Effective inhibition of mild steel corrosion by 6-bromo-(2,4-dimethoxyphenyl)methylidene]imidazo [1,2-a]pyridine-2-carbohydrazide in 0.5 M HCl: Insights from experimental and computational study

Article abstract of DOI:10.1016/j.molstruc.2021.130074

A new inhibitor, 6-bromo-(2,4-dimethoxyphenyl)methylidene]imidazo [1,2-a]pyridine-2-carbohydrazide (DMPIP) was evaluated as a corrosion inhibitor for Mild Steel (MS) in 0.5 M HCl solution at 303–323 K using potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) techniques. Both the techniques confirmed an increase in inhibition efficiency with the concentration of DMPIP but decrease with temperature. The highest inhibitive action (96.7%) was registered at 303 K for 500 ppm of DMPIP concentration. Polarization study revealed mixed inhibition action by DMPIP. Nyquist plot obtained for MS using EIS technique showed two capacitive loops on addition of inhibitor to HCl solution confirmed the inhibitory action of DMPIP via adsorption at the metal/solution interface. The surface morphology analysis was carried out by SEM, EDX and FTIR techniques. The adsorption process was demonstrated using Langmuir's adsorption isotherm model. The thermodynamic parameters (?G^o_ads, ?H^o_ads) indicated that the adsorption was spontaneous and done by physisorption. Further, quantum chemical studies using Density Functional Theory (DFT) elucidated that the formation of Fe-DMPIP complex presumably due to the interaction of protonated form of DMPIP with the empty d orbitals of the iron atom.

Full text of DOI:10.1016/j.molstruc.2021.130074

Journal of Molecular Structure 1232 (2021) 130074
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Effective inhibition of mild steel corrosion by
-bromo-(2,4-dimethoxyphenyl)methylidene]imidazo
1,2-a]pyridine-2-carbohydrazide in 0.5 M HCl: Insights from
experimental and computational study
6
[
a
b
a,∗
K Vranda Shenoy , Pushyaraga P Venugopal , P D Reena Kumari ,
b,∗∗
Debashree Chakraborty
a
Department of Chemistry, Shri MadhwaVadiraja Institute of Technology and Management, Bantakal, Udupi, Karnataka, India
b
Biophysical and Computational Chemistry Laboratory, Department of Chemistry, National Institute of Technology Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new inhibitor, 6-bromo-(2,4-dimethoxyphenyl)methylidene]imidazo [1,2-a]pyridine-2-carbohydrazide
Received 6 November 2020
Revised 1 February 2021
Accepted 3 February 2021
Available online 6 February 2021
(
DMPIP) was evaluated as a corrosion inhibitor for Mild Steel (MS) in 0.5 M HCl solution at 303–323 K us-
ing potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) techniques. Both the
techniques confirmed an increase in inhibition efficiency with the concentration of DMPIP but decrease
with temperature. The highest inhibitive action (96.7%) was registered at 303 K for 500 ppm of DMPIP
concentration. Polarization study revealed mixed inhibition action by DMPIP. Nyquist plot obtained for
MS using EIS technique showed two capacitive loops on addition of inhibitor to HCl solution confirmed
the inhibitory action of DMPIP via adsorption at the metal/solution interface. The surface morphology
Keywords:
Corrosion inhibitor
Mild steel
Electrochemical study
Langmuir’s adsorption isotherm
DFT studies
analysis was carried out by SEM, EDX and FTIR techniques. The adsorption process was demonstrated
using Langmuir’s adsorption isotherm model. The thermodynamic parameters (࢞Goads, ࢞Hoads) indicated
that the adsorption was spontaneous and done by physisorption. Further, quantum chemical studies us-
ing Density Functional Theory (DFT) elucidated that the formation of Fe-DMPIP complex presumably due
to the interaction of protonated form of DMPIP with the empty d orbitals of the iron atom.
©
2021 Elsevier B.V. All rights reserved.
1
. Introduction
the most practical, easily operative and cheapest methods [1,2].
By definition, corrosion inhibitors are substances which are sol-
uble in an aggressive solution, without participating in the over-
all reaction reduces the corrosion rate of the metal. From litera-
tures, it is found that various organic compounds have been uti-
lized as corrosion inhibitors for MS in HCl solution, such as organic
dyes [3], ionic liquids [4], nonionic surfactants [5], gemini surfac-
tants [6,7], natural polymers [8,9], carbaxmide ligands [10], natu-
ral compounds [11,12], polysaccharides [13,14],extracts of natural
substances [15–18], polypeptides [19]amino acids [20,21], Schiff
bases [22,23]. The inhibitor efficiency depends on the nature of
corrosion medium, the nature of metal surface, electrochemical po-
tential at the interface, and also on the geometrical configuration
of the inhibitor [24,25].
Mild steel (MS) consists of less than 0.3% of carbon content,
which is the most commonly used forms of steel in many in-
dustrial applications as a construction component in building in-
dustry, automobile industry, petroleum industry, power plants, etc.
due to its advantageous features like malleability, ductility, high
strength as well as low cost. But dissolution process is high in
presence of acidic media during industrial processing like clean-
ing, pickling due to low resistance of mild steel for an acid attack.
Among the various measures applied to protect metals from the
aggressiveness of acid, the use of inhibitors is considered as one of
∗
Corresponding author at: Chemistry laboratory, Department of chemistry, Shri
Heterocyclic compounds are considered as an efficient organic
corrosion inhibitor against metal in an aggressive medium [26].
Due to the presence of aromatic rings, electron-donating groups,
polar molecules, and pi bonds along with the electronegative het-
eroatoms such as nitrogen, oxygen, sulfur. They may act as barrier
by forming an adsorbed layer on the metal surface or retard the
MadhwaVadiraja Institute of Technology and Management, Bantakal, Udupi affili-
ated to VTU, Balagavi, Karnataka 574 115, India.
∗∗
Corresponding author at: Biophysical and Computational Chemistry Laboratory,
Department of Chemistry, National Institute of Technology Karnataka, Surathkal,
Mangalore 575025, India.
E-mail
addresses:
reena.chemistry@sode-edu.in
(P.D.
Reena
Kumari),
debashree@nitk.edu.in (D. Chakraborty).
https://doi.org/10.1016/j.molstruc.2021.130074
0
022-2860/© 2021 Elsevier B.V. All rights reserved.

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
cathodic, anodic or both the reaction.Thus blocking the active cor-
rosion centers and decreases the corrosion rate [27,28].
Imidazo [1,2-a] pyridines are the important class of hete-
rocyclic compounds, find great importance in the pharmaceuti-
cal industry as a bioactive molecule like antiviral [29], antifun-
gal [30], antibacterial [31], anti-inflammatory, analgesic and an-
tipyretic [32]. Schiff bases of imidazo pyridines are also important
compounds due to its biological activity such as antibacterial [33],
antimalarial [34,35], antifungal [36], and anticonvolusant [37]. But
recently imidazo pyridine derivatives and Schiff bases of imidazo
pyridines are also reported as corrosion inhibitors for mild steel in
HCl medium with different substituent position and group due to
its ability to donate electrons as well as a possible back donation
which affects the corrosion rate and increases inhibition efficiency
[
23,38–42].
A review of the literature reveals that the structural modifi-
cations by incorporating different functionalities in their structure
largely improve their applicability as potential corrosion inhibitors
[
43,44]. Therefore the objective of the present work is to inspect
the utilization of the newly synthesized imidazo [1,2-a] pyridine
derivative viz 6-bromo-(2,4-dimethoxyphenyl)methylidene]imidazo
[
1,2-a]pyridine-2-carbohydrazide (DMPIP) as corrosion inhibitor for
MS in 0.5 M HCl solution. Commonly, Hydrochloric acid is pre-
ferred as pickle liquor during the surface cleaning of iron alloys or
steel surfaces due to its desired surface cleaning effect. However,
at higher concentrations of HCl, hydrogen embrittlement becomes
a problem for mild steel and that leads to considerable degradation
of mechanical properties [45]. Hence, acid concentration and solu-
tion temperature must be kept under control to ensure the desired
pickling rates.
Thus
the
inhibitory
behavior
of
6-bromo-(2,4-
dimethoxyphenyl)methylidene]imidazo
[1,2-a]pyridine-2-
carbohydrazide (DMPIP) was systematically evaluated for MS
in 0.5 M HCl solution at 303–323 K using potentiodynamic po-
larization curves, electrochemical impedance spectroscopy (EIS),
scanning electron microscopy (SEM), Energy-dispersive X-ray spec-
troscopy (EDX), Fourier Transform Infrared Spectroscopy (FT-IR),
and quantum chemical calculations. To get an insight on the ad-
sorption mechanism of the inhibitor on metal surface temperature
Fig. 1. (a) Structure of 6-bromo-(2,4-dimethoxyphenyl)methylidene]imidazo[1,2-
a]pyridine-2-carbohydrazide (b) Reaction Scheme for the synthesis of 6-bromo-
effect, kinetic, and thermodynamic parameters (Ea, Kads, ࢞G˚ads
)
(
2,4-dimethoxyphenyl)methylidene]imidazo[1,2-a]pyridine-2-carbohydrazide (c) FT-
were considered. The correlation between molecular structure and
inhibition properties were investigated using quantum chemical
parameters. Also, various quantum indices related to the molecule
were studied and compared with the experimental inhibitive
efficiency data of DMPIP.
−1
−1
IR spectrum for DMPIP inhibitor in the frequency range 500 cm to 4000 cm .
terized by melting point (Digital melting point apparatus EQ 730
by Equiptronics) and FT-IR spectra as shown in Fig. 1(c) (Bruker
Instruments, frequency range 500 cm 1 to 4000 cm
−
−1
)
Melting Point: 252–258 °C. FTIR (ATR, cm 1): 3246 (N–H), 3083
−
2
. Experimental
(Ar-C), 2903 (O–CH ), 2806 (O–CH ), 1664 (C=O), 1533 (C=N),
3
3
1
485 (C=C), 1272 (C–O-C), 1190 (C–N), 1110 (C–O-C) and 1027 (C-
2
.1. Materials and methods
Br) (Fig. 1(c)).
Electrochemical studies were carried out in a conventional
three-electrode cell consisting of a saturated calomel electrode
(SCE) as the reference electrode, platinum electrode as an auxiliary
electrode (CE), and cylindrical MS coupons coated with epoxy resin
as a working electrode (WE), using CH electrochemical workstation
(Model No: CHI 608E) manufactured by CH Instruments, Austin,
USA. The finely polished test coupons were dipped in 0.5 M HCl
possessing 50 ppm, 100 ppm, 200 ppm and 500 ppm of DMPIP
at the temperature range of 303–323 K. Before each experiment,
the working electrode was immersed in the test solution for 1800
seconds to reach the quasi-equilibrium state. The relationships be-
tween open circuit potential (OCP) values and immersion time for
MS in 0.5 M HCl solution without and with DMPIP at 303 K are
shown in Fig. 2. It is clear that the OCP values remains almost con-
stant after 1000 s indicating the system under study has reached
the steady state. Thus all the electrochemical tests were performed
after 30 min immersion in the corresponding test solution.
The MS coupons with following composition (wt.%): C 0.18%,
Mn 0.6%, S 0.05%, P 0.04%, Si 0.1%, and remaining of iron were used
for electrochemical study. The epoxy resin embedded mild steel
strip of 0.502 cm2 of area was exposed to the medium for investi-
gation. Before the experiment, test coupon was abraded with 500
to 2000 grade silicon carbide papers successively, then washed and
rinsed with doubly distilled water, immersed in acetone followed
by air drying. The electrolyte solution, 0.5 M HCl, was prepared us-
ing analytical grade 37 wt% HCl (Merck) and suitable volume of pu-
rified water. The concentration of DMPIP used for this investigation
ranged from 50–500 ppm (mg/L) by weight in 100 mL of the test
solution. The testing solution was kept under aerated condition at
3
03–323 K successively by using a water thermostat.The molecular
structure of DMPIP is shown in Fig. 1(a). The molecule was syn-
thesized using the reported procedure [37]; the reaction scheme
for the synthesis is shown in Fig. 1(b). The molecule was charac-
2

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
hibitor and that of obtained from the surface of the MS copouns
immersed in 0.5 M HCl solution containing 500 ppm DMPIP for
15 h at 298 K were recorded.
3. Results and discussion
3
.1. Tafel polarization study
The electrochemical kinetics of corrosion process was studied
using Tafel polarization study in absence and presence of vary-
ing concentrations (50–500 ppm) of DMPIP in 0.5 M HCl solution.
The corrosion potential (Ecorr) and corrosion current density (icorr),
slope of the cathodic branch (βc) and slope of the anodic branch
(
β
a) were acquired from tafel polarization curves. The rate of cor-
rosion (νcorr) was calculated from icorr using expression 1 [48,49].
Fig. 2. Open Circuit Potential values versus Time for MS in 0.5 M HCl with and
ꢀ
ꢁ
3
270 × M × icorr
q × Z
−
1
without inhibitor.
v
corr mm y
=
(1)
where 3270 is a constant of corrosion rate, icorr refers corrosion
Polarization curves were obtained in the potential range from
−2
current density (A cm ), q is the corroding material density (g
−
200 to +200 mV after establishing the OCP with a scan rate of
−3
cm ), M is the metal atomic mass and Z is the number of elec-
−1
1
mVs . All the reported potential values were referred to SCE.
trons transferred/atom. Eq. (2) is used to calculate the percentage
The corrosion current density (icorr) values were obtained by the
Tafel extrapolation method. The electrochemical impedance mea-
surements were performed in the frequency range of 100 kHz to 10
mHz with 5 mV amplitude at OCP. EIS data were investigated and
then analyzed using Zsimpwin software. Reproducibility checked at
least three times for each experimental setup, and the average val-
ues were taken and presented.
of inhibition efficiency (η%).
0
corr
i
− icorr
η% =
× 100
(2)
o
icorr
where, i˚corr and icorr are values of corrosion current density val-
ues for blank solution and solutions with various concentrations
of DMPIP, respectively. From Fig. 3(a), it is visible that the nature
of polarization curves remains unchanged both in uninhibited and
inhibited solution. But in the presence of DMPIP and as the con-
centration was increased, the polarization branches shifted toward
lower current density region indicating slowdown of the electro-
chemical reactions. The related experimental parameters are listed
in Table 1, shows that as the concentration of the inhibitor in-
creases, icorr decreases, correspondingly, η% increases. However, an
increase in the νcorr with temperature suggests decreased inhibi-
tion efficacy by DMPIP molecules on MS surface at higher tem-
peratures. DMPIP recorded best inhibition efficiency (96.79%) at
500 ppm at 303 K, indicating that at this optimum concentra-
tion, DMPIP can provide more significant inhibitor protective effect
against metal corrosion. Higher temperature tafel plots for 313 K
and 323 K are given in Supplementary Information (Figure S1 and
Figure S2).
2
.2. Quantum chemical study
Density functional theory (DFT) calculations are considered as
the “green corrosion inhibition method” which helps to under-
stand the molecular structure and its corrosion inhibition behavior.
The electronic/molecular properties and reactivity indices of the
inhibitor can accurately predict its corrosion inhibition efficiency.
Correlation between experimental investigations regarding inhibi-
tion efficiency and theoretical calculations helps in understand-
ing the observed experimental behavior of the inhibitor by study-
ing the molecular structure. DFT was used to analyze the elec-
tronic properties of the DMPIP molecule in its neutral and pro-
tonated forms in aqueous phase by Becke’s three-parameter ex-
change functional theory with the Lee–Yang–Parr non local corre-
lation functional (B3LYP) [46] and 6–311++G(d, p) [47] basis set in
Gaussian09 program package using SCF approach. Quantum chem-
ical parameters associated with energies such as energy of highest
occupied molecular orbital (EHOMO), energy of lowest unoccupied
molecular orbital (ELUMO) and energy gap (ꢀEgap) have been as-
sessed. Other chemical parameters like electronegativity (χ), elec-
tron affinity (A), ionization energy (I), global softness (σ), global
hardness (η), dipole moment (μ) and the fraction of electrons
transferred (ꢀN) were also studied. Mulliken charge distribution
and Fukui indices were calculated to analyze the local reactive sites
on the inhibitor molecule.
Table 1, reveals that there was no noticeable shift in corro-
sion potential values (Ecorrinh) were observed after the addition of
DMPIP with concerning to the corrosion potential values (Ecorrblank
)
of blank solution. Normally, inhibitors can be arranged as either
anodic or cathodic if the shifting of Ecorrinh values exceeds ± 85 mV
compared to the Ecorrblank values [50]. However, in this work, max-
imum displacement was less than ± 85 mV; hence, DMPIP can
be classified as mixed-type inhibitor controlling both dissolutions
of metal and hydrogen liberation reactions [51,52]. Additionally, to
get more insight of the inhibition mechanism all the polarization
plots were displaced to the zero potential (Ecorr = 0) as shown in
Fig. 3(b). The displacement of both of the cathodic and anodic cur-
rent densities towards lower current densities at different concen-
trations of DMPIP evidence a mixed inhibition effect on the MS
[18,53]. At higher concentrations of inhibitor, the icorr values de-
crease more predominantly suggesting the more or less complete
isolation of metal surface from the aggressive media by forming a
protective barrier due to the accumulation of a higher number of
DMPIP molecules at the MS surface which provides wider surface
coverage area.
2
.3. Surface morphology investigations
SEM images and EDX images of the test samples were recorded
and analyzed after exposure to 0.5 M HCl solution in the absence
and presence of DMPIP for 15 h using SEM model Carl Ziess FE-
SEM and Oxford instruments respectively. The adsorption of DMPIP
on MS was elucidated by probing the surface of the working
electrodes using FTIR spectroscopic technique. Bruker instruments
−1
−1
spectrometer in the frequency range of 500 cm
to 4000 cm
As it can be noted from Table 1, compared with the blank so-
lution, the anodic Tafel slopes (βa) for inhibitors exhibit the ob-
used for FTIR analysis to study the FTIR data related to DMPIP in-
3

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 3. Tafel curves for MS corrosion without and with DMPIP inhibitor in 0.5 M HCl solution at 303 K (a) Original Ecorr, (b) Displacement of Ecorr to the zero potential.
Table 1
Potentiodynamic polarization parameters for the MS corrosion in 0.5 M HCl without and with DMPIP at 303–323 K.
icorr (μA cm 2)
−
ν
corr(mm y
−1
)
(ƞ%)
Temperature (K)
Inhibitor conc (ppm)
β
a (mV)
β
c (mV)
Ecorr (mV)
3
3
3
03
13
23
Blank
175
174
150
138
117
111
−510
−502
−524
−532
−551
2805
830
401
151
96.7
32.56
9.65
4.01
1.51
0.97
5
1
2
5
0
131.5
121.6
117
70.35
86.78
95.01
96.79
00
00
00
152
Blank
182
138
135
149
162
186
148
144
138
134
−501
−495
−512
−519
−544
3400
1100
620
39.54
12.79
7.21
5
1
2
5
0
67.64
81.76
92.20
95.14
00
00
00
265
3.08
165
1.91
Blank
194
175
147
160
188
197
181
153
143
151
−504
−511
−518
−526
−537
3800
1350
1010
594
44.19
15.70
11.74
6.90
5
1
2
5
0
64.47
73.42
84.36
89.21
00
00
00
410
4.76
vious decrease noticeably at lower concentrations of DMPIP (50–
1
00 ppm), which is more remarkable at T = 303 K. This suggests
for low concentrations, the adsorption is preferentially at the an-
odic sites leading tothe formation of Fe–Inh complex compound
which influences the dissolution of Fe. On the other hand, further
enhancement of η% with increasing inhibitor concentration (200–
5
00 ppm) suggests that DMPIP molecules impede iron dissolution
likely by blocking off the anodic reactive sites [54]. But, insignifi-
cant decrement of cathodic Tafel slopes (βc) concerning the blank
indicating that the hydrogen evolution process is activation con-
trolled, and the addition of DMPIP reduce the kinetics of cathodic
reaction by blocking the active cathodic sites without changing the
mechanism of this process [11,13].This is further supported by the
parallel cathodic plots of the inhibited solution to the cathodic
branch of the blank solution (Fig. 3(a)) while the shape and slope
of the anodic plots were changed significantly with the addition of
DMPIP, revealing the impact of the additives on the iron dissolu-
tion mechanism. [18].
Fig. 4. lnvcorrvs 1/T for MS corrosion in 0.5 M HCl solution with various concentra-
tions of DMPIP.
3.1.1. Thermodynamic and activation parameters
The data in Table 1 shows a linear association of corrosion rate
with temperature. The influence of temperature on corrosion rate
without and with inhibitors is expressed by Arrhenius Eq. (3). The
graphical representation of ln vcorr vs. 1/T at different concentra-
tions of DMPIP is given in Fig. 4. [55].
where Eais the energy of activation for MS dissolution in 0.5 M hy-
drochloric acid solution, R terms for universal gas constant, T refers
to absolute temperature (K), and A denotes the frequency factor.
The slope of the straight line obtained in Fig. 4 was used to cal-
culate the Ea value for the dissolution of MS in acid solution. The
data in Table 2 indicates that the Ea values in the solution with
−Ea
lnvcorr
=
+ lnA
(3)
RT
4

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Table 2
Activation energy parameters of DMPIP in 0.5 M HCl.
−1)
∗
−1
∗
−1
−1
K )
Inh. Conc (ppm)
Blank
Ea(kJ mol
࢞H (kJ mol
)
࢞S (J mol
14.675
19.621
36.506
54.024
47.572
11.35
17.27
33.98
51.49
44.23
−178.54
−168.86
−120.22
−70.81
5
1
2
5
0
00
00
00
−98.70
Fig. 6. Langmuir adsorption isotherm model of DMPIP on the MS in 0.5 M HCl at
303–323 K.
inhibitor DMPIP molecules on the MS surface from the hydrochlo-
ric acid solution retards the hydrogen ion discharge from the metal
surface, which is the rate-determining step.
3
.1.2. Adsorption isotherm
Two types of adsorption modes are generally possible during
Fig. 5. Plot of ln vcorr/T versus 1/T for MS in 0.5 M HCl solution with different con-
centrations of DMPIP.
the corrosion inhibition process. Physical adsorption or physisorp-
tion and chemisorption. In physisorption, there are weak electro-
static interactions between the organic ions or dipoles of the in-
hibitor and charged metal surface while in chemisorption, the elec-
tron sharing or transfer occurs between inhibitor and metal surface
to form a strong coordinate bond.
different concentrations of DMPIP exceed than that of the blank
solution. This suggests that the presence of DMPIP influences the
energy barrier for the corrosion process of MS by reducing corro-
sion rate and consequently increasing the ƞ%. A higher value of Ea
for inhibited solution with reference to blank suggests physical ad-
sorption of DMPIP at the MS surface. In addition, a low value of
Corrosion
inhibition
involves
interactions
at
the
metal/electrolyte interface which is totally a surface phenomenon.
The degree of surface coverage θ for various concentrations of the
additives was obtained from Eq. (5) and it was employed to fit
graphically for different adsorption isotherms such as Freundlich,
Frumkin, Temkin, Flory-Huggins, Langmuir, etc.
Ea (47.57 kJ mol 1) at the optimum concentration (500 ppm) of
the inhibitor suggests stability of corrosion product formed at the
metal/solution interface [56]. Further, higher values of Ea (19.62
−
to 47.57 kJ mol 1) in the presence of inhibitor could be attributed
for a considerable decrease in adsorption and increase in desorp-
tion/dissolution of the DMPIP molecules with the rise in tempera-
ture leading to increase the corrosion rate owing to the direct con-
tact of the bare metal surface with acid [57].
−
η%
θ =
(5)
100
From all the tested models, the best correlation between ex-
perimental data and isotherm function was obtained for the Lang-
muir isotherm with the slopes and linear regression coefficients
∗
The values of enthalpy of activation (ꢀH ) and entropy
R2) close to unity. The plot of Cinh/θ versus C
(
is represented in
inh
∗
of activation (ꢀS ) for metal dissolution were obtained from
Eq. (4). [55].
Fig. 6. Langmuir isotherm model is expressed using Eq. (6) [63–
5].
6
ꢂ
ꢃ
ꢂ
ꢃ
∗
∗
RT
ꢀS
ꢀH
Cinh
1
v
corr =
exp
exp
−
(4)
= K + Cinh
(6)
Nh
R
RT
θ
ads
where Cinh is the concentration of inhibitor, Kads is the adsorption-
desorption process equilibrium constant.
Where, h refers to Planck’s constant, and N gives the number
of Avogadro. The plots of lnvcorr/T versus 1/T gave straight line
as shown in Fig. 5. ꢀH and ꢀS were calculated from the slope
∗
∗
The Kads equilibrium constant, obtained from the intercepts of
the straight lines in the Fig. 6, represents the strength of inter-
action between adsorbed inhibitor and surface of the MS. Larger
values of Kads shows higher effectiveness of adsorption and bet-
ter protection efficiency obtained by the inhibitor. In the present
study, Kads value is more at room temperature (Table 3) showing
efficient corrosion inhibitive action by the inhibitor. Kads is related
to the standard free energy change during the adsorption process,
∗
∗
(
−ꢀH /R) and intercepts (lnR/Nh+ ꢀS /R) respectively are listed in
∗
Table 2. The positive values of ꢀH reflects the endothermic nature
of dissolution of MS in the presence of DMPIP [58,59] whereas
higher values in solution with varied inhibitor concentrations com-
pared to blank solution advocate its inhibitive actions to check the
electrochemical dissolution of MS in acid solution. The values of
∗
ꢀ
S
for the solutions in presence of DMPIP are more positive than
࢞
G˚ads, and using Eq. (7) [66]:
that of the blank solution indicates an increase in the randomness
of the dissolution process of metal in the presence of acid, which
is the driving force for the adsorption of inhibitor on MS surface
to form the activated complex [60–62]. For uninhibited solution,
the rate-determining step of the transition state represents the or-
derly arrangement than in the initial state. The adsorption of the
o
ads
ꢀ
G
= −RTln(Kads × 55.5)
(7)
K)
−1
−1
mol
Where, R is the universal gas constant (8.314 J
and T is the absolute temperature and 55.55 is the molar con-
centration of water in solution (mol L 1). The standard enthalpy
−
5

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Table 3
Adsorption parameters for MS for DMPIP inhibitor in 0.5 M HCl solution.
−1
−1
−1
−1
K )
Temperature (K)
Kads (104) M
Slope
࢞G˚ads(kJ mol
)
࢞H˚ads(kJ mol
−27.7
)
࢞S˚ads(J mol
−40.0
3
3
3
03
13
23
118.9
81.23
61.44
1.02
1.00
1.07
−39.5
−39.9
−40.3
Fig. 8. Interaction of metal with inhibitor showing physisorption and chemisorption
◦
process.
Fig. 7. Relationship between ꢀG ads and T.
change (࢞H˚ads), standard entropy change (࢞S˚ads) of adsorption
of DMPIP on MS surface is calculated using Eq. (8) [67]:
0
ads
0
ads
0
ads
ꢀ
G
= ꢀH − TꢀS
(8)
The plot of ࢞G0
equal to -࢞S0
vs T gives a straight line (Fig. 7), with slope
and intercept ࢞H0
ads
ads
ads.
The thermodynamic parameters are listed in Table 3 for the
◦
adsorption of DMPIP on MS surface. ꢀG
values are negative
ads
ensured adsorption of DMPIP molecules on the electrode surface
is spontaneous and strength of interactions is high between the
charged inhibitor and metal surface. Generally, if the values of
G˚ads are around −20 kJ mol− or more positive, it is said to be
1
࢞
−1
physical adsorption process and if it is around −40 kJ mol
or
more negative, then it is termed as chemisorption [68,69]. How-
ever, the value of −40 kJ mol− is referred to as threshold value
between physisorption and chemisorption [70].
1
Fig. 9. Nyquist plot for MS in 0.5 M HCl solution without DMPIP and with different
concentration of DMPIP at 303 K.
The calculated ꢀG˚
values (Table 3) are in the range of −39.5
ads
to −40.3 kJ mol−1 for the studied range of temperatures which
indicates that the adsorption involves both physisorption as well
as chemisorption process. Indeed, the corrosion inhibition by the
DMPIP molecule on mild steel attributed as physisorption process
since activation energy values are high for inhibited solution com-
pared to the blank. Initially, adsorption might occur by physisorp-
tion due to adsorbed water molecule on the MS surface, which is
followed by chemical interaction with the inhibitor molecules by
replacing the water molecules on the metal surface [71]. In hy-
tion process [73,74]. Therefore, though the above theory (based on
ꢀ
G˚ads values) suggests both physical and chemical adsorption of
DMPIP molecules on the MS surface the lower threshold value of
࢞
H˚ads (−27.7 kJ mol 1) confirms that DMPIP molecules are mainly
−
adsorbed on the MS surface through physisorption phenomenon.
The entropy of adsorption, ࢞S˚ads calculated is found to be nega-
tive, suggesting decrease in the disorderness from the reactant to
the adsorbed species. The adsorption nature of the inhibitor and
the guiding energy were further evaluated from short vaccum MD
simulations and the detailed results are given in Supplementary
Information (Figure S3 and Figure S4).
−
drochloric acid medium, Cl ions also compete for the adsorp-
tion along with the organic inhibitors and itis thought that ab-
sorbable anions like chloride ions are adsorbed on the electrode
surface through creating oriented dipoles [72]. Thus, the negatively
charged MS surface attracts the cationic inhibitor species resulting
in the electrostatic accumulation of protonated DMPIP molecules
3.2. Electrochemical Impedance spectroscopy (EIS)
−
onto the Cl adsorbed steel surface. While in chemisorption type,
In order to study the kinetics and characters of electrochemical
reactions for mild steel, the EIS analysis was done. Fig. 9 repre-
sents the Nyquist plot obtained at OCP, while Table 4 summarizes
the impedance data for MS in the absence and presence of differ-
ent concentrations of inhibitor DMPIP at 303 K in 0.5 M HCl so-
lution. Nyquist plots show depressed capacitive loops with center
under the real axis resulted due to roughness and inhomogeneities
the protonated inhibitor donates electrons to the metal by losing
proton as shown in Fig. 8 [10].
An exothermic adsorption process is confirmed by the negative
࢞
H˚adsvalues (Table 3). According to the earlier reported works, if
−1
the value of ࢞H˚ads is less than or equal to −40 kJ mol
refers
to physisorption and more than −100 kJ mol− for chemisorp-
1
6

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Table 4
ElS parameters for the corrosion of MS in 0.5 M hydrochloric acid medium having different concentrations of DMPIP at 303–323 K.
Rs (Ωcm2)
Rf (Ωcm2)
Rct (Ωcm2)
CPEdl (10 ) (μFcm
−4
−2
)
χ2 (10–3
)
η%
Temperature (K)
Inhibitor Concentration (ppm)
3
3
3
03
13
23
Blank
0.83
0.67
0.6
10.8
33.1
74.98
138
5.36
2.42
2.70
1.49
1.46
4.7
0.69
2.1
3.2
4.1
5
1
2
5
0
3.7
5.3
4.4
4.3
67.37
85.59
92.17
94.85
00
00
00
0.7
0.6
210
Blank
0.74
0.5
8.7
8.99
5.62
5.35
4.58
2.04
2.0
5
1
2
5
0
3.8
21.95
53.55
121
0.24
1.5
60.36
83.75
92.80
93.14
00
00
00
0.65
0.8
5.9
14.6
28.1
1.01
1.3
0.6
127
Blank
0.73
1.2
7.01
9.62
6.29
5.82
5.22
1.78
2.0
5
1
2
5
0
6.8
15.61
29.22
41.35
59.8
0.97
1.7
55.09
76.00
83.04
88.27
00
00
00
0.33
0.22
0.7
4.6
8.6
2.1
14.08
0.46
on the solid surface [75,76]. As compared to the blank solution,
the diameter of the semi-circles in the Nyquist plot increases for
inhibited solutions with increasing concentration of the inhibitor.
These results confirm the inhibitory action of DMPIP molecules at
the MS surface.
Fig. 10 represents the equivalent circuit for the mild steel dis-
solution process to analyze the electrochemical data using Zsim-
pwin 3.2.1 software. It is observed that an acceptable accuracy of
2
the fitting was obtained, as evidence by Chi-square (χ ) in the or-
der of 10 3 for all the experimental data as shown in Table 4. On
addition of DMPIP inhibitor, impedance response changes signif-
icantly in hydrochloric acid media showing two capacitive loops
in its Nyquist plot. The response of the EIS spectrum presented
here shows similarity with the previous reports [13,16,54,77,78].
Wang et al. [78] explained that one capacitive loop formed due
to the double layer capacitance at higher frequency ranges and
the other is due to the continuous adsorption and desorption pro-
cess at lower frequencies. Due to the non-ideal capacitive behavior,
which is commonly encountered during the electrochemical stud-
ies of solid/liquid interfaces, it is necessary to replace an ideal ca-
−
pacity (C ) by a constant phase element CPE while fitting the
dl
dl
EIS data as shown in Fig.10(b) and (d) [79]. The fitting result
Table 4) shows that Rct (charge transfer resistance) and Rf (pro-
(
tective film resistance) increases with increasing DMPIP concentra-
tion and CPEdl (double layer capacitance) values decrease at higher
frequencies inferring that adsorption of inhibitor molecule on the
MS surface of the electrode increases [80]. CPE is mathematically
expressed as following equation [54]:
ZCPE = Y− (jω)
1
−n
(9)
o
Where Yo is the magnitude of the CPE, j is the imaginary unit,
ω is the angular frequency, and n is the phase shift gives details
about the degree of surface inhomogeneity [54,79].
The CPEdl values decreased with the addition of DMPIP as it is
evident from Table 4 ascribed due to the reduction in local dielec-
tric constant or increase in the thickness of the protective layer at
the solid/solution interface region. In other words the gradual re-
placement of the adsorbed water molecules at the steel surface by
DMPIP molecules with lower dielectric constant leading to lower
value of CPEdl [81]. It is apparent from Fig. 9 that the radius of
the capacitive loop is increased with the increase of DMPIP con-
centration suggesting enhanced impediment of the surface film for
Fig. 10. (a) Experimental and fitted modelof MS dissolution in HCl medium with
inhibitor DMPIP (b) Equivalent circuitof MS dissolution in HCl medium with in-
hibitor DMPIP (c) Experimental and fitted model of MS dissolution in HCl medium
without inhibitor DMPIP (d) Equivalent circuit of MS dissolution in HCl medium
without inhibitor DMPIP.
charge transfer process at the metal-solution interface. Rct and Rf
values are found to increase with additives concentration presum-
ably due to the increased imperviousness of the protective film on
the steel surface. The η% was calculated from the charge transfer
7

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 11. (a) Bode phase angle plots and (b) Bode modulus plots for MS in 0.5 M HCl in the absence and presence of different concentrations of DMPIP inhibitor.
Table 5
resistance values using the Eq. (10).
Percentage atomic contents of elements derived from EDX spec-
o
ct
R
− Rct
tra.
η% =
× 100
(10)
o
R
ct
Fe
Si
Mn
O
C
Cl
where R°ct and Rct are the charge transfer resistance values in the
absence and presence of DMPIP. The results in Table 4 corrobo-
rate that largest inhibitory effect of DMPIP at 303 K at 500 ppm
Polished
88.94
51.78
75.38
0.88
0.48
0.69
0.71
0.19
0.44
6.16
3.08
2.1
–
Blank
43.59
15.62
0.69
–
DMPIP
7.8
(
94.85%) which is in good agreement with the Tafel extrapolation
results.
There is
a
single peak for blank/uninhibited solution in
in Fig. 13. The EDX results showed the presence of Fe, O, Cl and C
on the steel surface as shown in Table 5. The highest oxygen con-
tent was present in the uninhibited solution and also showed the
chlorine peaks confirming the formation of corrosion product(iron
oxide or iron chloride) on the surface of MS (Fig. 13(b)). In the
case of MS coupon dipped in the corrosive media with 500 ppm
of inhibitor DMPIP for 15 h, no chlorine peaks, considerable de-
crease in oxygen content were observed and highest carbon con-
tent seen due to the interaction of inhibitor molecule with MS sur-
face to form the protective film, which thus reduces the corrosion
rate (Fig. 13(c)).
Bode phase angle plot (Fig. 11a) shows that the electrochem-
ical impedance measurements were fit well in one time con-
stant equivalent circuit model and also one phase maximum in
Bode Modulus plot (Fig. 11b) indicates only one relaxation pro-
cess, which would be the charge transfer process taking place at
the metal/electrolyte interface. For the inhibited solutions, as con-
centration of the inhibitor is increased, higher values of Bode phase
angle and broadening of the Bode diagrams observed which reflect
the formation of protective film on the MS surface due to the ad-
sorption of inhibitor molecule which gives a more capacitive re-
sponse [16,77,82].
The above electrochemical experimental results prove that the
structurally modified, newly synthesized, imidazo pyridine deriva-
tive namely DMPIP shows better inhibition action (96.7%) compare
to the earlier reported studies [42,43] on MS surface in 0.5 M HCl.
3.3.3. FT-IR spectroscopic analysis
To observe the attachment of inhibitor molecules to the surface,
the insoluble material formed on the surface of the corroded steel
coupons were scraped, and subjected to IR spectra. FTIR spectra
shows new bonding information of product formed on the steel
surface with and without inhibitor.
3
3
.3. Surface morphological analysis
The FTIR spectra of DMPIP inhibitor, MS sample after immer-
sion in 0.5 M HCl and MS sample after immersion in 0.5 M HCl
and 500 ppm of DMPIP inhibitor for 15 h is depicted in Fig. 14.
FT-IR spectrum of the mild steel surface with inhibitor in corro-
sive medium shows similar FT-IR spectrum as that of inhibitor and
the peak regions of the protective film are basically consistent with
that of pure DMPIP inhibitor. FT-IR spectra of inhibitor shows some
.3.1. Scanning electron microscopy (SEM) analysis
The SEM images of the MS surface in the absence and presence
of 500 ppm concentration of DMPIP in 0.5 M HCl solution exposed
to 15 h were recorded. Fig. 12(a) portrays the surface image of the
mirror-finished sample; Fig. 12(b) depicts the image of the sample
surface is rough and damaged in aggressive media, where HCl cor-
rodes the surface highly in the absence of DMPIP. Fig. 12(c) depicts
the SEM image of mild steel sample in presence of 500 ppm of in-
hibitor DMPIP, shows roughness of the surface has been decreased
infers the adsorption of DMPIP molecules which hampers the elec-
trochemical reactions of the MS in HCl solution by adsorbing on
the surface and decreased corrosion rate.
−1
characteristic bands such as the band around 3246 cm
is at-
tributed to (N–H) and band around 1664 cm 1 is due to (−C=O)
−
of amide carbonyl vibration. The spectrum of DMPIP shows bands
at 2806 cm 1 and 2903 cm
−
−1
corresponding to (−C–H) aliphatic
−1
of methyl group (−CH3) of (-O-CH3) entity. 1485 cm of aromatic
−
1
(C=C) stretching and 1533 cm is attributed to the (-C=N) of im-
idazo pyridine ring. The spectrum of the absorbed DMPIP on mild
steel shows remarkable change in absorption bands of functional
groups of DMPIP molecule. We can clearly observe that the in-
tensity of most bands are significantly reduced compared to spec-
trum of DMPIP. The wave number of the bands located at 1664
3
.3.2. Energy-dispersive X-ray spectroscopy (EDX) analysis
EDX investigations were performed to identify the composition
of MS surface in the presence and absence of DMPIP in 0.5 M
HCl solution. EDX method was used to determine the percent-
age atomic content of various elements of the polished, MS sur-
face with and without the presence of DMPIP. The corresponding
EDX analysis of the selected areas on the SEM images as shown
cm 1 and at 1533 cm is considerably reduced to 1647 cm and
−
−1
−1
1512 cm 1 suggesting that these regions are mainly involved in the
−
adsorption through the donor-acceptor process. Indeed, the spec-
8

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 12. SEM images of (a) Polished surface of MS, (b) MS surface in 0.5 M HCl without inhibitor and (c) MS surface in presence of 500 ppm DMPIP.
trum of adsorbed DMPIP inhibitor also shows the disappearance of
ular orbital) distribution characterize the nucleophilic component
whereas LUMO (lowest unoccupied molecular orbital) distribution
indicate electrophilic component in a molecule.
−
1
bands observed at 3083 cm
associated to aromatic (C–H) and
those at 3246 cm 1 of (−N–H) amide, suggesting the participa-
−
tion of these group atoms in chemical interactions with mild steel.
The higher HOMO energy of the molecule is responsible for the
higher electron-donating ability to appropriate acceptor molecule.
This explains adsorption of the DMPIP molecule on MS surface by
delocalized pi electrons or by unpaired electrons present on het-
eroatom. The lower LUMO energy signifies the higher electron-
accepting ability of a molecule [85]. ࢞Egap (ELUMO − EHOMO) de-
termines the reactivity of DMPIP toward the adsorption on MS
surface. A large value of ࢞Egap implies high chemical stability for
the molecule in reactions [86]. The HOMO-LUMO electron den-
sity over protonated and neutral DMPIP molecules in the aqueous
phase are presented in Fig. 16.
−1
The intensity of NH band has shifted from 3246 cm
to 3350
−1
is due to the hy-
cm 1. The wide band observed at 3350 cm
−
drogen bonding between MS surface and inhibitor molecule thus
confirms the interaction between protonated inhibitor DMPIP and
MS through hydrogen bonding forming protective layer on the MS
surface [53,83]
4. Quantum chemical calculations
Quantum chemical calculations can accurately predict
As seen from Fig. 16, DMPIP has different HOMO-LUMO electron
density in the protonated and neutral forms of DMPIP. The popula-
tions of HOMO densities are focused on imidazo pyridine ring hav-
ing nitrogen atoms in protonated form, while the neutral form has
the HOMO densities mainly around dimethoxy substituted benzene
moiety. The LUMO distributions are, however, found all over the
imidazo pyridine ring as well as dimethoxy substituted benzene
ring for the protonated form but in the neutral form, it is more
distributed in imidazo pyridine ring.
the ground state of each atom as well as molecules, the ex-
cited states and the transition states in chemical reactions. Thus,
quantum chemical properties play a major role in evaluating the
correlation between geometrical configuration and the inhibi-
tion efficiency. DFT calculations are extensively used by various
researchers to interpret the inhibition mechanism. Quantum chem-
ical calculations gives the reactivity indices for both neutral and
protonated form of DMPIP in the aqueous phase. In the present
study, experimental results are correlated and confirmed with
molecular parameters such as orbital energies, dipole moment,
hardness, softness, electrophilicity index, ionization potential,
Mulliken distribution and Fukui indices. The optimized geometry
for both neutral and protonated form of DMPIP in the aqueous
phase are shown in Fig. 15.
The protonated form of the DMPIP donates electrons to the un-
occupied d orbitals of iron atom to form coordination bond. In neu-
tral form, a back-donating bond is preferred by DMPIP molecule by
accepting the electrons from anti-bonding orbitals of the iron atom
[
87].
From Table 6, it is evident that DMPIP has high EHOMO in pro-
The distribution of frontier molecular orbitals, natural atomic
charge and local reactivity descriptors (Fukui indices) are consid-
ered as influencing factors to study the active sites on the in-
hibitors. Generally, Frontier molecular orbital theory (FMO) is con-
sidered to predict the adsorption centers of inhibitors responsi-
ble for the interaction with surface metal atoms [84]. The bond-
ing between DMPIP and MS surface can be predicted by con-
sidering EHOMO and ELUMO. The HOMO (highest occupied molec-
tonated form than in neutral form, suggesting weaker electron do-
nation ability in neutral form compared to protonated form. It can
also be observed that the neutral form of DMPIP shows the lowest
ELUMO value, making the neutral form most likely to have interac-
tions with the mild steel. The calculations further show that the
protonated form has lower ࢞Egapvalue (1.712 eV), indicating that
DMPIP in protonated form is the most reactive formwhich can be
9

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 14. FT-IR spectra of DMPIP and MS in 0.5 M HCl with 500 ppm of inhibitor.
easily adsorbed on MS surface effecting higher surface coverage.
This fact correlates well with the experimental data, which shows
the protonated form of inhibitor interacts with MS through electro-
static interaction. Thus, the strong interaction between the cationic
form of DMPIP and empty d-orbital of the iron atom substantially
enhanced the inhibitive action of DMPIP on MS surface through
physisorption [85]. Also, DMPIP adsorption on the MS surface by
the neutral form of DMPIP is possible in the overall inhibition pro-
cess.
The molecular descriptors such as ࢞Egap(Energy Gap), I (Ioniza-
tion energy), A (Electron Affinity), μ (Dipole Moment), χ (Absolute
Electronegativity), ƞ (hardness), σ (Softness), ω (electrophilicity in-
dex) and ࢞N (Fraction) are calculated using following equations
[
66]:
ꢀ
E (Energy Gap) = ELU MO − EHOMO
(11)
(12)
(13)
I (Ionization Potent ial ) = −EHOMO
A (Electron A f f inity) = −ELU MO
(
I + A)
(EHOMO + ELU MO
)
χ (Absolute electronegativity) =
= −
2
2
(
14)
(
I − A)
(EHOMO − ELU MO
)
η (hardness) =
σ (softness) =
= −
(15)
(16)
(17)
(18)
2
2
Fig. 13. (a) EDX spectra of polished MS, (b) EDX spectra of MS in 0.5 M HCl and
(
c) EDX spectra of MS in presence of 500 ppm of DMPIP.
1
hardness
Table 6
B3LYP/6–311++G (d, p) level.
μ2
ω (electrophilicity index) =
Aqueous phase
Neutral form
2
η
Quantum chemical parameters
Protonated form
(
χFe − χinhibitor)
(ηFe + ηinhibitor)
Total energy, ꢀET (au)
EHOMO (eV)
−3673.29
−5.826
−2.061
3.764
−3676.49
−3.499
−1.787
1.712
ꢀN (Fraction) =
2
ELUMO (eV)
Dipole moment is one of the major electronic parameters to
evaluate the intermolecular interactions like dipole-dipole interac-
tion, van der Waals interactions etc. From Table 6, it is found that
the dipole moment for DMPIP molecule (8.955 debye) is higher
compared to the dipole moment of water (1.85 debye) proves
DMPIP has higher propensity to adsorb on MS surface than al-
ready existing water molecules. A large dipole moment makes the
molecule more polar, which helps to adsorb on MS surface, in-
creasing the inhibition efficiency [88]. Consequently, the reaction
࢞
Egap (eV)
Dipole moment (D)
8.955
9.022
Ionization potential, I (eV)
Electron affinity, A (eV)
Electronegativity (χ)
Hardness (η)
5.825
3.499
2.061
1.787
3.943
2.643
1.882
0.856
Softness (σ)
0.5313
0.81
1.168
Fraction of electrons transferred (࢞N)
2.544
2
Electrophilicity, ω (D /eV)
21.28
47.85
10

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 15. Optimized geometry for DMPIP molecule obtained by DFT at B3LYP/6–311++G(d, p) level in (a) neutral and (b) protonated form in aqueous phase.
between the charged sites on the DMPIP and MS results in elec-
trostatic dipole-dipole interaction. Thus, the correlation between
the dipole moment and inhibition efficiency of the inhibitor sug-
gests the process of physical adsorption [89] of DMPIP on the MS
surface, as revealed experimentally. Besides, higher μ increases the
volume of the inhibitor, thereby increasing the contact area be-
tween the DMPIP and the MS surface for adsorption phenomenon
tonated form of DMPIP has the lowest hardness, lowest ꢀEgap
and the highest softness, making the moiety more reactive. This
manifests a better inhibitive performance of DMPIP in the proto-
nated cationic form through electrostatic attraction between in-
hibitor molecule and mild steel vacant d orbital (physisorption).
This agrees well with the obtained experimental results.
The fraction of electrons transferred from DMPIP to the mild
steel surface is denoted as, ࢞N [93]. NuhaWazzen et al. [94] ex-
plain that when ࢞N > 0, higher the electron transfer from the in-
hibitor to mild steel and when ࢞N < 0, the transfer of electrons
from the inhibitor to metal is low. When ࢞N < 3.6, the electron-
donating capability of the inhibitor molecule to metal surface in-
creases which enhance the inhibition efficiency of molecule. ࢞N
values show that the inhibition effect results due to the donation
of the electrons [95,96]. ࢞N values for the inhibitor in both neu-
tral and protonated found to be much lesser than 3.6, thus making
inhibitor DMPIP as electron donor and mild steel surface as the
acceptor of electrons.
[
90]. Thus, a higher dipole moment enhances the corrosion inhibi-
tion ability of the inhibitor.
Absolute hardness and softness measure the chemical reactivity
and kinetic stability of the molecule to a great extent. According
to Pearson, hard molecules with large Egap cannot yield good cor-
rosion inhibition efficiency [91]. A soft molecule has a small en-
ergy gap, and a hard molecule has a large energy gap [92]. Since
Fe metal atoms are considered as soft acids, according to HSAB
theory, the inhibitor molecules should possess larger softness and
lower hardness value to interact and get adsorbed on the metal
surface more easily than the hard molecules. Soft molecules are
more reactive because they can easily provide electrons to an ac-
ceptor molecule due small energy gap. It is found that the pro-
The global electrophilicity index (ω) specifies the potential of
the inhibitor to withdraw the electrons from the metal atom [97].
11

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 16. HOMO-LUMO energy distribution over DMPIP molecule by B3LYP/6–311++G (d, p) level in aqueous phase (a) protonated and (b) neutral molecule.
It has been reported that the higher the value of ω, the superior is
the ability to withdraw the electrons [47,98]. The protonated form
of DMPIP molecule has high ω value compared to the neutral form
and in addition, shows the highest ability to accept electrons from
the mild steel. This attributes to the formation of back donating
bond where the inhibitor receives electrons from the iron atom
along with the formation of coordination bond between the un-
occupied/ empty d-orbitals of the metal atom and the inhibitor
molecule. The process of donation and back-donation fortify the
adsorption of DMPIP onto the mild steel surface [85]; thereby in-
creasing the adsorption ability of the inhibitor molecule on the
mild steel surface.
efficiently the atom donates its electrons to the unoccupied orbital
of the metal atom by forming donor-acceptor interaction [101].
From Fig. 17, it can be noted that most of the nitrogen atoms,
oxygen atoms as well as some carbon atoms carry negative charge
centers. These charge centers share the electrons to the metal to
form a coordinate bond. Some of the carbon atoms are electron de-
ficient with high positive charge due to the delocalization of elec-
tron on the system. It should be noted that the neutral form of
the DMPIP inhibitor has more negative charge centers compared
to the protonated form which boosts its adsorption process and in-
creases the corrosion-inhibiting properties. Further, the total nega-
tive charge values of the common heteroatoms are smaller in case
of protonated forms compare to the neutral forms. Thus, the pro-
tonation process decreases the adsorption strength of the molecule
through its active adsorption sites on the mild steel surface. In
aqueous solution for the neutral form, the total negative charge
measured from the partial Mulliken charges on the heteroatoms
nitrogen and oxygen gives the information about the active cen-
ters present on the inhibitor which interacts with the mild steel
surface. More negatively charged atom, its tendency to donate elec-
trons increases [89]. 3N, 9N, 12N, and 13O (Fig. 17(b)) are the neg-
atively charged centers in neutral form, indicating these are the
active sites interact with steel, get adsorbed on mild steel surface.
The change in total energy (ꢀE ) is associated with the adsorp-
T
tion process which describes the energy change associated with
the donation and back-donation processes occurring between the
inhibitor and metal [94,99,100]. From Table 5, it can be seen that
η > 0 and ꢀE < 0 for both protonated and neutral form of DMPIP
T
in the aqueous phase. This result suggests the charge transfer to
DMPIP molecule accompanied by back-donation process is energet-
ically favorable [89]. Anyhow it is essential to note that the back-
donation process occurs is not completely predicted by ꢀET values
acquired. It shows whether both the processes (charge transfer to
the inhibitor and back donation from the inhibitor) occur in the
inhibition mechanism. Also, the change in energy is directly pro-
portional to the absolute hardness of the molecule.
4
.2. Fukui indices
4
.1. Mulliken population analysis
To analyze the behavior of the different site, it is essential
to consider local reactivity descriptors like the Fukui function
[102,103] which demonstrate the reactivity of the frontier molec-
Further, Mulliken population analysis was carried out to pre-
+
−
dict the adsorption site of the inhibitor molecule for both neu-
tral and protonated forms of DMPIP to analyze the factor of anti-
corrosive properties of DMPIP for MS in hydrochloric acid medium.
Fig. 17 shows the Mulliken atomic charges calculated for neutral
and protonated form of DMPIP in aqueous phase. It is reported that
more negative the atomic charge of the inhibitor molecule, more
ular orbitals. The fk and fk were calculated to predict the most
probable atomic sites for nucleophilic and electrophilic attack re-
spectively. Table 7shows the values of natural population (P (N),
P (N-1), and P (N + 1)) with the corresponding Fukui functions
+
−
(fk , fk and fk˚) values of the inhibitor, DMPIP. The electrophilic
and nucleophilic Fukui functions for a site in a molecule can be
12

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
Fig. 17. Mulliken charge distribution for DMPIP molecule at B3LYP/6–311++G (d, p) level (a) protonated form and (b) neutral form.
Table 7
Calculated Mulliken charges and Fukui indices for heteroatoms of DMPIP using B3LYP/6–
11++G (d, p) basis Set.
3
Fukui indices
Anion
cation
neutral
qkN
Nucleophilic
electrophilic
Radical
+
−
qk(N + 1)
qk(N-1)
fk
fk
fk˚
3
9
1
1
1
N
0.122411
−0.18264
−0.21323
0.226147
−0.58011
0.013366
0.066638
0.131489
0.187015
0.094634
−0.05671
−0.25462
−0.43227
0.242675
−0.66619
0.179121
0.071979
0.21904
−0.070076
−0.321255
−0.563763
0.05566
0.054523
−0.12464
−0.17236
0.019566
−0.33737
N
2N
4N
3O
−0.016528
0.08608
−0.760821
determined by following equations [87]:
same vein, sites (3 N = 0.179 and 12 N = 0.219) with highest val-
+
ues of fk are the most vulnerable sites for nucleophilic attacks.
Nucleophilic attack : f + (r) = ρ N + 1(r) − ρ N (r)
Electrophilic attack : f − (r) = ρ N (r) − ρ N − 1(r)
Radical attack : f(r) = 1/2(ρN + 1(r) − ρN − 1(r))
(19)
(20)
(21)
5
. Conclusion
A new N-heterocyclic compound DMPIP was synthesized and its
inhibition ability on corrosion of MS in 0.5 M HCl was investigated
and correlated with the quantum chemical calculations.
Where ρN represents the density of electron around the
molecule at a point r. The number of electrons, N, N + 1 and N + 2,
corresponds to neutral molecule, anion (addition of an electron to
the LUMO of the neutral molecule) and cation (removal of an elec-
tron from the HOMO of the neutral molecule) respectively. Nitro-
gen atoms (3 N = −0.070 and 14 N = 0.056) with highest values
DMPIP acts as a potential corrosion inhibitor for MS in hy-
drochloric acid medium and its inhibition efficiency increases as
the concentration of the additive were increased but with the rise
in temperature corrosion rate increases. Potentiodynamic polariza-
tion curves demonstrated that DMPIP act as a mixed-type inhibitor
suppressing both metal dissolution and hydrogen evolution reac-
tion rates. EIS plots in the presence of DMPIP showed two capac-
−
of fk are the most susceptible sites for electrophilic attacks. In the
13

K. Vranda Shenoy, P.P. Venugopal, P.D. Reena Kumari et al.
Journal of Molecular Structure 1232 (2021) 130074
itive loops associated with the formation of protective film due
to the accumulation of inhibitors at the steel surface. The de-
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dl
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ity. The findings of theoretical calculations were in good agreement
with the experimental ones.
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Declaration of Competing Interest
1
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
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CRediT authorship contribution statement
K Vranda Shenoy: Methodology, Validation, Formal analysis, In-
vestigation, Writing - original draft, Visualization. Pushyaraga P
Venugopal: Software, Methodology, Validation, Formal analysis,
Writing - original draft, Visualization. P D Reena Kumari: Concep-
tualization, Methodology, Formal analysis, Resources, Writing - re-
view & editing, Supervision. Debashree Chakraborty: Formal anal-
ysis, Resources, Writing - review & editing, Supervision, Funding
acquisition.
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effect of mangifera indica leaves extract and zinc ions on the mild steel cor-
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