Synthesis and application of new acetohydrazide derivatives as a corrosion inhibition of mild steel in acidic medium: Insight from electrochemical and theoretical studies

Article abstract of DOI:10.1016/j.molliq.2015.05.005

Abstract Corrosion inhibition of mild steel in 15% HCl solutions by synthesized acetohydrazides namely, N′-[(1Z)-phenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)-4-chlorophenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (CPQA) wa

Full text of DOI:10.1016/j.molliq.2015.05.005

Journal of Molecular Liquids 208 (2015) 322–332
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and application of new acetohydrazide derivatives as a
corrosion inhibition of mild steel in acidic medium: Insight from
electrochemical and theoretical studies
a
a
b,c,
b,c
M. Yadav a, , R.R. Sinha , S. Kumar , I. Bahadur
⁎
⁎
, E.E. Ebenso
a
Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India
Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046,
b
Mmabatho 2735, South Africa
c
Department of Chemistry, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Corrosion inhibition of mild steel in 15% HCl solutions by synthesized acetohydrazides namely, N′-[(1Z)-
phenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)-4-chlorophenylmethylene]-2-
(quinolin-8-yloxy) acetohydrazide (CPQA) was studied using chemical (weight loss) and electrochemical
(potentiodynamic and electrochemical impedance spectroscopy) measurements. It was shown that PQA and
CPQA act as good corrosion inhibitor for mild steel protection. It was concluded that the inhibition efficiencies in-
creased with increase in the concentrations of the inhibitor. Tafel polarization studies showed that both the stud-
ied inhibitors act as mixed type inhibitor. The high inhibition efficiencies were attributed to the simple blocking
effect by adsorption of inhibitor molecules on the steel surface. Atomic Force Microscope (AFM), Scanning Elec-
tron Microscope (SEM) and Energy Dispersion X-ray Spectroscopy (EDX) observations confirmed the existence
of an absorbed protective film on the metal surface. The density functional theory (DFT) was employed for the-
oretical calculations and the obtained results were found to be consistent with the experimental findings.
© 2015 Published by Elsevier B.V.
Received 6 April 2015
Accepted 3 May 2015
Available online xxxx
Keywords:
Mild steel
Acetohydrazides
Corrosion inhibition
EIS
Polarization
Density functional theory
1. Introduction
determination of their inhibitive performance. Although, the protection
mechanism of organic corrosion inhibitors has not been clearly under-
Acethydrazide is an important organic intermediate which is mainly
used for the synthesis of nifuratrone in the pharmaceutical industry.
Substituted pyrazolones can be prepared by treatment with corre-
sponding hydrazide as strong alkalies [1] with hydrazide and its deriva-
tives were used as versatile synthons. Hydrazides are found to be more
reactive functional groups routinely used in protein and carbohydrate
chemistry [2,3] and it is also reported that oligonucleotides can be mod-
ified with hydrazide [4].
Hydrochloric acid is widely used for pickling, cleaning and descaling
of steel and ferrous alloys. The addition of corrosion inhibitors effective-
ly secures the metal against an acid attack. Inhibitors are generally used
in these processes to control metal dissolution. Most of the effective cor-
rosion inhibitors are organic compounds containing nitrogen, oxygen,
sulfur, aromatic rings and π-electrons in their structures [5–8]. The
presence of atoms such as N, O and S in heterocyclic compounds deter-
mines both the efficiency and adsorption mechanism [9–14]. The
molecular structure of these compounds plays an important role in
stood [15], it is generally accepted that the corrosion inhibition is
achieved due to the interactions between inhibitor molecules and the
metal surface; resulting in formation of an inhibitive surface film [9,
10,16,17]. The organic inhibitors decrease the corrosion rate by
adsorbing on the metal surface and blocking the active sites by
displacing water molecules and forming a compact barrier film on the
metal surface. Most of the organic inhibitors are toxic, highly expensive
and environment unfriendly. Research activities in recent times are
geared towards developing the cheap, non-toxic and environment
friendly corrosion inhibitors. A large number of organic compounds in-
cluding heterocyclic compounds [18–23] were studied as corrosion in-
hibitors for mild steel [24–26].
The aim of this research is to study the corrosion inhibition effect
of two synthesized compounds namely N′-[(1Z)-Phenylmethylene]-
2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)-4-
Chlorophenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (CPQA)
on mild steel in 15% HCl solution by using weight loss measurement,
electrochemical methods and theoretical calculations. Two derivatives
of acetohydrazide have been chosen to discuss the effect of electron
withdrawing (–Cl) substituent on inhibition efficiency of the inhibitor.
To the best of our knowledge there is no any open literature pertaining
on the studied compounds as a corrosion inhibitor on mild steel in
⁎
Corresponding authors.
E-mail addresses: yadavdrmahendra@yahoo.co.in (M. Yadav),
bahadur.indra@gmail.com (I. Bahadur).
http://dx.doi.org/10.1016/j.molliq.2015.05.005
0167-7322/© 2015 Published by Elsevier B.V.

M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
323
Scheme 1. Synthetic route of N′-[(1Z)-Phenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)-4-Chlorophenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide
(CPQA).
CPQA: Yield 70%, m.p. 132 °C; IR (KBr) cm−1: 3455 (N–H, CONH),
acidic medium. The present work is a continuation of our systematic
studies on corrosion inhibitor of organic compounds [27–29].
1670 (C = O), 1624 (C = N), 1065 (C–O–C), 1188 (N–N), 754 (C–
Cl); 1H NMR (DMSO d6) δ: 8.25 (s, 1H, CONH), 7.41 (d, 2H, Ar–H
near Cl), 7.11 (s, 1H, N = CH), 4.71 (s, 2H, OCH2); Anal. Calcd. for
C18H14N3O2Cl: C, 63.55; H, 4.07, N, 12.34. Found: C, 63.62; H, 4.12;
N, 12.37%.
2. Experimental
2.1. Synthesis of corrosion inhibitors
The compounds PQA and CPQA were synthesized by reported
method [30] as shown in Scheme 1. The purity of the compound
was checked by thin layer chromatography (TLC) and structure of
the compounds was confirmed by using physic-chemical studies.
The melting point, yield and IR data of the synthesized compounds
are given below:
The structure of inhibitors namely: N′-[(1Z)-Phenylmethylene]-
2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)-4-
Chlorophenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide
(CPQA) are given in Fig. 1.
2.2. Material and methods
PQA: Yield 70%, m.p. 105 °C; IR (KBr) cm−1: 3470 (N–H, CONH), 1674
(C = N), 1080 (C–O–C), 1181 (N–N), 1H NMR (DMSO d6) δ: 8.17 (s,
1H, CONH), 7.61 (m, 11H, ArH), 7.41 (s, 1H, N = CH), 4.98 (s, 2H,
OCH2); Anal. Calcd. for C18H15N3O2: C, 70.71; H, 4.84; N, 13.72.
Found: C, 70.81; H, 4.92; N, 13.77%.
2.2.1. Mild steel samples
Mild steel specimens of size 6.0 cm × 2.5 cm × 0.1 cm and
1.0 cm × 1.0 cm × 0.1 cm with composition (w%) C, 0.14; Mn, 0.98; Si,
0.033; S, 0.018; P, 0.026; Cr, 0.02 and remainder iron were employed
Fig. 1. Structure of N′-[(1Z)-Phenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (PQA) and N′-[(1Z)-4-Chlorophenylmethylene]-2-(quinolin-8-yloxy) acetohydrazide (CPQA).
Table 1
Corrosion parameters of mild steel in 15% HCl solution in the presence and absence of different inhibitors at different temperatures, obtained from weight loss measurements.
Conc. (ppm)
303 K
313 K
323 K
333 K
CR (mm y−1
)
θ
η%
CR (mm y−1
)
θ
η%
CR (mm y−1
)
θ
η%
CR (mm y−1
)
θ
η%
Blank
CPQA
50
100
200
300
400
28.2
–
–
58.1
–
–
98.9
–
–
144.5
–
–
8.70
6.32
4.51
2.46
1.63
0.691
0.776
0.840
0.912
0.942
69.13
77.58
84.02
91.25
94.21
19.56
14.47
10.79
6.17
0.663
0.750
0.814
0.893
0.925
66.33
75.08
81.42
89.38
92.59
37.37
27.74
21.54
14.36
9.43
0.622
0.719
0.782
0.855
0.904
62.21
71.95
78.22
85.48
90.46
62.64
46.64
38.40
26.97
19.34
0.566
0.677
0.734
0.813
0.866
56.65
67.72
73.42
81.33
86.61
4.30
PQA
50
100
200
300
400
7.75
5.62
3.42
1.88
1.35
0.725
0.800
0.887
0.933
0.952
72.52
80.06
87.84
93.32
95.18
17.59
12.94
8.15
5.17
3.50
0.697
0.777
0.859
0.911
0.939
69.72
77.72
85.97
91.10
93.97
33.55
25.81
17.70
12.98
9.17
0.660
0.739
0.821
0.867
0.907
66.07
73.90
82.10
86.87
90.72
57.84
44.69
32.97
24.53
17.80
0.599
0.691
0.772
0.830
0.876
59.9
69.07
77.18
83.02
87.68

324
M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
Table 2
for the weight loss and electrochemical studies respectively. The speci-
mens were abraded using emery papers of different grit sizes up to 1200
grit, polished with Al2O3 (1 μm and then 0.3 μm particle size), washed
with tap water followed by distilled water, degreased with acetone,
dried and stored in desiccator.
Activation parameter for mild steel in 15% HCl solution in the absence and presence of in-
hibitor obtained from weight loss measurements.
Inhibitor Concentration (ppm) Ea (kJ mol−1
)
ΔH*(kJ/mol) ΔS*(J mol−1 K−1
)
Blank
PQA
–
50
45.78
56.17
58.14
63.75
72.56
73.14
55.29
55.95
59.94
67.53
69.00
43.13
53.53
55.50
61.10
69.94
70.51
52.65
53.30
57.25
64.88
66.36
−74.45
−51.10
−47.27
−33.10
−9.47
100
200
300
400
50
100
200
300
400
2.2.2. Test solution
The test solution was prepared by dilution of AR grade HCl with
double-distilled water. The concentration range of inhibitors employed
was varied from 100 to 300 ppm (mg L−1). For weight loss and electro-
chemical studies 500 mL and 150 mL volume of the test solution were
used.
−8.78
CPQA
−53.01
−53.58
−43.24
−23.33
−21.80
2.2.3. Weight loss measurements
Weight loss measurements were performed in the absence and
presence of different concentrations (100 to 300 ppm) of inhibitors
at different temperatures (303 K–333 K) according to the standard
methods [19]. The corrosion rate (CR), inhibition efficiency (η%)
and surface coverage (θ) were determined by following equa-
tions [31]:
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).
CR0−CRi
θ ¼
ð2Þ
ð3Þ
CR0
8:76 ꢀ 104 ꢀ W
D ꢀ A ꢀ t
À
Á
CR mm y−1
¼
ð1Þ
CR0−CRi
CR0
ηð%Þ ¼
ꢀ 100
Fig. 2. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 15% HCl solution
Fig. 3. Transition state plot of log CR/T versus 1000/T for mild steel in 15% HCl solution at
(a) PQA, (b) CPQA.
different concentrations of (a) PQA, (b) CPQA.

M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
325
calomel electrode (SCE) as reference electrode, designed specifically for
potentiodynamic polarization measurements. A luggin capillary arrange-
ment filled with the same tested solution was used to keep the reference
electrode close to the working electrode. Potentiodynamic polarization
curves were obtained in the potential range from −250 to +250 mV vs.
SCE at OCP at a scan rate of 1 mV s−1. The linear Tafel segments of anodic
and cathodic curves were extrapolated to obtain corrosion current
densities (icorr) [32,33]. The percentage inhibition efficiency (η%) was
calculated using the equation
i0corr−icorr
i0corr
ηð%Þ ¼
ꢀ 100
ð4Þ
where, i0corr and icorr are the values of corrosion.
2.2.5. Electrochemical impedance measurements
Electrochemical impedance measurements were carried out using
the same electrochemical cell and electrochemical workstation as
used for potentiodynamic polarization measurements in the frequency
range from 10 mHz to 100 kHz using amplitude of 10 mV peak to
peak with an ac signal at the open-circuit potential. The impedance
data were obtained using Nyquist and Bode plots. All impedance data
were fitted to appropriate equivalent circuits using ZSimpWin.3.21 soft-
ware. The inhibition efficiency (η%) was calculated from charge transfer
Fig. 4. Langmuir plots of (Cinh/θ) versus Cinh for (a) PQA, (b) CPQA.
where, CR0 and CRi are corrosion rate in the absence and presence of
inhibitors. This experiment was repeated at different temperatures
of 30, 40, 50 and 60 °C by using water circulated Ultra thermostat
to determine the temperature dependence of the inhibition
efficiency.
2.2.4. Potentiodynamic polarization measurements
Potentiodynamic polarization measurements were carried out using
a computerized electrochemical analyzer model CHI 760D, manufactured
by CH Instruments, Austin, USA. The experiments were performed by
using of a three electrode cell consisting of mild steel specimen of
1.0 cm2 working electrode, a platinum counter electrode and a saturated
Table 3
Adsorption parameters for PQA and CPQA calculated from Langmuir adsorption isotherm
for mild steel in 15% HCl solution at a temperature range of 303–333 K.
Inhibitor
PQA
Temperature (K)
Kads (M−1
)
ΔG°ads (kJ mol−1
)
Slope
R2
303 K
313 K
323 K
333 K
303 K
313 K
323 K
333 K
7.6 × 105
8.5 × 105
9.6 × 105
1.2 × 104
8.3 × 105
9.3 × 105
1.0 × 104
1.2 × 104
−34.0
−34.8
−35.6
−36.0
−33.7
−34.6
−35.3
−35.9
0.967
0.972
0.981
0.992
0.974
0.987
0.982
1.014
0.992
0.989
0.991
0.995
0.986
0.997
0.995
0.999
CPQA
Fig. 5. Potentiodynamic polarization curves for mild steel in 15% HCl solution in the
presence and absence of inhibitor at 303 K. (a) PQA, (b) CPQA.

326
M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
Table 4
Electrochemical parameter and percentage inhibition efficiency (η%) obtained from polarization studies for mild steel in 15% HCl solution in the presence or absence of inhibitor at 303 K.
Conc. (ppm)
Tafel extrapolation data
EIS data
Ecorr (V vs SCE)
−495
β
a (mV dec−1
)
β
c (mV dec−1
)
Icorr (μA cm−2
)
η%
Rs (Ω cm2)
Rct (Ω cm2)
Y0 (μF cm−2
)
n
Cdl (μF cm2)
η%
Blank
PQA
50
100
200
300
400
103
182
10.0
–
1.02
2.7
483
0.852
168
–
−448
−447
−439
−443
−440
112
97
106
82
168
149
176
181
154
2.28
2.08
1.42
1.02
0.88
77.2
79.3
85.8
89.7
91.2
0.68
0.77
0.86
0.63
0.96
11.1
14.8
23.2
30.7
43.8
225
152
108
80
0.862
0.874
0.882
0.888
0.912
86.8
62.2
48.1
37.4
21.5
75.6
81.7
88.3
91.0
93.8
118
41
CPQA
50
100
200
300
400
−451
−449
−446
−441
−434
98
87
114
92
153
164
171
158
163
3.47
2.38
1.54
1.06
0.96
65.3
76.2
84.6
89.2
90.4
0.71
0.83
0.85
0.77
0.65
8.1
11.2
18.0
29.7
32.2
257
198
131
95
0.858
0.863
0.878
0.881
0.892
92.5
75.6
56.1
42.7
28.4
66.2
75.8
84.9
90.9
92.1
108
61
resistance values obtained from impedance measurements using the
following relation:
that inhibition efficiency increased with increasing the concentration
of the inhibitors. The inhibition efficiency of PQA and CPQA at
400 ppm was found to be 95.1% and 94.2% respectively, at 303 K
(Table 1). By increasing the inhibitor concentration, the part of metal
Rct ðinhÞ−Rct
ηð%Þ ¼
ꢀ 100
ð5Þ
Rct ðinhÞ
where Rct(inh) and Rct are charge transfer resistance in the presence and
absence of inhibitor respectively. The values of double layer capacitance
(Cdl) were calculated from charge transfer resistance and CPE parame-
ters (Y0 and n) using the expression [34]:
ꢀ
ꢁ
1=n
1−n
Cdl
¼
Y0Rct
ð6Þ
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.
2.2.6. Scanning electron microscopic and energy dispersive spectroscopy
analysis
For surface morphological study of the uninhibited and inhibited
mild steel samples, SEM and EDX images were recorded using the in-
strument HITACHI S3400N.
2.2.7. Atomic force microscopy
The AFM images of polished, uninhibited and inhibited mild steel
samples were carried out using a Nanosurf Easyscan2 instrument,
Model: NT-MDT, Russia; Solver Pro-47.
2.2.8. Quantum chemical study
Complete geometrical optimizations of the investigated molecules
are performed using density functional theory (DFT) with the Beck's
three parameter exchange functional along with the Lee–Yang–Parr
nonlocal correlation functional (B3LYP) with 6-31G (d, p) basis set is
implemented in Gaussian 03 program package [35,36]. Theoretical pa-
rameters such as the energies of the highest occupied and lowest unoc-
cupied molecular orbital (EHOMO and ELUMO), energy gap (ΔE) and dipole
moment (μ) were determined.
3. Results and discussion
3.1. Weight loss measurements
3.1.1. Effect of inhibitor concentration and temperature
Corrosion parameters namely, corrosion rate (CR), surface coverage
(θ) and inhibition efficiency (η %) of mild steel in 15% HCl solution in the
absence and presence of different concentrations (50–400 ppm) of in-
hibitor at different temperatures (303 K–333 K), obtained from weight
loss measurements are shown in Table 1. From Table 1, it is apparent
Fig. 6. Nyquist plot for mild steel in 15% HCl solution containing various concentrations of
(a) PQA, (b) CPQA at 303 K.

M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
327
immersion time. The apparent activation energy (Ea) for dissolution of
mild steel in 15% HCl was calculated by using the Arrhenius equation.
−Ea
2:303RT
logCR ¼
þ logA
ð7Þ
where Ea is the apparent activation energy, R is the molar gas constant
(8.314 J K−1 mol−1), T is the absolute temperature (K) and A is the Arrhe-
nius pre-exponential factor. Fig. 2(a, b) presents the Arrhenius plot of
log CR against 1/T for the corrosion of mild steel in 15% HCl solution in
the absence and presence of inhibitors PQA and CPQA at concentrations
ranging from 50 to 400 ppm. From Fig. 2(a, b), the activation energy was
calculated using the expression Ea = −(slope) × 2.303R. The calculated
values of Ea are summarized in Table 2. It is evident from Table 2 that
the values of the apparent activation energy for the inhibited solutions
were higher than that for the uninhibited solution, indicating that the
dissolution of mild steel was decreased due to formation of a barrier
by the adsorption of the inhibitors on metal surface [39].
Fig. 7. Equivalent circuit applied for fitting of the impedance spectra.
surface covered by inhibitor molecules increases and that leads to an in-
crease in the inhibition efficiencies [37].
It is also clear from Table 1 that the inhibition efficiency decreased
with increasing temperature from 303 K to 333 K. Such type of behavior
can be described on the basis that the increase in temperature leads to a
shift of the equilibrium constant towards desorption of the inhibitor
molecules at the surface of mild steel [38]. The inhibition efficiency of
PQA is greater than CPQA at all concentrations and temperatures.
The values of standard enthalpy of activation (ΔH*) and standard en-
tropy of activation (ΔS*) can be calculated by using the transition state
equation:
ꢂ
ꢃ
ꢂ
ꢃ
3.1.2. Thermodynamic and activation parameters
RT
Nh
ΔSꢁ
R
ΔHꢁ
RT
CR ¼
exp
exp
−
ð8Þ
To evaluate the adsorption and thermodynamic activation parame-
ters of corrosion processes of mild steel in 15% HCl solution, weight
loss measurements were carried out in the temperature range
303–333 K in the absence and presence of inhibitors after 6 h of
where, h is Planck's constant and N is the Avogadro number,
respectively.
A plot of log (CR/T) against 1/T (Fig. 3a, b) gave straight lines with a
slope of −ΔH*/2.303R and an intercept of [log(R/Nh) + ΔS* / (2.303R)],
from which the activation thermodynamic parameters ΔH* and ΔS*
were calculated, as listed in Table 2. The negative value of ΔS* for both
inhibitors indicates that the formation of the activated complex in the
rate determining step represents an association rather than a dissocia-
tion step, meaning that a decrease in disorder takes place during the
course of the transition from reactants to activated complex [40].
3.1.3. Adsorption isotherm
Information on the interaction between the inhibitor molecules and
the mild steel surface can be provided by adsorption isotherm. Plotting
Cinh/θ vs. Cinh yielded a straight line (Fig. 4a, b) with a correlation coeffi-
cient (R2) and slope values given in Table 3 at different temperatures.
The R2 and slope values in Table 3 are near to unity indicating that the
adsorption of these inhibitors obeys the Langmuir adsorption isotherm.
The values of Kads were calculated from the intercept of Fig. 4(a, b).
Large values of Kads obtained for both studied inhibitors suggesting
more efficient adsorption and hence better corrosion inhibition efficien-
cy. Using the values of Kads, the values of ΔG°ads were obtained by using
the following equation:
ΔG0ads ¼ −RT lnðKads
Þ
ð9Þ
where R is the gas constant and T is the absolute temperature (K). The
value of 55.5 is the concentration of water in solution in mol L−1. Calcu-
lated values of Kads and ΔG°ads are listed in Table 3. The negative values
of ΔG°ads reveal the spontaneity of adsorption process. In general, values
of ΔG°ads up to −20 kJ mol−1 are compatible with physisorption and
those which are more negative than −40 kJ mol−1 involve chemisorp-
tions [41]. The calculated ΔG°ads values for PQA and CPQA were found
in the range of −34.4 to −36.0 and −33.7 to −35.9 kJ mol−1, respectively,
at different temperatures (303–333 K), these values were between the
threshold values for physical adsorption and chemical adsorption,
indicating that the adsorption process of these inhibitors at mild steel
surface involves both the physical as well as chemical adsorption. Simi-
lar conclusion was also reported by Ozcan [42], who studied the use of
cystine as a corrosion inhibitor on mild steel in sulfuric acid.
Fig. 8. Bode plots for mild steel in a 15% HCl solution in the absence and presence of differ-
ent concentrations of inhibitors (a) PQA, (b) CPQA.

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M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
Fig. 9. SEM image of mild steel in 15% HCl solution after 6 h immersion at 303 K (a) before immersion (polished), (b) after immersion without inhibitor, (c) with 400 ppm PQA, (d) with
400 ppm CPQA.
3.2. Potentiodynamic polarization studies
presence and absence of inhibitors in the corrosive environment. The
Nyquist plots for mild steel obtained at mild steel 15% HCl solution in-
terface with and without the different concentrations of PQA and
CPQA at 303 K are shown in Fig. 6(a, b). The existence of a depressed
semicircle with its center below the axis (Z′) in Nyquist plots (Fig. 6a,
b) for both inhibitors suggests the non-homogeneity and roughness of
the mild steel surface. The EIS spectra of all tests were analyzed using
the equivalent circuit shown in Fig. 7, which is a parallel combination
of the charge transfer resistance (Rct) and the constant phase element
(CPE), both in series with the solution resistance (Rs). This type of elec-
trochemical equivalent circuit was reported previously to model the
iron/acid interface [45]. Constant phase element (CPE) is introduced in-
stead of pure double layer capacitance to give more accurate fit as the
double layer at interface does not behave as ideal capacitor.
The electrochemical parameters such as solution resistance, charge
transfer resistance and CPE constants (Y0 and n) obtained from the
fitting of the experimental data of Nyquist plots in the equivalent circuit
shown in Fig. 6 are presented in Table 4.
The data shown in Table 4 reveal that the value of Rct increases with
addition of inhibitors as compared to the blank solution, the increase in
Rct value is attributed to the formation of a protective film at the metal/
solution interface. The Cdl value decreases on increasing the concentra-
tion of both the inhibitors, indicating the decrease in local dielectric
constant and/or to an increase in the thickness of the electrical double
layer, suggesting that the inhibitor molecules are adsorbed at the
metal/solution interface.
Potentiodynamic polarization curves of mild steel in 15% HCl con-
taining PQA and CPQA at 303 K are shown in Fig. 5(a, b), respectively.
The addition of each compound in acidic medium causes a remarkable
decrease in the corrosion rate and shifts the both anodic and cathodic
curves to lower current densities. It is clear that the addition of inhibi-
tors hindered the acid attack on the steel electrode and a comparison
of curves in both cases, showed that, with respect to the blank, increas-
ing the concentration of the inhibitors gave rise to a consistent decrease
in anodic and cathodic current densities indicating that inhibitors act as
mixed type inhibitor [43]. Corrosion potential, anodic Tafel slope, ca-
thodic Tafel slope and corrosion current density values are given in
Table 4. It is clear from Table 4 that after increasing the concentration
of inhibitor, the inhibition efficiency increased, while the corrosion cur-
rent density decreased due to adsorption of PQA and CPQA on the metal
surface. The minor shift in Ecorr value (61 mV) towards positive direc-
tion in the presence of inhibitors as compared to the Ecorr value in the
absence of inhibitor indicate that PQA and CPQA act as mixed type in-
hibitor with predominant control of anodic reaction [44].
3.3. Electrochemical impedance spectroscopy studies
Adsorption of a protective inhibitor on the metal surface causes a
significant increase in impedance of the corrosion system, thus causing
an increase in the resistance to charge transfer process. Therefore, the
performance of an inhibitor can be determined by impedance measure-
ments of the corrosion system. The degree of the corrosion protection
can be determined by comparing the impedance obtained in the
The Bode phase angle plots (Fig. 8a, b) show single maximum (one
time constant) at intermediate frequencies, broadening of this maxi-
mum in the presence of inhibitors accounts for the formation of a

M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
329
Fig. 10. EDX spectra of mild steel specimens (a) polished, (b) after immersion without inhibitor, (c) with 400 ppm PQA, (d) with 400 ppm CPQA.
protective layer on the electrode surface. Moreover, there is only one
phase maximum in Bode plot (Fig. 8a, b) for both inhibitors, which indi-
cates only one relaxation process, which would be the charge transfer
process, taking place at the metal–electrolyte interface.
Fig. 8(a, b) shows that the impedance value in the presence of both
inhibitors is larger than in the absence of inhibitors and the value of im-
pedance increases on increasing the concentration of both studied in-
hibitors. These mean that the corrosion rate is reduced in the presence
of the inhibitors and continued to decrease on increasing the concentra-
tion of inhibitors. Electrochemical results (η%) are in good agreement
with the results (η%) obtained by weight loss experiment.
3.4. Scanning electron microscopy
SEM photomicrographs for mild steel in 15% HCl solution in the
absence and presence of 400 ppm of PQA and CPQA are shown in
Fig. 9(a–d). The morphology of the polished mild steel specimen
(Fig. 9a) is very smooth and shows no corrosion while mild steel speci-
men dipped in 15% HCl solution in the absence of inhibitor (Fig. 9b) is
very rough and the surface is damaged due to metal dissolution. Howev-
er, the presence of 400 ppm of inhibitor suppresses the rate of corrosion
and surface damage has been diminished considerably (Fig. 9c, d) as
compared to the blank solution (Fig. 9b) suggesting formation of a pro-
tective inhibitor film at the mild steel surface.
3.5. Energy dispersive spectroscopy
Table 5
Energy dispersive X-ray analysis (EDX) technique was employed in
order to get information about the composition of the surface of the
mild steel sample in the absence and presence of inhibitors in 15% HCl
solution. The results of EDX spectra are shown in Fig. 10(a–d). The per-
centage atomic content of various elements of the polished, uninhibited
and inhibited mild steel surface determined by EDX is shown in Table 5.
The percentage atomic content of Fe for mild steel immersed in 15% HCl
Percentage atomic contents of elements obtained from EDX spectra.
Inhibitors
Fe
C
S
Cr
Mn
Cl
N
O
Polished mild steel
Mild steel in blank HCl 83.12 15.68
Mild steel in CPQA
Mild steel in PQA
85.26 12.46
–
–
0.86 0.46
0.67 0.28 2.29
–
–
–
–
6.36
74.12 18.15 1.72 0.56
71.25 19.72 1.68 0.54
–
–
0.36 5.36 15.35
0.31 5.24 15.26

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M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
Fig. 11. Atomic force micrographs of mild steel surface (a) polished mild steel, (b) mild steel in 15% HCl solution and (c) in the presence of inhibitor CPQA (d) PQA.
solution is 83.12%, and those for mild steel dipped in an optimum con-
centration (400 ppm) of CPQA and PQA are 74.12% and 71.25%, respec-
tively. From Fig. 10(a–d) it is apparent that the spectra of inhibited
samples show suppressed Fe peaks, when compared with the polished
and uninhibited mild steel sample. This suppression of Fe lines is due
to the inhibitory film formed on the mild steel surface. The EDX spectra
of inhibited mild steel contains the peaks corresponding to all the ele-
ments present in the inhibitor molecules indicating the adsorption of in-
hibitor molecules at the surface of mild steel.
3.6. Atomic force microscopy
The three-dimensional AFM images of polished, uninhibited and
inhibited mild steel samples are shown in Fig. 11(a–d). The average
roughness values of polished mild steel sample (Fig. 11a) and mild
steel sample in 15% HCl solution without inhibitor (Fig. 11b) were
found as 25 and 650 nm. It is clearly shown in Fig. 11(b) that mild
steel sample is badly damaged due to the acid attack on surface. How-
ever, in the presence of optimum concentration (400 ppm) of CPQA
Fig. 12. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for molecules (a) CPQA, (b) PQA [Atom legend: white = H; gray = C; blue = N; red = O].

M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
331
Table 6
inhibition tendency than CPQA. Literature reveals that the use of Mulli-
ken population and HOMO population analysis can be used for the de-
termination of possible adsorption centers of the inhibitors [48–50].
The generalized interpretation given by several authors is that the
higher is the magnitude and the number of negatively charged hetero-
atom present in an inhibitor molecule, the higher is its ability to be
adsorbed on the metal surface via a donor–acceptor type bond [49]
and the more negatively charged regions with major distribution of
the HOMO are also interpreted as possible centers of adsorption. Mulli-
ken charges according to the numeration of corresponding atoms are
shown in Fig. 13(a, b). It is evident from Fig. 13(a, b) that both inhibitors
had a considerable excess of negative charge around the nitrogen
atoms, oxygen atoms, indicating that these are the coordinating sites
of the inhibitors.
Quantum chemical parameters for different inhibitors.
Inhibitor
EHOMO (eV)
ELUMO (eV)
ΔE (eV)
μ (D)
PQA
CPQA
−5.040
−5.899
−1.651
−1.526
3.389
4.373
4.32
3.66
and PQA as shown in Fig. 11(c, d), the average roughness values were
reduced to 88 and 75 nm, respectively. The lower value of roughness
for PQA than CPQA reveals that PQA protects the mild steel surface
more efficiently than CPQA in 15% HCl solution.
3.7. Theoretical calculation
The optimized structure, EHOMO and ELUMO for CPQA and PQA are
shown in Fig. 12(a, b). The quantum chemical parameters such as the
energy of the highest occupied molecular orbital (EHOMO), the energy
of the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE)
and dipole moment (μ) were determined and summarized in Table 6.
According to the frontier molecular orbital (FMO) theory of chemical re-
activity, the formation of a transition state is due to interaction between
HOMO and LUMO of reacting species. The smaller the orbital energy gap
(ΔE) between the participating HOMO and LUMO, the stronger the in-
teractions between two reacting species [46].
It was reported previously by some researchers that smaller values
of ΔE and higher values of dipole moment (μ) are responsible for higher
inhibition efficiency [47]. The lower values of the energy gap ΔE will
render good inhibition efficiencies since the energy to remove an elec-
tron from the last occupied orbital will be minimized. According to
HSAB theory hard acids prefer to co-ordinate to hard bases and soft
acid to soft bases. Fe is considered as soft acid and will co-ordinate to
molecule having maximum softness and small energy gap (ΔE =
ELUMO − EHOMO). From Table 6 it is clear that the highest value of
EHOMO (−5.04 eV), μ (4.32 D) and lowest values of ΔE (3.389 eV) and
ELUMO (−1.651 eV) are found for PQA, indicating that PQA has more po-
tency to get adsorbed on the mild steel surface resulting greater
3.8. Mechanism of inhibition
Corrosion inhibition of mild steel in 15% hydrochloric acid solution
by both inhibitors (PQA and CPQA) can be explained on the basis of mo-
lecular adsorption. These compounds inhibit corrosion by controlling
both anodic as well as cathodic reactions. In 15% hydrochloric acid solu-
tions these inhibitors exist as protonated species. In both inhibitors the
nitrogen atoms present in the molecules can be easily protonated in
acidic solution and convert into quaternary compounds. These proton-
ated species adsorbed on the cathodic sites of the mild steel and de-
crease the evolution of hydrogen. The adsorption on anodic site occurs
through π-electrons of quinoline and phenyl rings and lone pair of elec-
trons of nitrogen and oxygen atoms present in both the inhibitors which
decrease the anodic dissolution of mild steel.
4. Conclusions
➢ The synthesized acetohydrazide derivatives showed good inhibition
efficiencies for the corrosion of mild steel in 15% HCl solution and the
inhibition efficiency increased on increasing the concentration of in-
hibitor. The inhibiting performance of PQA is better than CPQA.
Fig. 13. The Mulliken charge density of (a) CPQA, (b) PQA.

332
M. Yadav et al. / Journal of Molecular Liquids 208 (2015) 322–332
[15] A. Kokalj, Electrochim. Acta 56 (2010) 745–755.
[16] V. Sastri, Corrosion Inhibitors: Principles and Applications, John Wiley & Sons, Inc.,
➢ Polarization studies showed that both tested inhibitors are mixed
type in nature.
Hoboken, New Jersey, 2011.
➢ EIS measurements show that charge transfer resistance (Rct) in-
creases and double layer capacitance (Cdl) decreases in the presence
of inhibitors, suggested the adsorption of the inhibitor molecules on
the surface of mild steel.
➢ The results obtained from SEM, EDX, AFM and Langmuir adsorption
isotherm suggested that the mechanism of corrosion inhibition is oc-
curring mainly through adsorption process.
[17] S.N. Raicheva, E.I. Sokolova, Russ. J. Electrochem. 42 (2006) 213–1223.
[18] E.E. Ebenso, U.J. Ekpe, B.I. Ita, O.E. Offiong, U.J. Ibok, Mater. Chem. Phys. 60 (1999)
79–90.
[19] K.S. Jacob, G. Parameswaran, Corros. Sci. 52 (2010) 224–228.
[20] E.A. Noor, Corros. Sci. 47 (2005) 33–55.
[21] M. Lebrini, M. Lagrenee, H. Vezin, L. Gengembre, F. Bentiss, Corros. Sci. 47 (2005)
485–505.
[22] F. Bentiss, M. Bovanis, B. Mernari, M. Traisnel, H. Vezin, M. Lagrenee, Appl. Surf. Sci.
253 (2007) 3696–3704.
[23] Z. Ait, D. Chebabe, A. Dharmraj, N. Hajjaji, A. Srhirri, M.F. Montemor, M.G.S. Ferreira,
➢ Quantum chemical results of PQA and CPQA showed higher value of
EHOMO, lower value of ELUMO, and smaller value of ΔE, indicating that
both inhibitors are good corrosion inhibitor for mild steel in 15% HCl
solution.
A.C. Bastos, Corros. Sci. 47 (2005) 447–459.
[24] M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, F. Bentiss, Corros. Sci. 44 (2002)
573–588.
[25] R. Solmaz, Corros. Sci. 52 (2010) 3321–3330.
[26] J. Aljourani, K. Raeissi, M.A. Golozar, Corros. Sci. 51 (2009) 1836–1843.
[27] M. Yadav, D. Sharma, S. Kumar, S. Kumar, I. Bahadur, E.E. Ebenso, Int. J. Electrochem.
Sci. 9 (2014) 6580–6593.
[28] M. Yadav, S. Kumar, N. Kumari, I. Bahadur, E.E. Ebenso, Int. J. Electrochem. Sci. 10
Acknowledgments
(2015) 602–624.
[29] M. Yadav, S. Kumar, I. Bahadur, D. Ramjugernath, Int. J. Electrochem. Sci. 9 (2014)
6529–6550.
[30] M. Ahmed, R. Sharma, D.P. Nagda, J.L. Jat, G.L. Talesara, ARKIVOC 1442 (2006) 66–75.
[31] G. Shukla, S.K. Dwivedi, P. Sundaram, S. Prakash, Ind. Eng. Chem. Res. 50 (2011)
1954–11959.
The authors acknowledge funding from North-West University and
Department of Science and Technology and the National Research
Foundation (DST/NRF) South Africa grant funded (Grant UID: 92333)
for postdoctoral scholarship of Dr. I. Bahadur.
[32] E. McCafferty, Corros. Sci. 47 (2005) 3202–3215.
[33] M.A. Amin, K.F. Khaled, S.A. Fadl-Allah, Corros. Sci. 52 (2010) 140–151.
[34] A.R.S. Priya, V.S. Muralidharam, A. Subramania, Corrosion 64 (2008) 541–552.
[35] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789.
[36] A.D. Becke, J. Chem. Phys. 98 (1993) 1372–1377.
[37] S.S. Abdel-Rehim, M.A.M. Ibrahim, K.F. Khaled, J. Appl. Electrochem. 29 (1999)
593–599.
[38] L. Fragoza-Mar, O. Olivares-Xometl, M.A. Domínguez-Aguilar, E.A. Flores, P.
Arellanes-Lozada, F. Jimenez-Cruz, Corros. Sci. 61 (2012) 171–184.
[39] I. Dehri, M. Ozcan, Mater. Chem. Phys. 98 (2006) 316–323.
[40] X. Wang, H. Yang, F. Wang, Corros. Sci. 53 (2011) 113–121.
[41] M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, M. Hamadanian, A.
Gandomi, Corros. Sci. 50 (2008) 2172–2181.
References
[1] P.E. Gagnon, B. Nolin, R.N. Jones, Can. J. Chem. 29 (1951) 843–847.
[2] S. Raddatz, J. Mueller-Ibeler, J. Kluge, L. Wab, G. Burdinski, J.R. Havens, T.J. Onofrey,
D.G. Wang, M. Schweitzer, Nucleic Acid Res. 21 (2002) 4793–4802.
[3] G.T. Hermanson, Bioconjugate Techniques, Academic Press, San Diego, 1996.
[4] T.D. Lumley-Woodyear, C.N. Campbell, A. Heller, J. Am. Chem. Soc. 118 (1996)
5504–5505.
[5] K.C. Emregul, O. Atakol, Mater. Chem. Phys. 83 (2004) 373–379.
[6] H.A. Sorkhabi, B. Shaabani, D. Seifzadeh, Electrochim. Acta 50 (2005) 3446–3452.
[7] M. Hosseini, S.F.L. Mertens, M. Ghorbani, R.M. Arshadi, Mater. Chem. Phys. 78 (2003)
800–808.
[8] H. Keles, M. Keles, I. Dehri, O. Serindag, Colloids Surf. 320 (2008) 138–145.
[9] M.M. Antonijevic, M.B. Petrovic, Int. J. Electrochem. Sci. 3 (2008) 1–28.
[10] J.O. Bockris, A.K.N. Reddy, Modern Electrochemistry, second ed. Kluwer Academic/
Plenum Publishers, New York, 2000.
[11] A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33–58.
[12] N.C. Subramanyam, B.S. Sheshadri, S.M. Mayanna, Corros. Sci. 34 (1993) 563–571.
[13] A. Kokalj, S. Peljhan, M. Finsgar, I. Milosev, J. Am. Chem. Soc. 132 (2010)
16657–16668.
[42] M.J. Ozcan, Solid State Electrochem. 12 (2008) 1653–1661.
[43] C. Cao, Corros. Sci. 38 (1996) 2073–2082.
[44] H. Amar, A. Tounsi, A. Makayssi, A. Derja, J. Benzakour, A. Outzourhit, Corros. Sci. 49
(2007) 2936–2945.
[45] E.E.F. El-Sherbini, S.M. Abdel Wahaab, M. Deyab, Mater. Chem. Phys. 89 (2005)
183–191.
[46] S. Xia, M. Qiu, L. Yu, L. Liu, H. Zhao, Corros. Sci. 50 (2008) 2021–2029.
[47] V.S. Sastri, J.R. Perumareddi, Corrosion 53 (1997) 617–622.
[48] N.K. Allam, Appl. Surf. Sci. 253 (2007) 4570–4577.
[49] G. Bereket, C. Ogretir, C. Ozsahim, J. Mol. Struct. 663 (2003) 39–46.
[50] J. Fang, J. Li, J. Mol. Struct. 593 (2002) 179–185.
[14] F. Touhami, A. Aouniti, S. Kertit, Y. Abed, B. Hammouti, A. Ramdani, K. Elkacemi,
Corros. Sci. 42 (2000) 929–940.