Investigation on corrosion protection behavior and adsorption of carbohydrazide-pyrazole compounds on mild steel in 15% HCl solution: Electrochemical and computational approach

Article abstract of DOI:10.1016/j.molliq.2020.113513

The behavior of synthesized (E)-5-amino-3-(4-methoxyphenyl)-N′-(1-(4-methoxyphenyl)ethylidene)-1H-pyrazole-4-carbohydrazide [AMPC] and (E)-5-amino-N′-(4-chlorobenzylidene)-3-(4-chlorophenyl)-1H-pyrazole-4-carbohydrazide [ACPC] towards the corrosion protection of mild steel (MS) in 15% HCl solution was studied by gravimetric and electrochemical methods. The inhibitor AMPC and ACPC at an optimum concentration of 300 ppm showed 98.26% and 96.21% inhibition efficiency, respectively at 303 K temperature. Thermodynamic results conquer that both inhibitors followed mixed adsorption and results from polarization suggests the retardation of the cathodic as well as anodic reactions in presence of inhibitors. The Langmuir adsorption isotherm suggests the monolayer adsorption of inhibitors on the MS facet. The formation of protective layers of inhibitors on MS surface was confirmed by the outcomes of FESEM and AFM topographical details. The composition of adsorbed layers on the MS surface was studied by XPS analysis. The energy related to molecular orbital of inhibitors, and adsorption behavior was computationally examined by Density Functional Theory (DFT), calculation of Fukui functions and Monte-Carlo simulation (MCS) studies.

Full text of DOI:10.1016/j.molliq.2020.113513

Journal of Molecular Liquids 314 (2020) 113513
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Investigation on corrosion protection behavior and adsorption of
carbohydrazide-pyrazole compounds on mild steel in 15% HCl solution:
Electrochemical and computational approach
a
a,
b
Priya Kumari Paul , Mahendra Yadav ⁎, I.B. Obot
a
Department of Chemistry, Indian Institute of Technology (ISM) Dhanbad, India
Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The behavior of synthesized (E)-5-amino-3-(4-methoxyphenyl)-N′-(1-(4-methoxyphenyl)ethylidene)-1H-
pyrazole-4-carbohydrazide [AMPC] and (E)-5-amino-N′-(4-chlorobenzylidene)-3-(4-chlorophenyl)-1H-
pyrazole-4-carbohydrazide [ACPC] towards the corrosion protection of mild steel (MS) in 15% HCl solution
was studied by gravimetric and electrochemical methods. The inhibitor AMPC and ACPC at an optimum concen-
tration of 300 ppm showed 98.26% and 96.21% inhibition efficiency, respectively at 303 K temperature. Thermo-
dynamic results conquer that both inhibitors followed mixed adsorption and results from polarization suggests
the retardation of the cathodic as well as anodic reactions in presence of inhibitors. The Langmuir adsorption iso-
therm suggests the monolayer adsorption of inhibitors on the MS facet. The formation of protective layers of in-
hibitors on MS surface was confirmed by the outcomes of FESEM and AFM topographical details. The composition
of adsorbed layers on the MS surface was studied by XPS analysis. The energy related to molecular orbital of in-
hibitors, and adsorption behavior was computationally examined by Density Functional Theory (DFT), calcula-
tion of Fukui functions and Monte-Carlo simulation (MCS) studies.
Received 21 March 2020
Received in revised form 31 May 2020
Accepted 2 June 2020
Available online 16 June 2020
Keywords:
Carbohydrazide-pyrazole inhibitor
EIS
AFM
XPS
DFT
MCS
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
some corrosion inhibitors are toxic in nature and produce environmen-
tal as well as health related problems. Therefore there is an urgent need
to develop eco-friendly corrosion inhibitors [7]. The inhibition efficiency
of organic corrosion inhibitors is mainly decided by the number of
2 4
The corrosive medium, most often acidic mediums (HCl and H SO
solutions) are used to remove the deposited scales on the inner part of
the tubes/pipelines used under the process of extraction, purification,
and transportation of the gas and petroleum products. When the corro-
sive medium comes in contact with tube walls, corrosion reaction starts
and results in the formation of pits and corrosion products [1,2].The de-
position of corrosion products on the inner wall of the tubes become in-
tense day by day and lead to a dreadful condition and creates a lethal
threat to the society. The MS is one of the most often used, cheapest, du-
rable and easily accessible form of steel in the market but its sensitivity
towards corrosion is a major drawback [3,4]. So, for the sake of environ-
mental concern and safety, it is desperately needed to adopt some effec-
tive corrosion protection methods to improve the efficiency of work and
durability of engineered structures. Among the numerous corrosion
protection methods, the application of corrosion inhibitors to mitigate
corrosion is most common and simple method [5,6]. Corrosion inhibi-
tors came into consideration due to their efficient corrosion inhibition
behavior, ease of accessibility and low cost. Along with inhibition action,
2
hetero-atoms in the molecule, bonded as -CN, -NH ,-S-, -CO, -SH, -C=
N- etc. and their ease of interaction to the metal surface [8]. The inhibi-
tors with planar geometry tend to produce better inhibition as they can
approach metal surface to the maximum closure arrangement and in-
teract effectively through all the active sites on the molecule along
with delocalized pi-electron density on aromatic rings. The compounds
with pyrazole and carbohydrazide moieties are basically known for
their biological applications. S. M. Gomha [9] and T.A. Farghaly [10] re-
ported the pyrazole compounds as an anti-microbial agent. M. Asif
[11] and Kajal [12] reported the carbohydrazide derivative as
anti-microbial and anti-inflammatory agent, respectively. S. Lian [13]
published his work on carbohydrazide-pyrazole derivative as an anti-
tumor and anti-proliferative agent. Besides the biological applications
pyrazole and carbohydrazide derivatives are also reported by some re-
searchers as corrosion inhibitors. Fouda et al. [14] and Kumar et.al [15]
reported the carbohydrazide derivative as corrosion inhibitor with
maximum 76 and 90% efficiency, respectively. Y. Filali [16] in 2016 pre-
sented a carbohydrazide derivative which showed maximum 90% cor-
rosion inhibition efficiency. The pyrazole derivative investigated by A.
⁎
Corresponding author.
E-mail address: mahendra@iitism.ac.in (M. Yadav).
https://doi.org/10.1016/j.molliq.2020.113513
167-7322/© 2020 Elsevier B.V. All rights reserved.
0

2
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Ouchrif [17] and R.S. Abdel Hameed [18] showed maximum corrosion
inhibition efficiency of 82 and 95%, respectively towards corrosion of
mild steel in HCl solution. M. Yadav in 2016, reported pyrazole deriva-
tive with 95% efficiency at 300 ppm [6]. H. Elmsellen [19] reported the
corrosion inhibition efficiency of 95% for the molecule containing both
carbohydrazide and pyrazole moiety. Keeping in view, the good corro-
sion inhibition behavior of carbohydrazide-pyrazole moiety, we have
planned to investigate the corrosion protection ability of synthesized
carbohydrazide-pyrazole derivatives namely, AMPC and ACPC by
using gravimetric and electrochemical methods.
2.2. Aggressive test medium
The experimental methods were carried out using aggressive me-
dium (15% HCl solution) with and without corrosion inhibitors. This ag-
gressive medium was prepared by diluting analytical grade 37% HCl
with double distilled water. The experimental solutions of inhibitor con-
centration: 10–300 ppm was prepared by mixing calculated amount of
inhibitors in aggressive medium. It is quite good to use freshly prepared
inhibitor solutions for each experiment to get better results.
2
2
.3. Methods
2
. Experimental details
.3.1. Surface preparation (electrode)
Mild Steel test coupons (MSTC) were used for corrosion study. For
2
.1. Synthesis of corrosion inhibitors
gravimetric measurements MSTC of dimension 3.0× 4.0 × 0.5 cm with
a hole centrally located at 0.5 cm below the top were used. This hole
helps to hold the MSTC with a hanger into the experimental solutions.
Polarization measurements were carried out on a working electrode
made up of MS with an exposed area of 1 × 1 × 0.2 cm and rest area cov-
ered with Araldite resin.
In each experimental corrosion measurements, the exposed area of
MSTC was first cleaned and then finely polished mechanically with dif-
ferent grits (up to 2000) of emery papers. Polishing of these samples to a
greater extent before carrying each experiment is a required condition
as this process enhances the appearance, smoothness, and consistency
of test coupons and improves experimental results. After polishing,
samples were cleaned, dried and used as such for experimentation.
Laboratory synthesis of corrosion inhibitors AMPC and ACPC was
done successfully by following step by step instructions given by A.S.
Salman [20] and the Scheme is represented in Fig. 1. All the chemicals
were purchased from Sigma Aldrich Corporation and used as such for
synthesis.
2
.1.1. AMPC
21 5 3
Molecular formula (C20H N O ) elemental analysis: Actual: C-
6
3.31%, H- 5.58%, N- 18.46%, Found: C- 63.25%, H- 5.53%, N- 18.44%.
−1
IR: 3390, 3305, 2945, 1640 cm
.
2
.1.2. ACPC
Molecular formula (C17
2
H13Cl N
5
O), elemental analysis: Actual: C-
2.3.2. Gravimetric measurement (GM)
5
4.56%, H- 3.50%, N- 18.71%. Found: C- 54.51%, H- 3.47%, N- 18.70%.
The Gravimetric measurements were performed as per the ASTM
standard procedure [21]. Gravimetric measurement shows the effect
−1
IR: 3320, 3050, 3245, 1625 cm
.
Fig. 1. Reaction Scheme for AMPC and ACPC.

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
3
of loss in weight of MSTC which were immersed in a corrosive medium
3. Result and discussion
with and without corrosion inhibitor for a period of 6 h. Measurement of
weight was taken from Mettler Toledo analytical weighing balance sta-
ble up to four decimal points. For this experiment, each finely polished
MSTC of dimension 3 × 4 × 0.5 cm was cleaned, dried and accurately
weighted and then dipped in the prepared test medium of concentra-
tion 0–300 ppm. After 6 h all the samples were carefully removed
from the test medium, washed, dried and again weighted. Since this
method is manually handled, so for the better observation this process
was repeated three times and the average of the findings was reported.
For determining the effect of corrosion at higher temperatures, thermo-
stat was used and correspondingly, the experiment was repeated for
higher temperature (303−333K) studies.
3.1. Gravimetric measurement
Corrosion parameters after the gravimetric measurements were
computed and listed in Table 1. On configuring the results, it was ob-
served that these corrosion parameters: corrosion rate (C
erage (θ) and inhibition efficiency (% η ) were dependent on the
amount of inhibitors added to the corrosive medium and temperature.
Observations were made for different concentrations where 10 ppm is
the lowest concentration and 300 ppm is the highest concentration (op-
timized concentration), beyond this range no considerable change was
observed. The results were computed with the help of given equations:
R
), surface cov-
g
4
CR ¼ 8:76 ꢀ 10 ðWI−W F Þ=Saρt
ð1Þ
ð2Þ
2
.3.3. Electrochemical measurement (EM)
The three-electrode system in a three-necked flat-bottomed flask
.
i
C
R
θ ¼ 1−
connected to the CHI-760D electrochemical analyzer set up was used
for polarization and electrochemical impedance studies. The outcomes
of the experiments were resolved by using CHI-760D electrochemical
software. The prepared MS electrode is used as a working electrode,
platinum electrode as auxiliary electrode and saturated calomel elec-
trode as reference electrode. This setup was kept on rest for 30 min
for ensuring the stabilization of open circuit potential (OCP) and after
the stabilization, the different electrochemical techniques were per-
formed. The obtained constant value of OCP is used as initial potential
value for running AC impedance technique keeping the higher and
lower frequency range as 100 kHz and 1 Hz, respectively, with 5 mV am-
plitude. After the completion of the AC impedance technique, the pa-
rameters were set for the polarization study. Polarization study was
performed in potential range of −250 to −250 mV with respect to
b
R
C
%η ¼ θ ꢀ 100
ð3Þ
g
where W and W is the weight (in mg) of the sample before and after
I
F
immersion in the corrosive test solution, t is the time of immersion in h,
S and ρ are the exposed surface area and density of MSTC respectively.
a
i
b
R
C
R
and C are the corrosion rates with and without inhibitors
respectively.
The inhibition efficiency consequently increases with inhibitor con-
centration and is ascribed as the mechanism of inhibition acquired by
inhibitors through forming an adsorbed thin layer on the exposed sur-
face of MS [23].The inhibition efficiency of AMPC and ACPC at
300 ppm concentration and 303 K temperature was found as 98.26
and 96.21%, respectively. The efficiency of these inhibitors (AMPC and
ACPC) are better than most of the reported similar types of inhibitors
−1
open circuit potential with a scan rate of 1 mVs
.
[
6,14–19,24,25]. The quantitative comparison of inhibition efficiency
2
.3.4. Surface characterization (FE-SEM, AFM & XPS)
of some reported similar type of inhibitors and our inhibitors (AMPC
and ACPC) are given in Table 2. From Table 2, it is clear that AMPC and
ACPC are better corrosion inhibitor as compared to the most of the al-
ready reported inhibitors.
The impact of temperature was also investigated to understand the
corrosion behavior on elevated temperatures and the experimental out-
comes were compared with the results found at room temperature. The
condition of metal surface in the corrosive medium at elevated temper-
atures becomes more adverse due to an increase in the metal dissolu-
tion process and shows better efficiency at room temperature
compared to other temperatures. For the better visual understanding,
Surface characterization was made using FESEM (Field Emission-
Scanning Electron Microscopy), AFM (Atomic Force Microscopy) and
XPS (X-ray Photoelectron Spectroscopy) analysis to understand the cor-
rosion damage and protection behavior of introduced corrosion inhibi-
tors into the corrosive medium. For the FESEM analysis, the
instrument with model Hitachi S-3400N was used. AFM analysis was
done by contact mode using an instrument from Bruker Corporation,
Berkovich type. Surface characterization by XPS analysis was done by
a PHI 5000 Versaprobe-II focus spectrometer from the facility provided
by IIT Kharagpur which uses Al Kα X-ray of 1486.6 eV monochromatic
radiation source [22].
g
the variation of inhibition efficiency (% η ) with concentration and tem-
perature is represented in Fig. S1 (Supporting information). The find-
ings in the corrosion rate with temperature might be connected to the
molecular rearrangement or fragmentation and disturbance on the
adsorbed layer of inhibitors on the surface [26].
Kinetic and thermodynamic parameters were extracted from the
slope and intercept of Arrhenius and Transition state plots represented
as Figs, S2 & S3 (Supporting information), respectively, and obtained pa-
rameters are portrayed in Table 3. The basic relation of temperature
with corrosion rate is mathematically represented by Arrhenius and
Transition state equations
2
.3.5. Computational studies
Computational methods were applied for both inhibitors to get more
structural and orbital energy related information. The quantum chemi-
cal calculations and energy optimization were observed with Gaussian
0
9 software program using the DFT method with B3LYP, 6–31 G (d,
p) basis set. The calculations of Fukui functions were carried out at
B3LYP /6–31 G (d, p) level. Monte Carlo simulation, was simulated for
getting the adsorption energy and to showing the interaction of the in-
hibitors with the steel surface, the adsorption locator module was ap-
plied based on the Metropolis Monte Carlo approach. The calculation
was performed using the COMPASS (Condensed-phase Optimized Mo-
lecular Potentials for Atomistic Simulation Studies) force field. The ad-
sorption of the inhibitors AMPC and ACPC was simulated on the Fe
lnCR ¼ ½lnA−ðEa=RTÞꢁ
ð4Þ
ð5Þ
ꢂ
ꢂ
R
C =T ¼ RT=Nh½ expðΔS =RÞ expðΔH =RTÞꢁ
(
110) crystal plane. The process involved cleaving from bcc Fe crystal,
followed by enlargement to a (10 × 10) supercell and, thereafter, build-
ing of a 30 Å thick vacuum slab on the Fe (110). For the simulating an-
nealing process, the maximum and minimum temperature was set at
where C
R
is the rate of corrosion, A is the preexponential factor, E
a
is the
energy of activation, R is the universal gas constant, T is the absolute
∗
temperature, h is the Planck's constant, ΔS is the deviation in entropy
5
00 K and 298 K respectively. The adsorption energy for the most stable
and ΔH ∗ represents the enthalpy change.
corrosion inhibitors–Fe (110) configuration was calculated by consider-
These mathematical expressions provide the activation energy (E
a
),
ing 50 molecules of water in order to account for the solvent effects.
some alteration in entropy and enthalpy during the corrosion process

4
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Table 1
Experimental outcomes from Gravimetric measurement for AMPC and ACPC corrosion inhibitors.
Conc.
ppm)
303 K
313 K
323 K
333 K
(
C
(
R
θ
% η
g
C
R
θ
% η
g
C
R
θ
% η
g
C
R
θ
% η
g
mmy−1)
(mmy−1)
(mmy−1)
(mmy−1)
Blank
AMPC
28.2 ± 2.48
58.1 ± 2.61
98.9 ± 2.97
144.5 ± 3.1
1
7
1
2
3
0
5
50
50
00
6.24 ± 0.09
4.40 ± 0.06
2.73 ± 0.04
1.51 ± 0.02
0.49 ± 0.01
0.778
0.843
0.903
0.946
0.982
77.86
84.38
90.32
94.63
98.26
13.90 ± 0.10
10.06 ± 0.08
6.49 ± 0.06
4.11 ± 0.03
1.82 ± 0.01
0.760
0.826
0.888
0.929
0.968
76.06
82.68
88.82
92.93
96.86
26.05 ± 0.12
19.70 ± 0.10
13.63 ± 0.07
9.56 ± 0.05
5.48 ± 0.02
0.736
0.800
0.862
0.903
0.944
73.66
80.08
86.22
90.33
94.46
43.26 ± 0.13
33.84 ± 0.11
24.68 ± 0.08
18.45 ± 0.05
12.91 ± 0.03
0.700
0.765
0.829
0.872
0.910
70.06
76.58
82.92
87.23
91.06
ACPC
1
7
1
2
3
0
5
50
50
00
7.37 ± 0.07
4.93 ± 0.05
4.05 ± 0.03
2.33 ± 0.02
1.06 ± 0.01
0.738
0.825
0.856
0.917
0.962
73.86
82.5
85.63
91.71
96.21
16.64 ± 0.08
11.56 ± 0.06
9.62 ± 0.04
6.15 ± 0.03
3.54 ± 0.01
0.713
0.801
0.834
0.894
0.939
71.36
80.10
83.43
89.41
93.91
32.47 ± 0.09
24.23 ± 0.07
20.44 ± 0.05
14.33 ± 0.04
9.98 ± 0.02
0.671
0.755
0.793
0.855
0.899
67.16
75.5
79.33
85.51
89.91
52.65 ± 0.1
40.31 ± 0.10
34.92 ± 0.08
25.56 ± 0.06
19.34 ± 0.03
0.635
0.721
0.758
0.823
0.866
63.56
72.10
75.83
82.31
86.61
Table 2
Comparison of inhibition efficiency of reported compounds with the compound investigated in the present study (AMPC and ACPC).
Corrosion inhibitors
Maximum Reference
%
η
no.
6
-Amino-4-(4-methoxyphenyl)-3-methyl-2,4 dihydropyrano[2,3-c]pyrazole-5-carbonitrile (AMPC) and
95
76
90
[6]
6-amino-4-(4-chlorophenyl)-3-methyl-2,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile (ACPC)
Amino-N′-(3-(hydroxyimino)butan-2-ylidene)-4,5,6,7- tetrahydrobenzo[b]thiophene-3-carbohydrazide), amino-N
[14]
[15]
′
-(thiophen-2-ylmethylene)-4,5,6,7 tetrahydrobenzo[b]thiophene-3-carbohydrazide and amino-N′-(1-(pyridin-2-yl)ethylidene)-4,5,6,7-
tetrahydrobenzo[b]thiophene-3-carbohydrazide
4
-(4-Bromophenyl)-N0-(2,4-dimethoxybenzylidene) thiazole-2-carbohydrazide (BDTC), 4-(4-bromophenyl)-N0-(4-methoxybenzylidene)
thiazole-2-carbohydrazide (BMTC) and 4-(4-bromophenyl)-N0-(4-hydroxybenzylidene) thiazole-2 carbohydrazide (BHTC)
-Oxo-N′-phenyl-1,2-dihydroquinoline-4-carbohydrazide
2
1
5
5
4
90
82
95
95
94
[16]
[17]
[18]
[19]
[24]
,3-Bis(3-hyroxymethyl-5-methyl-1-pyrazole) propane
-Chloro-1-Phenyl-3-methyle Pyrazolo-4 methinethiosemicarbazone
-Methyl- 1H -pyrazole-3- carbohydrazide
,4′-(Phenylmethylene)bis (3-methyl-1- phenyl-1H-pyrazol-5-ol) [PYR \\ H], 4,4′-((4-nitrophenyl)methylene)bis
(3-methyl-1-phenyl-1H-pyrazol-5-ol) [PYR-NO2], 4,4′-((4- hydroxyphenyl) methylene)bis (3-methyl-1-phenyl-1H-pyrazol-5-ol) [PYR-OH] and
4,4′-((2-hydroxy-4-methox-yphenyl)methylene)bis (3- methyl-1-phenyl-1H-pyrazol-5-ol) [PYR-OHOMe]
N1, N1-bis (2-(bis ((3,5-dimethyl-1H-pyrazol-1-yl) methyl) amino)ethyl)-N2, N2-bis ((3,5-dimethyl-1H-pyrazol-1-yl) methyl) ethane-1,2-diamine: 85
[25]
PAP and diethyl 1,1′-(((4-acetylphenyl) azanediyl) bis (methylene)) bis (5-methyl-1H-pyrazole-3-carboxylate): PAC
(E)-5-Amino-3-(4-methoxyphenyl)-N′-(1-(4 methoxyphenyl)ethylidene)-1H-pyrazole-4-carbohydrazide [AMPC] and (E)-5-amino-N
98.26
Our
′
-(4-chlorobenzylidene)-3-(4-chlorophenyl)-1H-pyrazole-4-carbohydrazide [ACPC]
inhibitors
Table 3
occurring at the metal-solution interface. The E
for 0 ppm solution is
a
Activation and thermodynamic parameters.
−1
4
5.78 kJmol and at an optimum concentration of AMPC and ACPC, it
−1
a
reaches to 91.82 and 81.90 kJmol respectively. The increase in E in
Concentration
ppm)
Arrhenius
parameters
Transition state parameters
(
presence of inhibitor as compared to in absence of inhibitor implies
the decrease in rate of metal dissolution and retardation of corrosion re-
action by increasing the energy barrier from reactant to corrosion prod-
uct formed at the metal surface.
R2
R2
E
a
ΔH*
ΔS*
Blank
AMPC
45.78
0.9810
43.14
−74.42
0.9782
The endothermic nature of metal dissolution process in the present
1
7
1
2
3
0
5
50
50
00
54.16
57.13
61.80
70.22
91.82
0.9918
0.9935
0.9954
0.9940
0.9936
51.52
54.48
59.16
67.58
89.18
−59.45
−52.60
−41.22
−18.23
43.78
0.9907
0.9927
0.9949
0.9934
0.9932
∗
study is well supported by the positive values of ΔH . The increase in
ΔS ∗ on increasing the concentration could be ascribed as increase in
disorderness due to replacement of more number of water molecules
by adsorption of one inhibitor molecule at the MS surface [27].
ACPC
1
7
1
2
3
0
5
50
50
00
55.29
59.27
60.73
67.53
81.90
0.9893
0.9899
0.9915
0.9899
0.9859
52.65
53.63
58.08
64.88
81.18
−54.31
−44.53
−41.40
−23.47
17.67
0.9880
0.9888
0.9906
0.9889
0.9848
3
.2. Electrochemical method
3.2.1. Variation of open circuit potential (OCP) with time
The variation of OCP of mild steel exposed in 15% HCl solution with
time in seconds without and with different concentrations of AMPC
and ACPC at 303 K is represented in Fig. S4 (Supporting information).
From the Fig. S4, it can be seen that after 30 min immersion, only
minor changes in the OCP are observed. Furthermore, the initial OCP
in the presence of AMPC and ACPC as compared to in absence of

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
5
AMPC and ACPC shift positively due to the adsorption of AMPC and
ACPC molecules on the mild steel surface [28].
Table 4
Electrochemical Polarization results (Tafel Curve) for AMPC and ACPC corrosion inhibitors.
Concentration Polarization data
(ppm)
3
.2.2. Tafel polarization curves
Tafel plots of AMPC and ACPC are represented in Fig. 2. Tafel extrap-
E
corr
β
a
β
c
i
corr
%ηtp
mV vs SCE) (mV dec−1) (mV dec
-1)
(μAcm−2)
(
olation of anodic and cathodic curves provides the corrosion potential
corr) and the corresponding corrosion current density (icorr) values.
Fig. 2, indicates that consecutive addition of inhibitor lowers the icorr
Blank
AMPC
−489
237
237
4893 ± 1.73
(
E
.
1
7
1
0
5
50
−463
−464
−460
−463
−452
303
322
353
371
396
299
321
354
377
401
1050 ± 0.46
78.54
The parameters extracted from Tafel polarization curves are repre-
sented in Table 4. The increase in the inhibition efficiency with the con-
centration is due to adsorption of more and more numbers of inhibitor
molecules on the MS surface and reduction of corrosion current density
caused by reduced release of electrons by metal dissolution.
On the basis of difference of corrosion potential of blank solution
with corrosion potential of solution with inhibitors at different concen-
trations, it can be concluded that the nature of both inhibitor is mixed
type [29].
667.0 ± 0.35 86.36
323.9 ± 0.36 93.38
211.8 ± 0.26 95.67
122.2 ± 0.24 97.50
250
300
ACPC
10
75
−466
−465
−470
−461
−472
297
316
323
347
376
296
307
321
349
372
1183 ± 0.47
75.82
831.8 ± 0.38 83.0
658.1 ± 0.39 86.55
364.2 ± 0.36 92.55
209.7 ± 0.25 95.71
150
250
300
a c
The anodic and cathodic slopes (β and β ) change with inhibitor
concentration which also suggest that the inhibitors affect the reactions
occurring at both anodic and cathodic sites. But the Tafel curves for me-
dium with inhibitors show a positive move in corrosion potential com-
pared to blank medium and this result can be depicted that in the
present study the inhibitors retard the metal dissolution at the anodic
site to some greater extent than the hydrogen evolution occurring at
the cathodic site. The %ηtp was computed on the basis of the following
equation:
a
(R ) of molecules are less pronounced. The resulted Nyquist plot shows
the ascending order of maximum Zre value and capacitive loop diameter
with an increase in inhibitor concentration, which determines the effec-
tiveness of inhibitor [31].
Fitting of the Nyquist plots to an equivalent circuit as represented in
Fig. S5 (Supporting information) provides the electrochemical parame-
ters such as solution resistance (Rs), charge transfer resistance (Rct),
hnꢀ
ꢁ
o
i
0
corr
−icorr =i0
f p
film resistance (R ), polarization resistance (R ), double layer capaci-
%ηtp
¼
i
ꢀ 100
ð6Þ
corr
tance (Cdl) and constant phase elements (CPE), which are helpful to de-
scribe the corrosion process occurring at the metal electrode-solution
interface. These obtained parameters are listed in Table 5. Fig. S5a, was
used for fitting the Nyquist plots in absence of inhibitors and Fig. S5b,
was used in presence of inhibitors.
3
.2.3. EIS study
Nyquist plots represent the response of the imaginary impedance
value (Zim) w.r.t. real impedance value (Zre) as shown in Fig. 3.
The corrosion parameters from EIS were calculated with the equa-
tions:
The imperfect depressed semicircles towards the x-axis were ob-
served as compared to the ideal semicircle which is ascribed as the fre-
quency dispersion may be resulting from contamination, porous and
heterogeneous surface [30]. Roughness in the electrode surface is the
most common issue because in the mechanical polishing method
achievement of 100% smooth surface can't be possible. These depressed
semicircles with a single time constant suggest that the corrosion reac-
tion occurring at the metal electrode-solution interface is mainly
marked by the charge transfer resistance (Rct). The participation of
Rp ¼ R þ Rct
ð7Þ
ð8Þ
f
hnꢀ
ꢁ=Ri
o
i
i
p
0
%ηnq
¼
R −R
ꢀ 100
p
p
ꢀ
ꢁ
1=n
0
1−n
ct
Cdl
¼
γ R
ð9Þ
d
other resistances produced by the diffused layer (R ) and accumulation
Fig. 2. Tafel polarization curves for inhibitors AMPC and ACPC.

6
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Fig. 3. Nyquist plots for inhibitors AMPC and ACPC.
Table 5
Electrochemical Impedance outcomes for AMPC and ACPC corrosion inhibitors.
Conc.
ppm)
R
s
CPEfilm
CPEdl
R
p
%ηnq
2
(
(Ωcm )
Y
(
0
n
1
R
f
C
f
Y
0
n
2
R
ct
C
dl
μFcm−2)
(Ωcm )
2
(μFcm−2)
(μFcm−2)
(Ωcm )
2
(μFcm−2)
Blank
AMPC
0.90
–
–
–
–
1924
0.811
6.269
17,168
6.269 ± 2.5
–
1
7
1
2
3
0
5
50
50
00
0.72
0.70
0.71
0.75
0.69
158.7
70.37
67.07
33.32
25.5
0.99
0.99
0.99
0.99
0.99
0.01153
0.0279
11.54
0.08732
0.3063
0.0159
0.0070
0.0071
0.0033
0.0026
712.3
647.9
611.5
472.1
453.1
0.4791
0.5332
0.5992
0.6385
0.6595
31.11
39.79
65.8
137.3
183.7
3778
471
73.5
25.0
15.7
31.12 ± 1.5
39.81 ± 1.3
77.34 ± 0.6
137.3 ± 0.3
184 ± 0.1
79.85
84.25
92.6
95.4
96.6
ACPC
1
7
1
2
3
0
5
50
50
00
0.82
0.82
0.79
0.90
0.76
95.82
80
71.41
31.24
27.3
0.99
0.99
0.99
0.99
0.99
5.349
4.89
7.031
2.65
0.0102
0.084
0.076
0.032
0.027
770.9
603.2
457.8
379.5
153.1
0.5
20.29
32.62
56.83
127.2
149
1205
603
89.5
20.7
0.63
25.63 ± 1.6
37.51 ± 1.3
63.86 ± 0.5
129.85 ± 0.4
149.05 ± 0.2
75.54
83.2
91.15
95.1
0.5177
0.5729
0.6311
0.7297
0.0589
95.8
It is evident from the results (Table 5) that the value of charge trans-
inhibitor MSTC was severely affected by corrosion and results in an un-
even rough surface with a large number of pits on it. The Fig. 4 (c) and
(d) shows the MSTC surface morphology after application of inhibitor
AMPC and ACPC respectively, which appears quite smooth as compared
to surface morphology shown in Fig. 4 (b). The findings from FE-SEM
points out that both inhibitors have the ability to protect MSTC from
corrosion by making a protective layer on it.
fer resistance increases and double layer capacitance decreases on in-
creasing the concentration of inhibitor due to adsorption of inhibitor
molecules on the MS surface. Table 5, shows the inversely proportional
nature of Cdl with Rct
.
Bode plots in Fig. S6 (Supporting information) shows single peak for
each concentration (0 ppm- 300 ppm) apprises presence of single time
0
constant. Ideally for a capacitor, value of phase angle should be 90 but
in the present study lower value of phase angle was found due to the
rough and uneven surface of mild steel. It can be seen from Fig. S6,
that on increasing the concentration, phase angle values increases due
to adsorption of inhibitor molecules on mild steel surface.
3
.3.2. AFM study
The 3-dimensional micrographs (Fig. 5) obtained in micro scales by
AFM analysis provides a better understanding of corrosion damage cre-
ated in 0 ppm concentration of inhibitor and corrosion protection at op-
timum concentration of both inhibitors. This method is very helpful for
visual interpretation of the outer most exposed surface and provides a
3
3
.3. Surface characterization
3
D representation of surface with proper height profiles (R
) for inhibited and uninhibited MSTC.
The height profile R shows the average surface roughness and its
lower value suggests the smoother surface. R and R are the height pro-
A Q
, R , and
.3.1. FESEM study
R
Z
The FESEM micrographs presented in Fig. 4 shows the surface mor-
A
phology of polished MSTC and MSTC after 6 h immersion in corrosive
medium without and with 300 ppm inhibitor concentration. The first
image (Fig. 4 a) of finely polished MSTC looks smooth and even with
some fine scratches on the surface generated during the polishing
method. The second image (Fig. 4 b) interprets that in absence of
Q
Z
files that show the average of maximum height and difference between
the valleys and peaks respectively over the analysis area. All these
height profiles help in interpreting results for the protected and dam-
aged surface. Lower the height profile values suggest the lesser

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
7
Fig. 4. FESEM images of MS test samples (a) Polished surface (b) Damaged surface in HCl medium (c) after application of AMPC (d) after application of ACPC.
damaged or smoother surface [32]. Comparing these profiles for
inhibited and uninhibited MSTC surfaces supports the outcomes from
GM and EPM experimental methods and accounts for the corrosion in-
hibition ability for both inhibitors. These are the values for the height
[34]. The N1s spectrum shows three deconvoluted peaks at B.Es.
397.8, 398.5 and 399.2 eV responsible for -N-C/-NH , -N=C/-N=N
2
and ammonium ions respectively [35]. The deconvoluted spectrum of
O1s shows three peaks at binding energies 529.7, 531.16 and
532.03 eV for O2 , -C-O/OH and -C-O-C/-C-OH groups, respectively
[36]. The Fe 2p3/2 spectrum was deconvoluted into four peaks at B.
Es.711.08, 713.72, 718.83 and 724.81 eV which are responsible for
−
−
profiles- for polished surface (Fig. 5a): R
Q
– 17.7 nm; R
12.1 nm., uninhibited surface (Fig. 5b): R – 211 nm; R
– 238 nm., surface with AMPC (Fig. 5c): R – 70.4 nm; R
5.6 nm; R – 78.0 nm and surface with ACPC (Fig. 5d): R
– 59.9 nm; R – 110 nm. On the basis of these height profiles it can
A
– 13.4 nm; R
Z
–
R
5
R
Q
A
– 167 nm;
Z
Q
A
–
2+
3+
Z
Q
– 76.6 nm;
FeOOH/Fe
2
O
3
, Fe /Fe
of Fe3 (FeCl
) ions [37].
3
3 4
O , Fe (Ferric ions) and Fe 2p1/2 or satellites
+
A
Z
be concluded that AMPC having the lower values than ACPC is better in-
hibitor than ACPC.
The XPS of ACPC + Fe (Fig. 7), consists of similar atoms with slightly
different binding energies having one extra Cl atom. The C1s spectrum
was deconvoluted into three peaks at binding energies 284.09, 285.11
and 287.49 eV responsible for groups -C-C/-C-H -, -C=C and C=N/-
C=O respectively. The N1s spectrum shows three deconvoluted peaks
at binding energies 398.09, 398.91 and 399.92 eV responsible for -N-
3
.3.3. XPS study
XPS is the surface examination procedure utilized for the assurance
of elements located on the surface. It produces the spectrum of each el-
ement with their specific binding energies by the discharge of the pho-
toelectrons from the outermost surface with a depth of 0.5 nm–7.5 nm
C/-NH
2
, -N=C/-N=N and ammonium ions respectively. The
deconvoluted spectrum of O1s shows two peaks at binding energies
2−
−
5
2
30.17 and 531.93 for O and -C-O/OH groups respectively. The Cl
3/2 spectrum was deconvoluted into two peaks, one observed for in-
[33]. The XPS surface analysis was performed after the adsorption of in-
p
hibitors on the MS test surface to ensure the presence of inhibitor film
formed on the MS surface. For achieving results to maximum accuracy,
any kind of contamination is avoided and it is needed to prepare sam-
ples carefully. The XPS analysis of the surface gives the spectrum of
each element present on the surface with their specific binding ener-
gies. The XPS spectrum was recorded for AMPC + Fe and ACPC + Fe
samples and the deconvoluted spectrum for each element was repre-
sented in Figs. 6 & 7. In XPS of AMPC + Fe (Fig. 6), C1s spectrum pro-
vides three curves at 288.11, 284.8 and 283.92 eV binding energies
responsible for groups -C=N/-C=O, -C=C and -C-C/-C-H respectively
organic chlorine (C l\ \Fe) at 199.18 and one for organic chlorine (Cl-ar-
omatic ring) at 200.29 eV [38]. The Fe 2p3/2 spectrum was deconvoluted
into four peaks at binding energies 710.5, 712.7, 718.7 and 724.5 eV
2+
3+
which are responsible for FeOOH/Fe
2 3 3 4
O , Fe /Fe O , Fe (Ferric ions)
3+
and Fe 2p1/2 or satellites of Fe (FeCl
3
) ions.

8
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Fig. 5. AFM 3D images of MS test samples (a) Polished surface (b) Damaged surface in HCl medium (c) after application of AMPC (d) after application of ACPC.
3
3
.4. Computational methods
adsorption of inhibitors. In the present investigation the energy gap be-
tween HOMO and LUMO orbitals are found higher for both inhibitors in
vacuum as compare to in water suggesting that the protonation of both
inhibitors and presence of both physical and chemical adsorption
(mixed adsorption).
.4.1. Quantum chemical calculations
In order to understand and explain the inhibition results of the ex-
perimental methods, the quantum chemical calculation was performed
by DFT. This method well describes and correlates the corrosion inhibi-
tion efficiency achieved by inhibitor molecules with their molecular
structure. This method was also employed to the inhibitors to examine
the effect of water as solvent and the effect of protonated sites on corro-
sion inhibition mechanism. The Figs. 8a & 8b show the molecular orbital
density distribution as HOMO (highest occupied molecular orbital),
LUMO (lowest unoccupied molecular orbital), and an optimized form
of both inhibitors in vacuum (neutral) and water (protonated) respec-
tively. Table 6 consists of the molecular orbital energy-related parame-
ters with dipole moment (μ), chemical hardness (η), chemical softness
The observations from the quantum study for HOMO and LUMO or-
bitals of both inhibitors (Figs. 8a & 8b) marks that the HOMO is concen-
trated over the side of pyrazole containing aromatic ring in vacuum but
reverses in water. For both inhibitors LUMO is concentrated over the
side of pyrazole carbohydrazide part in vacuum and remains same in
water medium. The reactivity of inhibitor towards the corrosion protec-
tion mechanism may be altered due to some change in dipole moment
as well as electronic density over the inhibitor molecule in both cases
(neutral and protonated). Both the inhibitors show increase in dipole
moment in water which may be due to acquiring more stability of inhib-
itors through interaction with water molecules. It is observed that the
inhibitors with higher dipole moment due to strong dipole- dipole in-
teraction to the MS surface produces better results [41]. Although
some researchers suggested from their study that effect of dipole mo-
ment for a molecule pronounced by its orientation and in some cases
lower dipole moment exhibiting molecules show better inhibition
[42]. Table 6 contains global hardness and softness of the inhibitors
and according to the stability of soft-soft and hard-hard interaction,
the molecules with higher softness value provide better efficiency
[43]. The basic relations for the determination of these informative
quantum chemical parameters were expressed as-
(
σ), absolute electronegativity (χ) and the fraction of electrons (ΔN)
transferred from inhibitors to unoccupied metal orbitals. All these pa-
rameters are related to the corrosion inhibition performed by both in-
hibitors [39]. The neutral and protonated forms of both inhibitors
provide the energies related to HOMO (E
very informative towards the behavior of furnishing electrons
HOMOinh–vacant metal orbitals) and accepting the electrons (occu-
h l
) and LUMO (E ) which are
(
pied metal orbitals-LUMOinh) respectively. The higher energy of
HOMO describes the better facility of providing electrons to the vacant
d-orbital of Fe on the surface and lesser energy of LUMO defines the fa-
cility of accepting electrons from filled orbitals of Fe on the surface to the
unoccupied orbitals of the inhibitor [40]. The lesser energy gap between
ΔE ¼ E −E
ð10Þ
l
h
h l
the E and E also supports the better inhibition of the MS surface by

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
9
Fig. 6. XPS spectrum of MS test sample + AMPC corrosion inhibitor.
χ ¼ −ðE þ E Þ=2
ð11Þ
ð12Þ
ð13Þ
h
l
The sites prone to nucleophilic and electrophilic attack on the inhibitor
molecule can be easily viewed by the electrostatic potential mapping
η ¼ −ΔE=2
(
EPM). The Figs. 8c & 8d present the total electron density and electro-
static potential mapping (EPM) for both inhibitors showing the red re-
gions (−ve potential) disposed for electrophilic attack and blue
regions (+ve potential) disposed for the nucleophilic attack [45].
σ ¼ 1=η
The one more informative term the fraction or number of electrons
transferred to the metal surface in corrosive medium through the
most active donation sites on inhibitor can be evaluated by the
expression-
3.4.2. Fukui functions calculation
Fukui function is used to know about the nucleophilic and electro-
philic attacking sites of the inhibitor molecules. The numbered struc-
tures of AMPC and ACPC are represented in Fig. 9. The Fukui indices
ꢂ
ꢃ
ΔN ¼ ðχ −χ Þ=2 η þ ηi
ð14Þ
+
−
Fe
i
Fe
k
F and Fk are calculated by the change in the electronic density when
the inhibitor accepts and donates an extra electron, respectively. The
calculated Fukui indices for AMPA and ACPC are enlisted in Table 7.
It can be seen from Table 7 that the nucleophilic attacking sites of
AMPC molecule are N2, N10, O5, C1, C4, C8, and C11 atoms having F
Here for getting this term the electronegativity and the hardness
value for iron were taken as 7 and 0 eV respectively. Lukovits et al.
found that the ΔN below 3.6 suggested the better efficiency with ease
of electron donation to the metal surface [44].
+
k
values 0.0577, 0.0339, 0.0607, 0.0583, 0.0961, 0.0303, and 0.0338, re-
The distribution of electrons all over the inhibitor molecule respon-
sible for their activity towards corrosion protection as it describes the
charge distribution, the dipole moment of the molecule and its polarity.
spectively, whereas the probable electrophilic attack site are N10, O26,
−
O34, C19, C20, C22, C31, C39 and C43 atoms with F
k
values 0.0358,
0.0483, 0.0347, 0.0206, 0.0208, 0.0206, 0.0200, 0.0299, and 0.0242,

10
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Fig. 7. XPS spectrum of MS test sample + ACPC corrosion inhibitor.

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
11
respectively. In ACPC, N2, N10, O5, C1, C4, C11, C31, C33, Cl26 and Cl34
atom are nucleophilic centers whereas N10, Cl26, Cl34, C1, C8 and C11
atoms are the probable electrophilic sites.
From the analyses, it has been seen that for both the molecules the
numbers of active sites are almost same for the donor-acceptor type in-
teractions with the metal surface. Thus, MD simulation has been per-
formed to investigate the details of the interactions of AMPC and ACPC
with the surface atoms of the mild steel.
3.4.3. Monte Carlo simulation
Adsorption is the main behavior of the inhibitor on which its capabil-
ity of inhibition depends and the MCS study for inhibitors was found
good enough to explain the relationship between inhibitors and metal
surface. This simulation well represents the proper orientation of inhib-
itor and mode of adsorption through the most probable active sites on it
[46]. The Fig. 10 shows the side view of the inhibitor- metal surface sys-
tem perturbed by 50 water molecules.
The energy released during the adsorption of inhibitor to the MS sur-
face was described as adsorption energy. Interaction energy was related
to the negative multiplication of B.E. Ease of adsorption of inhibitor mol-
ecules depends on the BE values. More strong interaction with metal
surface results increase in B.E and thus more will be the adsorption en-
ergy [47]. Adsorption energies for both molecules were computed and
listed in Table 8. The more negative adsorption energy
−1
(
−747.3 kJ mol ) for AMPC inhibitor specifies the stronger adsorption
on MS surface than ACPC inhibitor which also holds the outcomes from
experimental results.
3.5. Adsorption characteristic of inhibitor
Adsorption isotherms are needed to define the nature of adsorption
mechanism acquired by inhibitor molecules. Among the Frumkin,
Temkin and Langmuir adsorption isotherm tested for adsorption, the
best-fitted outcomes were found with the Langmuir adsorption iso-
therm. The efficiency of inhibitor and reduction in corrosion reaction
is associated with the adsorption capability of inhibitor molecules. The
nature of the interaction of inhibitor molecules with ionized iron on
the MS surface decides the type of adsorption as physical or chemical
[
48]. Here the best fitted Langmuir adsorption isotherm (Fig. 11) for
both inhibitors was provided with the least deviation (close to unity)
in the regression coefficient in Table 9. The isotherm gives the
adsorption-desorption constant (Kads) as a function of the surface cov-
ered by inhibitor molecules by the relation-
Kads ¼ 1=intercept
Kads
ð15Þ
ð16Þ
C
θ ¼ 1
þ Kads
C
A higher value of Kads and surface coverage (θ) with a parallel rela-
tionship suggests the more powerful adsorption [49]. The free energy
0
of adsorption (FEA or ΔGads) is directly related to the Kads as-
0
ads
ΔG ¼ −RT lnðKads ꢀ 55:5Þ
ð17Þ
0
The resulting ΔGads with a negative sign corresponds to the sponta-
neous nature of the adsorption process and firmness of the adsorbed
0
monolayer film on the exposed surface. A threshold range for ΔGads de-
termines the nature of adsorption where ΔG a0 ds b −20 kJmol
−1
de-
scribes the adsorption taking place by electrostatic interaction
0
between the charged atoms on inhibitors and metal. The ΔGads N −40
−
1
kJmol
shows the adsorption process by the interaction of donor-
acceptor active sites present on inhibitor molecule and metal surface.
0
In the present investigation the ΔGads value is in between the described
range and so the inhibitors are ascribed as mixed inhibitor showing
both kinds of adsorption [50]. The hetero-atoms and delocalized sp2

12
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Fig. 8a. Molecular orbital images resulted from Quantum chemical study in vacuum.
Fig. 8b. Molecular orbital images resulted from Quantum chemical study in water.
electrons on the inhibitor molecules block the active sites on the ex-
posed surface after metal dissolution and retard the further metal disso-
lution resulting improvement in the inhibition of the metal surface. The
substitution on the phenyl rings affects the inhibition, so here electron
space and its way of approach to the metal surface matters for the better
inhibition [51]. At first some active centers on the inhibitor get proton-
ated into the acidic medium and electrostatically interacted with the
negatively charged chloride ions (produced from medium) accumu-
lated near the metal-solution interface. This kind of interaction between
them represents physical adsorption. After the metal dissolution, the
positively charged iron on the metal surface gets coordinated with the
electron rich active centers on the inhibitor molecule. This kind of
chemical coordination involves the sharing of electrons between lone
pairs on heteroatoms and vacant d-orbitals on ionized iron atom and
vice versa and hence represents chemical adsorption. Along with this
the interaction between delocalized electrons on inhibitor and ionized
iron represents retro-donation [52].
3
releasing effect of the -CH group in inhibitor AMPC positively contrib-
utes to electron donation and hence act as a more effective corrosion in-
hibitor as compare to inhibitor ACPC containing electronegative
chlorine atoms substituted to the phenyl ring.
3
.6. Mechanism of corrosion inhibition
The major factor for the protection of MS surface in 15% HCl solution
is adsorption of inhibitor molecules on it. The adsorption of inhibitor
molecules depends on the chemical structure, number of electron rich
heteroatoms, aromatic rings and electron density on delocalized
bonds. Along with these the orientation of inhibitor molecules in the
In the present study the inhibitor ACPC interact with the N-atoms on
carbohydrazide-pyrazole ring, O-atom of carboxyl group and π-electron
clouds on the aromatic ring to the metal surface. Similarly, the inhibitor

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
13
Fig. 8c. Total charge density and EPM images for AMPC & ACPC in vacuum.
Fig. 8d. Total charge density and EPM images for AMPC & ACPC in water.
AMPC shows similar interaction with one extra O-atom of methoxy
group attached with aromatic ring near to carbohydrazide side. Also,
the electron withdrawing effect of two Cl ions attached to aromatic
ring in ACPC makes it less efficient than the inhibitor AMPC with two
electron donating methoxy groups. So, for the inhibition in acidic me-
dium we cannot confirm that only one kind of interaction is responsible
for adsorption and hence represents the mixed kind of adsorption. In
Fig. 12, the most probable orientation of both inhibitor molecules and
their possible interaction with MS surface in 15% HCl medium is
presented.
concentrations. It was concluded on the basis of all experimental sup-
ports that the inhibitor AMPC has the better capability of achieving
more efficiency than the inhibitor ACPC. Adsorption of both inhibitors
obeyed Langmuir adsorption isotherm and the outcomes of thermody-
namic parameters suggested mixed nature of adsorption. The outcomes
from electrochemical polarization method suggest that both the inhibi-
tors act as mixed type inhibitor. The protection of MS surface was
mainly achieved by adsorption of inhibitors and it was supported by to-
pographical results (FESEM, AFM & XPS) as well as results from compu-
tational methods (DFT & MCS). Experimental and computational results
are in good correlation.
−
4
. Conclusion
The inhibitors AMPC and ACPC have very good potential to inhibit
corrosion of MS in 15% HCl solution at different temperatures and

14
P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
Fig. 9. Numbered structure of inhibitors (a) AMPC (b) ACPC.
Author statements
CRediT authorship contribution statement
This is declared that the article is original, has been written by the
stated authors who are all aware of its content and approve its submis-
sion, has not been published previously, it is not under consideration for
publication elsewhere, no conflict of interest exists and the article will
not be published elsewhere in the same form, in any language, without
the written consent of the publisher. The work reported in the manu-
script is novel and useful for application in oil industries during
acidizing of oil wells.
Priya Kumari Paul:Data curation, Formal analysis, Investigation,
Methodology, Writing - original draft.Mahendra Yadav:Conceptualiza-
tion, Supervision, Writing - review & editing.I.B. Obot:Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.

P.K. Paul et al. / Journal of Molecular Liquids 314 (2020) 113513
15
Table 7
given in supplementary data. Supplementary data to this article can be
found online at https://doi.org/10.1016/j.molliq.2020.113513.
Fukui indices for AMPC and ACPC.
AMPC
ACPC
+
−
+
−
References
Atoms
F
k
F
k
Atoms
F
k
F
k
C1
N2
N3
C4
O5
C6
C8
N9
0.0583
0.0577
0.0116
0.0596
0.0607
0.0185
0.0303
0.0058
0.0339
0.0338
0.0011
0.0024
0.0071
0.0131
0.0017
0.0061
0.0202
0.0050
0.0051
0.0113
0.0231
0.0065
0.0472
0.0065
0.0245
0.0183
0.0122
0.0253
0.0189
0.0195
0.0186
0.0106
0.0187
0.0052
0.0195
0.0176
0.0358
0.0154
0.0196
0.0096
0.0055
0.0057
0.0206
0.0208
0.0239
0.0206
0.0126
0.0483
0.0142
0.0174
0.0200
0.0130
0.0112
0.0347
0.0299
0.0242
C1
N2
N3
C4
O5
C6
C8
N9
N10
C11
C13
N14
C15
C19
C20
C21
C22
C23
Cl 26
C29
C30
C31
C32
C33
Cl 34
0.0658
0.0631
0.0112
0.0477
0.0525
0.0162
0.0266
0.0055
0.0336
0.0305
0.0028
0.0018
0.0078
0.0029
0.0064
0.0095
0.0043
0.0067
0.0523
0.0150
0.0120
0.0302
0.0051
0.0324
0.1029
0.0233
0.0196
0.0208
0.0139
0.0255
0.0047
0.0220
0.0209
0.0438
0.0211
0.0162
0.0145
0.0002
0.0195
0.0132
0.0104
0.0138
0.0168
0.1256
0.0147
0.0104
0.0082
0.0088
0.0155
0.0960
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(kJ mol
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Langmuir Adsorption results for AMPC and ACPC corrosion inhibitors.
Langmuir Adsorption Isotherm
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ACPC
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1.031
34,800
−36.45
0.9947
Temp. (K)
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40,100
−36.81
0.9968
313
1.022
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0.9965
323
1.048
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−39.89
0.9959
313
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0.9944
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1.100
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0.9937
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1.140
26,200
−39.28
0.9930
K
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ΔG
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R
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