N–hydroxybenzothioamide derivatives as green and efficient corrosion inhibitors for mild steel: Experimental, DFT and MC simulation approach

Article abstract of DOI:10.1016/j.molstruc.2021.130648

In this research work, the thiohydroxamic acid derivatives included N–hydroxybenzothioamide (THA–H), 4–bromo–N–hydroxybenzothioamide (THA–Br), and 4–methoxy–N–hydroxybenzothioamide (THA–OCH₃) were first introduced as effective and green corrosi

Full text of DOI:10.1016/j.molstruc.2021.130648

Journal of Molecular Structure 1241 (2021) 130648
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
N–hydroxybenzothioamide derivatives as green and efficient corrosion
inhibitors for mild steel: Experimental, DFT and MC simulation
approach
Dakeshwar Kumar Vermaa,∗, Mohsin Kazib, Mohammed S. Alqahtanib, Rabbani Syedb,
Elyor Berdimurodovc,∗, Savas¸ Kayad, Rajae Salime, Ashish Asatkarf, Rajesh Haldharg
a Department of Chemistry, Government Digvijay Autonomous Postgraduate College, Rajnandgaon, Chhattisgarh, INDIA, 491441
b Department of Pharmaceutics, College of Pharmacy, PO Box 2457, King Saud University, Riyadh, 11451, Saudi Arabia
c Faculty of Natural Sciences, Karshi State University, Karshi, 180100, Uzbekistan
d Cumhuriyet University Health Services Vocational School, Department of Pharmacy, 58140 Sivas, Turkey
e Laboratory of Engineering, organometallic, Molecular and Environment (LIMOME), Faculty of science, University Sidi Mohamed Ben Abdellah, Fez, Morocco
f Department of Chemistry, Govt. Gundadhur Postgraduate College, Kondagaon, Chhattisgarh, INDIA
g School of Chemical Engineering, Yeungnam University, Gyeongsan 712749, SOUTH KOREA
a r t i c l e i n f o
a b s t r a c t
Article history:
In this research work, the thiohydroxamic acid derivatives included N–hydroxybenzothioamide (THA–H),
4–bromo–N–hydroxybenzothioamide (THA–Br), and 4–methoxy–N–hydroxybenzothioamide (THA–OCH3)
were first introduced as effective and green corrosion inhibitors for mild steel in 1M HCl.The inhibition
behaviours of THA–H, THA–Br and THA–OCH3 were first–fully characterized by weight loss (WL), poten-
tiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS).The obtained experi-
mental results suggested that the maximum inhibition efficiency of THA–H, THA–Br and THA–OCH3 were
over 90% at 300 ppm.The surface characterization of the metal surfacewas investigated by X–ray diffrac-
tion analysis (XRD), scanning electron microscope (SEM) and electron diffraction X–ray spectroscopy
(EDS) analysis; the obtained results confirmed that the selected inhibitors formed the protective filmon
the metal surface. The quantum chemical analysis and Monte Carlo (MC) simulationwere also performed
to determine the nature of adsorption, possible adsorption orientation of inhibitor molecules on the metal
surface, the correlation between the inhibition properties and molecular structures. Adsorption isotherm
suggests that the selected molecules are mixed type of corrosion inhibitors related to Langmuir isotherm.
Received 11 March 2021
Revised 2 May 2021
Accepted 4 May 2021
Available online 12 May 2021
Keywords:
Corrosion inhibitor
Thiohydroxamic acids
Adsorption
Electrochemical analysis
Theoretical calculations
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
properties of metal was dramatically reduced [1-3]. For example,
the metal pipes were made from mild steel in the gas and oil in-
Mild steel is a widely used metallic material in the gas and
oil industry.The corrosion of metallic materials is a large prob-
lem environmentally and economically [1]. The metal surface was
destroyed by the corrosion processes. As a result, the mechanical
dustry. The metal pipes were seriously corroded during oil and gas
transportation. Consequently, the surface of steel pipes was cov-
ered with corrosion deposits such as salts, hydroxide and oxides
[4]. The corrosion deposits inside metal pipes are difficult for gas
and oil transportation [5]. To clean the corrosion products from the
metal surface, 1 M HCl was used. The metal surface was corroded
during the cleaning processes. This is due to the acidic attacks on
the surface of the metal. Protecting the metal pipes from the corro-
sion processes is an important task in the gas and oil industry [6].
The corrosion inhibitor was added to the corrosion solution to pro-
tect the metal surface from corrosion destruction. In the inhibition
processes, the corrosion inhibitor adsorbed on the metal surface to
form the protective film [7]. As a consequence, the metal surface
was protected and corrosion deposits were reduced. The corrosion
inhibitors areorganic compounds, which have more heteroatoms,
Abbreviations: THAs, Thiohydroxamic acid derivatives; THA–H, N–
hydroxybenzothioamide; THA–Br, 4–bromo–N–hydroxybenzothioamide; THA–OCH3,
4–methoxy–N–hydroxybenzothioamide; WL, Weight loss; EIS, Electrochemical
impedance spectroscopy; PDP, Potentiodynamic polarization; MC, Monte Carlo;
SEM, Scanning electron microscopy; EDS, Electron dispersion x-ray spectroscopy;
HOMO, Highest occupied molecular orbital; DFT, Density function theory; LUMO,
Lowest unoccupied molecular orbital; INH, Inhibitor; XRD, X-ray diffraction; ADS,
Adsorption; MS, Mild steel.
∗
Corresponding authors.
E-mail addresses: dakeshwarverma@gmail.com (D.K. Verma), rsyed@ksu.edu.sa
(R. Syed), elyor170690@gmail.com (E. Berdimurodov).
https://doi.org/10.1016/j.molstruc.2021.130648
0022-2860/© 2021 Elsevier B.V. All rights reserved.

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
functional groups and benzoyl rings [8]. Generally, the corrosion
inhibitors contained nitrogen, sulfur, oxygen and phosphorous, and
aromatic rings. They are mainly responsible for corrosion protec-
tion [9]. The corrosion inhibitor chemical and physical adsorbed on
the metal surface by the heteroatoms, functional groups and ben-
zoyl rings [10].
orientation of inhibitor molecules on the metal surface, the corre-
lation between the inhibition properties and molecular structures.
2. Methods and materials
2.1. Metal specimen and test solution
It was known that the p–electron containing organic molecules
are excellent corrosion inhibitors because the p–electrons in the
corrosion inhibitor interact easily with the empty d–orbitals of
metal to form a stable complex on the metal surface;as a re-
sult, the metallic surface is protected from the corrosive attack [4-
9].Currently, there several types of heterocyclic compounds were
investigated as an efficient corrosion inhibitors. For example, the
imidazole derivatives [10], porphyrin, hydrazide [11,12], hydrox-
amic acid derivatives [13,14], tetrazole derivatives [15] and por-
phines derivatives [16] were mostly used.
In this research work, the mild steel (MS) sample was pur-
chased from the commercial company (Raipur, INDIA). It was a
composite material that included the following elements: ~98%Fe,
0.3% Сu, 0.3% Ni, 0.3% Сr, 0.25–0.5% Mn, 0.05% S, 0.09–0.22% С,
0.05–0.15% Si, 0.008% N, 0.04% Р and 0.08% As. These metal sam-
ples were applied for both electrochemical and mass loss measure-
ments. Hydrochloric acid (36.5%) purchased from the commercial
chemical company (MERCK grade, Raipur, INDIA)) and used to pre-
pare the 1M HCl for corrosive medium, in which different amounts
of inhibitors (150 to 300 ppm) were dissolved for the gravimet-
ric and electrochemical measurements. In all experiments, the mild
steel specimens should be were carefully scratched with different
grades (200–1200) of emery papers to get a clean and smooth sur-
face. After a scratch, the specimen was washed with double dis-
tilled and acetone for the removal of dust and grease. Then, the
metal specimen was kept in the desiccators before use for a suffi-
cient period (up to 15 minutes).
The novelty of this research work, the thiohydroxamic acid
derivatives (THAs) included [(N–hydroxybenzothioamide (THA–H),
4–bromo–N–hydroxybenzothioamide (THA–Br), and 4–methoxy–
N–hydroxybenzothioamide (THA–OCH3)] were first introduced as
effective corrosion inhibitors for mild steel in the 1M HCl corro-
sive solution. These derivatives contain the nitrogen, oxygen, and
sulphur heteroatom as well as p–electron and aromatic ring, which
support to become high inhibition efficiency of these inhibitors.
Also, the thiohydroxamic acid derivatives have been used as an
antioxidant and tyrosinase inhibitor [17], anticancer, antibacterial
agents [18] and in extraction and speciation of Plutonium [19].
These properties support that the selected corrosion inhibitors are
green and excellent. Additionally, THAs contain electron donor ni-
trogen and oxygen heteroatoms, which can promote form the sta-
ble complexes with iron metal ions on the metal surface. Also,
THAs are good chelating ligands. This is due to their complexation
behaviour with metal cation to form thermodynamically stable
five-membered chelate complexes [20]. Their high complexation
ability also supports the formation of a stable complex with iron
ions and adsorb on the metal surface.Therefore, hydroxamic acid
can be applied as a green alternative corrosion inhibitor with high
inhibition efficiency at low concentrations. Recently, few research
works suggested that hydroxamic acid is an efficient corrosion in-
hibitor for metals and alloys [14,21,22].Therefore,the hydroxamic
acids and their derivatives can be considered as environmentally
friendly alternatives to traditional toxic corrosion inhibitors. In ad-
dition to this, THA–H, THA–Br and THA–OCH3 easily formed the
complex with iron ions. These complex effectively adsorbed on the
steel surface. The formed complex are not toxic. This is due to the
active functional groups and heteroatoms in the THA–H, THA–Br
and THA–OCH3 are linked with the iron ions. As a result, all func-
tional groups are blocked and toxic properties of THA–H, THA–Br
and THA–OCH3 dramatically reduced. In addition to this, the cor-
rosion inhibitors rigid adsorbed on the steel surface and strongly
linked with metal surface. Consequently, the toxic properties of
THA–H, THA–Br and THA–OCH3 are importantly reduced. There-
fore, it was suggested that the THA–H, THA–Br and THA–OCH3 are
environmental-friendly and more efficient corrosion inhibitor for
steel metallic material.Therefore, they have been used in current
research as corrosion inhibitors for mild steel in an acidic medium
(1M HCl). Modern experimental methods such as gravimetric anal-
ysis, electrochemical impendence spectroscopy (EIS), and poten-
tiodynamic polarization (PDP) have been used for the characteri-
zation of inhibition efficiency and electrochemical behaviour. Sur-
face analysis techniques such as X–ray diffraction analysis (XRD),
scanning electron microscopy (SEM) and electron dispersion X–ray
spectroscopy (EDS) have also been performed to identify the sur-
face morphology. Theoretical calculations such as the density func-
tional theory (DFT) and Monte Carlo (MC) simulation have been
applied to describe the nature of adsorption, possible adsorption
2.2. Synthesis of inhibitors
All chemicals were purchased from a commercial chemical
company (MERCK grade, Raipur, INDIA). All chemical reactions
were monitored by pre-coated silica gel 60 F254 plates (250 mm
layer thickness thin layer) chromatography (TLC) analysis. Melting
points of all synthesised compounds were determined using a cap-
illary melting point apparatus (METTELAR TOLEDEO, Germany).
The N–hydroxybenzothioamide (THA–H) was synthesized re-
lated to the following procedures: 2.00
g (12.9 mmol) N–
hydroxybenzimidoyl chloride and 9.26 g (38.6 mmol) Na2S were
added in 172 mL (12.9 mmol) triethylamine solvent. Next, this mix-
ture was stirred for one hour. After one hour, 5 M HCl was added
to the formed mixture. As a result, the green solid crystal (THA–
H) was formed. Finally, the THA–H crystal was separated using the
separation funnel. The reaction yield of THA–H was 65%.
4–Bromo–N–hydroxybenzothioamide (THA–Br) was prepared
according to the following procedures: 0.43 g (5.5 mmol) 4–
bromo–N–hydroxybenzimidoyl chloride and 1.3 g (5.5 mmol) Na2S
were added in trimethylamine/water solvent system. The mixture
was stirred for one hour to form a yellow solid crystal. Finally, the
yellow crystal THA–Br was separated using the separation funnel.
The reaction yield of THA–Br was 48%.
N–Hydroxy–4–methoxybenzothioamide (THA–OCH3) was syn-
thesized related tothe following steps: 0.10 g (0.54 mmol) N–
hydroxy–4–methoxybenzimidoyl chloride, 0.39 g (1.6 mmol) Na2S
and 0.075 mL (0.54 mmol) trimethylamine were mixed and stirred
for one hour. As a result, the solid yellow crystal of THA–OCH3 was
formed. Finally, the yellow crystal was obtained with a 52% reac-
tion yield [23]. Additionally, Table 1 illustrates the structure, name,
and analytical spectral data of THAs derivatives.
2.3. Weight loss analysis
2.5 × 1.0 × 0.1 cm3 (H×W×T) mild steel specimens wereused
in the present investigation, and all experimentswere carried out
according to the standard procedure of ASTM G1 [24]. Weight
loss measurement was applied atthe various concentration (200,
250 and 300 ppm) of inhibitorin 1M HCl with the different tem-
peratures (from 298 K to 328 K). Before experimenting, the mild
steel samples were dipped carefully in the electrolytic solution
2

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Table 1
Structure, name and analytical spectral data of THAs derivatives
S.N.
IUPAC name
Structure
Analytical and spectral data
1
N-hydroxybenzothioamide
Mol. Formula: C7H7NOS
Yield (65%);
mp: 40 ( 1)°C;
1H NMR (400 MHz) δ 7.2 (d, 1 H), 7.9 (d 1H), 10.2 (m, 5 H)
IR (film); 3175, 1683.84, 1477.85, 1290.05, 1144.78 cm-1
(THA-H)
2
4-bromo-N-
Mol. Formula: C7H6BrNOS
hydroxybenzothioamide
Yield (48%);
mp: 108( 2)°C;
1H NMR
(400 MHz) δ 6.2 (d, 1H), 6.6 (m, 2H), 7.0 (2H)
IR (film); 3239.62, 3024.90, 1626.55, 1431.60, 1225 cm-1
(THA-Br)
3
4-methoxy-N-
Mol. Formula: C8H9NO2S
hydroxybenzothioamide
Yield (52%);
mp: 97 ( 1)°C;
1H NMR (400 MHz) δ 3.8 (s, 2 H), 6.1 (d, 1 H), 7.1 (s, 2 H); 7.5 (s
2H), 9.2 (s 1H)
IR (film); 3322.18, 1600.69, 1485.89, 1274.33, 1151.35 cm-1
(THA-OCH3)
Table 2
Weight loss parameters of THAs derivatives.
Corrosion Rate, ρ, (mg cm-2 h-1
)
Inhibition Efficiency (%η)
Inhibitor
Conc. (ppm)
298K
308K
318K
328K
298K
308K
318K
328K
THA-OCH3
0.0
1.080
0.188
0.143
0.095
0.062
1.135
0.193
0.151
0.110
0.077
1.280
0.255
0.209
0.173
0.111
1.905
0.346
0.294
0.251
0.201
2.166
0.444
0.374
0.291
0.241
2.311
0.502
0.432
0.356
0.306
4.046
0.755
0.656
0.601
0.558
4.160
0.888
0.773
0.635
0.575
4.313
0.962
0.841
0.696
0.638
5.170
1.396
1.322
1.101
0.982
5.372
1.692
1.574
1.2961
.102
-
-
-
-
150
200
250
300
0.0
82.59
86.95
91.21
94.26
-
81.83
84.57
86.82
88.45
-
81.34
83.79
85.14
86.20
-
73.00
74.43
78.70
81.01
-
THA-H
THA-Br
150
200
250
300
0.0
82.99
86.69
90.30
93.21
-
79.50
82.73
86.56
88.87
-
78.60
81.41
84.738
86.17
-
68.50
70.70
75.87
79.48
-
5.520
1.755
1.643
1.3651
.170
150
200
250
300
80.07
83.67
86.48
91.32
78.28
81.30
84.59
86.75
77.70
80.52
83.868
85.20
68.20
70.26
75.28
78.80
and inhibited solutions for 6 hours. After 6h, the specimen was
carefully taken out from the solutions and then the metal speci-
men was dried with distilled water and acetone. Finally, the dried
metal sample was weighed three times with METALLER TOLEDO
electronic equipment. To study the effect of temperature, the ex-
periments were performedat different temperatures (298K –328K)
in solutions with different concentrations. The obtained weight
loss results are presented in Table 2. The corrosion rate (ρ) at
mg/cm2×hour was calculated from Eq. 1 [25]:
where, wo and wi represent the weight loss of mild steel sample
in the corrosive and inhibited medium, respectively.
2.4. Electrochemical analysis
Echem analyst 5.0 (Gamy potentiostat software G–300) was
used in current research to perform the electrochemical measure-
ment [26]. Three–electrode sample was designed to do all electro-
chemical experiments at 298K temperature. Three–electrode sam-
ple contained SCE (reference electrode), mild steel (working elec-
trode), and platinum (counter electrode). The bottom side of mild
steel (1 cm2) electrodes were dipped into the electrolytic solution,
and the remaining parts of mild steelwere covered with the epoxy
resin of adhesive Araldite. The potentiodynamic polarization (PDP)
was recorded at the scan range 1m V s–1and the potential range
ꢀW
ρ =
(1)
At
where, ࢞W, A, and t represent the weight loss (mg/cm2×hour), the
metallic surface area (at cm2), and time (6h), respectively.
The inhibition efficiency (%η) was calculated according to
Eq. 2 [25]:
of
250 mV. It was used 1 mV/s polarization rate. This is due to
this rate is standard in corrosion inhibition investigations. To use 1
mV/s polarization rate supports to obtain more accurate results in
the polarization analysis.The inhibition performance was estimated
wo − w
%ηWL
=
i x 100
(2)
wo
3

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
according to Eq. 3 [21]:
1
1
iocorr − ii
χ = (IE + EA) = (−EHOMO − ELU MO
)
(9)
%ηPDP
=
corr x 100
(3)
2
2
iocorr
χFe − χinh
where, iocorr and ici orrrepresent the corrosion current densities in the
inhibitor absence and presence solutions, respectively.
ꢀN =
(10)
2 ηFe + η
(
)
inh
EIS analyses were applied from 100KHz to 0.01 MHz frequency
range with an amplitude of –10 mV. Then, the Nyquist curves for
the MS corrosion in 1M HCl solution were found with the best
equivalent circuit model (Fig. 5), which consists of a constant phase
(CPE) element or a charge transfer resistance (Rct) and solution re-
sistance (Rc).The inhibition efficiency (%ηEIS) of inhibitor was calcu-
lated from the values of charge transfer resistance using Eq. 4 [21].
where, the electronegativity was 4.88 eV for Fe has been applied
for the calculation of electron transfer (࢞N).
2.7. Monte Carlo simulation analysis
The Monte Carlo simulation study is an interesting technique,
which can provide us with more information about the interfa-
cial interactions between the steel surface and studied inhibitors
molecule [29].Therefore, the present investigation was conducted
to study the interaction with the aqueous phase (200 H2O, 5 H3O+,
and 5 Cl–), and a steel surface Fe(110) using the adsorption loca-
tor module in the Materials Studio 2017 software [30]. The most
suitable stable adsorbed configuration of the studied molecule on
the surface of Fe (110) is related to the higher negative adsorp-
tion energy values. All components of the simulation system were
optimized via the Forcite module using the COMPASS force field
Rict − Ro
%ηEIS
=
ct x 100
(4)
Rict
where, Rictand Rcot represents the charge transfer resistance in the
inhibited and uninhibited 1M HCl.
2.5. Surface analysis
SEM analysis was performed to analyse the surface morphology
of the metal specimen while EDS was used to find the composi-
tion of the metal surface [27]. In the present work, the mild steel
sample has been employed for surface analysis after 4h immersion
time. In the surface analysis, the 300 ppm inhibitor was used. After
an immersion time, the metal sample was taken out and washed
three times with distilled water and acetone. Next, the dried metal
sample was prepared for SEM–EDS analysis.1S kV accelerating volt-
age was performed to take the image with 500X magnification
and the spectrum of SEM–EDS. XRD analysis is another advanced
tool for surface characterization, which reveals the nature of the
deposit protective layer on the metal surface. The specimens ob-
tained from the weight loss measurement were scratched with a
soft hand using a new knife. PAN analytical 3 KW–expert powder
multifunction, Netherland X–ray diffractometer was used for XRD
analysis applying CuKα radiation with an angular range of 60°>
2θ> 0° at 298K.
˚
[27].The Fe (110) crystal was built with a 30 A edge to achieve
enough depth then enlarged to create a supercell (12 × 12).
3. Results and discussions
3.1. Weight loss measurement: Effect of concentrations
It is clear from the obtained values in Table 2 that when the
concentration increased, the value of corrosion rate decreased and
the value of inhibition efficiency rose. The main reason for this is
that the greater number of inhibitor molecules adsorbed on the
metal surface and covered a large part of the metal surface.These
findings are similar to previous research work [31]. Three inhibitors
used in the present investigation, namely THA–H, THA–Br and
THA–OCH3, which have H, Br and OCH3 substituents, respectively.
All inhibitors show very high efficiency and the order of inhibition
efficiency was found as follows: THA–Br ˂ THA–H ˂ THA–OCH3.
Among three inhibitors,THA–OCH3exhibited the highest inhibition
efficiency with 94.4% at 300 ppm and at 298K according to weight
loss measurement. The reason for the superior inhibition nature
of THA–OCH3 is the methoxy group.This is due to the methoxy
group is the good electron–donating group, and can easily donate
the electrons to the empty d–orbitals of iron.
2.6. Quantum chemical calculation analysis
Quantum chemical calculations based on DFT were applied
to identify the efficiency of organic molecules and observe the
quantum chemical parameters, which are responsible for inhibi-
tion characteristics [28]. In the present investigation, Gaussian 09
software was employed for DFT calculation with optimized struc-
tures of inhibitors. Li–Yang–Cross non–local correlation function
(B3L4P) and 21G based hybrid sets were applied to determine the
electronic properties of the inhibitors:THA–H, THA–Brand THA–
OCH3.The electronic affinity and ionization energy were deter-
mined using the energy of Frontier molecular orbitals (HOMO and
LUMO).Additionally, other important parameters such as the dipole
moment, global hardness, energy gap, electronegativity and frac-
tion of electron were also calculated from the Eqs. 5–10 [22]:
3.2. Adsorption mechanism
The adsorption isotherm is a very important tool to describe
the adsorption process between the inhibitor molecule and the
metal surface. In the present investigation, the Langmuir adsorp-
tion isotherm model fits were best to justify the adsorption. Usu-
ally, the Langmuir model expressed as given in Eq. 11 [32].
C
1
=
+ C
(11)
θ
Kads
IE = −EHOMO
(5)
Eq. 11 shows the correlationbetween the inhibitor concentra-
tion (Cinh) and the surface coverage (θ) on mild steel in 1M HCl at
different concentrations. Slope and intercept were calculated from
Cinh/θvs Cinh plots.The graph of Cinh/θ vs Cinh (Fig 1) obtained
at room temperature gives straight lines, revealing that Langmuir
isotherm is the best fitting. The Kads signifies resemblance among
the adsorbate and adsorbent system. The higher values of Kads for
selected inhibitors indicates the good adsorption property of the
molecule on the metal surface [5].The values of the adsorption
EA = −ELU MO
(6)
1
1
η = (IE − EA) = (−EHOMO + ELU MO
)
(7)
(8)
2
2
1
σ =
η
4

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 1. Langmuir adsorption plots for studied inhibitors (a) THA-H, (b) THA-Br and, (c) THA-OCH3 in 1M HCl solution at different temperatures.
Table 3
constant (Kads) were determined by the inverse value of the inter-
cept (Fig. 1). Generally, the higher value of the adsorption constant
showed that the adsorption efficiency of THA–H, THA–Br and THA–
OCH3 on the mild steel surface.It is also indicated that the selected
inhibitors are more efficient at 298K– 328K ranges. Next, the val-
ues of Kads and Gibbs free energy for adsorption (࢞Gads) were cal-
culated from Eq. 12 [32-34]:
The values of Gads and Kads for the MS in the inhibitor absence and pres-
ence of 1 M HCl at various temperatures.
-1
Inhibitor
THA-H
Temp. (K)
R2
Kads (M-1
)
࢞Gads (kJ mol
)
298
308
318
328
0.998
0.998
0.998
0.997
340
350
280
260
-24.38
-25.28
-25.51
-26.11
THA-Br
Average
200
-25.32
-23.07
-24.52
-25.21
-26.31
298
308
318
328
0.999
0.998
0.997
0.998
ꢀGads = − − 2.303RTlog 55.5K
(12)
(
)
ads
260
250
280
where R, T, and Kads represent the gas constant, temperature at K,
and absorption constant, respectively, 55.55 is the molar concen-
tration of water at mol/L.
Average
260
-24.78
-23.72
-24.42
-24.75
-26
THA-OCH3
298
308
318
328
0.978
0.998
0.998
0.977
250
All the calculated data are summarized in Table 3. The nega-
tive value of Gads shows the spontaneity of the absorption pro-
cess of the inhibitor on the mild steel surface. A previous inves-
tigation suggests that a ࢞Gads less than -20 kJ/mol indicates that
the adsorption mechanism is characterised by a physical nature; if
it is more than -40 kJ/mol, adsorption occurs chemically [35-40].
In the present study, it was found that ࢞Gads values were –25.32,
–264.78, and –24.72 kJ mol–1 for THA–H, THA–Br, and THA–OCH3,
respectively (Table 3), suggesting the studied inhibitors are mixed
type. The negative value of Gads (adsorption energy) shows that the
studied inhibitors adsorb spontaneously on the metallic surface.
In comparison to previous research works of [40-42], the studied
inhibitors i.e. thiohydroxamic acid derivatives follow the physico-
chemical behaviour as their values are more than 20 kJ mol–1 and
210
250
Average
-24.72
3.3. Electrochemical characterization
3.3.1. Potentiodynamic polarization(PDP) measurements
PDP analysis is applied to find out the adsorption property of
THAs derivatives that is THA–H,THA–Br andTHA–OCH3. Fig. 2 indi-
cates the PDP plots for the inhibited system. Tafel indices such as
the cathodic, anodic Tafel slopes (βc, βa), corrosion current den-
sities (icorr), and corrosion potential (Ecorr)were obtained from the
PDP study (Table 4).Table 4 reveals that the increase in the con-
less than 40 kJ mol–1
.
5

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 2. Tafel plots for studied (a) MS/1M HCl/THA-OCH3, (b) MS/1M HCl/THA-H and, (c) MS/1M HCl/THA-Br systems.
Table 4
Tafel polarisation parameters for THAs derivatives in 1 M HCl.
Inhibitors
Concentration (ppm)
Tafel polarisation parameters
-1
-1
-1
icorr (
Ecorr (mV)
βa
βc
θ
%η
cm
A
)
(mV dec
)
(mV dec
)
THA-OCH3
Blank
200
102.2
24.02
17.19
10.15
-513
-504
-499
-518
270.2
304.8
-
-
218.16
215.32
206.41
285.31
278.65
269.41
0.764
0.831
0.900
76.49
83.18
90.06
250
300
THA-H
THA-Br
200
250
300
25.13
18.97
12.86
-524
-513
-510
131.1
145.2
113.9
222.3
131.9
162.2
0.754
0.814
0.874
75.41
81.43
87.41
200
250
300
25.87
19.64
13.45
-510
-508
-522
122.5
118.7
110.3
211.1
198.7
227.6
0.754
0.807
0.868
75.41
80.78
86.83
centration of inhibitors promotes the enhance in the value of cor-
rosion current density, which indicates that the inhibitor molecules
are deposited on the mild steel surface effectively and protect the
steel surface from further corrosive attack. It is also indicated from
observed data that the studied corrosion inhibitors formed the po-
larization block for the cathodic and anodic electrochemical reac-
tions on the metal surface. The formation of a thin protective layer
on the metal surface is mainly responsible for enhancing in the
inhibition performance.Therefore, the presence of THAs molecules
enhances the % ɳPDP. The order of inhibition efficiencies measured
by PDP analysis are in agreement with the results of the weight
loss study; THA–Br ˂ THA–H ˂ THA–OCH3.It was indicated related
to the previous research articles [51-53] that if the value of Ecorr
between the inhibited and corrosive solution was greater than 85
mV, the inhibitor is usually anodic or cathodic. If Ecorr value is
less than 85 mV,the inhibition nature of the inhibitor is identified
as amixed type inhibitor [43-45]. In this investigation work, Ecorr
value was less than 85 mV for the three THA derivatives, demon-
strating that these inhibitors are the mixed type. The values of βc
and βa given in Table 4 show that the values of βc denote the ca-
thodic property of the inhibitors more than the values of βa. This
mixed type is mainly due to the cathodic nature of inhibitors, re-
vealing that the inhibitor molecules are predominantly adsorbing
on the cathodic site of the mild steel surface.
3.3.2. Electrochemical impedance spectroscopy (EIS) measurements
EIS measurement is a very effective tool to determine the ad-
sorption and interfacial properties of inhibitor molecules [46]. EIS
6

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Table 5
Impedance parameters for MS in 1 M HCl with and without THAs derivatives.
Inhibitor
Conc. (ppm)
Rs (ꢁ cm-2
)
Rct (ꢁ cm-2
)
Cdl (μF cm-2
)
θ
%η
THA-OCH3
Blank
200
250
300
200
250
300
200
250
300
1.566
1.013
1.470
1.988
0.982
1.503
0.976
1.012
0.969
1.004
06.52
25.43
78.22
120.4
22.89
73.51
163.7
75.54
122.4
171.6
127.4
90.53
83.33
66.31
82.58
76.65
66.32
82.22
75.31
64.48
-
-
0.744
0.916
0.950
0.715
0.911
0.948
0.914
0.937
0.940
74.36
91.67
95.08
71.51
91.13
94.85
91.37
93.67
94.04
THA-H
THA-Br
Fig. 3. Nyquist plot for studied (a) 1 M HCl (b) MS/1M HCl/THA-OCH3, (c) MS/1M HCl/THA-H and, (d) MS/1M HCl/THA-Br systems.
wasapplied to characterize THA–H, THA–Br, and THA–OCH3 in-
hibitors with different concentrations and temperatures ranging
from 298–328 K. The Rs (solution resistance) Rct (charge transfer
resistance) and Ri (inductive resistance) have been calculated and
the corresponding values are summarized in Table 5. The corre-
sponding Nyquist plots obtained from EIS are represented in Fig. 3.
The Nyquist plots are shown in Fig. 3 were depressed semicircle
rather than a true semicircle, which is associated with inhomo-
geneity and roughness of the metal surface [47]. It is next stressed
fact is that the Nyquist plots of the present study were one loop,
which is a capacitive loop at higher frequencies, revealing the effi-
cient adsorption characteristic of THAs derivative on the metal sur-
face.In the equivalent circuit model, the double–layer capacitance
(Cdl) was replaced by a constant phase element (CPE) to get a more
precisely optimized fitting. The equivalent circuit fitting model is
shown in Fig. 5. Eq. 13 represents the fitting of all experimental
data and can be represented as [48]:
ꢀ
ꢁ
ꢂ
ꢃ−1
1
N
)
ZCPE
=
jw
(13)
(
Y0
where,CPE represents capacitance, resistance, and impedance.
On the other hand, the value of Cdl was calculated for the corro-
sive and inhibited system. Eq. 14 indicates the double layer capaci-
tance for the corrosion of MS in 1 M HCl. Double layer capacitance
was estimated from the expression given in Eq. 14 [48]:
n−1
)
Cdl = Yo
w
(14)
(
max
where,Yo and n are the exponent and phase shift, respectively.
From Table 5, it is noted that the values of Cdl decreased gradu-
ally with the increase in the concentration, indicating the substitu-
tion of water molecules by the inhibitor molecule onthe metal–HCl
interface. In the present work, the values of Cdl were found. Sim-
7

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 4. Bode plots for studied (a) MS/1M HCl/THA-OCH3, (b) MS/1M HCl/THA-H and, (c) MS/1M HCl/THA-Br systems.
presence of protective layers of inhibitor molecules deposited on
the metal surface and formed a protective layer on the mild steel
surface and protect from further corrosive attack [51]. The EDS re-
sults suggested the formation of protective film on the metal sur-
face with the presence of inhibitor molecules [52].Values of ele-
mental composition (wt %) corresponding to EDS spectra are sum-
marized in Table 6. From Table 6, it can be seen that in the absence
of inhibitors a higher concentration of chloride ion (6.11 wt %) re-
ported. In comparison,the chloride concentration of 0.87%, 0.91%,
and 1.23 % was found with the presence of THA–H, THA–Br and
THA–OCH3, respectively. These results revealed that the presence
of high corrosive chloride ions on MS metal surface is responsible
for corrosion. The amounts of chloride ions were importantly re-
duced by the presence of inhibitor molecules. Therefore, both SEM
and EDS analyses confirmed the weight loss and electrochemical
experiments.
Fig. 5. Equivalent circuit model used for fitting the Nyquist curves
ilarly, Fig. 4 reveals the corresponding bode phase angle plots for
THAs inhibitors. In the presence of THA derivatives, the phase an-
gle value was greater than the value of 1M HCl solution (Fig. 4),
which confirmed the following facts: the more negative phase an-
gle value and the more capacitive behaviour support the increasing
smoothness of mild steel surface with the presence of inhibitor.
The above results suggested the efficient adsorption of inhibitors,
decreasing the roughness of the metal surface [49,50]. Finally, the
value of %ηEIS are were accounted for 94.85%, 94.04% and 95.08%
for THA–H, THA–Br and THA–OCH3, respectively, suggesting the
good correlation with the observed data of weight loss experiment.
3.4.2. XRD measurements
XRD is an advanced and prominent tool for the quantitative
analysis, phase characterizations, and determination of the crys-
talline nature of materials. Before doing XRD analysis, the surface
of the metal sample was cleaned. Fig. 8 represents the XRD pat-
tern of metal samples obtained in 1 M HCl solution without and
with inhibitor at the optimum concentration. It is stressed from
the spectrum of Fig. 8a that the corrosion product included mainly
oxide of iron accumulates on the metal surface in the corrosion
solution. In the scratched sample with 1 M HCl, the peaks have
appeared at 2θ = 36.8°, 53.2° and 53.5°, indicating the presence
of iron metal oxide such as FeOOH. Fe3O4 on the metal sample
while the value of spectra was 2θ = 55.1° with the presence of
THA–OCH3 (300 ppm), showing the decrease in the percentage of
iron oxyhydroxide (FeOOH) and ferric oxide (Fe3O4) [53]. From the
above XRD analysis,it is suggested that the formation of iron oxide
3.4. Surface characterization
3.4.1. SEM–EDS measurements
The SEM micrographs and EDS spectra of mild steel surface in
the 1 M HCl without and with the inhibitors at optimum con-
centration are shown in Fig. 6 a–d and Fig. 7 a–c, respectively.
Fig. 6 (a) exhibited the corrosion destruction on mild steel surface
in the absence of inhibitors (1 M HCl), which may be due to the at-
tack of corrosive chloride ion. Moreover, Fig. 6 (b–d) demonstrated
the less pitch, clean, and plane surface, which may be due to the
8

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 6. SEM microphotographs for (a) MS/1M HCl, (b) MS/1M HCl/THA-H, (c) MS/1M HCl/THA-Br and, (d) MS/1M HCl/THA-OCH3 systems.
Table 6
EDS parameters for THAs derivatives in 1 M HCl.
Composition (wt %)
Medium
Fe
Cl
O
S
C
N
MS/1M HCl system
92.58
89.79
90.41
90.74
6.11
0.87
0.91
1.23
0.84
1.69
2.71
2.54
0.31
1.12
0.97
0.92
0.16
6.02
4.52
4.13
-
MS/THA-OCH3/1M HCl system
MS/THA-H/1M HCl system
MS/THA-Br/1M HCl system
0.51
0.48
0.44
on the metal surface was significantly blocked with the formation
of protective film on the metal surface.
lower electron density (Fig. Fig. 9 c, f & i) for THA–H, THA–Br, and
THA–OCH3,respectively.
It is underlined fact from the amountsof HOMO (Table 7) that
the inhibition behaviour of THA–OCH3 is higher than that of other
thiohydroxamic acid derivatives. Experimental investigations also
show that the inhibition performance of THA–OCH3 is higher as
compared to other derivatives. This is due to the presence of
electron–donating methoxy substituent. Similarly, a low energy gap
(࢞E) between the LUMO and HOMO suggested the more reactive
and adsorption property of inhibitor molecules. The following as-
pect is that the positive values of ࢞N for all support that the elec-
tron flow from organic molecule to the metallic surfaceis easy and
effective [27,31]. The electron transfer from the inhibitor molecules
to the metal surface is also supported by the values of other pa-
rameters such as the electron fraction, global stiffness, softness,
electromagnetism and dipole moments.
3.5. Quantum chemical calculations
To performthe density functional theory (DFT), the optimiza-
tion structure, quantum chemical behaviour and reactivity of se-
lected inhibitors can be successfully studied [54]. Important pa-
rameters such as the energy gap (࢞E), frontier molecular energy
(highest occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO)), global hardness, softness, electron
fraction, electronegativity,etc.were calculated from the DFT calcu-
lation. All these parameters can explain the absolute properties
of any inhibitors such as nucleophile nature, reactivity, adsorp-
tion nature [55].The frontier molecular orbitals can be used to ex-
plain the electron–donating and electron–accepting properties of
a molecule.Typically, the lower the value of LUMO indicates the
more inhibition effective, and the higher the value of HOMO is
responsible for the good electron–donating property [56,57]. The
values of energy corresponding to HOMO and LUMO are shown
in Table 7. Fig. 9 (b, e & h) shows that the HOMO is more lo-
calized towards the electron–rich species in the THA–H, THA–Br,
and THA–OCH3, respectively.The electron density is more towards
the p–electrons and unbounded electron containing atoms while
LUMO is more localized towards the more electronegative atom
(S and O), heteroatom-containing unsaturated carbonylbond and
3.6. Fukui indices
The Fukui indices describe the most favourable sites for ad-
sorption in the optimised structure of THA–H, THA–Br and THA–
OCH3 molecules. The values of local molecular reactive proper-
ties of THA–H, THA–Br and THA–OCH3 were found and the ob-
tained data are represented in Tables 9,10 and 11. The most nu-
cleophilic ( fk−high), electrophilic ( fk+high) and neutral ( fk0high) at-
tacking sites in the THA–H, THA–Br and THA–OCH3 molecules was
9

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 7. EDS spectral images for (a) MS/1M HCl, (b) MS/1M HCl/THA-H, (c) MS/1M HCl/THA-Br and, (d) MS/1M HCl/THA-OCH3 systems.
Table 7
DFT parameters of neutral forms of THAs derivatives
THAs
EHOMO (eV)
ELUMO (eV)
ꢀE (eV)
IE
EA
η
χ
ω
σ
࢞N110
μ (Debye)
THA-OCH3
THA-H
-5.99
-6.16
-6.32
-1.47
-1.66
-1.92
4.52
4.5
5.99
6.16
6.32
1.47
1.66
1.92
2.26
2.25
2.2
3.73
3.91
4.12
15.721577
17.1991125
18.67184
0.44247788
0.44444444
0.45454545
0.24115044
0.20222222
0.15909091
3.6519
3.6519
3.6519
THA-Br
4.4
fk− = P N − P N − 1
(16)
(17)
k( )
(
)
k
fk0 = P N + 1 − P N − 1
(
)
(
k
)
k
where Pk(N + 1), Pk(N) and Pk(N − 1) are anionic, neutral and
cationic molecules, respectively.
Table 9, 10 and 11 compared the most nucleophilic ( fk−high),
electrophilic ( fk+high) and neutral ( fk0high) attacking atoms in the
THA–H, THA–Br and THA–OCH3.It is clear from Table 9 that the
11C and 14S atoms are most nucleophilic; the 2C and 17C are most
electrophilic; the 2C, 14S and 17C atoms are most neutral for THA–
H. Table 10 indicates that the 10C and 13S are more nucleophilic;
the 1C and 5C atoms are more electrophilic; the 1C, 5C and 13S
are more neutral sites for THA–Br. It is noted in Table 11 that the
10C and 13S are more nucleophilic; the 4C and 5C atoms are more
electrophilic; the 4C, 5C and 13S are more neutral attacking for
THA–OCH3. As concluded, the more nucleophilic and electrophilic
attacking sites in the inhibitor molecules are attributed to the rise
in the inhibition performance.
Fig. 8. XRD spectrafor (a) MS/1M HCl and, (b) MS/1M HCl/THA-OCH3(300 ppm)
systems.
measured through the Fukui function. In the Fukui analysis, the
natural population analysis (NPA) was found and the resulted data
3.7. Molecular electrostatic potentials
were performed to derivate the Fukui indices ( fk+, fk− fk0) of the
,
THA–H, THA–Br and THA–OCH3 by Fukui functions, as indicated in
The molecular electrostatic potentials (MEP) of the THA–H,
THA–Br and THA–OCH3were described in Fig. 10. MEP regions rep-
resented more reactive sites in the optimised structure of selected
Equations 15-17 [58]:
fk+ = P N + 1 − P
N
k( )
(15)
(
)
k
10

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 9. The optimized structures (left), HOMO (middle) and LUMO (right) for thiohydroxamic acid derivatives (A) THA-H, (B) THA-Br, and (C) THA-OCH3, respectively.
Fig. 10. Molecular electrostatic potential (MEP) of (A) THA-H, (B) THA-Br, and (C) THA-OCH3, respectively.
inhibitor molecules. These most reactive regions indicated the nu-
cleophilic and electrophilic attack positions in the optimised struc-
ture. In the resulted MEP, the red (negative) and blue (positive)
regions revealed the nucleophilic and electrophilic reactivity po-
sitions in the THA–H, THA–Br and THA–OCH3 molecules, respec-
tively. It is also indicated that the obtained MEP of selected in-
hibitors were located at more negative regions, confirming that the
nitrogen, oxygen and sulfur atoms make inhibitor to become more
effective.
3.8. Monte Carlo simulation
To deep understand the adsorption behaviour of the inhibitors
on the metal surface, Monte Carlo simulations were performed on
11

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 11. The most stable low energy configuration (left side view & right top view) for the adsorption of studied molecules (a) THA-H, (b) THA-Br and (c) THA-OCH3 on Fe
(110) surface obtained using MC simulation.
Table 8
Table 10
Adsorption energies for studied inhibitors on Fe(110)/ in-
hibitor system obtained using the Monte Carlo simulation
(all units in kcal/mol).
Fukui indices of THA-Br.
№
Atom
fk−
fk+
fk0
1.
C
0.0003
0.0011
0.0012
0.0001
0
0.5016
0.2509
0.0044
0.0007
0.0041
0.2398
0
System
Eads (inh)
Eads (H2O)
2.
C
0.0078
Fe(110)/THA-OCH3
Fe(110)/THA-H
Fe(110)/THA-Br
-3599.70
-3588.32
-3572.82
-10.27
-12.75
-13.76
3.
C
0.0002
4.
C
0.0081
5.
C
0.4795
6.
H
H
H
H
C
0
0
7.
0.0001
0.0003
0
0
0
Table 9
Fukui indices of THA-H.
8.
0
0.0001
0
9.
0
10.
11.
12.
13.
14.
15.
16.
17.
0.0577
0.0057
0.0001
0.9107
0.0007
0.01
0.0004
0.029
0.0029
0.0001
0.4554
0.0004
0.005
0.0062
0.001
№
Atom
fk−
fk+
fk0
N
H
S
0.0001
0
1.
C
0.0002
0.001
0.0011
0.0001
0
0.0085
0.4888
0.0076
0.0009
0.0005
0
0.0043
0.2449
0.0044
0.0005
0.0003
0
0
2.
C
O
H
C
0
3.
C
0
4.
C
0.0121
0
0.0003
0.002
5.
C
Br
6.
H
H
H
H
H
C
0
7.
0.0001
0.0003
0
0.0001
0
0.0001
0.0001
0
8.
9.
0
crease the surface coverage area. This adsorption model can be at-
tributed to becoming the strong interaction between the phenyl
rings of thiobenzohydroxamic acid and the metal surface. More-
over, these results also responsible for the strong interaction be-
tween the S, O atoms, several π–electrons and metal surface,
which can offer the electron densities to the unoccupied–orbitals
of iron to form coordinate bonds. Therefore, Monte Carlo simu-
lations results confirmed the adsorption of the studied inhibitors
and forming a stable barrier film on Fe (110) surface [59].The ab-
solute value of the adsorption energies of the studied inhibitors
on the simulated system follows the order: THA–OCH3> THA–H
> THA–Br. This order also supports by the experimental and DFT
results. In all examined systems, the adsorption energies of se-
lected inhibitors were far higher than that of water molecules.
This observation reflects the possibility of gradual substitution of
H2O molecules with inhibitor molecules on the Fe (110) surface
[30,31,60].
10.
11.
12.
13.
14.
15.
16.
17.
0
0
0
0.0574
0.0055
0.0001
0.9109
0.0007
0.0103
0.0122
0.0052
0.001
0
0.0313
0.0033
0.0001
0.4555
0.0004
0.0051
0.2497
N
H
S
0.0002
0
O
H
C
0
0.4872
Fe (110) surface taking into consideration the experimental condi-
tions (the presence of the studied molecules in hydrochloric acid
solution). The most stable low–energy adsorption forms of in-
hibitors on the Fe (110) / 200 H2O molecule system are shown
in Fig. 10. The adsorption energies for studied inhibitors on Fe
(110)/ inhibitor systemare listed in Table 8. It could be seen from
Fig. 11 that the investigated inhibitor molecules were preferentially
oriented parallel on the Fe surface to maximize contact and in-
12

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
Fig. 12. Plausible adsorption modes of a) THA-H, b) THA-Br and, c)THA-OCH3 molecules on MS surface.
Table 11
face with the presence of selected inhibitors [59,61].Fig. 12 (a–
c) demonstrated the plausible adsorption mechanism of THA–H,
THA–Br, and OCH3. Usually, inhibitor molecules acted as a Lewis
base. The electron-donating tendency of inhibitorson the metal
surface behaves similar to a Lewis acid. This is due to the pres-
ence of vacant d orbitals of metal cations.It also stressed that the
HOMO of inhibitor molecules and LUMO of metal cation interact
with each other to form a stable bond [7,62,63].
Fukui indices of THA-OCH3.
№
Atom
fk−
fk+
fk0
1.
C
0.003
0.0012
0.0012
0.0002
0.0001
0
0.0075
0.0039
0.0007
0.0043
0.2258
0.2633
0
2.
C
0.0002
3.
C
0.0073
4.
C
0.4514
5.
C
0.5266
6.
H
H
H
H
C
0
7.
0.0001
0.0003
0
0
0
8.
0
0.0002
0
9.0
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
0
5. Conclusion
0.057
0.0056
0.0001
0.9104
0.008
0.0107
0.012
0
0.0002
0.0286
0.0028
0
N
H
S
0
The present investigation reveals the inhibition properties of
THAs derivatives included N–hydroxybenzothioamide (THA–H), 4–
bromo–N–hydroxybenzothioamide (THA–Br), and 4–methoxy–N–
hydroxybenzothioamide (THA–OCH3) towards corrosion of mild
steel in 1 M HCl. The various experimental and computational
techniques were performed for the determination of the inhibition
nature of THAs derivatives. The following key points can be con-
cluded from the present analysis:
0
0
0.4552
0.0004
0.0053
0.0062
0.0032
0
O
H
C
0
0
0.0003
O
C
0.0064
0
0
0
0
0
H
H
H
0
0
0
0
0
0
1 The inhibition efficiencies of THA–OCH3, THA–H, and THA–Br
was 94.26%, 93.21%, and 91.32%, respectively, according to WL
analysis.
4. Adsorption mechanism
2 The presence of –OCH3 substituent group is responsible for
higher inhibition efficiency of THA–OCH3 onthe mild steel sur-
face.
The adsorption energies of the THAs on the MS surface fol-
lows the order: THA–OCH3> THA–H > THA–Br. This order is also
supported by the result of weight loss and electrochemical anal-
ysis. Moreover, these results can be ascribed to the strong inter-
action between the metal surface and heteroatom’s such S, O, N
atoms and phenyl ring, which can offer electron densities to the
unoccupied–orbitals of iron to form coordinate bonds. It is also
indicated that the protective film was formed on the metal sur-
3 PDP analysis confirmed the mixed type behaviours of all THAs.
4 The adsorption characteristics of all studied THAs were de-
scribed as related to the Langmuir adsorption isotherm.
5 SEM–EDS surface analysis demonstrated that the protecting
film on the MS surface was formed with the presence of an
inhibitor.
13

D.K. Verma, M. Kazi, M.S. Alqahtani et al.
Journal of Molecular Structure 1241 (2021) 130648
6 XRD results confirmed that the presence of inhibitor impor-
tantly reduced the concentration of chloride ions from 6.11 wt%
to around 1 wt%.
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Declaration of Competing Interest
The authors declare no conflict of interest
Acknowledgement
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of dendrimers as effective corrosion inhibitors for mild steel in 1 M HCl, Anal.
Bioanal. Electrochem. 8 (2016) 104–123.
The authors thank the Deanship of Scientific Research at King
Saud University for funding this work through research group no.
RG-1441-528.
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Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130648.
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