Understanding corrosion inhibition of mild steel in acid medium by new benzonitriles: Insights from experimental and computational studies

Article abstract of DOI:10.1016/j.molliq.2018.06.103

Two benzonitrile derivatives, namely 4-(isopentylamino)-3-nitrobenzonitrile (PANB) and 3-amino-4-(isopentylamino)benzonitrile(APAB) have been synthesized and evaluated as corrosion inhibitors for mild steel (MS) in 1 M HCl solution at 303 K by gravimetric

Full text of DOI:10.1016/j.molliq.2018.06.103

Accepted Manuscript
Understanding corrosion inhibition of mild steel in acid medium
by new benzonitriles: Insights from experimental and
computational studies
A. Chaouiki, H. Lgaz, Ill-Min Chung, I.H. Ali, S.L. Gaonkar, K.S.
Bhat, R. Salghi, H. Oudda, M.I. Khan
PII:
DOI:
Reference:
S0167-7322(18)31285-6
doi:10.1016/j.molliq.2018.06.103
MOLLIQ 9301
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
12 March 2018
14 May 2018
26 June 2018
Please cite this article as: A. Chaouiki, H. Lgaz, Ill-Min Chung, I.H. Ali, S.L. Gaonkar,
K.S. Bhat, R. Salghi, H. Oudda, M.I. Khan , Understanding corrosion inhibition of mild
steel in acid medium by new benzonitriles: Insights from experimental and computational
studies. Molliq (2018), doi:10.1016/j.molliq.2018.06.103
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ACCEPTED MANUSCRIPT
Understanding Corrosion Inhibition of Mild Steel in Acid Medium by new
Benzonitriles: Insights from Experimental and Computational Studies
A. Chaouiki1, 2, H. Lgaz3*, Ill-Min Chung3*, I. H. Ali4, S. L. Gaonkar5, K. S. Bhat5, R.
Salghi2*, H. Oudda1 and M. I. Khan6
1Laboratory separation processes, Faculty of Science, University Ibn Tofail PO Box 242,
Kenitra, Morocco
2Laboratory of Applied Chemistry and Environment, ENSA, University Ibn Zohr, PO Box 1136,
Agadir, Morocco
3Department of Applied Bioscience, College of Life & Environment Science, Konkuk
University, 120Neungdong-ro, Gwangjin-gu, Seoul 05029, South Korea
4Department of Chemistry, College of Science, King Khalid University, P. O. Box 9004, Postal
Code 61413, Saudi Arabia
5Department of Chemistry, Manipal Institute of Technology, Manipal Academy of Higher
Education, Manipal, Karnataka, India
6Chemical Engineering Department, College of Engineering, King Khalid University, Abha,
Kingdom of Saudi Arabia
*Corresponding Authors:
E-mail addresses: r.salghi@uiz.ac.ma (R. Salghi), lgaz.hassan@gmail.com (H. Lgaz),
imcim@konkuk.ac.kr (Ill-Min Chung).
ABSTRACT
Two benzonitrilederivatives, namely 4-(isopentylamino)-3-nitrobenzonitrile(PANB) and 3-
amino-4-(isopentylamino)benzonitrile(APAB) have been synthesized and evaluated as
corrosion inhibitors for mild steel (MS) in 1 M HCl solution at 303K by gravimetric,
potentiodynamic polarization (PDP) curves, and electrochemical impedance spectroscopy
(EIS) methods, as well as Density Functional Theory (DFT) and Molecular Dynamic (MD)
simulations. The results suggest that tested compounds are excellent corrosion inhibitors for
mild steel with PANB showing superior performance. Polarization measurements revealed that
PANB and APAB behaved as mixed type inhibitors. The polarization resistance, according to
EIS studies, found to be dependent on the inhibitor’s concentration. The adsorption of PANB
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and APAB on mild steel surface obeyed Langmuir’s adsorption isotherm. On the one hand,
DFT and MD simulations are being used to explain the effect of the molecular structure on the
corrosion inhibition efficiency and on the other hand to simulate the adsorption of benzonitrile
derivativeson mild steel surface. The protection of carbon steel in 1 M HCl was confirmed by
using scanning electron microscope (SEM) and Atomic Force Microscopy (AFM).
Electrochemical, DFT and MD simulations results are in good agreement.
Keywords: Benzonitrile derivatives; Corrosion inhibition; Mild steel; Acid solutions;DFT; MD
simulations.
1. Introduction
Hydrochloric acid is a strong inorganic acid that is extensively used in many industries, like oil,
gas and chemical industries. Unlike many other metals, mild steel is the most commonly-used
material in these industries; it came to be used for most purposes where wrought iron was
formerly used[1,2]. Nonetheless, the major problem associated with its use seems to reach its
high susceptibility to corrosion in acidic solutions, which results in a considerable loss in its
essential properties. In order to adopt effective preventive methods against corrosion of metals,
various strategies have been applied to control this undesirable phenomenon such as materials
selection, cathodic protection, coatings and the use of corrosion inhibitors. The inhibitors have
been considered as the most practical and easiest method. Typically, these organic compounds
along with heteroatoms and lone pair have shown good inhibition performance for many metals
and rapid progress in recent years has been made in the design and synthesis of a variety of
compounds such as quinoles, quinoxalines, pyridinium, triazoles and imidazolines...etc[3].
They seem to act by adsorbing onto the surface of metal, blocking one or more of the
undesirable reactions occurring at the solution/metal interface and therefore causing high
protection of metal against dissolution[4,5]. Benzonitriles and related compounds having nitrile
group as the substituent are known corrosion inhibitors. For example, a novel azo nitrile
derivative reported as an effective inhibitor of corrosion copper in nitric acid medium [6]. Here,
the structure of the inhibitors contains an additional azo group in addition to the nitrile group.
Simple aliphatic and aromatic nitrile compounds have been investigated for their inhibitive
action and found that effective in reducing hydrogen embrittlement on steel surface[7,8].
In recent years, to understand some unknown properties during the interaction of inhibitors with
the metal surfaces, theoretical chemistry and molecular simulation studies have been used in
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this context. Density Functional Theory based calculations have been proved to be a very
powerful tool for studying the electronic and structural properties of inhibitors. The geometry
of the inhibitor molecule in its ground state and the nature of their molecular orbitals (HOMO;
Highest Occupied Molecular Orbital and LUMO; Lowest Unoccupied Molecular Orbital) are
directly involved in the inhibitive properties of these molecules. Such calculations, though
instructive, are not always adequate for providing more clarification of the experimental results.
Modeling of an experiment by molecular dynamic (MD) simulations has been accepted as an
effective and authentic technique. MD simulations are particularly well-poised to investigate
the structure of the corrosion inhibitors and the mechanism of inhibition at an atomic level
under a variety of conditions[9].
In this contribution, weight loss measurements, potentiodynamic polarization and
electrochemical impedance spectroscopy have been successfully used for the analysis of the
anti-corrosive efficiency of novel benzonitrile derivatives in 1.0 M HCl. Meanwhile, MD
simulations and DFT calculations were carried out to explain the proposed inhibition
mechanism between the inhibitor and metal surface. These inhibitors are having structural
similarity, only differing in the nature of substituent groups attached with the phenyl. MS
samples were further studied by scanning electron and atomic force microscopies.
2. Materials and methods
2.1. Synthesis of inhibitors
General procedure for the Synthesis of novel benzonitrile derivatives is given below:
 Synthesis of compound 4-(isopentylamino)-3-nitrobenzonitrile (PANB):
Scheme 1.Preparation of 4-(isopentylamino)-3-nitrobenzonitrile (PANB)
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A stirred solution of 4-fluoro-3-nitrobenzonitrile (14.97 g, 9 mmol) in MeCN (15 mL) was
placed under an N2 atmosphere. Isopentylamine (1.58 g, 18 mmol) was added dropwise at rt
for a period of 30 min. The reaction mixture was stirred at room temperature for 1 hour after
which it was poured on water (60 mL) and stirred for 15 minutes. The resulting solid was
collected, washed with water (2 x 20 mL). The solid was dried in a vacuum oven at 40 OC to
give 4-(isopentylamino)-3-nitrobenzonitrile (PANB) (2.06 g, 98%) as a yellow solid.
1H NMR (400 MHz, DMSO-d6) δ ppm: 1.01(d, 2H, 2X–CH3), 1.60-1.69 (m, 3H, -CH and -
CH2), 3.4 (t, 2H, -CH2), 4.1 (s, 1H, -NH), 7.28-7.33 (m, 5H, ArH), 7.38 (d, 2H, ArH), 7.45 (d,
2H, ArH). 13C NMR (100 MHz, DMSO-d6) δ ppm: 23.1, 25.9, 39.9, 41.5, 101.2, 115.9, 118.9,
130.1, 135.0, 139.2, 149.8.
 Synthesis of compound 3-amino-4-(isopentylamino)benzonitrile (APAB):
Scheme 2. Preparation of 3-amino-4-(isopentylamino)benzonitrile (APAB).
A three neck RBF was charged with of Hydrochloric acid (10 mL), SnCl2 (1.6 g, 8.5 mmol).
Under stirring 4-(isopentylamino)-3-nitrobenzonitrile (PANB) (1.0 g, 4.2 mmol) was added for
a period of 30 min. The reaction mixture was stirred for 1 hour. The mixture was diluted with
water (10 mL) and basified with dilute NaOH. The solid formed was filtered and crystalized
from ethanol to give 3-amino-4-(isopentylamino)benzonitrile(APAB) as a brown solid to yield
(0.80 g, 91 %).
1H NMR (400 MHz, DMSO-d6) δ ppm: 1.02 (d, 6H, 2X –CH3)1.60-1.68 (m, 3H, -CH and -
CH2), 3.42 (t, 2H, -CH2), 4.1 (s, 1H, -NH), 6.41 (s, 2H, -NH2), 6.60 (s, 1H, ArH), 6.64 (d, 1H,
ArH), 6.81 (d, 1H, ArH). 13C NMR (100 MHz, DMSO-d6) δ ppm: 23.2, 25.9, 40.1, 42.8, 101.1,
115.9, 115.3, 118.4, 121.9, 123.1, 134.4, 140.8.
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2.2. Materials and corrosive solutions
The coupons used for the corrosion tests were made of mild steel with a chemical composition
of: 0.370 % C, 0.230 % Si, 0.680 % Mn, 0.016 % S, 0.077 % Cr, 0.011 % Ti, 0.059 % Ni, 0.009
% Co, 0.160 % Cu, and balance Fe. Prior to each individual experiment, test coupons were
mechanically abraded using a belt grinding polishing machine. To obtain a polished surface,
MS surface was grounded with emery papers of decreasing grit size from 600 to 1600 grades.
Afterwards, the samples were rinsed with distilled water, degreased in ethanol and dried in
warm air prior to use. The test solution (1 M HCl) was prepared from the reagent grade 37%
HCl and distilled water. Solutions of the inhibitors in 1 M HCl were prepared to obtain different
concentrations range of 1×10-4 M to 5×10-3 M at 303 K.The organic compounds tested were
PANB and APAB and are given in Figure 1.
2.3. Electrochemical measurements
Electrochemical measurements were conducted in an 80 mL three-electrode cell assembly with
a working electrode (mild steel, 1 cm2 dimension), platinum as auxiliary electrode and saturated
calomel as reference electrode (SCE) placed in a constant temperature bath. All potentials
within the text are were referred to the SCE. Electrochemical experiments were performed on
a Volta lab PGZ 100 Instrument with a Volta master 4.0 software package for experimental
applications. The results were fitted using EC-Lab software package. All electrochemical tests
were performed after 30 minutes at the temperature of 303K. Electrochemical impedance
spectroscopy (EIS) measurements were conducted over a frequency range of 100 KHz – 10
mHz with signal amplitude perturbation of 5 mV. Potentiodynamic polarization experiments
were carried out at the scan rate of 1 mV/s over the potential range from -800 to -200mV/SCE.
All experiments were run in triplicate to confirm the reproducibility.
2.4. Weight loss measurements
According to the ASTM standard [10], disk-shaped specimens (38 mm in diameter and 3 mm
in thickness with a hole of 8 mm diameter) were prepared for each weight loss test. The mild
steel samples were immersed in uninhibited and inhibited solutions for 6h at 303K. The MS
samples were prepared as mentioned previously and weighted using a precision balance with a
sensitivity of 0.1 mg. After each test, the MS specimens were taken out and rinsed thoroughly
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with distilled water and acetone, dried and weighted accurately again. The following equation
(Eq. (1)) was used to determine the corrosion rate in millimeters per year (mm y−1):
KW
CRW 
(1)
At
Where K= 8.76×104 was used as a constant. W and t are the mass loss in gram and the time of
exposure in hours. According to ASTM G1-03 standard[11], the density of mild steel is 7.86 g
cm−3. The exposed area, A (in cm2) was calculated from Equation (2)[12]:

A D2 d2 lDld
(2)


2
Where D, d and l are the diameter of mild steel pieces, the diameter of the hole for holding and
the thickness, respectively.The following equations were used to calculate the inhibition
efficiency WL (%) and the surface coverage ( )[13]:








CRW CRW
(3)
WL (%) 
100
CRW





RW 

CRW C
(4)
 
CRW
C
Where RW and CRW are the corrosion rates without and with various concentrations of the
inhibitors, respectively, θ is the degree of surface coverage of tested inhibitors.
2.5. Theoretical calculations
Our current understanding of electronic properties of many corrosion inhibitors has been shaped
by multiple theoretical studies[10]. The energy geometry optimization, quantum chemical
parameters and Fukui functions were obtained using Gaussian program package, module
version 9.0[10]. The calculations started without any geometry constraints until full geometry
optimizations. The calculation was performed using higher basis set denoted by 6–311G++
(d,p)[11,14]. All quantum calculations were carried out in aqueous phase using Self-Consistent
Reaction Field (SCRF) theory, with Polarized Continuum Model (PCM)[9]. The ionization
energy and the electronic affinity were determined by the values of the energies of the HOMO
and LUMO orbital[15]:
IPEHOMO
EAELUMO
(5)
(6)
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Then, electronegativity (χ) and chemical hardness (η) were evaluated, based on the finite
difference approximation, as linear combinations of the calculated IP and EA:
IPEA
(7)
 
2
IPEA
(8)
 
2
The fraction of electrons transferred (ΔN) from inhibitor to metallic surface was calculated
using the equation[16,17]:
 inh
N 
(9)
2(Fe  )
inh
Where

is the work function used as the appropriate measure of electronegativity of iron, and
=4.82 eV for Fe (110) surface which is reported to have higher
stabilization energy[10,18].

Fe = 0. The value of

The local reactivity of inhibitor molecules (nucleophilic and electrophilic attacks) were
obtained by condensed Fukui functions[19], using the following equations[13]:
fk qk(N1)qk(N)(10)
fk qk (N)qk (N 1)(11)
Whereqk (N) qk (N1) and qk (N1) represent charge values of atom k for neutral, anion
,
and cation, respectively.
In all molecular modeling and simulation procedures, molecular dynamics (MD) simulation
were carried out using Materials studio package[13] in a 10 x 10 super cell using the COMPASS
force field with a time step of 1 fs, NVT ensemble and simulation time of 2000 ps[13] in a
simulation box of (24.82×24.82×35.69 Å3). The Temperature was fixed at 303 K. The binding
and interactions energies were estimated by using following Equations[20]:
EBinding E
E
interaction  E (Esurface+solution +E
)
(12) and
(13)
interaction
total
inhibitor
Where
E
is the total energy of the entire system,
E
refers to the total energy of Fe
total
surface+solution
(110) surface and solution without the inhibitor and
E
represents the total energy of the
inhibitor
inhibitor. The calculations of Radial Distribution Function (RDF) were performed after MD
simulations, the RDF, g(r), is defined as the probability of finding particle B within the range
rdr around particle A. RDF is defined as follows[21]:
NA NB
(r r)
1
1
NA
ij
gAB (r) 

(14)


4r2
iA jB
B
local
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Where B local represents the particle density of B averaged over all shells around particle A.
2.6. Surface characterization
For SEM analysis, coupons were prepared following identical procedure as described for
weight loss. The surface was immersed for 6 h in 1 M HCl solution prepared without and with
the addition of optimum concentration of inhibitor at 303K. After 6 h the coupons were taken
out, and thoroughly cleaned with distilled water, air-dried and subjected to SEM analysis.
Atomic Force Microscopy (AFM) measurements were carried out using VEECO CPII to
observe the surface roughness over time in the absence and presence of inhibitors.
3. Results and discussion
3.1. Potentiodynamic polarization studies
To pool information concerning the kinetics of anodic and cathodic reaction, the addition effect
of both compounds at various concentrations in the aggressive electrolyte was investigated by
polarization method. The curves obtained are shown in Figure 2. Extracted electrochemical
parameters like corrosion current density (icorr) and Tafel slopes (βc and βa), obtained from
these graphs by extrapolation method are listed in Table 1. The corrosion inhibition efficiency
PDP(%)was calculated using the relation[21]:




icorr
ic°orr
PDP(%)  1
100
(15)


i
i°
Where corr and corr are the corrosion current density in the inhibited and uninhibited acid
respectively. The percentage of inhibition efficiencies PDP(%) derived from equation (15) are
also listed in Table 1.
From the Figure 2, we notice that in the presence of the inhibitors, the shapes of polarization
curves have no distinct changes compared to the blank one with a prominent decrease in
corrosion rate, which manifests the reaction mechanism of corrosion resistance process was not
changed for studied inhibitors. Interestingly, the results revealed that both anodic and cathodic
curves are being affected in the presence of both inhibitors suggesting that investigated
compounds retard the anodic metal dissolution as well as cathodic hydrogen evolution
reactions. The corrosion potential (Ecorr) does not change significantly in presence of both
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inhibitors, thus, the inhibitor molecules can be classified as mixed-type inhibitors, which
implies the inhibitors reduce the anodic mild steel dissolution and also retards the cathodic
hydrogen evolution reaction[22]. Upon inspection of Figure 2, it can be seen that when the
potential goes towards positive values ∼−300 mV, the anodic branch a slightly changed. This
result is well known as 'desorption potential' and it is consistent with other research which found
that an increase in anodic currents is mainly associated with desorption potential that alters
significantly the inhibitor film[23].
Results summarized in Table 1 indicate that, the current density values diminish in the order of
PANB<APAB and a more prominent decrease was observed at higher inhibitor concentration
which is attributable to the enhanced effectiveness in blocking the cathodic and anodic reaction
sites at higher concentration and thereby hindering the electrochemical reactions occurring at
those sites. Hence, we can infer that this phenomenon is due to the adsorption of the inhibitor
molecules on steel surface leading to the increase of the surface coverage. Evidently, the
inhibition efficiency reaches 95.42 and 93.86 at 5×10-3 M of PANB and APAB, respectively,
which indicates that the −NO2 substituted PANB has better blocking ability than that of the
−NH2 substituted inhibitor.
3.2. Electrochemical Impedance Spectroscopy measurements
In this paper, the electrochemical behavior at the interface of the mild steel in the free acid
solution and the inhibited solution was described by EIS measurements. Figure 3 compares the
Nyquist plots for mild steel immersed for 30 min without and with different concentrations of
PANB and APAB. The EIS data recorded at the open-circuit potential illustrate a capacitive
loop in form of depressed semicircles. That is to say that the charge transfer process mainly
controls the dissolution of mild steel in 1 M HCl solution[24]. Furthermore, the diameter of the
impedance spectra in presence of benzonitrile derivatives is much higher than that in the
uninhibited solution, and this increase became more pronounced as inhibitor concentration
increased, which suggests that the two studied inhibitors adsorbed on the metal surface and a
protective film is formed[25,26]. Using the EC-Lab software, the electrical equivalent circuit
illustrated in Figure 4[27] was used for analyzing the impedance spectra for the corrosion of
MS. The equivalent circuit comprises of the solution resistance (Rs) and a polarization
resistance (Rp), which is in parallel connection with the constant phase element (CPE). The
fitting results are tabulated in Table 2. As far as modeling of corrosion systems is concerned, it
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is important to note that the constant phase elements (CPE) is used in place of a pure double-
layer capacitor. The impedance of a CPE is expressed as[21]:
1
ZCPE

(16)
Q( j)n
Where Q is the CPE constant and CPE exponent, n is the phase shift which can be used as a
measure of surface inhomogeneity [28]; j is an imaginary number and ω is the angular frequency
in rad.s-1. However, the values of Cdl were computed from the Q and n values, using Eq. 17[29]:
1-n
(17)
n
C  QRp
dl
The corrosion inhibition efficiency of studied inhibitors using EIS data can be calculated from
the Rp values using the Eq. 18[21]:
i


o 
Rp Rp
(18)
 (%) 
100

EIS
Rip




Ro
Ri
Where p and p are the polarization resistance in absence and presence of inhibitors
respectively. Also Cdl and Rp are counted and presented in Table 2.
It is clearly seen from Table 2 that the polarization resistance exhibits an increasing trend with
increasing benzonitrile derivatives concentration and the Rp values in inhibited solution are
greater than that of uninhibited solution reached a maximum value of 382.5 Ω cm2 at 5×10-3
M in the case of PANB inhibitor. The increase in Rp values implied that a protective layer was
formed on the mild steel surface, thereby retarding the charge transfer[30]. On the Other hand,
adsorbed inhibitor molecules at the mild steel/electrolyte interface replace the surface adsorbed
water molecules which increase the thickness of the double layer, thus leading to the reduction
C
of dl values and therefore efficiently minimized dissolution of steel[31]. The n parameter is a
measure of surface inhomogeneity. The values of n obtained from this study ranged from 0.78
to 0.84 and it is observed to decrease in the presence of the inhibitors when compared to that
obtained in pure 1 M HCl, suggesting an increase in heterogeneity as a result of the adsorption
of the inhibitors on the electrode/ electrolyte interface. Also, it can be seen from Table 2 that
the inhibition efficiency values can be ranked as follows: PANB ˃ APAB. Therefore, we
conclude that the variation in inhibition efficiency is related to the nature of the substituent
attached to the inhibitor molecules. The reason for this difference in inhibition efficiency of
these compounds is better explained by DFT method and MD simulation (see theoretical
section).
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3.3. Weight loss study
3.3.1. Effect of concentration
By using weight loss method, the inhibition performance of organic inhibitors in studied
interface can be evaluated to further confirm the electrochemical techniques results. This
method is especially helpful because of its simple application and reliability. The inhibition
efficiency (WL (%)) of inhibitors, corrosion rate (
C
RW ) for mild steel specimen and the surface
coverage (θ) by inhibitors obtained from weight loss experiments at different concentrations of
inhibitors in 1 M HCl solution at 303K are reported in Table 3. From the results listed in Table
3, it can be observed that the corrosion rate decreases significantly with the increasing of
inhibitor concentration. The rate of corrosion in the absence of inhibitors is 130.4 mm/y. While
in the presence of 5×10-3 M of PANB and APAB, the corrosion rate reduces to 5.202 and 12.818
mm/y respectively. Decreasing of corrosion rate is due to the formation of the layer on the metal
surface which covers the surface metal. It is also obvious from the Table 3 that the inhibition
efficiency values for the two tested compounds attain 96.01% and 90.17% for PANB and
APAB, respectively. Thus, we deduce that PANB performance is considerably better than the
other inhibitor. Hence, this enhanced efficiency comes out from the adsorption of PANB
molecule on mild steel surface. In short, the presence of several heteroatoms in both compounds
facilitates their adsorption onto the metal surface and form a protective layer and henceforth
preventing the corrosion.
3.3.2. Effect of the temperature and kinetic parameters
The effect of the temperature can provide further information on the metal-inhibitor interactions
which is useful to understand the inhibition mechanism. Table 4 represents the temperature
effect on the inhibition efficiency in absence and presence of 5×10-3 M of APAB and all
concentrations of PANB. It is obvious, from this Table that the corrosion rate increased with
the increasing of the temperature which suggests that a protective film of inhibitor was formed
on the mild steel surface and was desorbed at a higher temperature. Arrhenius plots and
transition state plots for mild steel in 1 M HCl with 5×10-3 M of inhibitors were represented in
Figure 5 while the activation parameters derived from the slopes and intercepts of fitted lines
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were summarized in Table 5. The Arrhenius and transition state equations are defined as
follows[32]:
E
RT
a
(19)
CRW kexp(
)
RT
Nh
S
R
Ha
RT
CRW  exp( a )exp(
)
(20)
From the results in Table 5, it can be noticed that the values of activation energy, Ea in the
presence of inhibitors are generally larger than that of the uninhibited solution, which indicates
the increasing of the energy barrier for the corrosion reaction[33]. In addition, the increased
ΔSa values signify an increase in the degree of disorder compared with the blank. Furthermore,
we note from the Table 5 that the values of Ea and ΔHa are varied in the same way, verifying,
therefore the known thermodynamic relation:
Ea Ha  RT
(21)
3.4. Adsorption isotherm
It is important to study the adsorption isotherms to understand the mechanism of inhibition
corrosion reactions. To determine the most relevant isothermal model of our inhibitors, we have
plotted the different models of adsorption isotherm using their mathematical formulas. From all the
tested models, it can be concluded that the best fit is obtained with the Langmuir isotherm with
correlation coefficients close to unity. Figure 6 represents the Langmuir adsorption isotherms
of APAB and PANB at 303K and 303-333K respectively. The Langmuir adsorption isotherm
is given by[34]:
C

1
Kads
inh

C
(22)
inh
Where Kads is the adsorption-desorption equilibrium constant, Cinh is the concentration of the
benzonitrile derivatives in the solution. From the intercepts of the straight lines Cinh/θ vs. Cinh,
the Kads values were calculated. Kads is related to the standard Gibbs free energy of adsorption,
G° ,by using the equation (20)[35]:
ads
G° RTln(Kads 55.5)
(23)
ads
Where, 55.55 are the molar concentration of water, R is the universal gas constant and T is the
temperature (here, 303 K). The standard enthalpy change ( Ha°ds ), standard entropy change (
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Sa°ds ) of adsorption of APNB on the steel surface can be calculated by the following
equation[36]:
G° Ha°ds TSa°ds
(24)
ads
Figure 7 represents the variation of the standard Gibbs free energy as a function of the
temperature while the calculated thermodynamic parameters are listed in Table 6. Based on the
values of the correlation coefficient (R2 = 0.9999) and the slope (close to 1), we can confirm
that Langmuir model shows a good description of the adsorption behavior of inhibitors.
Generally, the high values of Kads indicate that the adsorption of inhibitors on the metal surface
is facile as well as strong. In our present study the Kads values (Table 6) of two tested inhibitors
follows the order: PANB ˃ APAB which is in accordance with the order of the inhibition
efficiency. Usually, inhibitors having the values of the adsorption free energy or lower near the
value −20 kJ mol−1 act by physical adsorption mechanism, while inhibitors with values of higher
adsorption energy or close to the value −40 kJ mol−1 act by a chemical adsorption
mechanism[37,38]. In our case, the G° values for APNB and APAB are between the
ads
threshold values for chemical and physical adsorption, signifying that both benzonitrile
derivatives adsorbed on the mild steel surface through chemical and physical adsorption[39].
In addition, this adsorption on mild steel is an exothermic process due to the negative Ha°ds
values. Besides, the value of Ha°ds for PANB is -54 kJ/mol, which is also between the threshold
values for chemical adsorption (close to -100 kJ/mol) and physical adsorption (lower than -40
kJ/mol), thus, confirming the mixed adsorption type[40]. Similar results have also been reported
by other authors[36]. It has been commonly accepted that simple adsorption on the surface is
usually an exothermic process, which has been accompanied by a decrease in entropy[41,42].
In our case, the same result was obtained.
3.5. Surface morphological studies
The mild steel surface was analyzed by SEM after immersion in 1.0 M HCl in absence and
presence of 5×10-3 M for 6h at 303K. Figure 8 shows the surface morphology by SEM for mild
steel in absence and presence of 5×10-3 M of tested inhibitors. It can be seen from this figure
that the sample of steel without inhibitors is badly corroded by aggressive solution and huge
corrosion products and cracks can be found on substrate surface. In the presence of inhibitors,
the surface roughness decreased and this can be attributed to protective effect of tested
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compounds. The morphology of steel surface in the presence of inhibitors is different and
smoother than that of the free acid.
Figure 9 shows the AFM micrographs of the mild steel in absence and presence of 5×10-3 M of
tested inhibitors. As is observed from this Figure, the surface of the mild steel is more corroded
with an average roughness of 1.3 µm. When adding the optimum concentration of inhibitors,
the average roughness was reduced to 461.4 nm and 790.8 nm for PANB and APAB
respectively. The AFM results further confirm that PANB and APAB molecules can effectively
protect the mild steel from corrosion.
3.6. Quantum chemical calculations
Generally, the inhibition effect of a compound can be well associated to its adsorption via
electronic interaction with the metal surface. In recent times, quantum chemical calculations
can provide more information about the structural parameters of the inhibitor molecule, while
the adsorption mechanism can be accounted for the chemical reactivity of the
compound[43,44]. In this case, the Frontier Molecular Orbitals (FMO) theory and geometrical
optimization is useful in predicting the nature as well as extent of adsorption of the inhibitor
molecules on the surface/inhibitor. The calculated quantum chemical parameters such as the
highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital
energy (ELUMO), the energy gap ( Egap = ELUMO − EHOMO) and the number of transferred
electrons (ΔN) are important and useful tools to compare the corrosion inhibition performance
of molecules and also to validate the experimental findings. The calculated theoretical
parameters are given in Table 7. Figure 10 presents the optimized and frontier molecular
orbitals pictures of the two inhibitors studied. Upon inspection of the geometry optimized of
molecules studied (Figure 10), results reveal that, the HOMO and LUMO orbitals are
distributed almost over the entire molecular structure except in acetate group. Meaning firstly,
the good ability of both inhibitors to donate electrons to appropriate acceptor and on the other
hand their good ability to accept electrons from the metallic surface. The HOMO energy
(EHOMO) is a measure of a molecule’s ability to give an electron to an acceptor, while the LUMO
energy (ELUMO) is a measure of a molecule’s proclivity to receive an electron from donor
species. The higher the EHOMO is, the better is the tendency of electron donation by a molecule
and vice versa[45]. Lower ELUMO suggests a better propensity of a molecule to receive electrons
and vice versa. Therefore, higher EHOMO and/or lower ELUMO favor(s) higher corrosion
14

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inhibition strength[46]. In contrast to ELUMO values, the values of EHOMO are not in conformity
with the trend of inhibition efficiency. From a theoretical point of view, the energy gap ( Egap
) is another important factor parameter to identify the chemical reactivity of an inhibitor and
their capability to be an effective corrosion inhibitor [47,48]. The adsorption performance
between the inhibitors and the metal surface increases when ΔEgap decreases, because the
energy to remove an electron from the last occupied orbital must be lower[49]. From the table
7, it is observed that the energy gap values for APAB and PANB are 4.820 eV and 3.813 eV
respectively. The compound PANB has lower energy gap (ΔE) compared to APAB. This result
indicates that the adsorption performance on the steel surface of PANB is greater than that of
APAB. ∆N values, on the other hand, indicate the ability of the tested inhibitor to transfer its
electrons to metal if ∆N > 0 and vice versa if ∆N < 0. According to this criterion, it is obvious
from the results in Table 7 that both compounds have higher tendency to donate electrons to a
metal surface. The ∆N values follow the same order of the ELUMO values which signifies that
the electron accepting ability of the inhibitor molecules plays the crucial role in the difference
between investigated compounds[50,51].
In corrosion inhibition research, the Fukui indices are an important descriptor to explore the
relationship of the inhibitors with MS surface in terms of structure-activity. This study provides
an exciting opportunity to strengthen our knowledge of electron donating capacity (i.e.
nucleophilic fk ), and electron accepting ability (i.e. electrophilic fk ) of inhibitors molecules.
The results are summarized in Table 8 from which we notice that in the case of APAB molecule,
the C (1), C(5), C(31) and N (32) atoms are the highest values of fk indicating that these atoms
are available to accept electrons from the metal surface. On the other hand, the N(10), N (28)
and N(32) atoms possess higher values of fk which means that they are responsible for the
electrophilic attack. In the case of the compound PANB, it is clear that the addition of acceptor
group -NO2 leads to the new distribution of the active sites. It is evidently clear from Table 8
that the C(3), N(12), O(13) and O(14) atoms are the active sites susceptible for nucleophilic
attacks. However, the electrophilic sites are present in N(11) and N(15) atoms. The theoretical
study shows that the introduction of the nitro group has a positive effect on the reactivity of the
compound PANB which is consistent with experimental observations.
3.7. Molecular dynamics simulations
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Since strong correlation exists between electronic properties of inhibitor and corrosion
effectiveness, DFT calculations are very useful for the overall understanding of electronic
properties of corrosion inhibitors [52]. On the other side, understanding the interactions of
inhibitor molecules with the metal surface are also of great importance. In this case, MD
simulations can be particularly useful for the determination of molecule's atoms with strong
interaction with the metal surface, which can be useful for the understanding of corrosion
inhibition process. The binding and interaction energies of the adsorbed inhibitors have been
approximated when the simulation system reached its equilibrium state[53]. The best adorable
adsorption configuration of the studied molecules on Fe (110) surface is shown in Figure 11
while the interaction and binding energies are placed in Table 9. Looking at Figure 11, what
stands out is that all inhibitor molecules had been moved in nearly parallel or flat disposition
which provide larger blocking area by the investigated inhibitors preventing the surface from
corrosion through a formation of a barrier layer between the metal surface and the aggressive
media. The large negative values of the interaction energies for both inhibitors indicating that
the interaction between inhibitor molecules and Fe(110) surface is spontaneous, strong and
stable [12]. On the other hand, the high magnitude of the binding energies suggests that the
adsorption system is more stable and that there is more than one bond to the iron surface per
inhibitor molecules [54].
In order to locate the atoms of inhibitor molecule with relatively significant interactions with
the metal surface, we have calculated RDFs after MD simulations [55]. According to the studies
in the literature, the length of the small links is indicated by the peak which produces at 1 Å ~
3.5 Å, which correlated to chemisorption, while the peaks longer than 3.5 Å prove that there is
a physical interaction[22,56]. The RDFs of all non-hydrogen atoms of the two inhibitors are
displayed in Figure 12. From this Figure, the radial distribution function of C, N and O atoms
in the case of PANB shows that the bonding length of Fe–N (3.1 Å), Fe–O (2.9 Å ) and Fe–C
(3.3 Å ) are all less than 3.5 Å, while for APAB, are 3.1 Å for Fe–C and 2.34 Å for Fe–N. This
extensive research has shown the chemical interaction between inhibitor compounds and MS
surface, and the highest protection of metal surface through an effective adsorption of
benzonitrile compounds. As the previously mentioned in the theoretical and experimental parts,
the presence of the nitro group provides good inhibitive properties. Therefore, the difference
between the two tested compounds may be attributed to the electron density and the number of
the reactive centers in each molecule.
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4. Conclusion
The present study was designed to evaluate the corrosion inhibition activity of novel
benzonitrile derivatives using weight loss, EIS, PDP measurements and theoretical calculations.
The results of both gravimetric and electrochemical experiments showed that the two
compounds inhibit mild steel corrosion in 1 M HCl solution and the inhibition efficiency
increases with increasing concentration of the inhibitors, reaching it maximum value at 5×10–3
M in the case of PANB ( ≈ 96.01%). The corrosion process is inhibited by the adsorption of
PANB and APAB on steel surface and the adsorption of these inhibitors fits a Langmuir
isotherm model. Based on the PDP results, PANB and APAB can be classified as mixed
inhibitors. The polarization resistance of these two inhibitors is highest with 5×10−3 M when
compared to the value obtained for the blank solution. The quantum chemical by DFT
calculations and molecular dynamics simulations give a better overview of the reactivity of
tested inhibitors towards mild steel. The results indicate that PANB had a stronger interaction
with the steel than APAB, which corroborate with experimental results. These outcomes are
important towards rational design of new corrosion inhibitors.
ACKNOWLEDGEMENTS
The authors extend their appreciation to the Deanship of Scientific Research at King
Khalid University for funding this work through research groups program under grant number
R.G.P-1.21-38.
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Figures captions:
Fig. 1: Schematic representation of the benzonitrile derivatives.
Fig. 2: Potentiodynamic polarization curves of mild steel in 1 M HCl at various concentrations
of PANB and APAB at 303K.
Fig. 3: Nyquist diagrams of mild steel in 1 M HCl in the absence and presence of different
concentrations of: (a) PANB and (b) APAB at 303K.
Fig. 4: Equivalent circuit employed to fitting and simulation of the impedance spectra.
Fig. 5: Arrhenius and transition state plots for corrosion inhibition of mild steel in absence and
presence of 5×10-3 M of inhibitor in 1.0 M HCl
Fig. 6: Plots of the Langmuir adsorption isotherm on mild steel in 1 M HCl containing various
concentrations of PANB and APAB at different temperatures.
Fig. 7. The variation of the standard Gibbs free energy as a function of the temperature.
Fig. 8: SEM images of MS: (a) exposed to 1 M HCl, (b) and (c) exposed to 1.0 M HCl + 5 ×
10−3 M of PANB and APAB respectively, after 6 h of immersion time at 303 K.
Fig. 9: AFM images of MS surface in absence and presence of tested inhibitors at 5 × 10−3 M
of inhibitors after 6 h of immersion time at 303 K.
Fig. 10: (A) The optimized geometry, (B) HOMO and (C) LUMO of the inhibitors molecules.
[atom legend: white=H; Gray=C; blue=N; red = O].
Fig. 11: Top and side views of the final adsorption of the benzonitrile derivatives on the Fe
(110) surface in solution.
Fig. 12: RDFs analysis of C, N, and O atoms from APAB and PANB adsorbed on the Fe (110)
surface.
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Fig. 1
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Fig. 2
PANB
100
10
1
0.1
Blank
0.01
0.001
1E-4
-3
5x10
1x10
5x10
1x10
M
M
M
M
-3
-4
-4
-800
-700
-600
-500
-400
-300
-200
E( mv/SCE)
APAB
100
10
1
0.1
Blank
-3
0.01
0.001
1E-4
5x10
1x10
5x10
1x10
M
-3
-4
-4
M
M
M
-800
-700
-600
-500
-400
-300
-200
E( mv/SCE)
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Fig. 3
400
35
Blank
(a)
-3
-3
-4
-4
30
25
20
15
10
5
5x10
1x10
5x10
1x10
M
M
M
M
Blank
300
200
100
0
0
0
5
10 15 20 25 30 35
2)
Z
(
ohm.cm
re
0
100
200
300
400
Zre (cm2)
400
Blank
(b)
-3
-3
-4
-4
5x10
1x10
5x10
1x10
M
M
M
M
300
200
100
0
0
100
200
300
400
Zre (cm2)
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ACCEPTED MANUSCRIPT
Fig. 4
26

ACCEPTED MANUSCRIPT
Fig. 5
27

ACCEPTED MANUSCRIPT
Fig. 6
0.006
0.005
0.004
0.003
0.002
0.001
0.000
303K_PANB
313K_PANB
323K_PANB
333K_PANB
303K_APAB
0.000 0.001 0.002 0.003 0.004 0.005
C ( M)
28

ACCEPTED MANUSCRIPT
Fig. 7
-32.0
-32.5
-33.0
-33.5
-34.0
-34.5
-35.0
300
305
310
315
320
325
330
335
T (K)
29