increasing attention.6,7,38
According to the chemical composition, a corrosion inhib-
itor is divided into two parts: an inorganic portion and an
organic inhibitor. The organic inhibitor molecules contain
a highly electronegative polar group (O, N, S, etc.), benzene ring,
heteroatom ring, double bond or triple bond, and are mostly
used in an acidic corrosion medium.4 The unpaired lone pair of
electrons or the high electron cloud density region of the bond
coordinates with the metal orbital in the form of a coordination
bond, and forms a protective lm by interface transformation,
polymerization and chelation.3,5,35
Most literature reports the use of a Mannich base as a corro-
sion inhibitor for effective oil and gas acidication.9,36,37,41
Although Mannich bases have a good inhibition effect, there are
still many problems that limit their application such as high
cost, toxicity and instability. With the enhancement of people’s
awareness of environmental protection, research on green
2. Experimental and methods
2.1 Preparation of samples
aState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest
Petroleum University, Chengdu 610500, Sichuan, China. E-mail: wdl_swpu@163.com
bCollege of Chemistry and Chemical Engineering, Chongqing University, Chongqing
400044, China
The composition of the mild steel samples (wt%) was C (0.26%),
Si (0.2%), Mn (0.49%), P (0.009%), S (0.004%), Si (0.2%) and Fe
26464 | RSC Adv., 2019, 9, 26464–26475
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
for balance. The specimens were tested for weight loss and samples were suspended from each jar, and the specimens were
surface topography analysis was carried out at a size of 50 mm immersed in a 15% HCl solution with different concentrations
ꢀ 10 mm ꢀ 3 mm. The mild steel specimen was cleaned of DEEQ for 4 h at 333, 343, 353 and 363 K. The temperature of
ultrasonically with anhydrous ethanol and acetone. Aer the solution was kept steady by the water bath, and at the end of
natural drying, the mass of the specimen was accurately the experiment the specimens were removed, the surface
weighed using an analytical balance, and the measurement was washed with water and the corrosion products removed. The
performed in parallel three times. The results are expressed as specimens were then cleaned with anhydrous ethanol and
average values.
acetone carefully, and aer natural drying the mass of the
The molecular structure of the inhibitor DEEQ is shown in sample was accurately weighed by an analytical balance, and
Fig. 1, and the synthesis was as follows. Dibenzylamine (3.94 g, measured in parallel three times, and the results are expressed
20 mmol) was added to a 250 mL single-necked ask with as an average value. The corrosion rate was calculated.
100 mL ethanol as the solvent. Epichlorohydrin (1.95 g, 21
mmol) was added dropwise, and the reaction was stirred at 2.4 Electrochemical measurements
50 ꢁC for 16 h. Then rotary evaporation was carried out to
Electrochemical tests can not only obtain instantaneous data
remove the solvent and a yellow viscous liquid was obtained.
from the metal corrosion process but can also continuously
Aerwards, the yellow viscous liquid (2.88 g, 10 mmol) dissolved
monitor the change in the metal electrode during the corrosion
in 150 mL ethanol was introduced to a 250 mL single-necked
process.2,31 Electrochemical experiments were carried out using
ask, and quinoline (1.42 g, 11 mmol) was added to the reac-
the traditional three-electrode system by Reference 3000
(Gamry). A graphite electrode and a saturated calomel electrode
tion system, and the system was equipped with a stirrer and
a reux condenser at 80 ꢁC for 12 h. Finally, rotary evaporation
(SCE) were used as the auxiliary and reference electrodes,
was carried out to remove the solvent and aer washing with
respectively.42,43 In addition,
a mild steel electrode was
1
ethyl acetate three times, a white solid was obtained. The H
employed as the working electrode with an exposed area of 1.0
cm2. Before starting the experiments, the polished working
electrode was cleaned with anhydrous ethanol and acetone and
then immersed in the test solution for 20 min to reach a steady
potential, and the measurements were performed in the
frequency range of 10ꢂ2 to 105 Hz with an amplitude of 5 mV at
open circuit potential (OCP). The EIS parameters were obtained
by tting the data to an equivalent circuit by using ZSimpWin
soware. The potentiodynamic polarization curves were ob-
tained in a potential range of ꢂ500 mV to 500 mV with a scan
NMR measurement was carried out using a Bruker AVANCE III
HD 400 NMR Spectrometer. 1H NMR for DEEQ: d (ppm): 1.1
(1H, m, CH–OH), 1.2 (2H, m, N–CH2), 2.1 (1H, m, CH2–CH–
CH2), 3.6–3.8 (4H, m, CH2–N–CH2), 5.2 (2H, m, N+–CH2), 7.2–8.1
(16H, m, Ar-H), 9.0 (1H, m, N+–H).
2.2 Contact angle measurements
Contact angles were measured using an automatic tensiometer
(GERMAN KRUSS DSA30S) at 20 ꢁC. The experiments were
performed under aqueous solution, and the dynamic contact
angle was measured by observing the change of contact angle
with time in the presence and absence of corrosion inhibitor.
rate of 0.5 mV sꢂ1
.
2.5 Scanning Kelvin probe
Compared with traditional electrochemical impedance spec-
troscopy (EIS) technology, scanning Kelvin probe (SKP) tech-
nology can analyze the potential distribution on the metal
surface, measure the electrochemical properties without con-
tacting the system, and more accurately reect the character-
istics of local corrosion of the metal, which is extremely
sensitive to small changes in the interface state. In addition,
SKP has high measurement accuracy, and the electrochemical
characteristics can be three-dimensionally presented.
2.3 Weight loss measurements
Weight loss measurements are one of the most reliable
methods for determining the uniform corrosion rate and the
corrosion inhibitor performance of metals, which is the basis of
other test methods.25–27 In the process of the experiment, three
The SKP experiment was carried out using a micro area
electrochemical scanning system (VersaScan of Princeton
Applied Research) at 298 K, using the scan step mode with
a scan step size of 100 mm, scan area of 4000 ꢀ 4000 mm2, probe
vibration amplitude of 30 mm, scan frequency of 80 Hz, and an
average distance of the probe from the sample surface of
approximately 100 mm. In addition, the SKP system was cali-
brated with a saturated calomel electrode (SCE) before testing.
2.6 Surface analysis
Surface morphology analysis experiments were carried out
using a scanning electron microscope (SEM) (FEI Quanta 450),
and the acceleration voltage was 20 kV and the magnication
Fig. 1 Synthetic route to DEEQ.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 26464–26475 | 26465

View Article Online
RSC Advances
Paper
was 150 to 1000 times. SEM can accurately reect the surface
morphology of the metal aer corrosion and provides a very
valuable reference for the evaluation of corrosion inhibitor
performance. Before starting the experiments, the surface
should be cleaned with anhydrous ethanol and acetone care-
fully before natural drying. In this process, the fresh specimen
and the specimens immersed in 15% HCl solution in the
absence or presence of 6.0 mM DEEQ for 4 h were examined.
2.7 Quantum chemical calculations
The molecular structure parameters of the corrosion inhibitor
and the interaction mode and active sites between the mole-
cules and the metal can be obtained through quantum chemical
calculations, which provide important information for
researching the mechanism of the corrosion inhibitor.19 In the
Gaussian 09 program, the geometry of the studied molecules
was optimized at the 6-31 g (d, p) basis set level, and the
frequency calculation was performed at the same basis level to
determine that the found stagnation point was the minimum
energy point. Aer the simulations, the energy of the highest
occupied molecular orbital (EHOMO), the energy of the lowest
unoccupied molecular orbital (ELUMO), the energy gap (DE ¼
ELUMO ꢂ EHOMO), the dipole moment (m), and the Mulliken
charge were calculated.
Fig. 2 The cross sectional optical images of oil droplets on (a) the
fresh steel surface, (b) the steel surface inhibited with 7 mM DEEQ after
1 h immersion, (c) the steel surface inhibited with 7 mM DEEQ after 2 h
immersion, and (d) the steel surface inhibited with 7 mM DEEQ after 3 h
immersion.
and 128ꢁ, respectively, which indicates that the surface of the
steel changed to lipophilic. The DEEQ molecules adsorbed on
the steel surface to form the aligned adsorption layer, reducing
the free energy of the interface so that water molecules can be
2.8 Molecular dynamic simulations
displaced more effectively, leading to
efficiency.
a high inhibition
The Forcite module of Materials Studio was used to model the
interaction between inhibitor and iron surface. The calculation
model consists of two layers: the surface layer of Fe (100) and
the water layer with corrosion inhibitor molecules. The
adsorption interface Fe (100) crystal plane thickness is 8 layers,
a total of 512 Fe atoms, and the system size is 2.30 nm ꢀ
2.30 nm ꢀ 1.15 nm. The solvent layer was established using the
amorphous Cel I module and consists of an inhibitor molecule
and 500 water molecules with a system size of 2.30 nm ꢀ
2.30 nm ꢀ 2.84 nm. In addition, the temperature was 298.15 K
and the integration time step was set to 1 fs. Aer the simula-
tions the interaction energy (Eint) between the inhibitor mole-
cule and the iron surface was calculated as follows:
Table 1 Weight loss measurements for mild steel in 15% HCl with
different concentrations of DEEQ for 4 h at different temperatures
Temperature
(K)
Concentration
(mM)
v (g mꢂ2 hꢂ1
)
h (%)
q
333
343
353
363
Blank
1.0
3.0
4.0
6.0
7.0
Blank
1.0
3.0
4.0
6.0
7.0
Blank
1.0
3.0
4.0
6.0
7.0
Blank
1.0
18.116
1.410
0.746
0.411
0.314
0.179
72.464
6.90
5.391
3.169
—
—
92.2
95.9
97.7
98.3
99.0
—
0.922
0.959
0.977
0.983
0.990
—
Eint ¼ Etot ꢂ Emol ꢂ Esur
(1)
90.5
92.6
95.6
97.3
98.0
—
0.905
0.926
0.956
0.973
0.980
—
where Etot is the total energy of the simulation system, Emol is
the energy of a free inhibitor molecule, and Esur is the energy of
the iron surface together with the water molecules.
1.948
1.476
271.739
47.582
30.170
14.875
9.814
3. Results and discussion
3.1 Contact angle measurements
82.5
88.9
94.5
96.4
98.2
—
0.825
0.889
0.945
0.964
0.982
—
It can be seen from Fig. 2 that the wettability of steel is greatly
changed in the presence of DEDB, and because the measure-
ments were performed under aqueous conditions the supple-
mentary angle of the results is the real contact angle. For the
fresh steel, the surface contact angle is 78ꢁ, which is consistent
with the previous reports.1,34 When adding corrosion inhibitor
aer 1, 2 and 3 hours, the contact angle of the steel is 118ꢁ, 127ꢁ
4.790
825.525
148.696
97.198
87.256
64.198
37.964
81.9
88.2
89.4
92.2
95.4
0.819
0.882
0.894
0.922
0.954
3.0
4.0
6.0
7.0
26466 | RSC Adv., 2019, 9, 26464–26475
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
concentration of corrosion inhibitor is 7 mmol, the corrosion
inhibition rate can reach over 95%, which indicates that DEEQ
is an effective inhibitor and can meet the requirements of
acidication operation of medium and low temperature oil and
gas well.15,24
3.2 Weight loss measurements
The results of the weight loss measurements for mild steel in
15% HCl with different concentrations of DEEQ for 4 h at
different temperatures are shown in Table 1. The experimental
parameters of corrosion rate (v), inhibition efficiency (h) and
surface coverage (q) were calculated by the following
equations:24
3.3 Potentiodynamic polarization curves
w0 ꢂ w
Fig. 3 shows the potentiodynamic polarization curves for mild
steel in 15% HCl solution with and without different concen-
trations of DHAC at 298 K. The electrochemical parameters of
the corrosion potential (Ecorr), corrosion current density (icorr),
anodic Tafel slope (ba) and cathodic Tafel slope (bc) obtained by
Tafel extrapolation are shown in Table 2, and the calculation
formula of the corrosion inhibition efficiency (h) is as follows:
v ¼
hð%Þ ¼
q ¼
(2)
(3)
(4)
St
v0 ꢂ v
ꢀ 100
v0
v0 ꢂ v
v0
ꢀ
ꢁ
where w0 and w are the weight values without and with inhib-
itor, S is the total surface area of the mild steel specimen, t is the
immersion time, q is the surface coverage, and v0 and v are the
corrosion rate of mild steel in the absence and presence of
inhibitor, respectively.30
From Table 1, it was found that with an increase in inhibitor
concentration, the corrosion rate decreases, and the inhibitor
efficiency and surface coverage increases, which indicates that
increasing the concentration leads to an increase in the amount
of adsorption. At the same concentration, when the tempera-
ture increases the corrosion efficiency decreases. When the
icorr
hð%Þ ¼ 1 ꢂ
ꢀ 100
(5)
icorr0
where icorr and icorr0 are the corrosion densities with and
without inhibitor, respectively. From Table 2, it is clear that with
the increase in DEEQ concentration, the corrosion current
density of the mild steel electrode in the 15% HCl solution
decreases, the corrosion efficiency increases, and the corrosion
potential shis negatively. During the linear regions in the
anodic and cathodic regions, we can calculate the corrosion
parameters, including the corrosion current density, anodic
and cathodic Tafel slopes and inhibition efficiency. The corro-
sion potential on the working electrode in the absence of
inhibitor is ꢂ0.4071 V. This value shis towards the cathodic
direction in the presence of DEEQ. The values of Ecorr are
ꢂ0.4382 V, ꢂ0.4346 V, ꢂ0.43285 V, ꢂ0.4284 V and ꢂ0.4447 V in
the presence of 1 mM, 3 mM, 4 mM, 6 mM and 7 mM, respec-
tively. The values of icorr are 0.6282 mA cmꢂ2, 0.3071 mA cmꢂ2
,
0.2450 mA cmꢂ2, 0.1708 mA cmꢂ2, 0.0749 mA cmꢂ2 and 0.0491
mA cmꢂ2 in the absence and presence 1 mM, 3 mM, 4 mM,
6 mM and 7 mM DEEQ, respectively. The reduction of the
current densities and the fact that the displacement in Ecorr
between the absence and presence of DEEQ is less than 85 mV
indicate that DEEQ is a mixed-type inhibitor.8,12
From Fig. 3, it was found that the addition of DEEQ had no
signicant effect on the slope of the cathodic polarization curve
of the mild steel electrode in the 15% HCl solution. The
approximately parallel cathodic polarization curve indicates
that the inhibitor DEEQ has no impact on the mechanism of the
reduction of hydrogen, and the process is under activation
control.43,44 The addition of DEEQ reduces the number of active
Fig. 3 Polarization curves for mild steel in 15% HCl solution with and
without different concentrations of DEEQ at 298 K.
Table 2 The electrochemical parameters for mild steel in 15% HCl without and with different concentrations of DEEQ at 298 K
Concentration
(mM)
Ecorr (VSCE
)
icorr (mA cmꢂ2
)
ba (mV decꢂ1
)
bc (mV decꢂ1
)
h (%)
Blank
1.0
3.0
4.0
6.0
ꢂ0.4071
ꢂ0.4382
ꢂ0.4346
ꢂ0.4285
ꢂ0.4284
ꢂ0.4447
0.6282
0.3071
0.2450
0.1708
0.0749
0.0491
62.475
110.0
121.1
110.3
125.4
124.0
ꢂ136.7
ꢂ138.5
ꢂ123.1
ꢂ128.7
ꢂ137.9
ꢂ144.9
—
51.1
61.0
72.8
88.1
92.2
7.0
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 26464–26475 | 26467

View Article Online
RSC Advances
Paper
sites on the surface of the electrode by adsorption on the mild
steel surface, and there was an obvious linear Tafel area on the
cathodic polarization curve. It can also be seen from the anodic
polarization curve that DEEQ has an evident inhibitory effect on
the mild steel electrode at a lower anode potential, which is
related to the adsorption of DEEQ on the surface of the mild
steel electrode and the formation of a protective layer of
inhibitor.10,38 However, when the polarization potential
continues to increase, it can be observed that the curves are
close to coinciding. This may be due to the rapid dissolution of
mild steel at the strong polarization potential, which causes the
desorption of DEEQ from the electrode’s surface.
3.4 Electrochemical impedance spectroscopy measurements
Fig. 5 shows the corrosion inhibitory properties of different
concentrations of DEEQ in 15% HCl solution at 298 K, as tested
using electrochemical impedance spectroscopy (EIS). The
equivalent circuit model is displayed in Fig. 4 by tting the
impedance data, which contain the solution resistance (Rs),
polarization resistance (Rp) and constant phase element (CPE),
and is given in eqn (6):
ZCPE ¼ Y0ꢂ1(ju)ꢂn
(6)
where the CPE is the constant phase element to replace a double
layer capacitance Cdl to obtain a more accurate t of the
experimental data set. n corresponds to the phase shi, which is
related to the inhomogeneities of the double layer. For n ¼ 0,
CPE represents a resistance, for n ¼ 1, a capacitance and for n ¼
ꢂ1 an inductance. The dimension of Y0 is sn Uꢂ1, while that of
a capacitance (C) is s Uꢂ1 or F. Despite this difference, Y0 is
oen used as if it were the capacitance of the corroding system.
Conversion of Y0 data into C is very important when experi-
mental capacitance data are to be used to determine quantita-
tively system parameters.14,40 The “double layer capacitance”
values (Cdl) are calculated as follows:
Fig. 4 EIS for mild steel in a 15% HCl solution with and without
different concentrations of DEEQ at 298 K: (a) Nyquist plots and (b)
Bode plots.
À
Á
nꢂ1
Cdl ¼ Y0 u0m0
(7)
where u0m0 is the angular frequency corresponding to the
maximum imaginary part of the impedance spectrum. The
impedance parameters are displayed in Table 3 and the inhi-
bition efficiency (h) was calculated by the following equation:
Rp ꢂ R0p
Rp
hð%Þ ¼
ꢀ 100
(8)
Fig. 5 Equivalent circuit used to fit the obtained impedance spectra
for mild steel in a 15% HCl solution.
Table 3 The EIS parameters for mild steel in 15% HCl solution without and with different concentrations of DEEQ at 298 K
C (mM)
Rs (U cm2)
Rp (U cm2)
Y0 ꢀ 104 (Uꢂ1 sn cmꢂ2
)
n
Cdl (mF cmꢂ2
)
h (%)
0
1.76
1.31
1.50
1.30
1.38
1.25
109.4
596.9
731.6
772.1
1037
1313
2.17
1.85
1.20
1.04
0.90
0.69
0.89
0.80
0.83
0.81
0.81
0.85
114.6
84.6
66.8
49.1
41.9
38.9
—
1.0
3.0
4.0
6.0
7.0
81.2
85.0
85.8
89.4
91.2
26468 | RSC Adv., 2019, 9, 26464–26475
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
where Rp and Rp0 are the polarization resistance (U cm2) with and addition of DEEQ.21,24 In addition, the fact that there is no
without DEEQ, respectively. From Fig. 4(a), it is clear that the change in the shape of the impedance spectra with the addition
Nyquist gure displays a depressed capacitive loop, and with of inhibitor indicates that the presence of the inhibitor has no
the increase in DEEQ concentration, the radius of the capacitive inuence on the corrosion mechanism. The Bode and phase
loop increases, which means that the charge transfer between angle plots for mild steel in the 15% HCl solution with different
the solution and mild steel is dramatically hindered by the concentrations of DEEQ are displayed in Fig. 4(b). The phase
Fig. 6 Three-dimensional diagrams of the potential distributions for the original state (a), and the absence (b) and presence (c) of DEEQ in 15%
HCl for 4 h at 298 K.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 26464–26475 | 26469

View Article Online
RSC Advances
Paper
angles of the blank and inhibited solutions are 66ꢁ at 197 Hz, 1313 U cm2 when the concentration is 7.0 mM, and h is 91.2%.
69ꢁ at 157 Hz, 70ꢁ at 60 Hz, 71ꢁ at 54 Hz, 72ꢁ at 200 Hz and 74ꢁ at In addition, with the increase in inhibitor concentration the
199 Hz, and with the increase in inhibitor concentration, the value of the electric double layer capacitance decreases, which is
absolute impedance gradually increases at low frequencies, attributed to the inhibitor molecules replacing the water
which indicates that DEEQ has the higher inhibitory effect.28 It molecules adsorbed on the mild steel surface.
can be found from Table 3 that with the increase in the DEEQ
concentration, R0p increases, which indicates that the inhibitor
has an evident inhibitory effect aer adsorption on the mild
3.5 Scanning Kelvin probe
steel surface. The minimum polarization resistance (Rp) of the
blank solution is 109.4 U cm2, the maximum value of R0p reaches
Fig. 6 shows the three-dimensional diagram of potential distri-
bution for the original state (a), and the absence (b) and pres-
ence (c) of DEEQ in 15% HCl for 4 h at 298 K. It is clear that the
potential distribution of the original state has a negative
potential value, and the uneven potential distribution was
mainly due to differences in roughness of the specimen surface.
Comparing the original state with the result in the absence of
DEEQ, the potential distribution of the specimen surface is
distinctly positive, which indicates that the surface of the spec-
imen was corroded, causing the potential to move in the positive
direction. For the result in the presence of DEEQ (c), the
potential distribution was again a negative value, which means
that the specimen’s surface was effectively protected with the
inhibitor, and proves that DEEQ has excellent performance as an
inhibitor, corresponding with the inhibitor performance ob-
tained from the weight loss and electrochemical experiments.
Fig. 7 is the GaussAmp tting curve of the SKP potential
distribution on the surface of the specimens in the original
state, and in the absence and presence of DEEQ in 15% HCl for
4 h at 298 K, and the tting formula is shown in the follow
equation:
Fig. 7 Histogram of potential distributions and fitting curves of the
original state (A), and in the absence (B) and presence (C) of DEEQ.
Fig. 8 Langmuir adsorption plots for mild steel in 15% HCl solution with different concentrations of DEEQ at different temperatures.
26470 | RSC Adv., 2019, 9, 26464–26475
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Table 4 Thermodynamic parameters of adsorption for mild steel in 15% HCl solution at different temperatures
T (K)
Kads (L molꢂ1
)
DG0ads (kJ molꢂ1
)
DH0ads (kJ molꢂ1
)
DS0ads (J Kꢂ1 molꢂ1
ꢂ77.1
)
333
343
353
363
9336
6234
3222
3128
ꢂ36.41
ꢂ36.35
ꢂ35.48
ꢂ36.39
ꢂ39.77
adsorption on mild steel surfaces, so adsorption isotherm
studies can provide more information regarding the interaction
between the inhibitor and the mild steel surface. In this part,
the classical Langmuir adsorption isotherm was applied to
describe the adsorption behavior of DEEQ, and the isotherm
can be represented as:
C
1
¼
þ C
(10)
q
Kads
where Kads is the equilibrium constant of the adsorption
process, C is the concentration of inhibitor and q is the surface
coverage. All of the parameters can be obtained from the weight
loss measurement. In addition, the thermodynamic parameters
of standard free energy (DG0ads), enthalpy (DH0ads) and entropy
(DS0ads) are calculated by the following equations:
ꢀ
ꢁ
1
DGa0ds
Fig. 9 The relationship between ln Kads and (1000/T) for mild steel in
15% HCl solution with different concentrations of DEEQ.
Kads
¼
exp
ꢂ
(11)
55:5
RT
1
DHa0ds DSa0ds
ln Kads ¼ ln
ꢂ
þ
(12)
2
55:5
RT
R
y ¼ y0 þ A eꢂðxꢂxcÞ
(9)
2w2
where R is the universal gas constant, T is the thermodynamic
temperature, and the value of 55.5 is the molar concentration of
water in the solution (mol Lꢂ1). From Fig. 8 it is clear that there
is an obvious linear relationship between C/q and C, and the
value of the regression coefficient (R) is close to 1 at different
temperatures, which indicates that the adsorption of DEEQ on
the mild steel surface obeys the Langmuir adsorption
isotherm.23,26,29 Table 4 shows the parameters of adsorption
thermodynamics, and it can be found that there is a negative
value of DG0ads at different temperatures, indicating that the
adsorption of DEEQ on the surface of mild steel is a sponta-
neous process. Furthermore, the values of DG0ads are concen-
where A is constant, y0 is the ordinate offset, xc is the concen-
trated position of the potential distribution, w2 is the degree of
concentration, and the smaller the value, the more concen-
trated the potential distribution at the xc value. From the tting
results we nd the xc values of the original state, and in the
absence and presence the DEEQ are ꢂ803 (mV), 98 (mV) and
ꢂ213 (mV), respectively.
3.6 Adsorption isotherm studies and thermodynamic
analysis
trated around ꢂ33 kJ molꢂ1
, which indicates that the
Many literature reports have indicated that organic corrosion
inhibitors mainly achieve their inhibitory effects through
Fig. 10 SEM micrographs of the mild steel surface before (a) and after immersion in 15% HCl solution with (b) and without (c) DEEQ for 4 h at 298
K.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 26464–26475 | 26471

View Article Online
RSC Advances
Paper
Table 5 Element analysis of the mild steel surface before and after Table 6 Quantum chemical parameters for the DEEQ cation
immersion in 15% HCl solution without DEEQ for 4 h at 298 K
HOMO (eV)
LUMO (eV)
DE (eV)
m (D)
Element
Before
C
N
—
—
2.41
7.59
O
Si
Mn
Fe
ꢂ8.43009172
ꢂ6.052359588
2.377732132
10.748355
wt%
at%
wt%
at%
00.15
00.63
0.11
0.42
03.99
12.53
7.39
20.38
00.67
01.20
0.61
0.96
01.72
01.57
1.87
1.50
93.47
84.06
87.60
69.15
Aer
benzene ring, indicating the benzene ring is the active site of
adsorption.39 On the other hand, because of the plane conju-
gated structure, the inhibitor DEEQ is considered to adsorbed
smoothly on the surface of the mild steel.
adsorption of DEEQ on the surface of mild steel is a mixed
adsorption, and the results correspond with the data obtained
from the electrochemical experiments. In addition, a negative
DH0ads reveals that the adsorption of DEEQ on the metal surface
in 15% HCl is a exothermic process (Fig. 9).32,33
According to frontier molecular orbital theory, the reactivity
of the molecule is closely related to the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molec-
ular orbital (LUMO).18 The HOMO and LUMO orbits are related
to the ability of the molecule to give and accept electrons,
respectively. A high EHOMO value means that the molecule has
a strong electron donating ability, and the lower the ELUMO value
the easier the molecule accepts electrons. Therefore, when the
corrosion inhibitor molecules have a high EHOMO and low
ELUMO, the electron transfer between the inhibitor molecules
and the metal surface is easy, and the inhibitor shows high
corrosion inhibition performance.45 From Table 6, both the
EHOMO and ELUMO values are lower, which means that the DEEQ
cation accepts electrons more easily than it donates electrons.
Compared with the tiny m value of H2O, the DEEQ cation is more
likely to replace the water molecules adsorbed on the surface of
mild steel.
Table 7 shows the Mulliken charges of the atoms in the
DEEQ cation. It was found that the charges of C2, C9, C11, C22,
C24 and C26 are positive, while the charges of the benzene ring
are negative, and the net charge of all the atoms of DEEQ is
negative. Therefore, the conjugated benzene ring can offer
electrons to form a coordinate bond with the metal orbital. In
addition, the site of the positive charge can accept electrons to
interact with the metal surface by electrostatic attraction, which
all indicates that DEEQ can easily adsorb on the metal’s surface.
3.7 Surface analysis
Fig. 10 shows the SEM micrographs of the fresh specimen (a)
and the specimens aer immersion in 15% HCl solution in the
presence (b) and absence (c) of 7.0 mM DEEQ for 4 h at 363 K. It
can be seen from the images that the surface of the specimen is
seriously damaged in the absence of inhibitor due to the
corrosive acid solution,22,23 and the specimen shows a smooth
surface in the presence of inhibitor. In addition, elemental
analysis was carried out by using energy dispersive spectroscopy
(EDS), and the results are shown in Table 5. Comparing the
samples in the absence and presence inhibitor, the content of
the element N increased signicantly, indicating that an
adsorption lm was formed on the surface of the specimen,16–18
which isolated the acid solution and protected the specimen.
These results are consistent with the inhibitor performance
results obtained from the weight loss, SKP, adsorption isotherm
and electrochemical experiments.
3.8 Quantum chemical calculations
In order to further understand the corrosion inhibition mech-
anism of DEEQ, quantum chemical calculations were per-
formed. The optimized geometry structure of DHAC and the
electron density distributions of the HOMO and LUMO are
3.9 Molecular dynamic simulations
displayed in Fig. 11 and Table 6 shows the associated quanti- In order to research the microscopic behavior of the inhibitor
zation parameters such as EHOMO, ELUMO, DE and the dipole adsorbed on the mild steel Fe (100) surface, the equilibrium
moment m. It can be seen that the electron density distributions conguration for DEEQ in an aqueous system was simulated,
of both the HOMO and LUMO are evenly distributed on the and the result is shown in Fig. 12. It can be seen that the DEEQ
Fig. 11 Optimized geometric structure (a) and the electron density distributions of the HOMO (b) and LUMO (c) for the DEEQ cation.
26472 | RSC Adv., 2019, 9, 26464–26475
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Table 7 The Mulliken charge of atoms in the DEEQ cation
Atom
N1
N2
O1
C2
C3
C4
C5
C6
C7
Mulliken charge
C8
ꢂ0.4845
ꢂ0.3892
ꢂ0.6470
0.1391
C12
ꢂ0.1806
ꢂ0.1271
ꢂ0.1345
ꢂ0.2145
ꢂ0.1296
C9
C10
C11
C13
C14
C15
C16
C17
ꢂ0.1634
0.1091
C19
ꢂ0.2291
C20
0.3403
C21
ꢂ0.1871
ꢂ0.1273
C23
ꢂ0.1275
C24
ꢂ0.1304
C25
ꢂ0.1132
C26
ꢂ0.2030
C27
C18
C22
ꢂ0.1297
ꢂ0.1650
ꢂ0.1567
ꢂ0.2037
0.1272
ꢂ0.1836
0.1337
ꢂ0.2036
0.1190
ꢂ0.0982
calculations revealed that the DEEQ molecules can provide
electrons to the metal atomic orbital with a strong interaction.
The results further demonstrated that DEEQ has an evident
inhibitory effect for oil and gas eld acidication.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 12 Equilibrium configuration of DEEQ in an aqueous system (side
view and top view).
This work was supported by the Sichuan Youth Science and
Technology Innovation Research Team Program (2017TD0013)
and the Major Program of the National Natural Science Foun-
dation of China (Grant No. 51490653).
cation is parallel with the Fe (100) surface, which indicates that
the planar structure of the dibenzylamine ring is the dominant
factor for the adsorption orientation. In an aqueous system, the
DEEQ molecules have a strong mutual attraction between the
active sites and the Fe (100) surface, causing the inhibitor
molecules to be immobilized on the metal surface. The kinetic
simulation results show that the adsorption energy of the
molecule is ꢂ2174.93 kJ molꢂ1, which is much larger than that
References
¨
1 A. Alagta, I. Felhosi, I. Bertoti, et al., Corrosion protection
properties of hydroxamic acid self-assembled monolayer
on carbon steel, Corros. Sci., 2008, 50(6), 0–1649.
2 M. T. Alhaffar, S. A. Umoren, I. B. Obot, et al., Isoxazolidine
derivatives as corrosion inhibitors for low carbon steel in
HCl solution: experimental, theoretical and effect of KI
studies, RSC Adv., 2018, 8(4), 1764–1777.
3 J. Aljourani, K. Raeissi and M. A. Golozar, Benzimidazole and
its derivatives as corrosion inhibitors for mild steel in 1 M
HCl solution, Corros. Sci., 2009, 51(8), 0–1843.
4 P. Arellanes-Lozada, O. Olivares-Xometl, N. V. Likhanova,
et al., Adsorption and performance of ammonium-based
ionic liquids as corrosion inhibitors of steel, J. Mol. Liq.,
2018, 151–163.
5 F. Bentiss, C. Jama, B. Mernari, et al., Corrosion control of
mild steel using 3,5-bis(4-methoxyphenyl)-4-amino-1,2,4-
triazole in normal hydrochloric acid medium, Corros. Sci.,
2009, 51(8), 0–1635.
of the water molecules on the Fe (100) surface of
ꢂ24.64 kJ molꢂ1
, indicating that the inhibitor molecules can
24
expel the water molecules and adsorbed on the Fe (100) surface.
It can be inferred that the inhibitor molecules can aggregate
and adsorb on the metal surface to form a dense molecular lm
layer in the liquid phase corrosive medium, separating the
metal surface from the water molecules and the corrosive
medium to achieve the effect of corrosion inhibition.
4. Conclusion
Weight loss and SEM measurements revealed that DEEQ is an
effective inhibitor through adsorption on the mild steel surface,
blocking contact between the acidic medium and metal surface,
and the inhibitor exhibited a 95.4% maximum inhibition effi- 6 D. Daoud, T. Douadi, H. Hamani, et al., Corrosion inhibition
ciency at 363 K, which can meet the requirements of acidica-
tion operation of medium and low temperature oil and gas well.
The contact angle experiment shows that the corrosion inhib-
of mild steel by two new S-heterocyclic compounds in 1 M
HCl: Experimental and computational study, Corros. Sci.,
2015, 94, 21–37.
itor can change the wettability of the steel sheet surface, thus 7 Q. Deng, N. N. Ding, X. L. Wei, L. Cai, X. P. He and Y. T. Long,
inhibiting acid corrosion of oil and gas string. Electrochemical
experiments showed that DEEQ can inhibit both the anodic and
cathodic corrosion processes, and indicated that DEEQ is
a mixed inhibitor. From the microscopic point of view, scanning
Identication of diverse 1,2,3-triazole-connected benzyl
glycoside-serine/threonine conjugates as potent corrosion
inhibitors for mild steel in HCl, Corros. Sci., 2012, 64(6),
64–73.
Kelvin probe (SKP) experiments further proved that DEEQ has 8 A. A. Farag, A. S. Ismail and M. A. Migahed, Environmental-
an obvious inhibitory effect, and the adsorption of DEEQ on
friendly shrimp waste protein corrosion inhibitor for carbon
mild steel obeys the Langmuir adsorption isotherm. Theoretical
steel in 1 M HCl solution, Egypt. J. Pet., 2018, 1187–1194.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 26464–26475 | 26473

View Article Online
RSC Advances
Paper
9 H. X. Feng, J. Li and X. H. Gao, Electrochemical Studies of
some New Mannich Bases Compounds as Corrosion
as mild steel corrosion inhibitors. Part I: Gravimetric and
voltammetric results, Corros. Sci., 2011, 53(2), 0–686.
Inhibitors for Mild Steel in Acidic Media, Adv. Mater. Res., 24 Q. Ma, S. Qi, X. He, et al., 1,2,3-Triazole derivatives as
2012, 538–541, 2329–2333.
corrosion inhibitors for mild steel in acidic medium:
Experimental and computational chemistry studies, Corros.
Sci., 2017, 129, 91–101.
10 A. S. Fouda, M. A. Ismail, A. S. Abousalem, et al.,
Experimental and theoretical studies on corrosion
inhibition of 4-amidinophenyl-2,20-bifuran and its 25 Y. Qiang, S. Zhang, B. Tan, et al., Evaluation of Ginkgo leaf
analogues in acidic media, RSC Adv., 2017, 7(73), 46414–
extract as an eco-friendly corrosion inhibitor of X70 steel
46430.
in HCl solution, Corros. Sci., 2018, 6–16.
11 H. J. Habeeb, H. M. Luaibi, R. M. Dakhil, et al., Development 26 Z. Salarvand, M. Amirnasr, M. Talebian, et al., Enhanced
of new corrosion inhibitor tested on mild steel supported by
electrochemical study, Results Phys., 2018, 1260–1267.
12 H. Hamani, T. Douadi, D. Daoud, et al., 1-(4-Nitrophenylo-
imino)-1-(phenylhydrazono)-propan-2-one as corrosion
inhibitor for mild steel in 1 M HCl solution: Weight loss,
corrosion resistance of mild steel in 1 M HCl solution by
trace amount of 2-phenyl-benzothiazole derivatives:
Experimental, quantum chemical calculations and
molecular dynamics (MD) simulation studies, Corros. Sci.,
2017, 114, 133–145.
electrochemical, thermodynamic and quantum chemical 27 T. A. Salman, K. F. Al-Azawi, M. I. Mahdi, et al., Experimental
studies, J. Electroanal. Chem., 2017, 801, 425–438.
13 K. Haruna, I. B. Obot, N. K. Ankah, et al., Gelatin: A green
corrosion inhibitor for carbon steel in oil well acidizing
environment, J. Mol. Liq., 2018, 264, 515–525.
14 F. Mansfeld and C. H. Hsu, Concerning the conversion of the
constant phase element parameter y0 into a capacitance,
Corrosion, 2001, 57, 747–748.
studies on inhibition of mild steel corrosion by novel
synthesized inhibitor complemented with quantum
chemical calculations, Results Phys., 2018, 10, 291–296.
28 S. M. Shaban, N-(3-(Dimethyl benzyl ammonio) propyl)
alkanamide chloride derivatives as corrosion inhibitors for
mild steel in
1 M HCl solution: experimental and
theoretical investigation, RSC Adv., 2016, 6(46), 39784–
15 Z. Hu, Y. Meng, X. Ma, et al., Experimental and theoretical
39800.
studies of benzothiazole derivatives as corrosion inhibitors 29 A. K. Singh and M. A. Quraishi, The effect of some bis-
for carbon steel in 1 M HCl, Corros. Sci., 2016, 112, 563–575.
thiadiazole derivatives on the corrosion of mild steel in
16 E. B. Ituen, O. Akaranta and S. A. Umoren, N-Acetyl cysteine
hydrochloric acid, Corros. Sci., 2010, 52(4), 0–1385.
based corrosion inhibitor formulations for steel protection 30 A. K. Singh and M. A. Quraishi, Investigation of the effect of
in 15% HCl solution, J. Mol. Liq., 2017, 112–118.
17 S. John and A. Joseph, Electro analytical, surface
disulram on corrosion of mild steel in hydrochloric acid
solution, Corros. Sci., 2011, 53(4), 0–1297.
´
morphological and theoretical studies on the corrosion 31 V. V. Torres, R. S. Amado, C. Faia de Sa, et al., Inhibitory
inhibition behavior of different 1,2,4-triazole precursors on
mild steel in 1 M hydrochloric acid, Mater. Chem. Phys.,
2012, 133(2–3), 1083–1091.
action of aqueous coffee ground extracts on the corrosion
of carbon steel in HCl solution, Corros. Sci., 2011, 53(7), 0–
2392.
18 O. Kaczerewska, R. Leiva-Garcia, R. Akid, et al., Efficiency of 32 X. Wang, H. Yang and F. Wang, An investigation of
cationic
Gemini
surfactants
with
3-
benzimidazole derivative as corrosion inhibitor for mild
steel in different concentration HCl solutions, Corros. Sci.,
2011, 53(1), 0–121.
azamethylpentamethylene spacer as corrosion inhibitors
for stainless steel in hydrochloric acid, J. Mol. Liq., 2017,
247, 6–13.
33 X. Wang, H. Yang and F. Wang, A cationic Gemini-surfactant
as effective inhibitor for mild steel in HCl solutions, Corros.
Sci., 2010, 52(4), 0–1276.
34 W. Wu, M. Chen, S. Liang, et al., Superhydrophobic surface
from Cu–Zn alloy by one step O2 concentration dependent
etching, J. Colloid Interface Sci., 2008, 326(2), 478–482.
19 L. Guo, S. Zhu, S. Zhang, et al., Theoretical studies of three
triazole derivatives as corrosion inhibitors for mild steel in
acidic medium, Corros. Sci., 2014, 87, 366–375.
20 S. Masroor, M. Mobin, M. J. Alam, et al., The novel iminium
surfactant p-benzylidene benzyldodecyl iminium chloride as
ˇ
ˇ ˇ
a corrosion inhibitor for plain carbon steel in 1 M HCl: 35 K. Xhanari, M. Finsgar, M. Knez Hrncic, et al., Green
electrochemical and DFT evaluation, RSC Adv., 2017, 7(37),
23182–23196.
corrosion inhibitors for aluminium and its alloys: a review,
RSC Adv., 2017, 7(44), 27299–27330.
21 P. Mayakrishnan, S. H. Kim, H. Venkatesan, et al., Rhus 36 Y. Xiu-Fang, T. Min-Min and M. A. Yang-Min, Mannich Base
verniciua as a green corrosion inhibitor for mild steel in
1 M H2SO4, RSC Adv., 2016, 6(62), 2046–2069.
as Corrosion Inhibitor for N80 Steel in HCl Solution, Surf.
Technol., 2017, 175–181.
22 N. A. Negm, A. M. A. Sabagh, M. A. Migahed, et al., 37 J.-Q. Xue, X.-N. Ma, L.-H. Yu, C.-B. Tang and J. Ma,
Effectiveness of some diquaternary ammonium surfactants
as corrosion inhibitors for carbon steel in 0.5 M HCl
solution, Corros. Sci., 2010, 52(6), 0–2132.
Development of a novel acidication corrosion inhibitor
for P110 steel at high temperature, Sci. Adv. Mater., 2012,
4(1), 61–67.
23 A. Popova, M. Christov, A. Vasilev, et al., Mono- and 38 X. Yang, F. Li and W. Zhang, 4-(Pyridin-4-yl) thiazol-2-amine
dicationic benzothiazolic quaternary ammonium bromides
as an efficient non-toxic inhibitor for mild steel in
26474 | RSC Adv., 2019, 9, 26464–26475
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
hydrochloric acid solutions, RSC Adv., 2019, 9(19), 10454– 42 D. Zhang, Y. Tang, S. Qi, et al., The inhibition performance
10464.
of long-chain alkyl-substituted benzimidazole derivatives
for corrosion of mild steel in HCl, Corros. Sci., 2015, 102,
517–522.
39 N. Yilmaz, A. Fitoz, ¨y. Ergun, et al.,
A Combined
Electrochemical and Theoretical Study into the Effect of 2-
((thiazole-2-ylimino) methyl) phenol as
a
Corrosion 43 X. Zheng, S. Zhang, W. Li, L. Yin, J. He and J. Wu,
Inhibitor for Mild Steel in a Highly Acidic Environment,
Corros. Sci., 2016, 110–120.
Investigation
of
1-butyl-3-methyl-1H-benzimidazolium
iodide as inhibitor for mild steel in sulfuric acid solution,
40 R. Yıldız, An electrochemical and theoretical evaluation of
Corros. Sci., 2014, 80(3), 383–392.
4,6-diamino-2-pyrimidinethiol as a corrosion inhibitor for 44 X. Zheng, S. Zhang, W. Li, M. Gong and L. Yin, Experimental
mild steel in HCl solutions, Corros. Sci., 2014, 90, 544–553.
41 Y. Pan, F. Zhan, Z. Lu and Y. Lin, A Mannich base 1-phenyl-3-
(1-pyrrolidinyl)-1-propanone: synthesis and performance
and theoretical studies of two imidazolium-based ionic
liquids as inhibitors for mild steel in sulfuric acid
solution, Corros. Sci., 2015, 95, 168–179.
study on corrosion inhibition for N80 steel in 15% 45 L. Zhou, Y. L. Lv, Y. X. Hu, et al., Experimental and
hydrochloric acid, Anti-Corros. Methods Mater., 2016, 63(2),
153–159.
theoretical investigations of 1,3,5-tris(4-aminophenoxy)
benzene as an effective corrosion inhibitor for mild steel
in 1, M HCl, J. Mol. Liq., 2018, 249, 179–187.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 26464–26475 | 26475