Hydroxy phenyl hydrazides and their role as corrosion impeding agent: A detail experimental and theoretical study

Article abstract of DOI:10.1016/j.molliq.2021.115605

This study presents synthesis of environmentally benign corrosion inhibitors, hydroxy acetophenone derivative namely, N′-(1-(2-hydroxyphenyl) ethylidene) acetohydrazide (ATOH), N′-(1-(2-hydroxyphenyl) ethylidene) benzohydrazide (BZOH), 2-(1-(2-hydroxyphen

Full text of DOI:10.1016/j.molliq.2021.115605

Journal of Molecular Liquids 330 (2021) 115605
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Hydroxy phenyl hydrazides and their role as corrosion impeding agent:
A detail experimental and theoretical study
a,
b
c
b
d
e
Ashish Kumar Singh ⁎, Bhawna Chugh , Manjeet Singh , Sanjeeve Thakur , Balaram Pani , Lei Guo ,
f
g
Savas Kaya , Goncagul Serdaroglu
a
Department of Applied Sciences, Bharati Vidyapeeth's College of Engineering, New Delhi 110063, India
Department of Chemistry, Netaji Subhas University of Technology, New Delhi 110078, India
Department of Chemistry, University of Petroleum and Energy Studies, Dehradun 248007, India
Department of Chemistry, Bhaskaracharya College of Applied Science, University of Delhi, New Delhi 110078, India
School of Material and Chemical Engineering, Tongren University, Tongren 554300, China
Sivas Cumhuriyet University, Health Services Vocational School, Department of Pharmacy, 58140 Sivas/Turkey
Sivas Cumhuriyet University, Math. and Sci. Edu., 58140 Sivas, Turkey
b
c
d
e
f
g
a r t i c l e i n f o
a b s t r a c t
Article history:
This study presents synthesis of environmentally benign corrosion inhibitors, hydroxy acetophenone derivative
namely, N′-(1-(2-hydroxyphenyl) ethylidene) acetohydrazide (ATOH), N′-(1-(2-hydroxyphenyl) ethylidene)
benzohydrazide (BZOH), 2-(1-(2-hydroxyphenyl) ethylidene) hydrazine-1-carbothioamide (TSCOH) and N′-
Received 15 December 2020
Received in revised form 1 February 2021
Accepted 5 February 2021
Available online 9 February 2021
(1-(2-hydroxyphenyl) ethylidene) hydrazinecarbothiohydrazide (TCBOH) and full investigation of their
protecting ability against corrosion of MS in 1 M HCl medium. A variety of electrochemical experimental tech-
niques viz., electrochemical impedance spectroscopy, and potentiodynamic polarization coupled with different
microscopic techniques viz., scanning electron microscopy, atomic force microscopy, and x-ray photoelectron
spectroscopy were run to evaluate the corrosion resistive behaviour of inhibitors. The surface feature of mild
steel was further analysed by measuring contact angle in different electrolytic solutions. Furthermore, these
wet-chemical experimental outcomes are well complemented from results analysed from ab initio DFT study
and MD simulation. Furthermore, the MD simulation outcomes helped in visualization of interaction and thereby
adsorption of inhibitors on Fe (110) surface.
Keywords:
Toxicity
Solubility
XPS
MD
EIS
Contact angle
©
2021 Elsevier B.V. All rights reserved.
1. Introduction
peoples to go for their intensive extraction which would be resulted in
to depletion of nation valuable resource.
Our current state, i.e. age of material has been evolved through peri-
Almost all the metals occur in the nature in the form of their ore [3].
The production or extraction of the metals involves addition of energy
to their ore. Thus the extracted metals with high system's Gibbs energy
are frequently susceptible to lower their system free energy due to in-
teraction with environment [4]. So, corrosion is inevitable process that
we intend to prevent but like death and taxes we must learn to deal
with it. Thus, most of the metals possess a strong propensity to revert
in to its most stable thermodynamic state [5]. This conversion of metals
in to their native state is actually corrosion. Although, corrosion is inev-
itable however, different control methods can be used to lower its rate.
This includes modification of metals/alloys and environment as well
as both.
odic development and wide use of different materials such as metals/al-
loys, polymers (blends and composites), ceramics etc. [1]. However, the
Materials Age can be regarded as successor of Stone Age.
With their progressive evolution man has learnt to use different
metals and their alloys. The technical culture of humanity has been
largely developed and formed because of exceptional role of metal [2].
The hardness and abundance of metals has made them as an indispens-
able material for the manufacturing of tools and other things. The his-
torical names such as Bronze and Iron Age speak itself about the
influence of metals on human life. Thus, metals are considered as vital
for our life.
In fact the metal reserve is a measure of economy of a nation. Thus
the metal can be considered as high demand natural resource for overall
development of a nation. The increasing demand of metals leads
Modern era uses mostly alloys in place of pure metals because of
their superiority over metal, as they are often harder than pure metals,
more versatile for conversion in to different forms and having more cor-
rosion resistance [6].
Iron is one of the most abundant, brittle and hard substance. How-
ever, its highly negative oxidation potential makes it susceptible to
⁎
Corresponding author.
E-mail address: ashish.singh.rs.apc@itbhu.ac.in (A.K. Singh).
https://doi.org/10.1016/j.molliq.2021.115605
167-7322/© 2021 Elsevier B.V. All rights reserved.
0

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
corrosion. Because of much susceptibility towards corrosion, pure iron
does not commonly used at commercial level. The pure iron is thus
need to be alloyed with some other metal. Mild steel (MS) is primarily
the most common alloy of iron possessing very less carbon content
JASCOFT/IR-5300 spectrophotometer, Jeol AL 300 FT NMR, JEOL 400
FT-NMR and CHNS analyser respectively.
2.3. Measurement of wettability of metal surface
(
also known as low carbon steel). High tensile strength, efficient mallea-
bility, light weight, cost effectiveness, high ductility and low brittleness
make mild steel an irreplaceable material across number of industries
The effect of the existence of an inhibitor on the wettability of MS
surface was studied by measuring wetting angle between electrolyte so-
lution and metal surface by sessile drop method using the instrument
DSA 100.
[
7]. However the low carbon content makes mild steel susceptible to
corrosion.
The corrosion inhibitors have proven as one of the best corrosion
combating methods compared to other methods because they make
possible the use of less-expensive materials in different corrosive envi-
ronments [8–11]. They offer one of the simplest solution for protection
of metal in different corrosive environment. Though, there are literally
number of corrosion inhibitors already studied and their selection im-
poses complicated problem before us. However, the toxicity and cost ef-
fectiveness of the inhibitor prompted corrosion scientists to explore the
possibility of new and effective options [12–15] which can combat
against the problem of metallic corrosion in different corrosive
environments.
2.4. Preparation of electrolyte and electrode
The standard electrolyte HCl solution was obtained by diluting 37%
HCl supplied by Merck India. The mild steel sheet was cut in to different
3
size (2.5 × 2.0 × 0.025 cm for weight loss measurement and
3
2
7.5 × 1.0 × 0.025 cm with 1 cm exposed area for electrochemical stud-
ies) to carry out experiments.
2.5. Electrochemical experimentation
In general, presence of hetero atoms (N, S, and O) and conjugated
unsaturation make the inhibitor appreciably effective for corrosion pro-
tection [16–18]. According to previous findings the ability of hetero
atoms to combat corrosion follow the order as S > N > O which is re-
lated to their electronegativity and some other factors [19]. The maxi-
mum protecting ability of S atom may be due to vacant 3d-orbital
which promotes retro-donation of electrons and lead to efficient chem-
isorption [20].
Recently, theoretical investigations have become popular these days
to study the chemical reactivity of inhibitors as well as the extent of in-
teraction with the metallic surface which is quite tedious for experi-
mental techniques [21–24]. However, the task to develop top premium
corrosion inhibitors still remain.
SP-240 Biologic electrochemical workstation equipped with three-
electrode cell was used to monitor the behaviour of redox reaction oc-
curring on metallic surface in electrolytic solution. The three electrode
cell contained MS strip, platinum cord and saturated calomel function
as working, counter and reference electrode respectively. During elec-
trochemical experimentation, the temperature of whole system was
thermostatically kept constant at 308 K. All the electrochemical experi-
ments were run after getting stabilised state of open circuit potential
(OCP) which can be attained by keeping the electrode cell system in
electrolyte solution for 30 min. Thereafter, electrochemical impedance
spectroscopy (EIS) was run at open circuit potential (OCP) with fre-
5
−2
quency ranging from 10 to 10 Hz and alternating current (AC) hav-
ing an amplitude of 20 mV.
This paper presents the synthesis of hydroxy acetophenone deriva-
tive namely, N′-(1-(2-hydroxyphenyl) ethylidene) acetohydrazide
The obtained EIS outcomes were extracted by EC lab software and
analysed by the use of Z-fit. Using EIS data, corrosion combating ability
of inhibitor can be compared with the help of Eq. (1) as [26]:
(
ATOH), N′-(1-(2-hydroxyphenyl) ethylidene) benzohydrazide (BZOH),
-(1-(2-hydroxyphenyl) ethylidene) hydrazine-1-carbothioamide (TSCOH)
and N′-(1-(2-hydroxyphenyl) ethylidene) hydrazinecarbothiohydrazide
TCBOH) and full investigation of their protecting ability against corro-
2
p
a
ct
R −R
ct
CCAEIS% ¼
ꢀ 100
ð1Þ
p
R
(
ct
sion of MS in 1 M HCl medium. The experimental techniques used for
the present work are electrochemical experiments, morphological
tests (SEM, AFM), x-ray photoelectron spectroscopy (XPS) to investi-
gate their protecting ability. In addition, simulation analyses were
done to gather information related to the mechanism of their adsorp-
tion. Moreover, the solubility and toxicity of the inhibitors have also
been accessed.
where, Rct signifies charge transfer resistance whereas superscripts p
and a represent presence and absence of inhibitor.
The EIS experiments were followed by potentiodynamic polariza-
tion with potential sweep ± 250 mV and 1 mV/s scan rate. The perfor-
mance of inhibitors can be compared on the basis of polarization data
as [27]:
a
p
corr
i
−i
corr
CCAPDP% ¼
ꢀ 100
ð2Þ
a
2. Experimental
i
corr
where, icorr described corrosion current density while the superscripts
a’ and ‘p’ described the absence and presence of inhibitor.
2.1. Synthesis
‘
The inhibitors were obtained by condensation of o-hydroxy
acetophenone and acetyl hydrazine, benzoyl hydrazine, thiosemicarbazide
and thiocarbohydrazide (1:1 ratio) in ethanol and acetic acid as a cata-
lyst according to the scheme shown in Fig. 1. The mixture were refluxed
separately for 5–6 h in a RB flask [25], the resultant solution were cooled
at room temperature for 1 h. The solid obtained was filtered out, washed
and recrystallized with ethanol. The recrystallized products were fur-
ther used for spectral characterization.
2.6. Weight loss investigation
MS of composition listed in Table 1 were used to prepare
2.5 × 2 × 0.025 cm3 sized coupons. These coupons were succes-
sively abraded with different grade emery paper to get desired MS
surface to run weight loss experiments. From the initial and ending
weight of MS coupons, inhibitor's performance can be assessed as
[28]:
a
i
2.2. Structural identification
w −w
CCAWL% ¼
ꢀ 100
ð3Þ
a
w
The structural characterization were performed by various spectral
techniques, for example, FT-IR, H NMR, C NMR and elemental analy-
sis (CHN). The instruments used for these characterizations are
1
13
where, w, superscripts a, and i are written for weight loss, absence and
presence of inhibitor respectively.
2

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 1. Synthetic route of different inhibitors, ATOH, BZOH, TSCOH, and TCBOH.
Table 1
2.9. Theoretical assessment
%
composition of mild steel used in the study.
2.9.1. Toxicity & solubility analysis
Elements
W%
The toxicity and solubility of the researched inhibitors were evalu-
C
0.05
ated. The toxic parameters were computed by the TOPKAT module of
Discovery Studio software, which is focussed on the quantitative
structure-activity relationship (QSAR) model, a mathematical formula-
tion employed to explain toxicity in relevance to the physical properties
of the chemical structure. While the aqueous solubilities were assessed
by the Advanced Chemistry Development (ACD/Labs 6.0) software.
Si
Mn
S
0.09
0.20
0.01
0.012
0.0025
0.001
Remaining
P
Ni
Cr
Fe
2.9.2. Quantum chemical calculations
All B3LYP [29,30] computations for ATOH, BZOH, TSCOH and TCBOH
compounds were performed at 6-31 g(d,p), 6–311 + g(d,p) and 6–311
+g(2df,2pd) basis sets by G09W [31] package. SMD [32] solvation
model was used to conduct the water phase calculations. After geome-
try optimizations and verifying these structures, the NBO (natural
Bond Orbital) computations [33,34] were employed at B3LYP/6–311+
2
.7. Topographical study of MS surface
.7.1. SEM & AFM
+
2
The effect of electrolyte solution on the topography of electrode sur-
face was analysed by SEM and AFM. The mild steel strips of defined size
were hanged in tested solutions which may or may not have inhibitor
for 6 h. After the experiment, the samples were taken out, cleaned and
used to scan through SEM (Hitachi TM 3000) and AFM (Model Bruker).
The AFM images were further examined by using a software
Nanoscope 8.5.
+
G(2df,2pd) level to analyse the intra-molecular interactions. Addition,
the DFT- based reactivity identifiers [35–37] were calculated in the gas
as well as water phases. Also, the nucleophilic and electrophilic attack
sites for each compound were demonstrated by HOMO, LUMO and
MEP plots. The optimized geometries and atom labelings of four com-
pounds were given in Fig. 2.
2
.8. XPS
Conceptual Density Functional Theory [38] called as DFT of chemical
reactivity provides great facilities to scientists in view of the practical
interpretation of chemical reactivity descriptors such as chemical po-
tential, electronegativity, hardness and softness. In this theory, the rela-
tion of the total electronic energy and the number of electrons at a
constant external potential aforementioned quantum chemical descrip-
tors are illustrated via the subsequent equations. The equations in
The nature and basic composition of inhibitor's film adsorbed on
metallic surface can be effectively examined by XPS. The X-ray photo-
electron spectra were scanned by Multiprobe Surface Analysis instru-
ment provided by Scienta Omicron, Germany. All the peaks were
analysed by CASA software.
3

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 2. The optimized structures of four compounds at B3LYP/6–311++G(2df,2pd) level in the water phase.
accordance with ionization energy and electron affinity parameters of
the descriptors have been obtained by Pearson and Parr using finite dif-
ferences approach [39].
Polarizability (α) that is one of the useful reactivity descriptors is
calculated depending on diagonal components of polarizability tensor.
ꢄ
ꢅ
ꢀ
ꢁ
ꢂ
ꢃ
〈α〉 ¼ 1=3 αxx þ αyy þ αzz
ð12Þ
∂
E
I þ A
μ ¼ −χ ¼
¼ −
ð3Þ
∂N
2
νðrÞ
"
#
2
2.9.3. MD simulations analysis
1
2
∂ E
I−A
2
η ¼
¼
ð4Þ
ð5Þ
The MD simulation was carried out employing the Forcite module of
Materials Studio 8.0 software developed by BIOVIA Inc. It was per-
formed in a simulation box (1.73 nm × 1.73 nm × 3.51 nm) with peri-
odic boundary conditions. The box comprises of a lower Fe slab and an
upper solvent layer (having 400 water molecules and 1 inhibitor mole-
cule of interest). For an iron substrate, Fe (110) was chosen as studied
surface as it has a density packed structure and is highly stable, which
was formulated with a six-layer Fe slab. In such model, there were 49
Fe atoms in every layer symbolising a (7 × 7) unit cell. The whole system
was conducted at 298 K regulated by the Andersen thermostat, NVT ca-
nonical ensemble, with a time step of 1.0 fs and simulation time of
2
∂N
νðrÞ
σ ¼ 1=η
Within the framework of Koopmans Theorem [35], ionization en-
ergy and electron affinity parameters are associated with frontier orbital
energies. This theory illustrates that the negative values of HOMO and
LUMO orbital energies of any compound is related to the ionization en-
ergy and electron affinity values of a compound. The relation could be
written in the form of following equations:
1
000 ps, using the COMPASSII force field. Electrostatic interactions
I ¼ −EHOMO
A ¼ −ELUMO
ð6Þ
ð7Þ
were computed using Ewald summation technique and atom based
summation approach for Van der Waals interaction.
3
. Result and discussion
Parr, Szentpaly and Liu modelled electrophilicity index [36] used to
describe their electrophilic power on the basis of the electronegativity
and chemical hardness values of chemical moeities via the subsequent
expression. Then Chattaraj [40] presented the nucleophilicity in terms
of the multiplicative inverse of electrophilicity index. Following these
studies, in recent years, Gazquez [41] and co-workers proposed some
expressions to predict electron donating and electron accepting powers
of the compounds. To calculate the Gazquez parameters, we need only
ionization energy and electron affinity values of the substances.
3.1. Synthesis protocol
The synthesis of inhibitors was confirmed using spectral characteri-
zation and elemental composition (CHN analysis). The data of FT-IR,
1
13
H NMR, C NMR and CHN analysis are presented in Table 2.
3.2. Analysis of wettability of metal surface
2
ω ¼ χ =2η
ð8Þ
ð9Þ
The contact angle measurement provided an all-important informa-
tion about the wettability of metal surface to the electrolytic solution in
presence of varying amount of studied compounds in electrolytic solu-
tion. The least contact angle of metal surface to the bare electrolytic so-
lution (Fig. 3) confirmed its most hydrophilic behaviour for metal
surface. Addition of different amounts of studied inhibitors increased
the contact angle and thereby lessened the wettability of metal surface
ε ¼ 1=ω
þ
2
ω
¼ ðI þ 3AÞ =ð16ðI−AÞÞ
ð10Þ
ð11Þ
ω− ¼ ð3I þ AÞ =ð16ðI−AÞÞ
2
4

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Table 2
Synthesis characterization data of inhibitors.
Inhibitors M. P. (o C) CHN (%)
FT-IR (ν cm−1
)
1
H NMR (δ ppm)
13
C NMR (δ ppm)
ATOH
BZOH
TSCOH
TCBOH
167
172
206
195
C = 62.5, 1665 (C=N), 3445 (OH; phenolic),
6.8–7.3 (4H; m, Phenyl); 13.1 (1H; s, OH); 116.7, 118.0, 121.1, 130.8, 132.0 (aromatic); 161.8
H = 6.3, 1680 (C=O), 3360 (-NH), 2880
N = 14.55 (CH ), 3078 (ArH)
C = 75.2, 1660 (C=N), 3440 (OH; phenolic),
1.6 (3H; s, CH ); 10.3 (1H; s, NH); 1.9 (3H; (aromatic C-OH); 166.8 (C=N); 172.1 (CONH); 18.2
s, CH -Acetyl group) (CH attached to C=N) and 20.5 (CH –acetyl group)
6.8–7.32 (4H; m, Phenyl); 13.2 (1H; s,
3
3
3
3
3
117.8, 118.5. 121.2, 127.5. 128.2, 132.1, 132.4
H = 5.5,
N = 5.8
1640 (C=O), 3350 (-NH), 2854
(CH ), 3072 (ArH)
OH); 1.75 (3H; s, CH
3
); 10.7 (1H; s, NH);
(aromatic); 162.5 (aromatic C-OH); 167.2 (C=N);
3
7.5–7.9 (5H; m,)
17.8 (CH
3
attached to C=N) and 162.8 (CONH)
C = 51.6, 1610 (C=N), 3450 (Phenolic OH),
H = 5.3, 3371 (NH ), 3240 (NH), 2877 (CH
N = 20.10 3066 (ArH), 1260 (C=S), 708 (CS)
C = 48.3, 1655 (C=N), 1245 (C=S), 710 (CS), 6.75–7.32 (4H; m, Phenyl); 13.2 (1H; s,
6.81–7.32 (4H; m, Phenyl); 13.15 (1H; s,
), OH); 1.8 (3H; s, CH ); 11.3 (1H; s, NH);
7.68 (1H, s, CSNH
116.3, 118.2, 121.0, 132.0, 132.4 (aromatic); 163.4
2
3
3
(aromatic C-OH); 168.1 (C=N); 16.8 (CH
C=N); 180.2 (C=S)
3
attached to
2
)
117.2, 118.5, 121.4, 132.0, 132.6 (aromatic); 163.8
(aromatic C-OH); 167.0 (C=N); 17.6 (CH attached to
2
); 4.3 (2H, d, C=N); 182.8 (C=S)
H = 5.4,
N = 24.99
2975 (CH
3
), 3485 (OH), 3005 (ArH) OH); 1.85 (3H; s, CH
=N-NH); 8.52 (1H, t, NH-NH
NH
3
); 10.5 (1H, s,
3
2
)
the EIS results achieved through fitting of the experimental data using
relevant equivalent circuit. The shape of Nyquist plots obtained after
running EIS experiments on MS dipped in different solutions reflected
that the impeding behaviour of the inhibitors ATOH, BZOH, TSCOH
and TCBOH was determined collectively by resistance, capacitance and
inductance. Though their impeding behaviour were controlled by resis-
tance, capacitance and inductance, but, since dominant part of Nyquist
plots occurred in +Y-direction which inferred the major role of resis-
tance and capacitance in controlling the impedance behaviour of these
inhibitors, ATOH, BZOH, TSCOH and, TCBOH.
The Nyquist curves attained in this experiment showed two loops;
one capacitive at higher frequency associated with a low-frequency in-
ductive loop. There may be two reasons behind the formation of induc-
tive loop; first one the strong adsorption of inhibitors or second one, the
relaxation phenomena's caused by adsorption of Clads, and H a+ ds [43].
However, a large size of inductive loop with increasing concentration
of inhibitors confirmed that it is the strong adsorption of inhibitor
which is responsible for the formation of inductive loop [44].
‐
The inhomogeneity or surface roughness of solid electrode resulted
in to depressed semi-circled Nyquist plots which would be perfect semi-
circle otherwise [45]. This results in to substitution of double layer ca-
pacitance (Cdl) by constant phase element (CPE) to get an appropriate
equivalent circuit (Fig. 6a) which can be more accurately fitted with
the experimental results. The fitting of experimental data with given
Fig. 3. Variation of contact angle of MS surface to the different electrolytic solutions.
equivalent circuit is represented in Fig. 6b-c (EIS data attained with
for electrolytic solution. The order of wettability of metal surface for dif-
ferent inhibitor solution is ATOH < BZOH < TSCOH < TCBOH. This order
confirmed their corrosion combating ability.
−1
3
00 mg L TCBOH).
Using Eq. (13), the impedance can be obtained as
ꢄ
ꢆ
ꢇꢅ
n
ZCPE ¼ Rs þ 1= ðjωÞ Q þ 1=ðRct þ 1=ðjωL þ 1=RLÞÞÞ
ð13Þ
3.3. Electrochemical experiment analysis
3
.3.1. Open circuit potential
The thermodynamic ability of a substance to be oxidized electro-
where, ZCPE, j, ω and n signify magnitude of CPE, an imaginary number
2
(j = −1), angular frequency and degree of inhomogeneity.
chemically could be assessed by measuring its OCP. The change of OCP
by the presence of inhibitors relative to that in blank acid is appreciably
supportive to interpret which electrochemical process whether ca-
thodic or anodic is largely controlled by the presence of various amount
of inhibitors [42]. The open circuit potential of MS immersed in different
inhibitor's solution is presented as Fig. 4. The presence of all the inhibi-
tors moved the OCP chiefly in negative direction which confirmed the
inhibitor's influence over cathodic reaction under the experimental
condition.
Angular frequency (ω) and double layer capacitance Cdl can be ob-
tained as [46]:
−
n
ω ¼ ðQRctÞ
ð14Þ
ð15Þ
ꢈ
ꢉ
1=n
−n
Cdl
¼
QRct1
3.3.2. Electrochemical impedance spectroscopy (EIS)
The value of n ranges within 0 and 1; It explains the deviation from
Potentiostatic EIS studies were run for MS working electrode im-
an ideal behaviour.
mersed in various solutions. The results of the experiment are presented
as Nyquist and Bode-Phase angle curves (Fig. 5). Table 3 contained all
The Cdl and thickness of protective film are related to each other as
per Gibb's Helmholtz model presented as [47]:
5

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 4. Variation of OCP against concentration of (a) ATOH, (b) BZOH, (c) TSCOH, and (d) TCBOH.
much similar. This is an indication of similar mechanism of inhibition
taking place at electrode surface after adsorption of different inhibitors.
Fig. 7a-d shows that both anodic and cathodic processes are
inhibited significantly for electrolytic solution having inhibitors. How-
ever, this suppression is more noticeable in case of cathodic reactions
which is further supported by a more pronounced change of cathodic
εε0A
d
Cdl
¼
ð16Þ
where, ε, ε
0
, A and d, are dielectric constant, vacuum permittivity, area of
electrode and thickness of electric double layer. The calculated Cdl data
are presented in Table 3.
A close overview of all the data attained from EIS (shown in Table 3)
makes it known that the increasing inhibitor's concentration increases
c a
Tafel slope (b ) compared to anodic Tafel slope (b ) [50].
The shift in the value of Ecorr related to blank solution decides the na-
ture of inhibitor. An Ecorr shift >85 mV confirms a cathodic/anodic na-
ture of an inhibitor whereas <85 mV corresponds to mixed nature
behaviour of inhibitor [51]. In this investigation, Ecorr shift was found
R
ct on one side and reduces Cdl on the other. This displays the deposit
of protective layer that slows down the process of charge transfer.
An observation of Bode plots obtained with different inhibitor's
amount confirms the presence of single time constant during the pro-
cess occurring on the electrode surface. Demonstration of phase angle
plots showed that the phase angle shifted towards more negative
value with increasing inhibitor's amount. It confirms the increasing ca-
pacitive behaviour of inhibitors at their higher concentrations [48].
<
85 mV that affirmed the mixed type behaviour of inhibitors, ATOH,
BZOH, TSCOH, and TCBOH.
The relation between corrosion current density and corrosion rate is
written as [43]:
ꢂ
ꢃ
Eq:wt:
d
3
:27icorr
¼ CR
ð17Þ
3.3.3. Tafel polarization
Polarization study offers primarily the best method to investigate
where all the symbols have their usual meaning.
The value of C obtained from above relationship are listed in Table 4
along with other polarization parameters. The obtained C value follow
R
corrosion of metals and their protecting abilities. The extrapolation of
polarization plots has made possible the computation of different polar-
ization parameters [49] listed in Table 4.
R
the same trend as calculated from other methods.
The potentiodynamic polarization plots of MS obtained in different
electrolytic solutions are presented as Fig. 7a-d. An overview of polari-
zation curves demonstrates clearly that polarization curves obtained
for different inhibitors, ATOH, BZOH, TSCOH, and TCBOH are very
The best performance of TCBOH compared to other inhibitors is re-
lated to presence of electron-rich hetero atoms which facilitates its ad-
sorption over the electrode surface and thereby reduces corrosion rate
up to maximum extent.
6

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 5. Nyquist plots of MS in acid solution in absence and presence of (a) ATOH, (b) BZOH, (c) TSCOH, and (d) TCBOH; Bode plots (e) ATOH, (f) BZOH, (g) TSCOH, and (h) TCBOH; Phase
angle plots (i) ATOH, (j) BZOH, (k) TSCOH, and (l) TCBOH.
Table 3
EIS parameters obtained for mild steel in HCl solution in absence and presence of inhibitors.
2
2
2
Q (μF.s (n−1)
dl (μF cm−2
Name of Inhibitor
M HCl
Conc. of Inhibitor (mg L−1)
R
s
(Ω cm )
R
ct (Ω cm )
R
L
(Ω cm )
L (H)
)
n
C
)
CCAEIS%
1
–
100
0.43
0.85
0.86
0.92
1.60
0.94
0.80
1.62
0.90
1.20
0.95
1.40
1.22
0.92
1.10
1.12
0.82
1.65
1.10
1.00
0.95
3.26
6.10
7.11
9.40
14.36
22.61
7.20
0.80
1.10
1.70
2.60
4.30
5.90
1.70
1.50
3.60
4.80
5.80
1.20
2.60
3.90
4.90
6.10
1.60
2.20
2.80
5.60
6.10
10.3
10.0
9.00
7.80
7.10
6.50
9.80
8.70
7.50
7.00
6.20
8.30
7.90
6.90
6.00
5.50
7.10
6.30
5.90
5.30
4.80
719.2
490.1
430.2
360.0
310.3
266.2
462.2
391.3
313.6
280.2
242.3
412.3
356.5
273.1
236.5
196.4
368.2
305.1
240.1
174.2
120.8
0.801
0.813
0.822
0.831
0.832
0.835
0.815
0.824
0.833
0.836
0.839
0.820
0.828
0.835
0.840
0.844
0.824
0.831
0.841
0.850
0.856
157.1
128.7
122.8
113.2
104.0
98.2
126.6
118.3
101.9
96.7
–
ATOH
BZOH
TSCOH
TCBOH
46.55
54.14
65.31
77.29
85.58
54.72
65.31
72.27
79.36
85.73
63.08
66.93
74.41
83.13
86.52
63.77
73.05
82.65
90.62
92.41
1
2
2
3
50
00
50
00
100
1
2
2
3
50
00
50
00
9.40
11.76
15.80
22.86
8.83
89.4
100
120.2
110.2
89.2
84.8
73.1
109.1
97.7
86.5
70.8
49.8
1
2
2
3
50
00
50
00
9.86
12.74
19.33
24.20
9.10
12.10
18.80
34.79
43.00
100
1
2
2
3
50
00
50
00
7

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 6. (a) proposed equivalent electrochemical circuit (b) and (c) Fitting of experimental data with proposed equivalent circuit obtained with 300 mg L− of TCBOH.
1
3.4. Weight loss investigation
3.4.2. Effect of inhibitor on corrosion kinetics
The role of inhibitor's presence on the corrosion kinetics can be
established by carrying out gravimetric analyses at different tempera-
tures ranging from 308 to 338 K with a difference of 10 K.
Applying Arrhenius and Eyring transition state equations on corro-
sion kinetics, different kinetic parameters were found as [52,53]:
3
.4.1. Corrosion rate vs. inhibitor's amount
Gravimetric analysis is one of the most common method used to find
out the role of inhibitor's amount on corrosion rate of metals when ex-
posed to aggressive corrosive solution. The results obtained from weight
loss investigation is shown as Fig. 8a which shows a steady decrease in
the weight loss with increasing inhibitor's concentration. Among all the
studied inhibitors, TCBOH shown excellent performance which is be-
cause of the existence of electron rich hetero atoms which ease its ad-
sorption on MS surface to lessen the corrosion rate.
−
Ea
log CR ¼ 2
þ log λ
ð18Þ
:303RT
Table 4
Polarization parameters obtained for mild steel in HCl solution in absence and presence of inhibitors.
Electrolyte solution
M HCl
Inhibitor
Concentration (mg L−1)
−Ecorr (mV vs. SCE)
i
corr (μA cm−2
)
β
a
(mV dec−1
119.2
)
β
c
(mV dec−1
)
C
R
(mmy−1
)
CCAPDP%
1
–
–
100
459
454
500
486
500
492
475
484
481
466
486
506
479
487
467
512
452
470
513
500
491
2600
1390
1192
902
590
375
1177
902
721
536
371
960
860
665
438
350
942
700
451
243
197
190.7
146.4
240.0
251.2
154.8
164.6
142.4
254.2
119.8
252.2
222.6
168.9
189.2
154.8
185.0
117.1
130.7
248.2
278.7
130.8
121.4
30.6
16.3
14.0
10.6
6.9
–
ATOH
126.8
103.0
136.4
107.0
133.1
98.6
130.4
98.7
107.3
97.0
92.3
98.8
97.0
80.7
85.8
94.8
100.9
132.6
101.2
93.3
46.5
54.1
65.3
77.3
85.5
54.7
65.3
72.3
79.4
85.7
63.1
66.9
74.4
83.1
86.5
63.8
73.1
82.6
90.6
92.4
1
2
2
3
50
00
50
00
4.4
BZOH
100
13.8
10.6
8.5
6.30
4.3
11.3
10.1
7.8
5.1
4.1
11.1
8.2
5.3
1
2
2
3
50
00
50
00
TSCOH
TCBOH
100
1
2
2
3
50
00
50
00
100
1
2
2
3
50
00
50
00
2.8
2.3
8

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 7. Potentiodynamic polarization curves of MS obtained in 1 M HCl in absence and presence of (a) ATOH, (b) BZOH, (c) TSCOH, and (TCBOH).
ꢂ
ꢃ
ꢂ
ꢃ
RT
Nh
ΔS⁎
R
ΔH⁎
RT
3
.4.3. Adsorption behaviour of inhibitors
Application of inhibitors lowers the attack of aggressive species
CR ¼
exp
exp
−
ð19Þ
mainly by adsorbing themselves over metallic surface. The inhibitor
molecules replaces aggressive species from metal surface and get
adsorbed themselves competitively. Thus, understand of adsorption of
inhibitor species is necessary to recognize the corrosion inhibitory
mechanism. This could be potentially done by the study of adsorption
isotherm.
where all the parameters have their usual meaning in context to corro-
sion kinetics described elsewhere [54].
A plot of log C
R
vs. 1/T (Fig. 8b) provides a straight line whose slope
, and Ar-
and intercept value enables us to calculate activation energy, E
rhenius factor, λ. The linear regression coefficient (R ≈ 1) value affirms
kinetic determination of corrosion and its inhibitory effect. The value of
a a
E , and λ thus attained are tabulated in Table 5. The value of both, E , and
a
2
The surface coverage, θ, for a concentration range used for this inves-
tigation was calculated as per Eq. (20).
λ are enhanced in the presence of inhibitor which affirms hindrance to
the corrosion process offered by inhibitor molecule [55].
a
i
w −w
θ ¼
ð20Þ
a
w
According to Eq. (18), a high activation energy and low Arrhenius
parameter value favours lowering of corrosion rate. The order of activa-
tion energy according to Table 5 is ATOH > TCBOH > TSCOH > BZOH >
blank. ATOH's lowest effectiveness is associated with its high Arrhenius
factor value.
Various standard adsorption isotherms (Frumkin, Temkin, Flory-
Huggins and Langmuir) were attempted to fit surface coverage values,
θ. By far, Langmuir isotherm (Eq. (21)) was found most fit because lin-
2
ear regression coefficient was closest to unity (R = 0.999).
The enthalpy and entropy of activation values can be known from
the value of slope and intercept of straight line achieved by plotting
Cinh
θ
1
Kads
log C
R
/T vs. 1/T (Fig. 8c) and are given in Table 5. The negative value of
¼
þ Cinh
ð21Þ
∗
ΔH is a result of association of inhibitor molecule with metal surface
compared to dissolution process [56], lead to reduced metal corrosion.
Though the value of ΔS ∗ was found negative in both conditions, how-
where Cinh and Kads are written for inhibitor's concentration and adsorp-
tion constant respectively.
A plot of Cinh vs. Cθinh (Fig. 8d) produces a straight line whose intercept
enabled us to get the value of Kads for different inhibitors, listed in
Table 5.
ever, presence of inhibitor raised the entropy of activation. Actually
the adsorption of inhibitor and desorption of solvent occur simulta-
neously at the same time. Thus, the increased entropy is an outcome
of desorption of water molecule from the metallic surface [57].
9

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 8. (a) weight loss of MS against amount of inhibitor in acidic solution, (b) Arrhenius plot for log C
R
R
vs. 1/T, (c) log C /T vs. 1/T, and (d) Langmuir isotherm.
ΔH −TΔSo ¼ ΔG
o
o
ð24Þ
Adsorption constant,Kads enables the calculation of Gibb's free en-
ergy of adsorption,ΔGads (see Table 5) with the help of Eq. (22) as:
ads
ads
ads
o
ꢂ−
ꢃ
A straight line attained by plotting either lnKads vs. 1/T (Fig. 9a) or
o
ads
ΔG
RT
o
o
o
ΔG vs. T (Fig. 9b) enabled us to calculate the value of ΔH , and ΔS
ads
ads
ads
.
−0:018 exp
¼ Kads
ð22Þ
Both methods led to consistence result of ΔHads, and ΔS oa ds (Table 5).
o
Generally, adsorption proceeds with negative enthalpy or heat of
Where R and T have their usual meaning as described earlier [58].
The spontaneity of adsorption is reflected by negative value of Gibb's
adsorption,ΔH oa ds, which must be further associated with decrease in en-
tropy or ordering of adsorbing species. But, here in this study, both
free energy of adsorption whereas extent of adsorption is determined
by Kads
ΔH ao ds, andΔS oa dsvalues are positive which is related to substitutional ad-
.
sorption of inhibitor which get adsorbed themselves replacing water
molecules (solvent) [60].
o
The values of Kads and ΔGads can be further used to get the value of
thermodynamic adsorption parameters with the help of Van't Hoff
and Gibb's Helmholtz equations as [59]:
3
.5. Topographical study of MS surface
−ΔHo
RT
ΔS
o
1
5:5
ads
ads
þ
þ
¼ ln Kads
ð23Þ
3.5.1. SEM
The role of adsorption of tested inhibitors on the surface morphology
of working electrodes have been investigated by capturing SEM images,
5
R
Table 5
Thermodynamic activation and adsorption parameters obtained for mild steel in HCl.
ΔH ∗
ΔS ∗
(J K
K
ΔHads
o
a
ΔSads
o
a
ΔHads
o
b
ΔSads
o
b
−ΔGads
o
R
Name of
inhibitor
E
a
λ
ads
L
(kJ mol−1)
(mg cm−2)
(kJ mol−1)
−1
mol−1
)
(mol−1)
(kj mol−1)
(J K
−1
mol−1
)
(kj mol−1)
(J K
−1
mol−1
)
(kj mol−1)
–
33.04
54.45
44.07
45.90
53.10
2.03 × 107
30.14
51.64
41.22
43.05
50.31
−114.76
−59.52
−94.83
−92.24
−75.33
–
–
–
–
–
–
–
10
4
4
4
4
ATOH
BZOH
TSCOH
TCBOH
1.50 × 10
0.29 × 10
0.48 × 10
0.63 × 10
1.58 × 10
21.54
14.72
13.49
29.43
44.20
58.63
60.44
4.74
22.65
16.45
15.20
32.10
43.70
58.30
60.10
3.78
30.71
32.04
32.72
35.04
0.18
0.14
0.09
0.04
2.18 × 108
8
2.98 × 10
2.22 × 109
a
Calculated from Eq. (23).
Calculated from Eq. (24).
b
10

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
o
Fig. 9. (a) plot of ln Kads vs. 1/T, and (b) ΔGads vs. T.
presented as Fig. 10a-f, of MS surfaces dipped in different acid solutions.
Fig. 10a shows smooth MS surface before immersion in electrolytic so-
lution which changed in to a highly degraded rough surface (Fig. 10b)
when dipped in mere acid solution without adding any inhibitor. This
deteriorated roughened surface of MS is due to attack of highly corro-
sive acid ions. Addition of inhibitor to acid solution prevents attack of
corrosive species on MS surface and thereby resulted in to compara-
tively smoother surfaces (Fig. 10c-f). A highly smooth, not attacked
MS surface (Fig. 10f) resulted when dipped in to acid + TCBOH solution.
It is related to excellent efficiency of TCBOH compared to other tested
inhibitors.
different useful AFM parameters viz. average roughness (R
mean-square roughness (R ), and maximum peak to valley height
(Rp-v) are enlisted in Table 6.
a
), root-
q
Inspection of Table 6 illustrates that these parameters are lowest for
unexposed MS surface while it increased by maximum extent for MS
surface exposed directly to 1 M HCl. Application of inhibitors to the
acidic solution controls the corrosion rate by adsorbing themselves on
MS surface replacing corrosive ions. This fact can be viewed by AFM pa-
rameters obtained for MS surface dipped in different inhibitor solutions.
The order of the value of AFM parameters are 1 M HCl > ATOH > BZOH
> TSCOH > TCBOH > unexposed MS. This order of corrosion inhibition
is supported by the results obtained from all other methods. This corro-
sion control by addition of inhibitors is due to the development of inhib-
itor layer on the MS surface.
3.5.2. AFM
The effect of incorporation of inhibitors to the acid solution on MS
surface could be visibly observed by using AFM technique as it helps
to study change in the surface morphology at nano level [61].
3.5.3. XPS
The MS surfaces, dipped in different electrolytic solutions, were
analysed by AFM and compared these AFM data with those obtained
for MS surface before immersion in any solution. 2D AFM images
along with longitudinal cross section graphs as well as 3D AFM images
of MS surfaces before exposure to corrosive solution and after exposure
to different acid solutions are illustrated as Figs. 11 and 12. These AFM
images were analysed by software Nanoscope 1.8 and obtained
The XPS spectra of the film adsorbed on the MS (Fig. 13) comprises
of several peaks for various elements such as C 1 s, O 1 s, Fe2p, N 1 s
and S 2p which affirms the adsorption of an inhibitor on the MS.
The deconvoluted C 1 s spectrum depicted three peaks i.e. 285 eV,
286.3 eV, and 288.1 eV. The primary peak located at 285 eV may be at-
tributed to the C–C, C=C, and C–H bonds [62], whereas the peak at
286.3 eV may be linked to the C=N and C=S bond in the inhibitor
Fig. 10. SEM images of MS (a) before immersion in acid solution, (b) after immersion in 1 M HCl, (c) after immersion in 1 M HCl + 300 mg L 1ATOH, (d) 1 M HCl + 300 mg L−1BZOH,
−
e) 1 M HCl + 300 mg L−1TSCOH, and (f) 1 M HCl + 300 mg L TCBOH.
−1
(
11

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
−1
Fig. 11. 2D AFM images and longitudinal section images the MS surface (a) before immersion in acid solution, (b) after immersion in 1 M HCl, (c) after immersion in 1 M HCl + 300 mg L
ATOH, (d) 1 M HCl + 300 mg L BZOH, (e) 1 M HCl + 300 mg L TSCOH, and (f) 1 M HCl + 300 mg L TCBOH.
−1
−1
−1
+
moiety. The third peak located at 288.1 eV is attributed to C=N . This
hydroxide species (FeOOH) respectively [64]. Peak obtained at
533.5 eV is result of adsorption of water molecules moisture on MS sur-
face [65].
+
contribution of C=N for this peak is resulted either from the proton-
ation of the C=N bond and/or due to coordination of N with the mild
steel surface [63].
Deconvolution of Fe2p spectra resulted in peaks located at 710 eV,
O 1 s spectra exhibited three peaks at B.E 529.8 eV, 531.5 eV, and
713 eV, 719 eV and 725 eV. The primary peak at 710 eV is ascribed to
(i.e., Fe3 oxide) and FeOOH (i.e., oxyhydroxide). The peak ob-
+
5
33.5 eV. The peaks at 529.8 eV and 531.5 eV may be attributed for ox-
Fe O
2 3
2
−
3+
ygen atom in O state coordinated with Fe (Fe
2
O
3
or Fe
3
O
4
) and
3
served at 713 eV indicates the existence of FeCl on the MS surface
Fig. 12. 3D AFM images of MS surface before immersion in acid solution, (b) after immersion in 1 M HCl, (c) after immersion in 1 M HCl + 300 mg L ATOH, (d) 1 M HCl + 300 mg L−1
−1
−1
−1
BZOH, (e) 1 M HCl + 300 mg L TSCOH, and (f) 1 M HCl + 300 mg L TCBOH.
12

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Table 6
inhibitors for MS in acid medium. Also, as depicted in Fig. 14b, several
typical toxicity descriptors including carcinogenicity, biodegradability,
and sensitization were given to represent the toxicities. Generally, prob-
ability values lying within 0.0 to 0.3 are thought to be low probabilities,
and are anticipated to give a negative response in an experimental
assay; whereas probability values with more than 0.7 are very high,
and are expected to give a positive response in an experimental assay.
Probabilities more than 0.3 but lower than 0.7 are treated indetermi-
nate. Careful examination of the attained probability values in Fig. 14b
one could conclude that the toxic level of involved inhibitors is very
low (all less than 0.09), and thus these compounds are environmentally
friendly.
AFM results obtained for mild steel in HCl solution in absence and presence of inhibitors.
Sample
R
a
(nm)
R
q
(nm)
Rp-v (nm)
MS before immersion
MS/1 M HCl
MS/1 M HCl + ATOH
MS/1 M HCl + BZOH
MS/1 M HCl + TSCOH
MS/1 M HCl + TCBOH
30.2
1335
231
140
75.6
54.4
38.2
1822
279
171
102
155
3714
1622
872
795
541
70.2
because of HCl corrosive solution [66]. The peak seen at 719 eV is attrib-
utable of Fe (III) satellite. The peak at 725 eV can be due to Fe2p1/2 rep-
2 3
resentative of the iron in the form of Fe O and FeOOH [67].
Deconvolution of N 1 s may be result into three peaks i.e. 399 eV,
00.5 eV and 402 eV. The primary peak at 399 eV is correspondent to
the unprotonated nitrogen atoms (=N- moiety), whereas the peak at
00.5 is a result of N\\Fe bond attributable to the bonding of nitrogen
in the inhibitor with the iron in the MS. The peak at 402 eV is because
of nitrogen in the oxidized state existing because of the protonation of
nitrogen that allows a positive polarization of the N atom [68].
The S 2p XPS spectra represented peaks located at 162 eV and
3
.6.2. Natural bond orbital (NBO) analysis
Table S1 displays the essential intramolecular interactions for ATOH,
4
BZOH, TSCOH and TCBOH compounds calculated at B3LYP/6–311++G
2df,2pd) level in the water phase. Accordingly, the LP(2) O11 → π* C2-
4
(
C3 (2) and the LP(2) O11 → π* C2-C3 (2) interactions have a vital role in
stabilization of all compounds: the perturbative energy of these interac-
tions is calculated at 27.15 and 24.86 kcal/mol for ATOH, 27.44 and
2
3.77 kcal/mol for BZOH, 27.26 and 24.84 kcal/mol TSCOH, and 27.79
1
68.5 eV. The first peak at around 162 eV may be because of disulfide
species, i.e. FeS while the peak situated at 168.5 eV is attributed to sul-
fur bonded to Fe to result into S − Fe complex [69].
and 24.84 kcal/mol for TCBOH, respectively. Furthermore, the π → π* in-
teractions have greatly affected the stabilization energy for all com-
2
(
2)
pounds and the calculated E values for the saturated ring of these
compounds are almost similar to each other. For instance, the resonance
energy for π C1-C6 (2) → π* C2-C3 interaction of ATOH, BZOH, TSCOH
and TCBOH compounds is calculated in 22.61, 22.67, 22.76 and
23.03 kcal/mol, respectively. Although its contribution to the stabiliza-
tion energy is lower than the other interactions, the hyperconjugative
interaction energy is worth to mention here. Namely, this energy for σ
C14-H15 (1) → π* C13-N18 (2) interaction for ATOH, BZOH, TSCOH
3
.6. Theoretical assessment
3.6.1. Solubility and toxicity analysis
As shown in Fig. 14a, we can see that all the four compounds possess
excellent water solubility in strong acid solution, which can be attrib-
uted to the hydroxyl, amino, and imine groups in their structures.
Therefore, they can behave as potential water soluble corrosion
i
and TCBOH compounds has estimated in 4.32 (ED = 1.97179e), 4.09
Fig. 13. XPS spectra of mild steel surface removed after immersion in 1 M HCl solution with 300 mg L−1 of TCBOH: (a) survey scan, (b) narrow scan spectra of C, (c) O, (d) N, (e) Fe, and (f) S.
13

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 14. (a) The solubility and (b) toxicity assessments for the studied inhibitors.
(
1
(
ED
.97179e), orderly. On the other hand, the energies of σ C23-H24
1) → π* C13-N18 (2) and σ C23-H24 (1) → π* C13-N18 (2) interactions
i
= 1.97179e), 4.09 (ED
i
= 1.97179e) and 3.84 kcal/mol (ED
i
=
Fig. 15 demonstrates the HOMO& LUMO and MEP graphs for ATOH,
BZOH, TSCOH and TCBOH compounds. For the ATOH and BZOH com-
pounds, HOMO is expanded over the whole molecular surface except
for ATOH compound are calculated in 5.59 and 4.92 kcal/mol, respec-
tively, since this compound has an additional methyl group attached
to aliphatic part of the compound. Moreover, the π → π* interactions
for BZOH compound have calculated in 15.43–22.12 kcal/mol contrib-
uted to the perturbative energy, remarkable. It is apparent that the –
for the -CH
is distributed on –(C=S)-NH
TCBOH. As known well, the HOMO indicates the nucleophilic attack
site for the interested molecules. In this context, the -CH for ATOH,
the phenylic group for BZOH, hydroxyphenyl and -CH groups for
3
for ATOH and phenylic group for BZOH while the HOMO
2
for TSCOH and on -(C=S)-N-NH for
2
3
3
(
C=S)-NH
2
and -(C=S)-N-NH
2
substituent groups for TSCOH and
TSCOH and TCBOH compounds have no essential role in the nucleo-
philic attack reactions. On the other hand, LUMO plots reveal that the
-OH group for all compounds has no effective role for the electrophilic
attacks because of no electron density on this group for all compounds.
TCBOH compounds have no important contribution to the perturbative
energy. These interactions and their quantities have an effect on the
chemical reactivity behaviour of the investigated compounds.
Fig. 15. HOMO& LUMO (isoval:0.02) and MEP (isoval:0.0004) pilots of ATOH, BZOH, TSCOH and TCBOH compounds, at B3LYP/6–311++G(2df,2pd) level in the water phase.
14

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Besides, the -CH
3
group for the ATOH and -NH
2
group for TCBOH have
in general. The electronegativity values of studied molecules presented
in Table 7 indicate that TCBOH and TSCOH molecules have the lowest
value of the electronegativity. As noted in the previous part, Parr's elec-
trophilicity index is related with electronegativity and hardness of
chemical species. Minimum Electrophilicity Principle [73] defines that
the natural direction of a chemical reaction is towards a state of mini-
mum electrophilicity, or in other words the molecules having lower
electrophilicity values are more stable in comparison to others. It is ap-
parent from the results given that the most stable inhibitor molecule is
ATOH while the most reactive one is TCBOH. Within the framework of
calculated electrophilicity values, the corrosion combating ability
order of investigated inhibitors can be written as: TCBOH > TSCOH >
BZOH > ATOH. This ranking supports the experimental data. Molecules
with high polarizability and dipole moment are good corrosion inhibi-
tors. Minimum Polarizability Principle [74] introduced encouraged by
Maximum Hardness Principle signifies that in a stable state polarizabil-
ity (α) is reduced. In the study, it is clear that the most polarized mole-
cules among studied compounds are BZOH and TCBOH. Few researchers
noted that dipole moment is a vital parameter for corrosion inhibition
analysis while some authors said it should not be considered in corro-
sion analysis. It is significant to notice that our dipole moment data sup-
port the usefulness of dipole moment in such studies. ΔN values given in
related table can be widely used in the prediction of corrosion inhibition
activities. This value represent the transferred electron amount be-
tween inhibitor molecule and metal surface. Corrosion inhibition per-
formances of molecules increase as their ΔN values increase.
Calculated ΔN values given in Table 7 show that corrosion inhibition
performances of the molecules obey the order: TCBOH > TSCOH >
BZOH > ATOH. The ranking is compatible with experimental order.
also no active role in the electrophilic attack reactions. In addition, the
red color density on oxygen atom (=O) around for ATOH and BZOH
compounds implies the electron-rich region whereas the blue color
over the hydrogen atom of hydroxyl group indices the electron-poor
site. Also, for TSCOH and TCBOH compounds, the similar H atom around
implies the region of positive electrostatic potential, whereas the orange
color over sulfur atom implies the negative electrostatic potential region
in medium size for these compounds.
Frontier orbital energies are extensively used in the theoretical esti-
mation of corrosion inhibition performances of molecules. It is signifi-
cantly noticeable that HOMO orbital energy describes the electron
donating tendency of molecules while LUMO orbital energy gives
clues about electron accepting ability of molecules. Molecules having
high EHOMO values act as good corrosion inhibitors. Calculated frontier
orbital energies presented in Table 7 show that corrosion inhibition per-
formances of studied molecules obeys the order TCBOH > TSCOH >
BZOH > ATOH. It is apparent that this ranking is very compatible exper-
imental results. Hard and Soft Acid Base Principle (HSAB) introduced by
Pearson [70] classifies the chemical species as hard and soft. Hard mol-
ecules possess higher HOMO-LUMO energy gap values. On the other
hand, energy gap values of soft compounds are quite low. For that rea-
son, quantities like hardness, softness as well as energy gap are closely
related each other. Chemical hardness [71] is known as the resistance
towards electron cloud polarization or detachment of atoms, ions and
molecules. In accordance to the Maximum Hardness Principle [72]
“there seems to be a rule of nature that molecules arrange themselves
so as to be as hard as possible.” This means that chemical hardness is a
measure of the stability and hard substances are more stable in contrast
to the soft ones. As can be seen from the definition given above of chem-
ical hardness that hard molecules are not efficient against the corrosion
of metallic surfaces. According to determined hardness, softness and en-
ergy gap values, one can say that corrosion inhibition efficiency order
for investigated molecules is stated as TCBOH > TSCOH > BZOH >
ATOH. This theoretical data are in good correlation with experimentally
obtained results.
Electronegativity is known as electron abstracting power of chemi-
cal molecules and this quantity signifies the electron accepting capabil-
ity of inhibitor molecules. Good corrosion inhibitors give easily
electrons to metal surfaces and they have low electronegativity values
3.6.3. Molecular dynamics simulation
The dynamics procedure was performed and the complete system
approached equilibrium till both temperature as well as energy of the
system are balanced. The low energy adsorption configurations of four
inhibitors adsorbed onto Fe (110) surface are represented in Fig. 16.
Fig. 16 represents that the adsorption centers of inhibitors on the Fe
(110) surface are the electrons of aromatic rings, O, N, and S atoms.
The inhibitor molecules adsorbed almost flat orientation on the Fe sur-
face to enhance surface coverage and contact, leading to a stronger co-
ordination for adsorbate/substrate system.
Table 7
The quantum chemical reactivity identifiers of ATOH, BZOH, TSCOH and TCBOH compounds.
Gas
631G(d,p)
6311 + G(d,p)
6311++G(2df,2pd)
ATOH (1) BZOH (2) TSCOH (3) TCBOH (4) ATOH (1) BZOH (2) TSCOH (3) TCBOH (4) ATOH (1) BZOH (2) TSCOH (3) TCBOH (4)
HOMO (−I)
LUMO (−A)
ΔE (L-H)
−0.2117
−0.0363
4.7742
−0.2143
−0.0448
4.6131
−0.2003
−0.0485
4.1304
−0.2002
−0.0519
4.0341
−0.2285
−0.0505
4.8447
−0.2300
−0.0584
4.6697
−0.2127
−0.0617
4.1089
−0.2141
−0.0651
4.0559
−0.2278
−0.0505
4.8259
−0.2295
−0.0573
4.6861
−0.2124
−0.0613
4.1127
−0.2107
−0.0644
3.9808
μ = −(I + A)/2 −3.3735
−3.5248
2.3066
2.6933
1.5282
4.7331
−3.3839
2.0652
2.7723
1.6385
6.8398
−3.4293
2.0170
2.9152
1.7002
7.0633
−3.7965
2.4224
2.9751
1.5673
5.0603
−3.9248
2.3349
3.2988
1.6810
5.0473
−3.7339
2.0545
3.3932
1.8175
6.7168
−3.7980
2.0279
3.5566
1.8729
6.9002
−3.7858
2.4130
2.9698
1.5689
4.9356
−3.9023
2.3430
3.2495
1.6655
4.9167
−3.7236
2.0564
3.3713
1.8108
6.5522
−3.7431
1.9904
3.5196
1.8806
6.7501
η = (I-A)/2
ω = μ*μ/2η
ΔN = -μ/η
D (debye)
α (au)
2.3871
2.3838
1.4132
4.6029
134.2220
185.5297 155.8550
161.6393
152.3950
210.8720 180.0797
187.8340
155.4503
213.9123 184.0643
191.9057
Water
ATOH (1) BZOH (2) TSCOH (3) TCBOH (4) ATOH (1) BZOH (2) TSCOH (3) TCBOH (4) ATOH (1) BZOH (2) TSCOH (3) TCBOH (4)
HOMO ( \\ I)
LUMO (−A)
ΔE (L-H)
−0.2140
−0.0376
4.8020
−0.2156
−0.0470
4.5857
−3.5729
2.2928
2.7837
1.5583
7.0325
262.0247 210.9500
−0.0207
−0.0431
−0.6086
−0.8680
−0.3043
−1.2379
−2.8524
9.4860
−0.2082
−0.0469
4.3895
−3.4701
2.1947
2.7433
1.5811
9.8167
223.6223
−0.2296
−0.0497
4.8961
−3.7994
2.4481
2.9483
1.5520
7.8668
218.8623
−0.2304
−0.0579
4.6940
−3.9236
2.3470
3.2797
1.6718
7.8278
306.0313 252.5450
−0.2196
−0.0551
4.4779
−3.7378
2.2390
3.1199
1.6694
9.6412
−0.2195
−0.0594
4.3574
−3.7937
2.1787
3.3029
1.7413
9.9065
270.3927
−0.2285
−0.0488
4.8883
−3.7723
2.4441
2.9111
1.5434
7.7161
225.0167
−0.2293
−0.0562
4.7084
−3.8840
2.3542
3.2040
1.6498
7.6923
313.2373 260.9693
−0.2186
−0.0543
4.4706
−3.7129
2.2353
3.0836
1.6610
9.4808
−0.2184
−0.0582
4.3598
−3.7628
2.1799
3.2475
1.7261
9.7468
278.4997
μ = −(I + A)/2 −3.4231
η = (I-A)/2
ω = μ*μ/2η
ΔN = -μ/η
D (debye)
α (au)
2.4010
2.4401
1.4257
6.9616
188.4700
15

A.K. Singh, B. Chugh, M. Singh et al.
Journal of Molecular Liquids 330 (2021) 115605
Fig. 16. Side and top views of most stable adsorption configurations for four inhibitors on Fe (110) surface.
The performance of corrosion inhibitors absorbed on Fe surface can
be formulated by an adsorption energy (Eads). The adsorption energy
in a solution could be determined using the subsequent equations:
Declaration of Competing Interest
There are no conflicts to declare.
Eads ¼ Etotal−ðEsurfþwater þ EinhþwaterÞ þ Ewater
ð25Þ
Acknowledgements
Authors BC and ST are thankful to the Netaji Subhas University of
Technology, New Delhi for providing financial assistance to carry out
the experimental work. Author AKS, MS and BP are thankful to their in-
stitutions for providing work platform. The theoretical study of this pro-
ject was sponsored by the National Natural Science Foundation of China
where, Etotal is the complete potential energy of the system, which com-
prises iron crystal, the.
adsorbed inhibitor and solution; Esurf+water and Einh+water denotes
the potential energies of the system with no inhibitor and the system
with no iron crystal, respectively; Ewater is the potential energy of the
water molecules.
(
2176195).
The adsorption energies in this work were compute from the
average adsorption energy of the attained equilibrium configurations.
The determined Eads values are −473.1, −475.4, −477.9, and
Appendix A. Supplementary data
−
1
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115605.
−
489.4 kJ mol for ATOH, BZOH, TSCOH, and TCBOH, respectively.
We can see that the adsorption energies are negative and thus sponta-
neous adsorption could be anticipated. The larger Eads value signifies
the stronger interaction among the inhibitor and metallic surface.
Clearly, it appears that TCBOH has more absolute values of Eads than
the other three inhibitors, and therefore it presented the best inhibiting
action for carbon steel, which is in correlation with the experimental
results.
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