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Accepted Manuscript
Excellent corrosion inhibition performance of novel quinoline
derivatives on mild steel in HCl media: Experimental and
computational investigations
Lu Jiang, Yujie Qiang, Zulei Lei, Jianing Wang, Zhongjian Qin,
Bin Xiang
PII:
DOI:
Reference:
S0167-7322(17)33980-6
https://doi.org/10.1016/j.molliq.2018.01.133
MOLLIQ 8587
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
30 August 2017
25 December 2017
23 January 2018
Please cite this article as: Lu Jiang, Yujie Qiang, Zulei Lei, Jianing Wang, Zhongjian Qin,
Bin Xiang , Excellent corrosion inhibition performance of novel quinoline derivatives
on mild steel in HCl media: Experimental and computational investigations. The address
for the corresponding author was captured as affiliation for all authors. Please check if
appropriate. Molliq(2017), https://doi.org/10.1016/j.molliq.2018.01.133
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ACCEPTED MANUSCRIPT
Excellent Corrosion Inhibition Performance of Novel Quinoline Derivatives on Mild
Steel in HCl media: Experimental and Computational Investigations
Lu Jianga,b, Yujie Qianga,b, Zulei Leia,b, Jianing Wanga,b, Zhongjian Qina,b, Bin Xianga*,b
a
b
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China.
National-municipal Joint Engineering Laboratory for Chemical Process Intensification and Reaction,
TEL: +86 23 65678934
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. * Corresponding authors: E-mail: [email protected]
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Chongqing University, Chongqing 400044, PR China
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Abstract: The inhibition performance of two synthesized quinoline derivatives: 6-benzylquinoline (BQ) and
6-(quinolin-6-ylmethyl) benzene-1,2,3,4,5-pentasulfonic acid (QBPA) on mild steel in 1 M HCl has been
investigated through weight loss, electrochemical measurements, scanning electron microscopy (SEM), and
atomic force microscopy (AFM). All experimental results indicated that BQ and QBPA extremely enhanced
the corrosion resistance of mild steel and QBPA showed a better inhibitive performance than BQ. The results
of potentiodynamic polarization illustrated that BQ and QBPA performed as mixed-type inhibitors. Langmuir
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adsorption isotherm was well fitted for the adsorption of BQ and QBPA on mild steel surface with a
competitive physisorption and chemisorption mechanism. The results of quantum chemical calculations and
molecular dynamic simulations showed that benzene rings of both BQ and QBPA adsorbed on the metal
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surface in distinct gradient direction and quinoline ring of both BQ and QBPA adsorbed nearly parallel on
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the steel surface.
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Key words: mild steel; inhibition corrosion; adsorption; electrochemistry; molecular modeling
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1 Introduction
Mild steel, which has excellent physical properties, affordability in economy and simple smelting
process, extensively applies in industrial field [1]. The main problem of mild steel is that it is easy to be
corroded in acid pickling process, causing severely financial and industrial losses. An effective way of anticorrosion for mild steel is the addition of corrosion inhibitors.
Many inorganic and synthetic organic compounds have been proved to be excellent corrosion inhibitors
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in recent years, such as chromate [2], polyphosphates [3] and benzothiazole [4]. However, these inhibitors
often have a negative effect to both human and environment, which hinders their application. Ghulamullah
khan et al. [5] stated that toxic inhibitors may cause temporary or permanent damage like kidney or liver or
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disturbing biochemical or enzyme system in body and often arise during synthesis or its applications, causing
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severely security risks and economic losses. Hence, the development of environmental-friendly inhibitors is
becoming a widely discussed issue. Quinoline derivatives are crucial ingredients of anti-malarial drugs and
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have special pharmacological properties [6], which has no significant toxicity to environment [7] .They are
also easy to be synthesized and cost-efficient with high inhibition performance [8]. So, quinoline derivatives
harbor remarkably practical applicability in metal anti-corrosion field.
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Some scientific studies on quinoline derivatives as corrosion inhibitors have been reported in recent
years. Achary et al.[9] stated 8-Hydroxy quinoline (HQ) and 3-formyl-8-hydroxy quinoline (FQ) to be two
effective corrosion inhibitors on mild steel in 1 M HCl. Gerengi al. [10] concluded the corrosion inhibition
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performance of 8-Hydroxyquinoline on copper in 0.1 M HCl and the best inhibition efficiency was only 79%
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at optimal concentration 0.016M. Singh et al. [7] successfully applied a series of 1, 4-dihrdroquinoline
derivatives as corrosion inhibitors for mild steel and demonstrated that methoxy group enhanced the
corrosion inhibition efficiency. However, to the best of author’s knowledge, most investigated quinoline
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derivatives exhibit low corrosion inhibition efficiencies.
Here, two quinoline-based inhibitors namely, 6-benzylquinoline (BQ) and 6-(quinolin-6-ylmethyl)
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benzene-1, 2, 3, 4, 5-pentasulfonic acid (QBPA) were synthesized and identified by Fourier Transform
Infrared Spectroscopy (FT-IR) and Hydrogen Nuclear Magnetic Resonance Spectroscopy (1H NMR).
Afterwards, weight loss and electrochemical measurements were implemented to investigate the performance
of the inhibitors on the corrosion of mild steel in 1 M HCl. The surface morphologies of samples were
analyzed utilizing scanning electron microscopy (SEM) and atomic force microscope (AFM) technique.
Quantum chemical calculations and molecules dynamics (MD) simulations were implemented to explain the
inhibition mechanism of these two organic molecules.
2. Experimental
2.1. Synthesis and characterization of corrosion inhibitors
Fig. 1 shows the synthetic route of the studied inhibitors. The intermediate product 6-benzylquinoline
(BQ) was prepared by an ice-water bath added quinoline and benzyl chloride (molar ratio 1:1). After 1 hour
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reaction, the reagents were washed with 10 mL ethyl acetate and a small amount of sodium iodide, then
heated to room temperature (298K) for 9 h and then heated to 338K for 27 h. The resulting solution was
purified by washing, filtering with ethyl acetate and dried in vacuum.
6-(Quinolin-6-ylmethyl) benzene-1,2,3,4,5-pentasulfonic acid (QBPA) was synthesized by reacting the
mixture of intermediate product BQ and concentrated sulfuric acid (molar ratio 1:5) in an ice-water bath for
2 h and then heated to 338K for 4 h. The product was percolated, swashed with ethyl acetate and desiccated
in vacuum.
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Fig. S1 shows the FT-IR and 1H NMR spectrums of BQ and QBPA. For BQ: light red, solid, yield =
82%. 1H NMR (400 MHz), δ (ppm) (Fig. 2a): 3.5144 (m, 2H, ArCH2), 7.1918 (t, 2H, ArCH2CCHCH),
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7.2924 (d, 2H, ArCH2CCH and NCHCH), 7.8075 (t, 1H, Ar-H), 7.9603 (m, 2H, Ar-H), 8.2243 (d, 2H, Ar-
cm-1 (ν
C-H,
aliphatic fatty chain), 1032.6-1230.6 cm-1 (ν
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H), 9.0438 (d, 1H, NCHCHCH-), 9.2104 (d, 1H, NCH-). FT-IR (KBr pellet) (Fig. 2c): 2922.9 and 2852.0
C-N,
fatty amines) and 3060.1-3254.2 cm-1 (ν
C-H,
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aromatic). For QBPA: red, liquid, yield= 63%. 1H NMR (400 MHz), δ (ppm) (Fig. 2b): 3.3532 (m, 5H, SO3H),
3.9652 (m, 2H, ArCH2), 7.1901 (t, 2H, ArCH2CCHCH), 7.2903 (t, 2H, ArCH2CCH and NCHCH) 9.0481
(d, 1H, NCHCHCH-), 9.2019 (d, 1H, NCH-). FT-IR (KBr pellet) (Fig. 2d): 1355.2 and 1192.8 cm-1 (νR-SO21022.8 cm-1 (ν
C-N,
fatty amines), 2920.6 and 2850.5 cm-1 (ν
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OH),
cm-1 (ν C-H, aromatic).
C-H,
aliphatic fatty chain), 2976.6-3260.9
2.2. Materials and reagents for inhibition tests
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The size of mild steel was 3 cm ×2 cm ×1 cm for weight loss measurements and 0.5 cm ×0.5 cm ×0.5
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cm for surface analysis while the exposed area of electrochemical experiments was 1 cm2. Before the tests,
all specimens were carefully polished by a sequence of emery papers of grade Nos. 400, 800, 1200 and 2000,
and then rinsed with distilled water, degreased with absolute ethyl alcohol and finally dried at room
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temperature.
Analytical grade HCl (37.5%, purchased in Chongqing Chuandong Chemical Co., LTD) was diluted
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with distilled water to acquire the test solution.
2.3. Weight loss experiments
In this study, weight loss experiments were taken according to the standard methods [11]. The corrosion
parameters (corrosion rate CRW, the surface coverage θ, corrosion inhibition efficiency ηW) were calculated
using the following equations [12, 13]:
ΔW W1  W2

St
St
ο
C C
θ  RW ο RW
CRW
CRW 
ηW % 
(1)
(2)
CοRW  CRW
 100
CοRW
(3)
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where W1(mg) and W2(mg) are the weight of samples before and after the immersion, respectively. S
represents the surface area of the test steel (cm2), t stands for the immersion time (h), CRW and CοRW are the
corrosion rate with and without quinoline derivatives, respectively.
2.4. Electrochemical approaches
Electrochemical measurements were implemented by CHI 660E electrochemical corrosion workshop at
room temperature (298K, controlled by thermostat bath). All electrochemical approaches were accomplished
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through a three-electrode system. Ag/AgCl electrode and Pt sheet (2 cm2 area) were utilized as reference
electrode (RE) and counter electrode (CE), respectively. Q235 was used as the working electrode (WE). A
stable system was obtained after 30 min’s monitoring of its open circuit potential in 1 M HCl. Afterwards,
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electrochemical impedance spectroscopy (EIS) was recorded at OCP in 100 kHz to 0.01 kHz frequency range
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with 5 mV peak amplitude AC signals. EIS data were matched and further analyzed through ZsimpWin 3.10
software. The values of charge transfer resistance acquired from fitted data were utilized to calculate the
inhibition efficiency [14]:
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R ct  R οct
ηEIS% 
 100
R οct
(4)
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where Rοct and Rct are charge transfer resistances in the absence and presence of different concentrations
of inhibitors, respectively.
After the EIS test, polarization curves were performed in a potential extent of ±0.25V away from the
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operating corrosion potential (Eocp) with 1.0 mV s-1 scan rate. The corrosion current density was acquired
ηp % 
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through Tafel plot. The inhibition efficiency ηp% was calculated by following equation [15]:
i οcorr  i corr
 100
i οcorr
(5)
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where iοcorr and i corr are the corrosion current density (A cm-2) with and without of inhibitors, respectively.
2.5. Surface morphology characterization
The surface morphology of mild steel samples immersed in 1 M HCl with and without studied quinoline
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derivatives were observed by scanning electron microscopy (Tescan Vega3 SEM instrument) at a high
vacuum and atomic force microscope (AFM，MFP-3D-BIO, Asylum Research, America) through tapping
mode, respectively.
2.6. Computational approaches
Gaussian 03W software was applied to conduct quantum chemical calculations. Geometry optimization
of BQ along with QBPA was performed by density functional theory (DFT) at B3LYP functional with 6311++G (d, p) basis set in gas phase [16]. Parameters of the two molecules including the total energy (ET),
the energy of highest occupied molecular orbital (EHOMO), the energy of lowest unoccupied molecular orbital
(EHOMO) along with dipole moment (μ) were calculated.
Moreover, molecular dynamics simulations have been implemented to explore the mutual effects
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between Fe (1 1 0) surface and the synthesized quinoline derivatives, since Fe (1 1 0) surface is regarded as
the heaviest packed as well as most steady surface [17]. The mutual effects between the inhibitors and Fe (1
1 0) surface was presumed in a simulation box by the COMPASS force field under periodic boundary
conditions. The number ratio of water molecules and inhibitors is 300:1, which are set spontaneously
cooperate with the steel surface. The temperature of MD simulation is 298K with a time step of 1 fs as well
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as a simulation time of 500 ps.
3. Results and discussions
3.1. Weight loss measurements
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3.1.1 Influence of inhibitor concentration
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Table S1 represents corrosion parameters obtained from gravimetric method for mild steel in 1 M HCl
with various concentrations of inhibitors. The variation trend of both corrosion rate and inhibition efficiency
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can be displayed by Fig.2. Apparently, with the increase concentration of the studied inhibitors, corrosion
rate decreases observably and the inhibition efficiency trend is the other way around, which indicates the
formation of a stable inhibitor-film on steel/solution interface[18]. Additionally, QBPA harbors a better
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prohibitive performance compared with BQ, which can be elucidated by the impact of sulfonic acid
functional group (-HSO3) and the higher molecular size of QBPA.
3.2.2 Influence of temperature
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The weight loss data at 298K-323K range are listed in Table S1. Obviously, the inhibition efficiency
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decreases with the rise of temperature, which can be attributed to a moderate mitigation of adsorption
molecular on metal surface in high temperature. In addition, ηw for QBPA shows a slower fall compared with
that of BQ at temperature range (decreased to 77.99% for BQ and 83.48% for QBPA at 1×10-3 M
than BQ.
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concentration in 323K), revealing that the inhibitive performance of QBPA has a better temperature stability
The relation between temperature and corrosion rate can be calculated from Arrhenius equation[19]：
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 Ea 

CRW  Ar exp 
 RT 
(6)
where R represents the gas constant, Ea stands for the apparent activation energy, T represents the absolute
temperature and Ar is the pre-exponential factor.
The apparent activation energy (Ea) of BQ and QBPA at different concentrations in 1 M HCl was
calculated from Arrhenius plot of Ln CRW verse 1/T (Fig. 3) and stated in Table S2. The value of Ea apparently
increases with the addition of QB and BQPA, revealing that more energy barriers are required for steel
corrosion than in their absence. Besides, the two synthesized inhibitors follows the order: Ea (QBPA) > Ea
(BQ), further demonstrating that QBPA can be regarded as a better inhibitor for mild steel.
According to transition state theory, enthalpy and entropy of activation can be calculated from[20]：
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CRW 
RT  Δ≠Sοa   Δ Hοa 
exp
exp 
Nh  R   RT 
(7)
Sa
where R is the gas constant, T stands for the absolute temperature,
and
Δ  Ha
are the entropy and
the enthalpy of activation, respectively. Additionally, h represents Planck’s constant and N signifies
Avogadro’s constant.
According to equation (7), the values of
plot of ln (CRW/T) verse 1/T (Fig. S2).
Δ H

a
Δ  Ha
and
Sa
given in Table S2 are calculated from the
has decisive influence on reaction rate. Higher values of
means that more energy barriers is needed for steel dissolution. For two inhibitors,
solution)> Δ  Ha (uninhibited solution) and
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
Δ  Ha
Δ  Ha (QBPA)> Δ  Ha (BQ),
Δ  Ha (inhibited
which further confirms that
dissolution happened owing to the positive values of
Δ  Ha
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QBPA has better inhibitive performance than BQ [21]. In addition, an endothermic reaction of steel
[15]. In the presence of inhibitors,
Sa
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values are much bigger than that without BQ and QBPA, which suggests a rise in disorder happening during
the adsorption process of the transition from reactants to activated complex [21].
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3.2.3 Adsorption isotherms
The mutual effects between inhibitors and the metallic superficies are able to be investigated through
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the adsorption isotherm since inhibitors generally adsorb upon the metal/solution surface [22]. Among
various adsorption isotherms (Langmuir, Frumkin, Temkin), Langmuir adsorption isotherm displays the
finest explanation for the adsorption behavior of the investigated inhibitors since the correlation coefficient
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(R2) were close to 1. Langmuir isotherm and corresponding adsorption thermodynamics parameters can be
  ΔGοads 
1

exp

55.5
RT


(8)
(9)
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Kads 
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conveyed by the following equation[18]:
C
1

C
θ
Kads
ΔHοads ΔSοads
1
(10)


55.5
RT
R
where C signifies the concentration of inhibitors; θ stands for surface coverage, whose value has been
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ln Kads  ln
calculated by weight loss test and stated in Table S1; Kads is the adsorptive equilibrium constant; T stands
for the thermodynamic temperature; R is the universal gas constant and 55.5 represents the molar
concentration of water in solution.
According to the above equations, the adsorption parameters, such as Kads ,
Gads
,
ΔHοads and
ΔSοads are calculated and presented in Fig.4. A high Kads value represents that inhibitors are strongly
adsorbed on metal and has a better inhibition behavior. As can be seen from Fig.4, with the increase of
temperature, Kads values decrease for both two investigated inhibitors. Meanwhile, it can be found that
Kads (BQ) is smaller than Kads (QBPA). Thus, it can be concluded that BQPA exhibit better inhibition
performance than QB, which matches the conclusion drew by 3.2.2.
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The negative value of the adsorption free energy shows a spontaneous adsorption process and strong
interaction between the two quinoline derivatives and mild steel surface [14]. A well-recognized judgment
method of adsorption on solid surface is
Gads . Values less than -20 kJ/mol means physical adsorption
(intermolecular forces between adsorbent and adsorbate) and values more than -40 kJ/mol indicates a
chemisorption process (the formation of coordination bonds between the inhibitor molecules and iron atoms
Gads
of steel surface) [21, 23, 24]. Compared to BQ, the
value of QBPA is more close to -40 kJ/mol,
indicating that QBPA adsorbed on the mild steel surface mainly through chemisorption accompanied with
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physical adsorption while the adsorption of QB is a mixed physical and chemical adsorption mechanism. As
a result, QBPA exhibits a stronger adsorption ability than BQ. This can be explained by the molecule structure
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of QBPA and BQ. Unlike BQ, QBPA contains sulfonic acid group, which forms back donating bonding with
the metal surface, resulting a more stable chemisorption.
ΔHοads values of QB and BQPA demonstrate that an exothermic process happens in the
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Negative
adsorption on the mild steel, which displays that the inhibition efficiency ηw declines with the temperature.
[25]. However, the values of
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This behaviors can be explained by desorption of some adsorbed inhibitor molecules from the steel surface
ΔSοads are all positive, which are opposite to what would be expected. In a real
the steel surface. Thus, the calculated
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situation, adsorption of inhibitor molecules is always accompanied by desorption of water molecules from
ΔSοads means the algebraic sum of inhibitor molecules adsorption and
water molecules desorption process [25].Since the adsorption of inhibitor molecules is exothermic process,
lower positive
ΔSοads value is attribute to the increase of solvent entropy Compared to QB, the
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Therefore, the positive
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endothermic process can occur for water molecules desorption, accompanied with an increase in entropy.
ΔSοads value of QBPA discloses lower driving force for QBPA to adsorb on mild steel [22,
26], further confirming QBPA harbors better inhibitive performance than QB.
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3.3. Electrochemical approaches
3.3.1 Polarization curve measurements
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The potentiodynamic polarization plots for mild steel in 1 M HCl with and without different
concentrations of BQ and QBPA at 298K are shown in Fig. 5.
As shown in Fig. 5, both anodic and cathodic plots move to poorer current densities, demonstrating that
the addition of BQ and QBPA conduce to an apparent suppression on corrosion process and the effect
enhances with the increasing concentration of inhibitors. The parallel cathodic polarization plots reveal that
BQ and QBPA have little effect on the mechanism of hydrogen reduction [27]. Thus, the addition of inhibitors
mitigate the reduction of H+ ions mainly though blocking the reaction sites on electrode surface. By contrast,
anodic polarization plots show an effective inhibitive influence at low anodic overpotentials while inhibitors
desorption is obvious at further potential values, especially at higher inhibitor concentrations [22]. This can
be regard as the crucial dissolution of steel, resulting desorption of inhibitors from the electrode surface [28].
Parameters such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (βc and βa) as well as
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corrosion current density (Icorr) obtained from polarization curves are recorded in Table 1. It can be found
that there is no distinct shift of Ecorr for both BQ and QBPA, disclosing that the investigated organics act as
mixed-type inhibitors [29]. The corrosion current density dramatically reduces from 1076 uA·cm-2 of bare to
274.2 uA·cm-2 for BQ and 205.8 uA·cm-2 for QBPA at the concentration of 1×10-4 M, respectively, showing
the effective inhibition performance for trace amounts of inhibitors. The corrosion inhibition efficiency ηp
increases with the growth of inhibitors’ concentration, till to its maximum value 94.74% for BQ and 97.15%
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for QBPA respectively at the concentration of 1×10-3 M.
In order to correlate the temperature results with weight-loss measurements, Fig. S3 shows polarization
curves for mild steel in 1 M HCl solution with and without 1 mM concentration of BQ and QBPA at different
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temperature (298-328K) and corresponding parameters are given in Table S3. As can be seen in Fig. S3,
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temperature has significant influence to both anodic and cathodic curves. With the increase of temperature,
anodic and cathodic curves move to higher current densities, leading to faster metal dissolution and hydrogen
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evolution reaction. And an apparent desorption process appears at sufficient positive potential conditions.
Meanwhile, Table S3 shows that BQ and QBPA still have high inhibition efficiency under elevated
temperature conditions, which is in accordance with weight loss measurements.
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3.3.2 Electrochemical impedance spectroscopy
Fig. 6 represents Nyquist and Bode diagrams of mild steel in 1 M HCl in the absence and presence of
two syudied quinoline derivatives.
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From the Nyquist plots, the depressed semicircle is relevant to frequency dispersion owing to the surface
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roughness, inhomogeneity of steel surface along with adsorption-desorption procedure of inhibitors on metal
surface [30]. Meanwhile, similar shapes for all experienced concentrations manifest that the addition of
inhibitors contributes no evident influence on corrosion mechanism. Furthermore, with the increase of
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inhibitors’ concentration, corrosion inhibition efficiency increases remarkably, which should ascribe to the
surface coverage’s growth of QB and BQPA on mild steel surface [31].
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For the Bode plots, the frequency range with maximum phase becomes larger with the rising inhibitors’
concentration, showing that inhibitors molecules effectively adsorb on the steel surface.
The equivalent circuit R(Q(R(QR))) （showed in Fig.7）was proposed to fit the impedance spectra since
ChiSq value was less than 10-4 [4]. Hence, two overlapped capacitive semicircles and two overlapped phase
maximum should exist in the Nyqusit diagrams and Bode diagrams, respectively [23]. The two time constants
should connected to the relaxation process of electrical double layer capacitor and adsorbed quinoline
derivatives [23]. As for the selected equivalent circuit, Rs and Rct are the solution and charge-transfer
resistance, respectively. The resistance of the film formed on the metal surface is represented by Rf. Qf and
Qdl stand for the constant phase angle elements constants (CPE), reflecting capacitance (Cf) and double layer
capacitance Cdl), respectively. The impedance function of the CPE is described as follows [32]:
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ZCP E 
1
Y0 jw 
（11）
n
where Y0 stands for the magnitude of the CPE, j denotes the imaginary number, w represents the angular
frequency. n is the deviation parameter in regard to phase shift. While n=0, the CPE stands for a pure resistor,
and for n=1, a pure capacitor.
The impendence parameters of BQ and QBPA are listed in Table 2. With the addition of inhibitors, both
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Rct and Rf value surge and the effect enhances with the increase of inhibitors’ concentration. This
phenomenon manifests the formation of inhibitors-adsorption films on metal surface; meanwhile, the
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adsorption inhibitor molecules and/or corrosion products consequently enlarge the pitting potential and
powerfully impedes the chloride-induced localized corrosion [33, 34]. The values of Cdl and Cf both display
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a decline tendency with the growing concentration of BQ and QBPA, which are caused by the decrease of
exposed area of electrode face, absorbed organic molecules replacement of water and the formation the thick
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electric double layer.
As a result, the highest Rct (515 Ω for BQ and 902.3 Ω for QBPA) have been acquired at optimum
concentration (1×10-3 M). Correspondingly, the highest corrosion inhibition efficiency ηEIS at 1×10-3 M is
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96.54% for BQ and 98.03% for QBPA. The order of inhibition ability is in perfect accordance with those
attained from polarization and weight loss experiments, revealing that the addition of sulfonic acid group in
3.4 Morphological analysis
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3.4.1 SEM analysis
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benzene ring enhances the effective performances of quinoline derivatives on mild steel corrosion.
The micrograms of the mild steel before and after the immersion in 1 M HCl with and without 1mM
QB and BQPA (the optimal concentration acquired by weight loss and electrochemical methods) for 4 hours
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at 298K are shown in Fig. 7. The immersed metal surface without inhibitor appears strictly damaged (Fig.
7b) compared to the smooth and quite uniform metal surface with only small scratches by abrasive paper
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before the immersion(Fig. 7a). After added 1mM inhibitors, the damage degree of metal surface is
remarkably reduced (Fig. 7c and Fig. 7d), demonstrating the excellent inhibitive ability of BQ and QBPA. In
addition, the metal surface of BQ is tougher than that for QBPA, which coordinates with the above
electrochemical results.
3.4.2 AFM observation
The 3D AFM images of uninhibited and inhibited mild steel surface with two synthesized inhibitors in
1 M HCl solution at 298K for 2 hours are shown in Fig. 8. The AFM images of freshly polished sample (Fig.
8a) appears uniform with some small scratches. The samples immersed in 1 M HCl without inhibitors (Fig.
8b) suffers damage from corrosion with rough surface. With the addition of inhibitors, the images are
relatively flat and smooth (Fig. 8c and d), which distinctly reveals that the corrosion of mild steel has been
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significantly suppressed. The visual results can be verified by the values of RMS roughness obtained from
AFM. The corresponding RMS values of the mild steel before HCl treatment, after unprotected treatment,
after protected treatment with BQ and QBPA are 9.313, 22.202, 11.276 and 7.446nm, respectively. Thus,
these images and parameters are indicative of the excellent inhibitive ability of two investigated inhibitors,
which is also in a well agreement with the results obtained from electrochemical measurement above.
3.5. Quantum chemical study
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Quantum chemical calculations were applied to explore the connection between electronic structures of
the inhibitors and their inhibition property. Optimized geometric structure and the electron density
distribution of both the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Orbital
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(LUMO) are presented in Fig.9. Table. S3 lists the related quantum chemical parameters including EHOMO,
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ELUMO,  E and μ.
As shown in Fig. 10, HOMO and LOMO of BQ are almost localized evenly on the quinoline ring and
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benzyl group (linked to quinoline ring), which indicates those parts act as the main sites to form coordinate
bonds and also dominant adsorption center to receive iron atoms’ electrons with its anti-bonding orbital to
constitute a feedback bond [35, 36]. By contrast, HOMO of QBPA is distributed in quinoline ring while its
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LUMO is located in the rest of molecule structure. Hence, rational assumption can be proposed that QBPA
includes more adsorption sites than QB [37, 38], issuing in a stronger adsorption capability.
Generally, higher EHOMO values means easier process of donating electrons to metal’s empty d-orbital
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[39]. In contrast, the LUMO energy (ELUMO) shows the electron accepting capacity of the molecules, the
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lower its value the easier transfer process of electron from the superficial metal to the inhibitors [40].
Rationally, the value of EHOMO: BQ<QBPA (Table. S3) is in full agreement with their inhibition efficiency
gained from electrochemical and weight loss experiments (η (BQ) < η (QBPA)). However, the sequence of
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ELUMO values shows an adverse result compared to the experimental data, revealing that a chelate nature of
interactions may involve in corrosive inhibition process [41]. Low ∆E usually represents greater polarization
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and easier adsorption of the molecule on the metal surface [42] and high dipole moment (μ) value relates to
the dipole-dipole interaction of inhibition effectiveness [43]. In this essay, the calculated energy gap of two
inhibitors follows the order: ∆E (QBPA) < ∆E (BQ), which is contrary to the order of dipole moment values.
Thus, the order of energy gap ∆E and high dipole moment (μ) indicates the strong adsorption of QBPA
molecules compare to that of BQ, confirming the rationalization of the experimental data
3.6. MD simulations
Molecular dynamics simulations have been implemented to explore the adsorption performance of these
quinoline derivatives on mild steel surface. Fig. 11 exhibits the side and top views of equilibrium
configuration for the adsorption of QB and BQPA on Fe (1 1 0) surface. It is evident that the adsorption of
benzene rings is in an obvious gradient direction for both BQ and QBPA, disclosing that the space structures
have substantial influence in adsorption process [21]. Furthermore, the quinoline ring of both BQ and QBPA
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adsorbed nearly parallel to the iron surface. This parallel position indicates that π-electrons on the quinoline
ring may offer electrons to the vacant d-orbital of Fe surface to constitute steady co-ordination bonds [4, 44].
Additionally, the anodic and cathodic active sites can be hindered by the adsorption of organic molecules.
Hence, high negative value of interaction energy represents good efficiency of corrosion inhibition. The
interaction energy value can be calculated as follows [17, 45]:
Einteract  E tot  (Esubs Einh)
(14)
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where Etot stands for the total energy of the whole system, Esubs represents the energy of mild steel substrate
together with water molecules and Einh is free inhibitor energy.
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According to the above equation, the calculated Einteract value is -131.7 Kcal/mol for BQ and -290.7
Kcal/mol for QBPA, disclosing that the addition of sulfonic acid functional group enhances inhibitive ability
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of molecules.
3.7. Inhibition mechanism
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Generally, the adsorption mechanism is widely acknowledged to explain corrosion inhibition property
of organic molecule and affected by the structural, chemical and electronic characteristics of inhibitors [14].
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The thermodynamic parameters of adsorption process in this study show that BQ adsorbed on the mild steel
surface through mixed physical and chemical adsorption while QBPA mainly adsorbed through
chemisorption accompanied with physical adsorption.
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Physical adsorption of organic inhibitors was firstly studied by Mann and his workers [46]. They stated
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that organic molecule contained central atom like N, S, O can be protonated to organic cation in strong acid
solution [46]. Meanwhile, the steel surface tends to be electronegative because of the specific adsorption of
Cl- in hydrochloric acid [47]. Hence, BQ and QBPA exist in a protonated form in acid solution and physically
adsorb on negative steel surface through electrostatic incorporation. After released H2 in steel surface, BQ
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and QBPA convert to their neutral state and follow a chemisorption process. The HOMO orbital for both BQ
and QBPA is mainly located in the quinoline ring and adjacent methylene group, indicating that these groups
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can donor electron to vacant d-orbital of iron atoms to form coordinate-covalent bond through nucleophilic
interaction [4]. These characters make these quinoline derivatives excellent corrosion inhibitors.
In this study, experimental results display that the inhibition efficiency of QBPA is better than BQ in
1M HCl solution. The quantum chemical results and molecular structure are contributed to such inhibition
efficiency difference. LUMO orbital of QBPA is mainly located in aromatic ring and sulfonic group (-SO3H),
forming feedback bonds between d orbitals of metal and inhibitor, which increases the chemical adsorption
oh QBPA molecules and thus enhances the inhibition efficiency. What’s more, literature [48] proved that the
larger size and high molecular weight can cause higher inhibition efficiency, confirming the greater inhibition
ability of QBPA.
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4. Conclusion
To conclude, systematic methods and characterizations were employed to explore the inhibitive ability
and mechanism of two novel quinoline-based molecules for mild steel corrosion in 1 M HCl. From
experimental as well as computational results, the following conclusion can be drew:
(1) The quinoline-based molecules QB and BQPA act as excellent inhibitors for mild steel in 1 M HCl
solution and their inhibition efficiency enhances with the increase of concentration. The maximum
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inhibition efficiency is 96.54% for QB and 98.03% for BQPA (obtained from EIS at 298K) at 1 mM,
respectively, which is higher than most reported quinoline derivatives inhibitors.
(2) The adsorption of QB and BQPA on steel surface obeys Langmuir adsorption isotherm with a mixed
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physisorption and chemisorption mechanism.
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(3) Electrochemical results indicate that QB and BQPA perform as both anode and cathodic type inhibitors
without changing hydrogen evolution’s mechanism. And the addition of inhibitors mainly hinders the
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active side on steel surface, contributing no evident influence on corrosion mechanism.
(4) A good coordination appears between the theoretical and experimental study, showing a deep insight into
the inhibition mechanism. Quantum chemical study demonstrates the inhibitive function of sulfonic acid
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group, theoretical explained why BQPA performed better inhibitive performance than QB.
Acknowledgment
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This work was supported by Chongqing Innovation Fund for Graduate Students (No. CYB16035) and
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the Fundamental Research Funds for the Central Universities (No. 106112016CDJXZ228803)
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Figures and captions:
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Fig. 1. Synthetic route and structure of BQ and QBPA
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Fig. 2. Inhibition efficiency (a) and corrosion rate (b) for mild steel in 1 M HCl with different concentrations
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of QBPA and QB at 298K.
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Fig. 3. Arrhenius plots for mild steel in 1 M HCl in the absence and presence of different concentrations of
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BQ and QBPA.
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Fig. 4. Langmuir isotherm plots for mild steel in 1 M HCl in the absence and presence of different
concentrations of BQ and QBPA.
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Fig. 5. Polarization curves for mild steel in 1 M HCl solution without and with different concentration of BQ
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and QBPA at 298K.
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Fig. 6. Nyquist (a) (b) and Bode (c) (d) plots for mild steel in 1 M HCl solution without and with different
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concentration of BQ and QBPA at 298K.
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Fig. 7. Electrochemical equivalent circuit applied to fit the impedance data
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Fig. 8. SEM images of steel samples (a) freshly polished without immersion and immersed in 1 M HCl (b)
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without and (c) with 1mM BQ or (d) with 1mM QBPA for 4 h at 298K.
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Fig. 9. Three-dimensional AFM images of mild steel surface (a) freshly polished without immersion and
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immersed in 1 M HCl (b) without and (c) with 1mM BQ or (d) with 1mM QBPA for 2 h at 298K.
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Fig. 10. Optimize structures, HOMO and LUMO of BQ and QBPA
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Fig. 11. Side and top views of equilibrium configuration for the adsorption of two investigated inhibitors on
Fe (1 1 0) surface: (a) and (c) BQ; (b) and (d) QBPA
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Table 1
Polarization parameters and corresponding inhibition efficiency for mild steel in 1 M HCl solution without
1076
22.7
-
103.1
274.2
74.5
74.52
112.9
100.7
158.8
146.0
85.24
-490
115.1
122.5
114.1
257.4
89.40
0.7
-480
107.1
109.3
62.3
341.0
94.21
1.0
-488
126.1
120.5
56.7
473.0
94.74
0.1
-489
98.3
85.7
205.8
96.7
80.87
0.3
-495
88.9
92.3
92.9
211.9
91.37
0.5
-488
94.3
93.2
70.9
287.2
93.41
0.7
-500
87.2
104.9
52.1
408.0
95.16
1.0
-486
92.8
107.2
30.6
616.3
97.15
Blank
-496
124.4
102.1
0.1
-492
108.2
0.3
-496
0.5
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ηp
(%)
-βc
(mV/dec)
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QBPA
βa
Icorr
(mV/dec) (uA·cm-2)
Ecorr
(mV)
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BQ
Rp
(ohm·cm2)
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(mmol L-1)
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and with different concentration of BQ and QBPA at 298K.
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Table 2
Equivalent circuit compatible with the experimental data in Fig. 6.
n1
Ydl (μF cm-2)
n2
107.51
1
1120.0
0.59
-
0.1
2.07
5.26
66.36
27.61
1
192.1
0.72
73.18
0.3
1.05
9.17
141.81
24.72
1
126.9
0.75
87.45
0.5
1.02
10.47
198.52
20.75
0.8
120.3
91.03
0.7
1.97
12.91
333.75
16.23
0.8
119.5
0.81
94.67
1.0
1.36
19.08
515.07
11.46
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0.81
1
0.68
96.54
0.1
1.15
6.32
113.13
39.69
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106.4
1
222.4
0.72
84.26
0.3
1.02
10.73
253.21
14.25
1
116.2
0.73
92.97
0.5
0.87
11.42
303.05
14.81
1
104.8
0.74
94.12
0.7
0.59
13.52
440.21
12.78
1
101.3
0.67
95.96
1.0
0.77
22.32
902.36
11.22
0.8
70.1
0.60
98.03
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17.80
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2.88
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Qdl
1.01
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QBPA
Yf (μF cm-2)
ηEIS
(%)
Qf
Blank
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BQ
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Rs
Rf
Rct
-1
2
2
(mmol·L ) (Ω cm ) (Ω cm ) (Ω cm2)
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Highlights：
●Two quinoline-based molecules QB and BQPA act as excellent eco-friendly inhibitors for mild steel in 1 M
HCl solution.
●The corrosion inhibition efficiency of QB and BQPA is 96.54% for QB and 98.03% for BQPA at 1mM
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(obtained from EIS at 298K).
●The order of inhibition performance obtained from electrochemical approaches is in a perfect agreement
with weight loss measurement.
●Theoretical calculations provide favorable support for the experimental data.
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