Available online at www.sciencedirect.com
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Journal of Taibah University for Science 10 (2016) 139–147
Investigation of the corrosion inhibition effect of
3-methyl-6-oxo-4-(thiophen-2-yl)-4,5,6,7-tetrahydro-2Hpyrazolo[3,4-b]pyridine-5-carbonitrile (TPP) on mild steel in
hydrochloric acid
Priyanka Singh a , M.A. Quraishi a,∗ , S.L. Gupta b , A. Dandia b
a
Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India
b Centre of Advance Studies, Department of Chemistry, University of Rajasthan, Jaipur 302004, India
Received 23 December 2014; received in revised form 9 July 2015; accepted 14 July 2015
Available online 10 November 2015
Abstract
The corrosion inhibition effect of the synthesized compound 3-methyl-6-oxo-4-(thiophen-2-yl)-4,5,6,7-tetrahydro-2Hpyrazolo[3,4-b]pyridine-5-carbonitrile (TPP) on the corrosion of MS in 1 M HCl was investigated using weight loss, potentiodynamic
polarization and electrochemical impedance spectroscopy (EIS) methods. The results obtained for TPP showed a good inhibition
effect on the MS in the 1 M HCl solution. The inhibition efficiency was found to increase with a corresponding increase in the TPP
concentration, and the best inhibition of 96.83% occurred at a TPP concentration of 200 ppm. The potentiodynamic polarization
studies revealed that TPP behaved as mixed-type of inhibitor that predominantly inhibited the cathodic reaction. The impedance
parameters were indicative of the high inhibitory efficiency and the adsorption of TPP on MS. Amongst the Frumkin, Temkin
and Langmuir isotherms, the studied compound more closely followed the Langmuir adsorption isotherm. The scanning electron
microscopy (SEM) examinations of the electrode surface confirmed the existence of such an adsorbed film.
© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article under
the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Mild steel (MS); Acid solution; Weight loss; Electrochemical measurements; SEM
1. Introduction
∗
Corresponding author. Tel.: +91 9307025126;
fax: +91 542 2368428.
E-mail
addresses:
[email protected],
[email protected] (M.A. Quraishi).
Peer review under responsibility of Taibah University.
Mild steel is the most widely used metal in industrial
applications due to its excellent mechanical properties
and low cost [1–3]. Acids are used in industry during
chemical cleaning, descaling, pickling, oil-well acidizing, petrochemical processes, erecting boilers, drums,
heat exchangers, tanks, etc., and this leads to a corrosive
attack [4–7]. Corrosion inhibitor solutions are widely
used to prevent or minimize material loss for materials
that are in contact with acid solutions. Heterocyclic
http://dx.doi.org/10.1016/j.jtusci.2015.07.005
1658-3655 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article under the
CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
140
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
NH 2
CN
S
CHO
+
COOE
Et
+
N
H
S
p-TSA
N
H 2O
CN
HN
N
thiop
phene-2carba
aldehyd e
etthyl cyano ac
cetate
3-am
mino-5metthylpyrazole
N
O
H
3-meth
hyl-6-oxo-4-((thiophen-2--yl)4,5,6,7-tetrahydro--2H-pyrazolo[3,4b]pyrid
dine-5-carbo
onitrile(TPP)
Scheme 1. Synthetic route for TPP.
compounds containing sulfur, oxygen, nitrogen and
aromatic rings are very effective and efficient inhibitors
for the metals in acidic medium [8–17].
This work was intended to state the inhibitory
properties of the synthesized TPP compound on the corrosion of MS in 1 M HCl, and these properties were
determined through the utilization of weight loss, potentiodynamic polarization, and electrochemical impedance
spectroscopy (EIS) techniques. The mild steel surface
morphologies in 1 M HCl with and without the presence of TPP were also studied. A survey of the literature
revealed that TPP had not been investigated as a corrosion inhibitor until now.
2. Experimental
2.1. Inhibitor
TPP was synthesized according a previously
described method [18]. The outline of the synthesis is
given in Scheme 1. The melting points were recorded on
a Toshniwal apparatus and are uncorrected. The purity
of the compounds was confirmed on thin layers of silica gel in various non-aqueous solvent systems, e.g.,
benzene: ethyl acetate (9:1) and n-hexane: ethyl acetate
(7:3). IR spectra (KBr) were recorded using a Shimadzu FT IR-8400s spectrophotometer, and 1 H NMR
and 13 C NMR spectra were recorded using a Bruker
DRX-300 instrument at 300 MHz and 75 MHz, respectively, in DMSO-d6 relative to tetramethylsilane as an
internal reference. The mass spectra of the representative
compounds were recorded using a Waters Xevo Q-Tof
spectrometer at 70 eV, as shown in Table 1.
2.2. Materials and solutions
The mild steel possessed the following composition
(wt %) and was used for the weight loss studies: C
0.17%; Mn 0.46%; Si 0.026%; Cr 0.050%; P 0.012%;
Cu 0.135%; Al 0.023%; Ni 0.05%; and balance Fe. The
dimensions of the steel for the weight loss study and the
electrochemical studies were 2.5 cm × 2 cm × 0.025 cm
and 8 cm × 1 cm × 0.025 cm, respectively, and the samples were subsequently abraded with SiC abrasive papers
of grade 600, 800, 1000 and 1200; washed with acetone; and finally dried at the ambient temperature. In the
electrochemical study, a 1 cm2 area of the MS strip was
exposed and was utilized as the working electrode; the
rest of the MS was covered with epoxy resin. The test
solution of 1 M HCl was prepared by diluting analytical
grade 37% HCl with double-distilled water.
2.3. Weight loss measurements
Weight loss experiments were performed in the
absence and presence of TPP at different concentrations
according to standard methods [19,20]. The corrosion
Table 1
Structure and analytical data for TPP.
Structure
Analytical data
S
CN
HN
N
N
O
H
3-methyl-6-oxo-4-(thiophen-2-yl)-4,5,6,7-tetrahydro-2Hpyrazolo[3,4-b]pyridine-5-carbonitrile(TPP)
White crystalline solid; IR (KBr) cm−1 : 3570, 3282, 2248, 1716, 1534;
1 H NMR (300 MHz, DMSO-d )?: 2.07 (s, 3H, CH ), 4.38 (d,
6
3
3J = 12.0 Hz, 1H, CH), 4.64 (d, 3J = 11.7 Hz, 1H, CH), 6.97–7.51 (m,
ArH, 3H), 10.99 (s, 1H, NHamid ), 12.10 (br s, 1H, NHpyrazole ) ppm. 13 C
NMR (75 MHz, DMSO-d6 )?: 10.01, 35.54, 42.39, 101.96, 117.07,
125.96, 126.21, 127.73, 135.07, 142.66, 146.84, 162.52 ppm; +ESI MS
(m/z): 259 [M+H]+
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
141
Table 2
Parameters obtained from weight loss measurements for MS in 1 M HCl containing different concentrations of TPP.
Inhibitors
Concentrations (ppm)
Corrosion rate (mm/y)
Surface coverage (θ)
η%
Blank
0.0
50
100
150
200
77.9
14.0
8.9
5.5
4.4
–
0.81
0.88
0.92
0.94
–
81.90
88.57
92.85
94.28
rate (CR ), inhibition efficiency (η%) and surface coverage (θ) were determined using the following equations:
CR (mm/y) =
87.6W
atD
C R − i CR
η% =
× 100
CR
θ=
CR − i CR
CR
3. Results and discussion
3.1. Weight loss measurements
(1)
(2)
(3)
where W is the average weight loss of MS specimens, a is
the total surface area of the MS specimen, t is the immersion time (3 h) and D is the density of MS in (g cm−3 ),
and in Eq. (2), CR and i CR are the corrosion rates of
the MS in the absence and presence of the inhibitors,
respectively.
2.4. Electrochemical measurements
The electrochemical experiments were performed
by using a three-electrode cell that was connected to
a Potentiostat/Galvanostat G300-45050 (Gamry Instruments Inc., USA). The Echem Analyst 5.0 software
package was used for the fitting of the data. MS was
used as the working electrode, a platinum electrode was
used as an auxiliary electrode, and a saturated calomel
electrode (SCE) was used as the reference electrode.
All potentials reported were measured vs. the SCE.
Tafel curves were obtained by changing the electrode
potential automatically from −0.25 V to +0.25 V vs. an
open circuit potential at a scan rate of 1.0 mV s−1 . EIS
measurements were performed under potentiostatic conditions over a frequency range from 100 kHz to 0.01 Hz
with an amplitude of 10 mV in the AC signal.
2.5. Surface analysis
The surfaces of the MS samples were studied by SEM
(model FEI Quanta 200F microscope) at 500× magnification. The MS samples were observed after 3 h of
immersion in the absence and presence of a 200 ppm
TPP solution.
3.1.1. Effect of inhibitor concentration
The corrosion inhibition efficiencies (η%) of the
inhibitor (TPP) at different concentrations were evaluated by the weight loss technique, and the results are
listed in Table 2. It is clear from the results that the
weight loss decreased and the η% increased with a corresponding increase in the concentration of the inhibitor
[21,22]. The maximum inhibition efficiency was 94.2%,
and it was obtained at 200 ppm.
3.1.2. Effect of temperature
The activation energy (Ea ), enthalpy (H*), and
entropy (S*) values shown in Table 3 were evaluated
by performing weight loss measurements in the temperature range of 308–338 K in the absence and presence
of the optimum concentration of TPP. The activation
energy (Ea ), enthalpy (H*), and entropy (S*) were
calculated using the following equations:
−Ea
+A
RT
S ∗
H ∗
RT
exp
exp −
CR =
Nh
R
RT
ln(CR ) =
(4)
(5)
where Ea is the activation energy for corrosion of MS
in 1 M HCl, R is the gas constant, A is the Arrhenius
pre-exponential factor, T is the absolute temperature, h
is Plank’s constant and N is Avogadro’s number.
A plot of the corrosion rate ln CR vs. 1000/T resulted
in straight lines, as shown in Fig. 1. The values of
Ea in 1 M HCl in the absence and presence of the
Table 3
Thermodynamic parameters in absence and presence of optimum concentration of TPP.
Inhibitors
Ea (kJ mol−1 )
H* (kJ mol−1 )
S* (J K−1 mol−1 )
Blank
Inhibitor
29.10
60.35
26.42
57.67
−123.27
−45.18
142
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
Fig. 1. Arrhenius plots of log CR vs. 1000/T for MS in 1 M HCl in the
absence and the presence of TPP at optimum concentration.
3.1.3. Adsorption isotherm
The adsorption isotherm can be determined by assuming that the inhibition effect was mainly due to adsorption
at the metal/solution interface. The adsorption process depends upon the electronic characteristics of the
TPP, the nature of the metal surface, the temperature,
steric effects and the varying degrees of the surface-site
activity. The surface coverage values, θ, for different concentrations of TPP were used to determine the best fitting
adsorption isotherm [25].
By fitting the degree of surface covered (θ) to the
adsorption isotherms, including the Frumkin, Temkin
and Langmuir isotherms, the best fit was obtained for
the Langmuir isotherm by plotting (C/θ) vs. C, which
resulted in a straight line with a coefficient (R2 ) value
close to unity, as shown in Fig. 3. The equation can be
represented as:
Kads =
θ
C(1 − θ)
(6)
The equation can be rearranged to:
inhibitor were determined from the slope. A plot of
ln (CR /T) against 1000/T, as shown in Fig. 2, shows
straight lines with a slope of (−H*/R) and an intercept of [(ln (R/Nh)) + (S*/R)], from which the values
of H* and S* were calculated.
The apparent increase in Ea and H* suggested the
creation of an energy barrier to the corrosion reaction in
the presence of the inhibitor. The higher value of S* for
the inhibited solution might be the result of the adsorption of the TPP molecules in the 1 M HCl solution (quasi
substitution) [23,24].
Cinh
1
+ Cinh
=
θ
Kads
(7)
where Cinh is the concentration of the inhibitor and θ is
the surface coverage.
Kads is related to the standard free energy of adsorption, Gads , through the following equation:
Gads = −RT ln(55.5Kads )
(8)
where R is the gas constant and T is the absolute temperature. The value of 55.5 is the concentration of water in
the solution in mol L−1 .
It has been reported in the literature that values of
Gads around −20 kJ mol−1 indicate electrostatic interactions i.e., physisorption. Conversely, a Gads value
near or greater than −40 kJ mol−1 indicates chemisorption [7,26]. The adsorption of the inhibitor molecules on
the metal surface cannot be considered to be a purely
physical or chemical phenomenon. In the present investigation, the calculated value for Gads at the optimum
concentration and 308 K was −38 kJ mol−1 , and this
suggests both a physical and chemical mode of adsorption [27,28].
3.2. Electrochemical measurements
Fig. 2. Arrhenius plots of log CR /T vs. 1000/T for MS in 1 M HCl in
the absence and presence of TPP at optimum concentration.
3.2.1. Potentiodynamic polarization measurements
Tafel polarization curves were recorded for the MS
in 1 M HCl solutions without and with the various concentrations of TPP; these curves are shown in Fig. 4.
The polarization data, including the corrosion potential
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
143
Fig. 3. (a) Frumkin, (b) Temkin and (c) Langmuir adsorption isotherms for of TPP on MS surface in 1 M HCl.
(Ecorr ), corrosion current density (Icorr ) and anodic (βa )
and cathodic (βc ) slopes, are listed in Table 4. The inhibition efficiency (η%) values were calculated from the
given equation:
η% =
Icorr − Icorr(i)
× 100
Icorr
(9)
where Icorr and Icorr(i) are the corrosion current densities
in the absence and presence of the inhibitor, respectively.
If the change in the Ecorr value was greater than
85 mV, a chemical compound could be recognized as
an anodic- or a cathodic-type inhibitor. The maximum
shift of Ecorr in the present case was 82 mV towards
the cathodic direction, which indicated that TPP was a
mixed type inhibitor and that it predominately inhibited
the cathodic reaction. It was clear from the results that
Fig. 4. Tafel curves for MS in 1 M HCl without and with different
concentrations of TPP.
Table 4
Polarization parameters of MS in 1 M HCl solution at different concentration of TPP derivatives.
Inhibitors
Concentrations (ppm)
Icorr (␮A cm−2 )
Ecorr (mV/SCE)
βa (mV/dec)
βc (mV/dec)
η%
Blank
TPP
0.0
100
150
200
1390
134
80
59
−445
−508
−516
−527
82
58
59
82
118
102
134
93
–
90.35
94.20
95.75
144
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
Fig. 6. Equivalent circuit model used to fit the EIS data.
Fig. 5. Nyquist plots for MS in 1 M HCl without and with different
concentrations of TPP.
the values of Icorr decreased with the increasing concentration of TPP, which indicated an effective corrosion
mitigation of the mild steel in 1 M HCl [29]. The maximum decrease in Icorr was 59 ␮A cm−2 at 200 ppm. The
anodic and cathodic Tafel slopes were found to change
with changes in the inhibitor concentration, indicating
that the anodic and cathodic corrosion were inhibited by
TPP [30,31].
3.2.2. Electrochemical impedance spectroscopy
(EIS)
The Nyquist loops and related parameters in the
absence and presence of TPP are shown in Fig. 5 and
Table 5. The Nyquist diagrams show a single semicircle
shifted along the real impedance axis (Zreal ), indicating
that the corrosion of MS in 1 M HCl was controlled by
a charge-transfer process. In the evaluation of Nyquist
plots, the difference in the real impedance at lower
and higher frequencies is commonly considered to be
a charge transfer resistance [32]. The equivalent circuit
model used for the analysis is shown in Fig. 6, and it consists of the solution resistance (Rs ), the charge-transfer
resistance (Rct ) and the constant phase angle element
(CPE). The inhibition efficiency was calculated using
the charge transfer resistance (Rct ) as follows:
η% =
Rct(inh) − Rct
× 100
Rct(inh)
(10)
where Rct(inh) and Rct are the values of the charge transfer
resistance in the presence and absence of inhibitors in
1 M HCl, respectively.
The results showed that the addition of different concentrations of TPP increased the Rct values and lowers
the Cdl [33,34]. The increase in Rct was attributed to the
increase in resistance. The decrease in the Cdl values was
attributed to the adsorption of TPP on the metal surface.
The values of the double layer capacitance, Cdl , can be
represented as:
Cdl = Y0 (ωmax )n−1
(11)
where Y0 is the CPE coefficient, n is the CPE exponent
(phase shift), and ω is the angular frequency. The thickness of this protective layer (d) is correlated with Cdl by
the following equation [35]:
Cdl =
εε0 A
d
(12)
where ε is the dielectric constant, ε0 is the permittivity
of the free space and A is the surface area of the electrode. A Bode plot [log |Z| vs. log f] and a phase angle
plot [α◦ vs. log f] are shown in Fig. 7. It is reported in
literature that an ideal capacitor phase angle and slope
value should be −90◦ and −1, respectively [36]. The
obtained results listed in Table 6, shows the deviations
of the Bode and phase angle values from the blank acidic
solution, which indicates the capacitive performance of
the inhibited system.
Table 5
Electrochemical impedance parameters and the corresponding inhibition efficiencies of mild steel in 1 M HCl in the absence and presence of different
concentrations of TPP.
Inhibitors
Concentrations
Rs ()
Rct ( cm2 )
n
Y0 (␮F cm−2 )
Cdl
η%
TPP
0.0
100
150
200
1.02
0.954
0.990
0.831
9.0
158.0
216.5
284.9
0.822
0.865
0.853
0.850
250
86
92
46
106
44
43
21
–
94.30
95.84
96.83
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
145
Fig. 7. Bode (log f vs. log |Z|) and phase angle (log f vs. α◦ ) plots
of impendence spectra for MS in 1 M HCl in presence of different
concentrations of TPP.
3.3. Surface analysis
The surface morphologies of the MS samples in the
presence and absence of TPP were observed through
SEM analysis. The changes that occurred in the MS samples are shown in Fig. 8(a–c). Fig. 8(a) shows the clean
polished surface of MS. Fig. 8(b) shows the MS surface
in 1 M HCl. Fig. 8(c) shows the MS surface in 1 M HCl
containing TPP. The smooth surface shown in Fig. 8(c)
of the MS was attributed to the adsorption of the inhibitor
molecules on the MS surface.
4. Mechanism of adsorption and inhibition
The mechanism of adsorption depends upon the interaction between the TPP molecules and the MS surface
in 1 M HCl. The adsorption of TPP on the MS surface
cannot solely be considered either physical or chemical. Because of this complex nature of adsorption, it is
difficult to explain it through a single adsorption mechanism. Therefore, the following adsorption and inhibiting
phenomena were proposed.
1. TPP in 1 M HCl exists in protonated form, which will
be in equilibrium with the corresponding neutral form
by the following equation:
[C12 H10 SN4 O] + xH+ ↔ [C12 H10+x SN4 O]x+
Table 6
Slopes of the Bode plots (−S) and maximum phase angles (α◦ ) for MS
in 1 M HCl solution with increasing concentrations of TPP.
Inhibitor concentration (ppm)
−α◦
−S
Blank
100
150
200
40.40
70.55
71.02
73.11
0.53
0.80
0.80
0.82
Fig. 8. SEM images of mild steel (a) freshly polished surface, (b)
exposed to 1 M HCl solution and (c) exposed to 1 M HCl containing
TPP.
146
P. Singh et al. / Journal of Taibah University for Science 10 (2016) 139–147
India, for the financial assistance and the facilitation of
our study.
References
Fig. 9. Pictorial representation of mechanism of adsorption of TPP on
MS surface.
The TPP molecules were adsorbed onto the mild
steel surface via physical adsorption between the
charged mild steel surface and the charged TPP
molecules.
2. The chemical adsorption of the TPP arises through
the donation of free electron pairs of heteroatoms to
vacant d-orbitals of the iron.
3. The donor–acceptor interactions between the ␲electrons of the aromatic ring and the d-orbitals of the
metallic surface atoms (retrodonation) [37–39]. The
skeletal representation of the adsorption of the TPP
molecule onto the mild steel surface is presented in
Fig. 9.
5. Conclusions
The results obtained for TPP showed that it acted as
a good inhibitor with a 96.8% inhibition efficiency at a
concentration of 200 ppm. The potentiodynamic polarization data indicated that the TPP was a mixed type
inhibitor with a predominance in the cathodic direction. EIS data revealed a decrease in the Cdl values that
resulted from either a decrease in the local dielectric
constant or an increase in the thickness of the electrical
double layer. The adsorption of the inhibitor molecules
on the mild steel surface was found to obey the Langmuir adsorption isotherm. All of these data supported
the theory of the good inhibition tendency of TPP.
Acknowledgement
Priyanka Singh would like to thank the Ministry of
Human Resource Development (MHRD), New Delhi,
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