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REGULAR ARTICLE
Corrosion inhibition, adsorption and thermodynamic propertiesof 2-aminopyridine derivatives on the corrosion of carbon steelin sulfuric acid solution
TARIK ATTARa,b,* , FATIHA NOUALIc, ZAHIRA KIBOUc,d, ABBES BENCHADLIa,
BOULANOUAR MESSAOUDIa,b, ESMA CHOUKCHOU-BRAHAMa and
NOUREDDINE CHOUKCHOU-BRAHAMc
aLaboratory of ToxicoMed, University of Abou Bekr Belkaid, B.P.119, 13000 Tlemcen, AlgeriabHigher School of Applied Sciences, P.O. Box 165 RP, 13000 Tlemcen, AlgeriacLaboratoire de Catalyse et Synthese en Chimie Organique, Faculte des Sciences, Universite de Tlemcen,
B.P. 119, 13000 Tlemcen, AlgeriadFaculte des Sciences et de la Technologie, Universite de Ain Temouchent, B.P. 284,
46000 Ain Temouchent, Algeria
E-mail: att_tarik@yahoo.fr; t.attar@essa-tlemcen.dz
MS received 24 March 2021; revised 23 June 2021; accepted 24 June 2021
Abstract. Corrosion inhibition of carbon steel in sulfuric acid solution was performed by three synthesized
products named 2-(butylamino)-4-phenylnicotinonitrile (BAPN), 2-(propylamino)-4-phenylnicotinonitrile,
and 2-(methylamino)-4-phenylnicotinonitrile using weight-loss method and scanning electron microscopy
(SEM). The temperature impact on the inhibition mechanism of the synthesized inhibitors of the carbon steel
surface was investigated at various temperatures (20–50 �C) where the inhibitive efficiency diminished with
increasing temperatures. The maximum IE of 97.45% was achieved at a temperature of 20 �C, the con-
centration of BAPN inhibitor of 5910-4 M, and H2SO4 acid concentration of 0.5 M. The adsorption of
inhibitors studied onto the carbon steel surface obeys the Langmuir adsorption isotherm. The SEM surface
analysis showed the formation of a protective organic film on the steel surface. The quantum chemical
calculations (DFT) supported the experimental results and showed that the inhibition efficiency also depends
on the structure of the inhibitor.
Keywords. Corrosion inhibition; 2-Aminopyridine derivatives; Carbon steel; Weight loss; DFT; SEM.
1. Introduction
In the chemical industry, carbon steel is the dominant
metal material because of its wide use and various
applications thanks to its low price and exceptional
mechanical and physical properties.1 The wastes and
residues of the metallic component damage in relation
to corrosion are countless. This includes cracks, time
loss and production arrest, structural and mechanical
damage, pollution, economic problems, and also large-
scale ecological damage.2 Hence, these negative
effects can be minimized or eliminated using diverse
techniques to protect the metal surface from aggres-
sive environments. Sulfuric acid is one of the strong
inorganic acid used in various industrial processes,
such as steel manufacturing, acid pickling, acid
descaling, gasoline, ion exchange resins regenerating,
sulfonation agents, cellulose fibers, sugar bleaching,
paper bleaching, pharmaceuticals, fertilizers, automo-
bile batteries, amino acid intermediates, water treat-
ment, coloring agents, oil well acidizing and acid
cleaning. The use of corrosion inhibitors is the most
practical alternative to block the active sites and
enhance the adsorption process, thus decreasing the
dissolution rate and extending the equipment life
span.3 The study of corrosion processes and their
inhibition by organic compounds is a very large field
of research. The corrosion inhibition using organic
*For correspondence
Supplementary Information: The online version contains supplementary material available at https://doi.org/10.1007/s12039-021-01971-w.
J. Chem. Sci. (2021) 133:109 � Indian Academy of Sciences
https://doi.org/10.1007/s12039-021-01971-wSadhana(0123456789().,-volV)FT3](0123456789().,-volV)
compounds is mainly because of the physical or
chemical adsorption resulting from the interaction of
the inhibitor’s polar centers with active sites on the
metal surface.4–6 They act through the adsorption
mechanism on the surface of alloys of these molecules
that form a barrier against corrosive solutions.7,8 On
the other hand, many syntheses of heterocyclic com-
pounds have nitrogen or oxygen; in addition to con-
jugated pi-system have been synthesized by our
research group.9–12 Application of pyridine derivatives
as corrosion inhibitors can be found in literature.
3-imino-4-methyl-2-(pyridin-3-ylhydrazono)pentanen-
itrile, 4-(3,4-dichlorophenyl)-2,6-dimethyl-1,2-dihy-
dropyridine-3,5-dicarbonitrile, 1,4-diamino-5-cyano-
2-(4-methoxyphenyl)-6-oxo-1,6-dihydropyridine-3-
carboxylic acid and ethyl 4-amino-5-cyano-2-(di-
cyanomethylene)-6-phenyl-1,2-dihydropyridine-3-car-
boxylate have been applied as corrosion inhibitors for
carbon steel in 2 M HCl, giving only about 69.5, 74.8,
82.1 and 91.0% inhibition efficiencies respectively at
1910-4 M.13 The corrosion inhibition efficiencies of
2-amino-6-methoxy-4-phenylpyridine-3,5-dicarboni-
trile, 2-amino-6-methoxy-4-(4-methyl-phenyl) pyr-
idine-3,5-dicarbonitrile and 2-amino-6-methoxy-4-(4-
methoxylphenyl) pyridine-3,5-dicarbonitrile have
been reported to inhibit mild steel corrosion in 1 M
HCl to the tune of 88, 95 and 97% efficiency respec-
tively at 1.6910-3 M concentration.14 Also, 6-(2,4-
dihydroxyphenyl)-4-phenyl-2-(phenylamino)nicoti-
nonitrile (DPPN), 6-(2,4-dihydroxyphenyl)-2-((4-hy-
droxyphenyl)amino)-4-phenylnicotinonitrile (DHPN)
and 6-(2,4-dihydroxyphenyl)-2-((4-methoxyphenyl)
amino)-4-phenylnicotinonitrile (DMPN) for mild steel
corrosion in 1 M HCl have been estimated to be 94.02,
95.42 and 97.18%, respectively at 20.20910-5 M
concentration of the inhibitors and 25 �C.15 The
present investigation is an attempt to explore three
novel inhibitors: 2-(butylamino)-4-phenylnicotinoni-
trile, 2-(propylamino)-4-phenylnicotinonitrile and
2-(methylamino)-4-phenylnicotinonitrile. They are
easily synthesizable, non-toxic and biologically
important,16 best suited for the development of non-
toxic inhibitors, and for an enhanced inhibition effi-
ciency at lower concentrations and at higher temper-
atures by introducing more heteroatoms (nitrogen) for
the mitigation of carbon steel corrosion. It was con-
sidered worthwhile to test these inhibitors for studying
the corrosion inhibition performance of carbon steel in
0.5 M H2SO4 solution by weight loss and surface
analysis. In addition, quantum chemical calculations
using density functional theory (DFT) were performed
to estimate theoretically the reactivity parameters of
the inhibitor.
2. Experimental
2.1 Spaceman preparation and weight lossmeasurements
Carbon steel samples used as test materials contain: C:
0.37%, Mn: 0.68%, Cu: 0.16%, Cr: 0.077%, Ni:
0.059%, Si: 0.023%, S: 0.016%, Ti: 0.011%, Co:
0.009% and the balance being Fe. Prior to tests, the
carbon steel material was abraded using a series of
emery paper sheets (from 600 to 1200 grains) to be
finally washed with water and acetone (99%, Sigma-
Aldrich, Germany). The solution of 0.5 M H2SO4 was
prepared by dilution of sulfuric acid (98%, Merck,
Germany) with distilled water. Various concentrations
of inhibitors (1910-6 to 5910-4 M) in 0.5 M H2SO4
solution were used at different temperatures which
were 293, 303, 313 and 323 K. The investigated
samples were washed with distilled water, acetone
then dried and weighed. All measurements were done
in triplicate, and the mean value of the weight loss was
reported and recorded. The corrosion rate ‘CR’ and the
inhibitor efficiency were calculated via the following
equations Eqs. 1 and 2:17,18
CR ¼ w
S� tð1Þ
where w denotes the weight loss (mg), S represents
the sample area (cm-2) and t symbolizes the immer-
sion time (h-1). The corrosion inhibition efficiency
(IE%) and the surface coverage (h) were calculated
from the CR values:
IE (% ) ¼ CR� CRInh
CR� 100 ð2Þ
where CR and CRinh denote the obtained corrosion
rates in the absence and presence of the inhibitor.
2.2 Characterization of carbon steel surface
Scanning electron microscopy (SEM) and TM1000
Hitachi Tabletop Microscope were used to investi-
gate the sample’s surface immersed in sulfuric acid,
and the presence and absence of corrosion inhibitors
for 24 h.
2.3 Computational studies
The effectiveness of inhibitors can be related to their
molecular electronic and spatial molecular structures.
The present work was; therefore, extended to establish
109 Page 2 of 10 J. Chem. Sci. (2021) 133:109
the effectiveness of three novel compounds as corro-
sion inhibitors using the tools of density functional
theory. DFT method at the B3LYP/6-31G* level of
theory was employed to test the inhibitor quantum
chemical calculations using the Gaussian-09 suit of
program. The values of the highest molecular orbitals
(EHOMO) and lowest occupied molecular orbitals
(ELUMO) were calculated. Other parameters, such as
electronegativity (v), global hardness (g), the affinity
of electrons (EA), and the ionization potential (IP)were determined by Koopmans’ theorem.19 The
HOMO energy is related to the ionization potential
(IP = -EHOMO) whereas the LUMO energy is linked
to the electron affinity (EA = -ELUMO).20 The energy
gap (DE) was determined as:21
DE ¼ ELUMO � E HOMO ð3ÞUsing electron affinity (EA) and ionization potential
(IP), the electronegativity (v) and the global hardness
(g) may be calculated as:22
v ¼ 1=2 IPþ EAð Þ ð4Þ
g ¼ 1=2 IP� EAð Þ ð5ÞThe number of transferred electrons (DN) was cal-
culated as:23
DN ¼ vFe � vInhð Þ=2 gFe þ gInhð Þ ð6Þwhere vFe and vInh are the absolute electronegativitiesof iron and the inhibitor, gFe and gInh are the absolute
hardness of iron and the inhibitor, respectively.
The theoretical values (v=7.0 eV mol-1 and g=0 eVmol-1) for iron were obtained from the literature.24
The value of the charges back-donation was calcu-
lated using the following expression:25
DEback�donation ¼ �g=4 ð7ÞThe charges transferred to the molecule, are ener-
getically favored when g[ 0 and DEback-donation\0.
3. Results and Discussion
3.1 Effect of concentration and temperatureon inhibition efficiency
The temperature can change the interactions between
the carbon steel and the sulfuric acid in the absence
and presence of the inhibitors. All the three inhibitors,
that is, BANP, PANP and MPAN are found highly
efficient for the corrosion inhibition of carbon steel.
On varying the concentration of the inhibitors used, a
change in the inhibition efficiency and the surface
coverage was observed as shown in Table 1. An
obvious increase was observed in the surface coverage
and the percentage inhibition with increasing con-
centrations of the inhibitors indicating the inhibitor
adsorption on the metal surface. The inhibition effi-
ciency obtained at optimum concentrations at 293 K
were 97.45, 95.43 and 91.73% for BAPN, PAPN and
MAPN, respectively. The inhibition efficiency
decreases as the temperature increases. This indicates
that the desorption of the adsorbed inhibitor due to the
enhanced solution agitation by higher rates of hydro-
gen gas evolution at elevated temperature is possible
and may cause the ability of the inhibitor to be
adsorbed on the carbon steel surface to reduce.26
BAPN (80.56%) shows a good inhibitor against cor-
rosion compared to other inhibitors. This inhibitory
power is more remarkable in a concentration of
1910-6 M. It is worthy of mentioning that the tests
were repeated three times to ensure reproducibility,
and the obtained results are the mean value (Table 1).
The evaluated inaccuracy did not exceed 5%.
3.2 Effect of immersion timeon inhibition efficiency
The immersion time is another important parameter
which ascertains the inhibitive effect on the metallic
surfaces. In order to study the effect of the immersion
time on the corrosion behavior of carbon steel, the
Table 1. Inhibition efficiency IE(%) and surface coveragefrom weight loss measurement for carbon steel corrosion in0.5 M H2SO4 without and with the addition of differentconcentrations of BAPN, PAPN and MAPN at differenttemperatures.
T (K) C (M) IE(%) h IE(%) h IE(%) hBAPN PAPN MAPN
293 1910-6 80.56 0.80 68.77 0.68 64.36 0.645910-6 87.51 0.87 78.58 0.78 76.74 0.761910-5 93.15 0.93 86.37 0.86 84.97 0.845910-4 97.45 0.97 95.43 0.95 91.73 0.91
303 1910-6 66.76 0.66 52.66 0.52 48.55 0.485910-6 75.38 0.75 63.75 0.63 58.31 0.581910-5 81.67 0.81 72.34 0.72 65.67 0.655910-4 93.64 0.93 85.28 0.85 80.96 0.80
313 1910-6 47.46 0.47 35.02 0.35 27.99 0.275910-6 60.16 0.60 42.95 0.42 35.92 0.351910-5 62.66 0.62 54.23 0.54 54.87 0.545910-4 81.49 0.81 76.09 0.76 73.94 0.73
323 1910-6 21.67 0.21 15.95 0.15 12.97 0.125910-6 27.97 0.27 21.89 0.21 28.59 0.281910-5 34.06 0.34 26.13 0.26 24.54 0.245910-4 47.42 0.47 45.02 0.45 41.17 0.41
J. Chem. Sci. (2021) 133:109 Page 3 of 10 109
inhibition efficiencies of three inhibitors were obtained
after 1, 2, 3, 6 and 24 h of immersion in 0.5 M H2SO4
solution at 303 K temperature as given in Table S1, SI.
From Table S1 (Supplementary Information), the
inhibition efficiency increases with an immersion time
from 1 to 6 h, thereafter it remains almost stable from
6 to 24 h. On the other hand, the IE(%) is higher 97%
for all inhibitors used from 6 h. The BAPN (93.64%)
shows a good inhibitor since its inhibitory ability
reached a maximum value from 3 hours immersion
compared to other inhibitors PAPN (85.28%) and
MAPN (80.96%). This behavior could be attributed to
the adsorptive layer of inhibitors that rests upon the
immersion time.27 The inhibitive film on the steel
surface firstly reaches a more compact and uniform
condition during a prolonging immersion time (3–6 h),
while the adsorptive film is in a saturated state within
6–24 h.
3.3 Activation parameters of the corrosionprocess
The corrosion rates evaluated at different temperatures
in the absence and presence of inhibitors were used to
calculate the activation energy (Ea), activation
enthalpy (DHa), and activation entropy (DSact) of themetal dissolution.
The activation energy values were calculated using
the Arrhenius equation Eq. 8:28
CR ¼ k exp �Ea=RT
� �ð8Þ
where CR is the corrosion rate, k is a constant, R is the
gas constant (R = 8.314 J�mol-1�K-1) and T is the
temperature, K.
The values of Ea (Table S2, Supplementary Infor-
mation)) for carbon steel in sulfuric acid without and
with different concentrations of inhibitors were
obtained from the slope of the ln(CR) plot versus 1/T
(Figure S1, Supplementary Information). The activa-
tion enthalpy values DHa and activation entropy DSacan be calculated by the slope (-DHa/R) and intercept
[(ln(R/Nh)?(DSa/R)] of the above plot ln(CR/T) vs. 1/T (Figure S2, Supplementary Information), using the
following transition state equation (9).29
ln CR=Tð Þ ¼ ln R=Nhð Þ þ DSa=Rð Þ½ � � DHa=RTð Þð9Þ
where N is Avogadro number, h is Plank’s constant.
The data (Table S2, SI) show that the activation
energies for the carbon steel corrosion in 0.5 M H2SO4
in the presence of different inhibitors and their
concentrations are higher than that of free acid. The
activation energy values indicate that they are more
important than the activation enthalpy values. This
result shows that the corrosion process must have
involved a gaseous reaction. The increase in DHa with
an increase in the concentration of the inhibitor for
carbon steel corrosion reveals that the decrease in
carbon steel corrosion rate is mainly controlled by
kinetic parameters of activation. The DHa positive
values in the absence and presence of the inhibitor
reflect the endothermic nature of a metal dissolution
process, suggesting the slow dissolution of carbon
steel.30
The higher values of free Gibbs energy (DGa) of the
process in the inhibitors presence when compared to
that in its absence is attributed to its physisorption,
while the opposite one is the case with chemisorptions.
The DGa[ 0 means a non-spontaneous corrosion
reaction which increases with increasing the inhibitor
concentration.31 With the concentration increase, the
free energy of activation increases, which is ascribed
to the formation of an unstable activated complex in
the rate-determining transition state. The entropy of
activation values are less negative for inhibited solu-
tions than that for the uninhibited solutions. This
suggests that an increase in randomness occurred
while moving from reactants to the activated complex.
3.4 Adsorption isotherms
Information on the interaction between the carbon
steel surface and the inhibitor compound can be pro-
vided by the adsorption isotherm. The adsorption of
inhibitor molecules at the metal solution interface
reduces the corrosion rate which is considered as a
substitution adsorption process where an organic
compound from the aqueous media moves the water
molecules associated with the surface (H2Oads).
OrgSol þ nH2Oads $ Orgads þ nH2O
where ‘n’ is the number of water molecules replaced
by the adsorption of one inhibitor molecule.
The inhibiting strength of the three investigated
organic compounds on the carbon steel corrosion in
0.5 M H2SO4 solution is based on their adsorption onto
the steel surface. The adsorption depends on the
chemical structure of the inhibitor and the chemical
composition of the steel, the type of the aggressive
acid, the pH value, the temperature and the exposure
time.
The isotherms of Temkin, Langmuir, Freundlich
and Frumkin were tested for fitting data and the best
109 Page 4 of 10 J. Chem. Sci. (2021) 133:109
results were found for Langmuir isotherm (Figure S3,
Supplementary Information), which can be expressed
in the following way:32
C=h ¼ 1=Kads þ C ð10Þwhere C denotes the concentration of inhibitors in
mol.L-1, h symbolizes surface coverage and
Kads represents the adsorption equilibrium constant.
The linear regression coefficient (R2) is almost
equal to 1, indicating that the adsorbed molecules
occupy only one site. The Kads value as obtained from
the intercept of the isotherm is given in Table 2. The
free energy of adsorption DGads can be calculated
from the following equation Eq. 11:33
DGads ¼ �RT ln 55:5 Kadsð Þ ð11Þwhere R is the universal gas constant (J mol-1 K-1), T
is the absolute temperature (K) and the constant value
of 55.5 is the concentration of water in solution (mol
L-1).
The adsorption enthalpy (DHads) can be calculated
using Van’t Hoff equation Eq. 12:34
lnðKadsÞ ¼ �ðDHads=RTÞ þ constant ð12ÞFigure 1 shows that there is a linear relationship
between ln(Kads) and 1/T, thus the value of DHads is
calculated according to Equation (5) and listed in
Table 2. The following equation was employed to
calculate the adsorption entropy (DSads):35
DGads ¼ DHads � TDSads ð13ÞThe obtained values for DSads are shown in Table 2.The adsorption equilibrium constants for all inhi-
bitors are positive, showing the feasibility of the
adsorption of the inhibitors to the carbon steel surface.
This result confirms the trend obtained with the inhi-
bitor which has better inhibition efficiency:36 The
adsorption equilibrium constant decreases with the
temperature rise.37 This indicates that at a higher
temperature, the adsorbed inhibitor tends to desorb
back from the metal surface.38 The negative value of
the free energy adsorption revealed the spontaneity of
its process.39 The thermodynamic parameters for the
inhibitors adsorption can furnish precious information
about the mechanism of corrosion inhibition. An
exothermic adsorption process DHads\0 may imply
either chemisorption or physisorption or a mixture of
both processes. The endothermic adsorption process
DHads[0 is assigned unequivocally to chemisorp-
tion.33 Based on the results of this study, the calculated
free energy and enthalpy of the adsorption values for
Table 2 The values of Kads, DHads, DSads and DGads forcarbon steel in the absence and presence of an optimum
concentration of inhibitors in 0.5 M H2SO4 at differentstudied temperatures.
ProductsT(K) R2
Kads
(104 L/mol) DHads (kJ/mol)DSads
(J/mol K) DGads (kJ/mol)
BAPN 293 1 233.828 -67.35 -74.58 -45.50303 0.999 90.542 -74.88 -44.67313 0.999 49.185 -72.85 -44.55323 0.999 16.437 -75.30 -43.03
PAPN 293 1 118.166 -63.86 -68.34 -43.84303 0.999 48.661 -68.52 -43.10313 0.999 28.693 -66.18 -43.15323 0.999 9.354 -69.18 -41.52
MAPN 293 1 96.846 -57.58 -49.45 -43.36303 0.999 42.023 -49.90 -42.73313 0.999 21.114 -49.52 -42.35323 0.999 10.641 -49.50 -41.86
3.1 3.2 3.3 3.411
12
13
14
15
BAPN PAPN MAPN
Ln (K
ads) (
L/m
ol)
1000/T (K-1)
Figure 1. ln(Kads) versus 1/T for adsorption BAPN,PAPN and MAPN on carbon steel surface
J. Chem. Sci. (2021) 133:109 Page 5 of 10 109
the three studied inhibitors show that the adsorption
mechanism is not completely chemical or physical and
that a combination of physisorption and chemisorption
exists between the inhibitor and the metal surface. The
negative values obtained for changes in the adsorption
entropy confirm that there is an association of the
inhibitor’s molecules.
3.5 Scanning electron microscope
In this examination, we used a scanning electron
microscope (SEM) in order to characterize the surface
state of the steel before and after immersion in sulfuric
acid. Figure 2 shows the SEM images of the carbon
steel samples after 24 h of immersion in 0.5 M H2SO4
solution without and with the addition of 5910-4 M of
the inhibitors produced (BAPN, PAPN and MAPN) at
a temperature of 30 �C. We noticed that the surface of
the carbon steel after 24 h of immersion in the cor-
rosive solution alone (Figure 2.b) was aggressively
damaged. This clearly shows that the steel has
undergone a strong dissolution in the absence of the
inhibitor, unlike the steel surface (Figure 2.a) which
appears smooth with few lines resulting from polish-
ing. It is very clear from Figure 2 (c, d and e) that the
addition of the three inhibitors, studied in sulfuric acid,
stops the dissolution and consequently the corrosion of
carbon steel. This difference in appearance is attrib-
uted to the formation of a protective adherent layer on
the surface of the steel in the presence of these
inhibitors.40,41 This protects the steel surface against
the aggressions of the corrosive environment. These
results are in agreement with the inhibition efficiency
obtained from the weight loss measurement.
3.6 Theoretical calculations
The electronic properties of the inhibitors were studied
by density function theory (DFT)- B3LYP/6-31G*.
Optimized structure, HOMO and LUMO were inves-
tigated for three molecules and displayed in Figure 3.
The investigated parameters energy of the frontier
molecular electrons, the global hardness, the gap
energy, the back-donation and the number of trans-
ferred electrons were displayed in Table 3. The high
values of HOMO energy tend to donate electrons to an
appropriate acceptor molecule of low empty molecular
orbital energy. The inhibitor does not only donate
electrons to the unoccupied orbital of the metal ion but
can also receive electrons from the metal, forming a
feedback bond, considered as an excellent inhibitor.
From Table 3 that by increasing the length of the alkyl
Figure 2. SEM images of carbon steel specimens corroded in 0.5 M sulfuric acid solution: before immersion (a), theabsence (b) and presence of BAPN (c) PAPN (d) and MAPN (e) at their optimum concentration.
109 Page 6 of 10 J. Chem. Sci. (2021) 133:109
chain (methyl, propyl and butyl), the EHOMO becomes
larger and larger, indicating the increased capability of
the electron donor of the molecule. By increasing the
length of the alkyl chain, ELUMO also has an increasing
trend, which shows that the capability of the electron
acceptor will weaken.42 The EHOMO values of the
molecules studied increase in the following order:
BAPN[PAPN[MAPN, which means that BAPN has
the highest inhibition efficiency and the best inhibition
performance, which agrees well with the experimental
measurement.43
In our case, the lower values of the global hardness (g)are pronounced to be a better corrosion inhibitor.44 The
gap energy levels of the molecules were another
important factor that should be considered. The order of
DEgap values MAPN[PAPN[BAPN is in good
agreement with that of experimental inhibition effi-
ciency MAPN\PAPN\BAPN. In the literature, the
lower the DEgap values are, the higher the inhibition
efficiency will be.45,46
If DN\3.6, the inhibition efficiency increases by
increasing the electron-donating ability of these inhi-
bitors to donate electrons to the metal surface.47 The
results indicate that DN values correlate strongly with
experimental inhibition efficiencies. This is in good
agreement with the experimental observations. In a
simple model of charge transfer for donation and back
donation of charges, an electronic back donation pro-
cess can occur as a result of the interaction between
the inhibiting molecule and the metal surface. The
energy of the back-donation involves that when (g[0)
and DEBack-donation\0, the charge transfer to a
HOMO Molecular structure LUMO
BAPN
PAPN
MAPN
Figure 3. Optimized structure, HOMO and LUMO orbitals for BAPN, PAPN and MAPN molecules using DFT approach
Table 3. Calculated quantum chemical parameters of the studied inhibitors with carbon steelin 0.5 M H2SO4, using the DFT method and weightloss.
SubstrateHOMO(eV) LUMO (eV)
g(eV) DEgap (eV)
DEback-donation
(eV)DN(eV)
IE(%)
BAPN -5.88 -1.51 2.17 4.37 -0.54 0.76 97.45PAPN -5.90 -1.52 2.19 4.38 -0.55 0.75 95.43MAPN -5.95 -1.54 2.23 4.41 -0.56 0.73 91.73
J. Chem. Sci. (2021) 133:109 Page 7 of 10 109
molecule, followed by a back-donation from the
molecule, is energetically favored.48
3.7 Mechanism of inhibition
The inhibition effect of 2-Aminopyridine derivatives
towards the corrosion of carbon steel in 0.5 M H2SO4
solution may be attributed to the adsorption of these
compounds at the metal-solution interface. From the
obtained experimental and theoretical results, we
suggest the adsorption mechanism presented in Fig-
ure 4. As all three inhibitors BAPN, PAPN, and
MAPN possess nitrogen atoms, they can be protonated
easily. During the inhibition process, the inhibitors get
protonated in solution and become positively charged
as well as the metal surface in the acid media. Sulfate
ions are specifically adsorbed onto the metal and cre-
ate an excess of negative charge on its surface.49 This
favours the adsorption of protonated BAPN on the
surface through electrostatic attraction and hence
reduces the dissolution of iron (physical adsorption).
In addition, the inhibitors were chemically adsorbed
onto the carbon steel surface by donor/acceptor inter-
action amongst free electron pairs of heteroatoms and
the vacant d-orbital of iron atoms of the surface.
Moreover, this type of electron transfer causes
excessive accumulation of negative charge on the
electron-rich metallic surface which renders it to
transfer its electrons to the vacant p* (anti-bonding)
molecular orbitals of the inhibitors through retro-do-
nation.50 In summary, the adsorption of 2-Aminopy-
ridine derivatives on the carbon steel surface is
comprehensive and includes both physisorption and
chemisorption.
4. Conclusions
The three organic compounds BAPN, PAPN and
MAPN showed a good inhibition efficiency against
carbon steel corrosion in an acidic sulfuric solution.
The inhibition efficiency was found to increase with
the rise of the concentration of the inhibitor and lower
with the temperature rise. The BAPN is the highest
one in inhibiting the steel corrosion which returns to
the latter that possesses a higher electronic cloud and
which enhances the adsorption propensity on the steel
surface. The Langmuir isotherm was found to be the
best isotherm describing the adsorption behaviors on
the steel interface. The results of the adsorption iso-
therm revealed that the three organic compounds
behaved as a mixed type of adsorption. SEM pictures
exposed that the inhibitors were adsorbed on the metal
surface and formed a film that protected the carbon
steel surface. The efficiency of the inhibitors obtained
from the weight loss method and density functional
theory are in good agreement.
Acknowledgements
The authors wish to thank Directorate-General for Scientific
Research and Technological Development (DGRSDT) and
the University of Tlemcen for their financial support.
References
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N
N
NH
Fe2+ Fe2+Fe2+Fe2+ Fe2+
SO42-SO4
2-SO42-
e-e-e- e-e-
SO42-
Fe FeSO4
2-SO42-
ChemisorptionPhysisorptionReterodonation
Figure 4. The suggested mechanism of 2-(butylamino)-4-phenylnicotinonitrile for carbon steel in sulfuric acid solution
109 Page 8 of 10 J. Chem. Sci. (2021) 133:109
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