Amino acid based imidazolium zwitterions as novel and green corrosion inhibitors for mild steel: Experimental, DFT and MD studies

Article abstract of DOI:10.1016/j.molliq.2017.08.049

Three novel amino acids based corrosion inhibitors namely 2-(3-(carboxymethyl)-1H-imidazol-3-ium-1-yl)acetate (AIZ-1), 2-(3-(1-carboxyethyl)-1H-imidazol-3-ium-1-yl)propanoate (AIZ-2) and 2-(3-(1-carboxy-2-phenylethyl)-1H-imidazol-3-ium-1-yl)-3-phenylpropa

Full text of DOI:10.1016/j.molliq.2017.08.049

Accepted Manuscript
Amino acid based imidazolium zwitterions as novel and green
corrosion inhibitors for mild steel: Experimental, DFT and MD
studies
V. Srivastava, Jiyaul Haque, C. Verma, P. Singh, H. Lgaz, R.
Salghi, M.A. Quraishi
PII:
DOI:
Reference:
S0167-7322(17)31227-8
doi: 10.1016/j.molliq.2017.08.049
MOLLIQ 7757
To appear in:
Journal of Molecular Liquids
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Revised date:
Accepted date:
21 March 2017
5 August 2017
12 August 2017
Please cite this article as: V. Srivastava, Jiyaul Haque, C. Verma, P. Singh, H. Lgaz,
R. Salghi, M.A. Quraishi , Amino acid based imidazolium zwitterions as novel and
green corrosion inhibitors for mild steel: Experimental, DFT and MD studies, Journal of
Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.049
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ACCEPTED MANUSCRIPT
Amino acid Based Imidazolium Zwitterions as Novel and Green Corrosion
Inhibitors for Mild Steel: Experimental, DFT and MD Studies
V. Srivastava1, Jiyaul Haque1, C. Verma1,3, P. Singh1, H. Lgaz4,5, R. Salghi4, M.A. Quraishi1,2,*
1Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi
-221005, India.
2Center of Research Excellence in Corrosion, Research Institute, King Fahd University of
Petroleum and Minerals, Dhahran 31261, Saudi Arabia.
3Material Science Innovation and Modelling (MaSIM) Research Focus Area, Department of
Chemistry, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng
Campus), Private Bag X2046, Mmabatho 2735, South Africa
4Laboratory of Applied Chemistry and Environment, ENSA, Universite Ibn Zohr, PO Box 1136,
80000 Agadir, Morocco.
5Laboratory of separation methods, Faculty of Science, Ibn Tofail University PO Box 242,
Kenitra, Morocco,
*Corresponding author:
Ph.no. +91-9307025126
E-mail: maquraishi.apc@itbhu.ac.in
Abstract
Three novel amino acids based corrosion inhibitors namely 2-(3-(carboxymethyl)-1H-
imidazol-3-ium-1-yl)acetate (AIZ-1), 2-(3-(1-carboxyethyl)-1H-imidazol-3-ium-1-yl)propanoate
(AIZ-2)
and
2-(3-(1-carboxy-2-phenylethyl)-1H-imidazol-3-ium-1-yl)-3-phenylpropanoate
(AIZ-3) were synthesized by condensing glyoxal, formaldehyde and amino acids, and
characterized. The corrosion inhibition performance of synthesized inhibitors was studied by
electrochemical impedance (EIS) and potentiodynamic polarization (PDP) methods. Among the
studied inhibitors, AIZ-3 showed the maximum inhibition efficiency (IE) of 96.08% at a
concentration as low as 0.55 mM (200 ppm). The results of potentiodynamic study reveal that
AIZ-1 acts as cathodic inhibitor while AIZ-2 and AIZ-3 act as mixed type inhibitors. The results
of EIS studies showed that in the presence of inhibitors, polarization resistance increased and Cdl
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decreased due adsorption of inhibitors at the metal surface. The adsorption of AIZs on the mild
steel surface followed the Langmuir adsorption isotherm. The result of scanning electron
microscope (SEM), atomic force microscope (AFM) and energy-dispersive X-ray spectroscopy
(EDX) supported the formation of inhibitors film on the metal surface. The quantum chemical
parameters and molecular dynamics (MD) simulations were used to study the reactivity and
adsorption behavior of zwitterions. The results of the theoretical investigation and experimental
studies well complimented each other.
Keywords: Mild Steel, Corrosion inhibition, Quantum chemical calculation, MD, imidazolium
zwitterions.
1. Introduction
Each country loses 3-5% of its GDP on account of corrosion. The global economic loss
due to corrosion is approximately $2.5 trillion estimated by National Association of Corrosion
Engineers (NACE) in 2016 [1]. Mild steel finds wide applications in various industries, because
of its low cost and excellent mechanical strength. However, it undergoes severe corrosion when
it is exposed to the acid solution during pickling and acid cleaning, descaling and oil-well
acidification [2, 3]. Inhibitors are added to acid solutions to prevent metal dissolution. The use of
inhibitors is one of the most practical methods to protect metals from corrosion. Most of the
synthetic inhibitors are toxic and cause health hazards [4]. Therefore, their replacement by the
environmentally benign inhibitors is desirable. Amino acids have been reported as effective
corrosion inhibitors [5-8]. They are naturally occurring chemicals, biodegradable and show the
wide range of pharmaceutical and biological activities [9, 10].
It is well documented in the literature that amino acids exhibit relatively low IE at the
higher concentration when they are used alone. Sorkhabi et al. [11] found that alanine and
glycine exhibit the maximum IE of 80% and 78.9%, respectively for steel corrosion in 0.1M HCl
at 10 mM. Amin et al. [12] tested alanine, s-methyl cysteine and cysteine as corrosion inhibitors
on iron in 1 M HCl solution. These compounds gave maximum inhibition efficiencies of 77.3%,
86% and 94.2%, respectively at 5.0 mM. Mobin et al. and Zhang et al. have studied the
synergistic effect of halides and surfactants for the improvement of inhibition efficiency of
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amino acids, but they did not find significant enhancement of inhibition efficiency [13-14]. In
our previous study, we have synthesized a condensation product of histidine and cysteine and
investigated its corrosion inhibition effect on mild steel in 1 M HCl. It gave the better inhibition
as compared to individual amino acids [15]. In view of the high performance of the condensation
product histidine and cysteine, we have synthesized three condensation products namely 2-(3-
(carboxymethyl)-1H-imidazol-3-ium-1-yl)acetate (AIZ-1), 2-(3-(1-carboxyethyl)-1H-imidazol-
3-ium-1-yl)propanoate (AIZ-2) and 2-(3-(1-carboxy-2-phenylethyl)-1H-imidazol-3-ium-1-yl)-3-
phenylpropanoate (AIZ-3) and evaluated them as corrosion inhibitors for mild steel in 1 M HCl.
The selection of these inhibitors is based on the facts that they are nontoxic, conveniently
synthesized in good yield and are soluble in aqueous acid solution. In addition to this, the
chemical structure of selected inhibitors (Fig. 1) consist of one imidazole ring and two
carboxylate groups, through which, they can adsorb on mild steel surface and are likely to give
effective corrosion inhibition. The corrosion inhibition effect of above three inhibitors has been
studied by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and
gravimetric methods. The atomic force microscopy (AFM), scanning electron microscopy (SEM)
and energy dispersive X-ray spectroscopy (EDX) were used for surface analyses. The quantum
chemical method and MD methods were also used to correlate the theoretical and experimental
results. The survey of literature reveals that these compounds have not been tested as corrosion
inhibitors.
2. Experimental procedure
2.1. Synthesis of corrosion inhibitors (AIZs)
The investigated inhibitors were synthesized by the previously reported method [16]. The
scheme of inhibitors synthesis is shown in Fig.1. The synthesized compounds were characterized
1
by their FT-IR and H/ 13C NMR techniques. The IR spectra were recorded using an FT-IR
1
13
(Perkin Elmer, Bruker) spectrophotometer and H/ C NMR spectra were recorded on a Bruker
instrument at 500 MHz, respectively. The characterization data of the synthesized compounds
are given in Table 1 and their corresponding FT-IR and 1H/ 13C NMR spectra are shown in Figs.
S1 and S2.
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2.1. Materials and test solution
The mild steel specimens having chemical composition (wt %) C 0.076%, Si 0.026%, Mn
0.192, P 0.012%, Cr 0.050%, Ni 0.050%, Al 0.023% and Cu 0.135% and Fe 99.30% were used
for gravimetric and electrochemical experiments. The specimens with dimension 2.5 cm × 2.0
cm × 0.025 cm were used for gravimetric test. The mild steel specimens were abraded by SiC
abrasive papers (from grade 600 to 1200), rinsed with distilled water, degreased in acetone and
dried in hot air and stored in moisture free desiccators before their use [17]. The test solution of
1M HCl was prepared by the dilution of analytical grade hydrochloric acid (HCl, 37 %, Fisher
Scientific) with double distilled water.
2.2 Corrosion tests
2.2.1 Gravimetric measurement
Gravimetric experiments were carried out at different temperature (308-338K) as
described in our earlier work [15]. The previously weighed mild steel specimens were immersed
in 100 ml of 1M HCl without and with different concentrations of AIZs inhibitors for 3 hours.
After immersion time, the sample was taken out, washed, dried and weighed again in order to
calculate the weight loss. Each experiment was repeated twice and mean value was reported in
order to achieve reproducibility of the results. The corrosion rate (CR), inhibition efficiency (IE
%) and surface coverage (θ) can be calculated by using the following equations:
w
(1)
CR 
At
CR C
IE(%) 
R(i) 100
(2)
CR
CR CR(i)
 
CR
(3)
where w, A and t are the weight loss (mg), the area of the specimen (cm2) and exposure time (h)
respectively. CR and CR(i) are the corrosion rate in absence and presence of AIZ molecules.
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2.2.2 Electrochemical test
The electrochemical tests were carried out to corroborate the results of gravimetric
measurement. A conventional three-electrode glass cell consisting of mild steel specimen with
the 1cm2 (one-sided) exposure area was used as working electrode (WE), platinum foil counter
electrode (CE) and saturated calomel reference electrode (RE) were used for all the
electrochemical experiments. All the electrochemical experiments were performed by using the
Gamry Potentiostat/Galvanostat (Model G-300) instrument and Gamry Echem Analyst 5.0
software was used for fitting and analyzing the electrochemical data. EIS and PDP experiments
0
were carried out at room temperature (35 ± 1 C) in the unstirred solution. The electrochemical
experiments were carried out after 30 min immersion time to stabilize the corrosion potential of
mild steel.
The EIS experiment was carried out in the frequency range 100 kHz to 0.01 Hz with a
sinusoidal amplitude of 10 mV AC signal. Polarization resistance (Rp), double layer capacitances
(Cdl) and other parameters were calculated by the fitting Nyquist plot. The inhibition efficiency
was calculated by applying the following equation:
Rpi R0
IE(%)
p 10
(4)
Ri
p
where R0 and Rip are the polarization resistances in the absence and presence of different
p
concentrations of AIZ molecules, respectively. Potentiodynamic polarization experiment was
performed after 30 min at a constant sweep rate of 1 mV/s in the potential range from ± 0.25 V
vs open circuit potential (OCP). Electrochemical parameters such as corrosion current density
(icorr), corrosion potential (Ecorr) and cathodic (c) and anodic (c) Tafel slopes were derived by
the extrapolation. The inhibition efficiency was calculated using following equation:
ic0orr ii
IE(%) 
corr 100
(5)
ic0orr
where the i0 and iicorr are the corrosion current density in the absence and presence of AIZ
corr
molecules.
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2.2.3 Surface analysis (AFM, SEM and EDX)
For the surface studies, mild steel specimens were prepared according to the previously
reported method [18]. The surface morphological studies were carried out on mild steel samples
after immersing in absence and presence of optimum concentration of AIZs. Scanning electron
microscope (SEM-EDX), Ziess Evo 50 XVP was used to study the changes occurring on metal
surface along with the elemental composition. The SEM images were taken at 500x
magnification. AFM technique was used to calculate the surface roughness for mild steel
samples immersed without and with AIZs. Atomic force microscopy, NT-MDT multimode
AFM, Russia, 111, was used to capture the micrographs.
2.2.4 Quantum chemical calculation
Quantum chemical calculations were performed by using Gaussian 09 suited program
[19]. The complete geometry optimization was conducted by the DFT with Becke’s three
parameter exchange functional with the Lee–Yang–Paar correlation functional (B3LYP) [20].
The calculation was carried out by using the basis set 6-311G(d,p) for protonated forms. The
self-consistent reaction field (SCRF) theory, with Tomasi’s polarized continuum model (PCM)
was applied to perform the computations in solution. The quantum chemical parameters such as
the energy of the highest occupied molecular orbital (EHOMO) and that of the lowest unoccupied
molecular orbital (ELUMO) have been calculated by optimized molecular structures of AIZs. Other
quantum chemical parameters such as energy gap (∆E), hardness (η), softness (σ),
electronegativity (χ) and the fraction of electrons transferred (∆N) were calculated by using the
following equations:
E  ELUMO EHOMO
(6)
1
2
(7)
  ELUMO EHOMO


1
2
(8)
(9)
  ELUMO EHOMO


Fe inh
inh
N 

2  
inh 
2(Fe  )

Fe
inh
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In equation (9), the electronegativity of iron metal surface χFe is replaced by the work function
(ϕ), previously described [18, 21]. The ϕ values obtained from DFT calculation are 3.91, 4.82
and 3.88 eV for the Fe (100), Fe (110) and Fe (111), respectively. Herein, we have chosen Fe
(110) surface due to its high stabilization energy and packed structure.
Parr et al. [22] introduced electrophilicity index (ω) (Eq. 11), which is the inverse of the
nucleophilicity. The electrophilicity of a chemical compound is associated with its
electronegativity and hardness. The equations are shown as:
1

 
(10)
2 2
4 4
(11)
 

2.2.5 Molecular dynamic details
MD simulations for investigated AIZ molecules were carried out in a simulation box with
periodic boundary conditions using Materials Studio 6.0 (from Accelrys Inc.) [23]. The iron
crystal was imported and cleaved along (110) plane and a slab of 5 Å was employed. The Fe
(110) surface was relaxed by minimizing its energy using smart minimizer method. Fe (110)
surface was enlarged to a (10 × 10) supercell to provide a large surface for the interaction of the
inhibitors. A vacuum slab with zero thickness was built. In 1 M HCl, the corresponding
proportion of water molecules to hydrogen chloride was 500/9, based on this, a supercell with a
size of a = b = 24.82 Å c = 25.14 Å, contains 491 H2O, 9Cl- , H3O+ and 1 inhibitor molecule was
created to simulate the corrosion process as much as possible [24]. The simulation was carried
out in a simulation box (24.82 × 24.82 × 35.69 Å3) using discover module with a time step of 1
fs and a simulation time of 500 ps performed at 303 K, NVT ensemble (constant number of
atoms, constant-volume, constant-temperature) and COMPASS force field [25]. In simulation
system, the interactions between inhibitors and Fe (110) surface can be understood by interaction
and binding energies calculated using equation (12) and (13) [26]:
E
interaction E (Esurface+solution +Einhibitor+solution)+E
total
solution
(12)
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EBinding E
interaction
(13)
surface+solution referred to the total energy of Fe
(110) surface and solution without the inhibitor and inhibitor+solution represent the total energy of
is the total energy of the solution.
where
E
is the total energy of the entire system
E
total
E
inhibitor and solution; and
E
solution
3. Results and discussion
3.1 Weight loss measurement
3.1.1 Influence of concentration
Variation of the inhibition efficiency with AIZs concentration is shown in Fig. S3 and
several weight loss parameters such as corrosion rate (CR), surface coverage (θ) and
corresponding inhibition efficiency (IE %) are listed in Table 2. It is apparent from the results
that the inhibition efficiency in presence of AIZs increases with increasing concentration of the
studied molecules in 1 M HCl. The studied inhibitors showed maximum inhibition efficiencies of
90%, 93.4% and 96% for AIZ-1, AIZ-2 and AIZ-3, respectively at 0.55 mM concentration. It can
be seen from Fig. S3 that after 0.55 mM, no significant change in the inhibition performance was
observed indicating that 0.55 mM is an optimum concentration in each case. The high IE of
investigated AIZ molecules is attributed to the presence of two N-atoms, π-electrons and two
carboxylate anions, which act as an adsorption centers and can effectively cover the metal
surface. The higher inhibition efficiency of the AIZ-3, AIZ-2 as compared to AIZ-1 is attributed
to the presence of benzyl and methyl substituents in AIZ-3, AIZ-2, respectively.
3.1.2 Effect of temperature
The variation of IE with temperature is shown in Fig. 3. It is clearly seen from the graph
that, IE decreases with rise in temperature from 308 to 338K. The decrease in IE may be
attributed to desorption of AIZ molecules from the metal surface at elevated temperature [27].
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The temperature effect on corrosion rate (CR) for mild steel in the absence and presence of AIZs
can be represented by Arrhenius equation [28]:
E
2.303RT
a
log(CR)
log
(14)
where A is the Arrhenius pre-exponent, CR is the corrosion rate (mgcm-2h-1), T and R are the
absolute temperature and universal gas constant, respectively. The activation energy can be
calculated by using the slope ( Ea 2.303R) values. The calculated Ea value for mild steel in
blank acid solution is 28.48 kJ mol-1, while in the presence of AIZ-1, AIZ-2, AIZ-3 the Ea values
were 81.33, 89.43 and 101.64 kJ mol-1 respectively (Table 3). The increased values of Ea in the
,
presence of AIZ molecules is attributed to increased energy barrier for the mild steel dissolution
in acid solution.
3.1.3. Adsorption isotherm
Adsorption isotherm gives the fundamental information about the interaction between
inhibitor and metal surface. The AIZ molecules adsorb on metal/solution interface by the
displacement of water molecules as shown [29]:
AIZs(sol)nH2O(ads)AIZs(ads)nH2O(sol)
(15)
where, n is the number of water molecules replaced from mild steel surface by each inhibitor
molecule. The surface coverage values were used to fit different commonly used adsorption
isotherms such as Langmuir, Temkin and Freundlich adsorption isotherms. The Langmuir
adsorption isotherm gave the best fit in the present study and is represented in Fig. 4, while
Temkin and Freundlich adsorption isotherms are presented in Fig. S4. The values of slope,
intercept and regression coefficient are listed in Table S1. The Langmuir adsorption isotherm can
be represented as follows:
C
1

C
 Kads
(16)
where, C is the concentration of inhibitor, θ is the fraction of surface coverage and Kads is the
adsorption equilibrium constant. A plot C/θ vs. C (optimum concentration of AIZ molecules)
gave a straight line with regressive coefficient (R2) values near to one at 308 K (Fig. 4). The
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slopes of the straight lines are unity, which indicates that AIZ molecules form a monolayer on
the mild steel surface [30]. The values of Kads were derived from the intercept (1/Kads) and using
the value of Kads, ΔGads values were calculated by using the following equation [31, 32].
Gο RTln(55.5Kads)
ads
(17)
where, R is the universal gas constant, T is the experimental temperature in Kelvin, 55.5 is the
concentration of water in acid solution in mol/L. The adsorption parameters ΔGads, Kads are listed
in Table 3. The negative values of ΔGads indicated that the studied inhibitors are spontaneously
adsorbed onto the mild steel surface [33]. In general, values of ΔGads around -20 kJ mol-1 or more
positive are associated with the electrostatic interaction between the opposite charge of AIZ
molecules and metal (i.e. physisorption) and ΔGads values around -40 kJ mol-1 or more negative
are related to sharing or transferring of electrons between the AIZ molecules and metals surface
to form a covalent type bond (i.e. chemisorption) [34, 35]. In the present study, the value of
ΔGads for AIZs ranges from -37 to -37.7 kJ mol-1 which are more close to -40 kJ mol-1 which
signifies that inhibitors are predominantly adsorbed by chemisorption mood [36].
3.2 Electrochemical study
3.2.1 Potentiodynamic polarization study
The potentiodynamic polarization curves of mild steel in 1 M HCl solution at 350C
without and with different concentrations of AIZ molecules are shown in Fig. 5. The
electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr),
anodic and cathodic Tafel slope (βa, βc) along with the percentage of inhibition efficiency (%IE)
are listed in Table 4. Polarization results (Table 4, Fig. 5) show that increase in the AIZs
concentration reduces anodic and cathodic currents suggesting that all the three zwitterions are
good inhibitors. The maximum decrease in current density from 1150 to 95 A/cm2 was
observed for AIZ-3 indicating that AIZ-3 is the best inhibitor. The order of efficiencies was
found AIZ-3 > AIZ-2 > AIZ-1. The presence of methyl group in AIZ-2 is resulted in its higher
inhibition performance as compared to the AIZ-1. The highest inhibition efficiency of AIZ-3
among the tested compounds is attributed to the presence of benzyl groups. More than 85 mV
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shift in the values of Ecorr in the presence of studied compounds, particularly for AIZ-1 indicate
that studied molecules behaved as cathodic type. On the basis of polarization study, inhibitor
molecules can be classified as anodic, cathodic or mixed type depending upon the displacement
in the values of Ecorr with respect to the Ecorr value of uninhibited metallic specimen. In the
present case, maximum displacements in the Ecorr values were 98, 77 and 81 mV, indicating that
AIZ-1 acts as cathodic type inhibitor, while remaining two behave as mixed type corrosion
inhibitors [37]. This finding was further supported by relatively more shifts in the values of βc as
compared to the values of βa. Inspection of the Fig. 5 reveals that potentiodynamic polarization
curves are parallel and similar in the absence and presence of different concentrations of the
studied inhibitors indicating that these molecules inhibit metallic corrosion by blocking the
actives sites present over the metallic surface without changing the mechanism of mild steel
corrosion [38].
3.2.2 Electrochemical impedance spectroscopy study
Fig. 6 and 7 show the Nyquist and Bode plots for mild steel with and without different
concentrations of AIZs. Nyquist plots consist of a depressed capacitive loop along with the real
axis and their size increases with increasing the concentration of inhibitor molecules (Fig. 6),
indicating that the corrosion of mild steel in 1M hydrochloric acid solution is controlled by
polarization resistance (Rp) [39]. For a metal corroding in acidic solution, Rp is associated with
several types of resistances such as film resistance (Rf), pore resistance (Rr), charge transfer
resistance (Rct) and diffuse layer resistance (Rd) etc. that is Rp = Rf + Rr + Rct + Rd [39]. This
finding suggests that mild steel corrosion inhibition occurs by adsorption of AIZ molecules at
metal/electrolyte interfaces. The equivalent circuit model used to interpret the impedance results
is given in Fig. 5. It consists of solution resistance (Rs), polarization resistance (Rp) and constant
phase element (CPE) [40]. CPE is used in place of capacitor in order to accurately fit the circuit.
CPE can be represented by the following equation:
 
1
1
ZCPE

( j)
n 
(18)

 
Y

0

where, Y0 and n are the magnitude and exponent (phase shift) of the CPE, respectively, j2 = -1 is
an imaginary number and ω is the angular frequency. Properties of CPE depends on the value
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of n. From the Table 5, it can be seen that the values of n vary from 0.827 to 0.886, which
suggest that the surface inhomogeneity decreases due to adsorption of AIZ molecules on the mild
steel surface [41]. The values of EIS parameters such as Rp, CPE, Y0 and n obtained by fitting
the EIS spectra are listed in Table 5. The values of Cdl can be calculated from the CPE
parameters Y and n using the following equation:
Yn1
Cdl 
(19)
sin n(/ 2)


As shown in Table 5, the value of Rp increases and Cdl decreases with increasing the
inhibitor concentration, which is attributed to increase in the thickness of the electric double
layer on the metal/solution interface and/or decrease in the value of dielectric constant due to the
displacement of pre-adsorbed water molecules by the inhibitors. From Table 5, the inhibition
performance order of AIZ molecules is AIZ-3 > AIZ-2 > AIZ-1. The same trend was obtained
from the weight loss and potentiodynamic polarization methods.
The increase in surface smoothness in the presence of AIZ molecules was further supported by
Bode plot. It can be seen from the Fig. 7 and Table S2 that phase angle (αᴼ) increases with
increase in inhibitors’ concentration and maximum increase in phase angle was -66.50 at
optimum concentration. However, for the ideal capacitor, the phase angle value is -90ᴼ and
constant slope is -1. In the present study, the values of slope range from -0.4805 to -0.7710 and
the phase angle ranges from -41.30 to -66.500, respectively. This finding suggests that in the
present analysis, mild steel interface did not behave as an ideal capacitor. The deviation from the
ideal capacitive behavior in is attributed to surface roughness [42].
3.3 Surface study
3.3.1 SEM/EDX analysis
Scanning electron microscopic (SEM) images of uninhibited and inhibited mild steel surfaces in
1 M HCl solution are shown in Fig. 8. Fig. 8a represents the SEM image of the uninhibited
metallic specimen which is drastically damaged and corroded due to free acid attack in the
absence of inhibitors. However, in the presence of inhibitors at their optimum concentration, the
surface morphologies of the metallic specimens (Fig. 8b-d) were remarkably improved, due to
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the formation of the protective film by AIZ molecules on the metallic surface which separates
the metals from corrosive environments and protects from corrosion. The adsorption of inhibitors
molecule on mild steel surface was further supported by EDX spectra of mild steel (Fig. S5). The
EDX spectrum of uninhibited metallic specimen (Fig. S5a) shows the presence of oxygen which
is attributed to the slow atmospheric oxidation of metallic surface and formation of oxides film
(Fe2O3, Fe3O3) during SEM/EDX analyses. However, the EDX spectra of inhibited metallic
specimens shown in Fig. S5b-d, give additional signal for nitrogen, which implies that the
investigated AIZ molecules are adsorbed on the metallic surface thereby forming a protective
surface film preventing further corrosion.
3.3.2 AFM analysis
The three-dimension AFM micrographs of these specimens in the absence and presence of
inhibitors are depicted in Fig. 9. In the absence of AIZ molecules (Fig. 9a), the metallic surface
was highly corroded and damaged owing to the free acidic corrosion. The calculated average
surface roughness for uninhibited specimen was 392 nm. However, in the presence of AIZ
molecules at their optimum concentration (0.55 mM), average surface roughness was 69 nm
(AIZ-1), 45 nm (AIZ-2) and 29 nm (AIZ-3) (Fig. 9b-d) due to the adsorption of inhibitor
molecules.
3.4. Theoretical studies
3.4.1 Quantum chemical calculation
Quantum chemical study using DFT method is an important theoretical tool to study the
reactivity of the inhibitor molecules [43]. AIZ molecules exist in cationic and anionic forms
depending upon the pH of the medium [44, 45]. In highly acidic solution such as 1M HCl, these
molecules exist predominantly in protonated forms. The protonation of carboxylic O-atom of
AIZs molecule was done on the basis of Mulliken charge (Mulliken charge on O-atom (-0.459) >
N-atom (-0.369). In view of this, DFT based quantum chemical calculations were carried out for
protonated forms of AIZ molecules in aqueous phase. The geometrically optimized structures are
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depicted in Fig. 10 and HOMOs, LUMOs with contour and molecular electrostatic potential
(MEP) of protonated form of AIZ molecules are shown in Figs.11. The quantum chemical
parameters such as EHOMO, ELUMO, ΔE = EHOMO-ELUMO, hardness (η), softness (σ),
electronegativity (χ), electrophilicity (ω), nucleophilicity (ε) and the fraction of electrons
transferred (ΔN110) for protonated form of inhibitors are presented in Table 6.
The inhibition effect of inhibitor molecule is usually attributed to the adsorption of AIZ
molecules on the metal surface. The analysis of frontier molecular orbital (HOMO and LUMO)
is useful to predict the adsorption center of AIZ molecules responsible for the interaction with
metal surface [46]. HOMO and LUMO are associated with the electron donor and acceptor
ability of AIZ molecules, respectively. Molecules with higher value of EHOMO (less negative) and
lower value of ELUMO (more negative) show more donor and acceptor tendency of electrons with
appropriate metal d-orbital, respectively [47]. From Fig. 11, it can be observed that HOMO are
distributed over the imidazole ring in all the AIZ molecules. Therefore, the imidazole ring is
responsible for donating the electron to the available vacant 3d orbital of Fe (110). LUMOs are
mainly distributed over the carboxylic group of the inhibitor molecules.
The reactive sites of inhibitors were further confirmed by the study of molecular
electrostatic potential (MEP). MEP is made the electron density visible and is a useful tool to
understanding sites of electrophilic and nucleophilic attack [48]. In MEP, the red (negative)
regions are subjected to nucleophilic attack, while blue (positive) regions are related to the
electrophilic reactivity. Fig. 11, red and blue region of MEPs are corresponding to the HOMO
and LUMO orbital distribution of inhibitor molecules, respectively.
Generally, higher the value of EHOMO suggests the better donor performance of inhibitor
molecules. Table 6, shows that the values of EHOMO, ELUMO and ΔE for studied inhibitors in their
protonated form obeyed the order AIZ-3 > AIZ-2 > AIZ-1, which supports the order of inhibition
efficiencies obtained experimentally. The electronegativity (χ) is another important parameter in
order to compare the inhibition efficiencies of structurally similar molecules. According to
Pearson, when two systems are brought in contact together, electron transfer will occur from
lower χ to higher χ until the chemical potential become equal [49]. Results showed that the
electronegativity of all the three protonated AIZ molecules is between 1.74-1.86 eV, those that
are lower than the work function of Fe (110) (4.8 eV). Hence, it is predicted that in the case of
protonated AIZ molecules, the AIZ molecules are mainly adsorbed on the metal surface of Fe
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(110) by the transfer of electrons from the metal orbitals to the suitable vacant inhibitor orbitals.
The values of electronegativity for the studied molecules obeys the trend: AIZ-3 > AIZ-2 > AIZ-
1, which is in good agreement with the order of experimentally obtained inhibition efficiency.
Electrophilicity (ω) is another important index to measure the electron-accepting ability of
inhibitor molecules and is physically related to nucleophilicity (ε) according to the relationship
given in Eq. (10). Therefore, a molecule with lower value of ε and higher value of ω is likely to
behave as good corrosion inhibitor [50]. From the Table 6, it was observed that both the
electrophilicity and nucleophilicity are supported by the experimental results.
3.4.2 Molecular dynamics (MD) simulation
MD simulations have been done to further study the interactions between tested inhibitors
and Fe (110) surface in the presence of all the concerned species, such as water molecules, Cl-,
H3O+ and one molecule of the investigated compounds. Based on the above discussion, in DFT
calculations, MD simulations were performed only using the protonated form of inhibitors. The
system reaches equilibrium only when energy and temperature fluctuation have reached their
balanced state [51]. When the adsorption process has reached equilibration, the interaction and
binding energies between tested inhibitors and Fe (110) surface can be calculated using the
equation (12) and (13). Table 7 listed the values of calculated energies. Fig. 12 presents side and
top views of the adsorption configuration of tested inhibitors derived from MD simulations. By
inspection of this figure, it could be observed that the investigated compounds adsorb nearly to
Fe (110) surface, where chemical bonds can possibly occur through heteroatoms and π-electrons
in phenyl rings. Looking at Fig. 12, it is apparent that in the case of AIZ-1 and AIZ-2, the
carboxyl groups are directed towards the solvent layer whereas the imidazolium ring moving
almost parallel to the surface suggesting that the significant interactions can occur between
nitrogen atoms and iron surface. In the case of AIZ-3, the phenyl ring covered more surface area
of the metal, likely due to the presence of π-electrons, on the other side, although the interactions
between the phenyl ring and metal surface are seen to be insignificant, the oxygen-iron
interactions may thus be the most important on this side. It can thus be suggested that the active
sites from phenyl ring and oxygen atoms are mainly responsible for the increase in the inhibition
efficiency. Otherwise, it can be seen from the data in Table 7 that the binding energies of the
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adsorption of inhibitors on Fe (110) surface are very high and positive suggesting the high
stability of the adsorbed inhibitors [52, 53]. The higher negative values of interaction energy can
be attributed to the strong adsorption of AIZ molecules on the Fe surface [54].
The radial distribution function (RDF) (or pair correlation function) g (r) is an effective
and useful method to estimate the bond length. The first peak occurs at 1 Å ~ 3.5 Å, indicating
the chemisorption whereas the physisorption is associated with the peaks longer than 3.5 Å [52,
55]. The radial distribution function of C, O and N atoms of AIZ-3 graphically presented in Fig.
13 shows that all the bond length of Fe-C (3.1 Å), Fe-N (3.1 Å) and Fe-O (2.9 Å) are less than
3.5 Å, so chemical interactions can occur between these atoms and the metallic surface.
As mentioned in the DFT calculations, once again it can be inferred that the protonated
form of the tested inhibitors plays a crucial role in the corrosion inhibition process.
4. Conclusion
In the present study, inhibition behavior of three amino acid derived imidazolium zwitterions
(AIZs) have been investigated by experimental and theoretical methods. Following conclusions
have been drawn:
(1) Both experimental and theoretical studies suggest that the studied imidazolium based
zwitterions act as good corrosion inhibitors for mild steel corrosion in 1 M HCl and their
inhibition efficiency increases with increase in concentration. The maximum values of %
IE for AIZ-1, AIZ-2 and AIZ-3 were 90%, 93.4% and 96%, respectively at concentration
as low as 0.55 mM (200 ppm).
(2) The studied zwitterions inhibit corrosion by adsorbing on the metallic surface at which
the adsorption obeyed the Langmuir isotherm.
(3) The results of potentiodynamic polarization study reveal that AIZ-1 acts as
predominantly cathodic type inhibitor while AIZ-2 and AIZ-3 behave as mixed type
inhibitors.
(4) The EIS study reveals that the studied zwitterions inhibit mild steel corrosion by getting
adsorbed at the metal/electrolyte interfaces thereby forming the surface protective film
which isolates the metals from the corrosive environment. The increased values of Rp and
decreased values of Cdl justify the observation.
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(5) The adsorption and film forming ability of the studied inhibitor molecules on the metallic
surface were supported by SEM, EDX and AFM analyses.
(6) Quantum chemical calculation results show that protonated inhibitors are better inhibitors
than the neutral inhibitors. The results of DFT parameters corroborated with experimental
results and a good agreement has been observed between them.
(7) Molecular dynamics simulation results reveal that inhibitor molecules adsorbed through
flat orientation and their order of effectiveness followed the experimental order of
inhibition efficiency.
Acknowledgements
Haque and Verma thankfully acknowledge the Ministry of Human Resource Development
(MHRD) and central instrumental facility center (CIFC) IIT (BHU) Varanasi for providing
financial and instrumental support, respectively.
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Fig.1: Synthetic scheme of studied AIZs.
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Fig.2: Variation of inhibition efficiency with temperature.
Fig. 3: Arrhenius plots for the corrosion rate of mild steel versus the temperature in 1M HCl.
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Fig. 4: Langmuir isotherm plot for the adsorption of AIZs on mild steel surface in 1M HCl.
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Fig. 5: Polarization curves for mild in absence and presence of different concentrations of AIZs in 1M
hydrochloric acid solution at 308K.
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Fig.6: Nyquist plot for mild steel in 1 M HCl without and with different concentrations of AIZs at 308K.
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Fig. 7: Equivalent circuit model used for fit and analyzed electrochemical data.
29