molecules

Article
Cucumber (Cucumis sativus L.) Leaf Extract as a Green
Corrosion Inhibitor for Carbon Steel in Acidic Solution:
Electrochemical, Functional and Molecular Analysis
Lijuan Feng 1, * , Shanshan Zhang 1 , Long Hao 2 , Hongchen Du 1 , Rongkai Pan 1 , Guofu Huang 1
and Haijian Liu 1

1 Shandong Engineering Research Center of Green and High-Value Marine Fine Chemical,
Weifang University of Science and Technology, Weifang 262700, China;
zhangshanshan87@wfust.edu.cn (S.Z.); duhongchen123@126.com (H.D.); panrk@wfust.edu.cn (R.P.);
hgflh007@wfust.edu.cn (G.H.); liuhaijian.1987@163.com (H.L.)
2 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, China; chinahaolong@126.com
* Correspondence: ljfeng@alum.imr.ac.cn; Tel.: +86-053-6510-7638

Abstract: An extract of cucumber leaves (ECSL) was prepared as a green corrosion inhibitor for
carbon steel. Its carbon steel corrosion inhibition performance against 0.5 mol L−1 H2 SO4 was inves-
tigated using electrochemical methods and scanning electron microscopy (SEM). Its composition was
analyzed by gas chromatography and mass spectroscopy (GC−MS). Quantum chemical calculations
and molecular dynamics simulations (MDS) were conducted to elucidate the adsorption mechanism
of the inhibitor molecules on the carbon steel surface. The results indicated that the inhibition
efficiency increases with its increasing concentration. The extract acted as a mixed type corrosion
inhibitor, and its inhibition properties were ascribed to the geometric coverage effect induced by
Citation: Feng, L.; Zhang, S.; Hao, L.;
its adsorption on the metal surface in accordance with Langmuir’s law. The active components in
Du, H.; Pan, R.; Huang, G.; Liu, H.
the extract were identified as mainly organic compounds with functional groups such as aromatic
Cucumber (Cucumis sativus L.) Leaf
moieties and heteroatoms. The inhibition activities of ECSL are delivered through the ability of the
Extract as a Green Corrosion
active components to adsorb on the metal surface through their functional groups to form a protective
Inhibitor for Carbon Steel in Acidic
Solution: Electrochemical, Functional
layer which hinders the contact of aggressive substances with carbon steel and thus suppresses its
and Molecular Analysis. Molecules corrosion. This research provides an important reference for the design of green corrosion inhibitors
2022, 27, 3826. https://doi.org/ based on plant waste materials.
10.3390/molecules27123826
Keywords: carbon steel; extract; green corrosion inhibitor; quantum chemical calculation
Academic Editor: Bogumil E. Brycki

Received: 12 April 2022

Accepted: 13 June 2022
Published: 14 June 2022 1. Introduction
Publisher’s Note: MDPI stays neutral As one of the basic industrial materials, carbon steel has been widely applied in vari-
with regard to jurisdictional claims in ous fields due to its excellent mechanical properties and low cost [1–8]. Nevertheless, it
published maps and institutional affil- is prone to suffer from scale and corrosion due to its chemical and electrochemical inter-
iations. actions with the surrounding environment—which leads to the degradation of metallic
structures [3,9–12]. Every year, a large amount of carbon steel equipment is damaged
and replaced owing to metal corrosion, causing enormous economic losses and huge
potential safety risks [13]. The most widely applied method for removing scale during
Copyright: © 2022 by the authors. facility maintenance activities is pickling [14,15]. Sulfuric acid is one of the most commonly
Licensee MDPI, Basel, Switzerland.
used pickling agents, as it can effectively remove dirt and oxidation products from the
This article is an open access article
metal surface [15–17]. However, the corrosion rate of carbon steel is so high in acidic
distributed under the terms and
conditions that the equipment may be damaged during the acidic cleaning process. There-
conditions of the Creative Commons
fore, applying corrosion inhibitors is considered to be an unavoidable approach, and has
Attribution (CC BY) license (https://
been proven to be a convenient, effective and economical technique for protecting metals
creativecommons.org/licenses/by/
4.0/).
during such processes [5,11]. It has been revealed that organic compounds containing

Molecules 2022, 27, 3826. https://doi.org/10.3390/molecules27123826 https://www.mdpi.com/journal/molecules

Molecules 2022, 27, 3826 2 of 18

heteroatoms, unsaturated bonds and planer conjugated systems usually display excellent
corrosion inhibition activities because they adsorb onto the metal surface via their func-
tional groups and form a protective film [3,5,18,19]. Unfortunately, traditional inhibitors
are often toxic to human beings or have a detrimental impact on the environment, and
so their use has been limited by more and more countries. Thus, researchers are keen to
develop environmentally benign “green” corrosion inhibitors [3,10–14,20–26]. A perusal
read of the literature has revealed that green inhibitors prepared from plant extracts are
readily available, nontoxic, biodegradable and economical, and can be considered to be
a most promising choice [16,23–26].
Cucumber (Cucumis sativus L.) is a vegetable that is widely planted in many coun-
tries. Its leaves are generally recognized as a biological waste material, causing serious
ecological and environmental pollution. Meanwhile, it has been found that there are many
valuable chemical constituents, including alkanoids, flavonoids, carbohydrates, proteins,
sugars and steroids, in the leaves of plants [20,27–30]. These organic compounds contain
adsorption centers—polar functional groups with N, S and O atoms or aromatic rings—in
their molecular structures, which suggests that leaf extracts could be potential corrosion
inhibitors [12,25,29–32]. However, there is no report to date on the corrosion inhibition
performance of an extract based on cucumber leaves. Thus, if a corrosion inhibitor can be
prepared from cucumber leaves, and if its corrosion inhibition mechanism can be deter-
mined, it will not only add a cheap new green product to the corrosion inhibitors list, but it
will also provide a novel solution for the utilization of plant waste such as cucumber leaves.
Therefore, in this study, an extract of cucumber leaves (ECSL) was prepared using an
impregnation method, and its corrosion inhibition performance on carbon steel subjected
to 0.5 mol L−1 H2 SO4 solution was evaluated through electrochemical measurements and
surface analysis techniques. The active components of the extract were analyzed using
gas chromatography and mass spectrometry. The adsorption performance of the active
constituents on the surface of carbon steel was further revealed through theoretical cal-
culations and simulations. The aims of the work were to assess the corrosion inhibition
effects of the biological waste product; to reveal its inhibition mechanisms; to pave a way
for the application of low-cost green plant extracts; and to promote the realization of
resources recycling.

2. Results
ECSL was prepared as described in “Materials and Methods”, Section 3. Based on the
experimental and theoretical analysis methods described in Section 3, the results are shown
and discussed as follows.

2.1. Potentiodynamic Polarization Curves

Potentiodynamic polarization curves obtained from immersing the electrodes in
0.5 mol L−1 H2 SO4 solution with and without various concentrations of ECSL are shown
in Figure 1. Curve fitting results obtained by Tafel extrapolation are shown in Table 1,
where Ecorr denotes the free corrosion potential; icorr , the corrosion current density; βc , the
cathodic Tafel slope; and βa , the anodic Tafel slope.
IE stands for the inhibition efficiency, which can be calculated according to the follow-
ing equation:
IE (%) = (i0 corr − icorr )/i0 corr × 100% (1)
where i0 corr and icorr are corrosion current densities for the uninhibited and inhibited
samples, respectively [5].
It can be seen from Figure 1 and Table 1 that the addition of ECSL led to both the
anodic and cathodic branches of the polarization curves moving toward lower current
density directions, indicating that both the anodic and cathodic reactions were inhibited by
ECSL [33,34]. Moreover, the corrosion current reduced with an increasing concentration
of ECSL, suggesting that the more inhibitor molecules that were adsorbed on the sample
surface, the more the contact between the metal surface and the corrosion medium was
Molecules 2022, 27, 3826 3 of 18

blocked. However, the changes in the anodic and cathodic Tafel slopes were very small,
and the fluctuation of the corrosion potential was much less than 85 mV. Thus, it could be
inferred that ECSL acted as a mixed type inhibitor which inhibited carbon steel corrosion
by forming a protective structure, obstructing both the anodic and cathodic electrochemical
reactions of the corrosion process [5,35,36].

Figure 1. Potentiodynamic polarization curves of the carbon steel samples in 0.5 mol L−2 H2 SO4
solution with and without different concentrations of ECSL.

Table 1. Curve fitting results for the potentiodynamic polarization curves for carbon steel in
0.5 mol L−1 H2 SO4 solution with and without different concentrations of ECSL.

Cinh (g L−1 ) βa (mV dec−1 ) βc (mV dec−1 ) icorr (mA cm−2 ) Ecorr (mV) IE (%)
0 58 125 0.73 −430 /
0.05 59 127 0.35 −420 51.7
0.10 59 126 0.19 −419 73.8
0.15 60 127 0.13 −422 82.1
0.20 61 127 0.066 −431 90.9
0.30 62 130 0.047 −434 93.5

2.2. Electrochemical Impedance Spectroscopy (EIS) Measurements

Figure 2 shows the electrochemical impedance spectrum for carbon steel electrodes in
0.5 mol L−1 H2 SO4 solution with and without various concentrations of ECSL. It is obvious
that only one capacity loop appeared in the Nyqusit diagram for all electrodes, indicating
that the carbon steel corrosion was mainly controlled by the charge transfer reaction [37].
However, the impedance behavior of the carbon steel sample was significantly changed
by the presence of ECSL. It is notable that the radius of the capacitive loop enlarged with
increasing ECSL concentration, suggesting that the anti-corrosion properties were gradually
enhanced due to the increasing surface coverage of the electrode with increasing inhibitor
concentration [33]. The EIS activities for the carbon steel samples in acidic solution, either
with or without ECSL, can be interpreted using a model that describes a solution resistor
element connecting with a unit of charge transfer resistor and double layer capacitor in
parallel [R(QR) equivalent circuit], as shown in Figure 3; where Rs represents the solution
resistance, Rct the charge transfer resistance and Cf the capacitance of the double electric
layer. Since all the capacitive semicircles were depressed (Figure 2), the constant phase
element CPE (Q) was used to replace C for the description of the frequency independent
Molecules 2022, 27, 3826 4 of 18

phase shift induced by the roughness of the electrode surface [38,39]. The value of CPE can
be assessed using the following equation:
Z (ω) = (Z0 )−1 (jω) − n (2)
j2
where Z0 is the CPE constant, ω is the angular frequency (in rad/s), = −1, is the imagi-
nary number and n is the CPE exponent. Depending on n, CPE can represent resistance
[Z (CPE) = R, n = 0], capacitance [Z (CPE) = C, n = 1], inductance [Z (CPE) = L, n = −1] or
Warburg impedance (for n = 0.5). Thus, the inhibition efficiency can be calculated from the
following formula:
η (%) = θ (%) = (1 − Rct /R0 ct ) × 100% (3)
where η denotes the inhibition efficiency and θ indicates the surface coverage [40–42]. The
curve fitting results based on the R(QR) circuit are displayed in Table 2.

Figure 2. Electrochemical impedance spectrum of carbon steel samples in 0.5 mol L−1 H2 SO4 solution
with and without different concentrations of ECSL.

Figure 3. Equivalent circuit to fit the impedance data for carbon steel in 0.5 mol L−1 H2 SO4 solution
with and without different concentrations of ECSL.

Based on Table 2, the capacity of the electrode was decreased in the presence of ECSL;
this is ascribed to the fact that the acidic solution with its larger dielectric constant was
replaced by the extract molecules with a smaller dielectric constant, which improved the
double layer structure [43,44]. As the concentration of the inhibitor was increased, the
double layer became thicker, reducing the local dielectric constant; which led to a decrease
in the double layer capacity [45]. Meanwhile, the charge transfer resistance increased
with increasing inhibitor concentration, indicating increasing mitigation of electrochemical
corrosion. This is because the active molecules in ECSL were adsorbed onto the carbon steel
surface and the charge transfer process was reduced, causing a corrosion inhibition effect.
Molecules 2022, 27, 3826 5 of 18

Table 2. Curve fitting results of the electrochemical impedance data for carbon steel in 0.5 mol L−1
H2 SO4 solution with and without different concentrations of ECSL.

Cinh (g L−1 ) Rs (Ω cm2 ) CPE−C (µF cm−2 ) CPE−n Rct (Ω cm2 ) η (%)
0 1.94 104 0.95 29.3 /
0.05 2.22 86.6 0.94 62.9 53.4
0.10 2.19 62.2 0.94 118 75.2
0.15 1.97 51.8 0.94 147 80.1
0.20 2.11 43.6 0.94 281 89.6
0.30 2.01 33.9 0.94 406 92.8

2.3. Scanning Electron Microscopy (SEM) Observations

SEM observations were conducted with the aim of confirming the changes in the
surface morphologies of carbon steel samples before and after the addition of ECSL. Figure 4
depicts the SEM images of samples immersed in 0.5 mol L−1 H2 SO4 solution in the absence
and presence of ECSL. For comparison purposes, an image of the morphology of the sample
before immersion is also included. It is clear that many large holes appeared in the surface
of the sample immersed in acid without ECSL, indicating that the surface of the carbon steel
was seriously damaged and the sample suffered from serious corrosion. In the presence
of ECSL, however, the surface of the sample was smooth, with only a few slight scratches
induced by emery papers visible. It had an appearance similar to that of the sample before
immersion, suggesting corrosion of the carbon steel was significantly inhibited by the plant
extract. This might be ascribed to the active constituents of ECSL interacting with the metal
surface and forming a protective layer, decreasing the contact area of the sample with the
aggressive medium and consequently mitigating its corrosion [27,46].

Figure 4. SEM images. (A) carbon steel sample before immersion; (B) carbon steel sample immersed
in 0.5 mol L−1 H2 SO4 solution; (C) carbon steel sample immersed in 0.5 mol L−1 H2 SO4 solution
with 0.20 g L−1 ECSL.

2.4. Adsorption Isotherm

Previous researchers have confirmed that the adsorption behavior of organic molecules
on metal surfaces has a decisive influence on their corrosion inhibition performance [47,48].
In order to further analyze the corrosion protection mechanism of ECSL, the surface
coverage degree (θ) of different concentrations of the inhibitor was tested by fitting to
several isotherms; the data fitted best with the Langmuir adsorption isotherm, as shown in
Figure 5. According to Langmuir’s adsorption isotherm law, the relationship between θ
and Cinh is shown by the following equation:
Cinh /θ = 1/K + C (4)
Molecules 2022, 27, 3826 6 of 18

where K represents the adsorption equilibrium constant, Cinh is the concentration of

the extract, and θ is the surface coverage degree, which is approximately equal to η in
Table 2 [5,49,50].

Figure 5. Langmuir adsorption isotherm for ECSL adsorbed on the carbon steel sample surface in
0.5 mol L−1 H2 SO4 solution.

It is clear from Figure 5 that Cinh /θ has a linear relationship with Cinh , with a slope
close to 1. The linear coefficient was 99.9% and the adsorption equilibrium constant K
was calculated to be approximately 22.3 L g−1 . This result indicated that the adsorption
performance of ECSL on the carbon steel surface conformed to Langmuir’s adsorption law,
and that the corrosion inhibition mechanism was the geometric covering effect.

2.5. Gas Chromatography and Mass Spectroscopy (GC–MS) Analysis

In order to further reveal the reasons for its corrosion inhibition effects, the constituents
of ECSL were analyzed by GC−MS. The spectrum obtained is depicted in Figure S1. After
preliminary analysis, it was identified that there are more than forty substances present in
the extract, of which eleven compounds were most likely to have corrosion inhibition effects.
These eleven chemical constituents, along with their molecular formula, abbreviations,
retention time and molecular weight are listed in Table 3. As can be seen in Figure S1
and Table 3, it is notable that ECSL contains a variety of organic substances with multiple
functional groups, such as rings, double bonds, π bonds and heteroatoms. Based on
previous theories, such compounds have extraordinary adsorption potential, and are very
promising corrosion inhibitors as they favor an ability to adsorb on the metal surface using
these groups as adsorption centers, and subsequently inhibit metal corrosion.
Molecules 2022, 27, 3826 7 of 18

Table 3. Compounds identified from the GC−MS chromatogram, and molecular information assigned
to the respective signals.

Name of the Compound Abbreviation Retention Time (min) Molecular Formula Molecular Weight
1-gala-l-ido-octose GIO 1.737 C8 H16 O8 240
Topotecan TO 2.215 C23 H23 N3 O5 421
Styrene ST 8.616 C8 H8 104
2-ethyl-1-hexanol EH 13.14 C8 H18 O 130
2-amino-5-[(2-carboxy)vinyl]-imidazole IACV 15.24 C6 H7 N3 O2 153
Adrenalone AD 19.62 C9 H11 NO3 181
Benzocycloheptatriene BT 22.02 C11 H10 142
Actinobolin AC 24.73 C13 H20 N2 O6 300
Pterin-6-carboxylic acid PCA 27.37 C7 H5 N5 O3 207
2,5-difluoro-β, 3, 4-trihydroxy-N-methyl-
BDTM 30.74 C8 H11 F2 N 219
benzeneethanamine
40 -methyl-2-hydroxystilbene MH 33.26 C15 H14 O 210

2.6. Quantum Chemical Calculations

The corrosion activities of organic substances are closely related to their molecular,
spatial and electronic structures. The adsorption of molecules mainly occurs relative to their
frontier orbitals, namely, the highest occupied orbital (HOMO) and the lowest occupied
orbital (LUMO). Further, their adsorption properties depend on the energy of the HOMO
(EHOMO ) and LUMO (ELUMO ) [51–53]. The higher the EHOMO of the molecule, the more
difficult it is for the nucleus to attract electrons to its orbital, and therefore, the easier it is to
donate electrons to form coordination bonds with the unoccupied orbital. The lower ELUMO
of the molecule, the easier it is to accept electrons and form feedback bonds. The difference
in energy between the LUMO and HOMO (∆E = ELUMO − EHOMO ) is also an important
parameter to describe the reactivity of the inhibitor molecule toward the surface, if the
difference in electronegativity between the molecule and the metal cannot be ignored [54].
In such cases, a higher ∆E suggests better stability of the molecule, and it will be more
difficult for the molecule to participate in the adsorption reaction. In contrast, a lower
∆E indicates that it is easier to adsorb the molecule onto the metal surface to construct
a protective film [21,55]. Generally, the effect on the molecular adsorption performance of
a molecule at the metal surface is reflected in the parameter of global hardness (H).
Figure 6 depicts the optimized molecular structures, and the HOMO and LUMO
distributions of the eleven identified components. The locations of the HOMO and LUMO
analyzed from the data in Figure 6 are shown in Table 4. Due to the density functional theory
(DFT) not being suitable for computing the ionization potential and the electron affinity
based on Koopman’s theorem [54–56], the Hartree–Fock (HF) method was subsequently
used to calculate the values of ELUMO , EHOMO , ∆E and other quantum chemical parameters
elucidated from ELUMO and EHOMO .
According to Koopman’s theorem, the ionization potential (I) and the electron affinity
(A) can be determined using the following equations, respectively:

I = −EHOMO (5)
A = −ELUMO (6)
Molecules 2022, 27, 3826 8 of 18

Figure 6. Optimized molecular structures and frontier molecular orbital density distributions of the
eleven compounds identified in ECSL.
Molecules 2022, 27, 3826 9 of 18

Table 4. HOMO and LUMO distributions of compounds identified in ECSL.

Compound HOMO Distribution LUMO Distribution

GIO O O
TO Rings Rings
ST C in the branch Rings
EH Branch with O atom Branch with O atom
IACV Pentatomic ring, O, N Pentatomic ring, O, N
AD Benzene ring, N, O Benzene ring, N, O
BT Rings Rings
AC Rings, N Rings, N
PCA Rings, O Rings, O
BDTM O, N, Benzene ring O, Benzene ring
MH Benzene rings Benzene rings

The global hardness (H), the electronegativity (X) and the number of electrons trans-
ferred (∆N) can be calculated according to the following relationships:
H = (I − A)/2 (7)
X = (I + A)/2 (8)
∆N = (XFe − Xinh )/2(HFe − Hinh ) (9)
where XFe = IFe = 7 eV, and HFe = 0 for iron, based on Pearson’s electronegativity scale
assumption [57–59]. However, their application of XFe = 7 eV for the calculation of ∆N
has been severely criticised because the value of 7 eV was obtained according to the free
electron gas Fermi energy of iron using the free electron gas model. In that case, the
electron–electron interaction is neglected, which causes differences from the state of the
bulk metal, and therefore the use of this value is conceptually inappropriate. Thus, Kokalj
suggested that the work function (ΦFe ) be used to replace XFe . Hence, ∆N can be calculated
as follows:
∆N = (ΦFe − Xinh )/2Hin ) (10)
Considering that metal corrosion most likely occurs at the densely packed surface,
ΦFe = 5.07 V obtained from the experimental result was applied to calculate ∆N [55,60].
The resulting structural parameters are listed in Table 5.

Table 5. Quantum chemical parameters of active components identified in ECSL.

Compound EHOMO (eV) ELUMO (eV) ∆E (eV) H (eV) X (eV) ∆N

GIO −11.03 2.66 13.69 6.85 4.18 0.07
TO −8.17 1.09 9.25 4.63 3.54 0.17
ST −8.39 2.65 11.04 5.52 2.87 0.20
EH −11.33 3.74 15.07 7.53 3.79 0.08
IACV −7.80 2.35 10.14 5.07 2.72 0.23
AD −8.64 2.42 11.05 5.53 3.11 0.18
BT −8.09 2.60 10.69 5.35 2.75 0.22
AC −9.09 2.75 11.83 5.92 3.17 0.16
PCA −9.14 1.55 10.69 5.34 3.80 0.12
BDTM −8.64 3.58 12.21 6.10 2.53 0.21
MH −7.82 2.21 10.03 5.01 2.81 0.23

By carefully analyzing the data in Figure 6 and Table 4, we can elucidate that the
HOMOs of the identified substances are mainly distributed in the aromatic rings (TO, AD,
BT, BDTM, MH), heterocycles (TO, IACV, AC, PCA) and heteroatoms (GIO, TO, EH, IACV,
AD, AC, BDTM, MH), which indicates that these sites have nucleophilic activities and are
the preferred adsorption centers. Thus, the molecules can donate electrons to construct
coordination bonds with iron and form a protective layer. The LUMOs of these substances
Molecules 2022, 27, 3826 10 of 18

are also mainly in benzene rings (TO, ST, AD, BT, BDTM, MH), heterocycles (TO, IACV, AC,
PCA), and heteroatoms (GIO, TO, EH, IACV, AD, AC, PCA, BDTM, MH), which suggests
that these compounds can accept electrons from the vacant d orbital of a metal to constitute
a feedback bond. Therefore, it is possible that the molecules in ECSL first physically adsorb
onto the surface of carbon steel, and then chemically interact with it to form a protective
structure trough, sharing electron pairs between the ECSL molecules and the iron [61].
From Table 5, the difference in electronegativity between the molecule and the metal
is visible in comparing the value of Xinh with that of ΦFe ; thus, the influence of ∆E on
the molecular adsorption properties should be considered [54]. Consequently, the global
hardness parameter (H), closely related to ∆E, indicates the charge transfer resistance,
which is directly proportional to the energy change, including both the charge transfer to
the molecule and the back donation from the molecule [62,63]. This indicates the reactivity
of the inhibitor molecule towards the adsorption on a metallic surface. Generally, the
reactivity of the molecule increases as the value of H or ∆E decreases. Thus, the inhibition
efficiency of the molecule increases, since it is more possible to form a bond-anti-bond
structure through offering and accepting electrons to and from the metal [21]. The results
in Table 5 show that TO displays the lowest value, 4.63 eV, which is ascribed to the fact that
this molecule contains the most rings and multiple heteroatoms. Most of the compounds
have H values ranging from 5.0 to 6.1 eV. These values of H are a little higher than those
calculated via DFT. This is reasonable since the H value calculated through DFT is lower
than that found using the HF method, which has been confirmed by the studies of other
researchers [57,64,65]. Accounting for the applicable condition of Koopman’s theorem, the
H value obtained by the HF method is more reliable, and an H value in the range from 5.0 to
6.1 eV is suitable for a molecule with an inhibition effect [55,66,67]. In combination with
Figure 6 and Table 4, these data indicate that these molecules all contain ring structures
(benzene moiety or heterocyclic) with dislocation electrons. In terms of HSAB (hard/soft–
acid/base) theory, these compounds can be recognized as soft bases which react with soft
acids [68,69]. The H values of GIO and EH are the highest (6.85 and 7.53 eV, respectively)
due to the fact that they only get negatively charged atoms and do not have conjugated
structures. These two compounds can be recognized as hard bases. The inhibition effect
of ECSL might be enhanced by the different types of compounds present. In the Fe–H2 O
system, there are different Lewis acids on the metal surface: the bare iron is categorized as
a soft acid and the metal ions produced in the corrosion process are hard acids [70]. Soft
bases such as TO can interact directly with the surface of the iron and form an adsorption
film, protecting the metal from corrosion. EH and GIO, as hard bases, can interact with the
Fe3+ or Fe2+ ions and assist the inhibition performance.
Electronegativity (X) reflects the ability of atoms to attract electrons toward themselves,
and can be used to assess the tendency of a molecule to retain its own electrons during
donor/acceptor interactions that lead to corrosion inhibition [59,62]. Usually, a compound
with a lower value of X means that it is shares electrons with metals more easily, and
can be expected to have higher inhibition performance [59,71]. From Table 5, there are
four compounds (GIO, EH, TO and PCA) with an X value higher than 3.5 eV. Considering
that GIO and EH are also the molecules with the highest H values, such high X values
suggest they interact with the metal solely via a physical electrostatic effect, rather than
by the chemical reaction of donating electrons. PCA and TO also have the lowest ELUMO ,
indicating that they are more likely to be electron acceptors rather than donors during their
interaction with metals. The X values of the other seven substances have no significant
differences, which may be ascribed to the fact that they all have planar ring structures with
conjugated π bonds. Thereby, when they interact with metals, they are probably able to
offer both electrons to the empty orbital of a metal molecule and receive electrons to form
back-donation bonds, and as a result, they can strongly adsorb on the metal surface and
constitute a protective layer, inhibiting its corrosion.
The fraction number of electrons transferred (∆N) indicates inhibition efficiency re-
sulting from electron donation [57]. If ∆N > 0, electrons can be transferred from organic
Molecules 2022, 27, 3826 11 of 18

molecules to metals and form feedback (back-donation) bonds. If ∆N < 3.6, the inhibition
efficiency increases with increasing ability to donate electrons to the metal surface [61,72].
Per Table 5, the ∆N values of all the compounds are in the range from 0 to 3.6, reveal-
ing that they can interact with the metal via electron donation from the inhibitor to the
metal surface.

2.7. Molecular Dynamics Simulations (MDS)

Molecular dynamics simulation is a comprehensive technology that combines mathe-
matics, chemistry and physics through computational simulations to calculate the prop-
erties of materials in the system [61,71–73]. In order to better understand the interaction
between ECSL and the carbon steel surface, and furthermore, to elucidate its corrosion
inhibition mechanism, MDS was performed to analyze the adsorption behaviors of the
active components of ECSL on the Fe(110) surface at the molecular level.
The optimized adsorption configurations of the molecular structures on the metal
surface are illustrated in Figure 7. The adsorption configuration of each molecule on the
Fe(110) surface in 0.5 mol L−1 H2 SO4 solution can be vividly observed by comprehensive
analysis of Figures 6 and 7, and Table 5. The GIO molecule adsorbs on the iron surface with
the O atoms on its one side supporting like feet. TO adsorbs on the Fe(110) surface with the
benzene rings and imidazole plane parallel to it. ST adsorbs with its benzene ring parallel
to the Fe(110) surface and its hydrophobic carbon chain tilting upward. EH adsorbs using
the O atom as the adsorption center with a C atom assisting. The adsorption centers of
IACV are an imidazole ring as well as the O atoms; while those of AD are O atoms and
the benzene ring. As to the adsorption of BT, the benzene ring is parallel to the Fe(110)
surface, with the C atom supported on one side. AC adsorbs with the aromatic moieties
parallel to the metal surface; meanwhile, the N and O atoms serve as adsorption centers to
strengthen its adsorption. The PCA molecule horizontally adsorbs on the Fe(110) surface
through the double ring structure, while the O and N atoms of the branched chain can
also perform as active adsorption centers, which facilitates its adsorption. Due to space
configuration constraints, BDTM adsorbs on the metal surface at a certain angle, yet its
heteroatoms can still act as the adsorption centers. MH molecules adsorb mainly through
their benzene rings.
In general, by careful examination of Figure 7, it can be noted that all eleven com-
pounds identified in ECSL are able to adsorb on the Fe(110) surface, therefore a barrier
structure can be formed to isolate contact between the carbon steel surface and the corrosive
medium, thus mitigating the corrosion of the carbon steel.
It should be mentioned that, owing to spatial structure limitations, the adsorption
configuration of an organic molecule on a metal surface is not the same as the optimal
geometric structure obtained by quantum chemical calculations; instead, it is adjusted to
a certain extent. As a result, the distance from the metal surface to the adsorbed molecules
varies. As shown in Figure 7, for molecules with less active groups, like EH and GIO,
the distance between the molecule and the metal surface is obviously larger than that
of other molecules. In fact, the high values (3.916 and 3.916 Å, respectively) indicate
their interactions with the metal are through physical adsorption mechanisms rather than
chemical ones [55]. Compounds containing planer moieties, especially benzene rings,
interact easily with the metal and can adsorb onto the Fe(110) surface at a short distance,
since the planer structure can be parallel to the Fe(110) surface. ST, IACV, AD, PCA, BDTM
and MH all display such adsorption configurations. However, if the branched chains
in the molecule are too many or too long, and they cannot adjust to the same side of
the planar structure, such as in BT and AC, the molecule will not adsorb on the metal
surface so closely. These results are expected to provide a reference for the design of new
high-efficiency corrosion inhibitors.
Molecules 2022, 27, 3826 12 of 18

Figure 7. Optimized adsorption configurations of the molecular structures on Fe(110) surface.

Molecules 2022, 27, 3826 13 of 18

The adsorption energy (Eads ) between the iron surface and the organic compound can
be calculated as follows:
Eads = Etotal − (Esurf+solu + Einh ) (11)
where Etotal is the total energy of the inhibitor molecule adsorption system on the Fe(110)
surface in 0.5 mol L−1 H2 SO4 solution, and Esurf+solu + Einh is the sum of the energy
including the metal surface, the corrosive solution and the inhibitor molecule before
adsorption [72,74]. The calculated results are listed in Table 6 from the lowest to the highest
adsorption energy value. It has been acknowledged that a negative adsorption energy
value indicates spontaneous adsorption of a molecule on a metal surface. Moreover, a lower
value of Eads implies the molecule is more likely to adsorb on the metal surface, thus, it
exhibits a better corrosion inhibition performance [58,59]. It can be observed from Table 6
that the adsorption energy on the Fe(110) surface for each molecule has a negative value,
indicating that all eleven components identified in ECSL are able to adsorb on the carbon
steel surface and exhibit corrosion inhibition effects.

Table 6. Interaction energies for each component identified in ECSL with Fe(110) surface.

Compound Esurf+solu + Einh (kcal mol−1 ) Etotal (kcal mol−1 ) Eads (kcal mol−1 )
TO −30,302.11 −31,204.32 −902.21
PCA −30,444.21 −31,331.86 −887.65
BDTM −30,397.84 −31,277.37 −879.53
AC −30,430.67 −31,246.72 −816.05
AD −30,390.48 −31,200.68 −810.19
ST −30,001.78 −30,759.53 −757.75
IACV −30,570.80 −31,195.94 −625.14
MH −30,411.15 −31,000.90 −589.75
BT −30,353.40 −30,924.26 −570.86
GIO −30,351.34 −30,850.19 −498.85
EH −30,431.33 −30,870.30 −438.97

2.8. Corrosion Inhibition Mechanism

The corrosion inhibition mechanism of ECSL for carbon steel can be analyzed from
the chemical structures of the identified components. The main chemical constituents
of ECSL are organic compounds containing many functional groups, such as benzene
rings, heterocycles and polar atoms, which have delocalized or unpaired electrons and
demonstrate high EHOMO . They are prone to provide electrons to the empty d orbital of
Fe and adsorb on its surface. Meanwhile, by virtue of these functional groups, they have
low ELUMO and easily accept electrons from the d orbital of Fe to form feedback bonds and
enhance the adsorption capacity. Thereby, the active components in ECSL can adsorb on
the metal surface and constitute a protective structure. Moreover, ECSL contains a variety
of active components, including both soft bases and hard bases, which can either directly
adsorb on the carbon steel surface or interact with Fe2+ and/or Fe3+ to form a protective
layer. Therefore, they complement each other, to a certain extent, to enhance the metal
corrosion inhibition properties. As a result, ECSL can spontaneously and effectively adsorb
on the carbon steel surface and inhibit its corrosion.

3. Materials and Methods

3.1. Materials
The metal samples were prepared from R235 carbon steel, which was machine cut
into cuboid samples with the dimensions of 10 mm × 10 mm × 5 mm. The composition
of the carbon steel is listed in Table 7. The sample was embedded in a PVC holder using
epoxy resin, leaving only a working surface with an area of 1 cm2 for electrochemical
measurements. Prior to each experiment, the working surface was abraded with sandpaper
from 100 to 1000 mesh, degreased with acetone, rinsed in distilled water, and air-dried. The
Molecules 2022, 27, 3826 14 of 18

corrosive medium was 0.5 mol L−1 H2 SO4 solution which was prepared from analytical
reagent grade H2 SO4 and bi-distilled water.

Table 7. Composition of carbon steel in wt. (%).

Composition C Si Mn P S Fe
Amount (%) 0.16 0.14 0.48 0.03 0.03 99.16

3.2. ECSL Preparation

Fresh cucumber leaves (collected from the green house of the agricultural experimental
base at Weifang University of Science and Technology, China) were cleaned under running
water and then distilled water to remove the surface dirt. Next, the leaves were dried in an
oven at 45 ◦ C for approximately 24 h. The dried leaves were then ground to a powder and
screened with a sieve with the pore size of 0.15 mm. After that, the dried and powdered
cucumber leaves were soaked in ethanol (75% by volume) for 2 h at 25 ◦ C. Then, the plant
extract was boiled at 45 ◦ C, naturally cooled to room temperature, and triple-filtered. The
excess ethanol was removed by vacuum distillation. Finally, the plant extract, a dark brown
solid residue, was obtained.

3.3. Chemical Composition Analysis

In order to identify the effective components of the plant extract, GC−MS analy-
sis was carried out using Agilent Technologies GC model 7890A and Mass Spectrom-
etry model 5977, coupled with an HP-5 capillary fused silica column of dimensions
30 µm × 320 µm × 0.25 µm. The carrier gas was helium, the pressure was controlled at
11.604 Psi and the flow rate was 1.5 mL min−1 . The oven temperature was programmed as
follows: the initial temperature was set as 35 ◦ C (isothermal for 5 min); then the temperature
was increased to 90 ◦ C at the rate of 6 ◦ C min−1 (isothermal for 3 min); then to 150 ◦ C
at 5 ◦ C min−1 (isothermal for 2 min); and finally, to 200 ◦ C at 5 ◦ C min−1 (isothermal for
3 min). All peaks were analyzed by matching with those in the NIST library to obtain the
exact information.

3.4. Electrochemical Techniques

Electrochemical measurements were conducted using an electrochemical work station
(CHI-660E, Chenhua) equipped with a three-electrode system, where the carbon steel
sample served as the working electrode, a platinum plate as the auxiliary electrode and
a saturated calomel electrode (SCE) as the reference electrode. The polarization curves
were determined from a cathodic potential of −0.25 V to an anodic potential +0.25 V, with
respect to the open circuit potential (OCP), at a sweep rate of 1 mV s−1 . Electrochemical
impedance spectroscopy (EIS) plots were acquired in the frequency range from 100 kHz
to 10 mHz with a perturbation amplitude of 10 mV. The EIS spectra were fitted using the
Corrview software. A fresh working electrode was used for each measurement. At least
three runs were performed for each measurement to obtain reproducible data.

3.5. Surface Morphological Observation

The surface morphologies of the carbon steel samples immersed in 0.5 mol L−1 H2 SO4
solution for 2 h with and/or without 0.20 g L−1 ECSL were observed by scanning electron
microscopy (SEM, Philip XL 30) at 5000× magnification. The energy of the acceleration
beam employed was 25 kV.

3.6. Computational Details

All of the quantum chemical calculations and molecular dynamics simulations were
performed using Materials Studio 2019 software supplied by Biovia Community. The
molecular structures of the inhibitors were fully geometrically optimized by DMol3 module
using the function of PBE (proposed by Perdew–Burke–Ernzerhof) with the double numeric
Molecules 2022, 27, 3826 15 of 18

basis set of DNP in the DFT framework. The convergence tolerances for energy, maximum
force and maximum atomic displacement were set as 1.0 × 10−5 Ha, 0.002 Ha Å−1 and
0.005 Å, respectively. The solvent effect was addressed using the COSMO implicit model of
water. Since the Koopmans’ approximation may lose its validity under the DFT framework,
the HF method was applied to calculate the structural parameters using cc-pVTZ by virtue
of the Gaussian 09W version D.01 software. The adsorption behavior of each compound on
the metal surface was investigated using the Forcite module with MDS. The simulations
of the interactions of the molecules with the Fe(110) surface were conducted in periodic
boxes with the dimensions of 17.2 Å × 16.2 Å × 75.5 Å using two layers. The bottom
Fe(110) surface was fabricated with a five-layer slab model and the top solvent layer was
constructed with 222 H2 O, 1 organic molecule, 4 H+ and 2 SO4 2− , where a 30 Å vacuum
region was set to ensure the decoupling of the repeated slabs. The MDS were carried out
under the conditions of 298 K, NVT ensemble, with a time step of 0.1 fs and simulation
time of 500 ps.

4. Conclusions
A green corrosion inhibitor, ECSL, can be prepared from a plant waste, namely, cu-
cumber leaves. It exhibits an excellent inhibition effect on the corrosion of carbon steel
in 0.5 mol L−1 H2 SO4 solution, and its inhibition efficiency increases with increasing con-
centration. ECSL reduces both the cathodic and anodic reactions of carbon steel in H2 SO4
solution. It is a mixed type corrosion inhibitor and its corrosion inhibition activities are
attributed to the geometric coverage effect. Its interaction with the carbon steel surface
is ascribed to spontaneous physical and chemical adsorption, which obeys the Langmuir
adsorption law. The corrosion inhibition properties of ECSL are closely related to the
unique molecular structures of its active components with many functional groups. The
molecules can easily adsorb on the carbon steel surface using these functional groups
as adsorption centers to form a protective layer, obstructing the corrosion medium and
suppressing the corrosion of the carbon steel.

Supplementary Materials: The following supporting information can be downloaded at: https://
www.mdpi.com/article/10.3390/molecules27123826/s1, Figure S1: GC−MS chromatogram of ECSL.
Author Contributions: Investigation, L.F. and S.Z.; Supervision, L.H.; Writing—original draft, L.F.;
Visualization, R.P.; Writing—review and editing, G.H. and H.L.; Data curation, L.F., H.D. and G.H.
All authors have read and agreed to the published version of the manuscript.
Funding: The APC was funded by Shandong Provincial Natural Science Foundation (No. ZR2019MEM046).
Data Availability Statement: Not Applicable.
Acknowledgments: The authors gratefully acknowledge the financial support from Shandong
Provincial Natural Science Foundation (No. ZR2019MEM046), Shandong Society for Environmental
Sciences Project (No. 202015), Doctoral Fund Project of Weifang University of Science and Technology
(No. KJRC2022005 and No. KJRC2022006) and the open experimental project of Shandong Peninsula
Engineering Research Center of Comprehensive Brine Utilization (No. 2018LS002). The authors also
specially thank for the technique support in the chemical calculations and molecular simulations
from National Supercomputer Center in Jinan and Institute of Metal Research, Chinese Academy
of Sciences.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of ECSL is available from the authors.

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