Electrochemical and theoretical study of the inhibition effect of two synthesized thiosemicarbazide derivatives on carbon steel corrosion in hydrochloric acid solution

Article abstract of DOI:10.1039/c4ra15808c

In this work, two thiosemicarbazide derivatives, 2-carbamothioyl-N-phenylhydrazine carboxamide (TSC1) and N-[(2-carbamothioylhydrazino)carbonothioyl]benzamide (TSC2) are synthesized and used as carbon steel corrosion inhibitors in 1.0 M HCl. Weight loss measurements indicate that the studied compounds reduce the corrosion rate of carbon steel in acidic solution and the inhibition effect increases with the inhibitors concentration. It was also shown that the adsorption of TSC1 and TSC2 followed a Langmuir adsorption isotherm. The Tafel polarization method reveals that the organic compounds act as mixed type corrosion inhibitors with a predominantly anodic effect. Electrochemical Impedance Spectroscopic measurements show that the inhibitors form an adsorptive layer on the metallic surface and increase the polarization resistance of the corrosion reaction. Fast Fourier Transform Continuous Cyclic Voltammetry is used to continuously monitor the inhibitors adsorption behavior on the carbon steel surface. The results indicate that TSC2 can better inhibit the carbon steel corrosion rather than TSC1. Some of the inhibitors' quantum chemical parameters are calculated with the DFT method at the B3LYP/6-31g(d,p) level of theory. Among the calculated parameters, the LUMO energy, energy gap between the HOMO and LUMO and the hardness could well describe the experimental observations. This journal is

Full text of DOI:10.1039/c4ra15808c

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Electrochemical and theoretical study of the
inhibition effect of two synthesized
Cite this: RSC Adv., 2015, 5, 20838
thiosemicarbazide derivatives on carbon steel
corrosion in hydrochloric acid solution
a
ab
and Mohammad Reza Ganjaliab
*
Shirin Shahabi, Parviz Norouzi
In this work, two thiosemicarbazide derivatives, 2-carbamothioyl-N-phenylhydrazine carboxamide (TSC1)
and N-[(2-carbamothioylhydrazino)carbonothioyl]benzamide (TSC2) are synthesized and used as carbon
steel corrosion inhibitors in 1.0 M HCl. Weight loss measurements indicate that the studied compounds
reduce the corrosion rate of carbon steel in acidic solution and the inhibition effect increases with the
inhibitors concentration. It was also shown that the adsorption of TSC1 and TSC2 followed a Langmuir
adsorption isotherm. The Tafel polarization method reveals that the organic compounds act as mixed
type corrosion inhibitors with a predominantly anodic effect. Electrochemical Impedance Spectroscopic
measurements show that the inhibitors form an adsorptive layer on the metallic surface and increase the
polarization resistance of the corrosion reaction. Fast Fourier Transform Continuous Cyclic Voltammetry
is used to continuously monitor the inhibitors adsorption behavior on the carbon steel surface. The
results indicate that TSC2 can better inhibit the carbon steel corrosion rather than TSC1. Some of the
inhibitors' quantum chemical parameters are calculated with the DFT method at the B3LYP/6-31g(d,p)
level of theory. Among the calculated parameters, the LUMO energy, energy gap between the HOMO
and LUMO and the hardness could well describe the experimental observations.
Received 4th December 2014
Accepted 12th February 2015
DOI: 10.1039/c4ra15808c
www.rsc.org/advances
surface and the corrosive medium. Generally, organic inhibitors
are adsorbed on the metallic surface through their hetroatoms
1 Introduction
or p-orbitals. However, their effectiveness depends on the
quality of adsorption and consequently their structural and
electronic properties. In recent years, in parallel with hardware
and soware developments, many attempts have been done to
relate the inhibition efficiency of inhibitors to their physico-
chemical and electronic properties, which obtained by theo-
retical calculations.15–20 Quantum chemical calculations are
used to calculate structural and electronic properties of mole-
cules and materials. DFT is presently among most successful
approaches to investigate the electronic structure of atoms,
molecules, solids and even nuclei and classical uids.21–24
The objective of this work is the investigation of inhibition
effect of two thiosemicarbazide derivatives on carbon steel
corrosion in 1.0 M HCl solution by weight loss measurements,
Tafel polarization method, Electrochemical Impedance Spec-
troscopy (EIS), Fast Fourier Transform Continuous Cyclic Vol-
tammetry (FFTCCV) and DFT calculations.
Metallic corrosion causes severe losses to both economy and
safety. Therefore, many research efforts have been made to
prevent or decrease the corrosion of metals in different
mediums.1–5 Hydrochloric acid (HCl) is an important inorganic
acid that is used in many industries. For example, one of the
main applications of HCl is its usage in the pickling baths to
remove rust or scale from iron or steel before subsequent pro-
cessing.6,7 However, when HCl is in contact with metals, it starts
to corrode the metallic surfaces. So, in spite of its benets, it is
necessary to nd a way for lessening the corrosion rate when
HCl is used. Carbon steel has been widely used in various
industries and consequently has been subjected to acid corro-
sion in its different situations.8–10 An economic and operative
solution to the corrosion problem is the use of corrosion
inhibitors. Inhibitors are chemicals that when added to the
corrosive solution at small amounts, decrease the corrosion rate
at noticeable extent.10–14 Inhibitor molecules are adsorbed on
the metallic surface resulting in formation a barrier between the
2 Experimental
2.1 Synthesize of inhibitors
aCenter of Excellence in Electrochemistry, University of Tehran, Tehran, Iran. E-mail:
norouzi@khayam.ut.ac.ir
bBiosensor Research Center, Endocrinology & Metabolism, Molecular-Cellular Sciences
Fig. 1 shows the molecular structures and IUPAC names of the
inhibitors, TSC1 and TSC2, which is used as carbon steel
Institute, Tehran University of Medical Sciences, Tehran, Iran
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2.3 Weight loss experiments
The weight loss of the carbon steel strips of 5.0 cm ꢀ 2.5 cm ꢀ
0.2 cm in 1.0 M HCl in the absence and presence of various
concentrations of the inhibitors are determined at 25 ꢂC. Before
doing each experiment, the carbon steel surface was abraded
with emery papers (up to 1200 grit), then cleaned in ultrasonic
bath with ethanol and acetone, and nally washed with distilled
water. Aer weighing accurately, the specimens were immersed
in 100 mL corrosive solution for 1 h. Then, the specimens were
taken out, cleaned, dried and reweighed. The experiments were
done in triplicate and the average value of the weight loss was
used.
2.4 Electrochemical measurements
A three-electrode set-up was composed of a Pt counter electrode
(CE), an Ag/AgCl reference electrode (RE), and carbon steel
working electrode (WE) was used for electrochemical
measurements. Polarization measurements were done by an
AUTOLAB model PGSTAT30, which was connected to a personal
computer to record and store the electrochemical data. Anodic
and cathodic polarization curves were recorded by sweeping the
potential from a more positive potential than Eocp to a more
negative potential at a scan rate of 1.0 mV sꢁ1. The polarization
data was analyzed using GPES soware. The inhibition effi-
ciencies (IE%) was calculated according the eqn (1).25
Fig. 1 Molecular structure of compounds TSC1 and TSC2 and their
IUPAC names.
corrosion inhibitors. A mixture of phenylisocyanate (1.0 ꢀ 10ꢁ2
mol), thiosemicarbazide (1.0 ꢀ 10ꢁ2 mol) and a catalytic
amount of acetic acid was reuxed in 70 mL ethanol for 2 h to
synthesize TSC1. Then the solvent was evaporated to 20 mL.
Aer cooling to room temperature, the product was obtained as
crystals. To synthesize TSC2, a mixture of ammoniumthiocya-
nate (2.0 ꢀ 10ꢁ3 mol) and HCl (2.0 ꢀ 10ꢁ3 mol) was warmedꢁa3t
about 50 ꢂC for 5 min. Then, thiosemicarbazide (2.0 ꢀ 10
mol) was added and stirred for 3 h at room temperature, and at
the end poured into 15 mL of water. The obtained precipitation
was separated by ltration and recrystallized from ethanol to
afford the pure compounds.
Icorr;uninhibit ꢁ Icorr;inhibit
IE% ¼
ꢀ 100
(1)
Icorr;uninhibit
where the Icorr,uninhibit and Icorr,inhibit are the corrosion current
density without and with inhibitors, respectively.
The EIS measurements were started at Eocp using an AC
signal (5 mV peak to peak) at the frequency range of 5.0 ꢀ 10ꢁ2
Hz to 1.0 ꢀ 105 Hz. EIS data were analyzed with FRA soware.
Polarization resistance (Rp) and constant phase element (CPE)
were obtained from Nyquist plots. The IE% values were calcu-
lated from the Rp values according eqn (2).26
The inhibitor compounds were characterized by 1HNMR and
13CNMR data the studied inhibitors are listed as below: For
TSC1: 1H NMR/DMSO/d ppm: 10.7 (s, 1H), 9.72 (s, 2H), 9.02
(s, 1H), 8.16 (s, 1H), 7.44 (d, J ¼ 7.5 Hz, 2H), 7.30 (t, J ¼ 7.3 Hz,
2H), 7.11(t, J ¼ 7.5 Hz IH). 13C NMR/DMSO/d ppm: 182.8, 151.4,
133.2, 127.6, 120.4, 118.9. For TSC2: 1H NMR/DMSO/d ppm:
11.15 (s, 1H), 10.06 (s, 2H), 9.44 (s, 2H), 7.82 (d, J ¼ 7.8 Hz, 2H),
7.61 (t, J ¼ 7 Hz, 2H), 7.27 (t, J ¼ 7.2 Hz, 1H). 13C NMR/DMSO/d
ppm: 180.3, 175.5, 164.2, 137.2, 133.5, 126.5, 122.5.
Rp;inhibit ꢁ Rp;uninhibit
IE% ¼
ꢀ 100
(2)
Rp;inhibit
where Rp,inhibit and Rp,uninhibit are the polarization resistance of
inhibited and uninhibited solutions, respectively.
To apply FFTCCV runs, a custom-made potentiostat was
connected to a PC equipped with a data acquisition board (PCL-
818HG, Advantech. Co.). The potential was repeatedly swept
between an initial and a nal potential, which are more negative
2.2 Carbon steel samples and solutions
A carbon steel sample containing (wt%) C 0.326%, Si 0.235%,
Mn 0.742%, P 0.016%, Cr 0.073%, Ni 0.0145%, Al 0.022%, S
0.017%, Cu 0.129%, V 0.001% and the rest iron, was used to
construct the working electrode. Before doing each run, the
electrode surface was polished with different grades of emery
papers (which ended with the 1200 grit), degreased in ultra-
sonic bath with ethanol and acetone and nally rinsed with
distilled water. The aggressive solution of 1.0 M HCl was
prepared by dilution of 36% HCl from Merck with distilled
water. In order to solve the solubility problems, the inhibitor
solutions were prepared in 1.0 M HCl containing 5% v/v DMSO.
Obviously, this volume of DMSO was also added to the blank
solution.
Fig. 2 Diagram of the applied potential waveform.
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and more positive than corrosion potential (Ecorr). As a result, immersion (mg), S is the total area of the carbon steel sheet
numerous cyclic voltammograms were recorded repeatedly. The (cm2), t is the corrosion time (h) and CR is the corrosion rate
sweep rate was equal to 1.0 V sꢁ1, which was the lowest sweep (mg cmꢁ2 hꢁ1). The surface coverage (q) and %IE are calculated
rate that could be accessible for the instrument. The potential using the following equations:33
waveform used in this technique is shown in Fig. 2.
CRuninhibit ꢁ CRinhibit
q ¼
(4)
(5)
CRuninhibit
2.5 SEM experiments
CRuninhibit ꢁ CRinhibit
A Hitachi 460 SEM was used to observe the surface morphology
of carbon steel samples before and aer 24 h immersion in
1.0 M HCl solution in the absence and presence of the
inhibitors.
IE% ¼
ꢀ 100
CRuninhibit
where CRuninhibit and CRinhibit are the values of carbon steel
corrosion rates in uninhibited and inhibited solutions, respec-
tively. From the table, it is obvious that with increasing the
inhibitors concentration, the corrosion rate decreases and
conversely, the inhibition efficiency increases. The data in Table
1 demonstrates that the maximum inhibition efficiency was
obtained for the solution containing 1.0 ꢀ 10ꢁ3 M TSC2. It is
concluded that for carbon steel corrosion inhibition in 1.0 M
HCl, TSC2 acts more efficiently than TSC1.
2.6 Quantum chemical calculations
Geometry optimizations were done using standard Gaussian-03
soware package27 with B3LYP functional and a 6-31G(d,p)
basis set.28,29 DFT methods are considered to be a standard
method for modeling many chemical processes.22,30 BLYP is a
popular functional which derived from the combination of
Becke31 for the exchange part and Lee, Yang and Parr32 for the
correlation part. The more broadly used, B3LYP, is a hybrid
functional in which the exchange energy, from Becke's three-
parameter functional (B3), is combined with the exact energy
from Hartree–Fock theory.
The calculated quantum chemical parameters are energy of
the Highest Occupied Molecular Orbital (EHOMO), energy of the
Lowest Unoccupied Molecular Orbital (ELUMO), energy gap
(DE ¼ ELUMO ꢁ EHOMO), dipole moment (D), electron affinity (A)
and hardness (h).
3.2 Polarization measurements
Fig. 3a and b show Tafel polarization plots of carbon steel in
1.0 M HCl without and with different concentrations of thio-
semicarbazide compounds. Fig. 3 represents that both anodic
and cathodic current densities decreased in the presence of the
investigated compounds. This decrease is more pronounced
with the growth in inhibitors concentration. This observation
demonstrates that the inhibitors are adsorbed on the carbon
3 Results and discussion
3.1 Weight loss measurements
The values of the inhibition efficiencies obtained from the
weight loss measurements are presented in Table 1 for different
concentrations of the inhibitors. The corrosion rate (CR) is
calculated from the following equation:33
m1 ꢁ m2
CR ¼
(3)
St
where m1 is the mass of the carbon steel sheet before immersion
(mg), m2 is the mass of the mass of the carbon steel sheet aer
Table 1 Weight loss results of carbon steel corrosion in 1.0 M HCl
solution in the absence and presence of various concentrations of the
inhibitors TSC1 and TSC2
Inhibitor
Cinh (M)
CR (mg cmꢁ2 hꢁ1
)
q
IE (%)
Blank
TSC1
—
5.31
0.92
0.81
0.73
0.53
0.60
0.47
0.43
0.39
—
1.0 ꢀ 10ꢁ4
5.0 ꢀ 10ꢁ4
8.0 ꢀ 10ꢁ4
1.0 ꢀ 10ꢁ3
1.0 ꢀ 10ꢁ4
5.0 ꢀ 10ꢁ4
8.0 ꢀ 10ꢁ4
1.0 ꢀ 10ꢁ3
0.83
0.85
0.86
0.90
0.89
0.91
0.92
0.93
82.7
84.7
86.3
90.0
88.7
91.1
91.9
92.7
TSC2
Fig. 3 Polarization curves of carbon steel electrode in 1.0 M HCl
solution without and with different concentrations of (a) inhibitor TSC1
and (b) inhibitor TSC2.
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Table 2 Polarization parameters for carbon steel in 1.0 M HCl solution
in the absence and presence of various concentrations of the inhibi-
tors TSC1 and TSC2
ꢁEcorr Icorr
ꢁbc
ba
IE
Inhibitor Cinh (M)
(mV) (mA cmꢁ2) (mV decꢁ1) (mV decꢁ1) (%)
Blank
TSC1
—
455
461.30
82.58
73.60
47.14
51.04
41.12
35.20
137
141
133
141
142
141
150
111
115
118
121
131
138
137
—
1.0 ꢀ 10ꢁ4 437
5.0 ꢀ 10ꢁ4 427
1.0 ꢀ 10ꢁ3 428
1.0 ꢀ 10ꢁ4 421
5.0 ꢀ 10ꢁ4 416
1.0 ꢀ 10ꢁ3 415
82.1
84.0
89.8
88.9
91.1
92.4
TSC2
steel surface. Consequently, this adsorption reduces both
anodic dissolution of iron at anodic sites and cathodic evolu-
tion of hydrogen at cathodic sites. As the inhibitors concen-
tration rises, the extent of adsorption increases, leading to
increased inhibition efficiency. The obtained corrosion
parameters including corrosion current density (Icorr), corrosion
potential (Ecorr) and electrochemical kinetic parameters i.e.
anodic and cathodic Tafel slopes (ba and bc) for different
concentrations of inhibitors are listed in Table 2.
Table 2 shows that the maximum and minimum inhibition
efficiencies were respectively obtained for the solutions con-
taining 1.0 mM of compounds TSC2 and TSC1, which is in
agreement with the weight loss results. The values of Tafel
slopes represent alteration in both anodic and cathodic slopes,
showing the effect of the inhibitors on both anodic and
cathodic processes. From Table 2, the maximum shi in Tafel
curves is equal to 40.0 mV, which is related to the presence of
1.0 mM of TSC2 in HCl solution. These observations approve
that all investigated compounds are mixed-type or adsorptive
inhibitors with primarily anodic inhibition effect.34–37
Fig. 4 Nyquist impedance plots of carbon steel electrode obtained in
1.0 M HCl solution in the presence of various concentrations of (a)
TSC1, (b) TSC2.
3.3 EIS measurements
Nyquist plots of carbon steel electrode in 1.0 M HCl in the
absence and presence of different concentrations of the inhibi-
tors TSC1 and TSC2 are represented in Fig. 4a and b, respectively.
It is obvious from Fig. 4 that the impedance diagrams of inves-
tigated inhibitors simply yield depressed semicircles. As seen in
the gure, the diameter of the semicircles increases with
increasing the inhibitors concentration. The simple –R(CR)–
model could well describe such situation at the metal/solution
interface. The capacitor element is referred to double layer
between the charged metal surface and the solution. Such
depressed semicircle shape in the Nyquist plots are indication of
Fig. 5 Electrical equivalent circuit used for demonstrating carbon
steel/solution interface.
phase element. Such equivalent circuit has been previously used
to model the metal/acid interface.44,45 A CPE has two parameters:
Q and n. The impedance of this element is given by the following
equation.44
ZCPE ¼ Qꢁ1(ju)ꢁn
(6)
non-ideality in capacitive behavior. this non-ideality is attributed where Q is the magnitude of the CPE, j is the imaginary number,
to frequency dispersion resulting from different physical u is the sine wave angular frequency (u ¼ 2pf, the frequency in
phenomena such as inhomogeneities, impurities, grain bound- Hz) and ꢁ1 # n # 1 is the phase shi, which gives details about
aries and also mass transport resistance.38–41 So, in the equivalent the degree of surface inhomogeneity. When n ¼ 0, the CPE
circuit, which was developed to t the experimental data, a converts to a pure resistor. If n ¼ +1, the CPE becomes a pure
capacitive element (ideal capacitor) is replaced by a constant capacitor where CPE characterizes Cdl, and if n ¼ ꢁ1, the CPE
phase element (CPE). Fig. 5 shows the corresponding equivalent becomes an inductor.43,44 In fact, when n is close to 1, the CPE is
circuit, where Rp is the polarization resistance, Rs is the solution similar to a capacitor, but the phase angle is not 90ꢂ. It is
resistance and as mentioned earlier, CPE represents a constant constant and somewhat less than 90ꢂ at all frequencies.
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Table 3 Electrochemical impedance parameters for carbon steel in
1.0 M HCl solution in the absence and presence of various concen-
trations of the inhibitors TSC1 and TSC2
Besides, the values of phase shi (n) in Table 3 are in the range
between 0.86 and 0.91. In view of that, there is not any
substantial change in the value of n in the absence and in the
presence of inhibitors under investigation, indicating that the
charge transfer process controls the dissolution mechanism of
carbon steel in 1.0 M HCl solution in the absence and presence
of studied inhibitors. Like weight loss and polarization results,
EIS measurements point out that the order of IE% for the
inhibitors is TSC2 > TSC1.
Inhibitor Cinh (M)
Rp (U cm2)
n
CPE (mUꢁ1 sn cmꢁ2
)
IE (%)
Blank
TSC1
—
57.8
0.86 305.7
0.86 35.2
0.86 30.1
0.88 26.5
0.89 29.5
0.88 18.7
0.91 17.4
—
1.0 ꢀ 10ꢁ4 334.2
5.0 ꢀ 10ꢁ4 378.8
1.0 ꢀ 10ꢁ3 538.8
1.0 ꢀ 10ꢁ4 484.7
5.0 ꢀ 10ꢁ4 596.9
1.0 ꢀ 10ꢁ3 861.1
82.7
84.7
89.3
88.1
90.3
93.3
TSC2
3.4 SEM studies
The surface morphology of the carbon steel samples was
examined by SEM aer the sample was immersed in 1.0 M HCl
solution in the absence and presence of the inhibitors. Fig. 6a
shows the morphology of freshly polished carbon steel surface.
It could be seen that there are some imperfections and
mechanical faults on the polished surface. The carbon steel
surface, which has been immersed in 1.0 M HCl solution in the
absence of inhibitors, is shown in Fig. 6b. The roughened
corroded surface and presence of considerable amounts of
corrosion products are indications for acidic corrosion of
carbon steel surface in noticeable extent. Fig. 6c and d represent
the SEM images of carbon steel surface aer immersion in
acidic solution containing 1.0 ꢀ 10ꢁ3 M of the inhibitors TSC1
and TSC2, respectively. These images clearly show the more
protected carbon steel surface, which results from the adsorp-
tion of inhibitors on the steel surface and reduction of the
corrosion rate. This protection is more obvious for the solution
containing the inhibitor TSC2.
The IE% values were calculated using Rp values by previ-
ously mentioned equation (eqn (2)). The polarization resis-
tance (Rp) must be corresponding to the resistance between
the metal and outer Helmholtz plane (OHP) and could be
simply calculated from the difference in impedance at lower
and higher frequencies.46–48 Calculated EIS parameters,
namely Q, Rp and n are presented in Table 3. These data
revealed that aer the addition of inhibitors Rp values
increased and Q values decreased. It can be concluded that the
adsorption of inhibitor molecules at the steel surface causes a
reduction in the local dielectric constant and growth in the
thickness of electrical double layer, leading to decrease in Q
values and increase in Rp values. In fact, replacement of water
molecules and other ions originally adsorbed on the surface
(with high dielectric constant) with organic inhibitor mole-
cules (with low dielectric constants) creates these changes in
electrical characteristics of the carbon steel/solution interface.
Fig. 6 SEM images of carbon steel for freshly polished surface (a) and after 24 h immersion in 1.0 M HCl solution without inhibitors (b) and with
1.0 ꢀ 10ꢁ3 M of TSC1 (c) and TSC2 (d).
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3.5 FFTCCV measurements
FFTCCV was used to continuously monitor the changes of
current passed through the electrode during the immersion
time. These changes are related to the adsorption behavior of
the inhibitors on the electrode surface. In this technique, the
potential waveform was continuously applied during the
experiment and variation of the charge under peak was moni-
tored. As a result, a large number of CV curves were recorded
and displayed in real time, which could be used to study the
changes in carbon steel surface and the adsorption process.
In order to extract the information about the adsorption
processes and the changes in the double layer at the electrode
surface,49 the background current in voltammetric measure-
ments must be removed from the recorded CV curves. The
following equation was used for the background subtraction in
the soware program.50–52
Di(s, E) ¼ i(s, E) ꢁ i(sr, E)
(7)
where s is the sweep number, i(s, E) represents the CV curve
recorded during the s-th sweep and i(sr, E) is the reference CV
curve. The reference CV curve was obtained by averaging three
CV curves recorded at the beginning of each experiment.
In practice, CV curves are recorded numerically by sampling
current in equal time intervals. As a result, we will have differ-
ential cyclic voltammograms, which have been obtained using
eqn (3). Fig. 7a shows the differential CV curves of the carbon
steel electrode in HCl solution without any inhibitors. Fig. 7b
and c show differential CV curves of the electrode in HCl solu-
tion in the presence of 5.0 ꢀ 10ꢁ4 M TSC1 and TSC2, respec-
tively. For each experiment, the electrode was immersed for
70 minutes in the acidic solution. As expected, the net current
change for the blank solution (up to 8.0 mA) is larger than the
electrode in the inhibitor containing solutions. The net change
of corrosion current for the electrode in the solution containing
inhibitors TSC1 and TSC2 are respectively around 4.3 mA and
3.5 mA, which obtained from the maximum of the differential
CV curves. These results approve that both of the investigated
thiosemicarbazide derivatives inhibit the corrosion of carbon
steel in HCl solution, and so reduce the change of current which
is due to the corrosion process in real time. Results also conrm
the weight loss, Tafel, EIS and SEM results which indicate better
performance of TSC2 rather than TSC1.
Moreover, Overlay differential CV curves in Fig. 7b and c
show that at the start of the experiment, the slope of the curves
maximums versus time is relatively large and at the end, it
progressively becomes smaller until reaches to unchanging
values. The time for achievement of such state for TSC1 and
TSC2 is about 3500 s and 2800 s, respectively. It could be
concluded that the adsorption of inhibitor TSC2 took place
considerably faster than the inhibitor TSC1. Therefore, at a
shorter time, TSC2 reaches to its maximum surface coverage.
For giving further information about the adsorption
behavior of studied inhibitors, the integration of current at the
specied potential range was performed in real time. The
following equation is theoretical approach for calculation of the
Fig. 7 Differential Continuous Cyclic Voltammogram of carbon steel
electrode recorded during 70 minutes immersion in 1.0 M HCl solution
without (a) and with 5.0 ꢀ 10ꢁ4 M of compound TSC1 (b) compound
TSC2 (c).
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Fig. 8 Plot of total charge loaded by the steel electrode (DQ) vs. time
in the blank solution and the solution containing 5.0 ꢀ 10ꢁ4 M of TSC1
and TSC2.
Fig. 9 Langmuir adsorption isotherm model for TSC1 and TSC2 on
carbon steel surface.
isotherm, which is shown in Fig. 9. The Langmuir isotherm is
charge under a cyclic voltammogram at the potential range E1 to
E2:
dened according to eqn (10):56,57
Cinh
1
ð
E2
¼
þ Cinh
(10)
1
q
Kads
Qt ¼
I
dE
(8)
ðEÞ
n
E1
where q denotes the surface coverage, Cinh is the inhibitor
concentration and Kads is the adsorption equilibrium constant.
The value of Kads process is related to the standard free energy of
adsorption, DGads, and equated as below:56,57
where n is the scan rate.
As previously mentioned, CV curves are recorded numeri-
cally by sampling current in equal time intervals. Therefore, we
used the following equation to integrate the current over the
potential range, E1 to E2, and calculate the total charge DQ:53–55
DGads ¼ ꢁRT ln 55.5Kads
(11)
!
E¼E2
X
X
ꢀ
DQðs; t ¼ Dt
Iðs; EÞ ꢁ E¼E2 iðsr; EÞ
(9)
E¼E1
E¼E1
In this equation, R is the gas constant (8.314 J Kꢁ1 molꢁ1), T
is the absolute temperature (K) and 55.5 is the concentration of
water in the solution (M).58 The thermodynamics parameters
derived from Langmuir adsorption isotherms for the studied
inhibitors are given in Table 4. The negative values of DGads
demonstrate a spontaneous adsorption process.59,60 Generally,
the energy values of ꢁ20 kJ molꢁ1 or less negative are related
with an electrostatic interaction between charged molecules
and charged metal surface (physisorption); the values of ꢁ40 kJ
molꢁ1 or more negative involve charge sharing or transfer
between the inhibitor molecules to the metal surface to form a
coordinate bond (chemisorption).58,61 The values of DGads
between ꢁ40 kJ molꢁ1 and ꢁ20 kJ molꢁ1 demonstrate mixed
type adsorption (both chemical and physical interactions). As
shown in Table 4, the values of DGads for TSC1 and TSC2 are
ꢁ36.6 kJ molꢁ1 and ꢁ39.1 kJ molꢁ1, respectively. These values
propose that the adsorption of these compounds involves two
types of interaction, chemisorption and physisorption.
where s is the sweep number, t is the time period between
subsequent sweeps, Dt is the time difference between two
subsequent points on the CV curves, i(s, E) represents the CV
curve recorded during the s-th sweep and I(sr, E) is the reference
CV curve.
Fig. 8 represents the plot of DQ as a function of time for
carbon steel electrode during 70 minutes immersion 1.0 M HCl
in the absence and presence of 5.0 ꢀ 10ꢁ4 M TSC1 and TSC2.
The slope of this plot in the solution containing inhibitors is
considerably more than the slope in the solution without any
inhibitors. This reduction seen in the slope demonstrates that
in the presence of the inhibitors, the corrosion rate of carbon
steel decays signicantly. The results, also, indicate that in the
presence of inhibitor TSC2, this decay is in its maximum value.
3.6 Adsorption isotherm
In order to gain more information about the mode of the
studied inhibitors adsorption on the carbon steel surface, the
data from weight loss measurements have been used to explain
the best isotherm that denes the adsorption process. In order
to nd the best adsorption isotherm, which describe the
adsorption of TSC1 and TSC2 on the carbon steel surface,
several isotherms Langmuir, Temkin and Frumkin, have been
tested. However, the best t is obtained with the Langmuir
Table 4 Thermodynamic parameters for the adsorption of TSC1 and
TSC2 in 1.0 M HCl on the carbon steel surface at 25 ꢂC
Inhibitor
R2
Kads (Mꢁ1
)
DGads (kJ molꢁ1
)
TSC1
TSC2
0.998
0.999
46220.9
130679.5
ꢁ36.6
ꢁ39.1
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However, the DGads values in this study, are more close to depends not only on the presence of HOMO and LUMO density
ꢁ40 kJ molꢁ1 demonstrating that the adsorption of TSC1 and on the special group, which is oriented toward the steel surface,
TSC2 occurs predominantly by chemisorption.39,61
but also on the approachability of the interaction site, the partial
charge and the local reactivity of the atom or group. In order to
nd a logical explanation for the relative inhibition effect of the
studied thiosemicarbazide derivatives, we need to consider the
value of quantum chemical parameters in parallel with the
spatial and electronic structure of the studied inhibitors. Table 5
shows some quantum chemical parameters of the studied
inhibitors, which calculated with DFT method at B3LYP/6-
31g(d,p) level of theory and some related equations.
EHOMO is related to the electron donating ability of the
molecule. A molecule with higher value of EHOMO is more
capable of giving electrons to an acceptor having appropriate
vacant molecular orbitals. On the other hand, ELUMO is related
to the electron accepting ability of the molecule. Alternatively,
the lower the value of ELUMO indicates that it is more probable to
receive electrons from an applicable donnor.63,64 Based on
experimental results, the order of inhibition efficiency for the
studied inhibitors is TSC2 > TSC1. From Table 5, the values of
EHOMO for the compounds TSC1 and TSC2 are ꢁ5.90 eV and
ꢁ6.21 eV, respectively. If we consider that the investigated
inhibitors just donate electrons to the Fe atoms at steel surface,
we will consequently expect that the molecule with highest
value of EHOMO is the most effective inhibitor. However,
3.7 Molecular geometries and molecular orbital distribution
The optimized geometries of the molecules under investigation
are presented in Fig. 10. These congurations are obtained by
Gaussian03 package using DFT method at B3LYP/6-31G(d,p)
level of theory, based on the absence of imaginary frequencies.
It could be seen that the inhibitor molecules are not fully
planar, which may affect their inhibition efficiencies in compar-
ison with similar structures with planar geometries. However, the
planarity is just one of the factors affecting the inhibition
performance of the molecules. Many other factors should be
considered to evaluate the inhibitors effectiveness. It was shown
that the effectiveness of an inhibitor depends on its electronic
structure, in addition to its spatial molecular structure.
Consistent with Frontier Molecular Orbital (FMO) theory,
developed by Kenichi Fukui,62 the frontier orbitals (HOMO and
LUMO) of chemical species play a major role in describing its
reactivity. Chemical reactions could be explained by interactions
between HOMO and LUMO on one or more molecules. FMO
theory uses these foundational ideas to describe the structure
and reactivity of molecules. In corrosion inhibition studies, FMO
theory is useful in guessing the adsorption centers of the inhib-
itor that are responsible for the interaction with metal surface.16,18
The HOMO and LUMO density distribution of the studied
molecules are shown in Fig. 10. For these inhibitors, the NH2
group at one end of the molecule has minimum or no density of
the frontier molecular orbital distributions. Other parts of the
molecules have contributions from the HOMO and LUMO pop-
ulations. We should consider that the center of adsorption
Table 5 Some quantum chemical parameters for the studied inhibi-
tors calculated using B3LYP/6-31G(d,p)
Inhibitor EHOMO (eV) ELUMO (eV) DE (eV) D (Debye) A (eV) h (eV)
TSC1
TSC2
ꢁ5.90
ꢁ6.21
ꢁ0.50
ꢁ1.75
5.41
4.46
1.65
4.23
0.50
1.75
2.70
2.23
Fig. 10 Optimized structure, HOMO, and LUMO distribution of (a) TSC1 and (b) TSC2, calculated using DFT method at B3LYP/6-31G(d,p) level of
theory.
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experimental results do not conrm this assumption. It could be
concluded that some other factors affect obtained experimental
results. In previous works, it was found that the inhibitors that can
accept electrons from metallic surface along with donate electrons
to the surface would be better inhibitors.42,65 As mentioned above,
the value of ELUMO is a good indicator for the electron accepting
ability of a molecule. The values of ELUMO for TSC1 and TSC2 are
ꢁ0.50 eV and ꢁ1.75 eV, successively. According to the calculated
values of ELUMO and experimental ndings, the reduction in
ELUMO values leads to increase in inhibition effect of the inhibi-
tors. So, the values of ELUMO could well describe the experimental
results. The next parameter, DE ¼ ELUMO–EHOMO, is an important
parameter to estimate the reactivity of a molecule. As DE
decreases, the reactivity of the molecule increases, leading to an
increase in adsorption of inhibitor molecules on the metallic
surface and consequently an increase in the IE%. Table 5 shows
that the value of DE for TSC2 is lower than TSC1 (4.46 eV < 5.41
eV), which is in good agreement with the experimental results.
The next calculated quantum chemical parameter is dipole
moment (D). Some authors state that %IE increases with
increasing the value of the dipole moment. However, in some
cases no signicant relationship has been found between the
dipole moment values and the IE%.66,67 In our study, increasing
the dipole moment of the molecules leads to increase their inhi-
bition efficiency as a result of increasing the intermolecular forces.
In order to nd proper quantum parameters to describe and
predict the experimental results, we calculated some parameters
using the parameters, which computed directly with DFT method.
Electron affinity (A) is the change in energy of an atom or molecule
(in the gaseous phase) when an electron is added to the atom or
molecule to form a negative ion. The electron affinity is directly
related to the energy of the LUMO:68
4 Conclusions
(1) The investigated thiosemicarbazide derivatives (TSC1 and
TSC2) inhibit the corrosion of carbon steel in 1.0 M HCl and
their inhibition efficiency follows the order: TSC2 > TSC1.
(2) The inhibitors are mixed type and adsorbed on the steel
surface to block its active sites. This adsorption extends the
double layer distance leading to a decrease in the double layer
capacitance.
(3) FFTCCV experiments show that the inhibitor TSC2 rea-
ches to its maximum surface coverage at a shorter time than the
inhibitor TSC1, indicating that the rate of adsorption of TSC2 is
faster than adsorption of TSC1.
(4) The adsorption of TSC1 and TSC2 molecules on the
carbon surface in 1 M HCl solution obeyed Langmuir adsorp-
tion isotherm.
(5) The experimental results are in good association with the
parameters ELUMO, DE, dipole moment and hardness.
Acknowledgements
The authors express their appreciation to the University of
Tehran Research Council for nancial support of this work.
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