Anti-corrosive property of bioinspired environmental benign imidazole and isoxazoline heterocyclics: A cumulative studies of experimental and DFT methods

Article abstract of DOI:10.1002/jhet.3752

In the present study, four imidazoline (IDZ) and four isoxazolines (ISO) heterocyclics differing in the nature of methoxy (-OCH₃) and aromatic (phenyl and naphthyl) moieties are synthesized, characterized and evaluated as corrosion inhibitors for mild steel in acidic solution of 1 M HCl. Results showed that imidazoline based heterocyclic compounds are better corrosion inhibitors than isoxazoline based heterocyclics and both classes of compounds showed inhibition efficiency of more than 85% at 20 mgL^?1 concentration. Results further showed that inhibitors containing methoxy, phenyl, and naphthyl moieties showed higher protection efficiency as compared to the inhibitors without these moieties. PDP Study revealed that investigated IDZs and ISOs acted as mixed type inhibitors and their adsorption on the metallic surface followed the Langmuir adsorption isotherm model. All the experimental results were corroborated by density function theory (DFT) based quantum chemical calculations. Numerous DFT based indices calculated for neutral as well as protonated forms of the IDZs and ISOs in order to get better insight about metal-IDZs/ISOs interactions. Outcomes of the DFT analysis showed that protonated (cationic) form of the all the inhibitors are more strongly adsorbed on the metallic surface as compared to their neutral form.

Full text of DOI:10.1002/jhet.3752

Received: 9 August 2019
DOI: 10.1002/jhet.3752
Revised: 6 September 2019
Accepted: 10 September 2019
A R T I C L E
Anti-corrosive property of bioinspired environmental
benign imidazole and isoxazoline heterocyclics: A
cumulative studies of experimental and DFT methods
Sowmya Ramkumar1 | D. Nalini1 | Mumtaz A. Quraishi2 | Eno E. Ebenso3,4
Chandrabhan Verma3,4
|
1Department of Chemistry, PSGR
Abstract
Krishnammal College for Women,
Coimbatore, Tamil Nadu, India
In the present study, four imidazoline (IDZ) and four isoxazolines (ISO) het-
2Center of Research Excellence in
Corrosion, Research Institute, King Fahd
University of Petroleum & Minerals,
Dhahran, Saudi Arabia
3Department of Chemistry, School of
Chemical and Physical Sciences, Faculty
of Natural and Agricultural Sciences,
North-West University, Private Bag
X2046, Mmabatho 2735, South Africa
4Material Science Innovation & Modelling
(MaSIM) Research Focus Area, Faculty of
Natural and Agricultural Sciences, North-
West University, Private Bag X2046,
Mmabatho 2735, South Africa
erocyclics differing in the nature of methoxy (-OCH3) and aromatic (phenyl
and naphthyl) moieties are synthesized, characterized and evaluated as corro-
sion inhibitors for mild steel in acidic solution of 1 M HCl. Results showed that
imidazoline based heterocyclic compounds are better corrosion inhibitors than
isoxazoline based heterocyclics and both classes of compounds showed inhibi-
tion efficiency of more than 85% at 20 mgL−1 concentration. Results further
showed that inhibitors containing methoxy, phenyl, and naphthyl moieties
showed higher protection efficiency as compared to the inhibitors without
these moieties. PDP Study revealed that investigated IDZs and ISOs acted as
mixed type inhibitors and their adsorption on the metallic surface followed the
Langmuir adsorption isotherm model. All the experimental results were cor-
roborated by density function theory (DFT) based quantum chemical calcula-
tions. Numerous DFT based indices calculated for neutral as well as
protonated forms of the IDZs and ISOs in order to get better insight about
metal-IDZs/ISOs interactions. Outcomes of the DFT analysis showed that pro-
tonated (cationic) form of the all the inhibitors are more strongly adsorbed on
the metallic surface as compared to their neutral form.
Correspondence
D. Nalini, Department of Chemistry,
PSGR Krishnammal College for Women,
Coimbatore 641004, Tamil Nadu, India.
Email: nalini@psgrkcw.ac.in
Chandrabhan Verma, Department of
Chemistry, School of Chemical and
Physical Sciences, Faculty of Natural and
Agricultural Sciences, North-West
University, Private Bag X2046, Mmabatho
2735, South Africa.
Email: chandraverma.rs.apc@itbhu.ac.in
corrosion inhibitors is attributed to the fact that they
can be easily synthesized from commercially available
1 | INTRODUCTION
Organic compounds, in general and heterocyclic com-
pounds in particular are used as first line of defense
against metallic corrosion in highly aggressive acidic
medium during numerous industrial processes such as
acid picking, acid cleaning, descaling and oil-well acidifi-
cation. The extensive use of heterocyclic compounds as
chemicals economically in high yield, and they provide
high inhibition efficiency even in small concentration.[1,2]
Literature study reveals that several classes of heterocyclic
compounds including imidazoline and isoxazoline deriva-
tives are fascinating class of heterocyclic compounds
which exhibit wide range of biological activity such as
J Heterocyclic Chem. 2019;1–17.
wileyonlinelibrary.com/journal/jhet
© 2019 Wiley Periodicals, Inc.
1

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RAMKUMAR ET AL.
anti-inflammatory, anti-colvulsant, anti-microbial, anti-
HIV, and so forth.[3–7] Literature study reveals that a few
imidazoline and isoxazoline derivatives have been evalu-
ated as effective corrosion inhibitors for different alloys
and metals in different corrosive environments.[8–12]
In view of this, we herein synthesized four imi-
dazoline and four isoxazoline derivatives and evaluated
them as corrosion inhibitors for mild steel in acidic envi-
ronment of 1 M HCl. Each class of compounds differ in
the nature of methoxy (-OCH3), phenyl/naphthyl ring
and in the nature of heteroatoms. Novelty of the work is
that all the studied heterocyclic compounds were synthe-
sized by environmental friendly microwave irradiation in
high yields, high purity, in short times without empl-
oying any toxic solvents and metal catalysts. The objec-
tive of the work is to demonstrate the effect of methoxy
(-OCH3) substituent phenyl and naphthyl rings on the
corrosion inhibition performance of the heterocyclic
compounds. Inhibition effect and adsorption behavior of
tested compounds was systematically evaluated by chem-
ical electrochemical methods DFT studies were carried
out to support the experimental results.
into crushed ice. The chalcones (i-iv) obtained were then
washed thoroughly with ice water, filtered, dried and rec-
rystallized from ethanol. (b) Cyclization of chalcones to
isoxazolines: Equimolar ratio of chalcones (1 mmol) and
hydrazine hydrate hydrochloride (1 mmol) with NaOH and
Ethanol were taken in a conical flask and irradiated under
microwave irradiation at 50% power. The reaction was
paused at intervals and checked by TLC. After the comple-
tion of the reaction, the mixture was added to ice, kept over-
night for complete precipitation. The obtained isoxazoline
derivatives (ISO1–ISO2a) was filtered, washed with water
and recrystallized with ethanol. Schemes for the synthesis
of heterocyclic compounds are presented as Scheme S1 and
S2 whereas as FT-IR spectra of the synthesized compounds
are shown in Figure S1.[3,13] Molecular structure and their
analytical data are given in Tables S1 and S2. Synthesized
compounds were under taken for the particle size determi-
nation using SEM analysis and results confirm their nano-
size (Figure 1). Inhibition effect of the investigated
imidazolines and isoxazolines was tested as on mild steel
corrosion having chemical composition of (wt%): P: 0.023,
Si: 0.04, Ni: 0.017, Mn: 0.37, C: 0.078, S: 0.02, Mo: 0.002, rest
Fe) in 1 M HCl at their different concentrations ranging in
between 2 and 20 mgL−1. Metallic rod mounted on a Teflon
coating with an exposed surface area of 1 cm2 (one sided)
was used for electrochemical measurements. The test solu-
tion, 1 M HCl was prepared by dilution of analytical grade
HCl with double distilled water.
2 | EXPERIMENTAL WORK
2.1 | Materials
All the chemicals of high purity were purchased from sigma
Aldrich and used without any further purification. Micro-
wave instrument, namely, Monowave 200 from Anton
Parr model was employed for the synthesis of imidazoline
and isoxazoline derivatives. In a typical synthesis of
imidazolines, suitable aldehydes, (1 mmol) and ethylene
diamine (1 mmol) were taken in a conical flask, to which
3 mmol potassium carbonate and 1 mmol iodine was added.
The reaction mixture was irradiated using microwave irra-
diation of 240 MHz energy at 100% power in a solvent free
condition for several seconds. The reaction was paused at
an interval of 5 seconds each and the progress of the reac-
tion was checked by thin-layer chromatography (TLC) on
precoated silica gel plates. After the completion of the reac-
tion, the reaction mixture was cooled and added to ice, kept
overnight for complete precipitation. The obtained imi-
dazoline derivatives (IDZa-IDZd) was filtered, washed with
water and recrystallized with ethanol. Purity of the com-
pounds was checked on precoated TLC plates using silica
gel G plates and iodine vapors as visualizing agent. Synthe-
sis of isoxazoline derivatives was achieved in two steps:
(a) Synthesis of chalcones: Aldehydes, (1 mmol) and
acetophenone (1 mmol) were irradiated under microwave
power (2%) for few seconds in the presence of ethanolic
sodium hydroxide. The reaction mixture was then poured
2.2 | Methods
2.2.1 | Computational details
DFT based quantum chemical (theoretical) calculations
were carried out using B3LYP functional 6-31 G (d, p)
basis set within the program package GAUSSIAN 09[14]
program package implemented on an Intel Pentium core
i5 166MHz computer. Several DFT indices including the
energy of frontier molecular orbitals (EHOMOs) and their
band gap (ΔE), ionization potential (IP), electron affinity
(AE), hardness (η), softness (σ), electronegativity (χ),
dipole moment (μ), fraction of electron transfer (ΔN),
and so forth were derived for neutral as well as proton-
ated forms of the imidazoline and isoxazoline derivatives
and corroborated with results of experimental analyses.
2.2.2 | Electrochemical measurements
A three electrode cell assembly with mid steel rod
as working electrode (WE), platinum as counter elec-
trode (CE) and saturated calomel as reference electrode
(RE) was used for electrochemical studies. All experi-
ments were conducted as per the ASTM procedure[15] in

RAMKUMAR ET AL.
3
molecules to donate or accept electrons based on the anal-
ysis of various reactivity parameters. The imidazoline
derivatives contain two nitrogen atoms and an extensive
π-electron system in their backbone whereas in isoxazoline
derivatives one of the nitrogen of five-membered heterocy-
clic rings is replaced more electronegative oxygen atom.
Among the studied heterocyclic compounds, IDZ1,
ISO1 & IDZ1a, and ISO1a contain styryl/substituted styryl
unit attached to the parent heterocyclic ring whereas
IDZ2, ISO2 and IDZ2a, ISO2a contain a naphthyl/
substituted naphthyl attached to the parent heterocyclic
ring. The inhibitive performance of the above-mentioned
compounds were studied in comparison to the previously
reported compound P2I and TMP2I, which contain a phe-
nyl/substituted phenyl moiety attached to the imidazoline
ring. All the substituted derivatives contain a methoxy
group, an electron donating group, at the same position.
The level of inhibition varies with the structure and sub-
stituent position on the inhibitor molecules.
3.1.1 | The calculation method
Theoretical (DFT) calculations were carried out using
restricted density function level using the Becke's Three
Parameter Hybrid Functional using the Lee-Yang-Parr
correlation functional theory (B3LYP) which employs the
6-31 G (d, p) basis set. Initial estimates for all the geome-
tries of all the structure were obtained by GAUSS
VIEW[16] program followed by full optimization of all the
geometric parameters (bond length, bond order, and
dihedral angle) without any symmetry constraints.
FIGURE 1 Particle size of (A) IDZ1 and (B) ISO1 derived
using SEM analysis
1 M HCl using an IVIUM potentiostat electrochemical
workstation. Before performing the experiments, the
working electrode was immersed in 1 M HCl for
30 minutes before each experiment in the absence and
presence of inhibitors in order to get stabilization of
OCP (open circuit potential). Impedance measurements
were performed at open circuit potential (Eocp vs SCE)
over a frequency range 100 KHz to 0.01 Hz with a
25 mV peak to peak amplitude. From the plot of Z' vs Z"
the charge transfer resistance (Rct) and double layer
capacitance (Cdl) were calculated. The potentiodynamic
polarization measurements were performed by changing
the electrode potential automatically from −200 to
3.1.2 | Reactivity parameters
Two different parameters were chosen on the inhibitor
molecule to include the most important electronic effect.
The first parameter is the hetero atom present and its
position while the second constrains considered is the
presence of the additional phenyl ring that helps in facili-
tating the electron donation by the inhibitor structure.
3.1.3 | Structural parameters of the non-
protonated molecules
The values of bond length derived from the optimized
minimum energy geometries of the investigated imi-
dazoline and isoxazoline derivatives are presented in
Figure 2. The lowest energy conformer shows the bond
lengths of the different bond in the studied compounds
as also given in the Figure 2. The C-C bond lengths (Å) of
the aromatic ring are in the range of 1.39-1.41 for investi-
gated imidazoline and isoxazoline derivatives. The bond
lengths derived for both classes of heterocyclic com-
pounds are comparable to the bond lengths in isolated
+200 mV vs SCE at OCP at a scan rate of 1 mVs−1
.
3 | RESULTS AND DISCUSSION
3.1 | Computational studies
DFT is one of the most useful computational techniques
in order to probe into the capability of the inhibitor

4
RAMKUMAR ET AL.
FIGURE 2 Optimized geometry for the studied series of heterocyclic compounds
benzene ring.[17,18] The longest bond length corresponds
to the C-C bonds of the imidazoline and isoxazoline ring
(1.55 Å). These bonds correspond to the weakest bonds
(lowest values of the bond order) in the studied systems.
The C-N bond lengths are relatively closer to the C-C
bond length while the C=N is the shortest. The C=N
bond length also has the highest bond order value and
therefore it is the strongest. The bond length of C-N is
shorter than the general C-N single bond (1.470 Å).[19,20]
The tendency of average in the bond length is indicative
of a conjugation effect in both the imidazoline and
isoxazolinederivatives. The conjugation effect restricts
the free rotation of C-N bonds in the rings.[21,22] This
restriction results in the rigid planar structure of the
inhibitor molecule in these compounds.[18] The opti-
mized geometry in imidazoline and isoxazoline deriva-
tives has a planar arrangement with the exception of the
methyl-hydrogen atoms. Thus, it is expected from this
result that these molecules may adsorb on the metal sur-
face in a horizontal state through the hetero atoms with
lone pair electrons supported by conjugation systems
with π electrons as well as supporting electron donation
to the vacant d-orbital of Fe atoms.
3.1.4 | Reactivity parameters of the
nonprotonated molecules
The frontier orbitals (HOMO and LUMO) of a chemical
species are very important in defining its reactivity. The
HOMO distribution predicts the region susceptible for
electron donation and in these cases it is localized mainly
over the hetero atoms in plane with the adjacent double
bond for all the compounds. Consequently, this is the
preferred zone of the molecule interaction with the metal
surface.[23,24] Higher HOMO energy (EHOMO) of the mole-
cule means a higher electron donating ability of the
inhibitor to appropriate acceptor molecules. Except in the
isoxazoline derivatives, HOMO distribution is localized
completely over the entire molecule. On the other hands,
in isoxazoline derivatives HOMO is localized mainly over

RAMKUMAR ET AL.
5
the heterocyclic ring as well as adjacent benzene ring.
Also, the substituents do not show any considerable dif-
ference in the HOMO energy distribution as shown in
Figure 3.[18,25]
Figure 3 summarizes the schematic diagram of fron-
tier molecular orbitals for the investigated imidazoline
and isoxazoline derivatives and related energy band gap
(ΔE). As ΔE decreases, the reactivity of the molecule
increases leading to increase the inhibition efficiency of
the inhibitor molecule. From the results, it can be seen
that ΔE values are greater for isoxazoline derivatives as
compared to the imidazoline derivatives which suggests
that imidazoline derivatives are more reactive and better
corrosion inhibitors as compared to the isoxazoline deriv-
atives. This also implies that imidazoline derivatives are
more potent to transfer their electrons into the d-orbital
of surface metallic atoms as well as they are also more
potent to accept the electrons from d-orbitals of the atom
FIGURE 3 Frontier molecular orbital density distributions of studied series of heterocyclic compounds
TABLE 1 Calculated quantum chemical parameters for the inhibitors
Inhibitor TE (eV) μ (D) EHOMO (eV) ELUMO (eV) ΔEg (eV) IP
EA
η
σ
χ
ω
ΔN
TNC
Imidazoline derivative
IDZ1
−14 558
−17 688
−18 232
−20 147
2.95
4.10
5.83
3.82
−5.53
−5.44
−4.94
−6.17
−1.41
−1.21
−1.00
−2.71
4.12
4.23
3.94
3.46
5.53 1.41 2.06 0.47 3.47 2.93 0.86 −2.27
5.44 1.21 2.12 0.49 3.33 2.62 0.87 −2.50
4.94 1.00 1.97 0.58 2.97 2.24 1.02 −2.82
6.17 2.71 1.73 0.51 4.44 5.69 0.74 −2.65
IDZ1a
IDZ2
IDZ2a
Isoxazoline derivatives
ISO1
−21403
−24519
−27250
−21403
3.54
4.41
5.88
3.54
−6.02
−5.68
−6.41
−6.02
−1.25
−1.20
−2.29
−1.25
4.77
4.48
4.12
4.77
6.02 1.25 2.38 0.42 3.64 2.77 0.71 −2.19
5.68 1.20 2.24 0.45 3.44 2.64 0.80 −3.11
6.41 2.29 2.06 0.49 4.35 4.60 0.84 −3.55
6.02 1.25 2.38 0.42 3.64 2.77 0.71 −2.35
ISO1a
ISO2
ISO2a

6
RAMKUMAR ET AL.
FIGURE 4 Mulliken charge density distributions for nucleophilic attack of the studied inhibitor molecules
TABLE 2 Protonation energies for protonated form of molecules calculated at B3LYP with 6−31G (d, p) basis set in gas phase
Compound
Protonation site
ΔEprot (kcal/mol)
ΔEprot (kcal/mol)
IDZ1a
ΔEprot (kcal/mol)
ΔEprot (kcal/mol)
IDZ2a
Imidazoline derivatives
IDZ1
−278
−261
ISO1
−256
−242
IDZ2
−285
−278
ISO2
−261
−256
7N
−269
−243
8NH
−244
−221
Isoxazoline derivatives
ISO1a
ISO2a
18N
19O
−238
−257
−223
−245
in to their empty antibonding molecular orbitals as com-
pared to the isoxazoline derivatives. In literature both
increasing and decreasing order of inhibition efficiency
are reported with the value of dipole moment. Similar to
previously reported numerous reports, dipole moment
values in present study do not furnished any regular
trend (Table 1).[26] According to the HF theorem, a soft
molecule is more reactive than a hard molecule. Results
showed that imidazoline derivatives have greater values
of softness and smaller values of hardness as compared to
the isoxazoline derivatives which implies that imi-
dazoline derivatives are more relative and stronger
corrosion inhibitors as compared to the isoxazoline
derivatives.[21,27] It is important to mention that DFT
parameters derived for protonated form of imidazoline
and isoxazoline derivatives showed better correlation
and interactions with the metallic surface as compared
to the neutral form of the inhibitor molecules. The
higher values of higher values of EHOMO, fraction of
electron transfer (ΔN) and softness (σ) and lower values

RAMKUMAR ET AL.
7
FIGURE 5 Correlation diagram of frontier molecular orbitals for the two heterocyclic compounds with their ΔE values
TABLE 3 Calculated quantum chemical parameters for the inhibitors in their stable protonated form in gas phase
Inhibitor TE (eV) μ (D)
E
HOMO (eV) ELUMO (eV) ΔEg (eV) IP
EA
η
σ
χ
ω
ΔN
TNC
Imidazoline derivative
IDZ1
−14586
−17715
−18232
−17715
5.30
3.64
2.02
7.73
−2.87
−3.06
−2.86
−3.06
−0.27
0.45
0.60
0.45
2.60
3.51
3.47
3.51
2.87
0.27 1.30 0.77 1.57 0.95 2.09 −3.27
IDZ1a
IDZ2
3.06 −0.45 1.76 0.57 1.31 0.49 1.62 −3.50
2.86 −0.60 1.73 0.58 1.13 0.37 1.69 −3.82
3.06 −0.45 1.76 0.57 1.31 0.49 1.62 −3.65
IDZ2a
Isoxazoline derivative
ISO1
−14586
−24519
−25470
−21403
5.30
4.41
5.30
3.54
−2.87
−4.04
−3.14
−6.02
−0.27
−0.38
0.27
2.60
3.66
3.41
4.77
2.87
4.04
0.27 1.30 0.77 1.57 0.95 2.09 −2.99
0.38 1.83 0.55 2.21 1.34 1.31 −3.51
ISO1a
ISO2
3.14 −0.27 1.71 0.59 1.44 0.60 1.63 −4.15
6.02 1.25 2.38 0.42 3.64 2.77 0.71 −3.96
ISO2a
−1.25
of energy band gap (ΔE), electronegativity (χ), hardness
(η), and so forth. for protonated form of the imidazoline
and isoxazoline derivatives as compared to their neutral
forms confirm that protonated form of the imidazoline
and isoxazoline derivatives interact strongly with metal-
lic surface as compared to their neutral forms.
Figure 4 shows the Mulliken charges calculated for
the imidazoline and isoxazoline derivatives. It is impor-
tant to mention that the atom containing more negative
Mulliken charge is associated with high electron donat-
ing ability during the metal-inhibitor interactions and
converse is also true for the positive or less negative
value of the Mulliken charge. From Figure 4 it can be
seen that more negative Mulliken charges are localized
mostly over the heteroatoms which implies that it is the
heteroatoms (N and O) which is pertaining in the
electron transfer during the isoxazoline and imidazoline
derivative's interactions with the metallic sur-
face.[24,28,29] It is noted that in all the studied com-
pounds, the aromatic ring's carbons also possess greater
negative charge. Comparing the total negative charge on
the molecule, the imidazoline derivatives contain more
negative centers as compared to the isoxazoline
derivatives.
3.1.5 | Electronic parameters of
protonated molecules
In acidic solution, heteroatoms easily undergo proton-
ation and there exists equilibrium between neutral and
protonated forms of heterocyclic compounds. These
compounds are known to accept protons on their

8
RAMKUMAR ET AL.
FIGURE 6 Open circuit potentials of mild steel
surface in 1 M HCl in the absence and presence of
imidazoline and isoxazoline derivatives
ΔEprot = EQ + −EQ
ð1Þ
where EQ+ is the energy of cationic form and EQ is the
energy of neutral form of investigated molecules. The
protonation energy values at different sites in the studied
inhibitor systems are given in Table 2. The sites suitable
for protonation have a high ΔEprot (Figure 5) value.
Based on DFT calculations, the most preferred site for
protonation is C=N nitrogen atom in all the inhibitors.
According to Raman spectroscopy results, protonation of
azo compounds preferably occurs on the nitrogen atom
that leads to the formation of quinoidal structure.[30] The
formed quinoidal structure is well stabilized by the pres-
ence of adjacent aromatic rings which facilitated delocali-
zation. All the studied inhibitor systems form stable
protonated cationic form as evidence from its resonance
structure (Figure S2). Thus, the existence of such stabi-
lized cationic forms of the compounds in acidic media
may lead to an electrostatic interaction between inhibitor
and negatively charged metal surface, forming a protec-
tive layer between the metal surface and corrosive media.
The quantum chemical parameters for the protonated
forms are given in Table 3. Data in Table 3 show that pro-
tonated inhibitors at N site has lower ΔEg and larger ΔN
relative to nonprotonated forms. These results show that
the inhibitors can also be in their protonated state, in
equilibrium with their neutral state, in acid medium and
exhibit
a
better inhibitive property than their
corresponding nonprotonated species.
FIGURE 7 Tafel curves for mild steel in 1M HCl in the
absence and presence of different concentration of IDZ1 and
ISO1 at 303 K
3.2 | Electrochemical measurements
3.2.1 | Open circuit potential
nitrogen or oxygen sites. Protonation energies of these
compounds were calculated to assign the favorable site
for the accepting proton. The calculation is as follows:
In order to determine the adequate immersion time, an
open circuit potential study was performed. Figure 6
shows the open circuit potentials (OCP) curves of mild

RAMKUMAR ET AL.
9
TABLE 4 Polarization parameters of mild steel for the imidazoline derivatives in 1 M HCl
Inhibitor
Conc (mg/L)
Ecorr (mV)
−474
−504
−529
−516
−509
−506
−495
−524
−519
−516
−509
−506
−495
−513
−507
−489
−505
−503
−501
−501
−508
−508
−503
−504
−511
Icorr (μA/cm2)
1100
667
536
484
348
291
241
367
336
284
248
191
149
281
230
182
163
153
126
116
104
96
IE (%)
−
ba (mV/dec)
ba (mV/dec)
111
Rp (Ω cm2)
16
IE (%)
−
Blank
2
67
106
99
80
79
76
74
106
99
80
79
76
74
81
80
73
77
74
64
76
67
78
70
72
75
IDZ1
39.36
51.27
55.96
68.35
73.59
78.09
66.64
69.45
74.15
77.44
82.68
86.45
74.46
79.08
83.45
85.22
86.14
88.55
89.42
90.59
91.24
92.50
93.47
95.28
103
27
38.54
44.99
53.37
58.72
66.86
72.37
61.52
71.57
82.72
85.01
88.29
90.73
78.51
81.28
83.87
86.6
6
111
30
10
14
18
20
2
126
35
132
40
133
50
135
60
IDZ1a
103
43
6
111
58
10
14
18
20
2
126
95
132
110
141
178
77
133
135
IDZ2
124
6
126
88
10
14
18
20
2
131
102
123
149
174
119
155
186
207
249
278
125
125
88.91
90.52
86.16
89.37
91.15
92.04
93.39
94.08
105
IDZ2a
125
6
133
10
14
18
20
124
83
130
72
131
52
128
steel surface in 1 M HCl in the absence and presence of
studied corrosion inhibitors. The potential for the blank
acid solution is lower as compared to that in the presence
of the inhibitor, which is due to the dissolution of the
metal surface. However, in the presence of inhibitors, the
potential value changed during the initial period of
10 minutes after which the potentials become almost sta-
ble.[31,32] Hence an immersion potential of 30 minutes is
selected to perform corrosion experiments.
steel corrosion in hydrochloric acid medium mainly point
out the following:
• icorr decreased with increase in concentration of imi-
dazoline and isoxazoline derivatives.
• Polarization study suggests that inhibition efficiency of
both classes of heterocyclic compounds followed the
order: imidazolines > Isoxazolines
• Ecorr value was shifted towards less negative (noble)
potential. In our study, maximum displacement in
Ecorr was around 30 mV which indicates that imi-
dazoline and isoxazoline derivatives are acted as mixed
type inhibitors. Further, values of Tafel slopes (βc and
βa) of inhibited curves did not show any appreciable
change values have changed with respect to inhibitor
free solution, which further supports our assumption
that imidazoline and isoxazoline derivatives are mixed
type inhibitors.[33–35]
3.2.2 | Potentiodynamic polarization
resistance
Typical potentiodynamic polarization curves for the com-
pound IDZ1 and ISO1 in 1 M HCl are as shown in
Figure 7 and their corresponding parameters are tabu-
lated in Tables 4 and 5. Potentiodynamic polarization
results obtained for the eight derivatives against mild

10
RAMKUMAR ET AL.
TABLE 5 Polarization parameters of mild steel for the isoxazoline derivatives in 1 M HCl
Inhibitor
Conc (mg/L)
Ecorr (mV)
−474
−489
−481
−495
−495
−492
−489
−501
−494
−493
−486
−472
−480
−497
−502
−497
−500
−509
−518
−457
−455
−461
−456
−461
−459
Icorr (μA/cm2)
1100
503
IE (%)
−
βa (mV/dec)
−βa (mV/dec)
111
Rp (Ωcm2)
IE (%)
−
Blank
2
67
55
42
55
60
57
54
61
54
53
54
44
50
67
69
68
73
85
93
71
72
78
77
82
81
16
ISO1
38.39
43.27
51.67
56.83
63.82
64.66
76.42
79.72
82.18
84.48
87.65
87.38
77.34
80.47
82.14
84.38
85.53
88.29
73.79
75.18
77.57
80.44
86.11
90.38
139
29
39.46
46.29
50.67
55.08
59.79
61.21
77.17
80.39
82.84
84.01
86.04
86.72
79.31
81.28
83.02
84.75
86.61
89.43
74.75
79.58
81.04
83.41
86.15
88.11
6
463
128
33
10
14
18
20
2
394
138
36
352
149
39
295
146
44
288
143
45
ISO1a
259
146
72
6
223
126
84
10
14
18
20
2
196
125
96
171
110
103
118
124
80
136
130
139
113
ISO2
249
125
6
215
126
88
10
14
18
20
2
196
118
97
172
116
108
123
131
63
159
110
140
107
ISO2a
291
107
6
275
106
78
10
14
18
20
249
104
84
217
107
96
154
103
115
134
107
108
• It can also be observed that shape of inhibited and
uninhibited anodic and cathodic polarization curve of
mild steel in HCl are almost similar. This indicates that
that imidazoline and isoxazoline derivatives inhibit the
metallic corrosion through adsorption without affect-
ing the mechanism of corrosion.[36]
• The polarization data clearly show that the current
density (icorr) values decreased in the presence of vari-
ous concentrations of the studied heterocyclic com-
pounds and it can be assumed that the adsorption of
the inhibitor molecules on metal surface makes a phys-
ical barrier for the mass and charge transfer, providing
a high degree of protection to the metal surface.
between 10 and 20 mV with respect to the corrosion poten-
tial (Ecorr) and the corresponding current was measured.
The slope of the plot of linear graph between (Eapp − Ecorr
)
vs current (I) near Ecorr is defined as the polarization resis-
tance (Rp). Percentage of the inhibition efficiency derived
for imidazoline and isoxazoline derivatives are tabulated
Tables 6 and 7. From the linear polarization measurements,
following conclusions were obtained:
• Rp values are increasing with imidazolines and
isoxazolines concentration which indicate that protec-
tion efficiency for both series of compounds is increas-
ing in the same sequence.
• An inverse relationship between the values of corro-
sion current density and polarization resistance is
obtained it can be assumed that the adsorption of the
inhibitor molecules on metal surface makes a physical
barrier for the mass and charge transfer, providing a
high degree of protection to the metal surface.[37,38]
3.2.3 | Linear polarization resistance
In order to study the linear polarization resistance (Rp),
potential of working electrode was allowed to is changed in

RAMKUMAR ET AL.
11
TABLE 6 Electrochemical impedance parameters of mild steel forimidazoline derivatives in 1 M HCl
Inhibitors
Conc (mg/L)
RS (Ωcm2)
3.51
3.02
2.5
Y0 (μF/cm2)
698
630
449
389
330
268
173
298
248
246
233
183
186
684
985
375
230
132
61
n
Rct (Ωcm2)
14
IE (%)
−
fmax (μHz)
104
75
Cdl (μF/cm2)
θ
Blank
2
0.25
0.28
0.32
0.34
0.38
0.39
0.41
0.35
0.37
0.39
0.41
0.43
0.44
0.38
0.39
0.42
0.45
0.44
0.46
0.39
0.44
0.45
0.48
0.5
185
36
35
34
32
32
31
71
68
66
61
58
42
32
29
28
27
26
20
38
29
27
26
20
14
−
IDZ1
21
35.26
57.56
68.39
74
0.8077
0.8108
0.8114
0.8219
0.8243
0.8292
0.6146
0.6314
0.6427
0.6697
0.6838
0.7703
0.8265
0.8405
0.8416
0.8486
0.8562
0.8892
0.7939
0.8409
0.8530
0.8588
0.8899
0.9232
6
32
69
10
14
18
20
2
5.38
2.96
5.48
6.7
44
59
53
57
65
78.63
82.39
69.33
76.57
80.03
81.62
85.4
58
78
47
IDZ1a
8.47
9.14
8.95
9.96
10.5
10.3
7.41
9.01
12.3
10.3
8.7
45
75
6
59
69
10
14
18
20
2
69
59
75
57
94
58
115
61
87.98
77.24
80.57
85.89
87.2
47
IDZ2
76
6
71
67
10
14
18
20
2
98
58
108
114
146
65
57
87.94
90.53
78.88
88.7
57
8.37
4.71
5.97
6.12
5.93
6.73
9.17
46
IDZ2a
251
121
113
101
92
78
6
122
141
162
200
221
71
10
14
18
20
90.21
91.49
93.11
93.76
65
56
54
72
0.5
46
presence of less electron-withdrawing nitrogen atom in
imidazole ring of the imidazoline derivatives at the place
of more electron-withdrawing oxygen atom in isoxazoline
derivatives. Nyquist and Bode pots for the test electrode
corrosion with and without imidazoline and isoxazoline
derivatives are shown in Figures 8 and 9. It can be seen a
single semicircle in the Nyquist plots with and without
imidazoline and isoxazoline derivatives which indicate
that corrosion of iron in given electrolyte involves the
single charge transfer phenomenon. This outcome is fur-
ther supported using single maxima in the Bode plots of
inhibited (by imidazoline and isoxazoline derivatives)
and uninhibited specimens. Furthermore, diameter of
the Nyquist curves increases with increasing the concen-
trations of imidazoline and isoxazoline derivatives which
is resulted due to increase in the magnitude in the resis-
tance for charge transfer phenomenon because of the
adsorption of imidazoline and isoxazoline derivatives on
metal-electrolyte interfaces.[39] The constant phase ele-
ment (CPE) is described by the expression:
3.2.4 | Electrochemical impedance
spectroscopy
Potentiodynamic polarization and linear polarization
resistance measurements for both series of heterocyclic
compounds are supported by EIS study and the outcomes
of the study are Tables 6 and 7. Representative Nyquist
plots obtained for the imidazoline and isoxazoline
derivatives are shown in Figure 8A,B. EIS data were
analyzed using an equivalent circuit presented in
Figure 8C. Protection efficiency of imidazoline and iso-
xazoline derivatives was analyzed at six concentrations
ranging from 2 to 20 mg/L. Further increase in the con-
centration (more than 20 mg/L) of imidazoline and iso-
xazoline derivatives did not caused any significant
change in their inhibition performance indicating that
20 mg/L concentration is optimum concentration in pre-
sent study. Results showed that imidazoline derivatives
exhibit higher protection efficiency as compared to the
isoxazoline derivatives which might be attributed due to

12
RAMKUMAR ET AL.
TABLE 7 Electrochemical impedance parameters of mild steel forisoxazoline derivatives in 1 M HCl
Inhibitors
Conc (mg/L)
RS (Ω cm2)
3.51
5.06
6.19
5.31
5.34
5.58
5.69
6.54
6.4
Y0 (μF/cm2)
698
n
Rct (Ωcm2)
14
IE (%)
−
fmax (μHz)
104
97
Cdl (μF/cm2)
θ
Blank
2
0.25
0.34
0.34
0.35
0.38
0.38
0.4
185
30
25
24
24
24
22
38
32
31
30
26
20
38
32
31
30
29
28
38
37
36
35
34
33
−
ISO1
599
39
64.89
67.95
71.89
74.68
77.09
80.66
71.59
73.97
76.09
78.76
80.35
82.73
74.11
77.88
82.48
83.39
85.03
86.96
69.34
79.17
83.06
86.77
89.11
90.19
0.8379
0.8609
0.8672
0.8662
0.8671
0.8770
0.6347
0.687
6
458
43
82
10
14
18
20
2
442
49
71
323
54
65
224
60
62
193
71
51
ISO1a
277
0.36
0.36
0.37
0.4
49
75
6
275
53
69
10
14
18
20
2
5.74
5.97
5.78
6.12
5.26
5.61
6.75
5.65
7.39
8.49
6.84
9.58
9.7
295
58
59
0.6976
0.7134
0.7486
0.8088
0.7926
0.8224
0.8284
0.8374
0.8398
0.8443
0.7896
0.7979
0.8005
0.8076
0.8121
0.8168
269
65
57
247
0.4
70
58
268
0.41
0.37
0.38
0.4
80
47
ISO2
252
53
75
6
234
62
69
10
14
18
20
2
216
79
59
211
0.42
0.42
0.43
0.35
0.38
0.4
83
57
188
92
58
202
106
45
47
ISO2a
298
75
6
220
66
69
10
14
18
20
209
81
59
11.3
11.8
12.8
168
0.44
0.45
0.46
104
127
140
57
137
58
152
47
−n
ZCPE = Y0−1ðiωÞ
ð2Þ
densities were determined by using the Stern-Grey equa-
tion. The values of Rct have been substituted in the Stern-
Grey equation to obtain the corrosion current.[40,41] The
impedance parameters, namely charge-transfer resistance
(Rct), the constant phase element (Yo) related to the
capacity of the double layer, corrosion current density
(Icorr) and the exponent (n) relevant to the capacitive
semicircle of the mild steel/acid/heterocyclic derivatives
system are given in Tables 6 and 7. Simulation of Nyquist
plots with above model shows excellent agreement with
derived using other experimental methods. It means that
the suggested equivalent circuit model could reasonably
represent the metal/solution interface features related to
the corrosion process of mild steel in acidic solution
containing studied imidazoline and isoxazoline
derivatives.[42,43]
where Yo is the CPE constant (in Ω−1Sn cm−2), ω is the
sine wave modulation angular frequency (in rad s−1),
i2 = −1 is the imaginary number and n is an empirical
exponent (0 ≤ n ≤ 1) which measures the deviation from
the ideal capacitive behavior.[39]
According to the electrochemical theory, Rct is
inversely proportional to the corrosion current density
(ie, corrosion rate), so it is analogous to the use of polari-
zation resistance in the Stern-Geary equation.
β
Icorr
=
ð3Þ
Rct
where
βa × βc
β =
:
ð4Þ
It is also evident that the impedance response of mild
steel in uninhibited acid solution has significantly chan-
ged after the addition of the studied imidazoline and iso-
xazoline derivatives.[44] The size of the semicircle
2:3030 × Rctðβa + βcÞ
In this work, we have also correlated impedance and
polarization methods. Thus, the corrosion current

RAMKUMAR ET AL.
13
converse is true for the higher values of n in their pres-
ence.[43] The change in Rct (increase) and Cdl (decrease)
values is caused by to the gradual replacement of water
molecules by the adsorption of the imidazoline and iso-
xazoline derivatives on the metal surface. This suggests
that the inhibiting action of these compounds occurs by
simple blocking the active sites present over the metallic
surface.
3.2.5 | Adsorption isotherms
Results derived using EIS studies were used to calculate
the adsorption isotherm parameters. Best fit for the inves-
tigated inhibitor in 1 M HCl was obtained for Langmuir
adsorption isotherm which can be presented as follows:
C
1
Kads
=
+ C
ð5Þ
θ
where θ is the degree of surface coverage, C is the inhibi-
tor concentration, and Kads is the equilibrium constant.
The standard Gibbs energy of adsorption (ΔGꢀads) values
were calculated using the values of Kads using following
relationship:
1
55:5
ΔG0ads
RT
Kads
=
+ exp
ð6Þ
where R is the universal gas constant and T is the absolute
temperature. Langmuir adsorption isotherm plots (between
C/θ against C) are shown in Figure 9. A very good fit is
observed (R2 up to 0.99), which suggests that the experi-
mental data are well described by Langmuir isotherm
model.[43,46] The Kads and ΔGꢀ values for the adsorption
ads
of imidazoline and isoxazoline derivatives are given in
Table 8. The slope of the straight line (Kads) is found to be
1.88 × 104 M−1 suggesting that the adsorbed imidazoline
and isoxazoline derivatives form the monolayer on the
mild steel surface and there is no interaction among the
adsorbed inhibitor molecules. The negative values of
ΔGꢀads ensure the spontaneity of the adsorption process
and the stability of the adsorbed layer on the mild steel
surface.[47] The ΔGaꢀ ds values is in the range of −8.9 to
−10.2 kJ/mol is indicating the physisorption process.[48]
FIGURE 8 Nyquist curves for mild steel in 1M HCl in
absence and presence of different concentration of (A) IDZ1 and
(B) ISO1 at 303 K. (C) Equivalent circuit used to fit EIS data
increases with concentration indicating that the corro-
sion is mainly a charge transfer process.[45] Also change
in the concentration of imidazoline and isoxazoline
derivatives did not alter the shape of the impedance cur-
ves, which suggests that imidazoline and isoxazoline
derivatives inhibit corrosion through adsorbing on the
surface without affecting the mechanism of mild steel
dissolution.[35] Lower “n” value for uninhibited solution
(n = 0.78) indicates that higher surface inhomogeneity
due to rapid and free corrosion of metallic surface in the
absence of imidazoline and isoxazoline derivatives while
3.2.6 | Bode plot
The Bode plots for mild steel corrosion in 1 M HCl in
absence and presence of the imidazoline and isoxazoline
derivatives is shown in Figure 10. According to
Figure 10, an increase of phase angle with imidazoline
and isoxazoline derivatives concentrations involves an

14
RAMKUMAR ET AL.
FIGURE 9 Bode plot for mild steel in absence and
presence of different concentration of (A) IDZ1 and
(B) ISO1 at 303 K
TABLE 8 Langmuir adsorption parameters obtained from EIS measurements for mild steel in 1 M HCl for imidazoline, isoxazolines,
and indoloimidazoline derivatives
Imidazolines
Isoxazolines
Inhibitors
Kads
ΔGads
9.2
R2
Kads
ΔGads
9.3
R2
1
0.693
0.626
0.911
0.929
0.983
0.996
0.657
0.979
0.716
0.816
0.883
0.681
0.983
0.996
0.657
0.979
1a
2
8.9
9.6
9.9
9.8
2a
9.9
9.2
increase of capacitive response associated to lower cor-
rosion rates. The maximum phase angle for a corroding
system represented by a simple RC parallel combination
should be 90o when RS should be equal to zero. How-
ever, depressed semi circles are usually obtained for an
electrode/solution interface, which has been known to
be associated with a rough electrode surface.[49] The cor-
rosion of mild steel in acid media increases the
roughness of the electrode surface and therefore reduces
the phase angle values. From the Bode plots, it can be
seen that values of phase angle are significantly changed
in the presence of imidazoline and isoxazoline deriva-
tives. This is an indication of the decrease in the surface
inhomogeneity due to the protection of the metal sur-
face by the adsorbed imidazoline and isoxazoline deriva-
tives on the metallic.[50,51]

RAMKUMAR ET AL.
15
FIGURE 10 Langmuir adsorption isotherms for the
imidazoline and isoxazoline derivatives in 1 M
FIGURE 11 A schematic
illustration of mode of adsorption of the
inhibitors on metal surface
TABLE 9 Comparative assessment on the corrosion inhibitive
potential of studied compounds in 1 M HCl
molecule PM2I, it is seen that all the eight heterocy-
clic compounds show enhanced inhibition perfor-
mance as corrosion inhibitor with an efficiency above
80% in 1 M HCl at a minimal concentration of
20 mg/L. The enhanced performance of all the com-
pounds may be attributed to the following reason:
(a) nanoparticle size of the synthesized inhibitors
which helps in the adsorption of more number of
inhibitor molecules, (b) flat planar structure of the
inhibitors, covering a larger surface area of the metal,
(c) the presence of extensive conjugation in the form
of aromatic ring which facilitates the stabilization of
charged species by resonance delocalization, and
(d) formation of a stable protonated species in acid
medium which helps in the stronger interaction
between the charged inhibitor moiety and charged
metal surface.
Imidazoline (IDZs)
Isoxazoline (ISOs)
PP
EIS
PP
EIS
1
78.09
86.45
88.55
95.28
82.39
87.98
90.53
93.76
64.66
87.38
88.29
90.19
80.66
82.73
86.96
90.36
1a
2
2a
3.3 | Comparative assessment of the
inhibitive performance of the inhibitors
The inhibitive potential of the different studied imi-
dazoline and isoxazoline derivatives in 1 M HCl acid
medium is summarized in Table 9. The following conclu-
sions can be drawn by analyzing the data.
2. In HCl medium, the presence of methoxy substituent
has increased the inhibition efficiency by 2% and 5%
for the imidazolines and isoxazolines, respectively.
1. On comparing the efficiency of the studied com-
pounds with that of the previous studies similar

16
RAMKUMAR ET AL.
• The adsorption of studied imidazoline and isoxazoline
derivatives on the metal surface followed the Langmuir
adsorption isotherm.
• DFT study showed that imidazoline and isoxazoline
derivatives easily undergo protonation in highly acidic
medium of 1 M HCl and they mostly adsorb on metal-
lic surface using physisorption phenomenon.
3.4 | Mechanism of inhibition
A clarification of mechanism of inhibition requires a full
knowledge of the interaction between the imidazoline
and isoxazoline derivatives and the metal surface. Most
organic inhibitors have polar substituents with hetero
atoms which are regarded as the reaction centers for the
adsorption process. On the other hand, iron has a greater
affinity to co-ordinate with ligands bearing unshared elec-
tron pairs of heteroatoms. In HCl solution, heteroatom(s)
of the studied heterocyclic compounds might be proton-
ated. Thus in aqueous acidic solution, the imidazoline
and isoxazoline derivatives may exist either in their neu-
tral or in their cationic (protonated) forms.[23,28,51,52] The
imidazoline and isoxazoline derivatives can also be
adsorbed on the metal surface via the donor-acceptor
interaction between the unshared electrons pairs of the
heteroatom and pi-electrons of the aryl ring with the
vacant d-orbital of iron. Both imidazoline and isoxazoline
derivatives contain nitrogen atoms that can easily
undergo protonation and get converted in to their cationic
forms.[53] In addition, owing to the presence of lone pair
of electrons on the hetero atoms, imidazoline and iso-
xazoline derivatives can act as ideal ligands for donor-
acceptor interactions with metallic surface.[28,54] Thus,
the adsorption of the imidazoline and isoxazoline deriva-
tives on the metal surface is assumed to be a mere physi-
cal process.[35,55] Schematic representation of corrosion
protection by one of the studied compounds showing bet-
ter protection in the acid medium is depicted in Figure 11.
ACKNOWLEDGMENTS
The authors thank UGC, New Delhi for the financial
support.
ORCID
Chandrabhan Verma https://orcid.org/0000-0001-9249-
7242
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4 | CONCLUSION
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• The presence of methoxy substituent (-OCH3)
enhances the inhibition efficiency for both series of
heterocyclic corrosion inhibitors.
• Polarization study showed that imidazoline and iso-
xazoline derivatives acted as mixed type corrosion
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