L-Proline-promoted synthesis of 2-amino-4-arylquinoline-3-carbonitriles as sustainable corrosion inhibitors for mild steel in 1 M HCl: Experimental and computational studies

Article abstract of DOI:10.1039/c5ra16982h

The inhibition of mild steel corrosion in 1 M HCl by 2-amino-4-(4-nitrophenyl) quinoline-3-carbonitrile (AAC-1), 2-amino-4-phenylquinoline-3-carbonitrile (AAC-2) and 2-amino-4-(4-hydroxyphenyl) quinoline-3-carbonitrile (AAC-3) has been investigated using

Full text of DOI:10.1039/c5ra16982h

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L-Proline-promoted synthesis of 2-amino-4-arylquinoline-3-carbonitriles as
sustainable corrosion inhibitors for mild steel in 1M HCl: experimental and
computation studies
a
a*
b, c
b, c
Chandrabhan Verma , M.A. Quraishi , L.O. Olasunkanmi and Eno E. Ebenso
a
Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi
21005,India
2
b
Department of Chemistry, NorthꢀWest University (Mafikeng Campus), Private Bag X2046,
Mmabatho 2735, South Africa
c
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
*
Corresponding author:
Ph.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 1M HCl by 2ꢀaminoꢀ4ꢀ(4ꢀnitrophenyl)
quinolineꢀ3ꢀcarbonitrile (AACꢀ1), 2ꢀaminoꢀ4ꢀphenylquinolineꢀ3ꢀcarbonitrile (AACꢀ2) and 2ꢀ
aminoꢀ4ꢀ(4ꢀhydroxyphenyl) quinolineꢀ3ꢀcarbonitrile (AACꢀ3) has been investigated using
weight loss, electrochemical (potentiodynamic polarization and electrochemical impedance
spectroscopy (EIS)), surface (SEM, EDX and AFM) and quantum chemical calculation methods.
The results indicated that the investigated inhibitors exhibited good inhibition efficiency against
the corrosion of mild steel in acidic solution. The weight loss and electrochemical results

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suggested that inhibition efficiencies were enhanced with increase in the concentration of 2ꢀ
aminoꢀ4ꢀarylquinolineꢀ3ꢀcarbonitriles (AACs). The results showed that inhibition efficiencies of
investigated AACs obeyed the order: AACꢀ3 (96.52%), AACꢀ2 (95.65%) and AACꢀ1 (94.78%).
Potentiodynamic polarization study revealed that investigated AACs act as cathodic type
inhibitors. Adsorption of the AACs on a mild steel surface obeyed the Langmuir adsorption
isotherm. Further, SEM, EDX and AFM studies clearly revealed the filmꢀforming ability of
AACs on the mild steel surface. Quantum chemical calculations were undertaken to provide
mechanistic insight into the mechanism of inhibition action of AACs. On the basis of
experimental and theoretical studies it was concluded that presence of electron releasing
hydroxyl (ꢀOH) group in AACꢀ3 increases inhibition efficiency whereas electron withdrawing
nitro (ꢀNO ) group in AACꢀ1 decreases the inhibition efficiency.
2
Keywords: Sustainable, mild steel, electrochemical, SEM/EDX/AFM, Quantum chemical
calculations
1. Introduction
Acidic solution is widely used in acid cleaning, acid descaling, oil well acidification and
enhanced oil recovery techniques in petroleum and other industries which causes severe
1
ꢀ3
economic and safety problems due to corrosion . Among the several available methods, the use
of synthetic corrosion inhibitors is the most effective, practical, and economic way to overcome
4
, 5
this problem . In this sense, heterocyclic compounds containing polar functional groups (ꢀOH,
ꢀ
NH , ꢀCN, ꢀC=Cꢀ, ꢀN=Nꢀ) and πꢀelectrons in form of double and triple bonds draw much
2
6
,
attention due to their high adsorption tendency in addition to their ease and economic synthesis
7
.
Generally, Nꢀheterocyclic inhibitors adsorb on metallic surface through unshared paired

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8
, 9
electrons of heteroatoms (N, O), conjugated double and/ or triple bonds, and aromatic rings
.
The adsorption of the heterocyclics on metal surface is influenced by several factors such as
presence of functional groups, electron density at donor heteroatoms (N, O, and S), and chemical
1
0, 11
structure
. Quinoline and its derivative possess potential applications for synthesis of
biologically important compounds such as DNA binding capabilities, antitumor and DNAꢀ
1
2
intercalating carrier . Previously, few reports have been published in which quinoline and its
12ꢀ19
derivatives emphasized as useful metallic corrosion inhibitors
.
Multicomponent reactions have immerged as a green and powerful technique in synthetic
organic chemistry and drug discovery in the sense that several biologically active complex
molecules can be synthesized from commercially available cheap starting materials in one step
2
0
.
Further, the consumption of environmentally benign solvents and chemicals are of particular
importance because they contribute many of the green chemistry principles. In asymmetric
organocatalysis, use of proline particularly in water and ionic liquids is the most sustainable
alternative method as it is directly isolated from natural biological sources without use of any
2
1ꢀ23
hazardous chemical and solvents such as DMSO, DMF and other chlorinated solvents
.
The main objective of present work was to compare the corrosion inhibition efficiency of
three synthesized 2ꢀaminoꢀ4ꢀarylquinolineꢀ3ꢀcarbonitriles (AACs) on mild steel corrosion in 1M
HCl. Several methods such as weight loss, electrochemical impedance spectroscopy (EIS),
scanning electron microscopy (SEM), atomic force microscopy (AFM), and quantum chemical
techniques were used for this purpose. The selection criteria of these compounds as corrosion
inhibitors was based on the facts that they: (a) can be easily synthesized from commercially
available cheap and green starting materials with high yield (b) exhibited excellent inhibition
efficiency even at low concentration (c) possess three aromatic rings, several heteroatoms (N, O)

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and polar functional groups (ꢀOH ꢀNO , ꢀCN, ꢀNH ) through which they can adsorb on the metal
2
2
surface (d) have high solubility in testing medium which is attributed due to presence of various
polar functional groups.
2
2
2
. Experimental procedures
.1. Materials
.1.1. Synthesis of 2ꢀaminoꢀ4ꢀarylquinolineꢀ3ꢀcarbonitriles
2
4
As described earlier , the investigated 2ꢀaminoꢀ4ꢀarylquinolineꢀ3ꢀcarbonitriles (AACs)
were synthesized by refluxing a mixture comprising of aniline (1mmol), aldehyde (1 mmol), and
malononitrile (1 mmol) and Lꢀproline catalyst (0.05 g, 20 mol %) in water (2 mL) was refluxed
for 12 h. The synthetic scheme of the investigated AACs is shown in Fig. 1. Completion of
reactions and formation of products were checked by TLC method. After completion of reaction,
the reaction mixtures were cooled to room temperature and solid crude products were filtered.
The solid crude precipitates were washed with water (10 mL) and finally with ethanol (5 ml) to
obtain pure AACs. The synthesized inhibitors were characterized by spectroscopic analysis. The
chemical structures, IUPAC name, abbreviation and analytical data of the investigated inhibitors
are given in Table 1. The stock solution of inhibitors was prepared in 1M HCl containing 2%
acetone.
2.1.2. Electrodes and reagents
For all weight loss and electrochemical experiments, working electrodes were cut from
the commercially available mild steel sheet having chemical composition (weight percentage): C
0.076), Mn (0.192), P (0.012), Si (0.026), Cr (0.050), Al (0.023), and Fe (balance). The exposed
(

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surface of the working electrodes were cleaned successively with emery papers of different grade
600, 800, 1000, 1200), washed with deionized water, degreased with acetone, ultrasonically
(
cleaned with ethanol and stored in moisture free desiccator before used in the experiments.
Hydrochloric acid (HCl, 37%, MERCK) and double distilled water were used for preparation of
test solution (1M HCl).
2.2. Methods
2.2.1. Weight loss measurements
Cleaned, dried and accurately weighted mild steel specimens having dimension 2.5 × 2.0
3
×
0.025 cm were immersed in 1M HCl without and with different concentrations of AACs for
3h. After elapsed time, these specimens were removed, washed with distilled water and
acetone, dried in moisture free desiccator, and again weighted accurately. To insure the
reproducibility of the weight loss results, each experiment was triply performed and means
values were reported at each concentration. From the calculated weight loss, inhibition efficiency
2
5, 26
(η%) was derived using following relationship
:
w − w
o
i
η
% =
×100
wo
(1)
where w and w are the weight loss values in the absence and presence of AACs at different
0
i
concentrations, respectively.
2.2.2. Electrochemical measurements
2
The mild steel specimens with exposed area 1 cm (one sided) were utilized for all
electrochemical measurements were performed under potentiodynamic condition using Gamry

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Potentiostat/Galvanostat (Model Gꢀ300) instrument. Gamry Echem Analyst 5.0 software
installed in the computer was used to fit and analyzed the all electrochemical data. The
instrument consist of a mild steel working electrode (WE), platinum as a counter electrode and a
saturated calomel electrode (SCE) as a reference electrode. Before starting the electrochemical
experiments, the working electrode allowed to corrode freely for sufficient time in order to attain
steady open circuit potential (OCP). During polarization measurements, the cathodic and anodic
Tafel slopes were recorded by changing the electrode potential inevitably from 0.25 to +0.25 V
ꢀ1
vs. corrosion potential (E ) at a constant sweep rate of 1.0 mV s . The corrosion current
corr
density (i ) was calculated by extrapolating the linear segments of Tafel slopes (cathodic and
corr
anodic). From the calculated icorr value inhibition efficiency was calculated using following
2
5, 26
relation
:
0
corr
i
i
−i
corr
η
% =
×100
0
i
corr
(2)
where, i°corr and icorr are corrosion current in the absence and presence of AACs, respectively
Electrochemical impedance measurements were carried out at open circuit potential in
the frequency range of 100 kHz to 0.01 Hz using AC signal of amplitude 10 mV peak to peak.
The charge transfer resistances were calculated from Nyquist plots. The inhibition efficiency was
2
5, 26
calculated using following equation
:
i
ct
0
ct
R − R
η
% =
×100
i
R
ct
(3)
0
i
ct
Where, R and R are charge transfer resistances in absence and presence of AACs,
ct
respectively.
2.2.3. SEM, EDX and AFM measurements

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For surface analysis, the cleaned mild steel specimens of above mentioned composion
were allowed to corrode for 3h in absence and presence of optimum concentration of AACs.
Thereafter, the specimens were taken out 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 composition of the
inhibited and uninhibited specimens was recorded by an EDX detector coupled to the SEM. NTꢀ
MDT multimode AFM, Russia, 111 controlled by solvers canning probe microscope controller
was employed for surface analysis by AFM method. The single beam cantilever having
resonance frequency in the range of 240–255 kHz in 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. Quantum chemical calculations
Quantum chemical calculations were carried out on the investigated AACs using the
density functional theory (DFT) method involving the Becke threeꢀparameter hybrid functional
2
7
together with the LeeꢀYangꢀPaar correlation functional (B3LYP) . The 6ꢀ31+G(d, p) basis set
was chosen for all the calculations. The calculations were carried out with the aid of Gaussian 09
2
8
software for Windows (Revision D.01) . The optimized geometries of the compounds were
confirmed to correspond to their true energy minima by the absence of imaginary frequency in
the computed vibrational frequencies. All quantum chemical parameters were derived based on
the electronic parameters of the most stable conformers of the molecules. The frontier molecular
orbital (FMO) energies, that is, the highest occupied molecular orbital energy (EHOMO) and the
lowest unoccupied molecular energy (ELUMO) were calculated. Other parameters such as the
energy gap (∆E), global hardness (η), global electronegativity (χ), and the fraction of electrons

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transfer (∆N) from the inhibitor to the metal atom were computed respectively according to the
2
9, 30
equations
:
ꢀ
η
E = ELUMO − EHOMO
(4)
(5)
1
=
(
ELUMO − EHOMO
)
2
1
χ
= −
(
ELUMO + EHOMO
)
(6)
2
χFe
−
+
χinh
ηinh
ꢀN =
(7)
2
(
η
)
Fe
where χFe and ηinh denote the electronegativity and hardness of iron and inhibitor, respectively.
A value of 7 eV/mol was used for the χFe , while ηFe was taken as 0 eV/mol for bulk Fe atom in
3
1
accordance with the Pearson’s electronegativity scale . The total energy change (∆E ) that
T
accompanies the donorꢀacceptor charge transfer process was calculated according to the
3
2
approximation made by Gomez et al. :
−
4
η
ꢀ
E =
(8)
T
where
η
was approximated to the chemical hardness of the inhibitor molecule. Selected dihedral
angles that might have some correlations with the experimental inhibition efficiency of the
compounds were also reported.
3
3
3
. Results and discussion
.1. Weight loss measurements
.1.1. Effect of AACs concentration

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The weight loss is a reliable, useful, and widely used technique to examine the corrosion
inhibition property of inhibitors. The Effect of AACs concentrations on mild steel corrosion at
308 K after 3 immersion time were investigated by weight loss method. Variation of the
inhibition efficiency with AACs concentration is shown in Fig. 2 and corrosion parameters such
as corrosion rate (C ), inhibition efficiency (η%), and surface coverage (θ) are given in Table 2.
R
From the results it is seen that the values of weight loss (mg) and corrosion rate (C ) were
R
decreased with an increased in AACs concentration and maximum efficiency was observed at 40
ꢀ1
mgL concentration. The AACꢀ1, AACꢀ2 and AACꢀ3 showed maximum efficiency of 94.78%,
ꢀ1
9
5.65% and 96.52%, respectively at 40 mgL concentration and 308 K temperature. The
increased AACs concentration causes increased in extent of adsorption and surface coverage due
3
3
to availability of large number of inhibitor molecules therefore increases inhibition efficiency .
The difference in the inhibition efficiency might be due to presence of different substituents and
34
molecular size of the AACs . The AACꢀ3 attained the highest inhibition efficiency among the
studied compounds due to presence of additional electron releasing hydroxyl (ꢀOH) group in the
phenyl moiety of aromatic aldehyde in AACꢀ3. Whereas, AACꢀ1 exhibited the lowest inhibition
efficiency which is attributed due to presence of electron withdrawing nitro (ꢀNO ) group in the
2
phenyl moiety of aromatic aldehyde.
3
.1.2. Kinetic parameters: effect of temperature
Variation of inhibition efficiency (η%) and corrosion rate (C ) with electrolyte solution is
R
given in Table 3. From the result depicted in Table 3 it seen that an increase in electrolyte
temperature leads to an increase in the rate of mild steel dissolution and corrosion for inhibited
and uninhibited acid solutions. The effect of temperature on metallic dissolution in inhibited
solution is highly complex, because at elevated temperature several changes such as rapid

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etching, desorption of inhibitor and decomposition and/or rearrangement of inhibitor takes place
3
5
.
The Arrhenius equation is successfully used to establish the relationship between corrosion
rate (C ) and temperature where the natural logarithm of the corrosion rate (C ) is a linear
R
R
function with 1/ T.
(9)
ꢀ2
ꢀ1
where C is the corrosion rate in mg cm h , A is the Arrhenius preꢀexponential factor, R is the
R
gas constant and T is the absolute temperature. The Arrhenius plots (Fig. 3) give a straight line
between log C versus 1/T. The values of activation energy (E ) for mild steel dissolution were
R
a
calculated from the slopes ( −ꢀE 2.303R ) of the Arrhenius plots and given in Table 4. From the
a
results depicted in Table 4 it is seen that values of apparent activation energies are higher in
ꢀ1
presence of AACs than in their absence (28.48 kJmol ). The higher values of apparent activation
ꢀ1
ꢀ1
ꢀ1
energy for AACꢀ1 (71.40 kJ mol ), AACꢀ2 (73.73 kJ mol ) and AACꢀ3 (75.65 kJ mol )
3
6
indicated the formation of energy barrier for mild steel dissolution in presence of AACs .
Further, it is assume that electrostatic interaction (physical adsorption) between charged
inhibitors and metal surface might cause increase in the values of apparent activation energy in
3
7
presence of AACs .
3.1.3. Thermodynamic parameters: Adsorption isotherm
Adsorption nature of inhibitor molecule provides good insight into their inhibition
3
8
mechanism . For this purpose several adsorption isotherms including the Langmuir, Temkin,
Frumkin and Flory–Huggins isotherms were tested, the best fit was obtained with the Langmuir
3
9
adsorption isotherm. The Langmuir adsorption isotherm can be best represented as :
θ
1−θ
K C =
ads
(10)

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where K is the equilibrium constant of the adsorption process, C is the inhibitor concentration
ads
and θ is the surface coverage. The Langmuir isotherm (Fig. 4) gives a straight line between log
ꢀ1
0
θ/1ꢀθ versus log C (molL ). The values of free energy of adsorption (ꢁG ads) can be calculated
using equation (11). In this equation numerical value of 55.5 represents the molar concentration
of water in acid solution:
ο
ads
ꢀ
G
= −RT ln(55.5Kads )
(11)
0
The calculated values of K and ꢁG at each studied temperature in presence of optimum
ads
ads
concentration of AACs are given in Table 5. The higher values of Kads in presence of AACs
4
0
suggest that the AACs adsorbed easily and strongly on the mild steel surface . Inspection of the
0
ꢀ1
ꢀ1
Table 5 reveals that the values of ꢁG ads varied from ꢀ34.62 kJ mol to ꢀ36.96 kJ mol , which
strongly suggest the physiochemical mode of adsorption by AACs on mild steel surface. Further,
0
the negative sign of ꢁG ads validated the spontaneous nature of adsorption by the AACs on mild
4
1ꢀ43
steel surface
.
3.2. Electrochemical measurements
3.2.1. Potentiodynamic polarization studies
Potentiodynamic polarization studied was carried out to understand the mechanism of the
mild steel dissolution at anode and evolution of hydrogen at cathode. The currentꢀpotential
curves for mild steel corrosion without and with several concentrations of three AACs are show
in Fig. 5. As reflected from the Tafel slopes, the presence of inhibitors causes significant shift in
the anodic and cathodic processes toward lower current relative to the Tafel slopes of blank
specimen. And, therefore it is concluded that presence of inhibitors affect the anodic mild steel
4
4
dissolution as well as the cathodic hydrogen evolution to some extent . Simultaneously, this
decrease in anodic and cathodic current densities becomes more significant with increasing

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inhibitors concentration. A successful extrapolation of linear segments of the anodic and Tafel
cathodic slopes provided some important electrochemical parameters such as corrosion potential
(
E ), corrosion current density (i ), anodic and cathodic Tafel slopes (β and β ), as well as
corr corr a c
corrosion inhibition efficiency (η%). The calculated parameters were listed in Table 6.
From the polarization results depicted in Fig. 5 and Table 6 it is observed that the
presence of three inhibitors significantly reduced the values of corrosion current densities
without causing any substantial change in the values of E . This finding suggests that
corr
investigated AACs inhibit mild steel corrosion in acid solution by forming a surface film without
4
5
changing the mechanism of mild steel corrosion . Further, the shift in E values is not very
corr
prominent, the maximum shift in Ecorr values were 59, 65 and 66 mV for AACꢀ1, AACꢀ2 and
AACꢀ3, respectively, toward cathodic polarization suggesting that investigated inhibitors acted
4
6
as mixed type inhibitors mainly cathodic inhibitors .
3.1.2. Electrochemical impedance spectroscopy (EIS) study
The electrochemical impedance spectroscopy is an important technique to study the
characteristics and kinetics of electrochemical reactions occurring on the metal surface in acid
solution. Nyquist plots for mild steel in absence and presence of different concentration of the
inhibitors are given in Fig. 6. The impedance spectrum at each condition gives depressed
capacitive imperfect semicircles in high frequency region which is attributed to the charge
4
7
transfer process and the formation of a protective layer by inhibitors on the metal surface . The
imperfect semicircle of the impedance spectrum is attributed to the mass transfer processes,
4
8
frequency dispersion as well as inhomogeneities and roughness of metal surface . Meanwhile,
the diameter of the imperfect semicircles increases on increasing AACs concentrations. This
finding suggests that the studied inhibitors strongly aggregate on the mild steel surface and block

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the active sites and therefore increases the charge transfer resistance as well as inhibition
4
9, 50
efficiency
. In the present case, the impedance nature of mild steel corrosion was studied
using previously described simple electrical equivalent circuit consisting of a resistor of solution
5
1
resistance (R ), a resistor of charge transfer (R ), and a double layer capacitance . The double
s
ct
layer capacitance was replaced by constant phase element (CPE) to find more precise and
5
1
accurate fit of impedance data. The impedance nature of CPE is defined as follows :

1 
−1
]
ZCPE
=
[
( jω
)n
 
Y

0

(12)
Where, Y is the CPE constant, ω is the angular frequency; j is the imaginary number and n is the
0
phase shift (exponent). The values double layer capacitance (C ) in absence and presence of
dl
5
2:
inhibitors was derived using following relationship
nꢀ1
C = Y (ωmax)
(13)
dl
0
Where, ωmax is the frequency at which the imaginary part of impedance has attained the
ꢀ1
maximum (rad s ) value. Generally, the value of n related to the surface inhomogeneity. The
electrochemical behavior of the CPE is related to the value of n. In general, the value of n equal
to 0, 1, ꢀ1 and 0.5 associated with the resistance, capacitance, inductance and Warburg
impedance nature of the metal / electrolyte interface, respectively. In our present work, the value
of n varies from 0.777 to 0.998. Upon inspection of the data in Table 7 it can be seen that Rct
increased with increase in the AACs concentration while values of C decreased with
dl
concentration signifying that the AACs functioned by adsorption at the metal/ electrolyte
5
3
interfaces . The inhibition efficiency was calculated using equation (6) and maximum observed

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efficiencies were 94.27%, 96.08%, and 97.42 for AACꢀ1, AACꢀ2and AACꢀ3, respectively, at 40
ꢀ1
mgL concentration and 308 K temperature.
The Bode plots, shown in Fig. 7aꢀc exhibited three different segments in absence of
0
AACs. In high frequency region the values of log /Z/ and phase angle (α ) tend to become zero
which is a characteristics response of resistive behavior and corresponds to solution resistance
enclosed between reference electrode (saturated calomel electrode) and working electrode (mild
5
4, 55
steel)
. However, in the medium frequency region, a linear relationship between log /Z/
0
against log f, with slop values close to ꢀ1 and the phase angle value tends to become ꢀ70 , were
observed. This is a characteristics response of capacitive behavior. An ideal capacitor is
0
56
associated with slope value of ꢀ1 and phase angle value of ꢀ90 . The deviation from ideal
capacitive behavior in the presence study is attributed due to rough working electrode surface
resulted due to corrosion. The careful examination of the Bode plots reveal that the values of
phase angle significantly increased and therefore, surface roughness remarkably decreased in
5
7
presence of AACs due to formation of protective surface film . Further, Bode plots give one
time constant, sigma maxima in the intermediate frequency region. The broadening of this
maximum in the Bode plots is attributed due to the adsorption and formation of protective film
5
8
by inhibitors at the metal/ electrolyte interface .
3.3. Surface measurements
3.3.1. Scanning electron microscopy (SEM) study
.
Fig. 8a reveals a SEM micrograph of mild steel surface before exposing to the test
solution which clearly shows the abrading scratches on surface formed during cleaning by SiC

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emery papers. The morphology of mild steel specimen surface in Fig.8b reveals that in the
absence of AACs, the surface is highly corroded and damaged with some area of localized
corrosion which is attributed due to the free acid corrosion of mild steel. However, in the
presence of AACs, at their optimum concentration, the surface morphologies (Fig. 8cꢀe) are
remarkably smoothed. By comparing the surface morphology of mild steel in absence and
presence of AACs at same magnification (500x), it is seen that in presence of AACs the mild
steel surfaces are comparatively less corroded and less damaged. This behavior is because of the
formation of protective surface film by AACs. This finding suggests that the AACs inhibit mild
steel corrosion in 1M HCl by forming a protecting surface film.
3.3.2. Energy dispersive Xꢀray (EDX) study
To determine the elements present on the mild steel surface before and after exposure to
the AACs solution, the EDX spectra of the specimens were taken after 3h immersion time. Fig.9
represents the EDX spectra of mild steel specimens in absence and presence of AACs at their
optimum concentration. In absence of AACs the EDX spectrum shown in Fig.9a gives
characteristics peaks of the elements (Fe, Carbon and Oxygen) constituting the mild steel
specimen. The presence of low intensity signal corresponding to oxygen (O) is might be due to
formation of iron oxide after removal of the specimen from the uninhibited solution during
SEM/EDX operation. However, in presence of optimum concentration of AACs (Fig.9bꢀd), the
EDX spectra show additional signals for nitrogen suggesting that in the inhibited solution AACs
adsorb on the metal surface and from protective surface film which is responsible for the

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appearances of pick for the nitrogen in the EDX spectra of inhibited mild steel specimens.
Moreover, the signals corresponding to the oxygen (O) for inhibited mild steel specimens show
comparatively high intensities as compare to uninhibited specimens. This finding again suggests
the adsorption of AACs on mild steel surface resulting into increased intensity of signals
corresponds to the oxygen. The elemental composition of the mild steel surface obtained from
EDX analysis is given in Table 8.
3.3.3. Atomic force microscopy (AFM) study
AFM is a powerful tool to study the surface morphology at nanoꢀ to microꢀscale and has
become a new choice to study the influence of inhibitors on the generation and the progress of
the corrosion at the metal/solution interface. The 3D AFM micrographs of mild steel specimens
after 3h immersion time in absence and presence of AACs at their optimum concentration are
shown in Fig. 10. Fig. 10a represents the surface morphology of the mild steel in absence of
AACs which reveals a very rough surface. This is concluded that in absence of AACs, the mild
steel surface highly corroded resulting in the highly surface roughness. The calculated average
surface roughness of the mild steel surface in absence of AACs was 392 nm. However, in
presence of AACs (Fig. 10 bꢀd), the surface morphologies are significantly improved might be
due to formation of protective surface film. The protective surface films isolated the specimens
from aggressive acid solution and therefore inhibit corrosion. The calculated average surface
roughnesses were 194, 139 and 108 nm in presence of inhibitor AACꢀ1, AACꢀ2 and AACꢀ3,
respectively.
3.4. Quantum chemical calculations

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The optimized structures of the studied compounds showing the dihedral angles between
the planes of the atoms labeled a, b, c, d are shown in Fig. 11. It is apparent from Fig. 11 that the
phenyl group attached to the 4ꢀposition of the quinoline ring is not coplanar with the quinoline
ring. Since the structural differences in the three AACs are mainly found on the 4ꢀphenyl group
substituent, it may be necessary to compare the degree of planarity/nonꢀplanarity of this phenyl
group in the three compounds. This is because molecules with high degree of planarity are
generally believed to exhibit higher inhibition efficiencies than their corresponding less (non)ꢀ
5
9ꢀ61
planar analogues
. A flat molecule has the tendency to cover the surface of the metal
optimally and also to offer enhanced electrostatic interactions between its coplanar atoms and the
metallic atom. The lower inhibition efficiency of AACꢀ1 despite its more number of N and O
atoms compared to the other two compounds could be as a result of the highly electronꢀ
withdrawing effect of the nitro group of the 4ꢀnitrophenyl, which reduces the electron density of
the aromatic ring, and also the relatively less planarity of the 4ꢀnitrophenyl group compared to
AACꢀ2, which lessens the possible electrostatic interactions between the electronꢀdensity of the
62,63
aromatic group and the charged or polarized metal atom
. AACꢀ3 on the other hand has –OH
substituent group at positions 4 making it a potentially better inhibitor than AACꢀ2 without any
substituent. The frontier molecular orbitals provide information about the prospective sites of the
molecules involved in donorꢀacceptor relationship between the inhibitor molecules and the metal
atom. The HOMO and LUMO electron density distributions of the studied AACs are shown in
Fig. 12. The HOMO electron densities of AACꢀ1 and AACꢀ2 are essentially distributed on the
atoms in the quinoline ring and extended to the N atom of the amino group attached to the
position 2 of the ring. This suggests that the quinoline rings in AACꢀ1 and AACꢀ2 play a major
role in donating πꢀelectrons to the vacant dꢀorbital of Fe. There is also the tendency of the 2ꢀ

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amino group donating σꢀelectrons to the vacant dꢀorbital of the Fe. For AACꢀ3, the HOMO
electron densities are distributed over the entire quinoline rings and extended to the 4ꢀ
hydroxylphenyl and the 2ꢀamino groups. This suggests that the quinoline ring, the 4ꢀ
hydroxylphenyl and the 2ꢀamino groups are likely to participate in donating electrons to the
appropriate vacant orbitals of the Fe for effective corrosion inhibition. The LUMO electron
densities in the three compounds are nearly distributed over the entire molecular areas. This
suggests that the compounds are capable of accepting electrons from the appropriate occupied
orbitals of the Fe atom during the donorꢀacceptor interactions.
The calculated quantum chemical parameters are listed in Table 9. The EHOMO is a
measure of the tendency of a molecule to donate its HOMO electrons to the appropriate vacant
orbital of an electronꢀacceptor during the donorꢀacceptor interactions. The higher the value of the
EHOMO the better will be chance of electron donation by the donor molecule. The results in Table
9
shows that the trend of the EHOMO for the studied compounds is: AACꢀ3 > AACꢀ2 > AACꢀ1,
which corresponds to the trend of the observed inhibition efficiency. The ELUMO on the other
hand is a measure of the tendency of a molecule to accept electrons into its LUMO from the
appropriate occupied orbital of an electronꢀdonor. That is, the lower the value of the ELUMO the
better the chance of electron acceptance by the acceptor molecule. The values of the ELUMO
obtained for the studied compounds do not correlate with the observed η%. The opposite trends
of the ELUMO values and the η% suggest that the inhibition potentials of the studied AACs are not
informed by their ability to receive electrons from the occupied orbitals of Fe. Similarly, the
values of the energy gap (∆E) and the global hardness (η) often employed as indices of relative
reactivity or stability of a molecule do not correlate with the observed η%. However, the values
of the global electronegativity (χ) show that AACꢀ3 has the least tendency to hold on to its

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electrons or the highest propensity to release its electrons in a favorable donorꢀacceptor
mechanism. The trend of the χ results is in complete agreement with the observed η%. The
fraction of electrons transferred (∆N) from an inhibitor molecule to the metal atom can be used
as the gauge of the relative performance of a set of inhibitor molecules in electron transfer
process. The results in Table 9 show that the trend of ∆N is AACꢀ3 > AACꢀ2 > AACꢀ1, which is
in conformity with the trend of the observed η%. According to Gomez et al., the change in
energy (∆E ) that accompanies the electron transfer process from an inhibitor to a metal atom
T
can be used to predict the favorability of the donorꢀacceptor interactions between the interacting
species. A negative value of ∆E though does not really predict the occurrence of backꢀdonation;
T
it is an indication that the charge transfer to a molecule, followed by a backꢀdonation from the
molecule is energetically favorable. Suppose there is any chance of charge transfer from Fe to
the studied inhibitors and also retroꢀdonation from the inhibitor molecules to the Fe, the more
negative the value of ∆E the better the stabilization of the product of the donorꢀacceptor
T
relationship. The results in Table 9 suggest that AACꢀ3 will form a more stable inhibitorꢀFe
complex compared to the AACꢀ1 and AACꢀ2, the trend of ∆E values is in agreement with the
T
experimentally observed η%.
4. Conclusion
The effect of electron donating hydroxyl (ꢀOH) and electron withdrawing nitro (ꢀNO2)
groups in 2ꢀaminoꢀ4ꢀarylquinolineꢀ3ꢀcarbonitriles (AACs) was studied on the corrosion behavior
of mild steel in 1M HCl. The weight loss and electrochemical results suggest that the inhibition
efficiency increases with increasing AACs concentrations. The EIS plots suggest that chargeꢀ
transfer resistance of the corrosion process increases on increasing AACs concentration due to
adsorption of these inhibitors at metal / electrolyte interfaces. The adsorption of the AACs on the

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mild steel surface obeyed the Langmuir adsorption isotherm. The values of activation energies
suggested that AACs inhibit mild steel corrosion by forming a protective surface film at metal/
electrolyte interfaces. The adsorbed layer of the AACs over the surface of mild steel has been
furthermore conformed by SEM, EDX and AFM analysis. The experimental results were well
supported by quantum chemical calculations.

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Figure Caption
Fig.1: Synthetic rout of studied AACs
Fig.2: Variation of inhibition efficiency with AACs concentration
Fig. 3: Arrhenius plots for the corrosion rate of mild steel versus the temperature in 1M HCl
Fig.4: Langmuir isotherm plots for the adsorption of AACs on mild steel surface in 1M HCl
Fig. 5 a-c: Polarization curves for mild in absence and presence of different concentration of
AACs
Fig.6. a-c: Nyquist plot for mild steel in 1 M HCl without and with different concentrations of
AACs
Fig. 7 a-c: Bode (log f vs log |Z|) and phase angle (log f vs α0) plots for mild steel in 1 M HCl in
absence and presence of different concentration of AACs.
Fig.8 (a-e): SEM images of mild steel surfaces: (a) abraded, (b) in absence of AACs and in
ꢀ1
the presence of 40 mgL of (c) AACꢀ1, (d) AACꢀ2, and (e) AACꢀ3
ꢀ1
Fig.9 (a-d): EDX images of mild steel: (a) in absence of AACs and in the presence of 40 mgL
of (b) AACꢀ1, (c) AACꢀ2, and (d) AACꢀ3
Fig.10 (a-d): AFM images of mild steel: (a) in absence of AACs and in the presence of 40
ꢀ1
mgL of (b) AACꢀ1, (c) AACꢀ2, and (d) AACꢀ3
Fig. 11: Optimized molecular structures of the studied compounds showing the dihedral angles
aꢀbꢀcꢀd (in degrees).

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Fig. 12: The frontier molecular orbital of studied AACs (a) AACꢀ1 (left, HOMO; right, LUMO),
(b) AACꢀ2 (left, HOMO; right, LUMO) and (c) AACꢀ3 (left, HOMO; right, LUMO)
Table Caption
Table 1. IUPAC name, molecular structure, molecular formula, melting point and analytical
data of studied AACs
Table 2: The weight loss parameters obtained for mild steel in 1 M HCl containing different
concentrations of AACs
Table 3: Variation of C and η% with temperature in absence and presence of optimum
R
concentration of AACs in 1M HCl
Table 4: Values of activation energies for mild steel dissolution in 1M HCl in the absence and
at of optimum concentration of AACs
◦
Table 5: The values of Kads and ∆G for mild steel in absence and presence of optimum
ads
concentration of AACs in 1M HCl at different studied temperature
Table 6: Tafel polarization parameters for mild steel in 1 M HCl solution in absence and at
different concentration of AACs
Table 7: EIS parameters obtained for mild steel in 1 M HCl in absence and presence of different
concentration of AACs
Table 8: Elemental composition of mild steel surface in absence and presence of optimum
concentration of AACs after 3h immersion time
Table 9: Quantum chemical parameters derived from the B3LYP/6ꢀ31+G(d,p) method

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Figure
Fig.1: Synthetic rout of studied AACs

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100
95
90
85
80
75
70
65
60
AAC-1
AAC-2
AAC-3
%
η
%
η
10
20
30
40
50
Conc (mol/L)
Fig.2: Variation of inhibition efficiency with AACs concentration