2,4-Diamino-5-(phenylthio)-5: H -chromeno [2,3- b] pyridine-3-carbonitriles as green and effective corrosion inhibitors: Gravimetric, electrochemical, surface morphology and theoretical studies

Article abstract of DOI:10.1039/c6ra04900a

The inhibition of mild steel corrosion in 1 M HCl by three newly synthesized 2,4-diamino-5-(phenylthio)-5H-chromeno[2,3-b]pyridine-3-carbonitriles (DHPCs) namely, 2,4-diamino-7-nitro-5-(phenylthio)-5H-chromeno[2,3-b]pyridine-3-carbonitrile (DHPC-1), 2,4-d

Full text of DOI:10.1039/c6ra04900a

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2
, 4-diamino-5-(phenylthio)-5H-chromeno [2, 3-b] pyridine-3-carbonitriles as
green and effective corrosion inhibitors: gravimetric, electrochemical, surface
morphology and theoretical studies
a
b,c,
d
b
Chandrabhan Verma , Lukman O. Olasunkanmi , I. B. Obot , Eno E. Ebenso
a,*
and M.A. Quraishi
a
Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi
21005,India
2
b
Department of Chemistry and Material Science Innovation & Modelling (MaSIM) Research
Focus Area, Faculty of Agriculture, Science and Technology, NorthꢀWest University (Mafikeng
Campus), Private Bag X2046, Mmabatho 2735, South Africa
c
Department of Chemistry, Faculty of Science, Obafemi Awolowo University, IleꢀIfe, 220005,
Nigeria
d
Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of
Petroleum and Minerals, Dhahran 31261, Saudi Arabia
*Corresponding author: Phone no. +91-9307025126; Fax: +91- 542- 2368428
E-mail: maquraishi.apc@itbhu.ac.in; maquraishi@rediffmail.com
Abstract:
The inhibition of mild steel corrosion in 1 M HCl by three newly synthesized 2,4ꢀ
diaminoꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀcarbonitriles (DHPCs) namely, 2,4ꢀ
diaminoꢀ7ꢀnitroꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀcarbonitrile (DHPCꢀ1), 2,4ꢀ
diaminoꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀcarbonitrile (DHPCꢀ2) and 2,4ꢀdiaminoꢀ
7ꢀhydroxyꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀcarbonitrile (DHPCꢀ3) was studied
using weight loss method, electrochemical techniques, surface morphology (SEM, AFM) studies
and theoretical (quantum chemical calculations and molecular dynamic simulation) methods.
The weight loss and electrochemical measurements showed that the inhibition efficiency
increases with increasing inhibitors concentration and the relative trend of inhibition
performances is DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1. Potentiodynamic polarization study reveals that

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the investigated DHPCs act as mixed type inhibitors. The adsorption of the DHPCs on the mild
steel surface obeys the Langmuir adsorption isotherm and involves both physisorption and
chemisorption modes. The presence of the electron releasing ꢀOH group at position seven on the
chromenopyridine ring is considered to be responsible for the highest inhibition efficiency of
DHPCꢀ3 among the studied compounds. Whereas, the presence of the electron withdrawing nitro
(
ꢀNO ) group at position seven on the chromenopyridine ring is responsible for the lowest
2
inhibitive strength of DHPCꢀ1. Quantum chemical calculations and molecular dynamic
simulation studies were undertaken to provide mechanistic insight into the roles of the different
substituents (ꢀOH and –NO ) on the corrosion inhibition behavior of the studied inhibitors.
2
Keywords: Chromenopyridines, mild steel, corrosion inhibition, SEM/AFM, EIS/Tafel, Quantum
chemical calculations and molecular dynamics
1
. Introduction
Iron and its alloys are widely used as construction material in the petroleum, food, power
production, chemical and electrochemical industries. This is due to their high thermal and
1
ꢀ3
mechanical stability, ease of fabrication and joining, and low cost.
However, these materials
become gradually destroyed by corrosion upon exposure to the environment due to chemical
or/and electrochemical reactions with the environment. Therefore, several efforts are being
channeled towards preventing these undesirable reactions. Among the several available methods
of corrosion protection, the utilization of synthetic corrosion inhibitors has become a popular
method because of the ease and economic viability of synthesis of inhibitors, high inhibition
4
ꢀ6
efficiency, and facileꢀfeasibility.
Most of the efficient corrosion inhibitors are organic
compounds containing polar functional groups and πꢀ electrons in form of triple or conjugated
double bonds. These synthetic compounds inhibit corrosion by adsorbing on metallic surface.
Generally, the adsorption of these inhibitors on the metal surfaces depends on numerous
physicochemical properties such as nature of functional groups, steric factors, aromaticity,
electron density at the donor atoms and pꢀorbital character of donating electrons and the
7
electronic structure of the inhibitors molecules. Previous literature had established that Sꢀ
containing compounds show better inhibition efficiency in sulphuric acid solution, while Nꢀ
8
containing compounds show better inhibition efficiency in hydrochloric acid solution. Whereas,

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compounds containing both Nꢀ and Sꢀ atoms generally give rise to even better inhibition
9
,10
efficiency.
The multicomponent reactions (MCRs) are of increasing importance in green organic and
medicinal chemistry because they involve processes in which three or more components react
1
1,12
directly to form a unique product, giving good atom economy.
Moreover, the MCRs are also
valuable for green chemistry due to their operational simplicity, small number of steps, facile
automation, minimum waste generation (as a result of reduced number of workꢀups), simple
1
3ꢀ15
purification procedure, which enhances synthetic efficiency, resources and time saving.
In view of this, we report on the corrosion inhibition efficiency of three
chromenopyridines namely, 2,4ꢀdiaminoꢀ7ꢀnitroꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀ
carbonitrile (DHPCꢀ1), 2,4ꢀdiaminoꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀcarbonitrile
(DHPCꢀ2)
and
2,4ꢀdiaminoꢀ7ꢀhydroxyꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2,3ꢀb]pyridineꢀ3ꢀ
carbonitrile (DHPCꢀ3) for mild steel in 1 M HCl. The investigated chromenopyridines were
synthesized by MCRs. The study was performed using weight loss, potentiodynamic
polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy
(SEM), atomic force microscopy (AFM), quantum chemical and molecular dynamics
calculations methods. The choice of these compounds as corrosion inhibitors is based on the
consideration that they can be easily synthesized via green method using readily available
chemicals in one step, in addition to their high solubility in the test solution, which enhances
their inhibition efficiency. Furthermore, literature survey reveals that the inhibitive property of
the studied chromenopyridines has not been reported earlier, which portrays the compounds to be
novel potential corrosion inhibitors.
2
. Experimental section
2.1. Materials
2.1.1. Electrode and reagents
The mild steel specimens for weight loss, electrochemical measurements and surface
morphology studies were cut form commercially available mild steel sheet having chemical
composition (wt%): C (0.076), Mn (0.192), P (0.012), Si (0.026), Cr (0.050), Al (0.023), and Fe
(balance). The exposed surface of the working electrodes were cleaned successively with emery
papers of different grades (600, 800, 1000, and 1200), washed with deionized water, degreased

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with acetone, ultrasonically cleaned with ethanol and stored in moisture free desiccator before
used in the experiments. Hydrochloric acid (37% HCl from MERCK) and double distilled water
were used for the preparation of 1 M HCl test solution.
2.1.2. Inhibitors synthesis
The chromenopyridines used in the present study were synthesized by one step MCRs as
1
6
previously described and the synthetic route is shown in Fig. 1. The progress of reaction was
checked by using thin layer chromatographic (TLC) method. After completion of reaction, the
products were dissolved in DMF and undissolved components were removed through filtration.
Addition of water to DMF results in the crystallization of pure product. The characterization data
of the synthesized compounds are given in Table 1.
2
.2. Methods
2
.2.1. Weight loss measurements
Cleaned, dried and accurately weighted mild steel specimens having dimension 2.5 × 2.0
3
×
0.25 cm were immersed in 1 M HCl without and with different concentrations of DHPCs for 3
h. The specimens were then removed, washed with distilled water and acetone, dried in moisture
free desiccator, and again weighed accurately. To ensure the reproducibility of the weight loss
results, each experiment was triply performed and average values were recorded at each
concentration of the inhibitors. From the observed average weight loss, the inhibition efficiency
1
7
(η%) was calculated as:
w − w
o
i
η
% =
×100
wo
(1)
where w and w are the weight loss values in the absence and presence of DHPCs at different
0
i
concentrations, respectively.
.2.2. Electrochemical measurements
The mild steel specimens with exposed area of 1 cm (one sided) were utilized for all
2
2
electrochemical measurements. The experiments were performed under potentiodynamic
condition using Gamry Potentiostat/Galvanostat (Model Gꢀ300) instrument. Gamry Echem
Analyst 5.0 software was used for fitting, simulation and analysis of all electrochemical data.
The instrument consist of mild steel working electrode (WE), platinum as counter electrode and
a saturated calomel electrode (SCE) as reference electrode. Before starting the electrochemical

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experiments, the working electrode was allowed to corrode freely for sufficient time in order to
attain steady open circuit potential (OCP). During the polarization measurements, the cathodic
and anodic Tafel slopes were recorded by changing the electrode potentials from ꢀ0.25 to +0.25
ꢀ1
V vs corrosion potential (Ecorr) at a constant sweep rate of 1.0 mVs . The corrosion current
density (icorr) was calculated by extrapolating the linear segments of the Tafel plots (cathodic and
anodic). From the calculated icorr values, the inhibition efficiency was calculated using the
1
7
relation:
0
corr
i
i
−i
corr
η
% =
×100
0
i
corr
(2)
i
where, i°corr and i
are corrosion current densities in the absence and presence of DHPCs,
corr
respectively.
Electrochemical impedance measurements were carried out at the OCP in the frequency range of
00 kHz to 0.01 Hz using AC signal of amplitude 10 mV peak to peak. The charge transfer
resistances were calculated from the Nyquist plots. The inhibition efficiency was calculated
1
1
7
using the equation:
i
ct
0
ct
R − R
η
% =
×100
i
R
ct
(3)
0
i
where, Rct and Rct are charge transfer resistances in the absence and presence of DHPCs,
respectively.
2.2.3. SEM, EDX and AFM measurements
For surface analysis, the cleaned mild steel specimens of were allowed to corrode for 3 h
in the absence and presence of optimum concentration of DHPCs. Thereafter, the specimens
retrieved, washed with water, dried and employed for SEM, EDX and AFM analysis. The SEM
model Ziess Evo 50 XVP was used to investigate the micromorphology of mild steel surface at
500x magnification. Chemical compositions of the inhibited and uninhibited specimens were
recorded by an EDX detector coupled to the SEM. NTꢀMDT multimode AFM, Russia, 111
controlled by solvers canning probe microscope controller was employed for AFM surface
analysis. The single beam cantilever having resonance frequency in the range of 240–255 kHz in

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semi contact mode with corresponding spring constant of 11.5N/m with NOVA programme was
used for image interpretation. The scanning area during AFM analysis was 5 mm x 5 mm.
2.2.4. Molecular Dynamics simulations
Forcite module in the Material Studio Software 7.0 from BIOVIAꢀAccelrys, USA was
adopted to carry out the quench molecular dynamic (MD) simulations. The simulation was
carried out with Fe (110) crystal with a slab thickness of 5 Å. The Fe (110) plane was enlarged to
a (8 × 8) supercell to provide a large surface for the interaction with the inhibitors. After that, a
vacuum slab with 30 Å thickness was built above the Fe (110) plane. For the whole simulation
procedure, the Condensedꢀphase Optimized Molecular Potentials for Atomistic Simulation
Studies (COMPASS) force field was used to optimize the structures of all components of the
system of interest. The MD simulations were performed in NVT canonical ensemble at 298K
with a time step of 0.1 fs and a total simulation time of 100 ps using Anderson thermostat.
2.2.5. Quantum chemical calculations
The quantum chemical studies in the present work were carried out by using similar
18,19
computational approach employed in some previous studies.
Quantum chemical calculations
were carried out on the studied compounds in order to provide corroborative explanations for the
inhibition activities of the DHPCs at molecular level. The optimized structures and electronic
2
0ꢀ22
energy parameters were obtained by using the B3LYP/6ꢀ31G(d) model
of the density
functional theory (DFT) method. The optimized structures were confirmed to correspond to true
energy minima by the absence of an imaginary frequency in the force constant calculations. All
the quantum chemical parameters used in providing theoretical explanations for the corrosion
inhibition properties of the studied compounds are those of the most stable ground state
geometries. Quantum chemical calculations were carried out with the aid of Gaussian 09 suite
2
3
software. The energy of the highest occupied molecular orbital (EHOMO), the energy of the
lowest unoccupied molecular orbital (ELUMO), the energy band gap (∆E = ELUMOꢀEHOMO), and the
dipole moment of the optimized structures were obtained and recorded. The absolute
19,20
electronegativity (χ) of the inhibitor molecule was calculated using the equations:
1
χ
=
(EHOMO + ELUMO )
2
(4)

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The fraction of electrons transferred from the inhibitor molecule to the metal, (∆N) was
1
9,24
calculated according to the equation:
χinh
ηinh
χFe
−
+
ꢀN =
2
(
η
)
Fe
(5)
where χFe and χinh are the electronegativity values of Fe and inhibitor respectively, while ηFe
and ηinh denote the hardness values of Fe and inhibitor respectively. Pearson’s electronegativity
6
scale assigns a 7 eV/mol for χFe , while ηFe is approximately equal to 0 eV/mol for bulk Fe.
Fukui functions are often utilized to predict the atomic sites that are susceptible to
nucleophilic and electrophilic attacks in an inhibitor molecule. The sites for nucleophilic and
+
–
electrophilic attacks are informed by the Fukui indices f and f respectively. Yang and
2
5
Mortier proposed the use of Mulliken population analysis (MPA) and finite difference (FD)
+
approximations for the calculation of Fukui functions, and the corresponding equations for f
–
18,19,25
and f respectively are:
+
f = ρk(N+1) (r) − ρk(N ) (r)
k
(
6)
7)
−
k
f = ρk(N ) (r) − ρk(N−1) (r)
(
where ρk(N +1)
, ρk(N ) and ρk(N −1) are the electron densities of the molecular species with N+1
+
−
electrons, N electrons and Nꢀ1 electrons respectively. The numerical values of f and f were
2
6,27
plotted as graphical electron density isosurfaces with the aid of Multiwfn software.
3
. Results and discussions
3
.1. Weight loss experiments
.1.1. Effect of inhibitors concentration
The weight loss experiments were carried out for 3 h immersion time at 308 K in the
3
absence and presence of different concentrations of the studied compounds. The calculated
weight loss parameters such as corrosion rate (C ), surface coverage (θ) and corresponding
R
inhibition efficiencies (η%) are given in Table 2. From the results shown in Table 2 it can be
seen that the inhibition efficiency of the three compounds increases with increasing
ꢀ5
ꢀ1
concentration. The maximum inhibition efficiency was obtained at 12.70 x 10 molL
concentration. Further increase in DHPCs concentration did not cause any significant change in

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the inhibition performance. The relative strengths of inhibition the studied compounds follow the
order DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1. The highest inhibition efficiency of the DHPCꢀ3 among
the studied inhibitors is attributed to the presence of the electron donating –OH group at position
seven of the chromenopyridine ring. In contrast, the lowest inhibition efficiency of the DHPCꢀ1
could be as a result of electron withdrawing nature of the ꢀNO group substituted at position
2
2
8
seven of the chromenopyridine ring.
.1.2. Effect of temperature
In order to study the influence of temperature on the corrosion inhibition efficiency of the
investigated inhibitors, the weight loss experiments were carried out at different temperatures
308ꢀ338K) in the absence and presence of optimum concentration of the DHPCs. It is observed
3
(
from the results depicted in Table 3 that the corrosion rate increases and inhibition efficiency
decreases with increasing inhibitors concentration. The increased corrosion rate at elevated
temperature could be as a result of increased average molecular speed of the inhibitor molecules
2
9
that weakens the adsorption tendency of the inhibitors on the metallic surface. The temperature
30
dependency of the corrosion rate can be represented by the Arrhenius equation:
(8)
ꢀ2 ꢀ1
where C is the corrosion rate in mgcm h , A is the Arrhenius preꢀexponential factor, R is the
R
gas constant and T is the absolute temperature. The Arrhenius plots shown in Fig. 2 give a
straight line between log C versus 1/T from the slopes ( −ꢀE 2.303R ) of which values of
R
a
activation energies were calculated for all the inhibitors and the results are listed in Table 4. It is
observed from the results in Table 4 that in presence of DHPCs, E attains the cooperatively
a
ꢀ1
ꢀ1
ꢀ1
higher values of 69.89 kJmol for DHPCꢀ1, 76.54 kJmol for DHPCꢀ2, and 84.38 kJmol for
ꢀ1
DHPCꢀ3 as compared to in their absence (E = 28.48 kJmol ). It was further observed that the
a
order of Ea was consisted with order of inhibition efficiency i.e. a higher value of E was
a
observed for a more efficient inhibitor. The increased values of E in the presence of DHPCs
a
suggested that higher energy barrier has been established for the corrosion reactions due to the
3
1
adsorption of the inhibitors on the metallic surface.
.1.3. Adsorption isotherm
The adsorption of inhibitors at metal/electrolyte interface is a very important topic in
corrosion inhibition study because it provides some insights into the inhibition mechanism. The
3

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adsorption of an organic adsorbate at metal/electrolyte interface depends upon the nature of the
testing media, the chemical structure of the inhibitor, the charge distribution of the inhibitor,
nature and surface charge of the metal. In the present study, the calculated surface coverage at
different concentrations of the investigated inhibitors was subjected to different adsorption
isotherm models in order to find best adsorption isotherm. However, the Langmuir adsorption
2
isotherm gives the best fit having values of regression coefficient (R ) very close to unity. The
32
Langmuir isotherm can be represented by the equation:
θ
1−θ
K C =
ads
(9)
where Kads is the equilibrium constant of the adsorption process, C is the inhibitors concentration
and θ is the surface coverage, which was derived from the weight loss experiments. The
Langmuir isotherm (Fig. 3) gives straight lines for the plots of log θ/1ꢀθ versus log C from which
values of Kads were calculated for the studied inhibitors. The Kads is related to the Gibbs’ free
energy of adsorption according to the equation:
ο
ads
ꢀ
G = −RT ln(55.5K )
ads
(
10)
where T is the absolute temperature and R is the universal gas constant. The numerical value of
55.5 represents the molar concentration of water in acid solution. The calculated values of Kads
0
and ꢁG are given in Table 5. Generally, the value of Kads represents the affinity of inhibitor
ads
for adsorption on the metallic surface and a high value of Kads is consistent with high adsorption
ability. In the present study the values of Kads for the studied inhibitors follow the order DHPCꢀ3
>
DHPCꢀ2 > DHPCꢀ1, which is in accordance with the order of inhibition efficiency. The large
0
negative values of ꢁG ads for the studied inhibitors indicate that these compounds possessed
3
3,34
strong tendency to adsorb spontaneously on the mild steel surface.
Literature survey reveals
0
ꢀ1
ꢀ1
that the values of ꢁG ads in between ꢀ20 kJ mol to ꢀ40 kJ mol are consistent with
0
ꢀ1
physiochemisorption. In our present case, the values of ꢁG ads varies from ꢀ34.66 kJ mol to
ꢀ1
3
8.19 kJ mol suggesting that the adsorption of the investigated inhibitors on mild steel surface
involves electrostatic interaction between charged inhibitors and charged mild steel surface
(physisorption) as well as charge sharing or transfer from the inhibitor molecules to the metal
3
5,36
surface to form a coordinate type of bond (chemisorption).
3.2. Electrochemical measurements

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3.2.1. Potentiodynamic polarization study
The potentiodynamic polarization study was carried out in the absence and presence of
different concentrations of the investigated inhibitors to gather information about kinetics of the
anodic and cathodic half reactions. The polarization curves for mild steel dissolution in 1 M HCl
with and without inhibitors are shown in Fig. 4. The values of potentiodynamic polarization
parameters namely, corrosion potential (Ecorr), corrosion current density (icorr), anodic and
cathodic Tafel slopes (β , β ) were obtained by extrapolation of the linear segments of the anodic
a
c
and cathodic Tafel curves. The polarization parameters along with the calculated inhibition
efficiency at various concentrations of all the studied inhibitors are given in Table 6. From the
results in Fig. 4 and Table 6, it is observed that all the three inhibitors considerably reduced the
corrosion current densities for both anodic and cathodic halfꢀreactions, suggesting that both
anodic dissolution of mild steel and cathodic reduction of hydrogen ions were inhibited.
Obviously, greater decrease in the values of icorr was observed at high inhibitors concentration. In
general, an inhibitor can be classified as anodic or cathodic type, if the shift in the Ecorr is higher
than 85 mV with respect to Ecorr of the blank, and as a mixed type inhibitor, if shift in Ecorr is
3
7,38
lower 85 mV.
In our present investigation maximum displacement in the Ecorr values were 72
mV for DHPCꢀ1, 70 mV for DHPCꢀ2 and 72 mV for DHPCꢀ3. It can also be observed from the
results in Table 6 that the change in the values of β are more prominent as compared to that of
c
βa, suggesting that the studied compounds mixed type inhibitors with predominantly cathodic
inhibitive effects.
3.2.2. Electrochemical impedance spectroscopy (EIS)
The EIS is a widely used technique to understanding the mechanism of corrosion,
passivation and charge transfer at the metal/electrolyte interface. The Nyquist curves for mild
steel corrosion in 1 M HCl without and with various concentrations of the studied inhibitors
obtained after 30 min immersion time are given Fig. 5aꢀc. It is noticeable that the shapes of
Nyquist plots are similar for all the systems, which suggest that presence of inhibitors did not
cause any significant change in the corrosion mechanism. In the present study, the Nyquist plots
give imperfect capacitive semicircle, which is often associated with different phenomena such as
roughness and other forms of inhomogeneities of the metal surface, distribution of the surface
3
9
active sites, grain boundaries and presence of impurities. The electrical equivalent circuit used
to analyze the Nyquist plots is shown in Fig. 5b. The electrical equivalent circuit consists of the

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solution resistance (R ), the charge transfer resistance (R ) and a constant phase element (CPE).
s
ct
In the present case, a CPE was used rather than pure double layer capacitance (C ) in order to
dl
take into account the mild steel surface roughness and heterogeneities, impurities, formation of
porous layer, dislocation, adsorption of inhibitors, and grain boundaries. The impedance of the
4
0,41
CPE (ZCPE) can be represented as:

1 
−1
]
ZCPE
=
[
( jω
)n
 
Y

0

(11)
where Y is the CPE constant, ω is the angular frequency, j is the imaginary number, and n is the
0
phase shift (exponent), which provides a measure of surface inhomogeneity resulted due to
inhibitors adsorption, porous film formation, surface roughness etc. On the basis of the value of
n, CPE can represent resistance (n = 0), capacitance (n = 1), inductance (n = 1) or Warburg
impedance (n = 0.5). The value of C with and without inhibitors was calculated using the
dl
4
2
equation:
C = Y (ωmax)
nꢀ1
dl
0
where ωmax is the frequency at which the imaginary part of impedance has attained the maximum
ꢀ1
(
rads ) value. The calculated EIS parameters such as R , R , n, C , η% and surface coverage (θ)
s ct dl
are reported in Table 7. The results in Table 7 show that the values of R increase with increase
ct
in inhibitors concentration. The increased value of R in the presence of inhibitors is as a result
ct
of the formation of protective adsorption layer on the steel surface, which isolates the metal from
43,44
the aggressive acid solution.
The decreased value of C in the presence of the investigated
dl
inhibitors suggests a decrease in the dielectric constant and an increase in the thickness of the
electrical double layer. Moreover, it is further observed that the increase in the values of R and
ct
4
3,44
decrease in the values of C is more pronounced at higher inhibitors concentration.
dl
The Bode phase and impedance plots for mild steel in the absence and presence of
different concentrations of the studied compounds are shown Fig. 6aꢀc. Careful inspection of the
Bode plots showed that the values of phase angle increase in the presence of inhibitors as

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compared to that of the blank acid solution. The relatively high values of phase angles in
presence of the inhibitors is attributed to increased surface smoothness of the mild steel
4
5,46
specimens, which may be due to adsorption of the inhibitors on the steel surface.
However,
the ideal capacitive behavior of electric double layer was not observed in the present study, as
0
45,46
the value of phase angle and slope could not achieved at 90 and ꢀ1, respectively.
The
deviation from ideal capacitive behavior in this study is due to surface inhomogeneities of
structural and interfacial origin.
3.3. Surface measurements
3.3.1. Scanning electron microscopy (SEM)
In order to further study the influence of the studied inhibitors on the corrosion of mild steel
in acid solution, the SEM micrographs of abraded mild steel surface, and mild steel surfaces in 1
M HCl solution in the absence and presence of optimum concentration of the studied DHPCs
were recorded after 3 h immersion time. Fig. 7a depicts the SEM micrograph of abraded mild
steel surface which shows only the abrading scratches and lining pits. The SEM image of the
mild steel specimen retrieved from blank acid solution (Fig. 7b) showed highly corroded and
damaged surface revealing a mountainꢀlike appearance, which is due to free acid corrosion of
mild steel in the absence of the inhibitors. However, in the presence of the inhibitors, the surface
morphology of the mild steel specimens (Fig. 7cꢀe) are remarkably improved, which may be as a
result of the formation of protective film of the inhibitor molecules on the steel surface.
3.3.2. Atomic force microscopy (AFM)
The AFM micrographs of abraded mild steel surface as well as after 3h immersion in 1M
HCl without and with optimum concentration of the investigated inhibitors are shown in Fig. 8.
Fig. 8a represents the AFM micrograph of abraded mild steel surface which is relatively smooth
with abrading scratches and pits which is resulted due to abrading cleaning of metallic surface

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from emery papers. The calculated average surface roughness for abraded mild steel surface was
5 nm. The surface of mild steel specimen in the absence of inhibitors was highly corroded,
8
rough and inhomogeneous as shown in Fig. 8b. For the mild steel specimen corroded in free acid
solution without inhibitors, the calculated surface roughness is 392 nm. However, in the presence
of optimum concentration of the studied inhibitors (Fig. 7cꢀe) there is significant improvement in
the surface smoothness, which is attributed to the adsorption of the inhibitors on mild steel
surface and consequently isolation of the metal surface from corrosive medium. The calculated
surface roughness decreased to 156, 134, and 108 nm in the presence of DHPCꢀ1, DHPCꢀ2 and
DHPCꢀ3, respectively. In other words, the degree of surface smoothness based on AFM analyses
is in the order DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1, which is in accordance with the order of the
experimental inhibition efficiencies.
3.4. Theoretical studies
3.4.1. Quantum chemical calculations
The ground state optimized structures of the studied compounds are shown in Fig. 9. The
most stable geometries of the three DHPCs correspond to the molecular structures in which the
benzenethiolyl group is twisted out of plane and nonꢀcoplanar with the chromenopyridine ring.
Electron density isosurfaces of the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of the studied DHPCs are also shown in Fig. 10. The
HOMO provides information about the molecular orbitals of the inhibitor that may interact with
atomic orbitals of the metal via charge donation to the appropriate vacant or partially filled
metallic orbitals. The HOMOs of the three compounds comprise both σꢀ and πꢀtype orbitals and
distributed over the entire rings contained in the molecules. In all the three DHPCs, the –NH2
group at the 4ꢀposition on the pyridine ring makes little or no contributions to the HOMO. In
DHPCꢀ3, the Nꢀatom in the pyridine ring does not contribute to the HOMO. The ꢀNO group on
2
the chromene ring of DHPCꢀ1 does not seem to contribute to the HOMO. In all the three
compounds, the Sꢀatom of the benzenethiolyl group contributes significantly to the HOMO and
shows the tendency of donating charges to the appropriate vacant orbitals of Fe via σꢀtype

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HOMOs. The LUMO provides information about the molecular orbitals of the inhibitor that may
interact with atomic orbitals of the metal by accepting charges from the occupied orbitals of the
metallic atom in a retroꢀdonation step. In all the three compounds, the Cꢀatoms in the chromene
ring are well involved in the LUMO. The LUMO is also well distributed on the benzenethiolyl
group in both DHPCꢀ2 and DHPCꢀ3. However, the benzenethiolyl group does not contribute to
the LUMO in DHPCꢀ1. This observation can be attributed to the electronꢀwithdrawing effect of
the –NO group on the chromene ring of DHPCꢀ1, which may pull the electron density of the
2
benzenethiolyl group towards the chromene ring, and inductively assisted by the highly
electronegative Sꢀatom. The Nꢀcontaining substituent groups on the pyridine ring in DHPCꢀ1 do
not contribute to the LUMO, suggesting that these groups cannot participate in backwardꢀ
donation of charges during the donorꢀacceptor interactions between DHPCꢀ1 molecule and Fe.
This generalization cannot be made for DHPCꢀ2 and DHPCꢀ3. The values of some relevant
quantum chemical parameters for the studied inhibitor molecules are listed in Table 8. The
donorꢀacceptor interactions between an inhibitor molecule and a metal atom are usually
correlated with the FMO energies and other related reactivity indices. Generally, the higher the
EHOMO, the better the tendency of an inhibitor molecule to donate electrons to the metal, and the
lower the ELUMO the better the tendency of an inhibitor molecule to accept electrons from the
metal. A low value of the energy gap (∆E) is also an indication of high reactivity of the inhibitor
molecule. For a group of inhibitor molecules with similar molecular architectures, the compound
with a higher EHOMO, a lower ELUMO and a lower ∆E usually exhibits higher inhibition efficiency.
The results in Table 8 show that the values of EHOMO for the three compounds are in the order:
DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1, which is in agreement with the order of inhibition efficiency of
the compounds. This observation suggests that the higher the tendency of a DHPC molecule to
donate its least stable electron(s) to the appropriate vacant orbitals of the metal atom, the higher
the inhibition efficiency. The values of ELUMO and ∆E listed in Table 8 do not agree with the
observed order of inhibition efficiency. Global electronegativity (χ) can be used as a measure of
the tendency of a molecule to retain its own electrons during donor/acceptor interactions that
lead to corrosion inhibition. It is expected that a low value of χ will inform high inhibition
performance. The results in Table 8 reveal that the order of increasing χ values is DHPCꢀ3 <
DHCPꢀ2 < DHPCꢀ1, which suggests highest inhibition efficiency for DHPCꢀ3, and the trend is
in agreement with the observed inhibition efficiency. The order of dipole moments of the studied

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inhibitors is DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1, which is in agreement with the order of inhibition
efficiency of the compounds and suggests that higher dipole moment favors higher inhibition
potential. The enhanced inhibition performance of a molecule with a high dipole moment has
been attributed to increased dipoleꢀdipole interactions between the inhibitor molecule and
1
8,47ꢀ49
charged metal surface.
The trend of the values of ∆N for the studied compounds as listed in
Table 8 is DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1. This trend is also in agreement with the order of
experimental inhibition efficiency of the compounds. This observation implies that the tendency
of a DHPC to inhibit metal corrosion is dependent on its ability to donate a high fraction of
electrons to the metal. The values of ∆N in the present study are less than 3.6, which according to
5
0
Lukovits’s study suggest that the inhibition efficiency of DHPCs will increase with increasing
electronꢀdonating ability to the metal surface.
+
ꢀ
The electron density isosurfaces for f
and f Fukui indices that correspond to
nucleophilic and electrophilic attacks respectively are shown in Fig. 11. The graphical
+
representations of f Fukui functions for the studied compounds show that the most susceptible
sites of nucleophilic attacks in DHPCꢀ1 are located on the –NO substituent on the chromene ring
2
and some neighboring Cꢀatoms in the ring especially those that are adjacent to the
electronegative Oꢀatom in the chromene ring. The ꢀC≡N substituent on the pyridine ring is also
susceptible to nucleophilic attacks in all the three DHPCs. The prospective sites of nucleophilic
attacks in DHPCꢀ2 and DHPCꢀ3 include mainly the Cꢀ4 atom of the pyridine ring, the SꢀC
region of the benzenethiolyl group, and the CꢀC region of the chromene ring adjacent to the
ꢀ
benzenthiolyl group. The electron density distributions for the f Fukui indices reveal that the
sites of electrophilic attacks in the studied DHPCs include essentially the Oꢀatom of the
chromene ring, the Nꢀatom of the –NH substituent at the 2ꢀposition of the pyridine ring, the –N
2
of the ꢀC≡N substituent and the adjacent Cꢀatom in the pyridine ring, the Sꢀatom of the
benzenthiolyl group, and the πꢀelectron (C=C) region shared by the fused pyridine and chromene
rings. The Oꢀatom of the –OH substituent on chromene ring in DHPCꢀ3 and the adjacent C=C
region are also susceptible sites for electrophilic attack.
3.4.2. Molecular dynamics simulations studies
The adsorption behavior of the studied inhibitors on mild steel surface was also
investigated using molecular dynamics simulation, which has emerged as a powerful tool to

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5
1,52
study the adsorption nature of the inhibitor molecule on metallic surface.
The molecular
dynamics simulation has been used to describe the most favorable conformation of the adsorbed
inhibitor molecule on the iron surface. Fig. 12 represents the side and top views of the most
stable adsorption models of (a) DHPCꢀ1, (b) DHPCꢀ2, and (c) DHPCꢀ3 on Fe (110) surface
using quench molecular dynamic and calculated parameters are listed in the Table 9. It is clear
from Fig. 12 that all the studied compounds adsorbed on the metallic surface by flat or parallel
orientation, which suggests the strong interactions between the inhibitor molecules and Fe (110)
surface. The values of interaction energy (Einteraction) for the studied inhibitors follow the order
DHPCꢀ3 > DHPCꢀ2 > DHPCꢀ1, which is agreement with the order of inhibition efficiency of the
molecules. The high negative values of interaction energies suggest that the DHPCs
1
8,53
spontaneously and strongly adsorb on Fe (110) surface.
The highest value of Einteraction
obtained for DHPCꢀ3 among the studied inhibitors implies that it is most strongly adsorbed
inhibitor on the metallic surface leading to highest inhibition efficiency. The trend of Einteraction
obtained for the studied compounds are in good agreement with the order of experimental
inhibition efficiency. The values of binding energies (Ebinding), which describe the strength of
binding between inhibitor molecules and Fe (110) surface are in the order DHPCꢀ3 > DHPCꢀ2 >
DHPCꢀ1. The values of binding energies (Ebinding) which describes the strength of binding
between inhibitor molecules and Fe (110) surface also obeyed the order: DHPCꢀ3 > DHPCꢀ2 >
DHPCꢀ1. This finding suggests that DHPCꢀ3 has maximum tendency to adsorb on mild steel
5
3ꢀ55
surface which resulted in the maximum inhibition efficiency obtained for DHPCꢀ3.
. Conclusion
In the present study, combined experimental and theoretical approaches are employed to
4
investigate the inhibition performance of three 2,4ꢀdiaminoꢀ5ꢀ(phenylthio)ꢀ5Hꢀchromeno[2, 3ꢀ
b]pyridineꢀ3ꢀcarbonitrile (DHPCs) on mild steel in 1 M HCl solution. The following conclusions
were drawn from the results:
1
2
3
. All the studied DHPCs act as good corrosion inhibitors and their inhibition efficiency
increases with increase in concentration, and decrease with increase in temperature.
. The studied inhibitors significantly elevate the activation energy associated with
corrosion reaction and thereby reduce the reaction rate.
. Polarization study revealed that studied compounds are mixed type inhibitors with
predominantly cathodic inhibitive capacity.

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4
5
6
. EIS study revealed that values of charge transfer resistance (Rct) increases in presence of
inhibitors due to the adsorption of inhibitors at metal/electrolyte interfaces.
. The adsorption of DHPCs on mild steel surface obey the Langmuir adsorption isotherm
and involves physisorption and chemisorption mechanisms.
. Quantum chemical calculations provide successful explanations to the observed
inhibition efficiencies of the molecules based on the frontier molecular energy
parameters.
7
. The electrophilic and nucleophilic sites of the inhibitor molecules were identified by
Fukui functions.
8
. The molecular dynamics simulation study revealed that all the studied inhibitors
spontaneously and strongly adsorbed on the Fe (110) surface by flat or parallel
orientation and the trend of predicted binding energies agree with experimental inhibition
efficiencies.

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Figure Caption
Fig.1: Synthetic route of the studied DHPCs
Fig. 2: Arrhenius plots for the corrosion of mild steel in 1 M HCl in the presence of the studied inhibitors
Fig.3: Langmuir adsorption isotherms for mild steel in 1 M HCl in the presence of the studied DHPCs at
different concentration
Fig. 4: Polarization curves recorded for mild steel in the absence and presence of different concentrations
DHPCꢀ1 (a), DHPCꢀ2 (b), and DHPCꢀ3 (c)
Fig.5: Fig.5 (a-c): Nyquist plots recorded for mild steel in 1 M HCl in the absence and presence of
different concentrations of (a) DHPCꢀ1, (b) DHPCꢀ2, and (c) DHPCꢀ3
(d) Equivalent circuit used for fitting and analyzing the electrochemical data
0
Fig. 6: Bode impedance modulus (log f vs log |Z|) and phase angle (log f vs
α
) plots for mild steel in 1 M
HCl in the absence and presence of different of different concentrations of (a) DHPCꢀ1, (b) DHPCꢀ2,
and (c) DHPCꢀ3
Fig.7: SEM images of abraded mild steel surface (a), mild steel surface in 1 M HCl in the
absence of DHPCs (b), and mild steel surfaces in 1 M HCl containing optimum
concentration of DHPCꢀ1 (c), DHPCꢀ2 (d), and DHPCꢀ3 (e)
Fig.8: AFM images of abraded mild steel surface (a), mild steel surface in 1 M HCl in the
absence of DHPCs (b), and mild steel surfaces in 1 M HCl containing optimum
concentration of DHPCꢀ1 (c), DHPCꢀ2 (d), and DHPCꢀ3 (e)
Fig. 9: Optimized molecular structures of (a) DHPCꢀ1, (b) DHPCꢀ2, and (c) DHPCꢀ3
Fig. 10: The frontier molecular orbital (leftꢀhand side: HOMO; and rightꢀhand side: LUMO) of
the studied DHPCs (a) DHPCꢀ1 (b) DHPCꢀ2, and (c) DHPCꢀ3
+
–
Fig. 11: Fukui indices f and f corresponding to the atomics sites for the nucleophilic and
electrophilic attacks respectively in (a) DHPCꢀ1, (b) DHPCꢀ2, and (c) DHPCꢀ3
(isosurface value = 0.003).

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Fig. 12: Side and top views of the most stable adsorption models of (a) DHPCꢀ1, (b) DHPCꢀ2, and (c)
DHPCꢀ3 on Fe (110) surface using quench molecular dynamic

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O
S
N
NH2
N
SH
CHO
CN
CN
Et N, C H OH
3
2
5
+
R1
+
R1
OH
Reflux
NH2
DHPC-1; R = -NO2
DHPC-2, R = -H
1
1
DHPC-3; R = -OH
1
Fig.1: Synthetic route of the studied DHPCs
Fig. 2: Arrhenius plots for the corrosion of mild steel in 1 M HCl in the presence of the studied inhibitors

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Fig.3: Langmuir adsorption isotherms for mild steel in 1 M HCl in the presence of the studied DHPCs at
different concentration

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Fig. 4: Polarization curves recorded for mild steel in the absence and presence of different concentrations
DHPC-1 (a), DHPC-2 (b), and DHPC-3 (c)

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(d)
Fig.5: Fig.5 (a-c): Nyquist plots recorded for mild steel in 1 M HCl in the absence and presence of
different concentrations of (a) DHPC-1, (b) DHPC-2, and (c) DHPC-3
(d) Equivalent circuit used for fitting and analyzing the electrochemical data

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