New pyrimidine derivatives as efficient organic inhibitors on mild steel corrosion in acidic medium: Electrochemical, SEM, EDX, AFM and DFT studies

Article abstract of DOI:10.1016/j.molliq.2015.06.063

New pyrimidine derivatives, namely, 7-methoxypyrido [2,3-d]pyrimidin-4-amine (MPPA) and 4-amino-7-methoxypyrido[2,3-d]pyrimidin-2(1H)-one (AMPO) were synthesized and their inhibitive action against the corrosion of mild steel in 15% HCl solution was studi

Full text of DOI:10.1016/j.molliq.2015.06.063

Journal of Molecular Liquids 211 (2015) 135–145
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
New pyrimidine derivatives as efficient organic inhibitors on mild steel
corrosion in acidic medium: Electrochemical, SEM, EDX, AFM and
DFT studies
a,
a
a
b,c,
b,c
M. Yadav ⁎, S. Kumar , R.R. Sinha , 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:
New pyrimidine derivatives, namely, 7-methoxypyrido [2,3-d]pyrimidin-4-amine (MPPA) and 4-amino-7-
methoxypyrido[2,3-d]pyrimidin-2(1H)-one (AMPO) were synthesized and their inhibitive action against the
corrosion of mild steel in 15% HCl solution was studied by weight loss, potentiodynamic polarization and electro-
chemical impedance spectroscopy (EIS) techniques. Polarization studies showed that both studied inhibitors
were of mixed type in nature. The adsorption of inhibitors on the mild steel surface obeys Langmuir adsorption
isotherm. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and atomic force
microscopy (AFM) were performed for surface study of uninhibited and inhibited mild steel samples. Density
functional theory (DFT) was employed for theoretical calculations.
Received 18 February 2015
Received in revised form 23 June 2015
Accepted 25 June 2015
Available online xxxx
Keywords:
Mild steel
Pyrimidine derivatives
EIS
© 2015 Elsevier B.V. All rights reserved.
Polarization
Corrosion inhibition
DFT
1
. Introduction
Acid solutions are commonly used for pickling, industrial acid
organic chemistry, because manifold implications viz, antibacterial, an-
tifungal, anticancer, antitumor, antiherpes, antiallergic, analgesic, anti-
inflammatory, and antineuplastic are well proved by a large number
of publication on it [14–16]. Due to presence of heteroatoms and
delocalized π-electrons, some pyrimidine derivatives are also reported
as good corrosion inhibitors [17–19].
In continuation of our research for developing corrosion inhibitors
[20–27] with high effectiveness and efficiency, the present paper
explores a systematic study to ascertain the inhibitive action of
synthesized pyrimidine derivatives, namely, 7-methoxypyrido[2,3-
d]pyrimidin-4-amine (MPPA) and 4-amino-7-methoxypyrido[2,3-
d]pyrimidin-2(1H)-one (AMPO) on corrosion of mild steel in 15% HCl
solution by using weight loss measurement, potentiodynamic polariza-
tion, AC impedance and quantum chemical calculations.
cleaning, acid descaling, and oil-well acidifying [1–5] processes. In
acidizing process of petroleum oil wells 15% of hydrochloric acid is
forced into the well through steel tubing to open up near-bore channels
in the formation and, hence, to increase the flow of oil. Because of the
aggressiveness of acid solutions, mild steel corrodes severely during
these processes, particularly with the use of hydrochloric acid, which re-
sults in terrible waste of both resources and money [6]. A corrosion in-
hibitor is often added to mitigate the corrosion of metal by acid attack.
Most well-known corrosion inhibitors are organic compounds contain-
ing polar groups including nitrogen, sulfur, and/or oxygen atoms and
heterocyclic compounds with polar functional groups and conjugated
double bonds [7–13]. These compounds can adsorb on the metal surface
and block the active sites on the surface, thereby reducing the corrosion
rate. The pyrido[2,3-d]pyrimidine derivatives are of great interest in
2. Experimental
2.1. Materials
⁎
⁎
Corresponding author.
Correspondence to: I. Bahadur, Material Science Innovation & Modelling (MaSIM)
2.1.1. Mild steel sample
⁎
Corrosion inhibition studies were performed on mild steel having
the composition (wt.%): C, 0.12; Mn, 0.11; Cu, 0.01; Si, 0.02; Sn, 0.01;
P, 0.02; Ni, 0.02 and balance Fe. Mild steel samples used in the weight
loss experiment were mechanically cut into 3.0 cm × 3.0 cm × 0.1 cm
Research Focus Area, Faculty of Agriculture, Science and Technology, North-West
University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa.
E-mail addresses: yadavdrmahendra@yahoo.co.in (M. Yadav),
bahadur.indra@gmail.com (I. Bahadur).
http://dx.doi.org/10.1016/j.molliq.2015.06.063
0167-7322/© 2015 Elsevier B.V. All rights reserved.

136
M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
dimensions, abraded with SiC abrasive papers of grade 320, 400 and 600
respectively. For potentiodynamic polarization and AC impedance stud-
ies, mild steel samples having dimension 1.0 cm × 1.0 cm × 0.1 cm were
mechanically cut and abraded similarly to a previous procedure, with an
compounds MPPA and AMPO were obtained by refluxing a mixture of
2-amino-3-cyano-6-methoxypyridine (0.04 mol) with formamide
(0.04 mol) and urea (0.04 mol) respectively, on an oil bath. The synthet-
ic route of inhibitors (MPPA and AMPO) is shown in Scheme 1 and the
structure of inhibitors is shown in Fig. 1. The purity of the inhibitors
2
exposed area of 1 cm (the rest covered with araldite resin) with 3 cm
1
long stem. Before starting the experiment, mild steel samples were
washed with distilled water, degreased in acetone, dried and stored in
vacuum desiccator.
was checked by thin layer chromatography (TLC). The IR and H NMR
spectra of inhibitors MPPA and AMPO are given in supplementary mate-
1
rials as Figs. S1 and S2, respectively. The melting point, yield, IR, H NMR
and 13C data of the synthesized compounds are given below:
2
.1.2. Test solution
The test solutions (15% HCl solution) were prepared by dilution of
MPPA
analytical grade 37% HCl with distilled water. The concentration range
Yield (74%), m.p. = 107–109 °C. Anal.: Calculated for C
54.54; H, 4.58; N, 31.80;
8 8 4
H N O; C,
−
1
of inhibitors was 50–200 ppm (mg L ) and the volumes of test solu-
tion used for weight loss measurement and electrochemical studies
were 250 mL and 150 mL, respectively.
Found: C, 54.51; H, 4.57; N, 31.79
−
1
IR (ν/cm ): 3260 (NH), 1665 (C_N), 2940 (CH alkyl)
1
H NMR (300 MHz, DMSO-d6) δ ppm: 7.72 (s, 2H, −NH
2
), 3.7
2
.1.3. Synthesis of corrosion inhibitors
The inhibitors MPPA and AMPO were synthesized by the method re-
(s, 3H, −OCH
3
), 6.52 (m, 2H, pyridine), 8.24 (m, H, pyrimidine)
13
3
C NMR (300 MHz, CDCl ) δ ppm: 164.8, 161.6, 156.4, 150.2, 142.3,
ported in literature [28]. Chalcone was synthesized by stirring a mixture
of benzaldehyde (0.1 mol), 4-methoxyacetophenone (0.1 mol) and
small amount of sodium hydroxide in ethanol (50 mL). A mixture of
the chalcone (0.05 mol), malononitrile (0.05 mol) and ammonium
acetate (0.4 mol) in ethanol (80 mL) was refluxed on a water bath for
114.6, 59.6.
AMPO
Yield (76%), m.p. = 115–118 °C. Anal.: Calculated for C
50.00; H, 4.20; N, 29.15.
8 8 4 2
H N O ; C,
1
4–16 h to synthesize 2-amino-3-cyano-6-methoxypyridine. The
Found: C, 49.97; H, 4.19; N, 29.14
Scheme 1. Synthetic route of inhibitors 7-methoxypyrido[2, 3-d]pyrimidin-4-amine (MPPA) and 4-amino-7-methoxypyrido[2, 3-d]pyrimidin-2(1H)-one (AMPO).

M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
137
Fig. 1. Structure of inhibitors 7-methoxypyrido[2, 3-d]pyrimidin-4-amine (MPPA) and 4-amino-7-methoxypyrido[2, 3-d]pyrimidin-2(1H)-one (AMPO).
IR (ν/cm−1): 3250 (NH), 1660 (C_N), 1720 (C_O), 2920 (CH alkyl)
range 100 kHz to 10 mHz. All impedance data were fitted to appropriate
circuits using ZSimpWin.3.21 software.
1
H NMR (300 MHz, DMSO-d6) δ ppm: 7.32 (s, 2H, −NH
2
), 8.12
(
s, 1H, −NH pyrimidine), 3.8 (s, 3H, −OCH
3
), 6.14 (m, 2H, pyridine)
1
3
3
C NMR (300 MHz, CDCl ) δ ppm: 162.4, 160.2, 154.8, 152.6, 145.5,
18.2, 64.5.
2.2.3. Scanning electron microscopic and energy dispersive
spectroscopy analysis
1
The mild steel specimens of size 1.0 cm × 1.0 cm × 0.1 cm were
abraded with a series of emery paper (grade 320–500–800–1200) and
then washed with distilled water and acetone. After 6 h immersion of
the specimen in 15% HCl solution in the absence and the presence of op-
timum concentration (200 ppm) of inhibitors (MPPA and AMPO) at
303 K, the specimen was taken out, cleaned with distilled water, dried
with a cold air blaster, and then the SEM and EDX images were recorded
using the instrument HITACHI S3400N.
2
2
.2. Corrosion tests
.2.1. Weight loss method
Weight loss measurements were performed at different tempera-
tures (303–333 K). The accurately weighed mild steel test coupons
were immersed for 6 h in 250 mL of 15% HCl solution in the absence
and presence of 50, 100, 150 and 200 ppm (mgL−1) of the inhibitors.
The test coupons were then removed from the15% HCl solution, washed
thoroughly with distilled water, dried and weighed. Triplicate experi-
ments were conducted for each concentration and temperature of the
inhibitor for the reproducibility and the average of weight losses were
taken to calculate different corrosion parameters. The corrosion rate
2.2.4. Atomic force microscopy (AFM)
The morphology of the polished, uninhibited and inhibited mild steel
surface was investigated by AFM. For AFM analysis the mild steel speci-
mens of size 1 cm × 1 cm × 0.1 cm were immersed in the test solution
for 6 h in the absence and presence of inhibitors at 303 K. Then the spec-
imens were taken out from the solution, cleaned with distilled water,
dried, and used for AFM images. The AFM analyses were carried out
using a Nanosurf Easyscan2 instrument, Model: NT-MDT, Russia; Solver
Pro-47.
(
CR), inhibition efficiency (η%) and surface coverage (θ) were deter-
mined by the following equations [20–22]:
4
À
Á
8:76 ꢀ 10 ꢀ W
D ꢀ A ꢀ t
−1
CR mmy
¼
ð1Þ
2
where, W = weight loss (g), A = area of specimen (cm ) exposed in so-
lution, t = exposure time (h), and D = density of mild steel (g cm ).
2.2.5. Quantum chemical study
−
3
Complete geometrical optimizations of the investigated molecules
were performed using density functional theory (DFT) in aqueous
phase with the Becke's three parameter exchange functional along
with the Lee–Yang–Parr nonlocal correlation functional (B3LYP) with
CR0−CRi
CR0
θ ¼
ηð%Þ ¼
ð2Þ
ð3Þ
6-31G (d, p) basis set as implemented in Gaussian 03 program package
CR0−CRi
CR0
ꢀ 100
[29]. Theoretical parameters such as the energies of the highest occu-
pied and lowest unoccupied molecular orbital (EHOMO and ELUMO), ener-
gy gap (ΔE) and dipole moment (μ) were determined.
0 i
where, CR and CR are corrosion rate in absence and presence of
inhibitors.
3. Results and discussion
2
.2.2. Electrochemical studies
The electrochemical studies were conducted in a conventional
3.1. Weight loss measurements
2
three-electrode cell consisting of mild steel sample of 1 cm exposed
area as working electrode, a platinum counter electrode and a saturated
calomel electrode (SCE) as reference electrode, using CH electrochemi-
cal workstation (Model No: CHI 760D, manufactured by CH Instru-
ments, Austin, USA) at 303 K. Before impedance and polarization
measurements, the working electrode was immersed in the test solu-
tion until a steady-state of the open-circuit potential corresponding to
the corrosion potential (Ecorr) of the working electrode was obtained.
Potentiodynamic polarization curves were obtained by changing the
electrode potential from −200 to +200 mV vs SCE at OCP at a scan
3.1.1. Effect of inhibitor concentration and temperature
The corrosion parameters such as corrosion rate (CR), surface cover-
age (θ) and corrosion inhibition efficiency (η%) obtained by weight loss
measurements for mild steel specimen immersed in 15% HCl solution in
the absence and presence of different concentration ranges of
(50–200 ppm) of inhibitors (MPPA, AMPO) for an immersion period of
6 h at different temperature ranges of (303–333) K and are listed in
Table 1. From Table 1, it is apparent that inhibition efficiency increased
with increasing concentration of the inhibitors. By increasing the inhibitor
concentration, the part of metal surface covered by inhibitor molecules
increases and that leads to an increase in the inhibition efficiencies [30].
Both the inhibitors are good inhibitors even at the concentration
as low as 50 ppm. The inhibition efficiency of MPPA and AMPO at
−
1
rate of 1 mVs . The linear Tafel segments of anodic and cathodic curves
were extrapolated to obtain corrosion current densities (icorr).
EIS measurements were carried out using AC signals of amplitude
1
0 mV peak to peak at the open circuit potential in the frequency

138
M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
Table 1
Corrosion parameters namely corrosion rate (CR), surface coverage (θ) and inhibition efficiency (η%) of mild steel in 15% HCl solution in the presence and absence of inhibitor at different
temperature, obtained from weight loss measurements.
303 K
313 K
323 K
333 K
Conc. (ppm)
Blank
CR (mmy−1)
θ
η %
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
CR (mmy−1
)
θ
η %
28.2
–
–
58.1
–
–
98.9
–
–
144.5
–
–
AMPO
5
1
1
2
0
3.76
2.56
1.72
0.87
0.86
0.90
0.93
0.96
86.6
90.9
93.9
96.9
8.88
6.62
4.88
3.13
0.83
0.87
0.91
0.94
84.7
88.6
91.6
94.6
16.51
12.16
9.29
0.81
0.85
0.90
0.93
83.3
87.7
90.6
93.8
26.15
20.08
15.46
10.98
0.81
0.84
0.91
0.93
81.9
86.1
89.3
92.4
00
50
00
6.13
MPPA
5
1
1
2
0
3.58
2.39
1.55
0.62
0.87
0.91
0.94
0.97
87.3
91.5
94.5
97.8
8.71
6.21
4.47
2.55
0.85
0.89
0.92
0.95
85.0
89.3
92.3
95.6
15.72
11.67
8.60
0.84
0.88
0.91
0.94
84.1
88.2
91.3
94.5
24.42
18.49
14.16
9.24
0.83
0.87
0.90
0.93
83.1
87.2
90.2
93.6
00
50
00
5.43
Experimental errors ± 3%.
2
00 ppm concentration was found to be 97.8% and 96.9% respectively,
while it was 87.3% and 86.6% respectively, at 50 ppm concentration at
03 K (Table 1). The inhibition efficiency of both the inhibitors de-
unity indicating that the adsorption of both the inhibitors obeys Lang-
muir adsorption isotherm represented by the following equation:
3
creased with increasing temperature from 303 to 333 K (Table 1). This
is due to the fact that at higher temperatures the metal dissolution pro-
cess is enhanced and the adsorbed inhibitor molecules are partially
desorbed from the metal surface [31].
Cinh
θ
1
¼ K þ Cinh
ð6Þ
ads
where Cinh is the inhibitor concentration, Kads is the equilibrium con-
stant for adsorption–desorption process.
3
.1.2. Thermodynamic and activation parameters
The apparent activation energy (E ) for dissolution of mild steel in
5% HCl solution was calculated by using Arrhenius equation:
a
1
−
:303RT
Ea
log CR ¼ 2
þ log A
ð4Þ
where R is the molar gas constant (8.314 J K− mol−1), T is the absolute
temperature (K) and A is the Arrhenius pre-exponential factor. Fig. 2
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
at concentrations ranging from 50 to 200 ppm. From the slope in Fig. 2,
1
a
the values of E were calculated and 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 forma-
tion of a barrier by the adsorption of the inhibitors on metal surface [32].
The values of standard enthalpy of activation (ΔH*) and standard en-
tropy of activation (ΔS*) were calculated by using the transition state
equation
ꢀ
ꢁꢁ
ꢀ
ꢁꢁ
RT
Nh
ΔS
R
ΔH
RT
CR ¼
exp
exp −
ð5Þ
where, h is the Planck's constant and N is the Avogadro number,
respectively.
A plot of log (CR/T) against 1/T (Fig. 3) 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, and are 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 dis-
sociation step, meaning that a decrease in disorder takes place during
the course of the transition from reactants to activated complex [33].
3
.1.3. Adsorption isotherm
The plots of Cinh/θ vs. Cinh yielded straight lines (Fig. 4) with correla-
2
tion coefficient (R ) and slope values given in Table 3 at studied temper-
atures. The correlation coefficient and slope values in Table 3 are near
Fig. 2. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 15% HCl solution
(a) AMPO (b) MPPA.

M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
139
Table 2
Activation parameters for mild steel in 15% HCl solution in the absence and presence of in-
hibitor obtained from weight loss measurements.
⁎
⁎
ΔS
Inhibitor
Concentration
ppm)
E
a
ΔH
(kJmol−1)
(kJ mol−1)
(Jmol K−1
−1
)
(
Blank
AMPO
–
50
45.7
53.1
55.3
57.9
61.8
55.0
60.5
63.5
71.0
43.0
51.5
54.2
58.2
67.0
58.7
71.9
43.0
50.7
−74.4
−63.2
−56.5
−47.2
−38.6
−54.4
−46.3
−44.7
−65.8
1
1
2
00
50
00
MPPA
50
1
1
2
00
50
00
Experimental errors ± 3%.
The values of Kads were calculated from the intercept of Fig. 4. Large
values of Kads were obtained for both studied inhibitors suggesting more
efficient adsorption and hence better corrosion inhibition efficiency.
Using the values of Kads, the values of ΔGads were obtained by using
the following equation:
0
ΔG ¼ −RT lnð55:5Kads
Þ
ð7Þ
ads
The value of 55.5 is the concentration of water in solution in mol L−1.
The calculated values of Kads and ΔGads are listed in Table 3. In
−
1
general, values of ΔGads up to −20 kJmol
are compatible with
−
1
physisorption and those which are more negative than −40 kJmol
involve chemisorptions [34]. The calculated ΔGads values for AMPO
Fig. 4. Langmuir plots of (Cinh/θ) versus Cinh for (a) AMPO (b) MPPA.
and MPPA were found in the range of −35.0 to −37.5 and −36.7 to
−
1
−
38.2 kJmol , 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 in-
hibitors at mild steel surface involve both the physical as well as the
chemical adsorption.
4. Electrochemical studies
4.1. Polarization studies
The potentiodynamic polarization curves for the mild steel in 15%
HCl solution in the absence and presence of different concentrations of
AMPO and MPPA at 303 K and are shown in Fig. 5a, b. The corrosion pa-
a
rameters such as corrosion potential (Ecorr), anodic Tafel slope (β ),
Table 3
Adsorption parameters for AMPO and MPPA calculated from Langmuir adsorption iso-
therm for mild steel in 15% HCl solution at temperature range of 303–333 K.
Inhibitor
Temperature
K)
K
ads
(M−1)
ΔG°ads
Correlation
coefficient (R )
Slope
values
(kJ mol−1)
2
(
4
4
4
4
4
4
4
4
AMPO
303
2.02 × 10
1.69 × 10
1.47 × 10
1.40 × 10
1.76 × 10
1.72 × 10
1.88 × 10
1.80 × 10
−35.0
−35.7
−36.5
−37.5
−36.7
−36.8
−37.2
−38.2
0.9989
0.9986
0.9981
0.9970
0.9989
0.9988
0.9989
0.9989
1.001
1.014
1.018
1.011
1.021
1.032
0.991
1.011
3
3
3
13
23
33
MPPA
303
3
3
3
13
23
33
Fig. 3. Transition state plot of log CR/T versus 1000/T for mild steel in 15% HCl solution at
different concentration (a) AMPO (b) MPPA.
Experimental errors ± 3%.

140
M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
thereby blocking the corrosion reaction [35] and increasing the inhibi-
tion efficiency. The anodic Tafel slope (β ) and the cathodic Tafel slope
) of AMPO and MPPA changed with inhibitor concentration, indicat-
a
(β
c
ing that these inhibitors controlled both anodic as well as cathodic reac-
tions and act as mixed inhibitor.
The presence of inhibitor caused minor change in Ecorr values with
respect to the Ecorr value in the absence of inhibitor. It was reported by
various researchers that if the change in Ecorr value in the presence of in-
hibitor with respect to the Ecorr value in the absence of inhibitor is more
than 85 mV, the inhibitor is recognized as an anodic or a cathodic type
inhibitor whereas if the change in Ecorr value is less than 85 mV, the in-
hibitor is recognized as mixed type inhibitor [36]. In the present inves-
tigation the maximum shift in Ecorr was 46 mV towards the cathodic
direction indicating that the inhibitors AMPO and MPPA act as mixed
type inhibitor with predominately cathodic effect.
4.2. EIS studies
The Nyquist plots for mild steel obtained at mild steel/15% HCl solu-
tion interface with and without the different concentrations of AMPO
and MPPA at 303 K and are shown in Fig. 6. The existence of a depressed
'
semicircle with its centre below the axis (Z ) in Nyquist plots (Fig. 6 a,
b) for both inhibitors is suggesting the non-homogeneity and roughness
of the mild steel surface [37]. The EIS spectra of all tests were analyzed
using the equivalent circuit shown in Fig. 7, which is a parallel combina-
tion of the charge transfer resistance (Rct) and the constant phase ele-
ment (CPE), both in series with the solution resistance (R ). This type
s
of electrochemical equivalent circuit was reported previously to model
Fig. 5. Potentiodynamic polarization curves for mild steel in 15% HCl solution in the pres-
ence and absence of inhibitor 303 K (a) AMPO (b) MPPA.
c
cathodic Tafel slope (β ) and corrosion current density (icorr) obtained
from these curves are given in Table 4. The percentage inhibition effi-
ciency (η %) was calculated using the equation
0
i
−icorr
corr
ηð%Þ ¼
ꢀ 100
ð8Þ
0
i
corr
0
where, i corr and icorr are the values of corrosion current density in the
absence and presence of inhibitors, respectively.
It is apparent in Fig. 5, that both anodic metal dissolution of iron and
cathodic hydrogen evolution curves shifted towards lower current den-
sity after the addition of inhibitors to 15% HCl solution. The lower corro-
sion current density (icorr) values in the presence of inhibitor suggesting
that the inhibitor molecules adsorbed on the surface of mild steel,
Table 4
Electrochemical parameter and percentage inhibition efficiency (Η %) obtained from po-
larization studies for mild steel in 15% HCl solution in the absence and presence of inhib-
itor at 303 K.
Inhibitor Conc.
-Ecorr
i
corr
β
a
β
c
θ
η
(%)
(
ppm) (mV/SCE) (μA cm−2) (mV dec−1) (mV dec−1)
Blank
AMPO
0
50
00
50
00
50
00
50
00
416
415
438
420
446
422
422
445
462
6733
862
586
397
175
821
539
323
128
332
277
357
324
319
319
278
370
339
338
339
286
291
271
283
269
285
306
–
–
0.87 87.2
0.91 91.3
0.94 94.1
0.97 97.4
0.87 87.8
0.92 92.0
0.95 95.2
0.98 98.1
1
1
2
MPPA
1
1
2
Fig. 6. Nyquist plots for mild steel in 15% HCl solution (a) AMPO (b) MPPA containing var-
ious concentrations (1) 0 ppm (2) 50 ppm (3) 100 ppm (4) 150 ppm (5) 200 ppm at
303 K.
Experimental errors ± 1%.

M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
141
the experimental data of Nyquist plots in the equivalent circuit shown
in Fig. 7 are presented in Table 5.
The inhibition efficiency (η %) was calculated from charge transfer
resistance values obtained from impedance measurements using the
following relation
R
−Rct
ct ðinhÞ
R
Fig. 7. Equivalent circuit diagram of a protective layer on the electrode surface.
ηð%Þ ¼
ꢀ 100
ð9Þ
ct ðinhÞ
where Rct(inh) and Rct are charge transfer resistance in the presence and
absence of inhibitor respectively.
Table 5
Electrochemical impedance parameters for mild steel in 15% HCl solution in the absence
and presence of inhibitor at different concentration at 303 K.
The values of double layer capacitance (Cdl) were calculated from
charge transfer resistance and CPE parameters (Y
pression [39]
0
and n) using the ex-
Inhibitor Conc.
R
s
R
ct
Y
o
n
C
dl
η%
(
ppm) (Ω cm ) (Ω cm ) (μF cm−2)
2
2
(μF cm−2)
ꢂ
ꢃ
1=n
Blank
AMPO
–
50
1
1
2
0.65
0.85
0.89
0.82
0.82
0.86
0.85
0.82
0.27
5.1
37.4
57.0
78.9
171.8
41.7
63.5
105.4
209.8
1471
981
730
510
218
803
487
278
114
0.87 721.1
0.70 250.0
0.71 199.5
0.72 146.6
–
Y0Rct1−n
Cdl
¼
ð10Þ
86.3
91.0
93.5
97.0
87.7
92.0
95.1
97.6
00
50
00
0
where Y is the CPE constant and n is the CPE exponent. The value of n
represents the deviation from the ideal behavior and it lies between 0
and 1.
The results showed that the charge transfer resistance for blank so-
lution was relatively low due to high conductance of HCl solution. Sim-
ilar value of Rct for blank solution was also reported by other researchers
0.77
82.7
MPPA
50
0.72 214.7
0.74 145.9
1
1
2
00
50
00
0.76
0.80
93.3
44.8
Experimental errors ± 1%.
[40]. The data shown in Table 5 reveal that the value of Rct increases
the iron/acid interface [38]. CPE is introduced instead of pure double
layer capacitance to give more accurate fit as the double layer at inter-
face does not behave as ideal capacitor.
The electrochemical parameters such as solution resistance, charge
0
transfer resistance and CPE constants (Y and n) obtained from fitting
with addition of inhibitors as compared to the blank solution, the in-
crease in Rct value is attributed to the formation of a protective film at
the metal/solution interface. The Cdl value decreases on increasing the
concentration of both the inhibitors, indicating the decrease in local di-
electric constant and/or to an increase in the thickness of the electrical
Fig. 8. Bode plots for mild steel in a 15% HCl solution in the absence and presence of different concentrations of inhibitors (a) AMPO (b) MPPA.

142
M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
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 inhibitor MPPA, (d) with
inhibitor AMPO.
Fig. 10. EDX spectra of mild steel specimens (a) polished, (b) after immersion without inhibitor, (c) with 200 ppm AMPO, (d) with 200 ppm MPPA.

M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
143
Table 6
Fig. 9a–d. The surface of the polished mild steel specimen (Fig. 9a) is
very smooth and shows no corrosion while mild steel specimen dipped
in 15% HCl solution in the absence of inhibitor (Fig. 9b) is very rough and
the surface is damaged due to metal dissolution. However, the presence
of 200 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 protective inhibi-
tor film at the mild steel surface.
Percentage atomic contents of elements obtained from EDX spectra.
Inhibitors
Fe
C
Cr
Mn
Cl
N
O
Polished mild steel
Mild steel in blank HCl
Mild steel in AMPO
Mild steel in MPPA
85.26
83.12
68.12
62.25
12.46
15.68
17.36
17.72
0.86
0.67
0.58
0.57
0.46
0.28
–
–
–
–
3.65
4.16
–
2.29
0.35
0.33
6.36
8.46
11.64
–
double layer, suggesting that the inhibitor molecules are adsorbed at the
metal/solution interface.
6. Energy dispersive spectroscopy
The Bode phase angle plots (Fig. 8b, d) shows single maximum (one
time constant) at intermediate frequencies, broadening of this maxi-
mum in the presence of inhibitors accounts for the formation of a pro-
tective layer on the electrode surface. Fig. 8a, c shows that the
impedance value in the presence of both inhibitors is larger than in
the absence of inhibitors and the value of impedance increases on in-
creasing the concentration of both studied inhibitors. These mean that
the corrosion rate is reduced in the presence of the inhibitors and con-
tinued to decrease on increasing the concentration of inhibitors.
Electrochemical results (η %) are in good agreement with the results
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 absence and presence of inhibitors in 15% HCl solu-
tion. The results of EDX spectra are shown in Fig. 10a, b, c, 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 6. The percentage atomic content of Fe for mild steel im-
mersed in 15% HCl solution is 83.12%, and those for mild steel dipped in
an optimum concentration (200 ppm) of AMPO, and MPPA are 68.12%
and 62.25%, respectively. In Fig. 10, the spectra of inhibited samples
show that the Fe peaks are considerably suppressed, 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 contain the peaks corresponding
to all the elements present in the inhibitor molecules indicating the ad-
sorption of inhibitor molecules at the surface of mild steel.
(η %) obtained by weight loss experiment.
5
. Scanning electron microscopy
SEM photomicrographs for mild steel in 15% HCl solution in the ab-
sence and presence of 200 ppm of AMPO and MPPA are shown in
Fig. 11. Atomic force micrographs of mild steel surface (a) polished mild steel, (b) mild steel in 15% HCl solution, and (c) in presence of inhibitor AMPO, (d) MPPA.

144
M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
7
. AFM
The three-dimensional AFM images of polished, uninhibited and
Table 7
Quantum chemical parameters for different inhibitors.
Inhibitor
-EHOMO (eV)
-ELUMO (eV)
ΔE (eV)
μ (D)
inhibited mild steel samples are shown in Fig. 11a–d. The average
roughness of polished mild steel sample (Fig. 11a) and mild steel sam-
ple in 15% HCl solution without inhibitor (Fig. 11b) were found as 25
and 650 nm. It is clearly shown in Fig. 11b that mild steel sample is
badly damaged due to the acid attack on surface. However, in the pres-
ence of optimum concentration of 200 ppm of AMPO and MPPA as
shown in Fig. 11c, d, the average roughness were reduced to 76 and
AMPO
MPPA
6.1630
6.0515
1.2979
1.4040
4.8715
4.6475
4.396
4.565
E
LUMO for MPPA as compared to AMPO are indicating that MPPA has
more potency to get adsorbed on the mild steel surface resulting in
greater inhibition tendency than AMPO.
6
4 nm, respectively. The lower value of roughness for MPPA than
AMPO reveals that MPPA protects the mild steel surface more efficiently
9. Mechanism of inhibition
than AMPO in 15% HCl solution.
Corrosion inhibition of mild steel in 15% hydrochloric acid solution
by both inhibitors (MPPA and AMPO) can be explained on the basis of
molecular adsorption. These compounds inhibit corrosion by control-
ling both anodic as well as cathodic reactions. In 15% hydrochloric acid
solutions 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 pyridine, and pyrimidine rings and lone pair of
electrons of nitrogen atoms present in both the inhibitors which de-
crease the anodic dissolution of mild steel.
8
. Theoretical calculation
In order to study the effect of molecular structure on the inhibition
efficiency, quantum chemical calculations were performed by using
DFT and all the calculations were carried out with the help of complete
geometry optimization. Optimized structures, EHOMO and ELUMO are
shown in Fig. 12a, b. The quantum chemical parameters such as the en-
ergy 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 7. Ac-
cording to the frontier molecular orbital (FMO) theory of chemical reac-
tivity, 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 interactions between two reacting species [41].
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 [42]. 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 =
10. Conclusions
The synthesized pyrimidine derivatives act as good corrosion inhib-
itor for mild steel in 15% HCl solution and inhibiting performance of
MPPA is better than AMPO. Polarization studies showed that both tested
inhibitors are mixed type in nature. EIS measurements show that charge
transfer resistance (Rct) increases and double layer capacitance (Cdl) de-
creases in the presence of inhibitors, which 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 occurring mainly through ad-
sorption process. Quantum chemical results of AMPO and MPPA
ELUMO − EHOMO). Higher value of EHOMO, μ and lower values of ΔE and
Fig. 12. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for molecules (a) AMPO (b) MPPA [H, Gray; C, Cyan; N, Blue; O, Red]. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)

M. Yadav et al. / Journal of Molecular Liquids 211 (2015) 135–145
145
[
13] H. Hamani, T. Douadi, M. Al-Noaimi, S. Issaadi, D. Daoud, S. Chafaa, Corros. Sci. 88 (2014)
34–245.
14] R.D. Goyal, L. Prakash, Indian J. Chem. 31B (1992) 719.
[15] A. Rahamana, H.N. Hafez, Bioorg. Med. Chem. 19 (2009) 3392–3397.
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.
2
[
[
16] H. Huang, D.A. Hutta, R.L. Desfarlais, C. Schuberh, L.P. Ptrounia, M.A. Chaikin, C. Manthey,
M.R. Player, Bioorg. Med. Chem. Lett. 18 (2008) 2355–2361.
Acknowledgments
[17] G.Y. Elewady, Int. J. Electrochem. Sci. 3 (2008) 1149–1161.
18] N. Caliskan, E. Akbas, Mater. Corros. 63 (2012) 231–237.
19] X.H. Li, X.G. Xie, Acta Phys. -Chim. Sin. 29 (2013) 2221–2231.
[20] M. Yadav, D. Behera, S. Kumar, R.R. Sinha, Ind. Eng. Chem. Res. 52 (2013) 6318–6328.
21] M. Yadav, D. Behera, U. Sharma, Corros. Eng. Sci. Technol. 48 (2013) 19–27.
22] M. Yadav, D. Behera, U. Sharma, Arab. J. Chem. (2012) http://dx.doi.org/10.1016/j.arabjc.
012.03.011.
[23] H. Vashisht, S. Kumar, I. Bahadur, G. Singh, Int. J. Electrochem. Sci. 8 (2013) 684–699.
24] H. Vashisht, I. Bahadur, S. Kumar, K. Bhrara, D. Ramjugernath, G. Singh, Int. J. Electrochem.
[
[
The authors gratefully acknowledge the North-West University, De-
partment of Science and Technology and the National Research Founda-
tion (DST/NRF) South Africa grant funded (grant UID: 92333) for the
postdoctoral scholarship of Dr I. Bahadur.
[
[
2
[
Sci. 9 (2014) 2896–2911.
[25] H. Vashisht, I. Bahadur, S. Kumar, G. Singh, D. Ramjugernath, E.E. Ebenso, Int. J.
Electrochem. Sci. 9 (2014) 5204–5221.
Appendix A. Supplementary data
[
26] M. Yadav, S. Kumar, D. Behera, I. Bahadur, D. Ramjugernath, Int. J. Electrochem. Sci. 9
2014) 5235–5257.
[27] M. Yadav, S. Kumar, D. Behera, I. Bahadur, D. Ramjugernath, Int. J. Electrochem. Sci. 9
2014) 3928–3950.
28] S. Bhargava, L.K. Rajwanshi, Indian J. Chem. 52B (2013) 448–452.
[29] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789.
30] M.A. Hegazy, A.M. Badawi, S.S. Abd El Rehim, W.M. Kamel, Corros. Sci. 69 (2013) 110–122.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2015.06.063.
(
(
[
References
[
[
[
[
[
[
[
1] E.E. Ebenso, I.B. Obot, Int. J. Electrochem. Sci. 5 (2010) 2012–2035.
2] P. Mourya, P. Singh, A.K. Tewari, R.B. Rastogi, M.M. Singh, Corros. Sci. 95 (2015) 71–87.
3] G.E. Badr, Corros. Sci. 51 (2009) 2529–2536.
4] Y. Ren, Y. Luo, K. Zhang, G. Zhu, X. Tan, Corros. Sci. 50 (2008) 3147–3153.
5] K.S. Jacob, G. Parameswaran, Corros. Sci. 52 (2010) 224–228.
6] M.A. Amin, S.S. Abd El-Rehim, E.E.F. El-Sherbini, R.S. Bayoumi, Electrochim. Acta 52 (2007)
588–3600.
7] D. Daoud, T. Douadi, H. Hamani, S. Chafaa, M. Al-Noaimi, Corros. Sci. 94 (2015) 21–37.
8] M.M. Antonijevic, G.D. Bogdanovic, M.B. Radovanovic, M.B. Petrovic, A.T. Stamenkovic, Int.
J. Electrochem. Sci. 4 (2009) 654–661.
9] A. Zarrouk, B. Hammouti, T. Lakhlifi, M. Traisnel, H. Vezin, F. Bentiss, Corros. Sci. 90 (2015)
72–584.
[31] 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.
[32] I. Dehri, M. Ozcan, Mater. Chem. Phys. 98 (2006) 316–323.
[33] X. Wang, H. Yang, F. Wang, Corros. Sci. 53 (2011) 113–121.
[34] M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, M. Hamadanian, A. Gandomi,
Corros. Sci. 50 (2008) 2172–2181.
3
[35] M.S. Morad, A.M. Kamal El-Dean, Corros. Sci. 48 (2006) 3398–3412.
[36] G.N. Mu, X.H. Li, Q. Qu, J. Zhou, Corros. Sci. 48 (2006) 445–459.
[37] M. Lebrini, M. Lagrene'e, H. Vezin, M. Traisnel, F. Bentiss, Corros. Sci. 49 (2007) 2254–2269.
[38] H. Gerengi, H.I. Sahin, Ind. Eng. Chem. Res. 51 (2012) 780–787.
[39] M. Lebrini, F. Robert, A. Lecante, C. Roos, Corros. Sci. 53 (2011) 687–695.
[40] P.B. Raja, A.K. Qureshi, A.A. Rahim, H. Osman, K. Awang, Corros. Sci. 69 (2013) 292–301.
[41] S. Xia, M. Qiu, L. Yu, F. Liu, H. Zhao, Corros. Sci. 50 (2008) 2021–2029.
[42] V.S. Sastri, J.R. Perumareddi, Corrosion 53 (1997) 617–622.
[
[
[
5
[
[
[
10] X. Li, X. Xie, S. Deng, G. Du, Corros. Sci. 87 (2014) 27–39.
11] M.V. Fiori-Bimbi, P.E. Alvarez, H. Vaca, C.A. Gervasi, Corros. Sci. 92 (2015) 192–199.
12] H.M. Abd El-Lateef, Corros. Sci. 92 (2015) 104–117.