3-Amino alkylated indoles as corrosion inhibitors for mild steel in 1M HCl: Experimental and theoretical studies

Article abstract of DOI:10.1016/j.molliq.2016.04.024

The present study describes the influence of ring and ring size of three 3-amino alkylated indoles (AAIs) namely, N-((1H-indol-3-yl)(phenyl)methyl)-N-ethylethanamine (AAI-1), 3-(phenyl(pyrrolidin-1-yl)methyl)-1H-indole (AAI-2) and 3-(phenyl(piperidin-1-yl)methyl)-1H-indole (AAI-3) on mild steel corrosion in 1M HCl solution using gravimetric, electrochemical, surface morphology (SEM, AFM), quantum chemical calculations and molecular dynamics simulations methods. Both experimental and theoretical results showed that the 3-amino alkylated indoles with cyclic amino groups exhibit higher inhibition efficiency compared to the one with opened-chain amino group. The results further suggested that the inhibition efficiency increases with increasing ring size of the amino group such that the piperidine-containing (6-membered ring) 3-amino alkylated indole showed higher inhibition performance than the pyrrolidine-containing (five membered) 3-amino alkylated indole. Experimental results revealed that the inhibition efficiency increases with increasing concentration of the inhibitors. Maximum inhibition efficiencies of 94.34% for AAI-1, 96.08% for AAI-2 and 96.95% for AAI-3 were obtained at 0.862 mM concentration. EIS measurements showed that the studied compounds inhibit mild steel corrosion by adsorbing on the steel surface. Polarization studies revealed that the compounds are cathodic type inhibitors. The adsorption of the studied compounds obeyed the Langmuir adsorption isotherm. SEM and AFM surface morphology analyses also provided evidence of formation of adsorbed film of the AAIs on the steel surface. Theoretical parameters such as E_HOMO and electronegativity derived from quantum chemical calculations as well as binding energy derived from molecular dynamics simulations studies adequately corroborate the trend of experimental inhibition efficiencies of the studied inhibitors.

Full text of DOI:10.1016/j.molliq.2016.04.024

Journal of Molecular Liquids 219 (2016) 647–660
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
3-Amino alkylated indoles as corrosion inhibitors for mild steel in 1M
HCl: Experimental and theoretical studies
Chandrabhan Verma a, M.A. Quraishi a, , E.E. Ebenso b,c, I.B. Obot d, A. El Assyry e
⁎
a
Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India
Department of Chemistry, School of Mathematical & Physical Sciences, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho
b
2735, South Africa
c
Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mma-
batho 2735, South Africa
d
Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Laboratoire d'Optoélectronique et de Physico-chimie des Matériaux, (Unité associée au CNRST), Université Ibn Tofail, Faculté des Sciences, B.P. 133, Kénitra, Morocco
e
a r t i c l e i n f o
a b s t r a c t
Article history:
The present study describes the influence of ring and ring size of three 3-amino alkylated indoles (AAIs) namely,
N-((1H-indol-3-yl)(phenyl)methyl)-N-ethylethanamine (AAI-1), 3-(phenyl(pyrrolidin-1-yl)methyl)-1H-indole
(AAI-2) and 3-(phenyl(piperidin-1-yl)methyl)-1H-indole (AAI-3) on mild steel corrosion in 1M HCl solution
using gravimetric, electrochemical, surface morphology (SEM, AFM), quantum chemical calculations and molec-
ular dynamics simulations methods. Both experimental and theoretical results showed that the 3-amino
alkylated indoles with cyclic amino groups exhibit higher inhibition efficiency compared to the one with
opened-chain amino group. The results further suggested that the inhibition efficiency increases with increasing
ring size of the amino group such that the piperidine-containing (6-membered ring) 3-amino alkylated indole
showed higher inhibition performance than the pyrrolidine-containing (five membered) 3-amino alkylated in-
dole. Experimental results revealed that the inhibition efficiency increases with increasing concentration of the
inhibitors. Maximum inhibition efficiencies of 94.34% for AAI-1, 96.08% for AAI-2 and 96.95% for AAI-3 were ob-
tained at 0.862 mM concentration. EIS measurements showed that the studied compounds inhibit mild steel cor-
rosion by adsorbing on the steel surface. Polarization studies revealed that the compounds are cathodic type
inhibitors. The adsorption of the studied compounds obeyed the Langmuir adsorption isotherm. SEM and AFM
surface morphology analyses also provided evidence of formation of adsorbed film of the AAIs on the steel sur-
face. Theoretical parameters such as EHOMO and electronegativity derived from quantum chemical calculations
as well as binding energy derived from molecular dynamics simulations studies adequately corroborate the
trend of experimental inhibition efficiencies of the studied inhibitors.
Received 18 January 2016
Received in revised form 30 March 2016
Accepted 9 April 2016
Available online xxxx
Keywords:
Mild steel
Acid corrosion
Electrochemical calculation
Quantum chemical calculations
Molecular dynamics simulations
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Adsorption of these compounds depends upon several factors including
molecular weight, nature of substituents, solution temperature, nature
Acid solutions are commonly used for pickling and also for the re-
moval of rust and scales in petroleum industries [1–4]. The use of acid
solutions for these industrial activities results in loss corrosion and
eventually loss of metals. Addition of organic corrosion inhibitors,
which are compounds that contain heteroatoms (e.g. N, O and S), dou-
ble and triple bonds and aromatic rings has been identified as one of
the most practical and economical ways of controlling metal corrosion
[5–8]. These compounds inhibit metallic corrosion by becoming adsor-
bate at metal/electrolyte interfaces in which polar functional groups
such –NH2, –OH, −CN, −NO2 etc. and pi-electrons of the double and
triple bonds and aromatic rings act as adsorption centers [9–11].
of inhibitor and electrolytes etc. [12,13]. However, most of the previ-
ously existing corrosion inhibitors are toxic and non-environmental
friendly [14,15]. The current strict measures on environmental regula-
tions and increasing ecological awareness have shifted the attentions
of corrosion control experts toward the development of efficient and
environmentally benign corrosion inhibitors [16–18]. In this direction,
multicomponent reactions (MCRs), which combine three or more sub-
strates is one of the most relevant current approaches that are able to
produce several bonds in one step [19–22]. In addition, the MCRs have
other several advantages including operational simplicity, facile auto-
mation and minimized waste generation, because of the reduction in
the number of work-up, extraction and purification stages [19,23]. In
spite of the theory of famous ancient philosopher of Greece, namely Ar-
istotle, the “No Coopora nisi Fluida” which means “No reaction takes
place in absence of solvent”, in recent years organic synthesis in solid
⁎
Corresponding author.
E-mail addresses: maquraishi.apc@itbhu.ac.in, maquraishi@rediffmail.com
(M.A. Quraishi).
http://dx.doi.org/10.1016/j.molliq.2016.04.024
0167-7322/© 2016 Elsevier B.V. All rights reserved.

648
C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
phase (solvent free condition) attracted great deal of attraction due to
their reduce pollution, low costs, and simplicity in process and handling
[24]. Certainly, in several cases, solvent free reactions takes place with
high yield and selectivity than does their solution counterpart because
of the more tight and regular arrangement of molecules in the crystal
form [20,25]. Furthermore, “green chemistry” emphasizes the optimiza-
tion of synthetic methodologies to reduce environmental pollution, cost
and tedious work-ups. This new challenge has led to a growing interest
in the field of organic synthesis using catalyst derived from natural re-
sources [26,27]. In asymmetric organocatalysis, consumption of L-pro-
line provides the means of upholding the essential principles of green
chemistry as it is directly isolated from natural biological sources with-
out use of any hazardous chemical and/or solvents such as DMSO, DMF
and other chlorinated solvents [28]. Literature survey reveals that indole
and its derivatives act as efficient metallic corrosion in different electro-
lytic media [29–33]. These compounds inhibit metallic corrosion by be-
coming adsorbate at the metal/electrolyte interface in which indole
moiety acts as adsorption center.
2.1.2. Inhibitors synthesis
3-amino alkylated indoles (AAIs) used in the present study were
synthesized by the method described earlier [34]. In a typical experi-
mental procedure, 1 mmol of the aldehyde, 1 mmol of the secondary
amine, 1 mmol indole and 30 mol% of the L proline were placed in a
round-bottom flask and stirred at room temperature. The progress
and completion of the reaction was monitor by TLC method. After, com-
pletion of the reaction, the reaction mixtures were diluted with water
and then extracted with ethyl acetate. The crude products were purified
by column chromatography to give the corresponding inhibitors. Syn-
thetic scheme of investigated inhibitors is given in Fig. 1 and chemical
structures, abbreviations, IUPAC name and analytical data of the synthe-
sized 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 cm × 2.0 cm × 0.025 cm were immersed in 1M HCl
without and with different concentrations of AAIs for 3 h. After
elapsed time, these specimens were 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 mean values
are reported at each concentration. From the calculated weight loss,
inhibition efficiency (η%) was derived using following relationship
[35]:
In the present study, the effect of the type of amine (opened chain or
cyclic) as well as ring size of cyclic amine on the corrosion inhibition ef-
ficiency of three newly synthesized 3-amino alkylated indoles (AAIs)
namely,
N-((1H-indol-3-yl)(phenyl)methyl)-N-ethylethanamine
(AAI-1), 3-(phenyl(pyrrolidin-1-yl)methyl)-1H-indole (AAI-2) and 3-
(phenyl(piperidin-1-yl)methyl)-1H-indole (AAI-3) on mild steel corro-
sion in 1M HCl solution is being investigated for the first time. The cor-
rosion inhibition performances of the three newly synthesized AAIs
were determined using weight loss, electrochemical impedance spec-
troscopy (EIS), potentiodynamic polarization, scanning electron micros-
copy (SEM), and atomic force microscopy (AFM) techniques. Quantum
chemical calculations and molecular dynamics simulations studies were
also carried out to provide more insights into the theoretical explana-
tions of the inhibition activities of the studied compounds.
w0−wi
w0
η% ¼
ꢀ 100
ð1Þ
where w0 and wi are the weight loss values in the absence and pres-
ence of AAIs at different concentrations, respectively.
2. Experimental section
2.2.2. Electrochemical measurements
The mild steel specimens with exposed area 1 cm2 (one sided) were
utilized for all electrochemical measurements were performed under
potentiodynamic condition using Gamry Potentiostat/Galvanostat
(Model G-300) instrument. Gamry Echem Analyst 5.0 software installed
in the computer was used to fit and analyzed all electrochemical data.
The instrument consist of a mild steel working electrode (WE), plati-
num as a counter electrode and a saturated calomel electrode (SCE) as
a reference electrode. Before starting the electrochemical experiments,
the working electrode were allowed to corrode freely for 30 min 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 from 0.25 to +0.25 V vs corrosion po-
tential (Ecorr) at a constant sweep rate of 1.0 mV s−1. The corrosion cur-
rent density (icorr) was calculated by extrapolating the linear segments
2.1. Materials
2.1.1. Electrode and reagents
The mild steel specimens for weight loss, electrochemical and sur-
face measurements 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 grade (600, 800, 1000, and 1200), washed with de-
ionized water, degreased with acetone, ultrasonically cleaned with eth-
anol 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).
Fig. 1. Synthetic route of studied AAIs.

C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
649
Table 1
IUPAC name, molecular structure, molecular formula, and analytical data of studied AAIs.
S·no. IUPAC name and abbreviation of inhibitor
Chemical structure
Molecular formula and analytical data
1
2
3
N-((1H-indol-3-yl)(phenyl)methyl)-N-ethylethanamine
(AAI-1)
Yield: 76%; C19H22N2 (mol. wt. 278.39), white colored solid, IR spectrum (KBr
cm−1):3534, 2956, 2856, 2628, 1658, 1622, 1426, 1241, 1163, 968, 854, 827, 827,
812, 745, 654; 1H NMR (300 MHz, DMSO) δ (ppm): 1.23, 2.43, 5.23, 6.97–7.11,
7.29–7.38, 7.63, 7.95, 10.65.
3-(phenyl(pyrrolidin-1-yl)methyl)-1H-indole
(AAI-2)
Yield: 86%; C19H20N2 (mol. wt. 276.37), white creamy solid, IR spectrum (KBr
cm−1): 3546, 3458, 2973, 2837, 2284, 1698, 1628, 1524, 1467,1286, 929, 874,
837, 816, 763, 624 1H NMR (300 MHz, DMSO) δ (ppm): 1.73, 2.28, 5.36,
6.94–7.06, 7.18–7.26, 7.59, 7.96, 10.43
3-(phenyl(piperidin-1-yl)methyl)-1H-indole
(AAI-3)
Yield: 87%; C20H22N2 (mol. wt. 290.40), creamy colored solid, IR spectrum (KBr
cm−1; 3542, 3454, 2982, 2864, 2253, 1678, 1648, 1624, 1469, 1383, 1247, 987,
966, 893, 852, 823, 712, 643; 1H NMR (300 MHz, DMSO) δ (ppm): 139–146, 2.42,
5.28, 6.88–6.94, 7.08–7.14, 7.74, 8.02, 10.63
of Tafel slopes (cathodic and anodic). From the calculated icorr value in-
hibition efficiency was calculated using following relation [35]:
[37]. The optimized geometries of the compounds were confirmed to
correspond to their true energy minima by the absence of imaginary fre-
quency 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
culated. Other parameters such as the energy gap (ΔE), global hardness
(η), global electronegativity (χ), and the fraction of electrons transfer
(ΔN) from the inhibitor to the metal atom were computed respectively
according to the equations [40,41]:
i0corr−iicorr
η% ¼
ꢀ 100
ð2Þ
i0corr
where, i0corr and icorr are corrosion current in the absence and presence
of AAIs, 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 calcu-
lated using following equation [35]:
) and the lowest unoccupied molecular energy (ELUMO) were cal-
ΔE ¼ ELUMO−EHOMO
ð4Þ
ð5Þ
Rict−R0ct
Rict
1
η ¼ ðELUMO−EHOMO
Þ
η% ¼
ꢀ 100
ð3Þ
2
1
2
where, R0ct and Rci t are charge transfer resistances in absence and pres-
ence of AAIs, respectively.
χ ¼ − ðELUMO þ EHOMO
Þ
ð6Þ
ð7Þ
χ
Fe−χinh
À
Á
ΔN ¼
2.2.3. SEM, and AFM measurements
2 ηFe þ ηinh
For surface analysis, the cleaned mild steel specimens of above men-
tioned composition were allowed to corrode for 3 h in absence and
presence of optimum concentration of AAIs. Thereafter, the specimens
were taken out washed with water, dried and employed for SEM and
AFM analysis. The SEM model Ziess Evo 50 XVP was used to investigate
the micromorphology of mild steel surface at 500× magnification. NT-
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 11.5 N/m with NOVA programme was used for image inter-
pretation. The scanning area during AFM analysis was 5 mm × 5 mm.
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 accordance with
the Pearson's electronegativity scale [40]. The total energy change
(ΔET) that accompanies the donor-acceptor charge transfer process
was calculated according to the approximation made by Gomez et al.
[41]:
−η
4
ΔET
¼
ð8Þ
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.
2.2.4. Quantum chemical calculations
Quantum chemical calculations were carried out on the investigated
AAIs using the density functional theory (DFT) method involving the
Becke three-parameter hybrid functional together with the Lee-Yang-
Paar correlation functional (B3LYP) [36]. The 6-31+G(d, p) basis set
was chosen for all the calculations. The calculations were carried out
with the aid of Gaussian 09 software for Windows (Revision D.01)
2.2.5. Molecular dynamics simulations
Forcite module in the Material Studio Software 7.0 from BIOVIA-
Accelrys, USA was adopted to carry out the quench molecular dynamics

650
C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
(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
(10 × 10) supercell to provide a large surface for the interaction with
the inhibitors. After that, a vacuum slab with 30 Å thicknesses was
built above the Fe (110) plane. For the whole simulation procedure,
the Condensed-phase 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 simula-
tions were performed in NVT canonical ensemble at 298 K with a time
step of 1.0 fs and a total simulation time of 500 ps using Anderson
thermostat.
size of the heterocyclic rings. AAI-3 possesses piperdine ring which
has comparatively larger molecular size and cover the larger surface
area as compare to pyrrolidine ring present in the AAI-2 [44,45]. From
these results it can be concluded that inhibition efficiency of 3-amino
alkylated indoles (AAIs) increases on introducing the ring in the inhibi-
tor molecule as well as on increasing the ring size (or decreasing the
ring strain). The increased inhibition efficiency of the AAIs, with intro-
ducing the ring and increasing the length of hydrophobic carbon/alkyl
chain is attributed due to repulsion between the non-polar hydrophobic
chain and the polar water phase, which in turn force the AAIs to adsorb
at metal and electrolyte interfaces in order to decrease that repulsion
[48,99]. Further, in presence of surface active molecules (inhibitors) in
the solution cause distortion of the solvent liquids structure and thereby
increase the free energy of the system. As a compromise, the AAIs con-
centrate at metal/electrolyte interfaces because there, the thermody-
namic best arrangement was possible. The AAIs adsorb on the metal/
electrolyte interface in such a way that polar part of the molecules ori-
ented toward metallic surface leaving the nonpolar hydrophobic alkyl
part into the solution which repel the aqueous corrosion fluid and
thereby inhibit corrosion [48–50].
The interaction energy (Eint) of molecules with Fe surface was ob-
tained using the following equation [42,43]:
Eint ¼ Etotal–ðEFe surface þ Emolecule
Þ
ð9Þ
where Etotal is the total energy of the molecules and the metal surface
system; Esurface is defined as the energy of metal surface without adsorp-
tion of molecules and Emolecule is the energy of isolated molecules. The
binding energy is the negative of the interaction energy and is given as:
Ebin ¼ −Eint
:
ð10Þ
3.1.2. Effect of solution temperature
In order to study the effect of temperature on the corrosion inhibi-
tion efficiency of 3-amino alkylated indoles on mild steel corrosion in
1M HCl, the weight loss experiments were performed in absence and
presence of optimum concentrations of the studied inhibitors at differ-
ent temperature ranging from 308 to 338 K. The values of corrosion
rates (CR) and percentage inhibition efficiency (η%) obtained at different
temperature for studied inhibitors are given in the Table 3. It can be seen
from the results that η% decreases and CR increases with increasing the
solution temperature. The increased CR at elevated temperature might
be attributed due to rapid etching, desorption and decomposition and/
or rearrangement of the inhibitors molecules [51]. The temperature de-
pendency of CR can be best represented by Arrhenius equation, where
the natural logarithm of CR is a linear function of 1/T [52]:
3. Results and discussions
3.1. Weight loss experiments
3.1.1. Effect of AAIs concentration
Table 2 represents the parameters derived from the weight loss ex-
periments after 3 h immersion time in absence and presence different
concentrations of the investigated inhibitors. It can be seen from the re-
sults that inhibition efficiency increases with increasing concentration
of all inhibitors and maximum values of inhibition efficiencies of
94.34% for AAI-1, 96.08% for AAI-2 and 96.95% for AAI-3 were obtained
at 0.862 mM concentration. However, careful examination of the results
depicted in Table 2 showed that very slight increase in the inhibition ef-
ficiency was observed on increasing inhibitors concentration from
0.689 mM to 0.862 mM suggesting that 0.689 mM is the optimum con-
centration. Increase in the inhibitors concentration resulted in the in-
creased surface coverage which ultimately increases the inhibition
efficiency. The higher inhibition efficiency of AAI-2 as compare to AAI-
1 could be as a result of larger surface coverage by the pyrrolidine ring
because of its ring structure present in AAI-2 as compare to its open
chain structural analogues of AAI-1. While, the higher inhibition effi-
ciency of AAI-3 as compare to AAI-2 can be explained on the basis of
−Ea
2:303RT
logðCRÞ ¼
þ logA
ð11Þ
where CR is the corrosion rate in mg cm−2 h−1, Ea is the apparent acti-
vation energy, A is the Arrhenius pre-exponential factor, R is the gas
constant and T is absolute temperature. The activation energy for metal-
lic corrosion is the minimum amount of energy that is required in order
to produce the corrosion products, such as rust and scales [7,53]. High
values of Ea are generally associated with low corrosion rates while
low values of Ea are associated with high corrosion rates. Fig. 2 repre-
sents the Arrhenius plots from the intercepts of which values of Ea are
calculated. The calculated values of Ea were 76.072 kJ mol−1
,
Table 2
The weight loss parameters obtained for mild steel in 1M HCl containing different concen-
trations of AAIs.
80.461 kJ mol−1 and 83.145 kJ mol−1 for AAI-1, AAI-2 and AAI-3, re-
spectively. These results showed that values of Ea are higher in presence
of inhibitors as compare to in their absence (28.48 kJ mol−1). The in-
creased values of Ea in presence of AAIs suggest the physical adsorption
that occurs during first stage of the adsorption processes. [54,55].
Inhibitor
Conc
Weight loss
(mg)
CR
Surface coverage
(θ)
η%
(mM)
(mg cm−2 h−1
)
Blank
AAI-1
0.0
230
91
37
21
14
13
68
28
16
10
9
7.66
–
–
0.172
0.345
0.517
0.689
0.862
0.172
0.345
0.517
0.689
0.862
0.172
0.345
0.517
0.689
0.862
3.033
1.233
0.700
0.466
0.433
2.266
0.933
0.533
0.333
0.300
1.933
0.833
0.433
0.266
0.233
0.6043
0.8391
0.9086
0.9391
0.9434
0.7043
0.8782
0.9304
0.9565
0.9608
0.7478
0.8913
0.9434
0.9652
0.9695
60.43
83.91
90.86
93.91
94.34
70.43
87.82
93.04
95.65
96.08
74.78
89.13
94.34
96.52
96.95
Table 3
Variation of CR and η % with temperature in absence and presence of optimum concentra-
tion of AAIs in 1M HCl.
AAI-2
AAI-3
Temperature (K) Corrosion rate (CR) (mg cm−2 h−1) and inhibition efficiency
(η%)
Blank
AAI-1
AAI-2
AAI-3
CR
η%
CR
η%
CR
η%
CR
η%
58
25
13
8
308
318
328
338
7.66
11.0
14.3
18.6
–
–
–
–
0.46
1.30
3.10
6.50
93.99
88.81
78.32
65.05
0.33
1.00
2.46
5.43
95.69
90.90
82.79
70.80
0.26
0.80
1.93
4.86
96.60
92.72
86.50
73.87
7

C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
651
Fig. 3. The values of Kads and ΔG0ads derived from Langmuir adsorption isotherms for mild
steel in 1M HCl in the presence of the studied AAIs at different temperatures.
Fig. 2. Arrhenius plots for the corrosion of mild steel in 1M HCl.
However, it is well known that the adsorption behavior of most of the
organic inhibitors involve both physical as well as chemical processes
[54,55]. Therefore, it is concluded that, the adsorption of these mole-
cules on the mild steel surface from HCl solution takes place through
both physical and chemical processes simultaneously with domination
of physical one.
inhibitor molecule and metallic surface (physisorption) [59,60]. Results
depicted in Table 4 showed that values of ΔG0ads in the present study at
different temperatures vary from −32.98 to −36.96 kJ mol−1 suggest-
ing that adsorption of the AAIs on mild steel surface is a combination of
chemisorption and physisorption [61,62].
3.2. Electrochemical measurements
3.1.3. Adsorption isotherm
Adsorption isotherm provides the information about the nature of
interaction between the inhibitor molecule and the metal surface and
therefore it is one of the most important topics in the field of corrosion.
The adsorption isotherm provides the structural information about dou-
ble layer in addition to the thermodynamic information's. The adsorp-
tion may be chemisorption, physisorption or mixed type adsorption.
In order to study the nature of adsorption of inhibitors undertaken in
the present study on metallic surface, several isotherms such as Lang-
muir, Temkin and Frumkin etc. were tested. Nevertheless, the values
of regression coefficient (R2) showed the Langmuir isotherm gave the
best fit as the numerical values of R2 were close to unity for all studied
inhibitors. According to the Langmuir isotherm, the degree of surface
coverage (θ) is related to the inhibitor concentration (C) by following
relation [56]:
3.2.1. Potentiodynamic polarization study
Potentiodynamic polarization studies were carried out in absence
and presence of different concentrations of the investigated inhibitors
in order to understand the process of anodic oxidative metallic dissolu-
tion and cathodic reductive hydrogen evolution. The potentiodynamic
polarization curves for mild steel in absence and presence inhibitors
are shown in Fig. 4. The values of potentiodynamic polarization param-
eters such as corrosion potential (Ecorr), corrosion current density (icorr),
anodic and cathodic Tafel slopes (βa, βc) were obtained from the polar-
ization curves through extrapolation method and are included in
Table 5. It can be seen from the results (Table 5 and Fig. 4) that presence
of AAIs significantly reduced the values of corrosion current densities
for both anodic and cathodic half reactions. This finding indicates that
inhibitors undertaken in the present study successfully inhibited both
anodic oxidative dissolution of mild steel and cathodic reductive evolu-
tion of hydrogen [63]. The decreased values of icorr in presence of inhib-
itors are attributed due to blocking of the active centers present on the
metallic surface [64]. It is obvious from the Fig. 4 that in presence of in-
hibitors the corrosion potential shifted toward more negative direction.
Further, it can be observed from the results shown in Table 5 that values
of βc are more affected by inhibitors as compare to the values of βa when
compared to the values of βa and βc of free acid solution indicating that
investigated inhibitors act as cathodic type inhibitors [65].
θ
KadsC ¼
ð12Þ
1−θ
where Kads is the equilibrium constant of the adsorption process, C is the
molar concentration of AAIs and θ is the degree of surface coverage of
AAIs at metallic surface. The values of surface coverage obtained at dif-
ferent concentrations of the AAIs from weight loss experiments were
used to obtain the Langmuir adsorption isotherm plots (Fig. 3) which
enabled the calculation of the values of Kads. The Kads related to the stan-
dard free energy of adsorption (ΔG0ads) by the following relation [57]:
3.2.2. Electrochemical impedance spectroscopy (EIS) study
The influence of the investigated compounds on the corrosion be-
havior of mild steel in 1M HCl was also studied by EIS method in
ΔG0 ¼ −RT lnð55:5KadsÞ:
ð13Þ
ads
The numerical value 55.5 represents the molar concentration of
Table 4
The values of Kads and ΔG0ads for mild steel in absence and presence of optimum concen-
water in acid solution. R is the gas constant and T is absolute tempera-
ture. The calculated values of Kads and ΔG0
are given in Table 4.
tration of AAIs in 1M HCl at different studied temperature.
ads
High values of Kads suggest that the adsorption of the AAIs on mild
Inhibitor
Kads (104 M−1
)
−ΔG0ads (k J mol−1
)
steel surface is easy and strong [58]. Previous, studies show that the
value of ΔG0 around −40 kJ mol−1 or more negative is consisted
308
318
328
338
308
318
328
338
ads
AAI-1
AAI-2
AAI-3
1.85
2.63
3.32
0.89
1.11
1.52
0.43
0.54
0.76
0.22
0.29
0.34
35.45
36.36
36.96
34.68
35.27
36.10
33.80
34.40
35.36
32.98
33.72
34.15
with charge sharing between metal and inhibitor molecule (chemisorp-
tion) [59,60]. While the value of ΔG0ads around −20 kJ mol−1 or less
negative is consisted with electrostatic interaction between charged

652
C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
order to support the results of weight loss and polarization studies. The
Nyquist plots obtained from the EIS for mild steel in 1M HCl in without
and with different concentrations of studied inhibitors is shown in
Fig.5a. Inspection of the Fig.5a shows that the value of impedance mod-
ulus for uninhibited specimen was much smaller than that for inhibited
specimen. Further, it is also observed that the impedance modulus grad-
ually increases with increasing the concentration of the inhibitors. It is
also obvious from It is obvious from Fig.5a that the Nyquist plot in ab-
sence of inhibitors shows a slightly depressed single semicircle capaci-
tive loop and only one time constant was obtained in the Bode plots
(single maxima in Bode plots). This finding suggests that corrosion of
mild steel in acid solution in the absence of inhibitors is mainly con-
trolled by charge transfer process [56,57]. In the Nyquist plots, the dif-
ference in real impedance at lower and high frequencies is commonly
considered as a charge transfer resistance (Rct) [66–68]. The charge
transfer resistance must be corresponding to the resistance between
metal and OHP (outer Helmholtz plane). And therefore, in the present
investigation, the polarization resistance (Rp) which includes charge
transfer resistance (Rct), diffuse layer resistance (Rd), accumulation re-
sistance (Ra), film resistance (Rf) etc. is taken into the account [66–70].
The equivalent circuit shown in Fig. 5b was used to fit the experimental
impedance data. The circuit comprises of a constant phase elements
(CPE), a charge transfer resistance (Rp) and a solution resistance (Rs).
Generally, double layer formed by adsorption of inhibitors on the metal-
lic surface behaves as CPE rather than pure capacitor and therefore, the
capacitor was replaced by CPE to fit the semicircle impedance data more
accurately [71]. The admittance (YCPE) and impedance (ZCPE) of the CPE
can be represented by following relation [72]:
n
YCPE ¼ Y0ðjωÞ
ð14Þ
and
ꢀ
ꢁ
Â
Ã
1
Y0
−1
ZCPE
¼
ðjωÞn
ð15Þ
where, Y0 is the CPE constant, ω is the angular frequency; j is the imag-
inary number (i.e. j2 = −1) and n is the phase shift (exponent) which is
related to the degree of surface inhomogeneity. The nature of CPE can be
expressed by the values of n, CPE can represent resistance (n = 0, Y0 =
R), capacitance (n = 1, Y0 = C), inductance (n = −1, Y0 = 1/L) or War-
burg impedance (n = 0.5, Y0 = W). Where W is the Warburg imped-
ance (WZ), which is related to the diffusion of the ions from the
passive films of inhibitors. The values of CPE constant (Y0), angular fre-
quency (ω) and phase shift (n) were used to evaluate the values of Cdl
with and without inhibitors as follows:
n−1
Cdl ¼ Y0ðωmax
Þ
ð16Þ
where, ωmax is the frequency at which the imaginary part of impedance
has attained the maximum (rad s−1) value. The derived impedance pa-
rameters are presented in Table 6. The inspection of the results shown in
Table 6 it is obvious that values of n are almost constant and are equal/or
more than 0.827 suggesting that the interface is of capacitive nature in
the present study. The careful examination of the results shown in
Table 6 shows that values of Rp gradually increase with increasing in-
hibitors concentrations which is attributed due to adsorption inhibitors
on metal surface which isolate the metal from electrolyte and protect
from corrosion [66,73]. The decreased values of Cdl in presence of inhib-
itors are attributed due to decrease in dielectric constant and/or en-
hancement of the thickness of the electrical double layer or by
combination of both [66,73].
Fig. 6 represents the Bode impedance and phase angle plots in ab-
sence and presence of different concentrations of the studied inhibitors.
An ideal capacitor is characterized by a fixed value of slope (unity) and
phase angle (−900) in Bode plot [73,74]. The deviation from the ideal
Fig. 4. Polarization curves recorded for mild steel in the absence and presence of different
concentrations of (a) AAI-1, (b) AAI-2, and (c) AAI-3.

C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
653
Table 5
roughness of mild steel surface decreased due to formation of protective
film by inhibitors [73,74].
Tafel polarization parameters for mild steel in 1M HCl solution in absence and presence of
different concentrations of AAIs.
Inhibitor Conc
Ecorr
βa
βc
icorr
η%
θ
3.3. Surface measurements
(mM) (mV/SCE) (μA/cm2) (mV/dec) (mV/dec)
Blank
AAI-1
–
−445
70.5
70.5
72.6
83.8
66.7
79.8
73.3
71.6
71.1
79.6
74.1
90.6
75.2
114.6
151.3
99.2
1150
473.4
201.7
136.2
85.6
332.8
178.6
96.0
–
–
3.3.1. Scanning electron microscopy (SEM)
0.172 −498
0.345 −472
0.517 −549
0.689 −508
0.172 −489
0.345 −498
0.517 −479
0.689 −495
0.172 −498
0.345 −496
0.517 −514
0.689 −410
58.83 0.5883
82.46 0.8246
88.15 0.8815
92.55 0.9255
71.06 0.7106
84.46 0.8446
91.65 0.9165
94.87 0.9487
74.20 0.7420
87.28 0.8728
93.53 0.9353
97.78 0.9778
The adsorption of the AAIs on metallic surface was also supported by
SEM analysis. The SEM images of mild steel specimens in absence and
presence of optimum concentrations of the studied inhibitors after 3 h
immersion time are given in Fig. 7. Fig. 7a represents the SEM image
of abraded mild steel specimen which is characterized by visual appear-
ances of cracks and lines might be caused during abrasion by SiC emery
papers. Further, it can be seen that SEM micrograph of mild steel in ab-
sence of inhibitors (Fig. 7b) is more corroded and damaged showing
Mountain like appearance due to free acid corrosion in absence of inhib-
itors. However, in presence of optimum concentrations of inhibitors, the
SEM micrographs show very remarkable change in the surface mor-
phologies. The increased surface smoothness in presence of inhibitors
is attributed due to adsorption of inhibitors on the metallic surfaces.
126.8
125.3
107.5
143.9
100.9
157.2
107.4
88.1
AAI-2
AAI-3
58.9
296.7
146.2
74.4
128.4
108.3
25.5
capacitive behavior can be attributed to the surface inhomogeneity of
structural and interfacial origin. Generally, the value of phase angle in-
creases with increasing the metal surface smoothness. Careful visualiza-
tion of the Bode plots shows that value of phase angle for free corroding
specimen is much smaller (≈400) as compare to the values of phase an-
gles for inhibited specimens. The increased values of phase angle in
presence of different concentrations of the suggest that surface
3.3.2. Atomic force microscopy (AFM) study
Fig. 8 shows the three dimensional views of the mild steel surfaces in
the absence and presence of optimum concentration of each of the three
studied inhibitors. Fig. 8a, which is highly corroded, rough, and inhomo-
geneous depicts the surface of mild steel retrieved from the uninhibited
Fig. 5. (a–c): Nyquist plots recorded for mild steel in 1M HCl in the absence and presence of different concentrations of (a) AAI-1, (b) AAI-2, and (c) AAI-3 (d) Equivalent circuit used for
fitting and analyzing the electrochemical data.

654
C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
acid solution. The calculated average surface roughness of the uninhib-
ited mild steel specimen was 392 nm. However, in the presence of opti-
mum concentration of studied inhibitors, surface morphology of mild
steel specimens from the inhibitors containing aggressive media
(Fig. 8b–d) remarkably improved owing to the formation of protective
film on the surface. The calculated average surface roughnesses were
176, 143, and 118 nm in the presence of optimum concentration of
AAI-1, AAI-2and AAI-3, respectively. The increased surface smoothness
further suggested the formation of protective film of the inhibitors mol-
ecules on the steel surface, which prevents the metal from aggressive
(direct) acid attack.
3.4. Theoretical studies
3.4.1. Quantum chemical calculations
The optimized molecular structures and the corresponding HOMO
and LUMO electron density surfaces of the studied compounds are
shown in Figs. 9 and 10. The electron distribution of HOMO gives infor-
mation about the centers or the sites that are most likely to donate the
electrons to the corresponding orbital of the acceptor molecule,
whereas the electron distribution of the LUMO provides information
about the centers of the molecule that are more likely to accept the elec-
tron from an appropriate donor molecule. Fig. 10 shows that the elec-
tron density of the HOMO mainly localized only over the indole rings
in the AAI-1 and AAI-2, while for AAI-3 the electron density localized
over the indole as well as phenyl rings. From the frontier molecular elec-
tron distribution it can be seen that as compare to AAI-1 and AAI-2, the
involvement of the phenyl ring of AAI-3 helps in a greater electron
transfer and therefore could adsorb more effectively on steel. The
diethylamine and pyrrolidine residues of AAI-1 and AAI-2, respectively
makes small contributions to the LUMO, as the electron density mainly
localized over indole and phenyl rings. While the effectiveness of AAI-3
is due to the fact that piperdine ring make huge contribution to LUMO
distribution.
The calculated values of some common quantum chemical calcula-
tions parameters are listed in Table 7. On the basis of earlier reports
available in the literature it can be concluded that the value of EHOMO
of a molecule is a measure of the tendency to donate its HOMO electrons
to the corresponding acceptor molecule [75,76]. The value of EHOMO of
the studied compounds follows the order: AAI-3 N AAI-2 N AAI-1,
which is in accordance with the order of inhibition efficiency obtained
experimentally. On the other hand, the ELUMO is a parameter that defines
the affinity of molecule to accept electrons into its LUMO from appropri-
ate donor molecule [75,76]. The energy gap ΔE (ELUMO–EHOMO) is an-
other very important parameter which can be used to predict the
reactivity of molecules. Generally a molecule with low value of ΔE asso-
ciated with high chemical reactivity and therefore high inhibition
Table 6
EIS parameters obtained for mild steel in 1M HCl in absence and presence of different con-
centrations of AAIs.
Inhibitor Conc Rs
(mM) (Ω
Rp
n
Y0
(μF
cm−2
Cdl
(μF
cm−2
)
η%
θ
(Ω
cm2)
cm2)
)
Blank
AAI-1
–
1.12
10.7
0.827 482.2
106.21
53.58
50.65
44.19
42.22
51.71
48.41
43.52
41.07
48.25
43.48
32.16
29.82
–
–
0.172 0.796
0.345 0.765
0.517 1.059
0.689 1.181
0.172 0.987
0.345 0.743
0.517 0.892
0.689 0.811
0.172 0.810
0.345 0.741
0.517 0.696
0.689 1.362
24.73 0.846 140.3
70.02 0.832 128.5
107.0
175.4
31.97 0.842 138.7
75.89 0.843 115.8
121.3
234.3
41.72 0.839 131.8
90.44 0.832 110.3
155.5
256.6
56.73 0.5673
84.71 0.8471
90.00 0.9000
93.89 0.9389
66.53 0.6653
85.90 0.8590
91.17 0.9117
95.43 0.9543
74.35 0.7435
88.16 0.8816
93.11 0.9311
95.83 0.9583
0.848 108.3
0.832 91.8
AAI-2
AAI-3
0.866
0.845
96.21
84.3
Fig. 6. Bode impedance modulus (log f vs log |Z|) and phase angle (log f vs α0) plots for
mild steel in 1M HCl in the absence and different of different concentrations of (a) AAI-
1, (b) AAI-2, and (c) AAI-3.
0.829
0.836
87.8
63.7

C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
655
Fig. 7. SEM images of mild steel surfaces: abraded (a), in 1M HCl in the absence of AAIs (b), and in 1M HCl in the presence of optimum concentration of AAI-1 (c), AAI-2 and (d), AAI-3(e).
efficiency [77,78]. The trends of the ΔE obtained in the present study for
investigated inhibitors are in good agreement with the order of experi-
mental inhibition efficiencies. The global electronegativity (χ) is an-
other important reactivity parameter that can be utilized to explain
the electron holding capacity of the molecule. Generally, higher value
of χ is consistent with low electron transfer by the molecule and there-
fore low inhibition efficiency. In our present study, the values of χ obey
the order (Table 7): AAI-1 N AAI-2 N AAI-3, which indicates that AAI-1

656
C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
Fig. 8. AFM images of mild steel surfaces in 1M HCl in the absence of AAIs (a), and in 1M HCl in the containing optimum concentration of AAI-1 (b), AAI-2 and (c), AAI-3(d).
has lowest and AAI-3 has highest tendency of electrons donation to the
appropriate acceptor (Fe) molecule. Further, inspection of the tabulated
data show that the values of χ decreases with increasing the softness
(σ) and decreasing the hardness (η) of the inhibitors molecules. And
therefore it can be concluded that electrons donation from inhibitors
to metal increases with increasing the softness and decreases with in-
creasing the hardness of the inhibitors. The values of the fraction of elec-
tron transferred (ΔN) form inhibitors to the metal were calculated and
Fig. 9. Optimized molecular structures of studied AAIs, (a) AAI-1, (b) AAI-2 and (c) AAI-3.

C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
657
Fig. 10. The frontier molecular orbital (left-hand side: HOMO; and right-hand side: LUMO) of the studied APQDs (a) AAI-1 (b) AAI-2 and (c) AAI-3.
listed in Table 7 for all studied inhibitors. In the present case the ΔN
values obey the order: AAI-3 N AAI-2 N AAI-1, which suggests that the
maximum fraction of electron transfer occur in case of AAI-3 which
the lowest fraction of electrons transfer occur in case of AAI-1 among
the studied inhibitors suggesting that AAI-3 would absorb most effec-
tively on steel surface. The entire quantum chemical calculations results
Table 7
Quantum chemical parameters derived from the B3LYP/6-31+G(d,p) method.
Parameters→
Inhibitors↓
μ (Debye)
EHOMO
(eV)
ELUMO
(eV)
ΔE
(eV)
η
(eV)
σ
(eV)
χ
(eV)
ΔN
ΔET
AAI-1
AAI-2
AAI-3
1.6927
1.9680
2.3671
−8.5748
−8.5628
−7.8036
−5.1214
−5.1124
−5.1236
3.4534
3.4504
2.6800
1.7267
1.7252
1.3350
0.57913
0.57964
0.74903
6.8481
6.8376
6.4636
0.04398
0.04706
0.20014
−0.43167
−0.43130
−0.33375

658
C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
Fig. 11. Side view equilibrium adsorption of AAI-1, AAI-2, and AAI-3 on Fe (110) surface (left hand side: before; and right hand side: after molecular dynamics simulations).
show that corrosion inhibition in the present study occur via electron
transfer from high electron density of the inhibitors to the surface Fe
atoms.
on the metallic surface [79]. MD simulations can reasonably provide in-
formation of the most favorable configuration of adsorbed inhibitor
molecule on the metal surface and have become a well-established
tool in computational chemistry. In our present paper, MD simulations
were carried out to study the adsorption characteristics of investigated
inhibitors on the Fe (110) surface. The equilibrium configurations for
all inhibitors are represented in Fig. 11. Careful inspection of the
Fig. 11 shows that during the MD simulation process, the AAIs moved
gradually near the Fe (110) surface with almost flat orientation. And,
therefore it has been concluded that investigated inhibitors can be
adsorbed on the mild steel surface through the phenyl and indole
rings moieties. Furthermore, it has been reported that empty d-
orbitals of the surface Fe atoms can facilitate the adsorption process by
accepting the electrons from the inhibitors molecules. In the studied
compounds, the nonbonding electrons of the N atoms and π- electrons
3.4.2. Molecular dynamics simulations
Recently, MD simulation has emerged as an indispensable tool in the
characterization of the adsorption behavior of the inhibitor molecules
Table 8
Interaction energies between the inhibitors and Fe (110) surface (Kcal/mol).
Inhibitor
Ebinding
Einteraction (−Ebinding)
AAI-1
AAI-2
AAI-3
137.53
142.79
144.82
−137.53
−142.79
−144.82

C. Verma et al. / Journal of Molecular Liquids 219 (2016) 647–660
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steel surface to form protective film with essentially capacitive behavior.
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chain amino group. The inhibition potential was also found to increase
with increasing ring size of the cyclic amine and this was attributed to in-
creasing molecular size/volume of the molecule, which corresponds to in-
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Acknowledgment
Chandrabhan Verma, gratefully acknowledges Ministry of Human
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