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Exploration of 8-piperazine-1-ylmethylumbelliferone for application as a corrosion inhibitor for mild steel in hydrochloric acid solution

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The name umbelliferone is from the Umbelliferae plants family, which in turn is named after the umbrella-shaped inflorescences, each of which is called an umbel. It was found in carrot, coriander, and garden angelica, mouse-ear hawkweed, and bigleaf hydrangea. In this study, a new umbelliferone derivative, namely 8-piperazine-1-ylmethylumbelliferone (8P), was synthesized, and its structure was characterized and confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The potential of using 8P as an eco-friendly corrosion inhibitor for mild steel in hydrochloric acid solution was examined using gravimetric techniques confirmed by topology characterization through scanning electron microscopy technique. The effect of inhibitor concentration, solution temperature, and immersion time on the inhibition efficiency of 8P was also investigated. The results from weight loss techniques reveal that 8P exhibits significant anti-corrosive characteristics toward mild steel dissolution in 1 M hydrochloric acid environment. 8P with a concentration of 0.0005 M exhibited the highest inhibition efficiency of 93.42% at 303 K. Scanning electron microscopy exhibited that 8P inhibited mild steel corrosion. The adsorption isotherm indicated that 8P exhibited physical and chemical sorption mechanisms and obeyed the Langmuir adsorption isotherm model. The relationship between the inhibition behavior of the 8P and its molecular structure was examined by density functional theory (DFT) which confirmed that 8P possessed significant inhibition efficiency which was consistent with experimental findings.
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Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 368
Exploration of 8-piperazine-1-ylmethylumbelliferone for
application as a corrosion inhibitor for mild steel in hydrochloric
acid solution
A.M. Resen,1 M.M. Hanoon,1 W.K. Alani,2 A. Kadhim,3 A.A. Mohammed,4
T.S. Gaaz,4 A.A.H. Kadhum,5,6 A.A. Al-Amiery2,6 * and M.S. Takriff6
1Production engineering and metallurgy department, University of Technology Iraq,
Baghdad, Iraq
2Energy and renewable energies technology center, University of Technology Iraq,
Baghdad, Iraq
3Laser & optoelectronics engineering department, University of Technology Iraq,
Baghdad, Iraq
4Al-Mussaib Technical College, Al-Furat Al-Awsat Technical University, 51006 Babylon,
Iraq
5College of Medicine, University of Al-Ameed, Karbala, Iraq
6Department of Chemical Engineering, Faculty of Engineering and Build Environment,
Universiti Kebangsaan Malaysia, Bangi, Malaysia
*E-mail: dr.ahmed1975@gmail.com
Abstract
The name umbelliferone is from the Umbelliferae plants family, which in turn is named after
the umbrella-shaped inflorescences, each of which is called an umbel. It was found in carrot,
coriander, and garden angelica, mouse-ear hawkweed, and bigleaf hydrangea. In this study, a
new umbelliferone derivative, namely 8-piperazine-1-ylmethylumbelliferone (8P), was
synthesized, and its structure was characterized and confirmed by Fourier transform infrared
spectroscopy and nuclear magnetic resonance spectroscopy. The potential of using 8P as an eco-
friendly corrosion inhibitor for mild steel in hydrochloric acid solution was examined using
gravimetric techniques confirmed by topology characterization through scanning electron
microscopy technique. The effect of inhibitor concentration, solution temperature, and
immersion time on the inhibition efficiency of 8P was also investigated. The results from weight
loss techniques reveal that 8P exhibits significant anti-corrosive characteristics toward mild
steel dissolution in 1 M hydrochloric acid environment. 8P with a concentration of 0.0005 M
exhibited the highest inhibition efficiency of 93.42% at 303 K. Scanning electron microscopy
exhibited that 8P inhibited mild steel corrosion. The adsorption isotherm indicated that 8P
exhibited physical and chemical sorption mechanisms and obeyed the Langmuir adsorption
isotherm model. The relationship between the inhibition behavior of the 8P and its molecular
structure was examined by density functional theory (DFT) which confirmed that 8P possessed
significant inhibition efficiency which was consistent with experimental findings.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 369
Keywords: 8-piperazine-1-ylmethylumbelliferone, corrosion, DFT, HOMO, Langmuir
isotherm.
Received: November 26, 2020. Published: March 12, 2021 doi: 10.17675/2305-6894-2021-10-1-21
Introduction
Coumarins have been considered as an important class of heterocyclic molecules due to their
common medicinal and biological applications [1]. Coumarins exhibited considerable
biological activities such as antitumor, antimicrobial, and anticoagulant activities [2]. These
observations led us to assess the corrosion inhibition features of 8-piperazine-1-
ylmethylumbelliferone (8P) as a newly synthesized coumarin. 8P has been selected in this
study as a novel corrosion inhibitor due to its molecular formula that has tertiary and
secondary amines besides hydroxy and carbonyl groups and possesses one O atom in the
ring, in addition to -electrons, through which they can easily bind to the mild steel surface
and increase the inhibition efficiency [3]. The characteristic feature for the synthesis of 8P
is that it is a one-pot reaction providing a high yield [4]. In the present effort, the density
functional theory (DFT), as a theoretical technique, was used to understand the mechanism
of corrosion inhibition of the synthesized compound in comparison with conventional
techniques, and help stimulate the experimental findings with calculated molecular
characteristics such as the energy of the highest occupied molecular orbital (HOMO), energy
of the lowest unoccupied molecular orbital (LUMO), energy gap E), electronegativity (χ),
softness (σ), hardness (η) and fraction of transferred electrons (ΔN) from 8P to mild steel.
This technique is cheap and time-saving [58]. In a joint work of the Malaysian National
University (Malaysia) with the University of Technology (Iraq), some novel coumarins that
exhibit corrosion inhibition efficiency of 83% to 97% at 0.005 M were previously reported
[917]. Herein, we have synthesized a novel corrosion inhibitor 8P to enhance the inhibitive
performance compared to that of the previously published coumarins at lower concentrations
because this one has more hetero atoms than in the previous work on coumarins. Comparison
of the inhibitory efficacy of 8P with other recently published inhibitors confirms that the
current inhibitor has better corrosion inhibition performance. Gupta et al. have published a
synthesis of new corrosion inhibitors derived from α-aminophosphonate for mild steel, and
they found that the inhibition efficiencies were 5896% at concentrations of 141564 ppm.
On the other hand, Wang et al. published a synthesis of S-benzyl-O,O´-bis(2-naphthyl)-
dithiophosphate derivatives as corrosion inhibitors for mild steel, which demonstrated
inhibition efficiencies of 7095% at concentrations of 40100 mg/L [18, 19]. To the best of
our knowledge, there is no previous study that uses 8P as a corrosion inhibitor of mild steel
in a hydrochloric acid solution. This work presents an investigation of a new eco-friendly
synthesized coumarin for the corrosion inhibition of mild steel coupons in hydrochloric acid
solution through weight loss techniques. The surface topology studies of mild steel coupons
in the absence and presence of 8P were used to characterize the protective inhibitor layer.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 370
DFT Da was also conducted to study the correlations between the experimental findings and
the quantum chemical values.
1. Experimental Section
1.1. Materials
The starting materials used in this investigation were provided by Sigma/Aldrich in
Selangor, Malaysia. The composition of the mild steel specimen is as follows (wt.%):
carbon, 0.210; manganese 0.050; silicon 0.380; aluminum 0.010; sulfur 0.050; phosphorus
0.090; balance iron. Mild steel pieces were mechanically cut into 4.2.0.025 cm
specimens for gravimetric techniques. The surface of the mild steel specimens was abraded
with emery papers and washed with acetone and double distilled water. A 37% hydrochloric
acid solution of analytical grade was diluted with double distilled water to prepare the 1 M
hydrochloric acid test solution.
1.2. Inhibitor
8P was synthesized by refluxing a solution of piperazine (0.01 mol) in absolute ethyl alcohol
(70 mL) with formalin (37%, 0.02 mol) for 1 h at 60°C. An ethanolic solution of
umbelliferon (0.01 mol) was added and refluxing was continued for 12 h. The precipitate
was recrystallized from CH2Cl2. Yield 45%, m.p. 233°C. The target compound (Scheme 1)
was characterized by NMR, FTIR, and CHN techniques. Nuclear magnetic resonance
spectroscopy (NMR) was conducted with an AVANCE III 600 MHz spectrometer (Bruker,
Billerica, Massachusetts, United States of America), with dimethyl sulfoxide as the internal
standard. The delta values were expressed in ppm. Fourier-transform infrared spectroscopy
(FTIR) was conducted with a Shimadzu 8300 spectrometer. CHN elemental analysis was
carried out with a Carlo Erba 5500 CHN elemental analyzer. FT-IR (cm1): 3413.8 for O
H, 3097.4 for NH), 17328.9 for C=O lactone; 1H NMR, DMSO-d6, δ, ppm: 2.57 (4H, d,
CH2), 2.89 (4H, d, CH2), 3.99 (2H, s, NCH2), 6.33 (1H, d, =CH), 6.88 (1H, d, =CH), 7.29
(1H, d), 7.30 (1H, d), and 7.44 (1H, d). 13CNMR, DMSOd6, δ, ppm: 161.8, 159.7, 153.8,
144.9, 130.2, 126.5, 114.8, 113.2, 109.7, 55.3, 54.1, 52.6 and 44.9. Elemental analysis
(C14H16N2O3): C, 64.60 (64.76); H, 6.20 (6.62); N, 10.76 (10.59).
Scheme 1. Synthetic route for the preparation of 8P.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 371
1.3. Weight Loss Techniques
Weight loss measurements were conducted with mild steel specimens by exposing them in
1 M hydrochloric acid without and with the addition of 8P at various concentrations
(0.00010.0005 M). The mild steel specimen was taken out after the immersion period (1,
5, 10, 24 h), washed, dried, and weighed accurately. The corrosion rate (CR) was evaluated
using equation (1):
( )
Rmm/year 87.6W
CatD
=
(1)
where W is weight loss, a is surface area of the mild steel coupon, t is exposure time, and D
is the density of the mild steel (g·cm3).
1.4. Scanning Electron Microscopy
Surface morphology investigations were conducted on mild steel specimens after exposure
in hydrochloric acid solution in the absence and presence of 8P at the optimum concentration
(0.0005 M). A Scanning electron microscope TM1000 Hitachi Tabletop Microscope was
utilized for the analyses.
1.5. Quantum Chemical Calculations
Quantum chemical calculations using the density functional theory (DFT) method were
conducted for the 8P molecule. The 631+G(d,p) basis set was selected for the calculations
which were conducted with Gaussian 09 software. The quantum chemical parameters were
determined to estimate the electronic parameters of the most stable conformers of the
inhibitor molecule. The energy of the highest occupied molecular orbital (EHOMO) and the
energy of lowest unoccupied molecular orbital (ELUMO) as the energies of frontier molecular
orbital (FMO), energy gap (ΔE), global hardness (η), softness (σ), absolute electronegativity
(χ), and a number of transferred electrons N) were calculated using equations (26)
[20, 21].
HOMO LUMO
ΔEEE =
(2)
HOMO LUMO
2
ηEE



=
(3)
1
η
σ=
(4)
HOMO LUMO
2
χEE



=
(5)
( )
Fe inh
Fe inh
χ -χ
2η +η
ΔN=
(6)
where χFe and ηFe were 7 eV/mol and 0 eV/mol, respectively.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 372
2. Results and Discussion
2.1. Weight Loss Measurements. Effect of Inhibitor Concentration
The experimental findings obtained from gravimetric techniques, such as the corrosion rate
and inhibition efficiency, are demonstrated in Figures 16, which exhibits that in the
presence of various concentrations of 8P for immersion periods 1, 5, 10 h, the corrosion rate
decreases and the inhibition efficiency increases. At 24 h immersion time the corrosion rate
increases and the inhibition efficiency decreases.
010 20 30 40 50
16
18
20
22
24
26
28
0.0001 M
Time (h)
CR (mm/y)
30
32
34
36
38
40
42
44
46
48
50
IE (%)
Figure 1. Gravimetric measurements for mild steel in the absence and presence of 0.0001 M
8P in 1 M hydrochloric acid solution at 303 K.
010 20 30 40 50
10
12
14
16
18
20
22
0.0002 M
Time (h)
CR (mm/y)
35
40
45
50
55
60
IE (%)
Figure 2. Gravimetric measurements for mild steel in the absence and presence of 0.0002 M
8P in 1 M hydrochloric acid solution at 303 K.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 373
Figure 3. Gravimetric measurements for mild steel in the absence and presence of 0.0003 M
8P in 1 M hydrochloric acid solution at 303 K.
010 20 30 40 50
6
8
10
12
14
16
0.0004 M
Time (h)
CR (mm/y)
50
55
60
65
70
75
80
85
IE (%)
Figure 4. Gravimetric measurements for mild steel in the absence and presence of 0.0004 M
8P in 1 M hydrochloric acid solution at 303 K.
This elucidates that the corrosion inhibitive performance of the investigated compound
is concentration-dependent [22] and this is reflected in the inhibition effectiveness plot
versus concentration as demonstrated in Figures 15. The gravimetric technique is a reliable
method to optimize the inhibitor concentration effect on corrosive inhibitive performance
[23].
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 374
010 20 30 40 50
2
4
6
8
10
12
14 0.0005 M
Time (h)
CR (mm/y)
55
60
65
70
75
80
85
90
95
100
IE (%)
Figure 5. Gravimetric measurements for mild steel in the absence and presence of 0.0005 M
8P in 1 M hydrochloric acid solution at 303 K.
0.0001 0.0002 0.0003 0.0004 0.0005
30
40
50
60
70
80
90
100
IE (%)
Concentration (M)
immersion time 5 h
immersion time 48 h
Figure 6. A comparison between the effect of different concentrations of 8P on the inhibition
efficiency for mild steel in 1 M hydrochloric acid solution at 5 and 48 h immersion times.
The inhibitive performance of the investigated 8P was studied for mild steel in 1 M
hydrochloric acid utilizing various concentrations of the inhibitor (0.0001−0.0005 M) and
different immersion times of 1, 2, 5, 10, and 24 h. The inhibition efficiency increased sharply
with an increase in the concentration of the studied inhibitor. The efficiency remained almost
constant at the optimum immersion time (between 5 and 10 h) and the inhibition efficiency
of the 8P at 0.0005 M was 93.4% for 5 h immersion time. The inhibition efficiency (η, %)
was evaluated according to Equation 7:
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 375
RR(i)
R100ηCC
C
=
(7)
where CR is the corrosion rate (mg·cm2·h1) in the absence of 8P and CR(i) is the corrosion
rate in the presence of 8P.
The investigated inhibitor controls the corrosion through the surface coverage (θ) of
mild steel surface which increases the inhibition efficiency. The relationship between
inhibition efficiency and surface coverage can be calculated according to Equation 8:
η
100
θ=
(8)
The increase in 8P concentration led to an increase in the surface coverage and
adsorption extent due to the presence of many inhibitor molecules, which leads to increased
inhibition efficiency. The high inhibition efficiency of the studied compound is attributed to
the presence of many heteroatoms and the presence of aromatic rings as well as double bonds
and thus an increase in the inhibition efficiency.
2.2. Effect of Temperature
The effect of temperature (308−338 K) on the corrosion inhibition efficiency for mild
steel at the optimum concentration is demonstrated in Figures 711. It is obvious from the
plots that the inhibition efficiency of 8P decreases with an increase in temperature. This is
due to the desorption of adsorbed 8P molecules from the surface of mild steel at a higher
temperature of the acidic solutions.
300 305 310 315 320 325 330 335
16
18
20
22
24
26
28
30
32
34
36 0.0001 M
Temp (oC)
CR (mm/y)
28
30
32
34
36
38
40
42
44
46
IE (%)
Figure 7. Effect of temperature (308−338 K) on the inhibitive efficiency for mild steel in the
presence of 0.0001 M of 8P in 1 M hydrochloric acid solution.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 376
300 305 310 315 320 325 330 335
10
12
14
16
18
20
22
24
26
28
30
0.0002 M
Temp (oC)
CR (mm/y)
30
35
40
45
50
55
IE (%)
Figure 8. Effect of temperature (308−338 K) on the inhibitive efficiency for mild steel in the
presence of 0.0002 M of 8P in 1 M hydrochloric acid solution.
300 305 310 315 320 325 330 335
5
10
15
20
25
0.0003 M
Temp (oC)
CR (mm/y)
35
40
45
50
55
60
65
70
75
IE (%)
Figure 9. Effect of temperature (308−338 K) on the inhibitive efficiency for mild steel in the
presence of 0.0003 M of 8P in 1 M hydrochloric acid solution.
2.3. Adsorption Isotherm
To understand the mode of interactions between the inhibitor molecules and the metal
surface, adsorption isotherms were studied. Inhibitor molecules that are adsorbed on a mild
steel surface never attain the actual equilibrium but tend to attain steady state adsorption.
When the rate of corrosion is reduced upon addition of an inhibitor, the process of adsorption
tends to reach a quasi-equilibrium state. The quasi-equilibrium adsorption nature of inhibitor
molecules can be studied utilizing the proper adsorption isotherm. The experimental data
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 377
were fitted to different adsorption models, and the plots are demonstrated in Figures 1214.
The main equations (911) of the adsorption isotherms are listed accordingly.
300 305 310 315 320 325 330 335
4
6
8
10
12
14
16
18
20
22
0.0004 M
Temp (oC)
CR (mm/y)
40
45
50
55
60
65
70
75
80
85
IE (%)
Figure 10. Effect of temperature (308−338 K) on the inhibitive efficiency for mild steel in the
presence of 0.0004 M of 8P in 1 M hydrochloric acid solution.
300 305 310 315 320 325 330 335
2
4
6
8
10
12
14
16
18
20 0.0005 M
Temp (oC)
CR (mm/y)
55
60
65
70
75
80
85
90
95
IE (%)
Figure 11. Effect of temperature (308−338 K) on the inhibitive efficiency for mild steel in the
presence of 0.0005 M of 8P in 1 M hydrochloric acid solution.
Temkin isoterm:
( )
ads
fθ
exp KC=
(9)
Freundlich isotherm:
ads
θCK=
(10)
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 378
Langmuir isotherm:
ads
1
θ
CC
K+=
(11)
where Kads is the equilibrium constant for the adsorption process and C is the concentration
of the corrosion inhibitor studied.
Figure 12. Temkin adsorption isotherm plot for mild steel in the presence of 8P.
Figure 13. Freundlich adsorption isotherm plot for mild steel in the presence of 8P.
-4.0 -3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3.3
0.4
0.5
0.6
0.7
0.8
0.9
θ
Log C
-4.0 -3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3.3
-0.35
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
Log θ
Log C
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 379
Figure 14. Langmuir adsorption isotherm plot for mild steel in the presence of 8P.
It can be seen from the plot (log(θ/1−θ) vs. log C) that the Langmuir isotherm provided
the best fit to a straight line and R2 value close to 1 (Figure 14). The equilibrium constant for
the adsorption process (Kads) has a relation with the standard free energy of adsorption
Gads) which is represented by equation (12).
( )
ads ads
ln 55.5GRT K=−
(12)
where 55.5 value is the molar water concentration whereas the R and T are the gas constant
and absolute temperature respectively.
It has been published recently that the ΔGads value has great importance in determining
the nature of the interaction between the inhibitor molecules and the metal surface. If this
value is equal to or less than −20 kJ mol1, it suggests that the adsorption interaction is
physisorption, while if the ΔGads value is approximately equal to or greater than −40 kJ
mol1, the adsorption interaction is chemisorption [24]. The value of ΔGads obtained in this
investigation is 40 kJ mol1, which implies that the nature of adsorption interaction of the
investigated compound on mild steel surface is generally chemisorption. Usually, the
chemical interactions between the inhibitor and the surface of the metal are initiated by the
decomposition reaction. Thus, the adsorption isotherms of the investigated inhibitor
molecules on mild steel surface might be firstly by a physisorption process but the adsorption
dominant model as proposed by the value of ΔGads can be categorized as chemisorption [25].
2.4. Surface Characterization
The SEM images for mild steel in the absence and presence of 8P are displayed in Figure 15.
The SEM micrograph in the absence of the inhibitor shows deep corrosion (Figure 15a),
resulting in a harsh and unsmooth mild steel surface in a 1 M hydrochloric acid environment.
-4.0 -3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3.3
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Log θ/1- θ
Log C
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 380
On the other hand, upon addition of the investigated inhibitor, the mild steel surface is
protected and looks smoother (Figure 15b), which enhances the inhibition performance.
Figure 15. SEM images (a) in the absence (b) and presence of 5.104 M 8P.
2.5. Quantum Chemical Calculations
The optimized structure of the studied inhibitor molecules and the respective frontier
molecular orbitals called HOMO and LUMO electron density surfaces of the investigated
inhibitor are demonstrated in Figure 16. HOMO is the Highest Occupied molecular orbital,
the highest-energy molecular orbital that has electrons in it. LUMO is the Lowest
Unoccupied molecular orbital, the lowest-energy molecular orbital that does not have any
electrons in it. The optimized structure in Figure 16 implies that the inhibitor molecules
adopt near-planar geometry in which the N-methylene groups in the piperazine and coumarin
rings are only twisted. The near-planar geometry of the investigated inhibitor molecule may
contribute to the high inhibitive efficiency due to the high planarity degree that was proved
to support the best adsorption of the tested compound on the mild steel surface leading to an
increase the inhibitive performance [2628]. The highest occupied MO electron density
surface suggests data about the most donating electron species of the inhibitor molecules to
the favorable orbital of the acceptor sites. On the other hand, the lowest unoccupied MO
electron density surface provides the active groups of the inhibitor molecules that possess
accepting electrons with higher chances from donor sites.
Figure 16. The optimum structure, HOMO and LUMO of 8P.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 381
To define more about the HOMO and LUMO orbitals and their importance in predicting
the nature of the interference between the molecules and the type of the interaction and
bonds, wherein the redox reaction. In a redox reaction, the reductant has a HOMO with a
high value and the oxidant has a LUMO with a low value, but the reductant HOMO is higher
than the oxidant LUMO so that the electrons pass completely to the oxidant LUMO from the
reductant HOMO (Figure 17). Predominantly the match of energy is poor, and hence no
covalent bond is formed, just electrons are transferred. Sometimes a covalent bond is also
formed. This depends on the energies of the orbitals, which is reflected by electronegativity.
Figure 17. The HOMO and LUMO energies.
Molecules with a high LUMO, a low HOMO, and a big energy gap (HOMO-LUMO),
such as hydrocarbons, are not quite reactive. Hydrocarbons do not react at room temperature.
The HOMO of the studied molecules is widely distributed over piperazine and is well
delocalized over the methylene group but not the coumarin moiety, which does not contribute
to the HOMO. The values of quantum chemical factors obtained for the investigated
compound are displayed in Table 1. The energy of the highest occupied MO is the propensity
measure of donating HOMO electrons of a molecule to the sufficient unoccupied orbital of
the metal surface, so a molecule with high EHOMO value have the best ability to donate electrons
[29]. The EHOMO value of the investigated inhibitor was 8.667 eV, which agrees with the
experimental findings of inhibition efficiency. On the other hand, the ELUMO is the affinity
measure of the tested molecule to accept electrons, so that a low ELUMO value indicates the
tendency of the inhibitor molecule to accept electrons from the d-orbitals of iron atoms. The
findings of the energy of low unoccupied MOs in Table 1 shows a significant agreement
with the tendency of the observed inhibitive efficiency, which proposes that the energy of
low unoccupied MOs may be a significant descriptor for the relative inhibitive efficiency of
the investigated inhibitor molecules. The energy gap (ΔE=EHOMOELUMO) is a considerable
reactivity factor of inhibitor molecules for which a molecule with a low value of ΔE is
generally more reactive and possesses higher inhibitive activity. The energy gap value
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 382
obtained for the investigated inhibitor molecules is in good agreement with the inhibitive
efficiency found in the experiments. The electronegativity, χ, measures the propensity of an
atom to attract a shared pair of electrons, and it is the index of reactivity which predicts the
range to which an inhibitor molecule possesses their electrons. The electronegativity with a
high value represents the lower electron donation chance of the molecule. The trend of
electronegativity value obtained for the investigated inhibitor molecules is listed in Table 1,
which proposes that the studied inhibitor molecules have the possibility of donating electrons
to the d-orbital of iron atoms of the mild steel surface. The electronegativity trend is agreed
with the inhibition efficiency results. The fraction of electrons transferred N) value is listed
in Table 1 and represents the fraction of electrons transferred from the studied inhibitor
molecules to the iron d-orbital atoms of the mild steel surface, also in agreement with the
trend of the experimental results of inhibition efficiency. The dipole moment (μ) is used to
relate the inhibition efficiency of an organic molecule to different perspectives. The first
point of view is that an increase in the dipole moment of an organic molecule leads to a
decrease in the efficiency of corrosion inhibition in corrosive solutions, with an explanation
that a decrease in dipole moment supports the preference for the accumulation of the
inhibitor molecules on the surface layer of the metal. The second point of view is, in general,
the opposite of the first view, as it assumes that with a higher value of the dipole moment,
the inhibition efficiency is enhanced due to the increase in dipole-dipole interactions between
the damper molecules and the metal surface [3033]. The dipole moment value obtained for
the investigated corrosion inhibitor molecules is listed in Table 1, which agrees with the
authors opinion who consider that the high value of dipole moment leads to high inhibitive
efficiency. The quantum chemical indexes (HOMO, LUMO, χ, ΔN and μ) that demonstrated
significant agreement with the methodological findings are those that propose the increase
of inhibitive efficiency for an inhibitor molecule with a considerable ability to electron
donation. The practical application of this assumption is also based on the fact that the
surface of mild steel in a corrosive environment is said to be filled with positive charges,
such that the interaction of the electron donation inhibitor molecules with the positively
charged mild steel surface is an appropriate process.
This indicates that the adsorption of the investigated corrosion inhibitor molecules on
the surface of mild steel mostly occurs by donating the electron from the electronic high-
density atoms or active groups to the d-orbital of iron atoms.
The global hardness (η) and global softness (σ) are other significant parameters that
give data about the stability of the molecule and its reactivity [34]. The hard molecule has a
large energy gap, but the soft molecule has a low energy gap. Thus, the inhibitor molecules
which have a low value of global hardness, and a high value of global softness propose a
significant inhibition efficiency [35, 36].
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 383
Table 1. Quantum chemical parameters of the studied molecules.
Parameter
Value
HOMO LUMO
ΔEEE =
8.667+(4.506) = 4.161 eV
HOMO LUMO
2
ηEE



=
2.805 eV
1
η
σ=
0.3565 eV
HOMO LUMO
2
χEE



=
6.414 eV
( )
Fe inh
Fe inh
χχ
2ηη
ΔN
+
=
1.058
μ
5.713
3. Conclusions
A new umbelliferon derivative, namely 8P, was synthesized, and its structure was
characterized and confirmed by Fourier transform infrared spectroscopy and nuclear
magnetic resonance spectroscopy. 8P has been studied for its corrosion inhibitive
performance on mild steel in 1 M hydrochloric acid solution. The experimental findings
demonstrated that the investigated inhibitor inhibited mild steel corrosion in the corrosive
solution and the inhibition efficiency increased with increasing concentration. The
experimental data revealed that the highest inhibition efficiency was 93.42% at 0.0005 M.
Also, the inhibition efficiency increased with increasing immersion time. SEM micrographs
demonstrated that the studied inhibitor molecules prevented the mild steel surface from
having direct contact with the hydrochloric acid environment. The adsorption of corrosion
inhibitor molecules on mild steel surface in a hydrochloric acid environment follows the
Langmuir isotherm. The tendency of quantum chemical parameters such as energy gap,
HOMO, LUMO, electronegativity, hardness, softness, and ΔN for the synthesized corrosion
inhibitor is in good agreement with the inhibition efficiency.
Acknowledgments
This study was supported by the University of Technology, Baghdad, Iraq.
Conflicts of Interest
The authors declare no conflict of interest.
Int. J. Corros. Scale Inhib., 2021, 10, no. 1, 368387 384
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