N-heterocycles as corrosion inhibitors for mild steel in acid medium

Article abstract of DOI:10.1016/j.molliq.2015.12.096

The corrosion behavior of mild steel in 1 M H₂SO₄ was studied using 2,3-diphenylpyrazine (DP), 2,3-di(furan-2-yl)pyrazine (FP) and 2,3-di(furan-2-yl)quinoxaline (FQ) as inhibitors using weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) methods were utilized for surface characterization. The results showed that the three inhibitors possess excellent inhibition effect toward mild steel corrosion. The inhibitor molecules are adsorbed on the mild steel surface, blocking the reactive sites available for acid attack. Adsorption of the inhibitor was found to obey Langmuir isotherm. Electronic structure calculations were used to study the inhibition efficiency of the inhibitor molecules on Fe (100) surface. The calculated results are in agreement with the experimental findings.

Full text of DOI:10.1016/j.molliq.2015.12.096

Journal of Molecular Liquids 216 (2016) 42–52
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
N-heterocycles as corrosion inhibitors for mild steel in acid medium
Jagadeesan Saranya a, Murugaiyan Sowmiya c, Palanisamy Sounthari b, Kandhasamy Parameswari a,
c,
Subramanian Chitra a, , Kittusamy Senthilkumar
⁎
⁎
a
Department of Chemistry, PSGR Krishnammal College for Women, Coimbatore, India
b
Department of Chemistry, PSG College of Arts and Science, Coimbatore, India
Department of Physics, Bharathiar University, Coimbatore, India
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The corrosion behavior of mild steel in 1 M H2SO4 was studied using 2,3-diphenylpyrazine (DP), 2,3-di(furan-2-
yl)pyrazine (FP) and 2,3-di(furan-2-yl)quinoxaline (FQ) as inhibitors using weight loss, potentiodynamic polar-
ization and electrochemical impedance spectroscopy (EIS) studies. Scanning electron microscopy (SEM) and
atomic force microscopy (AFM) methods were utilized for surface characterization. The results showed that
the three inhibitors possess excellent inhibition effect toward mild steel corrosion. The inhibitor molecules are
adsorbed on the mild steel surface, blocking the reactive sites available for acid attack. Adsorption of the inhibitor
was found to obey Langmuir isotherm. Electronic structure calculations were used to study the inhibition effi-
ciency of the inhibitor molecules on Fe (100) surface. The calculated results are in agreement with the experi-
mental findings.
Received 28 October 2015
Accepted 24 December 2015
Available online xxxx
Keywords:
Charge transfer resistance
Adsorption
Inhibition mechanism
Mild steel
Density functional theory
Impedance
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
structure, which facilitates electrophilic attack. Quinoxaline derivatives
are a very important class of nitrogen-containing compounds; it has
Iron and its alloys are extensively used in industry; therefore, it is
necessary to explore the corrosion inhibition of these materials in vari-
ous aggressive environments. Mineral acids are regularly used in the in-
dustrial areas such as acid cleaning of boilers, acid pickling, acid
descaling and oil well acidizing. Due to the aggressive nature of the
acid solutions, various corrosion protection methods are used to reduce
its attack on the metallic materials. Application of corrosion inhibitors in
these processes controls the metal dissolution as well as the acid con-
sumption [1]. Inhibitors are adsorbed on the metal surface, forming a
protective barrier and interact with anodic or/and cathodic reaction
sites to decrease the oxidation or/and reduction of corrosion reactions.
Most of the well-known acid inhibitors are organic compounds contain-
ing electronegative functional groups and π electrons in conjugated
double or triple bonds and hence exhibit good inhibitive properties by
supplying electrons through π orbitals. There is also a specific interac-
tion between functional groups containing heteroatoms like nitrogen,
sulfur, oxygen having free lone pair of electrons and the metal surface,
which play an important role in inhibition. When both of these features
combine, increased inhibition can be observed [2–4].
pyrazine cycle annulated with the benzene ring. The anti-tuberculotic,
anti-cancer activities, anti-mycobacterial, anti-trichomonas, anti-
candida data, dyes, pharmaceuticals and electrical/photochemical ma-
terials of quinoxaline derivatives have also been reported [5–11].
Many of the substituted quinoxaline compounds have been recently
studied in detail as effective corrosion inhibitors for steel and copper
in acidic medium [12–17]. The good inhibitory effect of quinoxaline de-
rivatives has incited us to test quinoxaline derivatives as corrosion in-
hibitors for mild steel in 1 M H2SO4.
The pyrazine ring is a part of many polycyclic compounds of biolog-
ical and/or industrial significance. Examples are quinoxalines, phena-
zines and bioluminescent natural products pteridines, flavins and their
derivatives. All these compounds are characterized by a low-lying unoc-
cupied π-molecular orbital and by the ability to act as bridging ligand.
Due to these two properties pyrazine possess a characteristic reactivity
[18]. Pyrazines have been intensively investigated as effective corrosion
inhibitors in different acid medium [19–21]. Pyrazines and its different
substituted derivatives e.g., bromo, methyl and amino, have different in-
hibition efficiency depending on the availability of lone pair of electrons
on hetero atoms and also based on metal–heteroatom interactions of
concerned metal in acid solutions [22]. Quantum chemical calculations
found to be quite useful to investigate the electronic properties and re-
action mechanisms of the inhibitor molecules [23,24]. Recently, molec-
ular simulation studies reveal the mechanism of corrosion inhibition
through interaction of molecules with surface metal atoms.
The selection of quinoxaline and pyrazine derivatives as corrosion
inhibitors is based on the presence of two nitrogen atoms in their
⁎
Corresponding authors.
E-mail addresses: rajshree1995@rediffmail.com (S. Chitra), ksenthil@buc.edu.in
(K. Senthilkumar).
http://dx.doi.org/10.1016/j.molliq.2015.12.096
0167-7322/© 2015 Elsevier B.V. All rights reserved.

J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
43
The objective of the present work is to study the corrosion inhibition
of mild steel in 1 M sulphuric acid solution by quinoxaline and pyrazine
derivatives by weight loss and electrochemical techniques. The experi-
mental findings are discussed with various adsorption parameters.
The protective film formed on the metal surface was characterized by
scanning electron microscopy (SEM) and atomic force microscopy
(AFM) techniques. Electronic structure calculations based on density
functional theory methods were used to study the inhibition efficiency
of the FQ, FP and DP molecules on Fe (100) surface.
weight was calculated. The experiment was repeated for various inhib-
itor concentrations (0.5 mM, 1 mM, 2.5 mM, 5 mM, 7.5 mM, 10 mM) in
1 M H2SO4.
The inhibition efficiency, corrosion rate and surface coverage were
calculated from the weight loss results using the formulas,
ðWblank−Winhibitor
Þ
Inhibitor Efficiency; ð%IEÞ ¼
ꢀ 100
ð1Þ
Wblank
where Wblank is the weight loss of the metal without inhibitor; Winhibitor
is the weight loss of the inhibitor with inhibitor.
2. Experimental methods
Cold rolled mild steel specimen of size 3 cm × 1 cm × 0.08 cm having
composition 0.084% C, 0.369% Mn, 0.129% Si, 0.025% P, 0.027% S, 0.022%
Cr, 0.011% Mo, 0.013% Ni and the reminder iron were used for weight
loss measurements. For electrochemical methods, a mild steel rod of
same composition with an exposed area of 0.785 cm2 was used. The
specimens were polished with 1/0, 2/0, 3/0 and 4/0 grades of emery
sheets, degreased with trichloroethylene and dried using a drier. The
plates were kept in a desiccator to avoid the absorption of moisture.
534 ꢀ Weight loss ðgÞ
Corrosion Rate; CR ¼
ð2Þ
ð3Þ
Density ꢀ Area ðcmÞ ꢀ Time ðhÞ
%IE
100
Surface Coverage; θ ¼
2.3. Evaluation of inhibition efficiency by electrochemical techniques
2.1. Synthesis of the inhibitor
Electrochemical impedance spectroscopy (EIS) and potentiodynam-
ic polarization were carried out with IVIUM Compactstat Potentiostat/
Galvanostat. EIS measurements were carried out at a frequency range
of 10 KHz to 0.01 Hz with a superimposed sine wave of amplitude
10 mV. From the plot of Z′ vs Z″ the charge transfer resistance (Rct)
and double layer capacitance (Cdl) were calculated. The inhibition effi-
ciency was calculated using the formula,
A solution of o-phenylenediamine/ethylenediamine (1 mmol) and
furil (1 mmol) in ethanol:water (7:3, 10 ml) was stirred at room tem-
perature in the presence of catalytic amount of phenol (20 mol%,
0.01 g). The progress of the reaction was monitored by TLC (n-hex-
ane-ethyl acetate 20:1). After completion of the reaction, water
(20 ml) was added to the mixture and was allowed to stand at room
temperature for 30 min. During this time, crystals of the pure product
were formed which were collected by filtration and dried. For further
purification, the products were recrystallized from hot ethanol [25].
Melting point of the synthesized compounds were determined by
LABINDIA-Visual Melting Range Apparatus (MR-VIS+). The structure
of the synthesized compounds were confirmed by FTIR spectra recorded
using FTIR Affinity 1 (Schimadzu) spectrophotometer.
Rꢁ −Rct
ct
Inhibition Efficiency ð%Þ ¼
ꢀ 100
ð4Þ
Rꢁct
⁎
where Rct and Rct are the charge transfer resistance obtained in the ab-
sence and presence of the inhibitors.
The potentiodynamic polarization curves were made after EIS for a
potential range of −200 mV to +200 mV (versus OCP) with a scan
rate of 1 mV/s. The data were collected and analyzed by IVIUM Soft soft-
ware. The inhibition efficiency was calculated from the Icorr using the
formula
2,3-di(furan-2-yl)quinoxaline (FQ): Yield: 88%, melting point =
130–132 °C, pale brown solid, IR spectrum (γ/cm−1): 1687.53
(NC=N–) 1606.54–1566.51 (aromatic ring stretching), 1080–1275
(NC–O–Cb) (Scheme 1).
2,3-di(furan-2-yl)pyrazine (FP): Yield: 86%, melting point = 142–
144 °C, dirty white solid, IR spectrum (γ/cm−1): 1649.32 (NC=N–),
1601.51–1589.04 (aromatic ring stretching), 1074–1280 (NC–O–Cb)
(Scheme 2).
Icorr−I
Inhibition Efficiency ð%Þ ¼
corrðinhÞ ꢀ 100
ð5Þ
Icorr
2,3-diphenylpyrazine (DP): Yield: 96%, melting point = 119–122 °C,
pale yellow solid, IR spectrum (γ/cm−1): 1661.10 (NC=N–), 1609.71–
1561.06 (aromatic ring stretching) (Scheme 2).
where Icorr and Icorr(inh) signify the corrosion current density in the ab-
sence and presence of inhibitors.
2.4. Surface morphology
2.2. Evaluation of inhibition efficiency by weight loss method
2.4.1. Scanning electron microscope-energy dispersive X-ray spectroscopy
(SEM with EDS)
The initial weight of the degreased and dried mild steel plates of size
1 cm × 3 cm × 0.08 cm were taken. The plates were suspended in trip-
licates into the solution using glass hooks in 1 M H2SO4 taken in a 100 ml
beaker. Care was taken to ensure the complete immersion of the speci-
men. After a period of 3 h the specimens were removed, washed with
distilled water, dried and weighed. From the initial and final mass of
the plates (i.e., before and after immersion in the solution) the loss in
Mild steel specimen of dimensions 3 cm × 1 cm × 0.08 cm were im-
mersed in 1 M H2SO4 in the absence and presence of inhibitors with op-
timum concentration for 3 h. The specimens were removed, washed
with distilled water and dried. The chemical composition of the layer
formed was analyzed with an EDS coupled with SEM using Biomedical
Research Microscope (Mumbai, India).
Scheme 1. Synthesis of quinoxaline derivative.

44
J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
Scheme 2. Synthesis of pyrazine derivatives.
2.4.2. Atomic force microscopy (AFM)
3. Results and discussion
The AFM analysis was used to observe the surface of mild steel im-
mersed for 3 h in 1 M H2SO4 in the absence and presence of inhibitors
using NOVA software by Multimode Scanning Probe microscope
(NTMDT, NTEGRA prima, Russia) with cantilever length, width and
thickness 135, 30 and 2 μm respectively and 0.35–6.06 N/m force con-
stant. Mild steel specimens of dimensions 3 cm × 1 cm × 0.08 cm
were cleaned and immersed as described in above procedure and
used for AFM.
3.1. Weight loss method
Table 1 shows corrosion rate, surface coverage and inhibition effi-
ciencies obtained at room temperature in the presence of different con-
centrations of inhibitors in 1 M H2SO4. The results of the weight loss
experiments from Table 1 show that the mild steel corroded severely
in the uninhibited lM H2SO4 solution, whereas the presence of inhibitors
reduced the dissolution rate considerably. Shahin et al., pointed out in
their work [33] that the relative inhibition efficiencies of the compounds
can be rationalized qualitatively under the assumptions that:
2.5. Computational details
(i) the inhibition is essentially based on the coverage of the metal
surface by the inhibitor molecules, thus making the contact of
the corroding species difficult;
(ii) attachment of the molecules on the metal surface is facilitated
through the coordination of π electron system to the metal
atom; and
To study the inhibition efficiency of Fe with the inhibitors 2,3-
di(furan-2-yl)quinoxaline (FQ), 2,3-di(furan-2-yl)pyrazine (FP) and
2,3-diphenylpyrazine (DP), the density functional theory calculations
are performed using the projector augmented wave (PAW) method as
implemented in the Vienna ab initio simulation package (VASP)
[26–29]. The exchange–correlation interactions are treated by general-
ized gradient approximation (GGA) with Perdew–Burke–Ernzerhof
(PBE) [30,31] functional. The valence orbitals are calculated as linear
combinations of plane waves, and the size of the basis set is determined
by a cutoff energy of 400 eV. In all the DFT calculations, a 1 × 1 × 1 (Г-
point) Monkhorst–Pack [32] k-point grid is used for sampling the
Brillouin zone. To accelerate the convergence, a Methfessel–Paxton
smearing of the Fermi surface is employed with a smearing width of
0.2 eV. In the present study, we utilized a 4 × 3 surface supercell con-
taining four Fe layers, which is allowed to relax until the total energy
difference between the loops is less than 1 meV and force is less than
0.02 eV/Å. A vacuum region of 10 Å is used to avoid periodic image in-
teractions between the slabs of Fe surface. The geometry optimization
for the isolated molecules is performed by placing the molecule in a
cell with the side length of 25 Å. During the geometry optimization of
adsorbed systems, the adsorbate molecule and two top surface layers
are allowed to relax, while the rest of the atoms in the slab are fixed at
their bulk lattice positions. The interaction of inhibitor molecule on Fe
surface is characterized through the adsorption energy, Eads and is cal-
culated by using the following equation:
(iii) the stability of the complex is somewhat related to the molecules
being planar.
Table 1
Inhibition efficiencies at various concentration of the inhibitors for corrosion of mild steel
in 1 M H2SO4 obtained by weight loss measurement at 303 K.
Name of the
inhibitor
Con
(mM)
Weight loss
(g)
Inhibition
efficiency
(%)
Surface
coverage
(θ)
Corrosion
rate
(g cm−2 h−1
)
BLANK
FQ
–
0.2212
0.1552
0.1284
0.0919
0.0621
0.04
0.0054
0.1596
0.1114
0.0937
0.0686
0.0523
0.0198
0.1649
0.1445
0.1124
0.0849
0.0617
0.0451
–
–
21.92
15.38
12.73
9.11
6.15
3.96
0.5
1.0
2.5
5.0
7.5
10.0
0.5
1.0
2.5
5.0
7.5
10.0
0.5
1.0
2.5
5.0
7.5
10.0
29.84
41.95
58.45
71.93
81.92
97.56
27.85
49.64
57.64
68.99
76.36
91.05
25.45
34.67
49.19
61.62
72.11
79.61
0.2984
0.4195
0.5845
0.7193
0.8192
0.9756
0.2785
0.4964
0.5764
0.6899
0.7636
0.9105
0.2545
0.3467
0.4919
0.6162
0.7211
0.7961
0.54
FP
15.82
11.04
9.29
6.80
5.18
1.96
Eads ¼ EInhibitor‐Fe−EFe−EInhibitor
ð6Þ
DP
16.34
14.32
11.14
8.41
6.11
4.47
where, EInhibitor–Fe is the total energy of the adsorbed system, i.e., inhibitor/
Fe adsorbed system, EFe is the total energy of the Fe surface and EInhibitor is
the total energy of the isolated inhibitor molecule.

J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
45
Table 2
The inhibitor FQ is found to be the best inhibitor with %IE 90 at a con-
centration of 10 mM. As the concentration of the inhibitor is increased
though %IE also increases. At higher concentration, the %IE is almost
leveled off and only slight variation is observed. This can be explained
as follows: the primary action of inhibition is the adsorption of the in-
hibitors onto the mild steel surface. Increase in concentration of the in-
hibitor lead to the gradual formation of multilayers that further reduce
the rate of corrosion beyond what can be achieved with monolayer cov-
erage below that concentration. In the present work only minor changes
in %IE were observed at higher concentration of the inhibitors. This is
because above a particular concentration only additional coverage be-
yond the monolayer results which is already sufficient for significant in-
hibition. The situation is analogous to adding a second coating of paint
to protect a surface that is already protected by an initial coating [34].
Adsorption parameters of the studied inhibitors on the mild steel surface at 303 K obtained
from Langmuir adsorption isotherm.
Name of the inhibitor
Langmuir isotherm
R2
Slope
K
−ΔG
(mol l−1
)
(kJ mol−1
)
FQ
FP
DP
0.9905
0.9917
0.9849
0.9787
1.024
1.111
1686.99
1499.34
1929.04
28.55
28.84
29.18
adsorption of inhibitors on the mild steel surface is spontaneous. Gener-
ally, the values of −ΔGads around or less than 20 kJ mol−1 are associat-
ed with the electrostatic interaction between charged molecules and
the charged metal surface (physisorption); while those around or
higher than 40 kJ mol−1 mean charge sharing or transfer from the in-
hibitor molecules to the metal surface to form a coordinate type of
metal bond (chemisorption). The values of Kads and ΔGads are listed in
Table 2. The negative values of ΔGads and the higher values of K reveal
the spontaneity of adsorption process and they are characteristic of
strong interaction and stability of the adsorbed layer with the mild
steel surface. The values of free energy of adsorption for all the systems
lie between −20 and −40 kJ mol−1 signifying both physical adsorption
and chemical adsorption would take place. Similar values of ΔGads were
also obtained by several researchers [35,36]. The R2 values and slope are
very close to unity, indicating strong adherence to Langmuir adsorption
isotherm [37,38].The possible mechanism of adsorption of the studied
inhibitors shown in Fig. 2.
3.2. Adsorption isotherm and adsorption parameters
It is generally assumed that the adsorption of the inhibitors on the
metal surfaces is the essential step in the mechanism of inhibition. The
establishment of isotherms that describe the adsorption behavior of
corrosion inhibitors is essential because they provide important clues
about the nature of metal–inhibitor interaction. The experimental data
were tested graphically by fitting to Langmuir adsorption isotherm.
The equation is given by
C
θ
1
Kads
¼
þ C
ð7Þ
where θ is the degree of surface coverage, C is the concentration of the
inhibitor, and Kads is the equilibrium constant of the adsorption–desorp-
tion process. The linear relationships of C/θ versus C, depicted in Fig. 1,
suggest that the adsorption of inhibitors on the mild steel surface
obeyed the Langmuir adsorption isotherm. Langmuir isotherm assumes
that the adsorption of organic molecule on the adsorbent is monolayer,
the adsorbed molecules occupy only one site, and there are no interac-
tions with other adsorbed species. The standard free energy of adsorp-
tion of inhibitor (ΔGads) on a mild steel surface can be evaluated with
Eq. (8):
3.3. Electrochemical methods
3.3.1. Electrochemical impedance spectroscopic technique
Electrochemical impedance spectroscopy (EIS) is an excellent tech-
nique that has been used in understanding the mechanism of corrosion
and passivation phenomena of metals and alloys in their surrounding
environments. EIS experiments were performed to report the effect of
quinoxaline and pyrazine derivatives on the inhibition of mild steel
and to determine the kinetic parameters for electron transfer reactions
at the mild steel/electrolyte interface.
Electrochemical impedance spectroscopy simultaneously provides
details about the kinetics of electrode processes and the surface proper-
ties of the investigated systems. From the shape of the impedance dia-
gram, mechanistic information can be derived. The impedance results
obtained in the presence and absence of inhibitors are presented as
Nyquist plots in Fig. 3. It is evident from the figure that MS exhibited
typical impedance behavior in 1 M H2SO4 medium for all the inhibitors
screened and displayed marked changes in impedance response for
each concentration studied. This can be attributed to charge transfer
of the corrosion process, the diameter of the semicircle increases with
increasing inhibitor concentration. The semicircles in the impedance di-
agram indicate that the corrosion process is mainly controlled by the
charge-transfer process. The main impedance parameters obtained are
listed in Table 3. As can be seen from the Table 3, with increasing
ΔGads ¼ −RT lnð55:5 Kads
Þ
ð8Þ
The value of 55.5 in the above equation is the concentration of water
in solution in mol l−1. The negative values of ΔGads suggest that the
Fig. 1. Langmuir plot for the studied inhibitors for mild steel in 1 M H2SO4.
Fig. 2. Possible mechanism of adsorption of studied inhibitors.

46
J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
Fig. 4. Polarization curves for mild steel recorded in 1 M H2SO4 for selected concentrations
of the inhibitor FQ.
Fig. 3. Nyquist diagram for mild steel recorded in 1 M H2SO4 for selected concentrations of
the inhibitor FQ.
concentration of inhibitors, the measured values of charge-transfer re-
sistance (Rct) increased and the values of the double-layer capacitance
(Cdl) decreased. The increase in the values of Rct and inhibition efficien-
cy are due to the formation of a protective surface film on the metal sur-
face caused by the gradual replacement of water molecules by adsorbed
inhibitor molecules. Also, the adsorption of the inhibitors results in the
formation of a protective film on the mild steel surface, which becomes
a barrier to hinder the mass and charge-transfer processes. This results
in an increase in the thickness of the electronic double layer, and a de-
crease in electrical capacitance [39]. The decrease in Cdl occurred as a re-
sult of the decrease in the local dielectric constant, which suggests that
the inhibitor molecules function by adsorption [40]. The inhibition effi-
ciencies obtained from different testing methods used in this study are
comparable and in fair agreement.
with inhibitor concentration, the inhibitors affected both these reac-
tions and acted as mixed-type inhibitors. A close examination of Fig. 3
reveals that the addition of inhibitors to 1 M H2SO4 affects both the an-
odic and cathodic parts of the curve. The adsorption of inhibitor can af-
fect the corrosion rate in two ways:
(i) by decreasing the available reaction area, the so-called geometric
blocking effect and
(ii) by modifying the activation energy of the cathodic and/or anodic
reactions occurring in the inhibitor-free metal in the course of
the inhibited corrosion process.
It is a difficult task to determine which aspect of the inhibiting effect
is connected to the geometric blocking action and which are connected
to the energy effect. Theoretically, no shifts in Ecorr should be observed
after addition of the corrosion inhibitors if the geometric blocking effect
is stronger than the energy effect [41]. The change observed in the Ecorr
values upon addition of the inhibitors therefore indicates that the ener-
gy effect is stronger than the blocking effect. It can be seen from the
Table 4 that the corrosion current density Icorr decreased noticeably
with increase in inhibitor concentration and the corrosion potential
Ecorr of mild steel shifts toward less negative direction, which suggests
that the inhibitors behave as very good corrosion inhibitors for mild
steel in 1 M H2SO4 solution. The higher values of bc compared to those
3.3.2. Potentiodynamic polarization technique
In order to know the kinetics of anodic and cathodic reactions, polar-
ization experiments were carried out potentiodynamically in unstirred
1 M H2SO4 solution in the absence and presence of different concentra-
tion of inhibitors and the obtained polarization curves are shown in
Fig. 4. The corrosion kinetic parameters derived from these curves are
presented in Table 4.
In acidic solutions, the anodic reaction is the movement of metal ions
from the working electrode into the solution, and the cathodic reaction
is the discharge of hydrogen ions to hydrogen gas or reducing the con-
centration of oxygen. The inhibitor may affect either the anodic or the
cathodic reaction or both. Since the anodic Tafel slope (ba) and cathodic
Tafel slope (bc) of quinoxalines and pyrazines were found to change
Table 4
Corrosion parameters for corrosion of mild steel with selected concentrations of the inhib-
itors in 1 M H2SO4 by potentiodynamic polarization method.
Table 3
Name of the
inhibitor
Con
(mM)
Tafel
slopes
(mV
−Ecorr
(mV/SCE)
Icorr
Inhibition
efficiency
(%)
(μA cm−2
)
AC-impedance parameters for corrosion of mild steel for selected concentrations of the in-
hibitors in 1 M H2SO4.
dec−1
)
Name of the inhibitor Con
Rct
Cdl
Inhibition efficiency
(%)
(mM) (Ohm cm2) (μF cm−2
)
ba
bc
BLANK
FQ
–
14.36
78.01
107.59
132.3
62.42
85.62
96.62
28.7
36.8
21.3
20.5
20.1
29.7
29.6
28.1
33.7
26.6
22.7
–
BLANK
FQ
–
67
49
43
47
47
55
47
42
28
29
142
144
132
128
110
115
142
148
173
177
514.9
468.5
470.8
457.2
501.8
478.0
468.2
469.4
460.8
448.1
562.4
297.5
221.0
124.5
365.7
306.7
136.1
344.1
280.3
246.6
–
0.5
5.0
10
0.5
5.0
10
0.5
5.0
10
81.59
86.65
89.15
76.99
83.23
85.14
49.97
64.14
73.73
0.5
5.0
10.0
0.5
5.0
10.0
0.5
5.0
10.0
47.10
60.70
77.86
34.98
45.47
75.82
38.82
50.16
56.15
FP
FP
DP
DP
40.04
54.67

J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
47
Fig. 5. SEM micrograph for (a) polished mild steel in the absence of 1 M H2SO4 and inhibitor and (b) mild steel in 1 M H2SO4 in the absence of inhibitor and (c) mild steel in 1 M H2SO4 in the
presence of inhibitor (FQ).
of ba at each concentration of the inhibitor further confirm that the in-
hibitor is predominantly cathodic. The anodic Tafel curves in the acidic
solution containing the inhibitor is shifted to the direction of the
lower current density, which suggests that the inhibitor could also sup-
press the anodic reaction [42]. An inhibitor can be classified as an anodic
or cathodic type when the change in Ecorr value is larger than 85 mV
[43]. However, in the present study, the largest displacement exhibited
by the inhibitors was less than 85 mV, from which it can be concluded
that all the inhibitors act as mixed type inhibitors. On the other hand,
the anodic and cathodic slope values of inhibited solution have changed
with respect to uninhibited solution, which also restates that the inhib-
itors are of mixed type. The obtained inhibition efficiency values are in
good correlation with the values of weight loss measurements.
3.4. Surface morphology
3.4.1. Scanning electron microscopy (SEM) studies
Panels (a), (b) and (c) in Fig. 5 represent the micrograph obtained
for polished mild steel in the absence of 1 M H2SO4 and inhibitor, mild
steel plates in the absence and in the presence of inhibitor (FQ) at
10 mM exposure for 3 h immersion in 1 M H2SO4. It is clear that the
mild steel surface had suffered from severe corrosion attack in the
blank sample. It is important to stress out that when the compound is
present in the solution, the morphology of mild steel surface is quite dif-
ferent from the previous one, and the specimen surfaces were smooth-
er. We noted the formation of a film, which is distributed in a random
way on the overall surface of the mild steel. This may be due to the
Fig. 6. EDS analysis for (a) mild steel in 1 M H2SO4 in the absence of inhibitor and (b) mild steel in 1 M H2SO4 in the presence of inhibitor (FQ).

48
J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
Table 5
inhibitor treatment. The EDS analysis of the specimen in the absence of
the inhibitor indicates the presence of Fe and O, which shows that the
passive film on the mild steel surface contained only Fe2O3. Panels
(a) and (b) in Fig. 6 represent the EDS analysis of mild steel in 1 M
H2SO4 and in the presence of 10 mM of the inhibitor (FQ). The spectrum
showed additional lines, due to N, O & C (due to the carbon atoms of
quinoxaline derivative). The data confirms the presence of the inhibitor
on the specimen surface. This is entirely due to the inhibitor, because
the carbon and N signals are absent on the specimen surface exposed
to uninhibited H2SO4. A comparable elemental distribution is shown
in Table 5. The obtained atomic weight% values of the elements from
the EDS spectra of inhibited mild steel surface show more intensity of
nitrogen and carbon which are present in FQ. The values of atomic
weight% of Fe in the inhibited acid is less compared to blank. This con-
firms the presence of inhibitor on the mild steel surface [46].
Surface composition of mild steel after 3 h of immersion in 1 M H2SO4 without and with
the optimum concentrations of the studied inhibitor FQ.
Name of the inhibitor
Element
Weight
(%)
Atomic weight
(%)
BLANK
FQ
O
Fe
C
O
N
Fe
33.80
66.20
11.81
9.08
8.33
70.79
64.05
35.95
28.75
18.96
15.22
37.07
dsorption of the inhibitor onto the mild steel surface incorporating into
the passive film in order to block the active sites present on the mild
steel surface. On the other hand, due to the involvement of inhibitor
molecules in the interaction with the reactive sites of mild steel surface,
there is a decrease in the contact between mild steel and the aggressive
medium which sequentially resulted in excellent inhibition effect [44,
45].
3.4.3. Atomic force microscopy (AFM) studies
For characterizing at the microstructure level, AFM is used to study
the influence of inhibitor on the metal surface and the progress of the
corrosion at the metal/solution interface. Panels (a) to (d) in Fig. 7 com-
prise the two-dimensional (2D) and three-dimensional (3D) AFM im-
ages of mild steel surface in 1 M H2SO4 solution without and with
2 mM concentration of the inhibitor FQ. Fig. 6(a) shows the mild steel
surface immersed in 1 M H2SO4 without inhibitor due to the aggressive
attack of acid. AFM microstructure of Fig. 6(d) shows that there is a
3.4.2. Energy dispersion spectroscopy (EDS) studies
The EDS spectra were used to determine the elements present on the
surface of mild steel after 3 h of exposure in the uninhibited and
inhibited 1 M H2SO4. Panels (a) & (b) in Fig. 5 show the EDS analysis re-
sult on the composition of mild steel immersed in the acid only and after
Fig. 7. AFM photographs for (a) 2D image of mild steel in 1 M H2SO4 in the absence of inhibitor, (b) 3D image of mild steel in 1 M H2SO4 in the absence of inhibitor, (c) 2D image of mild steel
in 1 M H2SO4 in the presence of inhibitor (FQ) and (d) 3D image of mild steel in 1 M H2SO4 in the presence of inhibitor (FQ).

J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
49
wrapping zonal film adsorbed on mild steel surface in 1 M H2SO4 with
the addition of inhibitor FQ. The calculated Sa values of mild steel in
1 M H2SO4 without inhibitor was 530 nm. After the addition of inhibitor
in 1 M H2SO4, the Sa value was decreased to 126 nm. The reduction in Sa
values in the presence of inhibitor in acid solutions is due to the de-
crease in surface roughness. This can be attributed to the formation of
protective layer by the inhibitor molecules [47].
3.5. Theoretical studies
Theoretical calculations based on DFT are used to investigate the ad-
sorption of three inhibitor molecules 2,3-di(furan-2-yl)quinoxaline
(FQ), 2,3-di(furan-2-yl)pyrazine (FP) and 2,3-diphenylpyrazine (DP)
on Fe (100) surface. The optimized geometry of FQ, FP, DP and Fe
(100) surface is shown in Fig. 8. For each molecule, we have studied
two different adsorption configurations, perpendicular and parallel.
In the perpendicular adsorption configuration, the inhibitor mole-
cule is oriented perpendicular to the Fe (100) surface, whereas in the
parallel adsorption configuration, the inhibitor molecule is aligned par-
allel to the Fe (100) surface. The optimized adsorption configurations of
inhibitor molecules on Fe (100) surface are shown in Figs. 9 and 10. The
adsorption energy is calculated by using Eq. (6) and the results are sum-
marized in Table 6. The selected geometrical parameters are listed in
Tables S1, S2 and S3 (Supporting information). The adsorption energy
of FQ, FP and DP molecules in the perpendicular adsorption configura-
tion is −0.26, −0.21 and −0.14 eV, respectively. The structural param-
eters given in Tables S1, S2 and S3 clearly show that there is no
significant change in the geometrical parameters of inhibitor molecules
due to perpendicular adsorption on Fe (100) surface. Further, no cova-
lent or ionic bonds are formed between the molecules and the surface
Fe atoms, that is, the adsorption is physisorption.
As shown in Fig. 10, in parallel adsorption configuration of FQ mole-
cule, the oxygen and carbon atoms of FQ molecule are directly
interacting with the Fe atoms of Fe (100) surface. In the FP/Fe (100)
adsorbed system, carbon, oxygen and nitrogen atoms of FP molecule
Fig. 9. Optimized perpendicular adsorption configuration of FQ, FP and DP molecules on Fe
(100) surface.
are interacting with Fe atoms in the surface. But, in the case of DP mol-
ecule, carbon atoms of DP molecule alone is interacting with surface Fe
atom. From the Table 6, it has been observed that the FQ adsorbed sys-
tem is having the maximum adsorption energy of −3.01 eV. The calcu-
lated adsorption energy of FP in the parallel adsorption configuration is
−2.96 eV. While comparing FQ, FP and DP molecules, the DP/Fe (100) is
having lesser adsorption energy of −1.03 eV. The above results show
that the oxygen and nitrogen atoms of inhibitor molecule are acting as
the reactive sites to enhance the inhibition efficiency on Fe surface
and is in agreement with the previous studies [48–50]. Among the stud-
ied inhibitor molecules, the FQ molecule interacts strongly with the Fe
surface and is in agreement with the experimental findings. From the
Tables S1, S2 and S3, it has been found that the geometry of the inhibitor
molecules gets altered by the adsorption on Fe (100) surface in the par-
allel adsorption configuration. The average bond length between adsor-
bate oxygen atoms and the bonding surface Fe atoms is 1.90 Å, which is
close to the computed Fe–O bond distance of 1.94 Å [51]. Also the aver-
age bond length between adsorbate carbon atoms and bonding surface
Fe atoms is 1.86 Å. The C–O distance in furan rings of adsorbed FQ is
1.42 Å, which is slightly higher than that of the initial isolated molecule.
The C–C bond length of pyrazine ring in adsorbed FQ molecule is elon-
gated by about 0.05 Å from that of the isolated molecule. As shown in
Fig. 10, the Fe atoms which interacting directly with the O and C
atoms of the FQ molecule move slightly upward by 0.1 Å. It has been ob-
served that the (C9–O1–C10) and (C13–O2–C14) angles in the FQ ad-
sorption configuration is increased by about 8° from that of isolated
molecule. This is because the strong interaction between the oxygen
atoms and surface Fe atoms. For FP, the average bond length between
adsorbate oxygen atoms and bonding surface Fe atoms is 1.89 Å and
the bond length between adsorbate carbon atom and bonding Fe atom
is 2 Å. In the FP/Fe (100), the calculated bond length between adsorbate
Fig. 8. Optimized structure of (a) 2,3-di(furan-2-yl)quinoxaline (FQ), (b) 2,3-di(furan-2-
yl)pyrazine (FP), (c) 2,3-diphenylpyrazine and (DP) and (d) Fe surface. The C, O, N, H
and Fe atoms are shown in green, red, violet, blue and brown colors, respectively. (For
interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)

50
J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
Fig. 11. Electron density difference plot of (a) FQ/Fe (100), (b) FP/Fe (100) and DP/Fe
(100) in parallel configurations. Yellow isosurface represents the increase in electron
density and cyan isosurface represents the decrease in electron density upon the
adsorption. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
plotted with isosurface value of 0.006 e/Å3 and is shown in Fig. 11. In
these plots, the yellow and cyan isosurfaces indicates the gain and loss
of electron density, respectively upon the adsorption. It has been ob-
served that the charge accumulation and depletion are mainly confined
to the area between interacting atoms of adsorbate and the surface,
which suggests that covalent bonds are formed between interacting
atoms of adsorbate and surface Fe atoms. The yellow region is mainly
found at inhibitor molecule and the cyan region is found at the surface
Fe atoms. The above result clearly indicates that the charge is trans-
ferred from the surface Fe atoms to the molecule.
In order to quantify the charge redistribution, the Bader charge anal-
ysis is performed for the optimized adsorption configurations. The
Bader charge analysis uses the atoms in molecule (AIM) approach, in
which the charge on each atom is calculated by integrating the electron
density within its Bader volume [52]. The calculated total charge on
molecule and surface in the isolated and adsorbed systems are summa-
rized in Table S4. In the FQ adsorption configuration, the interacting ox-
ygen and carbon atoms gain charge by about 0.18 to 0.2 e and 0.21 to
0.25 e, respectively, while the interacting surface Fe atoms losses the
charge by about 0.2 e. In the FP adsorption configuration, the carbon
and oxygen atoms of FP molecule gains the charge by about 0.2 e, and
the nitrogen atom gains the charge by about 0.3 e upon the adsorption.
In the DP adsorption configuration, the carbon atoms gain the charge by
about 0.22 to 0.29 e, while the interacting surface Fe atoms losses the
charge by about 0.18 to 0.24 e. The above results clearly indicate that
the charge is transferred from surface Fe atoms to inhibitor molecule,
confirming the results obtained from the electron density difference
plots. The calculated adsorption energy, structural parameters, electron
density difference and Bader atomic charges clearly show that the FQ is
the efficient inhibitor and DP is the lowest efficient inhibitor on Fe
surface.
Fig. 10. Optimized parallel adsorption configuration of FQ, FP and DP molecules on Fe
(100) surface.
nitrogen atom and bonding surface Fe atom is 1.97 Å. The bond length
and bond angles corresponding to the interacting atoms in the FP mol-
ecule significantly changes upon the adsorption, and the other structur-
al parameters are slightly changing from that of the isolated systems
(see Table S2). For DP adsorbed system, the bond length between adsor-
bate carbon atom and bonding surface Fe atom is 1.71 Å. In order to re-
lease the stress between the molecule and the surface, the C1–C2 bond
is elongated by about 1.60 Å. After the adsorption, the (C2–N1–C3)
angle of DP molecule is changed by 34° after the adsorption, showing
that the C2 atom of DP molecule is strongly interacting with the surface
Fe atom. The calculated adsorption energy and structural parameters of
the adsorbed systems indicate that in parallel adsorption configurations
the adsorption is chemisorption.
3.6. Electronic structure analysis
To study the electrostatic interaction between inhibitor molecules
and Fe (100) surface, we have analyzed the electron density difference
and Bader atomic charges. While comparing perpendicular and parallel
adsorption configurations inhibitor molecules on Fe (100) surface, it has
been observed that the parallel adsorption configuration exhibits strong
interaction between inhibitors and Fe surface. Therefore, the electron
density difference map for parallel adsorption configurations has been
Table 6
Calculated adsorption energy of FQ, FP and DP molecules on Fe (100) surface using PBE
method.
Adsorbate molecules
Adsorption energy
(Eads) in eV
4. Conclusions
Perpendicular
Parallel
This study has revealed that the inhibition efficiencies of the
studied compounds 2,3-di(furan-2-yl)quinoxaline (FQ), 2,3-di(furan-
2-yl)pyrazine (FP) and 2,3-diphenylpyrazine (DP) tended to increase
with increasing inhibitor concentration. The order of inhibition
FQ
FP
DP
−0.26
−0.21
−0.14
−3.01
−2.9
−1.03

J. Saranya et al. / Journal of Molecular Liquids 216 (2016) 42–52
semiconductor applications, J. Mater. Chem. 11 (2001) 2238–2243.
51
efficiency is FQ N FP N DP. Polarization measurements show that they are
mixed-type inhibitors. Nyquist plots established that the inhibitors re-
duced the mild steel corrosion through their effective adsorption of in-
hibitive layer, which is further evidenced from SEM and AFM. The
adsorption of the studied inhibitors on mild steel followed the Langmuir
adsorption. The inhibition of 2,3-di(furan-2-yl)quinoxaline (FQ), 2,3-
di(furan-2-yl)pyrazine (FP) and 2,3-diphenylpyrazine (DP) on Fe
(100) surface is studied by using first principle calculations. Theoretical
results confirm the experimental findings that FQ molecule is the most
efficient inhibitor molecule on Fe surface with the adsorption energy of
−3.01 eV in the parallel adsorption configuration. Further, the electron-
ic structure calculations show that the charge is transferred from the
surface Fe atoms to the inhibitor molecules.
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Acknowledgment
The authors compete no financial interest.
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