Experimental and theoretical studies on the corrosion inhibition performance of 4-amino-N,N-di-(2-pyridylmethyl)-aniline on mild steel in hydrochloric acid

Article abstract of DOI:10.1039/c5ra09173j

The corrosion inhibition properties of a pyridine derivative with more than one pyridine ring, 4-amino-N,N-di-(2-pyridylmethyl)-aniline, on mild steel in 1.0 M hydrochloric acid were estimated by potentiodynamic polarization, electrochemical impedance spectroscopy and surface analysis methods. The results showed that 4-amino-N,N-di-(2-pyridylmethyl)-aniline was a mixed type inhibitor and the inhibition efficiency decreased with the rise in temperature. The adsorption process followed the Langmuir adsorption isotherm. The thermodynamic parameters responsible for the adsorption process were also discussed. Quantum chemical calculations were performed to correlate electronic structure parameters of 4-amino-N,N-di-(2-pyridylmethyl)-aniline with its inhibition performance. Molecular dynamics simulations were also used to investigate the equilibrium configurations of 4-amino-N,N-di-(2-pyridylmethyl)-aniline on the iron surface.

Full text of DOI:10.1039/c5ra09173j

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PAPER
Experimental and theoretical studies on the
corrosion inhibition performance of 4-amino-N,N-
di-(2-pyridylmethyl)-aniline on mild steel in
hydrochloric acid†
Cite this: RSC Adv., 2015, 5, 56049
a
a
a
a
a
*
Bin Xu, Yan Ji, Xueqiong Zhang, Xiaodong Jin, Wenzhong Yang
and Yizhong Chenb
The corrosion inhibition properties of a pyridine derivative with more than one pyridine ring, 4-amino-N,N-
di-(2-pyridylmethyl)-aniline, on mild steel in 1.0 M hydrochloric acid were estimated by potentiodynamic
polarization, electrochemical impedance spectroscopy and surface analysis methods. The results
showed that 4-amino-N,N-di-(2-pyridylmethyl)-aniline was a mixed type inhibitor and the inhibition
efficiency decreased with the rise in temperature. The adsorption process followed the Langmuir
adsorption isotherm. The thermodynamic parameters responsible for the adsorption process were also
discussed. Quantum chemical calculations were performed to correlate electronic structure parameters
of 4-amino-N,N-di-(2-pyridylmethyl)-aniline with its inhibition performance. Molecular dynamics
simulations were also used to investigate the equilibrium configurations of 4-amino-N,N-di-(2-
pyridylmethyl)-aniline on the iron surface.
Received 16th May 2015
Accepted 18th June 2015
DOI: 10.1039/c5ra09173j
www.rsc.org/advances
(phenylthio) pyridine,10 2-amino-3,5-dicarbonitrile-4-phenyl-6-
1. Introduction
(phenylthio)
nitrophenyl)-6-(phenylthio) pyridine,10 2-amino-5-(3-pyridyl)-
1,3,4-thiadiazole,11 2-amino-5-(4-pyridyl)-1,3,4-thiadiazole,11
pyridine,10
2-amino-3,5-dicarbonitrile-4-(4-
Mild steel is considered as one of the most important
construction and engineering materials because its price is
relatively low while it provides material properties that are
acceptable for many applications. Generally, hydrochloric acid
is widely employed in industrial processes, such as acid clean-
ing, acid descaling and oil well acidication.1–3 Unfortunately,
the corrosion of mild steel in the aggressive acid solution leads
to huge economic losses. Among the corrosion prevention tech-
niques, the use of inhibitors is the most efficient and practical
method to overcome corrosion.4 It is well known that heterocyclic
organic compounds, containing heteroatoms, electronegative
functional groups and p electrons, are oen found to work as
efficient corrosion inhibitors.5–7 Meanwhile, the adsorption
characteristics which determine the inhibition performance of
compounds are oen affected by the molecular size, functional
groups, the electron density at the donor atoms, etc.8,9
pyridine-2-thiol,12 2-pyridyl disulde,12 2-pyridinecarbonitrile,13
2-amino-4-methylpyridine,14 and 2-(4-pyridyl)-benzimidazole.15
The presence of nitrogen atom and electron rich structure can
facilitate the adsorption of pyridine derivatives onto metal
surface. However, a perusal of literature10–15 reveals that the
most studied pyridine derivatives contain only one pyridine ring
and the studies on the pyridine derivatives with two or more
pyridine rings are still scarce. In the current study, the studied
inhibitor, 4-amino-N,N-di-(2-pyridylmethyl)-aniline (APMA),
contains two pyridine rings and an aniline ring. Recently,
theoretical calculation methods have been widely used to
investigate the mechanism of corrosion.16 Quantum chemical
calculations have been proved to be a useful tool for studying
the correlation of the electronic properties and the corrosion
inhibition efficiency.17,18 In addition, the molecular dynamics
simulations can provide insights into the conguration and
adsorption energy of inhibitor molecules adsorbed on the metal
surface at a molecular level.19,20
In recent years, many researchers have oriented their interest
to the corrosion inhibition behavior of pyridine derivatives,
including
2-amino-3,5-dicarbonitrile-4-(4-methoxyphenyl)-6-
Consequently, the aim of the present work is to evaluate the
corrosion inhibition effectiveness of APMA on mild steel in 1.0
M hydrochloric acid by utilizing electrochemical techniques,
surface analysis and theoretical calculation methods. Besides,
an attempt is made to elucidate the corrosion inhibition
mechanism.
aDepartment of Applied Chemistry, School of Science, Nanjing Tech University, Nanjing
210009, China. E-mail: yangwznjtech@163.com; Tel: +86 25 83172359
bSchool of Environmental and Safety Engineering, Jiangsu Polytechnic University,
Changzhou 213164, China
† Electronic supplementary information (ESI) available: 1H, 13C NMR, MS and
FTIR
spectra
of
4-amino-N,N-di-(2-pyridylmethyl)-aniline.
See
DOI:
10.1039/c5ra09173j
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(100) M+ + Na. IR n (KBr, cmꢁ1) 3401, 3303, 3204, 3012, 1621,
1589, 1517, 1470, 1433, 1388, 1353, 813, 762.
2. Experimental section
2.1. Materials
The mild steel performed in this study possesses the following
chemical composition (wt%): 0.37% Mn, 0.17% C, 0.20% Si,
0.03% S, 0.01% P and remainder Fe. The mild steel disk used for
electrochemical measurements was mounted in epoxy resin
with an area of 0.785 cm2 exposed to the electrolyte. Prior to the
experiments, the mild steel disk was abraded with different
emery papers (grades 500, 1000, 1500 and 2000), rinsed with
double distilled water, degreased in ethanol, and nally dried at
room temperature. The acid solution (1.0 M HCl) was prepared
by dilution of analytical grade 37% hydrochloric acid with
double distilled water. The concentration range of APMA
employed was varied from 0.5 mM to 2.0 mM.
2.2. Electrochemical measurements
All the electrochemical measurements were conducted using
ZAHNER IM6ex electrochemical workstation, controlled by
ZAHNER THALES soware. A traditional three-electrode cell
assembly was carried out in all electrochemical measurements.
The mild steel disk was used as the working electrode and a
platinum plate with a surface area of 1.0 cm2 was served as the
auxiliary electrode, respectively. A saturated calomel electrode
(SCE) coupled to a ne Luggin capillary was used as the refer-
ence electrode. The Luggin capillary was kept close to the
working electrode to decrease IR drop. All the electrochemical
measurements were carried out in non-deaerated solution
under unstirred condition at constant temperature controlled
by a thermostatic water bath. Before the electrochemical
measurements, the working electrode was immersed in the test
solution for 30 min to reach a steady-state potential. The
potentiodynamic polarization curves were recorded in the
potential range ꢁ700 to ꢁ300 mV (vs. SCE) with a scan rate of
0.001 V sꢁ1. Electrochemical impedance spectroscopy (EIS) were
carried out in a frequency range from 10ꢁ1 to 105 Hz with a 5 mV
peak-to-peak voltage excitation at open circuit potential.
The studied inhibitor, APMA, shown in Fig. 1 was synthe-
sized as reported earlier.21 A solution of di-tert-butyldicarbonate
(0.046 mol) in 1,4-dioxane was added dropwise to a solution of
p-phenylenediamine. Triethylamine (0.046 mol) was then added
to the reaction mixture, and the solution was heated at 60 ꢀC for
24 h. Solvent was removed in vacuo to yield a brown oil. Crude
product was puried via ash column chromatography (ethyl
acetate/petroleum ether ¼ 3 : 1) to give the desired product, (4-
aminophenyl) carbamic acid tert-butyl ester. To a solution of it
(0.019 mol) in anhydrous ethanol was added Na2CO3 (0.077
mol). 2-(Chloromethyl)-pyridine hydrochloride (0.038 mol) was
2.3. SEM-EDX studies
ꢀ
then added, and the mixture was heated to reux at 80 C for
The morphologies of specimens aer immersed in 1.0 M HCl
with and without 2.0 mM APMA for a duration of 12 h at 30 ꢀC
were analyzed by using scanning electron microscopy (JEOL
JSM-6510) at high vacuum. Chemical composition of the spec-
imens was recorded by an EDX detector (NORAN VANTAGE DSI)
coupled to the SEM.
24 h. The resulting brown suspension was ltered, and the
solvent was removed in vacuo to yield a brown oil. The brown oil
was diluted with CH2Cl2 and treated with 2 M NaOH. Product
was extracted with CH2Cl2, then the organic layer was washed
with brine and dried over Na2SO4, and the CH2Cl2 was removed
under vacuum. The crude product was run through a silica plug
to give the desired product as an orange solid. The orange solid
(2.56 mmol) was dissolved in ethyl acetate and then treated
dropwise with 12 M HCl. The reaction mixture was le to stir for
1 h. The resultant solution was combined with 5 M NaOH, and
the organic layer was extracted with CH2Cl2. Solvent was
removed in vacuo to yield the APMA. Identication of the
2.4. X-ray photoelectron spectroscopy (XPS)
XPS experiments were carried out by a PHI 5000 VersaProbe
spectrometer with the monochromatized Al Ka X-ray source
(hn ¼ 1486.4 eV) and an X-ray beam of around 100 mm. The
analyser was operated in constant pass energy of 58.7 eV using
an analysis area of 1 cm ꢂ 1 cm. The binding energy scale was
initially calibrated using Ag 3d5/2 (368.2 eV) level and pressure
was in the range of 6.7 ꢂ 10ꢁ10 mbar during the experiments.
Then XPS Peak soware was employed to t the experimental
curve. The analyses were practiced on the mild steel specimen
immersed in 1.0 M HCl with 2.0 mM APMA for 12 h at 30 ꢀC. The
specimen was pre-treated by the same procedure described in
Section 2.1.
1
structure of the synthesized APMA was performed by H NMR,
13C NMR, MS and FTIR (see ESI, Fig. S1–S4†). 1H NMR (500
MHz, DMSO-d6): d 4.32 (s, 2H), 4.62 (s, 4H), 6.40 (d, J ¼ 7.75 Hz,
2H), 6.47 (d, J ¼ 7.85 Hz, 2H), 7.19 (m, 2H), 7.27 (d, J ¼ 7.7 Hz,
2H), 7.67 (m, J ¼ 14.6 Hz, 2H), 8.49 (d, J ¼ 3.25 Hz, 2H). 13C NMR
(500 MHz, DMSO-d6): d 159.68, 148.95, 139.74, 139.60, 136.38,
121.78, 121.23, 115.22, 114.73, 57.89. MS (ESI) m/z (%) 313.1
2.5. Quantum chemical calculations
Quantum chemical calculations were conducted with DMol3
module in Materials Studio soware 7.0. All electron calcula-
tions were performed with the generalized gradient approxi-
mation (GGA) functional of BLYP in conjunction with double
zeta plus polarization (DNP) double-numeric basis set.22,23 BLYP
derived from the combination of Becke for the exchange part
and Lee, Yang and Parr for the correlation part is a popular
Fig. 1 Chemical structure of APMA.
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function.24 The DNP basis sets are of comparable quality to 6-
31G Gaussian basis sets.25 Fine convergence accuracy and global
orbital cutoffs were employed on basis set denitions.
Frequency analysis was performed to ensure that the calculated
structure is the minimum point on potential energy surface
(without imaginary frequency).
2.6. Molecular dynamics simulations
Molecular dynamics (MD) simulations were performed using
the Discover module in Material Studio 7.0 to model the
adsorption process of inhibitor molecules onto Fe (1 1 0)
surface. The MD simulations were conducted in a three-
Fig. 2
E
ocp–t curves for mild steel in 1.0 M HCl in the absence and
˚
˚
˚
dimensional simulation box (22.34 A ꢂ 22.34 A ꢂ 38.11 A)
with periodic boundary conditions to model a representative
part of the interface without any arbitrary boundary effects. The
Fe (1 1 0) surface was rst built and relaxed by minimizing its
energy according to molecular mechanics, then the surface area
was enlarged by constructing an appropriate super cell. Aer
presence of APMA at 30 ꢀC.
measurements.27,28 Fig. 3 represents the potentiodynamic
polarization curves for mild steel immersed in 1.0 M HCl at
ꢀ
30 C in the absence and presence of APMA.
˚
that, vacuum slab with 30 A thickness was built on the Fe (1 1 0)
From Fig. 3, it is clear that both anodic and cathodic Tafel
curves shi towards the lower current densities in the presence
of APMA. This phenomenon implies that both anodic metal
dissolution and cathodic hydrogen evolution reactions are
retarded. Meanwhile, this suppression effect becomes more
pronounced with the increasing APMA concentration. The
electrochemical parameters such as corrosion potential (Ecorr),
corrosion current density (icorr), anodic Tafel slope (ba) and
cathodic Tafel slope (bc), obtained by extrapolating the anodic
and cathodic Tafel curves to the point of intersection, are
summarized in Table 1. The corrosion inhibition efficiency (hp)
calculated by the following equation is also listed in Table 1.
surface and the system was optimized using COMPASS
(condensed-phase optimized molecular potentials for atomistic
simulation studies) force eld.11,15,18,22 During the simulations,
the Fe slab was kept “frozen”, and the inhibitor molecule was
allowed to reach the Fe surface freely. MD simulations were
performed at 303 K, NVT (dynamics at xed volume with a
thermostat to maintain a constant temperature) ensemble, with
a time step of 1.0 fs and simulation time of 100 ps. The trajec-
tory was recorded every 5000 fs. The interaction energy (Eint
)
between the inhibitor molecule and the Fe (1 1 0) surface was
calculated as follows:
Eint ¼ Etotal ꢁ (Esurface + Einh
)
(1)
hp ¼ (1 ꢁ icorr/icorr0) ꢂ 100%
(3)
where Etotal is the total energy of the simulation system, Esurface
and Einh are the energies of iron crystal and free inhibitor
molecule, respectively. The binding energy (Ebin) is the negative
value of the interaction energy.17,19,22
where icorr0 and icorr represent the corrosion current densities of
mild steel in the absence and presence of APMA, respectively.
According to the literature,29–31 it has been reported that if the
displacement in Ecorr in the presence of inhibitor is more than
85 mV with respect to Ecorr of the blank, the inhibitor can be
Ebin ¼ ꢁEint
(2)
3. Results and discussion
3.1. Open circuit potential measurements
The variation of open circuit potential (Eocp) of working elec-
trode with time in 1.0 M HCl in the absence and presence of
ꢀ
APMA at 30 C is graphically represented in Fig. 2. Apparently,
Eocp moved to more positive value with time and gradually
remained unchanged, which was similar to the previously
reported articles.23,26 It took about 1500 s to reach the steady-
state, so the working electrode was immersed into the test
solution for 30 min before each electrochemical measurement.
3.2. Potentiodynamic polarization measurements
Fig. 3 Potentiodynamic polarization curves for mild steel in 1.0 M HCl
in the absence and presence of different concentrations of APMA at 30
The information about the kinetics of corrosion reactions can
be
provided
by
the
potentiodynamic
polarization ꢀC.
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Table 1 Potentiodynamic polarization parameters for mild steel in 1.0 M HCl in the absence and presence of different concentrations of APMA at
30 ꢀC
C (mM)
Ecorr (mV vs. SCE)
Icorr (mA cmꢁ2
)
ꢁbc (mV decꢁ1
)
ba (mV decꢁ1
)
hp (%)
qp
Blank
0.5
1.0
1.5
2.0
ꢁ467.1 ꢃ 6.4
ꢁ475.8 ꢃ 0.6
ꢁ487.6 ꢃ 5.1
ꢁ485.4 ꢃ 4.2
ꢁ481.9 ꢃ 2.1
0.783 ꢃ 0.092
0.303 ꢃ 0.027
0.225 ꢃ 0.011
0.187 ꢃ 0.020
0.149 ꢃ 0.026
113 ꢃ 8
117 ꢃ 5
115 ꢃ 3
115 ꢃ 3
114 ꢃ 7
87.4 ꢃ 0.8
88.8 ꢃ 2.9
90.8 ꢃ 3.1
93.4 ꢃ 2.8
80.1 ꢃ 6.1
—
—
61.3
71.3
76.1
81.0
0.613
0.713
0.761
0.810
recognized as cathodic or anodic type. On the contrary, if the the capacitive loops increase sharply with the increasing
displacement in Ecorr is less than 85 mV, the inhibitor can be concentration of APMA and the shape of the loops doesn't
classied as a mixed type. In the present study, the maximum modify. The Bode plots recorded for mild steel immersed in
shi is 20.5 mV and the Ecorr shis towards negative side in the 1.0 M HCl with and without APMA are shown in Fig. 5. It is clear
presence of APMA, which indicates that APMA acts as a mixed that only one phase peak at the middle frequency range can be
type inhibitor and has more inuence on the cathodic polari- observed, conrming that there is only one time constant.
zation plots. Inspection of Table 1 reveals that the corrosion Meanwhile, the increasing concentration of APMA conduces to
current density decreases signicantly in the presence of APMA, the increase in the impedance modulus at low frequency and
and hp increases sharply with the increase in the concentration the more negative values of phase angle at intermediate
of APMA. Furthermore, the values of bc and ba exhibit no frequency, demonstrating the adsorption of APMA onto mild
obvious changes, which suggests that the inhibition effect steel surface.17
comes from the reduction of the reaction area on the corroding
According to the above results, The EIS data are analyzed via
metal surface.23 Namely, the corrosion inhibition is caused by the Randle's equivalent circuit (Fig. 6) that includes the solution
geometric blocking of the active reaction sites on the metal resistance (Rs), polarization resistance (Rp) and constant phase
surface without modifying the corrosion mechanism of mild element (CPE), which can be represented as follows:22,37
steel in hydrochloric acid.32,33
ZCPE ¼ Y0ꢁ1(ju)ꢁn
(4)
3.3. Electrochemical impedance spectroscopy
where Y0 is the CPE constant, n is the phase shi, j is the
imaginary unit and u is the angular frequency. Thus the values
Electrochemical impedance spectroscopy (EIS) is a valuable tool
to evaluate the protective properties of inhibitors on metal
surface.17 Fig. 4 presents the Nyquist plots for mild steel in
1.0 M HCl at 30 ꢀC in the absence and presence of APMA. All the
impedance spectra displayed in Fig. 4 reveal a single depressed
capacitive semicircle across the studied frequency range, which
denotes that the dissolution process is related to the charge
transfer process.10,34 Noticeably, these impedance loops are not
perfect semicircles, which can be termed as frequency disper-
sion effect as a result of the roughness and inhomogeneity of
the metal electrode surface.35,36 Furthermore, the diameters of
Fig. 5 Bode plots for mild steel in 1.0 M HCl solution in the absence
and presence of different concentrations of APMA at 30 ꢀC.
Fig. 4 Nyquist plots for mild steel in 1.0 M HCl in the absence and
presence of different concentrations of APMA at 30 ꢀC.
Fig. 6 Randle's equivalent circuit model used to fit the EIS data.
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of the double layer capacitance (Cdl) and inhibition efficiency interface by replacing water molecules. It is worth noting that
(he) are calculated according to the following equations:
the inhibition efficiencies calculated from the EIS measure-
ments show the same trend as that obtained from potentiody-
namic polarization measurements.
Cdl ¼ Y0(um)nꢁ1
(5)
(6)
he ¼ (1 ꢁ Rp0/Rp) ꢂ 100%
3.4. Adsorption isotherm
It is crucial to know the mode of adsorption as well as the
adsorption isotherm which can provide basic information
about the corrosion inhibition action between APMA molecules
and the mild steel surface. The adsorption process of organic
inhibitor molecules can be considered as a result of replace-
ment of water molecules adsorbed on the metal surface
according to the following process:
where um represents the angular frequency at the maximum
value of the imaginary part. Rp0 and Rp stand for the polariza-
tion resistance in the absence and presence of APMA, respec-
tively. The representative example of using the equivalent
circuit to t the experimental data for mild steel in 1.0 M HCl
with 2.0 mM APMA is shown in Fig. 7 and the electrochemical
parameters are listed in Table 2.
It can be seen that Rs is small, which supports that the IR
drop can be negligible.23 It is apparent that the value of Rp
increases signicantly with the increasing APMA concentration
while the value of Cdl decreases. The larger Rp and the smaller
Cdl are related to an increase in the surface coverage by APMA
molecules, resulting in an increase in inhibition efficiency. The
maximum he reaches up to 85.7% at 2.0 mM. According to
Helmholtz model:
APMA(sol) + nH2O(ads) / APMA(ads) + nH2O(sol)
(8)
where APMA(sol) and APMA(ads) represent the APMA molecules
in the solution and adsorbed on the metal surface, respectively.
n is the number of water molecules replaced by one organic
molecule. In order to characterize the adsorption of APMA,
various isotherms were employed to t the experimental data,
such as Langmuir, Temkin, Frumkin, Flory–Huggins and
Freundlich adsorption isotherms. Thus the surface coverage (q)
is dened as:
Cdl ¼ 303A/d
(7)
where 3 is the local dielectric constant, 30 is the free space
permittivity, d is the thickness of the interfacial layer and A is
the electrode area. The decrease in Cdl is due to a decrease in the
local dielectric constant or an increase in the thickness of the
electrical double layer, which in turn justify that APMA reduces
the metal dissolution by effective adsorption. Therefore, it is
suggested that APMA molecules adsorb at metal/solution
q ¼ h/100
(9)
In order to yield the best t isotherm model, the values of h
are obtained from potentiodynamic polarization and EIS
measurements, respectively. According to the data, it can be
concluded that the best description of the adsorption behavior
of APMA can be explained by Langmuir adsorption isotherm:
Fig. 7 The representative figure of using the equivalent circuit to fit the experimental data of EIS for mild steel in 1.0 M HCl with 2.0 mM APMA (a)
Nyquist plot (b) Bode plot.
Table 2 Impedance data for mild steel in 1.0 M HCl in absence and presence of different concentrations of APMA at 30 ꢀC
C (mM)
Rs (U cm2)
Rp (U cm2)
n
Cdl (mF cmꢁ2
)
he (%)
qe
Blank
0.5
1.0
1.5
2.0
0.36 ꢃ 0.03
0.32 ꢃ 0.08
0.18 ꢃ 0.04
0.22 ꢃ 0.06
0.21 ꢃ 0.05
13.6 ꢃ 2.0
55.9 ꢃ 8.8
76.7 ꢃ 6.7
90.7 ꢃ 10.2
95.0 ꢃ 11.4
0.88 ꢃ 0.02
0.81 ꢃ 0.03
0.80 ꢃ 0.04
0.80 ꢃ 0.05
0.80 ꢃ 0.02
269.4 ꢃ 14.5
250.3 ꢃ 26.4
240.0 ꢃ 10.5
226.1 ꢃ 13.5
199.6 ꢃ 18.1
—
—
75.7
82.3
85.0
85.7
0.757
0.823
0.850
0.857
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ꢀ
ꢁ
ꢀ
ꢁ
C/q ¼ 1/Kads + C
(10)
RT
Nh
DH*
DS*
icorr
¼
exp ꢁ
exp
(13)
RT
R
where C is the concentration of APMA, Kads is the equilibrium
constant for the adsorption–desorption process.
where h is the Planck's constant, N is Avogadro's number. The
transition state plots of ln(icorr/T) versus 1/T are depicted in
Fig. 11.
The plots in Fig. 8 were found to be linear, with slopes of
1.106 (line a) and 1.114 (line b), respectively. The interactions
between the APMA species on the metal surface and the changes
in the adsorption heat conduce to the divergence of the slopes
from unity.37 According to Langmuir adsorption isotherm, the
value of Kads can be obtained from the intercept of the straight
line in Fig. 8 and Kads is also related to the standard free energy
of adsorption (DG0) by the following equation:
Corresponding electrochemical results and thermodynamic
parameters are listed in Table 3. As seen from Fig. 10 and Table
3, the icorr increases with the rise in temperature while the
inhibition efficiency decreases, which may be due to the fact
that higher temperature accelerates the desorption capacity of
APMA on the mild steel surface.29,38 It is apparent that the value
of Ea for the inhibited solution (82.2 kJ molꢁ1) is higher than
that for uninhibited solution (71.9 kJ molꢁ1), indicating that the
dissolution of metal is decreased by an energy barrier made by
the adsorption of inhibitors.4 The positive sign of DH* reects
the endothermic nature of mild steel dissolution process and
more energy barrier is required for the dissolution of metal in
the presence of APMA.29 The increase in the value of DS* implies
an increase in disordering takes place during the adsorption
process on going from reactants to the activated complex.39
DG0 ¼ ꢁRT ln(55.5Kads
)
(11)
where T is the absolute temperature, R is the universal gas
constant. The calculated values of Kads and DG0 are 3540 Mꢁ1
,
ꢁ30.7 kJ molꢁ1 (line a) and 9884 Mꢁ1, ꢁ33.3 kJ molꢁ1 (line b),
respectively. The high negative DG0 values ensure strong and
spontaneous adsorption of APMA lm on the mild steel
surface.11,37
3.6. Surface analysis
3.5. Effect of temperature
SEM micrographs and corresponding EDX spectra of mild steel
specimens before and aer immersed in 1.0 M HCl with and
without APMA are shown in Fig. 12. Fig. 12a shows an SEM
micrograph of the mild steel specimen before immersed in
1.0 M HCl and the abrading scratches can be clearly observed.
The corresponding EDX analysis (Fig. 12b) shows that the Fe is
the main element in addition to C. It can be clearly seen from
Fig. 12c that the mild steel surface was seriously damaged in
hydrochloric acid solution. However, in the presence of APMA,
the damage of the mild steel was signicantly reduced
(Fig. 12e). A cleaner and smoother mild steel surface can be
observed in comparison to that in the absence of APMA. The
percentages of the elements (weight%) found on the mild steel
surface in Fig. 12d and f are 11.26% O, 7.86% C, 80.87% Fe and
3.62% O, 9.96% C, 82.95% Fe, 3.47% N, respectively. Close
examination of the corresponding EDX analysis reveals that
the decrease in O and the presence of N on the inhibited mild
steel surface, conrming the adsorption of APMA on the metal
surface.
To evaluate the effect of temperature on the inhibitive perfor-
mance of APMA on mild steel surface in 1.0 M HCl, potentio-
dynamic polarization measurements were carried out in the
ꢀ
temperature range of 30–50 C in the absence and presence of
2.0 mM APMA, as shown in Fig. 9. The activation energy (Ea) of
the corrosion process could be determined by the Arrhenius
equation:18,19
ln icorr ¼ ꢁEa/RT + ln A
(12)
where Ea is the apparent activation energy, A is the Arrhenius
pre-exponential factor and T is the absolute temperature.
Fig. 10 represents the Arrhenius plots of ln icorr versus 1/T for
mild steel in 1.0 M HCl with and without APMA. The change in
the enthalpy of activation (DH*) and the entropy of activation
(DS*) can be calculated by using the transition state equation:
It is well known that XPS is a powerful tool for probing the
adsorption of inhibitors on metal surface.40,41 Thus we per-
formed XPS experiments on the mild steel immersed in 1.0 M
HCl with 2.0 mM APMA. High resolution XPS spectra of the
protected mild steel are illustrated in Fig. 13. The C 1s spectrum
is shown in Fig. 13a and is tted to three peaks. The peaks of
binding energy at 284.2 and 284.9 eV can be attributed to the C–
C, C]C and C–H aromatic bonds in benzene ring and pyridine
ring. The peak at 287.9 eV maybe assigned to the carbon atom
bonded to nitrogen in CH2–N. The deconvolution of the O 1s
spectrum is tted into two main peaks (Fig. 13b). The rst peak
(529.6 eV) is assigned to O2ꢁ, and in principle could be related
to oxygen atoms bonded to ferric oxides.42 The second peak
(531.1 eV) can be ascribed to OHꢁ of hydrous iron oxides, such
as FeOOH and/or Fe(OH)3.43 The Fe 2p spectrum (Fig. 13c)
Fig. 8 Langmuir adsorption plots for mild steel in 1.0 M HCl with APMA
at 30 ꢀC. (a) Potentiodynamic polarization data, (b) EIS data.
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Fig. 9 Potentiodynamic polarization curves for mild steel in 1.0 M HCl in the absence and presence of 2.0 mM APMA at different temperatures:
(a) blank, (b) APMA.
to the presence of small concentration of FeCl3 on the metal
surface.44 The last peak at 718.3 eV can be ascribed to the
satellites of the ferric compounds. From these previous obser-
vations, the XPS results support the adsorption of APMA on the
mild steel surface.
3.7. Quantum chemical calculations
The effectiveness of an inhibitor is inuenced by its spatial
molecular structure as well as its molecular electronic structure.
In this regard, quantum chemical calculations were performed
to investigate the structural and electronic properties of APMA.
The optimized structure and the frontier molecular orbital
Fig. 10 Arrhenius plots for mild steel in 1.0 M HCl and 1.0 M HCl + 2.0
mM APMA.
density distributions of APMA are given in Fig. 14 and 15,
respectively. All the quantum chemical parameters are listed in
Table 4.
According to the frontier molecular orbital theory, the
density distributions of HOMO and LUMO are very critical for
the prediction of the reactivity of a chemical species. The EHOMO
(the highest occupied molecular orbital energy) is associated
with the electron donating ability of a molecule, whereas the
ELUMO (the lowest unoccupied molecular orbital energy) char-
acterizes the electron receiving tendency of a molecule. It can be
seen from Fig. 15 that HOMO is spread principally on the
aniline and N8 atom, while the population of LUMO is mainly
distributed on the one pyridine ring and N8 atom. Thus, N8
atom is the main adsorption center and one pyridine ring acts
as the main sites to accept electrons from the iron atoms with
its anti-bonding orbital to form a feedback bond. Also the
aniline part behaves as the dominant site to donate electrons to
Fig. 11 Transition state plots for mild steel in 1.0 M HCl and 1.0 M
HCl + 2.0 mM APMA.
the unoccupied d-orbital of the iron atom to form coordinate
bonds.4,18 It is evident from Table 4 that APMA has a high
EHOMO, indicating that APMA has a tendency to adsorb on the
mild steel surface. A molecule with small value of DE is easily
polarized, resulting in easily adsorbing of APMA onto the metal
surface.45 The increase of the dipole moment (m) probably
depicts a double peak prole located at a binding energy around
711 eV (Fe 2p3/2) and 725 eV (Fe 2p1/2) together with an associ-
ated ghost structure.43 Deconvolution of the high resolution Fe
increases the inhibition efficiency, which could be related to the
2p3/2 XPS spectrum consists three peaks. The rst peak located
dipole–dipole interaction of the molecules and the metal
at 711.1 eV can be assigned to Fe3+ due to the presence of the
Fe2O3 and/or FeOOH. The second peak at 714.0 eV is attributed
surface.46 The values of m follow the order: APMA > H2O (1.85 D),
indicating that APMA can adsorb on the mild steel by displacing
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Table 3 Effect of temperature on electrochemical parameters of the corrosion in 1.0 M HCl in the absence and presence of 2.0 mM APMA
DEa
DSa
DHa
Inhibitor
Blank
Temp. (ꢀC)
Ecorr (mV vs. SCE)
Icorr (mA cmꢁ2
)
hp (%)
(kJ molꢁ1
)
(J molꢁ1 Kꢁ1
)
(kJ molꢁ1
)
30
35
40
50
30
35
40
50
ꢁ467.1 ꢃ 6.4
ꢁ458.1 ꢃ 5.8
ꢁ452.5 ꢃ 7.7
ꢁ426.4 ꢃ 9.7
ꢁ481.9 ꢃ 2.1
ꢁ471.8 ꢃ 2.7
ꢁ466.1 ꢃ 4.4
ꢁ475.0 ꢃ 2.2
0.783 ꢃ 0.092
1.452 ꢃ 0.191
1.923 ꢃ 0.127
4.828 ꢃ 0.102
0.149 ꢃ 0.026
0.299 ꢃ 0.039
0.442 ꢃ 0.034
1.180 ꢃ 0.032
—
—
—
—
81.0
79.4
77.0
75.6
71.9
82.2
39.4
59.8
69.3
79.6
APMA
Fig. 12 SEM micrograph (a) and EDX analysis (b) of mild steel before immersed in HCl; SEM micrograph (c) and EDX analysis (d) of mild steel after
immersed in 1.0 M HCl without APMA; SEM micrograph (e) and EDX analysis (f) of mild steel after immersed in 1.0 M HCl with APMA.
H2O molecules, thereby inhibiting the mild steel against
corrosion.
A perusal of literature4,48,49 reveals that the work function (F)
of the metal surface is an appropriate measure of its electro-
The quantities of ionization potential (I) and electron affinity negativity and the fraction of electron (DN) transferred from the
(A) are dened as I ¼ ꢁEHOMO and A ¼ ꢁELUMO, respectively.22 inhibitor to iron can be calculated as follows:
Then the absolute electronegativity (c) and global hardness (g)
are approximated as follows:47
DN ¼ (F ꢁ cinh)/[2(gFe + ginh)]
(16)
where gFe z 0 eV, F values obtained from DFT calculation are
c ¼ (I + A)/2
(14)
3.88 eV, 4.82 eV, and 3.91 eV for Fe (1 1 1), (1 1 0), (1 0 0) planes,
g ¼ (I ꢁ A)/2
(15) respectively.22,48 Accordingly, if DN < 0, the electrons transfer
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Fig. 13 High resolution XPS of C 1s (a), O 1s (b) and Fe 2p (c) for mild steel after immersed in 1.0 M HCl with 2.0 mM APMA.
The system reaches equilibrium only if both of the energy and
temperature reach balance.17,18,22 As can be seen from Fig. 16,
one pyridine ring adsorbs nearly parallel to the Fe (1 1 0)
surface, while the rest two aromatic rings adsorb onto the Fe (1
1 0) surface in apparent tilted orientation. Therefore, the non-
planar adsorption of APMA indicates that the steric effect
plays an important role in the adsorption mode.
The values of both interaction energy and binding energy of
APMA are given in Table 5. The negative value of Eint indicates
attractive interactions.
Fig. 14 The optimized structures of APMA.
3.9. Mechanism of corrosion inhibition
Generally, the adsorption process depends on many factors,
including the nature of the metal, electronic structure, molec-
ular size and chemical properties of the inhibitor.29 Based on
the results obtained from the experimental and theoretical
analysis, a credible inference of the corrosion inhibition
mechanism can be proposed. It is well known that the metal has
an affinity towards heterocycles and abundance p electrons.
Hence, APMA may adsorb on the mild steel surface via the
chemisorption on the basis of donor–acceptor interactions
between the vacant d-orbital of the iron atoms and the lone pair
Fig. 15 Frontier molecular orbital density distributions of APMA.
from the metal to the inhibitor and vice versa if DN > 0. In this
study, all the values of DN are positive, indicating that APMA is
an electron donator, favoring the formation of adsorption lm
on metal surface.
3.8. Molecular dynamics simulations
In order to better understand the adsorption behavior of APMA
on the mild steel surface, the MD simulations were carried out.
Fig. 16 Equilibrium adsorption configurations performed for APMA on
Fe (1 1 0) surface.
Table 4 Quantum chemical parameters of APMA
Inhibitor
APMA
EHOMO (eV)
ꢁ4.096
ELUMO (eV)
ꢁ1.382
DE (eV)
m (D)
DN100
DN110
DN111
2.714
2.94
0.43
0.77
0.42
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Table
5 Interaction and binding energies between the inhibitor
¨
¨
2 E. Bayol, T. Gurten, A. A. Gurten and M. Erbil, Mater. Chem.
molecules and Fe (1 1 0) surface
Phys., 2008, 112, 62.
˙
˘
3 H. Kele¸s, M. Kele¸s, I. Dehri and O. Serindag, Mater. Chem.
Systems
Eint (kJ molꢁ1
ꢁ603.0
)
Ebin (kJ molꢁ1
)
Phys., 2008, 112, 173.
4 S. Kumar, D. Sharma, P. Yadav and M. Yadav, Ind. Eng. Chem.
Res., 2013, 52, 14019.
Fe + APMA
603.0
5 J. J. Fu, H. S. Zang, Y. Wang, S. N. Li, T. Chen and X. D. Liu,
Ind. Eng. Chem. Res., 2012, 51, 6377.
electrons on the N atoms as well as p electrons in the aromatic
rings. N8 atom is the main adsorption center and one pyridine
ring acts as the main sites to accept electrons from the iron
atoms with its anti-bonding orbital to form a feedback bond.
Meanwhile, the aniline part behaves as the dominant site to
donate electrons to the unoccupied d-orbital of the iron atom to
form coordinate bonds. Some researchers have reported that
6 J. J. Fu, S. N. Li, Y. Wang, X. D. Liu and L. D. Lu, J. Mater. Sci.,
2011, 46, 3550.
7 N. A. Negm, F. M. Ghuiba and S. M. Tawk, Corros. Sci., 2011,
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8 A. K. Singh, Ind. Eng. Chem. Res., 2012, 51, 3215.
9 H. Gerengi and H. I. Sahin, Ind. Eng. Chem. Res., 2012, 51,
780.
mild steel surface carries positive excess charge in the acidic 10 Sudheer and M. A. Quraishi, Ind. Eng. Chem. Res., 2014, 53,
solution,17–19,50 and APMA molecules should be protonated in
2851.
hydrochloric acid solution. The protonated APMA molecules 11 Y. M. Tang, X. Y. Yang, W. Z. Yang, R. Wan, Y. Z. Chen and
can hardly approach the metal surface because of the electro-
X. S. Yin, Corros. Sci., 2010, 52, 1801.
static repulsion. Thus the hydrated chloride ions are specically 12 A. Kosari, M. H. Moayed, A. Davoodi, R. Parvizi, M. Momeni,
adsorbed onto the mild steel surface to create an excess negative
charge at rst, favoring the adsorption of protonated APMA. 13 R. Yıldız, A. Doner, T. Dogan and I. Dehri, Corros. Sci., 2014,
H. Eshghi and H. Moradi, Corros. Sci., 2014, 78, 138.
˙
¨
˘
Above all, the adsorption of APMA onto the mild steel surface
includes both physisorption and chemisorption.
82, 125.
¨
14 B. D. Mert, A. O. Yuce, G. Kardas and B. Yazıcı, Corros. Sci.,
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15 F. Zhang, Y. M. Tang, Z. Y. Cao, W. H. Jing, Z. L. Wu and
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16 I. B. Obot and Z. M. Gasem, Corros. Sci., 2014, 83, 359.
17 B. Xu, W. Z. Yang, Y. Liu, X. S. Yin, W. N. Gong and
Y. Z. Chen, Corros. Sci., 2014, 78, 260.
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193.
In summary, 4-amino-N,N-di-(2-pyridylmethyl)-aniline (APMA)
was synthesized and evaluated as a corrosion inhibitor for mild
steel in 1.0 M HCl. The experimental results showed that APMA
was a mixed type inhibitor, retarding both anodic metal disso-
lution and cathodic hydrogen evolution reactions. The EIS
results specied that dissolution of mild steel was prevented by
the adsorption of APMA on the metal surface. The adsorption
behavior obeyed Langmuir adsorption isotherm. Quantum
chemical calculations showed that N8 atom is the main
adsorption center and one pyridine ring acts as the main sites to
accept electrons from the iron atoms with its anti-bonding
orbital to form a feedback bond. Meanwhile, the aniline part
behaves as the dominant site to donate electrons to the unoc-
cupied d-orbital of the iron atom to form coordinate bonds. The
molecular dynamics simulations results revealed that the non-
planar adsorption of APMA molecules on the Fe (110) surface.
The surface analysis experiments (SEM-EDX, XPS) conrmed
the adsorption of APMA on the mild steel surface.
19 B. Xu, Y. Liu, X. S. Yin, W. Z. Yang and Y. Z. Chen, Corros.
Sci., 2013, 74, 206.
20 K. F. Khaled and M. A. Amin, Corros. Sci., 2009, 51, 2098.
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23 S. D. Deng, X. H. Li and X. G. Xie, Corros. Sci., 2014, 80, 276.
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Financial supports from the National Major Science and Tech-
nology Program for Water Pollution Control and Treatment
(2013ZX07210-001) are highly appreciated.
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