Gravimetric, electrochemical and surface studies on the anticorrosive properties of 1-(2-pyridyl)-2-thiourea and 2-(imidazol-2-yl)-pyridine for mild steel in hydrochloric acid

Article abstract of DOI:10.1039/c8nj01762j

1-(2-Pyridyl)-2-thiourea (TP) and 2-(imidazol-2-yl)-pyridine (IP) are described here for the first time as inhibitors of mild steel corrosion in acidic medium based on investigations such as weight loss tests, potentiodynamic polarization measurements and

Full text of DOI:10.1039/c8nj01762j

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Gravimetric, electrochemical and surface studies on the
anticorrosive properties of 1-(2-pyridyl)-2-thiourea and 2-
(imidazol-2-yl)-pyridine for mild steel in hydrochloric acid
Received 00th January 20xx,
Accepted 00th January 20xx
Weiwei Zhang, Hui-Jing Li*, Yiwei Wang, Ying Liu, Qing-Ze Gu, and Yan-Chao Wu*
DOI: 10.1039/x0xx00000x
Abstract: The 1-(2-pyridyl)-2-thiourea (TP) and 2-(imidazol-2-yl)-pyridine (IP) are described here for the first time as
inhibitors for mild steel in acid medium based on investigations with weight loss, potentiodynamic polarization and
electrochemical impedance spectroscopy (EIS). The experimental results revealed that 1-(2-pyridyl)-2-thiourea and 2-
(imidazol-2-yl)-pyridine are effective corrosion inhibitors for mild steel in acid medium, and their maximum corrosion
inhibition efficiency at 4 × 10-4 M are 93.57% and 96.66%, respectively. TP and IP are determined as mixed-type inhibitors
based on polarization studies, and their adsorption on mild steel surface follows Langmuir adsorption isotherm and
physical adsorption is dominant. The formation and characteristic of protective layer on the steel surface were verified
from their spectra of scanning electrochemical microscope (SECM), UV–visible, FT-IR and X-ray photoelectron
spectroscopy (XPS) methods. Besides, the correlation between inhibition efficiency and molecular structure of inhibitor
was theoretically studied via quantum chemical calculations.
www.rsc.org/
behaviors of three chromenopyridines for mild steel in 1 M HCl
solution and found that their inhibition ability was obviously
enhanced with polar groups on the chromenopyridine ring, in
Introduction
Mild steel is widely used in multitudinous industrial fields due
to its excellent mechanical properties, easy availability and low
price. As acidic solutions are widely used in various industrial
processes, such as acid pickling, acid cleaning, acid descaling
and oil well acidification, anticorrosion of mild steel in acid
solutions is required.1-3 Otherwise, the mild steel corrosion
caused by acid solutions will induce huge economic losses and
serious environmental pollution. Corrosion inhibitors are
widely appreciated in the field of mild steel protection because
of their excellent inhibition efficiency and considerable
economic benefit. Accordingly, various corrosion inhibitors
have been developed for the protection of mild steel in acid
medium, which are usually organic heterocyclic compounds
containing high electronegativity atoms, such as nitrogen,
oxygen, sulfur and phosphorus, unsaturated bonds or a large
conjugation system.4-7 For example, azoles,8 Schiff bases,9
quinolines,10 thiourea derivatives11 and pyrimidines12 have
been reported as effective corrosion inhibitors for mild steel in
hydrochloric acid solutions. Nevertheless, corrosion scientists
are not very comfortable with corrosion inhibitors as they are
usually not readily available, expensive, or water-insoluble.
Recently, pyridine-type mild steel corrosion inhibitors have
attracted increasing attention due to their properties of facile
synthesis, water-soluble, low cost and high inhibition
efficiency. Verma investigated the corrosion inhibition
which the inhibition efficiency of the functional groups in the
preferential order of -OH > -NO2.5 The result is in good
agreement with reports in the literature that organic inhibitors
with polar functional groups can be easily adsorbed on the
metal surface and cause stronger corrosion inhibition
capability.4,13 Elmsellem studied the corrosion inhibition
performance of 2,6-bis-(hydroxy)-pyridine and diethyl 1,1’-
(pyridine-2,6-diyl)bis(5-methyl-1H-pyrazol2-3-carboxylate for
mild steel in 1 M HCl solution and found that corrosion
inhibition occurs due to the adsorption of inhibitors on steel
surface, and the differences in the inhibitive properties were
attributed to the different molecular structures of inhibitors.7
Various researches on the use of corrosion inhibitors help to
narrow the gap between anticorrosion experiments and
practical mild steel protection. However, pursuing practical
corrosion inhibitors is still at its early stages and further
research is needed to fully understand the role that corrosion
inhibitors play. Various different corrosion inhibitors are
required to search for more practical corrosion inhibitors,
which make the development of new mild steel corrosion
inhibitors a high priority.
In this paper, 1-(2-pyridyl)-2-thiourea (TP) and 2-
(imidazol-2-yl)-pyridine (IP) were smoothly synthesized from
readily available starting materials, and their inhibition effect
on the corrosion behavior of mild steel in 1 M HCl solution was
investigated by means of weight loss test, potentiodynamic
polarization measurement and electrochemical impedance
spectroscopy
(EIS).
Surface
analysis
of
scanning
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electrochemical microscope (SECM), UV–visible, FT-IR preparation of
Journal Name
1
M
HCl corrosion test solutions. The
spectroscopy and X-ray photoelectron spectroscopy (XPS) concentration of 1-(2-pyridyl)-2-thiourea and 2-(imidazol-2-yl)-
were also used to confirm the formation and characteristic of pyridine ranged from 1 × 10-4 M to 4 × 10-4 M in 1 M HCl
protective film on steel surface in presence of inhibitor. solution. The solubility of TP and IP in 1 M HCl solution is 104
Several quantum chemical calculations have been conducted mg/ml and 241mg/ml, respectively.
to correlate the inhibition efficiency and mechanisms with the
Weight loss experiments
molecular properties of TP and IP compounds. Moreover, the
The steel specimens with a size of 1.0 × 1.0 × 0.1 cm were
mechanism of corrosion inhibition on mild steel surface in
immersed in glass beakers containing 100 ml 1 M HCl solution
hydrochloric acid solution by TP and IP was proposed.
without and with different concentrations of TP and IP
inhibitors at ambient temperature [(25±0.1) °C] for 24 h. After
soaking, the specimens were removed from the test solutions,
Experimental
carefully rinsed the surface of the test piece with deionized
Inhibitor synthesis and characterization
water, dried under nitrogen and then re-weighed. Each group
As shown in Fig. 1, 1-(2-pyridyl)-2-thiourea (TP) inhibitor was
synthesized in our laboratory and 2-(imidazol-2-yl)-pyridine
(IP) was synthesized following the reported procedure in the
literature (Fig. 1).14 The molecular structures of the inhibitors
were confirmed by 1H NMR and FTIR spectroscopic, and the
characterization data were shown as below:
of samples was repeated three times under the same
conditions, and the standard deviation values of weight loss
results were less than 5% (Fig. 2), indicating that the
reproducibility of the experiment is acceptable. The corrosion
rate (v) and inhibition efficiency (ηw) were calculated by the
average weight loss of three parallel specimens.13,15 In
addition, weight loss tests were also carried out for mild steel
in 1 M HCl in absence and presence of 4 × 10-4 M TP and IP at
different temperatures from 25 °C to 55 °C.
1
TP, yield: 64%; mp: 146-148°C; H NMR (400 MHz, d6-DMSO):
δ/ppm = 10.56 (d, J=22.5Hz, 2H), 8.88 (s, 1H), 8.23 (d, J=4.0Hz,
1H), 7.76 (t, J=7.4Hz, 1H), 7.16 (d, J=8.3Hz, 1H), 7.03 (m, 1H);
13C NMR (100 MHz, d6-DMSO): δ/ppm =174.8, 158.6, 148.2,
138.3, 113.3, 109.9; IR (film) νmax: 3225, 3079, 1594, 1538,
1435, 1331, 1233, 1151, 1076, 1042, 995, 821, 767, 679 cm-1.
Electrochemical measurement
Electrochemical tests were performed by CHI 760E
electrochemical analyzer with a conventional three-electrode
system including working electrode (0.25 cm2 working area
was exposed), auxiliary electrode (platinum plate with 1 cm2
surface area) and reference electrode (saturated calomel
electrode). All the potentials in this paper were referred to
saturated calomel electrode (SCE). To obtain a stable working
surface, the working electrode was immersed in test solution
at open-circuit potential (OCP) for 0.5 h before measurement.
The potentiodynamic polarization curves were measured in
potential range from −550 to −150 mV (vs. SCE) with a scan
rate of 1 mV s-1. EIS measurement was performed at EOCP in a
frequency range from 100 kHz to 100 mHz and the amplitude
was 5 mV. The corresponding EIS parameters and the
appropriate equivalent circuit can be obtained by fitting the
experimental results with ZSimpWin software. All the
electrochemical tests were carried out under air atmosphere
without stirring and the test temperature was controlled by
thermostatic water bath.
1
IP, yield: 92%; mp: 135-136°C; H NMR (400 MHz, d6-DMSO):
δ/ppm = 12.79 (s, 1H), 8.59 (d, J=4.8Hz, 1H), 8.04 (d, J=7.9Hz,
1H), 7.87 (t, J=7.7Hz, 1H), 7.35 (m, 1H), 7.16 (s, 2H); 13C NMR
(100 MHz, d6-DMSO): δ/ppm =166.2, 155.1, 149.3, 137.2,
127.8, 124.2, 120.8; IR (film) νmax: 3314, 3048, 1598, 1483,
1462, 1418, 1386, 1142, 1113, 996, 957, 786, 736, 706 cm-1.
Electrode and electrolytes
The experimental material for mild steel with chemical
composition (wt.%): 0.047 C, 0.020 Si, 0.14 Mn, 0.025 P, 0.005
S and balance Fe. The dimensions of mild steel strips for
electrochemical and surface analysis experiments were
mechanically cut into 2.5 × 0.5 × 0.5 cm and 0.5 × 0.5 × 0.05 cm,
respectively. For electrochemical experiment, the working
electrode was embedded in epoxy resin with an exposed area
of 0.25 cm2. Prior to all the measurement, the strips were
polished using SiC abrasive papers with different grades (400,
800, 1000 and 1200), degreased with acetone and washed
with ethanol, then dried at room temperature. Analytical
grade 37% HCl and ultrapure water were used for the
Scanning electrochemical microscopy (SECM)
Scanning electrochemical microscope (CHI900C) uses a four-
electrode system: 25 μm platinum ultra-microelectrode as the
probe, working electrode (mild steel), Ag/AgCl/KCl (saturated)
reference electrode and platinum counter electrode. The area
of surface scanning was 600 μm × 600 μm, the distance
between probe and substrate was 25 μm. The probe tip
potential was 0.6 V vs. the reference electrode. Before the test,
the platinum probe was polished with Al2O3 powder with a
particle size of 0.05 μm and then washed with ultrapure water.
2-thioureyl pyridine (TP)
HCl, H2O
reflux, 6 h
N
N
S
+
NH4SCN
NH2
N
NH2
N
H
2-imidazolyl pyridine (IP)
Bromosuccinimide
N
N
+
NH2CH2CH2NH2
H2O, 70 °C, 2 min
CHO
HN
Figure 1 Synthetic route of the studied TP and IP.
UV–visible and FT-IR spectroscopy analysis
2 | J. Name., 2012, 00, 1-3
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(a)
UV–vis Spectrophotometer (Perkin Elmer, Lambda 25) was
employed to confirm the Inh-Fe complex formed between
inhibitor molecules with mild steel. The absorption spectra of
corrosion inhibitors before and after 24 h immersion in 1M HCl
for steel were recorded and hydrochloric acid was used as
blank control. FT-IR spectrum was performed with Nicolet 700
FT-IR spectrometer with ranging from 4000 to 400 cm-1. The
absorption spectrum was recorded from the mild steel surface
without and with 4 × 10-4 M pyridine derivatives inhibitors
after 24 h immersion in 1 M HCl solution at ambient
temperature (~25 °C).
X-ray photoelectron spectroscopy (XPS)
(b)
X-ray photoelectron spectroscopy (XPS) spectra were recorded
by a Thermo VG Scientific ESCALAB 250 spectrometer with a
monochromatic Mg Κα X-ray source (1486.6 eV) and an X-ray
beam of around 1 mm. The binding energy of the C 1s peak at
285.0 eV was used as internal reference.16 The XPS analyses
were deconvolved using a nonlinear least squares curve-fitting
program (XPSPEAK software, Version 4.1) with
a
Gaussian/Lorentzian product function. XPS spectra were
practiced on protected mild steel surface with IP after 24 h of
immersion in 1 M HCl solution at 25 °C.
Quantum chemical calculations
All the quantum chemical calculations were performed by
density function theory (DFT) with B3LYP/6-31G(d, p) level
within Gaussian 09 program package. The quantum chemical
parameters of studied inhibitor molecules were calculated,
including the lowest unoccupied molecular orbital energy
(ELUMO), the highest occupied molecular orbital energy (EHOMO),
energy gap (ΔE = ELUMO – EHOMO), fraction of electrons
transferred (ΔN) and Mülliken charges. Besides, the condensed
Fukui functions (f(r)-: electrophilic attack, f(r)+: nucleophilic
attack) have been also computed using the natural population
analysis on the system with N, N+1 and N-1 electrons.
Figure 2 Variation of corrosion rate(a) and inhibition efficiency(b) with different
concentrations of pyridine derivatives by weight loss. Error bars (
standard deviations (%).
┳) represent the
of mild steel decreased significantly and the ηw values
increased with the addition of inhibitor. At 4 × 10-4 M, the
maximum ηw values of TP and IP were 93.57% and 96.66%,
respectively, indicating that the presence of TP and IP inhibitor
could effectively inhibit the corrosion of steel in 1 M HCl, and
the ηw was related to the mount of inhibitor molecules
adsorbed on metal surface. In addition, the values of v
followed the order: v(TP) > v(IP) at any given inhibitor
concentration, revealing that the molecular structure of
inhibitor affected the adsorption property of organic
molecules on steel surface. It could be explained that the
strong conjugation between imidazolyl and pyridine ring
significantly promotes the adsorption of imidazole group, so IP
will have greater surface coverage and more adsorption active
centers on steel surface, thereby increasing the ηw. The
mechanism of the adsorption between the inhibitor molecules
Results and discussion
Weight loss study
Weight loss method was often used to evaluate the corrosion
inhibition performance of organic corrosion inhibitors as the
simple monitoring method and high reliability. Fig. 2 shows the
corrosion rate (v) and inhibition efficiency (ηw) of steel in 1 M
HCl in the presence of different concentrations of TP and IP at and the metal surface will be further elucidated in the
following Section, Mechanism of corrosion inhibition.
25 °C. The v and ηw were obtained from the following
equations:
Effect of temperature on inhibitive performances of TP and IP
ꢂ ꢄ ꢂꢅ
Weight loss measurements at different temperature provide more
detailed insights into the kinetics and thermodynamics of metal
corrosion in aqueous solutions. Corrosion rate (v) and inhibition
efficiency (ηw) obtained from weight loss measurements at different
temperatures (25-55°C) are presented in Table 1. As showed in
Table 1, the values of v increased with increase of temperature in
both uninhibited and inhibited solutions, which is due to desorption
of initially adsorbed inhibitor molecules, resulting in exposure of
greater surface area of metal to corrosive medium leads to
ꢃ
ꢀ ꢁ
ꢉ1ꢊ
ꢉ2ꢊ
ꢆ ꢇ ꢈ
ꢀꢍ ꢄ ꢀ
ꢋꢌ
ꢁ
ꢇ 100%
ꢀꢍ
where v is corrosion rate, W1 and W2 (mg) are the steel sample
weight before and after immersion, s (cm2) is the exposed area
of steel specimens, t (h) is immersion time, vo and v indicate
corrosion rate in absence and presence of inhibitors,
respectively. It can be clearly seen from Fig. 2 that the v values
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Table1 Weight loss results of mild steel in 1 M HCl with different concentrations of TP
and IP at different temperatures.
T
(°C)
c
TP
IP
(10-4 M)
v
ηw
(%)
v
ηw
(%)
(mg cm-2 h-1)
15.910±0.082
4.240±0.035
2.231±0.037
1.262±0.015
1.023±0.021
19.172±0.067
5.719±0.043
3.315±0.014
2.410±0.056
1.858±0.023
25.810±0.045
8.479±0.032
5.415±0.012
3.985±0.062
3.404±0.024
41.465±0.033
15.089±0.042
9.313±0.011
7.045±0.026
6.282±0.022
(mg cm-2 h-1)
15.910±0.082
3.441±0.023
1.542±0.015
0.789±0.017
0.531±0.016
19.172±0.067
4.479±0.025
2.605±0.037
1.576±0.019
0.980±0.016
25.810±0.045
6.804±0.021
4.210±0.032
3.002±0.041
1.907±0.034
41.465±0.033
25
35
45
55
Blank
1
2
3
-
-
73.35
85.98
92.07
93.57
-
70.17
82.71
87.43
90.31
-
67.15
79.02
84.56
86.81
-
63.61
77.54
83.01
84.85
78.37
90.31
95.04
96.66
-
76.64
86.41
91.78
94.89
-
73.64
83.69
88.37
92.61
-
4
Blank
1
2
3
4
Blank
1
2
3
Figure 4 Arrhenius plots of ln(vcorr /T) vs. 1/T for steel in 1 M HCl in the absence and
presence of inhibitors.
4
presence of inhibitors. Ea values calculated from the slopes of
straight lines are shown in Table 2. As depicted in Table 2, all the
regression coefficients are almost close to unity, indicating that the
corrosion of mild steel in 1 M HCl solution can be using the kinetic
model. The values of Ea in presence of two inhibitors are higher
than that for the uninhibited solution, it may be correlated with the
existence of physical adsorption phenomenon by forming an
adsorptive film of an electrostatic character on mild steel surface,
which has been reported in the literatures.3,8
Blank
1
2
3
4
12.033±0.028 70.98
8.231±0.019
6.124±0.022
3.782±0.013
80.15
85.23
90.88
decrease in efficiency of the inhibitors at high temperatures. That is
to say, TP and IP are temperature-dependent corrosion inhibitors.
In addition, it can be seen that the inhibition efficiency of IP is still
higher than TP in the studied temperature range.
The enthalpy and entropy of activation (ΔH*, ΔS*) of the
process was calculated from transition state Eq. (4) which listed in
Table 2:
Kinetic parameters
ꢀꢎꢏꢐꢐ ꢔꢕ
Δꢙ∗
ꢔ
ꢄꢛꢜ∗
ꢔꢕ
To gain a better understanding of the inhibition mechanism
affected by temperature on corrosion inhibition, the kinetic analysis
of mild steel in 1 M HCl in absence and presence of 4 × 10-4 M TP
and IP at different temperatures from 25 °C to 55 °C was studied.
The activation energy (Ea) of the corrosion process can be obtained
by Arrhenius Eq. (3):
ꢁ
exp ꢘ
ꢚ exp ꢘ
ꢚ
ꢉ4ꢊ
ꢕ
ꢖꢗ
where h is Planck’s constant and N is Avogadro’s number. The plot
of ln(vcorr /T) vs. 1/T for steel in 1 M HCl in the presence and
absence of inhibitors at different temperatures, as shown in Fig. 4.
As can be seen from Table 2, the values of ΔH* were positive, which
means that the activation process is an endothermic corrosion
process. This has also been reported to be indicative of retarded
dissolution of metal in inhibitor-containing solution.8 Therefore, the
values of ΔH* obtained in the present study (Table 2) indicate that
mild steel dissolution is an endothermic process and was retarded
by the inhibitors. The values of ΔS* were lower for the uninhibited
solution than that for the inhibited one. This might be attributed
that the activated complex in the rate-determining step represents
an association rather than dissociation step, meaning that decrease
in disordering takes place on going from reactants to the activated
complex, similar results have been reported elsewhere.3,8
ꢒꢓ
ꢉ ꢊ
3
lnꢀꢎꢏꢐꢐ ꢁ lnꢑ ꢄ
ꢔꢕ
where vcorr represents the rate of corrosion, R is the gas constant, T
is the absolute temperature and A is the pre-exponential factor. Fig.
3 shows the Arrhenius plots for steel in 1 M HCl in the absence and
Adsorption isotherms
Adsorption isotherms can provide information on the adsorption
behavior of inhibitor molecules on metal surface. Most organic
Table 2 Activation parameters for the steel dissolution in 1M HCl in the absence and
the presence of inhibitors at 4× 10-4 M.
Inhibitors
R2
Ea (kJ mol-1)
Δ
H* (kJ mol-1)
ΔS* (J mol-1 K-1)
Blank
TP
IP
0.997
0.999
0.998
44.99
68.23
72.59
42.40
65.94
70.00
-80.35
-23.73
-15.69
Figure 3 Arrhenius plots of lnvcorr vs. 1/T for steel in 1 M HCl in the absence and
presence of inhibitors.
4 | J. Name., 2012, 00, 1-3
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corresponding results are listed in Table 3. From Table 3, the
∆G0 values of two inhibitors were -35.11 and -35.64 kJ/mol,
ads
respectively. Usually, the inhibitor molecules are mainly
adsorbed on the surface of the metal substrate by electrostatic
attraction when |∆G0ads| is less than 20 kJ/mol, which belongs
to physisorption. On the contrary, when |∆G0ads| is above 40
kJ/mol, the absorption of organic molecules on the metal
surface accompanied by charge sharing or transfer to form
chemical bonds belongs to chemisorption.4,10 As mentioned
above, the values of ∆G0ads for studied TP and IP inhibitor were
-35.11 and -35.64 kJ/mol, respectively, denoting that the
adsorption of inhibitor molecules on the surface of mild steel is
not a single physical adsorption or chemical adsorption, which
is the result of two kinds of adsorption interaction. Obviously,
pyridine derivatives exist in the form of either neutral or
cationic molecules in acidic media. In the form of neutral
molecules, the TP and IP can adsorb on mild steel by the
formation of coordinate bond between the molecules and Fe
atoms. In the form of cationic molecules, the adsorption is
predominantly related to the electrostatic interaction between
the protonated compounds and mild steel surface negatively
charged by the specific adsorption of Cl- ions. Therefore, the
adsorption of these compounds involves two modes of
Figure 5 Langmuir adsorption isotherm of the inhibitors in 1 M HCl solution with
different concentrations of TP and IP.
inhibitors act by adsorption on the metal surface, depending on the
nature of the interaction between the inhibitor molecule and the
metal surface, which contains two main modes: (a) electrostatic
interaction between the charged molecules and the charged metal
(physisorption) and (b) formation of coordinate bond between the
molecules with unshared electron pairs and/or π-electrons and the
iron with vacant d-orbital (chemisorption). Several adsorption
isotherms including Frumkin, Langmuir, Temkin and Freundlich
were deduced by fitting the surface coverage as a function of the
concentration of two inhibitors at 25 °C. The Langmuir isotherm
was found to be the most suitable model to fit the results of weight
loss measurements, with linear regression coefficients and slopes
close to 1 (Table 3), as shown in Fig. 5, indicating that the
experimental data are well described by Langmuir isotherm and
exhibit single-layer adsorption characteristic. This kind of isotherm
involves the assumption of no interaction between the adsorbed
species on the electrode surface.16 The Langmuir isotherm is given
by Eq. (5):
interaction. Besides, the absolute values of ∆G0 for IP is
ads
higher than that of TP, further illustrating that IP has stronger
adsorption capacity on the metal surface to produce better
corrosion inhibition. However, it is difficult to distinguish
between chemisorption and physisorption only based on these
criteria, especially when charged species are adsorbed. The
possibility of Coulomb interactions between adsorbed cations
(protonated form) and specifically adsorbed anions (Cl-) can
increase the Gibb's energy even if no chemical bond
appears.16,17 The model of adsorption will be further discussed
in the following Section, Adsorption thermodynamic
parameters.
ꢝ
ꢞ
1
ꢟꢓꢠꢡ
ꢁ
ꢢ ꢝ
ꢉ5ꢊ
Adsorption thermodynamic parameters
where c is inhibitor concentration, θ represents the surface
coverage and Kads is adsorption equilibrium constant. Kads can
be calculated from the intercepts of the straight lines, then the
standard adsorption free energy (ΔG0ads) of the adsorption
process associated with Kads can be calculated by the following
Thermodynamic parameters are important to further
understand the adsorption process of inhibitor on
steel/solution interface. The standard enthalpy (∆H0ads) and
entropy (∆S0ads) of dissolution process could be calculated by
Eqs. (7) and (8):
∆ꢜꢤ
Eq. (6):
ꢤ
lnꢟ ꢁ
∆ꢙꢓꢤꢠꢡ
ꢓꢠꢡ ꢢ ꢥ
ꢉ7ꢊ
ꢉ8ꢊ
ꢉ
ꢊ
∆ꢣꢓꢠꢡ ꢁ ꢄꢔꢕln 55.5ꢟꢓꢠꢡ
ꢉ6ꢊ
ꢔꢕ
where ∆G0 is free energy of adsorption, T is thermodynamic
∆ꢜꢓꢤꢠꢡ ꢄ ∆ꢣꢓꢤꢠꢡ
ads
ꢁ
temperature and R represents the gas constant. The
ꢕ
where B is an integration constant. The fitted line of lnK versus
1/T was show in Fig. 6. The values of ∆H0 and ∆S0 are
Table 3 Thermodynamic parameters of inhibitors on mild steel surface in 1 M HCl
solution.
ads
ads
listed in Table 3. As seen from Table 3, the ∆H0 values are
Inhibitors T (°C)
R2
k
ΔG0
ΔH0
ΔS0
ads
ads
ads
ads
(kJ mol-1) (kJ mol-1) (J mol-1 K-1)
negative, which show that the adsorption is an exothermic
process. Further, in the exothermic process the physisorption
TP
IP
25
35
45
55
25
35
45
55
0.999 0.967
0.999 1.003
0.999 1.037
0.999 1.046
0.999 0.953
0.999 0.969
0.999 0.989
0.997 1.001
-35.11
-36.09
-37.08
-37.93
-35.64
-36.53
-37.44
-38.21
-6.88
94.73
94.84
94.97
94.66
86.28
86.49
86.64
86.34
value ∆H0 is around -40 kJ mol-1 or less negative, while the
ads
value around -100 kJ mol-1 or more negative is associated with
chemisorption.2 Therefore, the values of ∆H0 obtained in
-9.89
ads
this study for TP (-6.88 kJ/mol) and IP (-9.89 kJ/mol) suggest
that can be considered physically adsorbed. The positive value
of ∆S0ads can be attributed to the increased randomness at the
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(a)
Figure 6 lnK vs. 1/T for the corrosion of mild steel in 1 M HCl in the absence and
presence of different concentrations of inhibitors.
(b)
mild steel/solution interface during the adsorption of inhibitor
molecules onto the mild steel surface.2 This increase of
disorder is due to more water molecules which can be
desorbed from the mild steel surface by TP and IP molecules.
Open circuit potential (EOCP) curves
The variation of open circuit potential (EOCP) of mild steel electrode
with time in 1 M HCl solution in absence and presence of inhibitor
at 25 °C was shown in Fig. 7. For all cases, the initial potential was
(a)
Figure 8 Potentiodynamic polarization curves of mild steel in 1 M HCl solution without
and with different concentrations of inhibitors: (a) TP and (b) IP.
moved positive value by time and gradually remains constant,
which is similar with the previous reported EOCP-time curves of steel
in 1 M HCl.12 In the presence of inhibitor solutions, the open circuit
potential values shifted towards more positive potentials during the
immersion, which can be explained by the adsorption of inhibitor
on the mild steel. By increasing the concentration of the inhibitor,
the shift in EOCP increases in the positive direction, indicating that
inhibitor was more active for anodic reactions as compared to
cathodic reactions. It took about 20 min to reach the steady state,
so we chose the immersion time of 30 min for electrochemical
measurements.
(b)
Potentiodynamic polarization curves
Fig. 8 shows the polarization curves of mild steel in 1M HCl
solution without and with different concentrations of inhibitor
TP and IP. The calculated electrochemical parameters such as
corrosion potential (Ecorr), anodic and cathodic Tafel slopes (βa,
βc), corrosion current density (icorr) as well as inhibition
efficiency (η) were summarized in Table 4. The calculation
formula of inhibition efficiency was calculated as Eq. (9):
ꢦꢎꢤꢏꢐꢐ ꢄ ꢦꢎꢏꢐꢐ
ꢉ
ꢊ
ꢉ ꢊ
9
ꢋ % ꢁ
ꢇ 100
ꢦꢎꢤꢏꢐꢐ
where, i0 and icorr represent the corrosion current densities
corr
Figure 7 EOCP – t curves for mild steel in 1 M HCl solutions with different concentrations
of mild steel in the absence and presence of inhibitor solution,
of inhibitors: (a) TP and (b) IP.
6 | J. Name., 2012, 00, 1-3
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Table 4 Polarization curve parameters for the corrosion of mild steel in 1 M HCl
solution without and with different concentrations of inhibitors.
(a)
Inhibitors
c
Ecorr
icorr
η
(%)
βa
(mV dec-1)
116.2
74.3
63.2
79.3
70.6
42.5
33.2
39.4
βc
(mV dec-1)
-207.4
-241.1
-251.9
-239.1
-245.8
-200.4
-226.5
-215.1
-204.5
(mM) (mV) (µA cm-2)
-
TP
Blank -349
450.9
124.0
75.6
44.2
39.3
111.8
57.1
25.8
19.1
-
0.10
0.20
0.30
0.40
0.10
0.20
0.30
0.40
-328
-324
-317
-314
-347
-342
-335
-323
72.50
83.23
90.19
91.28
75.21
87.34
94.28
95.76
IP
34.1
respectively. As can be seen from Fig. 8 that all of the curves
shift towards low current density direction after TP and IP
inhibitor were added, and the trend is more pronounced at
higher inhibitor concentrations. In the presence of inhibitors,
the icorr values shown in Table 4 are significantly reduced
relative to the blank, and the values of icorr gradually decreased
with incremental concentrations, thus increasing the inhibition
efficiency. Furthermore, all the cathodic polarization curves
were almost parallel and have no significant changes with each
other, which suggested the mechanism of hydrogen evolution
reaction had not changed, still controlled by active site. The
inhibitor molecules firstly absorbed on the electrode surface,
merely hinder the active site of hydrogen evolution reaction
on the metal surface, so that the available of H+ ions on the
electrode surface was reduced, while the actual reaction
(b)
mechanism had not effected. In the anodic domain,
a
transitional passivity region was observed after adding TP or IP
inhibitor. And the current density decreased rapidly with the
increase of potential after passing the peak of activity state,
which could be due to the dissolution of iron on the electrode
surface formed a layer of high resistance and corrosion-
resistant oxide film based on Fe (II). At higher potentials, the
lowest peak of transition passivity followed by an increase in
anodic current density, which may be due to the dissolution of
the protective film to produce Fe(III) species or the discharge
of oxygen by the water molecules.4
Figure 9 Nyquist plots for mild steel in 1 M HCl solution without and with different
concentrations of inhibitors: (a) TP and (b) IP.
Electrochemical impedance spectroscopy (EIS)
To further study the steel surface interface changes in
presence and absence of corrosion inhibitor, electrochemical
impedance measurements were conducted. The Nyquist plots
of mild steel in 1 M HCl solution with different concentrations
of TP and IP were displayed in Fig. 9. As can be seen from Fig. 9
that in presence of inhibitor the diameter of the capacitive
loop increased with increasing TP or IP concentration
demonstrate an improvement in charge transfer resistances
and inhibition efficiencies. In addition, the imperfect and
approximately compressed semi-circle capacitive loops were
shown in Nyquist diagram was generally believed to be
"dispersion effect" caused.19-21 Ordinarily, such phenomenon
can be traceable in the roughness of the metal electrode
surface and other in-homogeneities of the metal electrode
surface. In addition, the diameter of the capacitive loop in
presence of inhibitor is larger than that in the uninhibited
acidic solution and the diameter of the high frequency loop
increases notably with increasing the concentration of
inhibitor, which indicates that the corrosion of mild steel in 1
M HCl is retarded by the inhibitor. The corresponding Bode
plots are exhibited in Fig. 10. It is apparently seen from Fig.10
that the phase angle of TP and IP have two maximum peaks
within the studied frequency range, which means the double-
Simultaneously, it is worth noting that the values of η for
two compounds follow the order: TP < IP, and maximum
inhibition efficiency reached 91.28% and 95.76%, respectively.
As seen in Table 4, the corrosion potential was also positive
shift with the maximum displacement at 39 mV which was less
than 85 mV compared with the blank, indicating that the
investigated TP and IP belonged to mixed-type inhibitors which
predominantly suppress the anodic reaction. The similar
results of other N/S-heterocyclic compound inhibitors have
been reported for the corrosion of mild steel in 1 M HCl
solution.16-18 Another finding from Table 4 was that the values
of βa and βc changed after addition of inhibitor, which
indicates that inhibitors have the function of controlling the
anodic and cathodic reactions in the corrosion process. In
addition, the inhibition efficiency of IP is higher than that of TP
at the same concentration, which is consistent with the results
obtained by weight-loss method.
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(a)
(a)
(b)
(b)
Figure 12 Equivalent circuit fit for mild steel in 1 M HCl solution.
(a)
Figure 10 Bode plots for mild steel in 1 M HCl solution without and with different
concentrations of inhibitors: (a) TP and (b) IP.
(a)
(b)
(b)
Figure 13 Equivalent circuit fit for mild steel in the presence of IP at 2 × 10-4 M.
impedance increased significantly as inhibitor concentration
increased at low frequency. This is due to the fact that more
molecules were adsorbed on the surface of the steel with
increasing concentration, and the thicker the adsorption film
was, more difficult the charge transport process become. Since
Figure 11 Equivalent circuit used to fit the EIS experiment data: (a) without inhibitor
and (b) the presence of inhibitor.
layer impedance does not performance ideal capacitive
behavior due to the dispersing effect. Besides, the absolute
8 | J. Name., 2012, 00, 1-3
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Table 5 EIS parameters for the corrosion of mild steel in 1 M HCl solution without and with different concentrations of inhibitors.
Inhibitors
c
Rs
Cd
nd
Rct
Cf
nf
Rf
ηz
(%)
(10-4 M)
(Ω cm2)
(μF cm-2)
595±2.32
318±3.06
192±3.21
85±1.89
84±1.94
191±2.44
125±2.46
70±1.71
65±1.36
(Ω cm2)
(μF cm-2)
-
(Ω cm2)
-
-
TP
Blank
0.91±0.03
0.73±0.05
1.26±0.04
1.39±0.03
1.84±0.05
1.21±0.04
0.76±0.03
0.84±0.05
0.96±0.04
0.89±0.05
0.91±0.07
0.84±0.03
0.87±0.02
0.88±0.02
0.89±0.04
0.91±0.06
0.85±0.03
0.87±0.02
52.4±1.61
182.9±3.58
356.9±6.13
593.2±4.52
719.4±7.25
228.8±4.48
497.2±3.61
1010.1±7.12
1286.3±8.36
-
-
1
2
3
4
1
2
3
4
47±1.21
34±0.81
31±1.02
27±0.98
40±1.13
33±1.69
19±0.91
14±0.61
0.87±0.03
0.86±0.02
0.84±0.02
0.83±0.05
0.91±0.07
0.88±0.02
0.88±0.04
0.86±0.03
26.1±1.92
33.2±1.79
42.9±1.57
45.0±2.21
34.8±2.48
43.1±1.63
57.8±1.49
40.5±1.42
74.93
86.57
91.76
93.14
80.12
90.30
95.09
96.05
IP
Table 6 Comparison of the inhibition efficiency of TP and IP with the literature data as corrosion inhibitors for mild steel in 1 M HCl solution.
Inhibitor
10-4 c (M)
η (%)
93.14
96.05
90
91
90
Ref.
This paper
This paper
[28]
[29]
[29]
[30]
[31]
[31]
[32]
[33]
[33]
[22]
[22]
[34]
[35]
1-(2-pyridyl)-2-thiourea
2-(imidazol-2-yl)-pyridine
4
4
5
5
5
3-pyridinecarboxaldehyde thiosemicarbazone
3-pyridinecarboxaldehyde-4-phenyl thiosemicarbazide
4-pyridinecarboxaldehyde-4-phenylthiosemicarbazide
2-hydroxybenzaldehyde-thiosemicarbazone
methylthiourea
4
94
10
10
10
5
92.4
96.8
87
83
70
89.0
90.8
86
phenylthiourea
thiourea
(5-methyl-1-pyridin-2-yl-1H-pyrazol-3-yl)methanol
5-methyl-1-pyridin-2-yl-1H-pyrazol-3-carboxylate
2-phenylimidazo[1,2-a]pyridine
2-(mmethoxyphenyl) imidazo[1,2-a]pyrimidine
2-(3-methyl-1H-pyrazol-5-yl) pyridine
2,5-bis(4-pyridyl)-1,3,4-thiadiazole
5
10
10
10
4
79.2
the adsorption properties of corrosion inhibitor molecules
For the purpose of better analysis of electrochemical
have profound influence on charge transfer process, constant impedance characteristics, ZSimpWin software had been
phase element (CPE) was usually used to optimize the electric utilized to fit the data and the EIS results are simulated by an
double layer capacitance for obtaining more accurate equivalent circuit revealed in Fig. 11 obtained in accordance
experimental data.15,22 The impedance of the CPE (ZCPE) can be with the data fitting curve illustrated in Figs. 12 and 13. The
calculated by Eq. (10):
small values of χ2 (2.42/8.24 × 10-5) indicates a better fit. The
electrochemical parameter Rs is electrolyte resistance, Rct is
charge transfer resistance, Rf is membrane resistance, Cd
represents double-layer capacitance on the electrode surface
and Cf is membrane capacitance. The electrochemical
impedance parameters obtained by equivalent circuit were
listed in Table 5. The inhibition efficiency (ηz) can be calculated
by the ac polarization resistance Rp (Rp = Rf + Rct) in Eq. (12):
ꢵꢔꢶ ꢄ ꢔꢶꢤꢷ
ꢉ
ꢊꢯ
ꢧꢨꢩꢪ ꢁ ꢫꢬ ꢭꢮ ꢰꢱꢃ
ꢉ10ꢊ
ꢤ
where Y0 is the magnitude of the CPE, j the imaginary number,
ω the angular frequency, and n the phase shift. The CPEd
element was considered to be a double layer capacitance (Cd)
model which contained H2O and other ions adsorbed on steel
surface, and CPEf served as a model for the inhibitor
membrane (Cf).23,24 The values of Cd and Cf are calculated from
ꢉ
ꢊ
ꢋꢴ % ꢁ
ꢇ 100
ꢉ12ꢊ
ꢔꢶ
Y0 and n as follows:
where R0 and Rp are the ac polarization resistance of steel
ꢉ ꢊꢯꢱꢃ
ꢬ ꢮ
sinꢉn ꢳ/2 ꢊ
p
ꢤ
ꢲ ꢁ
ꢉ11ꢊ
electrode in the uninhibited and inhibited solutions,
respectively. As can be seen from Table 5, the values of Rct
rose sharply with increasing the concentration of inhibitor,
indicating the corrosion inhibitor TP and IP could effectively
inhibit the charge transfer reaction in the protective film.
ꢉ
ꢊ
where wmax = 2πfmax , and fmax is the frequency at the maximum
value of the imaginary component of the impedance spectrum.
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Meanwhile, with the increasing concentration of studied
inhibitors, the values of Cf and Cdl were gradually decreased
due to the decrease in exposed electrode surface area, local
dielectric constant and/or an increase in the thickness of the
electric double layer capacitor.25-27 It expressed that the
corrosion inhibitor molecules instead of water molecules
adsorbed on the surface of steel, forming a layer of adsorption
film which could prevent the metal matrix and corrosive media
contact, then slowing down the corrosion. Furthermore
，
another diverting discovery we could find in Table 5 was that
the value of n were close to unit and didn’t show significant
fluctuations, which revealed the interface behavior was nearly
capacitive and the charge transfer controlled the dissolution
mechanism of the steel. Finally, it is worth pointing out that
the value of ηz calculated by EIS increased with the addition of
inhibitor and the maximum ηw reached 93.14% (TP) and 96.05%
(IP) at 4 × 10-4 M, coinciding with that obtained by weight loss
and polarization curves studies.
Figure 14 Schematic diagram of SECM tip working in the generator–collector mode.
(a)
Compared with the earlier studied pyridine and/or
thiourea base corrosion inhibitors by EIS method in 1 M HCl
was shown in Table 6 (Ref. 22,28-35), TP and IP exhibited
better corrosion inhibition behaviour. This phenomenon is
related to the steric hindrance effect and the number of
heteroatoms in the structure.
(b)
SECM experiments
SECM technique was used to obtain the change of Faraday current
distribution on the surface of the substrate by micro-probe to scan
at various positions on the substrate surface, thus directly
characterizing the morphology and electrochemical activity of the
matrix surface. The distance from the tip above the sample is
determined by approach curves.36,37 When a mild steel electrode
was under open circuit potential, the substrate undergoes self-
corrosion reaction resulting in the release of Fe2+ ions. The probe
potential of 0.60 V can oxidize ferrous ions to Fe3+ and be detected
at the UME in the generator–collector mode, as shown in Fig. 14.37
The SECM area scans images of mild steel specimens surface in 1 M
HCl solution without and with 4 × 10-4 M of TP and IP were
displayed in Fig. 15. As can be seen from Fig. 15a, multiple current
peaks appear in the SECM surface scanning map in acid solution
with a maximum value of 5100 nA, indicating faster metal corrosion
at these active sites. In addition, the current fluctuations were
caused by anodic dissolution on the surface of steel sample.
Howbeit, in the presence of corrosion inhibitors (Fig. 15b and c), the
current density of the entire scanning surface was distributed
uniformly. The current maximum value for TP and IP inhibitors
were23 and 11 nA, respectively, which again shows that IP has
better inhibitive performance than IP. This is due to the obstacles
(c)
encountered in the diffusion field around tip to reduce the tip Figure 15 SECM images of mild steel after immersion in 1 M HCl solution for 1h: (a)
Blank, (b) 4 × 10-4 M TP, (c) 4 × 10-4 M IP.
current, indicating that the addition of inhibitor reduced the acid
dissolution of mild steel and formed uniform adsorption film. Thus,
inhibitors (TP and IP) before and after the steel immersion for 24 h
were shown in Fig. 16. The absorption spectra of TP and IP before
the mild steel immersion display shorter wavelength absorption
bands at 226 nm, 263 nm and 251 nm, respectively, attributable to
the π→π* transition of aromatic ring in compounds. And the
broadband at around 311 and 283 nm was n→π* electron
transition duo to the lone pair electrons of N and/or S atom.38,39
the above analysis reveals that the adsorption film of pyridine
derivatives molecules have formed on metal surface.
UV–Visible and FTIR spectroscopy analysis
To confirm the formation of Inh-Fe complex on steel surface, UV-vis
absorption spectra of 1 M HCl solution containing 4 ×10-4 M of
10 | J. Name., 2012, 00, 1-3
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(a)
(a)
(b)
(b)
Figure 16 UV–visible spectra of pure inhibitor and pyridine derivatives inhibitors
Figure 17 FTIR spectrum of pure inhibitor and pyridine derivatives inhibitors adsorbed
adsorbed on MS surface: (a) TP and (b) IP.
on MS surface: (a) TP and (b) IP.
After immersion in a solution containing mild steel for 24 h, it
is clearly seen from Fig. 16 that the band at 263 nm has
completely disappeared and the maximum band shift of n→π*
transition was red shifted, which was probably the charge-
transfer process from ligand to metal, indicating that N and/or
S interacted with Fe to form a complex. At the same time, the
absorbance at short wavelength absorption band becomes
larger and the absorbance at long wavelength absorption band
becomes smaller. Generally speaking, changes in the position
of absorbance maximum and/or changes in absorbance values
indicated that a complex was formed between two substances
in solution, as reported in the literature.40,41 Compelling
evidence was provided for the possibility of Fe2+ and inhibitor
forming complex in 1 M HCl solution by these experimental
results.
adsorptive membrane on steel surface, it can be found that
the peaks of stretching vibration at N-H (TP: from 3225 cm-1 to
3266 cm-1; IP: from 3314 cm-1 to 3222 cm-1), C=N (TP: from
1594 cm-1 to 1551 cm-1; IP: from 1598 cm-1 to 1572 cm-1) and
C=S (TP: from 1233 cm-1 to 1206 cm-1) were shifted
significantly, which shows that they were mostly involved in
the adsorption process to form coordination bond with Fe
atoms.42-44 In addition, the C = C peaks at 1435 cm-1 and 1479
cm-1 for TP were merged into a broad peak at 1443 cm-1 and
the IP peaks at 1462 cm-1 and 1483 cm-1 were merged at 1469
cm-1, suggesting that π electrons of pyridine ring were involved
in the inhibitor adsorption process. The above findings clearly
illustrated that the functional group of 1-(2-pyridyl)-2-thiourea
and 2-(imidazol-2-yl)-pyridine were involved in formation a
coordination bond with iron metal, using a solitary pair of
electrons on N/S atoms and aromatic π-electrons.
In order to support the complex formation between corrosion
inhibitor and steel surface, we analyzed the pure inhibitor and the
sample scratched from steel surface by FTIR, the related spectrum
was shown in Fig. 17a and b, respectively. It can be seen from Fig.
17 that the FTIR of the protective film formed on steel surface
showed all characteristic peaks of thepure inhibitor, as the
molecular vibrations moved towards higher or lower wave numbers,
possibly due to the formation of inhibitor and Fe2+ complex on steel
surface. Comparing the FTIR spectrum of pure inhibitor with
Surface analysis by XPS
In order to confirm the nature of the organic film adsorbed on the
steel surface and the adsorption mode of the inhibitor, XPS test was
used to investigate the composition of the organic adsorbed layer
on the steel surface in 1 M HCl solution in presence of best inhibitor
(IP). As shown in Fig. 18a, the XPS spectra with three elements (C
1s, N 1s and Fe 2p) were detected, which give evidence for the
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(a)
N
1s and Fe 2p3/2) on the protected steel surface. The
deconvoluted C 1s spectrum was fitted into three peaks (Fig. 18b).
The first peak located at 285.0 eV can be attributed to the C–C, C=C
and C–H aromatic bonds, the second peak at 286.3 eV may be
associated with the presence of C-N and C=N bonds in the
heterocyclic ring. The last peak at higher binding energy (288.6 eV)
may be ascribed to C=N+ structures, resulting probably from the
protonation of the =N– structure in the imidazole ring.17,19 As
shown in Fig. 18(c), the N 1s spectrum consists of three peaks
located at 399.4 eV, 400.7 eV and 401.5 eV, respectively. The first
component is attributed to the bonds of C–N and the unprotonated
N atoms (=N– structure) in the heterocyclic ring,10,16 while the
second component is mainly ascribed to N–Fe bond due to the
coordination of nitrogen in IP ring with the steel surface. Indeed,
the N–Fe bond complex can make the peak shift to higher binding
energy compared to uncoordinated =N– structure.10 The last peak
at 401.4 eV is probably due to the oxidized nitrogen originating
from the protonation of nitrogen in the pyridine and/or imidazole
ring, which leads to a positive polarization of the nitrogen atom,
and therefore the binding energy is increased.10 The Fe 2p3/2
spectrum exhibits three peaks as displayed in Fig. 18(d). The first
peak at about 710.9 eV is assigned to ferric compounds, such as
Fe2O3 (i.e., Fe3+ oxide) and FeOOH (i.e., oxyhydroxide). The second
peak located at around 713.4 eV indicates the presence of a small
concentration of FeCl3 on the steel surface due to the hydrochloric
acid medium, while that appeared at 714.6 eV may be ascribed to
the satellite of Fe(III).16
(b)
(c)
The results obtained by XPS analysis supports the adsorption
of inhibitors on mild surface. Especially, the presence of the
nitrogen species on the steel surface indicates that the IP
compound can adsorb on steel by physisorption simultaneously
accompanied by chemisorption in 1 M HCl, and corroborates the
thermodynamic study. Furthermore, the Fe 2p3/2 XPS results show
that the formation of a stable and insoluble layer (Fe2O3, FeOOH)
can reduce ions diffusion, and therefore improves the corrosion
resistance of mild steel in 1 M HCl medium.
Quantum chemical calculations
To further study the correlation between the molecular structures
of investigated inhibitors and inhibitive effect, quantum chemical
calculations are carried out in this work. The donor-acceptor
interactions between the inhibitor molecules and iron atoms can be
analyzed by Mülliken charges and Fukui indices. Fig.19 shows the
strictly optimized molecular structures of TP and IP. Zhang et al.
reported that Mülliken charges distribution can be used to analyze
the adsorption of inhibitor molecules on the metal surface, and an
atom with a large negative charge is easily adsorbed on the metal
surface.4 In this study, the Mülliken charges of the selected C, N and
S atoms for studied inhibitors were calculated with the dates listed
in Table 7. As can be clearly found in Table 7, the larger negative
atoms are N6, N11, N13 and S17 for TP, N6, N16 and N17 for IP,
these atoms are active adsorptive atoms and will donate electrons
to the empty d-orbital of Fe atom to form coordination bonds. Thus,
the pyridine, imidazole rings and thiourea group act mainly as the
adsorption centers of TP and IP molecules on mild steel. In addition,
Fukui function was employed to predicting the local site selectivity,
(d)
Figure 18 XPS spectra for mild steel surface treated with 4 × 10-4 M of IP in 1 M HCl: (a)
survey scan spectra and narrow scan spectra of (b) C, (c) N, and (d) Fe.
adsorption of the inhibitor on the mild steel surface. Fig. 18(b-d)
shows the high-resolution XPS spectra of the major elements (C 1s,
12 | J. Name., 2012, 00, 1-3
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(a)
(a)
(b)
(b)
Figure 20 HOMOs of two studied inhibitors of TP (a) and IP(b).
(a)
Figure 19 Optimized structures of TP (a) and IP (b).
and the Fukui function f(r)+ and f(r)- were related to nucleophilic
Table 7 Mulliken charge, f(r)+and f(r)- for TP and IP.
f (r)+
f (r)-
Compounds
Atoms
C1
Mulliken
0.486503
-0.120076
-0.054153
-0.102592
0.09872
TP
0.08802
0.00541
0.08291
0.10743
0.00488
0.07626
-0.01597
0.05001
0.09242
0.23223
-0.02451
0.0295
C2
C3
0.0324
C4
0.05906
0.00788
0.04606
0.05354
0.04721
-0.00801
0.533
(b)
C5
N6
-0.443916
-0.577954
-0.588564
0.32548
N11
N13
C16
S17
C1
-0.239796
0.250797
-0.114908
-0.058391
-0.10043
0.100965
-0.460309
0.072061
0.031937
0.454113
-0.466019
-0.545989
IP
0.13638
0.01501
0.07631
0.14426
0.00869
0.09406
0.08403
0.02004
0.00919
-0.00157
0.07591
0.00399
0.11717
0.03314
0.03659
0.18846
0.0949
C2
C3
Figure 21 LUMOs of two studied inhibitors of TP (a) and IP(b).
C4
C5
attack and electrophilic attack, respectively, which can be
calculated by Eqs. (13) and (14):4
N6
C11
C12
C15
N16
N17
ꢸ ꢉꢺꢊꢻ ꢁ ꢼꢹ ꢖ ꢢ 1 ꢄ ꢼꢹ ꢖ
ꢉ ꢊ
ꢉ13ꢊ
ꢉ14ꢊ
ꢉ
ꢊ
ꢹ
ꢸ ꢉꢺꢊꢱ ꢁ ꢼꢹ ꢖ ꢄ ꢼꢹ ꢖ ꢄ 1
ꢊ
ꢉ
ꢊ
ꢉ
0.09647
ꢹ
0.08639
0.04051
0.15996
0.0815
where qi (N+1), qi(N) and qi (N–1) are the charge of atom i in
cationic, neutral and anionic, respectively. The calculated Fukui
indices for the inhibitor are displayed in Table 7. As an extensively
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accepted point of view, the higher the values of f(r)+ and f(r)- are,
the stronger the ability of atom to gain and lose electron,
respectively. It can be seen from Table 7, the larger f(r)+ values are
found on C4, C16 and S17 atoms for TP, and C1, C4 and N6 for IP,
which can accept electrons from steel surface to form back-
donating bond. While, the preferred sites for an electrophilic attack
are C4, N11 and S17 for TP, and C4, C11 and N16 for IP, which can
form coordination bonds by donating electrons to steel surface.
Consequently, from the above calculated results, it can be inferable
that the most active reaction region in the studied inhibitors should
be the pyridine, imidazole ring and thiourea which can both donate
and accept electrons to form back-donating bonds and coordinate
bonds to realize the adsorption of these inhibitors on steel surface.
The frontier molecular orbitals were useful to study the
adsorption activity of inhibitor molecules by electrons distribution.
HOMO is often associated with the capacity of a molecule to donate
electrons, whereas LUMO represents the ability of the molecule to
accept electrons.4 Figs. 20 and 21 show the electron density
distribution of HOMO and LUMO for TP and IP inhibitor molecules,
respectively. For IP the densities of both HOMO and LUMO are on
the pyridine and imidazole ring, which means that they could be
both the acceptor of the electron and the donor of the electron.
That is, the electron transfer occurs when inhibitor molecules
interact with the metal surface. For TP, LUMO is located on the
pyridine ring and thiourea group, but HOMO absent on the pyridine
ring except nitrogen atom, which reveals that the pyridine ring of TP
could be mainly accepting electrons instead of donating electrons.
Thereby, comparing with TP, it is reasonable to assume that the
pyridine ring of IP has additional ability to donate electrons to the
empty d-orbital of Fe atom to form coordination bonds, which may
be the main reason of the inhibition efficiency of IP is higher than
Table 8 The calculated quantum chemical parameters for TP and IP.
Compounds EHOMO (eV)
ELUMO (eV)
ΔE (eV)
ΔN
μ (D)
5.31
4.92
TP
IP
-5.84
-5.42
-1.02
4.82
0.74
0.85
-0.91
4.51
the adsorption capacity on the metal surface. According to Pearson
theory, the fraction of electrons transferred to the metal surface is
calculated as Eq. (15):4
ꢽꢾꢿ ꢄ ꢽꣀꢯꣁ
2ꢉꢋꢾꢿ ꢢ ꢋꣀꢯꣁ
∆ꢖ ꢁ
ꢉ15ꢊ
ꢊ
where χ is absolute electronegativity and η is global hardness,
which are given by Eqs. (16) and (17):
ꢄꢒꣂꣃ꣄ꣃ ꢄ ꢒꣅ꣆꣄ꣃ
ꢽ ꢁ
ꢉ16ꢊ
2
ꢒ
ꣅ꣆꣄ꣃ ꢄ ꢒꣂꣃ꣄ꣃ
ꢉ
ꢊ
ꢋ ꢁ
17
2
For Fe, the theoretical values of χFe and ηFe are 7 and 0 eV mol-1,
respectively. According to previous reports, if ΔN < 3.6, the
inhibition efficiency increases with the increase in electron-
donating ability to the metal surface.32 In the present study, it can
be clearly found in Table 8 that the ability to donate electrons to
the metal surface follows the order of ΔNIP > ΔNTP, which is also in
agreement with the experimental results of weightlessness and
electrochemical studies (see Tables 1 and 4).
Besides, there’s another calculated parameter in Table 8, μ,
which has been considered as a significant index to associate with
the inhibition efficiency. The large value of dipole moment probably
increases the inhibitor adsorption through electronic force.45 In
Table 8, μ (TP) > μ (IP), while η (TP) < η (IP), which indicates that the
better inhibitive performance of IP would not be arisen from
intermolecular electrostatic force.
that of TP. The values of quantum chemical parameters (EHOMO
,
ELUMO and ΔE) were calculated and listed in Table 8. Where, EHOMO
and ELUMO are related to the capability of inhibitor molecules to
donate electrons and accept electrons, respectively.32 As reported
previously, the higher the values of EHOMO are, the more likely of the
molecule to donate electrons. On the contrary, the lower the values
of ELUMO are, the stronger the abilities of inhibitor molecule to Mechanism of corrosion inhibition
accept electrons.4,32 It is clear in Table 8 that the EHOMO values obey
The corrosion inhibition mechanism of organic inhibitor on
the order of EHOMO,IP > EHOMO,TP >, which is consistent with the
inhibition efficiency order of η (IP) > η (TP). Thus, there is a good
correlation between the η and EHOMO. However, the ELUMO values
obey the order of ELUMO,TP < ELUMO,IP, which disagree with the
inhibition efficiency obtained from experimental results. This result
is usually interpreted as having complex interactions that may play
metal surface has received extensive attention in the
literature.10,42 It’s generally believed that these compounds are
adsorbed on metal surface, suppressing anode and cathode
reactions to avoid further corrosion of the metal. Therefore,
the corrosion mechanism of steel in HCl solution can be
explained by adsorption theory, and a plausible mechanism of
adsorption model was proposed in Fig. 22.
a
crucial role in the adsorption process. Another important
parameter in Table 8 is the separation energy (ΔE = ELUMO – EHOMO),
that usually varies with the reactivity of the inhibitor molecule
towards its adsorption on metallic surface. Usually, if ΔE decreases,
the reactivity of inhibitor molecules is enhanced, and more easily
adsorbed on steel surface. As showed in Table 8, the ΔE values
follow the order of ΔETP > ΔEIP, which indicated that the adsorption
of IP on steel surface is the most stable and has a higher corrosion
inhibition performance, and corroborates the experiment results.
Calculating electrons fraction transferred from inhibitor
molecule to a metal surface can also analyze the relationship
between corrosion inhibition and molecular structure, to determine
In hydrochloric acid solution, the steel will form several
tiny corrosive batteries that cause electrochemical corrosion.
The dissolution of iron at the anode was given as:
Fe ꢢ Clꢱ ⇌ ꢉFeClꢱꢊ꣇꣈꣉
ꢉFeClꢱꢊ꣇꣈꣉ ⇌ ꢉFeClꢊ꣇꣈꣉ ꢢ eꢱ
ꢉFeClꢊ꣇꣈꣉ → ꢉFeClꢻꢊ ꢢ eꢱ
FeClꢻ ⇌ Feꢅꢻ ꢢ Clꢱ
ꢉ18ꢊ
ꢉ19ꢊ
ꢉ20ꢊ
ꢉ21ꢊ
Generally, chloride ions in the corrosive medium should be
preferentially adsorbed on the positively charged metal
14 | J. Name., 2012, 00, 1-3
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surface, and the N and/or S atoms in inhibitor molecule that
have been adsorbed on the metal surface were not coordinate
with iron atoms can also chelated with Fe(II) species.16,33 Thus,
the stability and compactness of the adsorption film on the
metal surface were improved and the corrosion of steel was
effectively prevented from acid solution. Based on above
analysis, it can be concluded that the adsorption process first
occurs physical adsorption, followed by chemical adsorption,
and the large negative values of ∆Gads also indicate that the
adsorption of TP and IP molecules were mainly chemically
adsorbed in the present study.
Figure 22 Schematic illustration of the adsorption mechanism of inhibitor on mild steel
surface in 1 M HCl solution.
surface in virtue of its hydration degree.46,47 Besides, the TP
and IP molecules can form cationic species in acidic medium,
which will be in equilibrium with the corresponding neutral
form by the following equation:
Conclusions
1-(2-Pyridyl)-2-thiourea (TP) and 2-(imidazol-2-yl)-pyridine (IP)
have been synthesized and their corrosion inhibitions for mild
steel in 1 M HCl solution have been investigated. The main
conclusions can be drawn as follows:
Inh ꢢ ꣊Hꢻ ↔ ꢉInhHꢊ꣋ꢻ
ꢉ22ꢊ
Therefore, the protonated inhibitor can be electrostatically
adsorbed on the surface of negatively charged mild steel due
to the specific adsorption of Cl- anions (physical adsorption)
(1) TP and IP were effective corrosion inhibitors for mild steel
corrosion in 1 M HCl. Their inhibition efficiency increased
with the increase of inhibitor concentration, in which the
corrosion inhibition performance of IP was better than
that of TP.
and formed
a , namely,
protective layer (FeCl-InhH+)ads
protonated organic compounds have produced synergistic
effect with Cl-, then the anodic dissolution reaction of iron is
prevented. Thus, it can be concluded that the reaction
pathway in the presence of inhibitor appears to form a
protective layer (FeCl-InhH+)ads by electrostatic action rather
than steps 18-21. The hydrogen evolution reaction occurs at
the cathode could be shown as follows:
(2) Tafel polarization data showed that TP and IP belonged to
mixed-type inhibitors, which predominantly suppressed
the anodic reaction. EIS measurements revealed that the
values of Rct increased in presence of pyridine derivatives
due to the adsorption of inhibitors at the
electrode/solution interface.
Fe ꢢ Hꢻ ⇌ ꢉFeHꢻꢊꢓꢠꢡ
ꢉ23ꢊ
ꢉFeHꢻꢊ꣇꣈꣉ ꢢ eꢱ → ꢉFeHꢊ꣇꣈꣉
ꢉ24ꢊ
(3) The adsorption of TP and IP on mild steel surface follows
Langmuir adsorption isotherm and the calculated
thermodynamic parameters suggest the adsorption is
mainly the physisorption.
ꢉFeHꢊ꣇꣈꣉ ꢢ Hꢻ ꢢ eꢱ → Fe ꢢ Hꢅ ꢉ25ꢊ
In hydrochloric acid solution, the protonated inhibitors are
also adsorbed at the cathodic sites of mild steel, competing
with H+ for electrons to reduce the evolution of H2 without
affecting the hydrogen evolution mechanism.
(4) The SECM, UV–visible, FT-IR and XPS analysis showed that
the adsorption film of TP and IP was formed on metal
surface.
Apart from physisorption, chemisorption process can
occur through the displacement of water molecules from the
metal/solution interface and the free electron pairs of N, S
heteroatoms and/or π electrons of inhibitor molecule with the
unoccupied d-orbital of iron atoms will form coordination
bonds. Also, the excess negative charge accumulated on metal
surface can be transferred from the d-orbit of Fe to the empty
π* orbital of inhibitor molecules (retro-donation), thus
enhancing the adsorption of inhibitor molecule on steel
surface.48 In addition, the non-polar groups in the molecule
may be tiled on metal surface to form a relatively complete
hydrophobic protective layer. Thus, a barrier between acid
solution and metal was formed to prevent the diffusion of iron
from the corrosion product into solution and the solution of H+
shift to the metal corrosion reaction process, then reducing
the rate of metal corrosion process. Besides the above
reaction, TP and IP may undergo chelation reaction with Fe2+
generated by the dissolution of steel in acid solution (Fig. 22).
It can be reasonably speculated, the N and S atoms in inhibitor
molecule can coordinate with Fe(II) species to form stable
chelates that were deposited on the active sites of steel
(5) Quantum chemistry calculation results showed that the
pyridine, imidazole rings and thiourea group in the studied
inhibitors will play an important role in donating and/or
accepting electrons to form back-donating bonds and
coordination bonds for the adsorption of these inhibitors
on mild steel surface. There is a good correlation between
inhibition efficiency and the quantum parameters of
EHOMO, ΔE and ΔN.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21672046, 21372054), and the
Fundamental Research Funds for the Central Univercities
(HIT.NSRIF.201701).
Notes and references
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Table of contents entry
The 1-(2-pyridyl)-2-thiourea and 2-(imidazol-2-yl)-pyridine is described here for
the first time as an inhibitor for mild steel in acid medium.