Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl

Article abstract of DOI:10.1007/s11164-014-1579-8

A new Schiff base compound named 4-(4′-benzoylhydrazine)-pyridinecarboxaldehyde hydrazone (BPBH) was synthesized and investigated as a corrosion inhibitor for mild steel in 1 M HCl using weight loss measurements, electrochemical techniques, and adsorption studies. The structure of BPBH was characterized by elemental analysis, infrared spectroscopy (IR), and ¹H nuclear magnetic resonance (NMR). The results clearly suggest that the compound acts as a mixed-type inhibitor of acid corrosion of mild steel. The adsorption of BPBH on the metal surface in 1 M HCl was found to agree with the Langmuir isotherm with a standard free energy of adsorption (Δ G ads 0) of -31.60 kJ/mol. Scanning electron microscopy images indicate that BPBH exhibits good corrosion inhibition performance for mild steel in 1 M HCl. Quantum chemical calculations were also carried out to verify the inhibition efficiencies obtained from all the experiments.

Full text of DOI:10.1007/s11164-014-1579-8

Res Chem Intermed
DOI 10.1007/s11164-014-1579-8
Schiff base compound as a corrosion inhibitor for mild
steel in 1 M HCl
Hong Liu
•
Lin Zhu Qilong Zhao
•
Received: 2 November 2013 / Accepted: 27 February 2014
Ó Springer Science+Business Media Dordrecht 2014
0
Abstract A new Schiff base compound named 4-(4 -benzoylhydrazine)-pyri-
dinecarboxaldehyde hydrazone (BPBH) was synthesized and investigated as a
corrosion inhibitor for mild steel in 1 M HCl using weight loss measurements,
electrochemical techniques, and adsorption studies. The structure of BPBH was
1
characterized by elemental analysis, infrared spectroscopy (IR), and H nuclear
magnetic resonance (NMR). The results clearly suggest that the compound acts as a
mixed-type inhibitor of acid corrosion of mild steel. The adsorption of BPBH on the
metal surface in 1 M HCl was found to agree with the Langmuir isotherm with a
0
standard free energy of adsorption (DGads) of -31.60 kJ/mol. Scanning electron
microscopy images indicate that BPBH exhibits good corrosion inhibition perfor-
mance for mild steel in 1 M HCl. Quantum chemical calculations were also carried
out to verify the inhibition efficiencies obtained from all the experiments.
Keywords Mild steel ꢀ Schiff base ꢀ Electrochemical technique ꢀ Quantum
chemical calculations ꢀ Acid corrosion
Introduction
Acid solutions are widely used in many industrial processes, such as industrial
cleaning, acid descaling, oil-well acidification, and petroleum processes, in which
H. Liu ꢀ L. Zhu (&) ꢀ Q. Zhao
School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500,
People’s Republic of China
e-mail: zhulinswpu@163.com
H. Liu
State Key Laboratory of Oil and Gas Reservoir Geology and Exploration, Southwest Petroleum
University, Chengdu 610500, People’s Republic of China
123

H. Liu et al.
corrosion of equipment made from metals and alloys is regarded as a serious
problem [1–6]. One of the most effective and economic approaches to protect
equipment against acid corrosion is use of organic compounds as corrosion
inhibitors, which is becoming increasingly popular [7–11]. Such organic compounds
containing heteroatoms (oxygen, nitrogen, and sulfur) and multiple bonds exhibit
excellent corrosion inhibition performance as they can easily interact with the metal
surface via their adsorption behavior to assemble a protective film that can
simultaneously block active sites [12–14]. It has been widely acknowledged that
electron transfer from organic inhibitors to the metal occurs to form chelates on the
metal surface [15]. The adsorption ability of an organic inhibitor on a metal surface
depends on its physical and chemical properties and also relies on the nature and
surface charge of the metal, the type of aggressive medium, and the electrochemical
potential at the interface [16–18].
Due to their great flexibility and structural diversity, a wide range of Schiff
bases have been synthesized and their complexation behavior studied [19, 20].
Based on the presence of nitrogen atom and the imine functional group in their
structure, many Schiff bases have been reported to be effective corrosion inhibitors
for various metals and alloys in acid media [21, 22]. Schiff bases can interact with
metals and alloys easily to form self-assembled monolayers (SAMs). Some
research works have revealed that the inhibition efficiencies of Schiff bases are
much higher than those of corresponding amines and aldehydes [23]. Hydrazone
compounds, one kind of Schiff base, are condensation products of a hydrazide and
a ketone or aldehyde. Because of the p–p conjugation formed between the lone
pair of electrons of the nitrogen atom in secondary amine, acyl group, and the
double bond in imino group, hydrazone compounds are characterized by a stable
structure and resistance to hydrolysis. Therefore, it is of significant and industrial
importance to synthesize and investigate new hydrazone compounds as corrosion
inhibitors.
0
In this study, a new hydrazone compound named 4-(4 -benzoylhydrazine)-
pyridinecarboxaldehyde hydrazone (BPBH) was synthesized. The structure of
1
BPBH was characterized by elemental analysis, IR, and H NMR. The corrosion
inhibition of the studied compound was investigated by the weight loss method and
electrochemical techniques. The surface of the metal was also analyzed by scanning
electron microscopy. In addition, quantum chemical calculations were carried out to
explore the relationship between the molecular structure and the corrosion inhibition
efficiency.
Materials and experimental techniques
Synthesis and characterization of the inhibitor
BPBH was synthesized by condensation reaction of terephthalhydrazine (4 mmol)
with 4-pyridinecarboxaldehyde (4 mmol) in acetic acid (30 mL) by heating under
reflux with stirring for 8 h [24–26]. The precursor compound terephthalhydrazine
was synthesized according to the procedure described in the literature [27]. The
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
solvent was evaporated under reduced pressure, and the resulting solid was
recrystallized from ethanol. The synthetic route and chemical structure of BPBH are
shown in Fig. 1. Calcd. for C H N O : C, 59.36; H, 4.63; N, 24.72; O, 11.30.
1
4 13 5 2
-
1
Found: C, 59.02; H, 4.51; N, 23.96; O, 11.05 %. IR (KBr, cm ): 3,238, 3,044,
1
1
–
,738, 1,663, 1,541, 1,271. H NMR (DMSO-d , 400 MHz, d ppm): 12.29 (s,
CONHN=, 1H), 8.67–8.68 (m, Ar–H, 2H), 8.47 (s, CH=N, 1H), 8.05–8.12 (m,
6
Ar–H, 4H), 7.69–7.70 (m, Ar–H, 2H), 3.90 (s, –CONHNH , 3H).
2
Metal specimens
Corrosion tests were performed on mild (N80) steel with composition (w %):
C = 0.039 %, Mn = 1.57 %, Si = 0.18 %, S = 0.023 %, Mo = 0.12 %, and
Cr = 0.16 %, with remainder iron.
Weight loss measurements
N80 mild steel samples (30 mm 9 15 mm 9 3.0 mm) were abraded with emery
paper (grade 600–800–1,000–1,200) and washed with bidistilled water and acetone,
then dried at room temperature (298 K). Each experiment was carried out at least
three times simultaneously, and the average results were recorded.
Electrochemical measurements
Electrochemical tests (potentiodynamic polarization, electrochemical impedance
spectroscopy) were carried out in a conventional three-electrode corrosion cell with
platinum sheet as auxiliary electrode, saturated calomel electrode (SCE) as
reference electrode, and N80 mild steel as working electrode (WE). The exposed
2
surface area of WE was a square of size 1 cm . Before testing, the WE was polished
with emery paper, then washed with acetone and bidistilled water. The WE was
immersed in the test solution at open-circuit potential for 30 min until steady state
was reached. Electrochemical measurements were carried out with an IviumStat
electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was
carried out with an alternating-current (AC) sine wave with amplitude of 10 mV
root mean square (RMS) in the frequency range from 10,000 Hz to 0.01 Hz. The
potentiodynamic current–potential curves were recorded by changing the electrode
potential automatically from -300 to ?300 mV versus open-circuit potential at
scan rate of 0.3 mV/s.
Scanning electron microscopy (SEM)
Study of the surface morphology of the metal specimens in both absence and
presence of inhibitor was carried out by using a scanning electron microscope (FEI
Inspect F) at accelerating voltage of 20 kV. Samples were prepared by immersing
the steel samples for 2 h in 1 M HCl or 1 M HCl containing 1.0 mmol/L inhibitor.
All micrographs of samples were taken at magnification of 5,0009.
123

H. Liu et al.
Fig. 1 Synthetic route and chemical structure of BPBH
Quantum chemical calculations
All calculations were performed at the B3LYP/6-311??G** level using the
Gaussian 09W program package. The optimized molecular structure and highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) surfaces were visualized using GaussView.
Results and discussion
Weight loss measurements
Effect of inhibitor concentration
Weight loss studies were performed in the absence and presence of different
concentrations of BPBH by immersion for 2 h at room temperature. The corrosion
2
rate (CR, mg/cm h) was calculated as
W
Corrosion rate (CR) ¼
;
ð1Þ
ð2Þ
At
the percentage inhibition efficiency (g) was calculated as
W0 ꢁ W
gð% Þ ¼
ꢂ 100;
W0
and the degree of surface coverage (h) was calculated as
W0 ꢁ W
h ¼
;
ð3Þ
W0
where W and W are the weight losses of N80 mild steel with and without inhibitor,
0
respectively, and A and t stand for the total corroding area and the immersion
period, respectively. The weight losses of the steel samples were obtained from
different experiments upon addition of different concentrations (0.05–1.0 mmol/L)
of BPBH, as listed in Table 1. It is conspicuous that the inhibition efficiency
increases and the corrosion rate decreases with increasing inhibitor concentration
(
Fig. 2). The inhibition efficiencies of BPBH obtained from the weight loss mea-
surements are in reasonably good agreement with those extracted from the elec-
trochemical methods below.
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
Table 1 Corrosion parameters obtained from weight loss measurements for N80 steel in 1 M HCl in
presence of different concentrations of BPBH at 298 K
2
CR (mg/cm h)
Inhibitor concentration
mmol/L)
Weight loss
2
(mg/cm )
g (%)
h
(
Blank
0.0154
0.0122
0.0086
0.0027
0.0015
0.0010
–
–
0.65
0.51
0.36
0.11
0.06
0.04
0
0
0
0
1
.05
.1
21.4
44.6
82.6
90.3
93.6
0.214
0.446
0.826
0.903
0.936
.5
.8
.0
Fig. 2 Variation of inhibition efficiency (IE) and corrosion rate (CR) in 1 M HCl versus BPBH
concentration
Effect of temperature
To explore the relationship between the inhibition efficiency of the studied
compound and temperature, several experiments were carried out on the inhibition
performance of the optimum concentration (1.0 mmol/L) of BPBH for N80 steel in
1
M HCl at temperatures ranging from 298 to 328 K. The immersion time of the
weight loss measurements was 2 h. The results are shown in Fig. 3, clearly showing
that the inhibition efficiency gradually decreased with increasing temperature.
Effect of immersion time
Figure 4 shows the relationship between the stability of the adsorbed inhibitor film
in mild steel/acid solution and the immersion time ranging from 2 to 24 h (2, 4, 6,
123

H. Liu et al.
Fig. 3 Variation of inhibition efficiency versus temperature of the solution for N80 steel in presence of
BPBH (1.0 mmol/L)
1
2, and 24 h) with 1.0 mmol/L inhibitor at room temperature. As shown in Fig. 4, it
was found that the inhibition efficiency increased slightly from 93.6 to 95 % during
–12 h in 1 M HCl, which can be explained by the formation of a protective film on
2
the mild steel surface with increasing time [28]. However, the inhibition efficiency
gradually decreased to 87 % during 12–24 h due to decreasing adsorption and
increasing desorption [22, 29].
Polarization measurements
The potentiodynamic polarization curves of N80 mild steel in 1 M HCl in the
absence and presence of different concentrations of inhibitor are shown in Fig. 5.
Table 2 presents the electrochemical corrosion kinetic parameters, i.e., the corrosion
potential (Ecorr), the cathodic and anodic slopes (b , b ), and the corrosion current
c
a
density (icorr). The inhibition efficiency was calculated from the measured icorr
values using the following equation:
0
corr
ði
ꢁ icorrÞ
corr
g ð%Þ ¼
ꢂ 100;
ð4Þ
0
i
0
where i
corr
and icorr are the corrosion current density of mild steel without and with
inhibitor, respectively. Table 2 reveals that the inhibition efficiency increased and
icorr decreased as the inhibitor concentration was increased, as the adsorption pro-
cess taking place on the metal surface gradually strengthened with increasing
inhibitor concentration. The values of b changed slightly on addition of BPBH,
c
suggesting that the cathodic process was activation controlled, whereas addition of
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
Fig. 4 Variation of inhibition efficiency versus immersion time for N80 steel in presence of BPBH
(1.0 mmol/L)
BPBH did not modify the mechanism of the hydrogen evolution process. The shift
of b may be due to adsorption of inhibitor molecules on the metal surface. The
a
inhibition efficiency reached 85.9 % at the optimal concentration (1.0 mmol/L) of
BPBH, so we can conclude that BPBH is an efficient corrosion inhibitor for mild
steel in HCl solution [30]. Decreases in both the cathodic and anodic currents are
noted in Fig. 5. Simultaneously, the change in the Ecorr value calculated from
Table 2 is smaller than 85 mV. The above facts confirm that BPBH can be regarded
as a mixed-type inhibitor [31].
Electrochemical impedance measurements
Figure 6 shows the Nyquist plane plots for N80 steel in 1 M HCl in the absence and
presence of various concentrations of inhibitor at room temperature. The impedance
diagrams obtained have the appearance of a semicircle in all experiments, indicating
that a charge-transfer process mainly controls the corrosion of mild steel and a
protective film is formed on the metal surface. The Nyquist diagrams in Fig. 6 are
not perfect semicircles as a result of the roughness and other inhomogeneities of the
metal surface. Generally, the shape of impedance diagrams can be used to deduce
kinetic information regarding the electrode interface between the metal and acid
solution [32]. The electrochemical corrosion kinetic parameters of the studied
compound are listed in Table 3. R is solution resistance. The inhibition efficiency
s
was calculated from the following equation:
123

H. Liu et al.
Fig. 5 Potentiodynamic polarization curves for N80 steel in 1 M hydrochloric acid in the absence and
presence of different concentrations of BPBH at 298 K
Table 2 Electrochemical data for N80 steel with different concentrations of BPBH at 298 K
2
Inhibitor concentration
mmol/L)
Ecorr (V/SCE)
i
corr (lA/cm )
b
a
(V/dec)
b
c
(V/dec)
g (%)
(
Blank
-0.4621
-0.4521
-0.4551
-0.4416
-0.4326
-0.4269
157
106
64
0.214
0.212
0.193
0.217
0.226
0.224
0.234
0.227
0.225
0.223
0.213
0.221
–
0
0
0
0
1
.05
.1
32.3
59.2
67.5
80.3
85.9
.5
51
.8
31
.0
22
0
ðRct ꢁ R Þ
ct
g ð%Þ ¼
ꢂ 100;
ð5Þ
R
ct
0
where R and R are the values of the charge-transfer resistance observed in the
ct
ct
absence and presence of inhibitor, respectively.
As can be seen from Fig. 6, the size of the depressed semicircle increased with
increasing inhibitor concentration, reaching a maximal value for inhibitor concen-
tration of 1.0 mmol/L. The corresponding Bode plots are shown in Fig. 7, and the
best-fit electrical equivalent circuit for the systems is shown in Fig. 8. The
impedance of the constant-phase element (CPE) is given by the following equation:
1
ZCPE ¼ Qꢁ ðjxÞꢁn
;
ð6Þ
where Q is the magnitude of the CPE, j is the imaginary unit, x is the angular
frequency (x = 2pf, where f is the frequency in Hz), and n is the phase shift, which
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
Fig. 6 Nyquist plots of EIS measurements of N80 steel in 1 M hydrochloric acid at 298 K for different
concentrations of BPBH
Table 3 Electrochemical impedance parameters obtained for N80 steel in 1 M HCl in the absence and
presence of BPBH
2
2
2
Inhibitor concentration
mmol/L)
R
s
(X cm )
R
ct
Q (lF/cm )
n
Cdl (lF/cm ) g (%)
(
2
(
X cm ) Error (%)
Blank
0.366
0.426
0.527
0.476
0.460
0.556
70.10
89.96
2.39
2.66
3.69
2.96
4.44
2.40
242.8
241.4
142.5
132.0
109.0
97.0
0.86 123
0.82 105
–
0
0
0
0
1
.05
.1
22.1
73.2
75.4
82.6
84.5
262.1
285.2
402.9
452.9
0.82
0.82
0.83
0.83
70.5
.5
65.3
60.6
55.7
.8
.0
gives details about the degree of surface inhomogeneity. When n = 1, this is the
same equation as for the impedance of a capacitor, where Q = C . In fact, when
dl
n is close to 1, the CPE resembles a capacitor, but the phase is not 90°, being
constant and somewhat less than 90° at all frequencies. To obtain values for the
double-layer capacitance (C ), the frequency values corresponding to the maximum
imaginary component of the impedance -Zmax were found and used in the fol-
lowing equation:
dl
Cdl ¼ QðxmaxÞnꢁ1
;
ð7Þ
where xmax is the angular frequency at which the imaginary component of the
impedance reaches a maximum.
123

H. Liu et al.
Fig. 7 Bode plots of N80 steel in 1 M hydrochloric acid in the absence and presence of BPBH
Fig. 8 Equivalent circuit for
BPBH used to fit the EIS data of
N80 steel in 1 M hydrochloric
acid
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
It is clearly seen that C decreases while the inhibition efficiency increases with
dl
increasing BPBH concentration. The decrease in C can be attributed to a decrease
dl
in the local dielectric constant and/or an increase in the thickness of the electrical
double layer, suggesting that inhibitor molecules (with low dielectric constant)
displace water molecules (with high dielectric constant) originally adsorbed on the
N80 steel surface. The thickness of the protective layer (d) is related to Cdl
according to the Helmholtz model as follows:
ee0A
C ¼
;
ð8Þ
dl
d
where e is the local dielectric constant of the medium, e is the permittivity of
0
vacuum, d is the thickness of the film, and A is the effective surface area of the
electrode.
The fact that Rct increases while Cdl decreases with increasing inhibitor
concentration indicates considerable surface coverage by the inhibitor and strong
bonding to the surface [33]. In the absence and in the presence of BPBH, the phase
shift values remain more or less identical, indicating that a charge-transfer process
controls the dissolution mechanism of N80 steel in 1 M HCl solution without and
with BPBH [22].
Adsorption isotherm
To understand the mechanism of corrosion inhibition by an organic inhibitor, its
adsorption behavior on the metal surface should be determined [33]. Generally,
basic information on the interaction between the inhibitor and the mild steel surface
can be effectively obtained through the adsorption isotherm. In this work, the results
obtained by the weight loss method were used to investigate the interaction of the
inhibitor with the metal surface. To obtain the isotherm, the degree of surface
coverage (h), which can be calculated from Eq. (3), must be obtained. The values of
h and inhibitor concentrations were fit to the Temkin, Frumkin, Freundlich, and
Langmuir isotherms, respectively. Finally, it was found that adsorption of the
synthesized compound on N80 mild steel surface in 1 M HCl obeyed the Langmuir
adsorption isotherm given by the following equation:
Cinh
1
¼
þ Cinh
;
ð6Þ
h
Kads
where Cinh is the inhibitor concentration and Kads is the equilibrium constant of the
inhibitor adsorption process. A plot of Cinh/h against Cinh is shown in Fig. 9. The
regression coefficient is close to 1 (0.9963), so the best fit was obtained with the
Langmuir adsorption isotherm, indicating that the adsorbed inhibitor molecules
form a monolayer on the mild steel surface and that there is no interaction among
the adsorbed inhibitor molecules [23].
3
-1
The value of Kads (6.248 9 10 M ) was calculated from the intercept of the
0
fitting line. The relationship between the standard free energy of adsorption (DGads
and Kads satisfies the following equation:
)
123

H. Liu et al.
Fig. 9 Langmuir adsorption plots for N80 steel in 1 M hydrochloric acid at different concentrations of
BPBH
ꢀ
where R is the universal gas constant, T is the thermodynamic temperature, and 55.5
ꢁ
0
1
DG
ads
Kads ¼ 5 exp ꢁ
;
ð7Þ
5:5
RT
is the molar concentration (mol/L) of water in the solution. In the condition studied,
0
ads
0
the calculated value of DG
was -31.60 kJ/mol. A negative value of DG
ads
indicates stability of the adsorbed layer on the steel surface and spontaneity of the
0
adsorption process [30]. As the value of DGads is between -20 and -40 kJ/mol,
both physical and chemical adsorption were involved in the adsorption process, with
chemical adsorption predominating [30, 34–36].
Scanning electron microscopy (SEM)
SEM analysis was carried out to study the changes and the effect of corrosion on the
steel surface in the presence and absence of inhibitor. Figure 10a shows a polished
N80 mild steel sample. Figure 10b shows a SEM image of the surface of a steel
specimen immersed for 2 h in 1 M HCl, which is porous and rough; it can be
deduced that the steel surface is strongly corroded in HCl. Figure 10c shows an
image of the surface of a steel specimen immersed for 2 h in 1 M HCl containing
1
.0 mmol/L BPBH, which is flat and smooth; this can be explained by the fact that
the corrosion of the steel surface was efficiently inhibited when BPBH was present.
Therefore, it can be speculated that an effective protective adsorption film was
formed on the steel surface, indicating that BPBH exhibits good corrosion inhibition
performance for N80 mild steel.
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
Fig. 10 SEM images of N80
steel samples: (a) polished
surface, (b) after 2 h immersion
in 1 M hydrochloric acid, and
(
c) after 2 h immersion in 1 M
hydrochloric acid ?
.0 mmol/L BPBH
1
123

H. Liu et al.
Quantum chemical calculations
To investigate the relationship between the molecular structure and the corrosion
inhibition efficiency, quantum chemical calculations were undertaken [15, 37, 38].
According to frontier molecular orbital theory, only frontier molecular orbitals are
involved in interactions between reactants. Hence, only the HOMO and LUMO of
reactants are considered when analyzing the chelation processes of chemical
adsorption [39].
Figure 11 shows the full geometry optimization of the inhibitor molecule.
Figure 12a and b show the frontier molecular orbital (FMO) density distribution and
energy levels of the HOMO and LUMO orbitals computed at the B3LYP/6-
311??G** level for BPBH. The values of the calculated quantum chemical
parameters, such as EHOMO (the energy of the highest occupied molecular orbital),
ELUMO (the energy of the lowest unoccupied molecular orbital), DE (ELUMO
EHOMO), and l (the dipole moment) of BPBH, are given in Table 4.
-
EHOMO is often associated with the electron-donating ability of a molecule.
Generally, the higher the EHOMO value of the inhibitor, the greater its tendency to
offer electrons to unoccupied d-orbitals of the metal, and the higher the corrosion
inhibition efficiency observed for mild steel in acid solution. On the other hand,
ELUMO indicates the ability of the molecule to accept electrons. The lower the
ELUMO value of the inhibitor, the easier acceptance of electrons from the metal
surface becomes. As DE (ELUMO - EHOMO) decreases, the interactions occurring
on the steel surface become stronger and the efficiency of the inhibitor increases.
Regarding l, a higher value favors strong interaction of inhibitor molecules on the
metal surface [28–42]. The EHOMO, ELUMO, DE, and l values presented in Table 4
corroborate those obtained in the above experiments and show that BPBH adsorbed
onto the metal surface easily. It can also be deduced that a protective layer was
formed by adsorption at the mild steel/acid solution interface [28, 43].
The absolute hardness (g) and softness (d) play an important role in measuring
molecular stability and reactivity. Hard molecules have a large energy gap, whereas
soft molecules have a small energy gap. Because soft molecules can easily offer
electrons to an acceptor, they are more reactive than hard molecules. For the
simplest transfer of electrons, adsorption can occur at the part of the molecule where
the softness is highest [44]; the softness of a molecule is a local property. Generally,
an inhibitor acts as a Lewis base and the metal as a Lewis acid in a corrosion
system. Soft base inhibitors are most effective for acidic corrosion protection of
bulk metals, which are soft acids [45]. The chemical potential (p) can measure the
tendency of electrons to flow from high-potential to low-potential regions until
reaching a constant state. The hardness, softness, and chemical potential are listed in
Table 4. The number of electrons transferred (DN) was calculated from the quantum
chemical method using the following equation:
ðv ꢁ v
Fe
Þ
inh
DN ¼ 2
;
ð8Þ
ðgFe þ g
Þ
inh
where vFe and vinh denote the absolute electronegativity of iron and the inhibitor
molecule, respectively, and gFe and ginh denote the absolute hardness of iron and the
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
Fig. 11 Optimized structure of BPBH
Fig. 12 Frontier molecular orbital (FMO) density distribution of BPBH: (a) HOMO and (b) LUMO
123

H. Liu et al.
Table 4 Calculated quantum chemical parameters of BPBH
Inhibitor
E
HOMO
E
(eV)
LUMO
DEL-H
(eV)
l
(Debye) (eV)
Electronegativity Hardness Softness Chemical DN
(eV)
(eV)
(1/eV)
potential
(eV)
BPBH
-7.115 -2.462 4.653
10.864
4.789
2.327
0.430
-4.788
0.475
inhibitor molecule, respectively; a theoretical value of vFe & 7.0 eV was taken for
iron, and gFe = 0 was taken, assuming that I = A for bulk metals [46, 47].
The v and g values of the inhibitor molecule were calculated from the following
equations:
ðI þ AÞ
ðI ꢁ AÞ
v ¼
; g ¼
;
ð9Þ
2
2
where the ionization potential (I) and the electron affinity (A) are related in turn to
EHOMO and ELUMO as follows:
I ¼ ꢁEHOMO; A ¼ ꢁELUMO
:
ð10Þ
The softness of the inhibitor molecule is simply the inverse of the hardness:
d = 1/g. If DN \ 3.6, the inhibition efficiency increases with increasing electron-
donating ability at the metal surface [37]. In the present study, BPBH is a donor of
electrons, while N80 steel surface atoms are acceptors. BPBH was bound to the N80
steel surface, and thus it formed an inhibitive adsorption layer against corrosion.
Mechanism of inhibition
The adsorption of the corrosion inhibitor at the mild steel/hydrochloric acid solution
interface depends on several factors, such as the number of adsorption sites, the
molecular size, and the mode of interaction with the metal surface and extent of
formation of metallic complexes [48]. The adsorption process can take place
through its active centers that are unshared electron pairs of heteroatoms and
aromatic rings.
In acidic solutions, it is known that inhibitor molecules can be protonated as
follows:
þ
xþ
C14H13N5O2 þ xH ! ½C14H13þxN5O2ꢃ
ð11Þ
In aqueous acidic solutions, either neutral molecules or cationic forms of BPBH
molecules exist [28, 48]. It has been reported that the mechanism of corrosion
inhibition for dissolution of mild steel is as follows [49]:
The anodic dissolution of iron may follow the steps:
ꢁ
ꢁ
Fe þ Cl ! ðFeCl Þ
ð12Þ
ð13Þ
ð14Þ
ð15Þ
ads
ðFeCl Þ ! ðFeClÞ þeꢁ
ꢁ
ads
ads
þ
ꢁ
ðFeClÞ ! ðFeCl Þ þe
ads
ads
FeCl ! Fe þ Clꢁ
þ
2þ
1
23

Schiff base compound as a corrosion inhibitor for mild steel in 1 M HCl
The cathodic hydrogen evolution reaction may follow the steps:
þ
þ
ads
Fe þ H ! ðFeH Þ
ð16Þ
ð17Þ
ð18Þ
þ
ꢁ
ðFeH Þ þe ! ðFeHÞ
ads
ads
þ
ꢁ
ðFeHÞadsþH þ e ! Fe þ H2
It was assumed that chloride ion was first adsorbed onto the positively charged
metal surface by coulombic attraction. Then, the protonated BPBH adsorbed
through electrostatic interactions between the positively charged molecules and the
negatively charged metal surface [50]. These adsorbed molecules interact with
(
-
FeCl ) species to form a monolayer on the surface by forming a compound. The
ads
N80 steel surface was protected from attack by aggressive chloride ions through the
monomolecular layers.
Conclusions
A new hydrazone Schiff base compound BPBH was synthesized. It was found that
BPBH shows good inhibition efficiency for N80 mild steel in 1 M HCl. The
inhibition efficiency of BPBH increases with increasing inhibitor concentration and
decreases with higher temperature and longer exposure period. The polarization
curves imply that BPBH acts as a mixed-type inhibitor. Under investigation of the
steel surface, the adsorption of BPBH was found to obey the Langmuir adsorption
isotherm, and its adsorption at the mild steel/hydrochloric acid interface involves
both physical and chemical adsorption, with the latter being stronger.
References
1
2
3
4
5
6
7
8
9
. J.I. Bregman, Corrosion Inhibitors (MacMillan, New York, 1963)
. S. Ghareba, S. Omanovic, Corros. Sci. 52, 2104–2113 (2010)
. Z. Tao, W. He, S. Wang, S. Zhang, G. Zhou, Corros. Sci. 60, 205–213 (2012)
. D. Gopi, K.M. Govindaraju, L. Kavitha, J. Appl. Electrochem. 40, 1349–1356 (2010)
. L.J. Li, X.P. Zhang, J.L. Lei, J.X. He, S.T. Zhang, F.S. Pan, Corros. Sci. 63, 82–90 (2012)
. A.O. Y u¨ ce, G. Karda s¸ , Corros. Sci. 58, 86–94 (2012)
. K.C. Emreg u¨ l, E. D u¨ zg u¨ n, O. Atakol, Corros. Sci. 48, 3243–3260 (2006)
. A. D o¨ ner, E.A. Sahin, G. Kardas, O. Serindag, Corros. Sci. 66, 278–284 (2013)
. M. Bobina, A. Kellenberger, J.P. Millet, C. Muntean, N. Vaszilcsin, Corros. Sci. 69, 389–395 (2013)
10. I. Danaee, O. Ghasemi, G.R. Rashed, M. RashvandAvei, M.H. Maddahy, J. Mol. Struct. 1035,
247–259 (2013)
11. A.S. Patel, V.A. Panchal, G.V. Mudaliar, N.K. Shah, J. Saudi, Chem. Soci. 17, 53–59 (2013)
12. S.S. Abd-El-Rehim, M.A.M. Ibrahim, K.F. Khalid, J. Appl. Electrochem. 29, 593–599 (2002)
13. S. Rengamani, S. Muralidharan, M. AnbuKulamdainathan, I.S. Venkatakrishna, J. Appl. Electro-
chem. 24, 355–359 (1994)
1
1
1
1
1
4. M. Ajmal, A.S. Mideen, M.A. Quraishi, Corros. Sci. 36, 79–84 (1994)
5. H. Ju, Z.P. Kai, Y. Li, Corros. Sci. 50, 865–871 (2008)
6. I. Ahamad, M. Quraishi, Corros. Sci. 51, 2006–2013 (2009)
7. I. Ahamad, M. Quraishi, Corros. Sci. 52, 651–656 (2010)
8. S.K. Shukla, M. Quraishi, Corros. Sci. 52, 314–321 (2010)
123

H. Liu et al.
1
2
2
2
2
9. A.D. Azazl, S. Celen, H. Namlil, O. Turhanl, Proc. Ind. Acad. Sci. 32, 884–888 (2007)
0. H.D. Yin, H. Liu, M. Hong, J. Organomet. Chem. 713, 11–19 (2012)
1. I. Ahamad, R. Prasad, M.A. Quraishi, Corros. Sci. 52, 933–942 (2010)
2. I. Ahamad, C. Gupta, R. Prasad, M.A. Quraishi, J. Appl. Electrochem. 40, 2171–2183 (2010)
3. P. Pelagatti, M. Carcelli, F. Franchi, C. Pelizzi, A. Bacchi, A. Fochi, H.-W. Fr u¨ hauf, K. Goubitz, K.
Vrieze, Eur. J. Inorg. Chem. 3, 463–475 (2000)
24. H. Meng, Z.F. Xie, J. Hu, F.M. Liu, Chin. J. Org. Chem. 28, 1423–1427 (2008)
25. D.Q. Long, S.S. Chen, D.J. Li, J. JiangXi Normal Univ. Nat. Sci. 30, 372–374 (2006)
26. X.H. Sun, Y. Tao, Y.F. Liu, B. Chen, Acta Chim. Sin. 66, 234–238 (2008)
27. D.Q. Long, S.S. Chen, H. Chen, Chin. J. Synth. Chem. 14, 069–071 (2006)
28. A. Singh, J.N. Avyaya, E.E. Ebenso, M.A. Quraishi, Res. Chem. Intermed. 39, 537–551 (2013)
29. D.F. Shriver, P.W. Atkins, C.H. Langford, Inorganic Chemistry, 2nd edn. (Oxford University Press,
Oxford, 1994)
3
0. S. S¸ afak, B. Duran, A. Yurt, G. T u¨ rko g˘ lu, Corros. Sci. 54, 251–259 (2012)
1. Y. Abboud, B. Hammouti, A. Abourriche, A. Bennamara, H. Hannache, Res. Chem. Intermed. 38,
3
1591–1607 (2012)
3
3
3
3
3
3
3
3
4
4
4
4
2. W.J. Lorenz, F. Mansfeld, Corros. Sci. 21, 647–672 (1981)
3. M.A. Quraishi, D. Jamal, Mater. Chem. Phys. 68, 283–287 (2001)
4. E.A. Noor, Int. J. Electrochem. Sci. 2, 996–1017 (2007)
5. M. Bhupendra, S.J. Mistry, Res. Chem. Intermed. 39, 1049–1068 (2013)
6. S. John, K.M. Ali, A. Joseph, Bull. Mater. Sci. 34, 1245–1256 (2011)
7. I. Lukovits, E. Kalman, F. Zucchi, Corrosion 57, 3–8 (2001)
8. I. Ahamada, R. Prasadb, M.A. Quraishia, Mater. Chem. Phys. 124, 1155–1165 (2010)
9. J. Fang, J. Li, J. Mol. Struct. Theochem. 593, 179–185 (2002)
0. G. Gece, Corros. Sci. 50, 2981–2992 (2008)
1. R.M. Issa, M.K. Awad, F.M. Atlam, Appl. Surf. Sci. 255, 2433–2441 (2008)
2. B.D. Mert, M.E. Mert, G. Kardas, B. Yazici, Corros. Sci. 53, 4265–4272 (2011)
3. N. Missoum, A. Guendouz, N. Boussalah, B. Hammouti, A. Chetouani, M. Taleb, A. Aouniti, S.
Ghalem, Res. Chem. Intermed. 39, 3441–3461 (2013)
44. S. Martinez, Mater. Chem. Phys. 77, 97–102 (2002)
45. L.F. Mar, O.O. Xometl, M.A.D. Aguilar, E.A. Flores, P.A. Lozada, F.J. Cruz, Corros. Sci. 61,
171–184 (2012)
4
4
4
4
5
6. J.C. Slater, Introduction to Chemical Physics (Dover, New York, 1970)
7. M.J.S. Dewar, W. Thiel, J. Am. Chem. Soc. 99, 4899–4907 (1977)
8. I. Ahamad, R. Prasad, E.E. Ebenso, M.A. Quraishi, Int. J. Electrochem. Sci. 7, 3436–3452 (2012)
9. W. Li, Q. He, C. Pei, B. Hou, Electrochim. Acta 52, 6386–6394 (2007)
0. M.A. Quraishi, M.Z.A. Rafiquee, S. Khan, N. Saxena, J. Appl. Electrochem. 37, 1153 (2007)
1
23