Corrosion inhibition performance of pyranopyrazole derivatives for mild steel in HCl solution: Gravimetric, electrochemical and DFT studies

Article abstract of DOI:10.1016/j.molliq.2015.12.106

Two pyranopyrazole derivatives namely, 6-amino-4-(4-methoxyphenyl)-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (AMPC) and 6-amino-4-(4-chlorophenyl)-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (ACPC) were synthesized and investig

Full text of DOI:10.1016/j.molliq.2015.12.106

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Journal of Molecular Liquids 216 (2016) 78–86
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Corrosion inhibition performance of pyranopyrazole derivatives for mild
steel in HCl solution: Gravimetric, electrochemical and DFT studies
a
a
b
Mahendra Yadav a, , Laldeep Gope , Nilam Kumari , Premanand Yadav
⁎
a
Department of Applied Chemistry, Indian school of Mines, Dhanbad 826004, India
Department of Physics, Post Graduate College, Ghazipur 233002, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two pyranopyrazole derivatives namely, 6-amino-4-(4-methoxyphenyl)-3-methyl-2,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile (AMPC) and 6-amino-4-(4-chlorophenyl)-3-methyl-2,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile (ACPC) were synthesized and investigated as an inhibitor for mild steel corrosion in
15% HCl solution by using weight loss measurement and electrochemical techniques. The experimental results
showed that the inhibition efficiency of studied inhibitors increased with increasing inhibitor concentration
whereas decreased with an increase in temperature. Corrosion inhibition efficiency of 96.1 and 94.6 was obtained
with 300 ppm of AMPC and ACPC, respectively, at 303 K. Potentiodynamic polarization studies showed that both
studied inhibitors were mixed type in nature. The potential of zero charge (EPZC) for the mild steel was
determined by electrochemical impedance spectroscopy method. The adsorption of both inhibitors on mild
steel surface obeyed Langmuir adsorption isotherm. The surface morphology of the uninhibited and inhibited
mild steel specimens was studied by using scanning electron microscopy (SEM), energy dispersion X-ray
spectroscopy (EDX) and atomic force microscopy (AFM). The Density Functional Theory (DFT) was applied for
theoretical studies of the inhibitors.
Received 17 October 2015
Received in revised form 18 December 2015
Accepted 30 December 2015
Available online xxxx
Keywords:
Mild steel
EIS
Polarization
Corrosion inhibition
Density functional theory
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
with the metal surface. The pyranopyrazole derivatives consist of
pyran and pyrazole rings as well as sufficient number of π- electrons,
Mild steel is widely applied as a constructional material in a large
number of industries due to its excellent mechanical properties and
low cost. Although, mild steel finds wide range of technological applica-
tions, its poor corrosion resistance in acid environment, restrains its
utility. Hydrochloric acid solutions are commonly used for pickling,
industrial acid cleaning, acid de-scaling, and oil well acidifying process-
es. Because of the aggressiveness of acid solutions, mild steel corrodes
severely during these processes, particularly with the use of hydrochlo-
ric acid, which results in terrible waste of both resources and money.
Therefore a corrosion inhibitor is often added in the acid solutions to
minimize the corrosion of mild steel in these processes. The selection
of a good corrosion inhibitor is controlled by its economic availability,
its efficiency to inhibit the substrate material and its environmental
side effects. The organic compounds containing nitrogen, sulfur and/or
oxygen atoms have been reported as good inhibitor [1–7] for mild
steel corrosion in acid environment. The inhibiting action of these
compounds is due to the adsorption of these compounds to the
metal/solution interface. The adsorption process depends upon the
nature and surface charge of the metal, the type of aggressive
media, the structure of the inhibitor and the nature of its interaction
so it was expected that these compounds will easily adsorbed on the
metal surface and protect the metal from corrosion. Therefore in the
present investigation two pyranopyrazole derivatives AMPC and
ACPC were synthesized and their corrosion inhibition performance
was studied by means of weight loss measurement, potentiodynamic
polarization, electrochemical impedance spectroscopy (EIS) and
quantum chemical studies.
2.Materials and Methods2.2.Materials2.1.1.Mild steel sample
The mild steel used in the present study was having composition
(wt.%): C, 0.12; Mn, 0.11; Cu, 0.01; Si, 0.02; Sn, 0.01; P, 0.02; Ni, 0.02
and Fe balance. For weight loss and electrochemical measurements
mild steel specimens having dimension 3.0 cm × 3.0 cm × 0.1 cm and
1.0 cm × 1.0 cm × 0.1 cm respectively, were mechanically cut and
abraded with different grade emery papers (120, 220, 400, 600, 800,
1500 and 2000 grades). Before starting the experiment, specimens
were washed with distilled water, degreased in acetone, dried and
stored in vacuum desiccator.
2.1.2.Test solutions
For the present study, 15% HCl solution was prepared by dilution of
analytical grade 37% HCl (Ramkem) with double distilled water. For
weight loss and electrochemical studies 250 mL of the test solution
⁎
Corresponding author.
E-mail address: yadav_drmahendra@yahoo.co.in (M. Yadav).
http://dx.doi.org/10.1016/j.molliq.2015.12.106
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79
was used and the concentrations of the studied inhibitors ranged from
50 ppm to 300 ppm (mg L−1).
2.2.2.Electrochemical methods
Electrochemical studies were conducted in a three-electrode cell
consisting of mild steel specimen of 1 cm2 exposed area as working
electrode, a platinum counter electrode and a saturated calomel
electrode (SCE) as reference electrode, using CH electrochemical work-
station (Model No: CHI 760D) at 303 K. Potentiodynamic polarization
curves were obtained at a scan rate of 1 mV s−1 in the potential range
of −250 to +250 mV vs. SCE at open circuit potential. The linear Tafel
segments of anodic and cathodic curves were extrapolated to obtain
corrosion current densities (icorr). The percentage inhibition efficiency
(η%) was calculated using the equation.
2.1.3.Synthesis of corrosion inhibitor
The studied inhibitors, namely, AMPC and ACPC were synthesized by
reported experimental procedure [8] as shown in Scheme 1. The purity
of the synthesized compounds was checked by thin layer chromatogra-
phy (TLC).The melting point, yield, IR and 1HNMR data of the synthe-
sized compounds are given below:
AMPC
Yield (78%), m.p. = 174 °C.
i0corr − icorr
IR (ν/cm−1): 3120 (NH), 1680 (C = N), 1480 (C = C), 760 (C-N).
1H NMR (DMSO-d6) δppm: 11.2 (s, 1 H, N–H), 1.6 (s, 3 H, CH3), 4.6
(s, 1 H), 3.5 (s, 3 H, OCH3), 7.1–8.3 (m, 4 H, phenyl), 6.9 (s, 2 H, NH2).
ηð%Þ ¼
ꢀ 100
ð4Þ
i0corr
where, i0corr and icorr are the values of corrosion current density in the
absence and presence of inhibitor, respectively.
ACPC
EIS studies were performed in the frequency range of 100 kHz
to 10 mHz using AC signals of amplitude 10 mV peak to peak at the
open circuit potential. The inhibition efficiency (η%) was calculated
from charge transfer resistance values obtained from impedance
measurements using the following relation.
Yield (80%), m.p. = 176 °C.
IR (ν/cm−1): 3160 (NH), 1640 (C = N), 1460 (C = C), 740 (C-N).
1H NMR (DMSO-d6) δppm: 11.6 (s, 1 H, N–H), 1.62 (s, 3 H, CH3),
4.8 (s, 1 H), 7.4–8.2 (m, 4 H, phenyl), 6.8 (s, 2 H, NH2).
RctðinhÞ − Rct
2.2.Methods
ηð%Þ ¼
ꢀ 100
ð5Þ
RctðinhÞ
2.2.1.Weight loss measurements
Weight loss measurements were performed in the absence and
presence of different concentrations (50 to 300 ppm) of inhibitors at dif-
ferent temperatures (303–333 K) according to the standard method [1].
The corrosion rate (CR), inhibition efficiency (η%) and surface coverage
(θ) were determined by following equations:
where Rct(inh) and Rct are charge transfer resistance in the presence
and absence of inhibitor, respectively. The values of double layer capac-
itance (Cdl) were calculated from charge transfer resistance and CPE
parameters (Y0 and n) using the expression [9]:
ꢀ
ꢁ
1=n
1−n
Cdl
¼
Y0Rct
ð6Þ
8:76 ꢀ 104 ꢀ W
D ꢀ A ꢀ t
À
Á
CR mmy−1
¼
ð1Þ
where Y0 is CPE constant and n is CPE exponent. The value of n repre-
sents the deviation from the ideal behavior and it lies between 0 and 1.
where, W = weight loss (g), A = area of specimen (cm2) exposed in
acidic solution, t = exposure time (h), and D = density of mild steel
(g cm−3).
2.3.SEM and EDX analysis
The SEM and EDX images of the uninhibited and inhibited mild steel
specimens were recorded using the instrument HITACHI S3400N.
CR0 − CRi
θ ¼
ð2Þ
ð3Þ
CR0
2.4.Atomic force microscopy
CR0 − CRi
ηð%Þ ¼
ꢀ 100
CR0
The AFM images of uninhibited and inhibited mild steel specimens
were carried out using a Nanosurf Easyscan2 instrument.
where, CR0 and CRi are corrosion rates in the absence and presence of
inhibitors, respectively.
2.5.Quantum chemical study
Quantum chemical calculations were carried out in aqueous phase
using Density Functional Theory (DFT) method, B3LYP combined with
6-31G (d, p) basis set implemented in Gaussian 03 program package
[10,11]. Theoretical parameters such as the energy of the highest
occupied and lowest unoccupied molecular orbital (EHOMO and ELUMO),
energy gap (ΔE), dipole moment (μ) absolute electronegativity (χ),
global hardness (γ) and softness (σ), and fraction of electrons
transferred (ΔN) were determined.
3.Results and discussion
3.1.Weight loss measurements
3.1.1Effect of inhibitor concentration and temperature
Corrosion parameters for mild steel in 15% HCl solution were deter-
mined from weight loss measurement data in the absence and presence
of different concentrations (50–300 ppm) of the studied inhibitors at
Scheme 1. Synthetic route and structure of inhibitors.

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M. Yadav et al. / Journal of Molecular Liquids 216 (2016) 78–86
Table 1
Corrosion parameters obtained from weight loss measurements of mild steel in 15% HCl solution in the presence and absence of inhibitors at different temperatures.
Conc. (ppm)
303 K
313 K
323 K
333 K
CR (mmy−1
37.7
)
θ
η %
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
Blank
–
–
66.4
–
–
115.2
–
–
169.8
–
–
AMPC
50
100
150
200
300
9.38
4.93
2.33
1.54
1.47
0.751
0.869
0.938
0.959
0.961
75.1
86.9
93.8
95.9
96.1
18.72
10.49
5.57
3.98
3.78
0.718
0.842
0.916
0.940
0.943
71.8
84.2
91.6
94.0
94.3
37.90
23.15
14.05
10.59
10.13
0.671
0.799
0.878
0.908
0.912
67.1
79.9
87.8
90.8
91.2
66.56
43.80
29.54
23.43
22.75
0.608
0.742
0.826
0.862
0.866
60.8
74.2
82.6
86.2
86.6
ACPC
50
100
150
200
300
12.10
6.93
4.41
3.50
2.03
0.679
0.816
0.883
0.907
0.946
67.9
81.6
88.3
90.7
94.6
23.57
14.14
9.42
7.50
4.91
0.645
0.787
0.858
0.887
0.926
64.5
78.7
85.8
88.7
92.6
46.42
29.49
20.39
17.04
12.44
0.597
0.744
0.823
0.852
0.892
59.7
74.4
82.3
85.2
89.2
79.29
53.31
39.56
33.45
26.65
0.533
0.686
0.767
0.803
0.843
53.3
68.6
76.7
80.3
84.3
different temperatures (303–333 K) and listed in Table 1. Inspection
of the data in Table 1 reveals that the inhibition efficiency of both
inhibitors increased with increasing concentration of inhibitor whereas
decreased with an increase in temperature. Such behavior can be
interpreted on the basis that the inhibitors exert their action by
adsorbing themselves on the mild steel surface and an increase in tem-
perature resulted in desorption of some adsorbed inhibitor molecules
leading to a decrease in the inhibition efficiency.
in the present study is less than 49 mV/SCE, which indicates that AMPC
and ACPC act as mixed type inhibitor.
3.3.Electrochemical impedance spectroscopy
The experimental (symbols) and fitting (solid lines) Nyquist plots
for mild steel corrosion in 15% HCl solution in the absence and presence
of different concentrations (50–300 ppm) of AMPC and ACPC at 303 K
are shown in Fig. 2 (a, b). The Nyquist plots for both inhibitors showed
single semicircles with one time constant. The capacitive loops are not
perfect semicircles due to non-homogeneity and roughness of the
mild steel surface [13]. The impedance spectra were analyzed by fitting
the experimental data to the equivalent circuit model shown in Fig. 3.
The CPE is used in place of Cdl to compensate deviations from ideal
dielectric behavior arising from the inhomogeneous nature of the
electrode surfaces. The impedance of the CPE is given by:
3.2.Potentiodynamic polarization measurements
Fig. 1 (a, b) shows the potentiodynamic polarization curves for the
corrosion of mild steel in 15% HCl solution in the absence and presence
of various concentrations of AMPC and ACPC respectively, at 303 K.
The electrochemical parameters such as corrosion potential (Ecorr),
corrosion current density (icorr), anodic Tafel slope (βa), cathodic Tafel
slope (βc) and percentage inhibition efficiency (η %) were determined
and summarized in Table 2. It is apparent from Fig. 1(a, b) that the
nature of the polarization curves for both inhibitors remains the same
in the absence and presence of inhibitor but the curves shifted towards
lower current density in the presence of inhibitor, suggesting that
the inhibitor molecules retard the corrosion process. It is reported in
literature that if the displacement in Ecorr in the presence of inhibitor
as compared to blank solution is more than 85 mV/SCE, the inhibitor
is classified as cathodic or anodic type [12]. The maximum displacement
−n
−1
ZCPE ¼ Y0 ðjωÞ
ð7Þ
where Y0 and n stands for the CPE constant and exponent, respectively,
j = (−1)1/2 is an imaginary number, and ω is the angular frequency in
rad s−1 (ω = 2πf), where f is the frequency in Hz. The electrochemical
parameters obtained from fitting the experimental data of Nyquist
plots in the equivalent circuit shown in Fig. 3 are presented in Table 2.
The data shown in Table 2 revealed that Rct increased and Cdl decreased
in presence of inhibitor as compared to in the absence of inhibitor,
Fig. 1. Potentiodynamic polarization curves for mild steel in 15% HCl in the presence and absence of inhibitor at 298 K (a) AMPC (b) ACPC.

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Table 2
Electrochemical parameters for mild steel in 15% HCl solution in the presence and absence of inhibitor at 303 K.
Conc. (ppm)
Tafel extrapolation data
EIS data
n
Ecorr (mV vs SCE)
−497
β
a (mV dec−1
)
-βc (mV dec−1
)
icorr (μAcm−2
)
η %
Rs (Ω cm2)
Rct (Ω cm2)
Y0 (μFcm−2
)
Cdl (μF cm2)
η %
Blank
95
140
653
–
1
2.5
800
0.825
214
–
AMPC
50
100
150
200
300
−457
−468
−466
−457
−448
102
95
93
85
115
162
157
135
147
189
165
73.9
88.2
94.1
95.3
95.5
1.1
1.2
0.9
0.9
0.9
9.9
19.8
40.1
60.1
69.0
180
130
100
71
0.83
0.85
0.86
0.90
0.91
49.2
45.4
40.7
38.7
37.4
74.7
87.3
93.7
95.8
96.3
82.5
40.7
27.2
23.7
64
ACPC
50
100
150
200
300
−478
−467
−452
−449
−450
85
77
76
92
97
135
165
138
145
160
202
117
81.7
54.1
30.7
69.0
82.1
87.5
91.7
95.3
1.0
1.0
1.5
0.8
0.9
8
13
20.5
28.5
43.1
208
150
106
73
0.86
0.87
0.89
0.91
0.92
52.3
48.2
44.8
42.4
40.3
68.7
80.7
87.8
91.2
94.9
47
suggesting the formation of a protective film of inhibitor at the
metal/solution interface. The decrease in Cdl values on increasing
the concentration of inhibitor, suggesting the decrease in local di-
electric constant and an increase in the thickness of the electrical
double layer due to the adsorption of the inhibitor molecules at the
metal/solution interface [14].
as the value of EPZC of the mild steel. The net surface charge of
the mild steel at the open circuit potential was evaluated according to
the equation:
Er ¼ Eocp − EPZC
ð8Þ
The single maximum obtained in experimental (symbols) and fitting
(solid lines) Bode phase angle plots (Fig. 4 a, b) confirmed the presence
of one time constant at intermediate frequencies, broadening of this
maximum in the presence of inhibitors accounts for the adsorption of
inhibitor on the electrode surface. The magnitude of impedance in the
presence of both inhibitors is larger than in the absence of inhibitors
and the value of impedance increases on increasing the concentration
of both studied inhibitors suggested that the corrosion rate is reduced
in the presence of the inhibitors and continued to decreasing on increas-
ing the concentration of inhibitors.
where Er is the Antropov's “rational” corrosion potential [15]. Table 3
shows the values of Eocp, EPZC and Er for both inhibitors at 300 ppm
concentration. The positive value of Er for both the inhibitors with
respect to EPZC, as shown in Table 3, suggesting that mild steel surface
is positively charged at open circuit potential in presence of 300 ppm
of these inhibitors. Since mild steel surface is positively charged
therefore anions (Cl− ions) in aqueous hydrochloric acid solution gets
adsorbed on the mild steel surface. After the adsorption of the Cl−
ions, the N80 steel surface becomes negatively charged. Hence, the pro-
tonated positively charged form of inhibitor (AMPC+ and ACPC+
)
Electrochemical results (η %) are in good agreement with the results
(η %) obtained by weight loss experiments.
adsorbed on the surface of mild steel via electrostatic interactions
with the Cl− ions already adsorbed on mild steel surface. Other sides,
the inhibitor molecules may also adsorbed on the mild steel surface
via the unshared pair of electrons present on N and O atoms.
3.4.Potential of zero charge (EPZC
)
The potential of zero charge (EPZC) of mild steel in 15% HCl solution
in presence of inhibitors was determined by using electrochemical
impedance spectroscopy method. The minimum on the double layer
capacitance (Cdl) versus applied voltage (E) plots (Fig. 5) is considered
3.5.Adsorption isotherm
Basic information on the interaction between the inhibitor and mild
steel surface were obtained from various adsorption isotherms. In the
Fig. 2. Experimental (symbols) and fitting (solid lines) Nyquist plot for mild steel in 15% HCl acid containing various concentrations of (a) AMPC (b) ACPC at 303 K.