Corrosion inhibition of carbon steel pipelines by some novel Schiff base compounds during acidizing treatment of oil wells studied by electrochemical and quantum chemical methods

Article abstract of DOI:10.1016/j.molstruc.2016.10.078

Three novel Schiff bases compounds were prepared and their structures were characterized by X-ray, ¹³C-NMR, ¹H-NMR, mass, UV–Vis, FT-IR, spectral data and elemental analyses. The corrosion inhibition of the investigated inhibitors to

Full text of DOI:10.1016/j.molstruc.2016.10.078

Accepted Manuscript
Corrosion inhibition of carbon steel pipelines by some novel Schiff base during
acidizing treatment of oil wells studied by electrochemical and quantum chemical
methods
Hany M. Abd El-Lateef, Ahmed M. Abu-Dief, Mounir A.A. Mohamed
PII:
S0022-2860(16)31135-8
10.1016/j.molstruc.2016.10.078
MOLSTR 23079
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 22 June 2016
Revised Date: 25 October 2016
Accepted Date: 25 October 2016
Please cite this article as: H.M. Abd El-Lateef, A.M. Abu-Dief, M.A.A. Mohamed, Corrosion inhibition
of carbon steel pipelines by some novel Schiff base during acidizing treatment of oil wells studied by
electrochemical and quantum chemical methods, Journal of Molecular Structure (2016), doi: 10.1016/
j.molstruc.2016.10.078.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical abstract
Corrosion inhibition of carbon steel pipelines by some novel
Schiff base during acidizing treatment of oil wells studied by
electrochemical and quantum chemical methods
*
Hany M. Abd El-Lateef , Ahmed M. Abu-Dief, Mounir A. A. Mohamed
Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag,
Egypt
E-mail address: Hany_shubra@yahoo.co.uk (Hany M. Abd El-Lateef)

ACCEPTED MANUSCRIPT
Corrosion inhibition of carbon steel pipelines by some novel
Schiff base compounds during acidizing treatment of oil wells
studied by electrochemical and quantum chemical methods
*
Hany M. Abd El-Lateef , Ahmed M. Abu-Dief and Mounir A. A. Mohamed
Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt
Abstract
Three novel Schiff bases compounds were prepared and their structures were
1
3
1
characterized by X-ray, C-NMR, H-NMR, mass, UV-Vis, FT-IR, spectral data and
elemental analyses. The corrosion inhibition of the investigated inhibitors towards
carbon steel in 15% HCl was investigated by using electrochemical measurements
(EIS, LPR corrosion rate and Tafel plots), SEM, EDX and quantum chemical
methods. The results showed that, the inhibitors are efficient mixed type corrosion
inhibitors, and their inhibition performance increased with the rise of inhibitor
concentration and temperature. The adsorption of the inhibitors on steel surface was
found to obey Langmuir's adsorption isotherm and chemisorption. Quantum chemical
calculations provide good support to empirical results.
Keywords: Carbon steel alloy; Corrosion inhibition; Schiff base compounds; EIS;
SEM; Quantum chemical calculations
*
Corresponding author: Fax: (+2)-093 -4601159, Tel: (+2)-012-28-137-103
E-mail address: Hany_shubra@yahoo.co.uk (Hany M. Abd El-Lateef)
1

ACCEPTED MANUSCRIPT
1
. Introduction
The electrochemical properties of carbon steel are the subject of many studies
due to its wide applications in industry such as heat exchange and boiler systems,
construction material for chemical reactors, storage tanks, and oil and gas transport
pipelines [1-4]. Hydrochloric acid solution is extensively used in industrial-cleaning
processes, such as: industrial acid cleaning, acid pickling, oil well acidizing, acid
descaling and in petrochemical processes [5]. In petroleum industry, 15% HCl is
generally used for acidizing treatment because it leaves no insoluble products after the
handling. The use of HCl leads to destructive effects on the metal surface. The
corrosion protection of the equipments and vessels is one of chief concerns of the
maintenance and design engineers. The use of organic molecules as corrosion
inhibitors to decrease the corrosion rate processes has been the focus of very many
efforts within the chemical process industry especially in acidic environments [6-8].
Schiff base (imines) are organic compounds containing azomethine group (–
CH=N–) which resulted from condensation of a primary amine with a carbonyl
compounds (aldehyde or ketone) [9]. Due to the presence of the azomethine group,
sulphur and/or oxygen atoms and electronegative nitrogen in the molecule, Schiff
bases compounds should be good corrosion inhibitors. The action of such compounds
depends on the specific interaction between the metal surface and the functional
groups. So it is very important to clarify the interactions between metal surfaces and
the inhibitor molecules in order to find new and efficient corrosion inhibitors [10-12].
Many Schiff bases had been investigated as efficient corrosion inhibitors for carbon
steel [13], zinc [14], aluminium [15], aluminium alloys [16] and copper [17] in acidic
media. These compounds generally become effective by adsorption on metal surfaces.
The adsorption species help to protect metals from aggressive medium which causes
2

ACCEPTED MANUSCRIPT
decomposition of the metal [18]. Abd El-Lateef [3] studied the corrosion behavior of
carbon steel in hydrochloric acid solution with and without two Schiff bases namely
Sodium
hydroxybenzenesulfonate and Sodium 3-{[(1-carboxy-2-phenylethyl) imino] methyl}-
-hydroxybenzenesulfonate synthesized based on amino acids and found that, these
3-[{[1-carboxy-3-
(methylthio)
propyl]
imino}
methyl]-4-
4
compounds had excellent inhibition efficiency [3].
Temperature has significant effectiveness on the process of metal corrosion
[19]. Temperature variation supplies the possibility to obtain the thermodynamic
adsorption and kinetic corrosion parameters [20], and the obtained results can help to
deduce the type of adsorption of the inhibitor on the metal surface.
Nowadays, the choice of effective inhibitors has been mostly achieved by
using the experimental information based on their electron donating ability and
macroscopically physicochemical properties. At this time the expansion of theoretical
calculations emphasized the scientific approaches involved in the branch of inhibitors
by correlating the empirical data with quantum chemical properties [21, 22]. Thus, the
purpose of the present work is to investigate the inhibition effect of three novel Schiff
bases on the corrosion of carbon steel pipelines during acidizing treatment (15% HCl)
in petroleum industry using electrochemical techniques. The effects of inhibitor
concentration, temperature and immersion time were investigated. In order to examine
the relation between the structure of studied Schiff bases and the inhibition efficiency,
some quantum chemical indices such as EHOMO and ELUMO, ꢀE, charge density of
adsorption centers and dipole moment have also been calculated.
3

ACCEPTED MANUSCRIPT
. Experimental Details
2
2
.1. Chemical composition of carbon steel C1018 alloy
Carbon steel C1018 (which is representative of a pipeline material) used for
electrochemical measurements and immersion tests has the following composition
wt%): C 0.18 %, Si 0.17 %, Mn 0.70 %, S 0.03 %, Ni 0.01 %, Cr 0.01 % and Fe
(
balance. The data was provided by European Corrosion Supplies Ltd. The cylindrical
2
shaped working electrode has a diameter of 0.5 cm, length of 3.0 cm and 4.55 cm
exposed area to fluid.
2
.2. Synthesis of the investigated inhibitors
Starting materials (2-amino-4, 6-dimethylpyridine, 3, 5-dimethylaniline, 3-
methoxy-4-nitroaniline,
Tripropylamine,
2-hydroxynaphthaldehyde
and
5-
bromosalicylaldehyde) were purchased from Sigma-Aldrich and all manipulations and
used directly without further purification in the preparation of the investigated
inhibitors.
Synthesis of 4-bromo-2-{(Z)-[(3, 5-dimethylphenyl) imino]methyl }phenol (HA-1)
HA-1 was synthesized by the condensation between 5-dimethylaniline (1.9 ml,
5
mmol) in 30 ml of dry ethanol treated with (1.1 g, 5 mmol) of 5-
о
bromosalicylaldehyde under vigorous stirring at 80 C for 2 h. Then, the reaction
mixture was left standing at room temperature to cool. The yellow crystals were
collected and washed several times with ethanol to get pure product. . The obtained
crystal was used for X-ray single crystal measurements.
Synthesis of 1-{(Z)-[(4, 6-dimethylpyridin-2-yl)imino]methyl}naphthalen-2-ol (HA-
2
)
The title inhibitor was obtained in a high yield (92 %) by reflux condensation
of equimolar of 2-amino-4, 6-dimethylpyridine and 2-hydroxynaphthaldehyde in
4

ACCEPTED MANUSCRIPT
ethanol. The reaction mixture was refluxed for 2 h, cooled by keeping overnight to
obtain yellow colored crystals filtered, washed, and dried (cf. Fig. 1).
<<Figure 1>>
Synthesis of 1-{(Z)-[(2-methoxy-4-nitrophenyl)imino]methyl}naphthalen-2-ol (HA-
3
)
The investigated inhibitor was synthesized by condensation of 3-methoxy-4-
nitroaniline with 2-hydroxynaphthaldehyde in ethanol. Drops of tripropylamine were
added as a catalyst. The obtained mixture was refluxed for 3 h. The obtained yellow
solution left at room temperature over night. Then the obtained yellow crystals were
recrystallized from commercial ethanol.
2
.3. Identification of the synthesized inhibitors
2
.3.1. Crystal structure determination of the investigated inhibitors
X-ray crystal determination: single crystal of the investigated compounds
suitable for X-ray diffraction was obtained by slow evaporation method. The
diffraction data from a selected single crystal was collected at room temperature on
Oxford Diffraction Xcalibur Gemini S diffractometer equipped with Copper kα
radiation source for the investigated inhibitors. The structures were solved by direct
methods and refined on F by full-matrix least-squares procedures using the
2
SHELXL-97 program. The hydrogen atoms were included in the structure factor
calculation at idealized positions by using a riding model, but not refined. Images
were created with ORTEP [23-25].
2
.3.2. Melting points and spectral measurements
Melting points for the prepared inhibitors were carried out on a melting point
apparatus, Cimarec 3 Thermolque. C, H and N analyses were performed with a
-1
Perkin-Elmer 2400 series II analyzer. Infrared spectra (4000-400 cm ) were recorded
5

ACCEPTED MANUSCRIPT
from KBr pellets on a Bruker IFS-125 FT-IR spectrophotometer. NMR spectra were
measured and recorded in the Central Lab., Chemistry Department, Faculty of
Science, Sohag University at 25 °C on a multinuclear FT-NMR spectrometer Bruker
1
13
1
13
ARX400 at 400.1 ( H) and 100.6 ( C and dept) MHz. The H and C chemical shifts
δ are given in ppm. Mass spectra were performed on a Shimadzu GCeMS-QP 1000
mass spectrometer at 70 eV. Absorption spectra were recorded with 1 nm resolution
for all cases on JASCO UV–Visible spectrophotometer.
HA-1 data
-1
Yield: 95%. Melting: 130 ºC. IR (cm ): 3258 m,3052 m, 2913 m, 1640 s, 1558
m, 1499 s, 1391 s, 1205 m, 1161 m, 739 m, 628 m. Elemental analysis calculation for
HA-1 (C H N O Br ): C, 59.21; H, 4.60; N, 4.60 %, Found: C, 59.57; H, 4.49; N,
3
0
28
2
2
2
1
4
7
.72 %. H NMR (400 MHz, DMSO-d ): δ 13.21 (s, 1H, OH), 8.91 (s, 1H,=CH),
6
.85-6.43 (m, 6H, CH aromatic), 2.31(S, 6H, 2 CH3) (Fig. S1a; Supporting data);
1
3
CNMR (400 MHz, DMSO-d ): δ 161.99, 159.91, 148.09, 139.15, 135.82, 134.46,
6
1
29.22, 121.63, 119.54, 119.5, 110.92;21.32 (Fig. S1b; Supporting data); ESI MS
+
(
M/Z): 304.1(M ).
HA-2 data
-1
Yield: 92 %. Melting: 170 ºC. IR (cm ): 3346 m, 3076 m, 2918 m, 2866 m,
612 s, 1536 s, 1432 s, 1392 m, 12 s, 829 m, 753 w. Elemental analysis calculation for
1
HA-2 (C H N O): C, 78.17; H, 5.79; N, 10.13%, Found: C, 77.93; H, 5.65; N, 9.98
1
8
16
2
1
%
. H NMR (400 MHz, DMSO-d ): δ 15.2 (s, 1H, OH), 9.79 (s, 1H,=CH), 8.24-6.83
6
(
m, 8H, CH aromatic), 2.2 (S, 3H, CH ), 2.12 (S, 3H, CH ) (Fig. S2a; Supporting
3 3
1
3
data); CNMR (400 MHz, DMSO-d ): δ 157.94, 150.82, 150.63, 139.52, 136.11,
6
1
2
35.26,134.88, 129.73, 129.12, 124.94, 124.26, 122.32, 119.95, 113.03, 112.66, 106.18,
+
4.22, 21.05(Fig. S2b; Supporting data); ESI MS (M/Z): 276.30 (M ).
6

ACCEPTED MANUSCRIPT
HA-3 data
-1
Yield: 87 %. Melting: 210 ºC. IR (cm ): 3379 m, 3056 m, 2936 m, 2841 m,
622s, 1526s, 1329 s, 1267 s, 1075 s, 842 m, 735 w. Elemental analysis calculation for
1
HA-3 (C H N O ): C, 66.85; H, 4.95; N, 8.67%, Found: C, 66.64; H, 5.05; N, 8.81
1
8
16
2
5
1
%
. H NMR (400 MHz, DMSO-d ): δ 15.5 (s, 1H, OH), 9.5 (s, 1H,=CH), 8.46-6.78
6
1
3
(
m, 10H, CH aromatic), 4.1(S, 3H, OCH ) (Fig. S3a; Supporting data); CNMR (400
3
MHz, DMSO-d ): δ 179.43, 150.48, 150.08, 144.95, 140.17, 136.86, 133.95, 126.84,
6
1
25.28, 124.58, 120.95, 117.58, 117.42, 109.64, 107.38, 57.33(Fig. S3b; Supporting
+
data); ESI MS (M/Z): 323.10(M ).
2
.4. Test solution and inhibitors
The aggressive solution 15% HCl which is generally used for acidizing
treatment was prepared by dilution of AR grade (BDH) HCl (Assay 37 % , density
.017 g/ml at 25 ºC). The concentration range of inhibitors employed was 0.01 to 1.0
1
mM. All Schiff bases inhibitors were dissolved in 15% HCl solution. Figure 2 shows
the chemical and optimized structures of the inhibitors HA-1, HA-2 and HA-3.
<<Figure 2>>
2
.5. Corrosion Measurements
All the electrochemical measurements under unstirred conditions were
performed at atmospheric pressure in a one liter glass cell which consisted of a in a
three electrode cell, A carbon steel C1018 specimen was used as the working
electrode (This electrode was used for one time), a platinum wire and a saturated
calomel electrode (SCE) served as counter and the reference electrodes, respectively.
All electrochemical measurements were carried out using VersaSTAT4
potentiostat/galvanostat.
7

ACCEPTED MANUSCRIPT
A stabilization time of 60 minutes was allowed before each run were
completed and this time was deemed to be sufficient to attain a stable open circuit
potential (E ). Polarization measurements were carried out at potential ±250 mV vs.
ocp
-1
the corrosion potential (E ) at a sweep rate of 1 mV s at 45 and 60 ºC.
corr
Electrochemical impedance spectroscopy (EIS) measurements were performed
with a phase-sensitive amplifier (Model-5208) driven by using Versa-STAT4
potentiostat/galvanostat with a frequency response analyzer (FRA) contained in a
single unit and connected computer system. The EIS measurements were conducted at
open circuit potential, after 1 h of immersion in the test solution in the
frequency range from 100 kHz to 0.5 Hz. The amplitude of the voltage perturbation
was 10 mV RMS. Parameters such as the double-layer capacitance (C ), the solution
dl
resistance (R ) and the resistances of charge transfer (R ) were obtained from the EIS
s
ct
study by fitting the experimental data to the equivalent model circuit using the fit and
simulation tool of the Z-View software.
The immersion tests were performed by using the linear polarization resistance
(LPR) corrosion rate method implicated evaluating the corrosion of carbon steel in
1
5%HCl solution at 45 ºC as described in Abd El-Lateef [3]. Fresh solution and fresh
specimens were used in each experiment and three measurements were performed to
ensure the reliability and reproducibility of the data.
2
.6. Surface characterization by SEM/EDX
In order to get insight into the changes on surface of corrosive samples before
and after introducing of inhibitors, the substrates were initially immersed in the
corrosive solution (15% HCl) in the absence and presence of 1.0 mM HA-1 and HA-3
for 24 h at 45 ºC, respectively, then taken out from the test solutions, cleaned with bi-
distilled water and acetone, and dried with air. The morphology of the tested samples
8

ACCEPTED MANUSCRIPT
was observed by using energy dispersive spectroscopy (EDX) on the scanning
electron microscope (SEM) (SEM/EDX) (JEOL, model 5300). The tested samples
obtained from LPR corrosion rate measurements were used in investigations.
2
.7. Quantum chemical calculations
MOPAC2000 with WinMOPAC 2.0 as a graphic interface was used to carry
out the quantum chemical calculations [26, 28]. At the beginning, structure the
starting material was optimized with the eigenvector-following routine (EF) [28]
using the semi-empirical PM3 method [29]. Quantum chemical calculations were
performed using Gaussian-09 software package at B3LYP/6-31G (d, p) basis set [30].
The quantum chemical indices calculated included, the energy of the highest occupied
molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital
(ELUMO), the energy gap (ꢀE =ELUMO-EHOMO), the dipole moment (µ), the absolute
hardness (η), the electronic energy (E ), core-core repulsion (C-CR), heat of
e
formation (H ) and ionization potential (IP).
f
3
. Results and discussions
3
.1. Structural conformation for the investigated compounds
.1.1. FT-IR and molecular electronic spectra
3
Infrared spectra for the investigated inhibitors system were recorded in the range
-1
of 400 – 4000 cm . A broad hydroxyl (-OH) peak is observed within the region
−
1
3
258-3379 cm . The broadening of these bands and the low frequency could be
attributed to the formation of strong hydrogen bonding (O-H…..N) in the solid state
−
1
−1
[
31]. The peaks appear within the regions 3052-3076 cm and 2913-2936 cm refer
to aromatic and aliphatic –CH respectively. Moreover, the strong peaks lie within the
−
1
region 1612-1640 cm , could be rationalized to ν(C=N) stretching indicating that
condensation between aldehyde and amine has taken place, resulting in the formation
9

ACCEPTED MANUSCRIPT
of the desired the investigated inhibitor compounds [32]. The appearance of bands
−
1
within the range of 1205- 1292 cm for, respectively are mainly due to the bending of
C-O bond.
The molecular electronic spectra of HA-1 inhibitor and its complex with Fe(II)
was carried out in ethanol at room temperature at concentration 1.0 mM for each
compound. The investigated HA-1 inhibitor has an absorption bands in ultraviolet
region (200–400 nm), these bands extends over to higher wavelength region due to
conjugation. (cf. Fig. 3). The spectrum of the investigated HA-1 exhibits band at 379
nm were observed and assigned to n→π* transitions originating from the imine or
amide group of the Schiff base ligand [33]. These bands disappeared via
complexation and new band attributed to the donation of the lone pair of electrons of
the nitrogen atoms of the Schiff base to metal ion (M→N) appear [34]. Moreover, the
appearance of bands at 443 nm in HA-1 inhibitor due to extra conjugation between
the amine and aldehyde ring [8]. Furthermore, the [Fe-HA-1] complex show
additional band at 480 nm due to d-d transition [32-34].
1
13
3
.1.2. H, CNMR spectra and elemental analalysis for the prepared compounds
Data are reported along with the possible assignments in experimental. The
1
HNMR spectra of the prepared Schiff bases inhibitors showed the characteristic
NMR signals. Signals of phenolic proton are appeared at 15.50-13.20 ppm. The
characteristic proton of azomethine appeared at 9.5-8.91 ppm. The signals in the
1
3
range of 7.80-6.70 ppm are characteristic to the aromatic protons. The CNMR
spectra of the free ligand (nap) have showed a peak at 179.00-157.94 ppm due to
characteristic azomethine carbon. The signals at 150-106 ppm may be attributed to
phenyl aromatic carbons and 57-21.05 for aliphatic carbon.
1
0

ACCEPTED MANUSCRIPT
The micro analytical data for the prepared inhibitors are in a good agreement
with calculated values. This confirmed the proposed structures of the prepared
inhibitors (cf. Fig. 2)
<<Figure 3>>
3
.1.3. X-ray crystallographic
The molecular structures and the packing arrangements of the molecules with
hydrogen bonds of compounds HA-1, HA-2 and HA-3 are presented in Figure 4. The
crystal data and structure refinement of investigated synthesized inhibitors are given
in Table 1. Some selected geometrical parameters (bond lengths and bond angles) are
listed in Table 2. Compounds HA-1 crystallized in the orthorhombic Pn2 a space
1
groups and the determined unit cell parameters are a = 21.8290 (3) Å, b = 16.9484 (3)
3
Å, c = 7.11447 (13) Å, α = β= γ= 90 and V= 2632.11 (7) Å . The ORTEP diagram
of the HA-1 compound with atomic labeling scheme in Figure 4 showed that, the
molecule form a dimer sustained by N−H···N hydrogen bonds. The crystal structure
is settled by intermolecular C—H…N interactions. The compound HA-2 crystallizes
in monoclinic crystal system with the space group P2 . The unit cell parameters are a
1
=
8.8848(2) Å, b = 6.3713(1) Å, c = 12.6507(2) Å, α = γ= 90°, β= 94.429° and V =
3
7
13.99(2) Å . Furthermore, the planarity of molecule is stabilized by a strong
intramolecular hydrogen bond. The compound HA-3 (Fig.4) crystallizes in triclinic
crystal system with the space group P-1. Summary of the crystallographic data and
refinement parameters is given in Table 1. The important interatomic distances and
angles are listed in Table 2. The crystal structure is stabilized by intermolecular C—
H…N interactions. The packing diagram is given in the Fig. 4b.
The molecular structures of compounds HA-1, HA-2 and HA-3 consist of an
electronic π-system from the hydroxy-aldehyde derived fragment connected to an
1
1

ACCEPTED MANUSCRIPT
aromatic aniline ring. The bond distances from N to C (-C=N-) were found to be
1
.286 (7) Å, 1.327(4) Å and 1.332(3) Å for compounds HA-1 (N1—C7), HA-2 (N1—
C11) and HA-3 (N1—C11), respectively. Bond angles C7—N1—C8, 119.3(4) °;
C11—N1— C12, 123.8(3) ° and C11—N1—C12, 127.02 (17) ° were also found for
compounds HA-1, HA-2 and HA-3, respectively. The bond distances and angles are
in accordance with the normal ranges previously reported for other Schiff bases
derived from cinnamaldehyde [35].
<<Figure 4>>
<<Table 1>>
<<Table 2>>
3
.2. Potentiodynamic polarization measurements
.2.1. Effect of inhibitor dose
3
In order to estimate the inhibition ability of the investigated Schiff base
compounds for carbon steel pipelines, polarization measurements were carried out on
in 15% HCl solution in the absence and presence of the studied inhibitors at
concentrations range 0.01-1.0 mM. Figure 5 shows the polarization curves of carbon
steel in the test solution containing different dose of inhibitors HA-1, HA-2 and HA-3
at 45 ºC. It is clear that from Fig. 5, introduction of inhibitors to the system causes a
significantly minimize in the rate of corrosion, i.e. shifts the cathodic curves to more
negative potentials and the anodic curves to more positive potentials. This may be
ascribed to adsorption of the inhibitors over the metal surface [36]. The pertinent
Tafel indices such as the corrosion potential (E ), anodic and cathodic Tafel slopes
corr
(
β β ) and corrosion current densities (I ) were obtained by extrapolating the
a, c corr
1
2

ACCEPTED MANUSCRIPT
polarization curve. The surface coverage (θ) and the inhibition efficiency (P %) can
be calculated using Eq. (1) [1-4] :

I 
inh
(1)
P% =
θ
×100 = 1−
×100


Iuninh


where Iuninh. and Iinh. are the corrosion current densities in the free solution and the
presence of inhibitors, respectively. An assessment of Table 3 shows that, with
increasing the concentration of inhibitors, the corrosion current density (Icorr)
decreased and the inhibition efficiency (P%) increased for carbon steel at studied
temperatures. The high P% value suggested a full coverage of the studied steel
surface with adsorbed inhibitor molecules. The above results indicated that retardation
of the samples corrosion processes at cathodic and anodic reaction sites occurred, and
the results were attributed to the adsorption of these sites by inhibitor molecules [1-4].
However, the addition of inhibitors shifted the corrosion potential E , with
corr
respect to the blank, but this shift is not more than 85 mV (maximum ꢀE = -29
corr
mV). According to Riggs [37], the classification of compounds as anodic or cathodic
inhibitors is feasible when the potential displacement is at least 85 mV in relation to
that measured for the free inhibitor system, it may be concluded that, these inhibitors
molecule should be considered as a mixed-type inhibitors. Also with the addition of HA-
1
, HA-2 and HA-3 inhibitors both β
a
and β values almost unchanged with respect to
c
blank, indicating that, the hydrogen evolution mechanism do not modify by the
addition of inhibitors to HCl solution, but acts as an adsorptive inhibitors [38]. On the
other hand, the anodic Tafel slopes (β ) are also found to be smaller than the
a
respective cathodic Tafel slopes (β ). The θ value found to be near unity (θ=0.96,
c
Table 3), indicates almost a whole coverage of the steel surface with the adsorbed
inhibitor molecules. Categorically, when the inhibitor has high θ value is considered
1
3

ACCEPTED MANUSCRIPT
as a perfect physical barrier shielding the corroding steel surface from the aggressive
solution. Correspondingly, P% increases with the inhibitor concentration. P% for 1.0
mM inhibitor reaches up to a maximum of 96.2% for HA-3, 91.6% for HA-2 and 89.2
%
for HA-1. The results revealed that the investigated inhibitors had inhibition
effectiveness on C1018 steel at studied temperatures, and P% follows the order: HA-3
HA-2 > HA-1.
>
<<Figure 5>>
<<Table 3>>
3
.2.2. Effect of temperature
The influence of the reaction temperature on the potentiodynamic polarization
curves in the absence and presence of 1.0 mM of HA-1 for C1018 carbon steel in 15
HCl is represented in Fig. 6. There is a clear acceleration of both the cathodic and
%
anodic reactions with an increase in temperature. As it is observed from the
polarization parameters (Table 3), for carbon steel, the corrosion current densities
increased with the increase of temperature in 15% HCl solution without and with 1.0
mM of the investigated inhibitors. These results could be attributed to the acceleration
of all the processes during the corrosion, such as electrochemical, chemical, transport
with increase in temperature [39]. It is noted that from Table 3, the inhibition
efficiency and surface coverage degree increased with the rise of temperature,
indicating chemical adsorption of the Schiff base molecule on the metal surface [40].
To clarify the effect of temperature on the inhibition characteristics of the
inhibitors, the apparent activation energies (E ) of the corrosion process in the absence
a
and presence of 1.0 mM of studied inhibitors at 45 °C and 60 °C were determined
from the condensed Arrhenius equation as follows [41]:
1
4

ACCEPTED MANUSCRIPT
Icorr(2)
Icorr(1)
Ea  1 1 
 − 
log
=
(2)


2.303R T T

1
2

where Icorr(1) and Icorr(2) are the corrosion current densities at temperatures T (45 °C)
1
and T (60 °C), respectively, and R is molar gas constant. The calculated E value of
2
a
carbon steel in the absence of inhibitors was 45.49 kJ/mol. However, in the presence
of 1.0 mM of HA-1, HA-2 and HA-3 inhibitors, the E values were decreased to
a
2
5.11, 24.87 and 21.71 kJ/mol, respectively. The decrease in activation energy after
the addition of inhibitors could be interpreted as a chemical adsorption of the
inhibitors on the steel surface [42].
The heat of adsorption was evaluated from the surface coverage and
temperature, as follows [43]:






θ2
θ1
T1T2
∆
Hads = 2.303R log
 − log
 


(3)






 
1
−
θ2
1−θ1
T2 −T




1

where θ and θ are the values of surface coverage at temperatures T (45 °C) and T
2
1
2
1
(60 °C), respectively. The calculated values of the heat of adsorption for the carbon
steel were 22.31, 22.69 and 25.36 kJ/mol in 15% HCl in the presence of 1.0 mM of
HA-1, HA-2 and HA-3 inhibitors, respectively. The obtained positive values indicated
that, the adsorption of the inhibitors on the metal surface was endothermic, and the
results were in a good agreement with the phenomenon of the proposed chemisorption
mechanism [44].
<<Figure 6>>
3
.3. Electrochemical impedance spectroscopy (EIS) measurements
To further study the corrosion inhibition of the investigated Schiff base
inhibitors, EIS measurements were carried out at 45 °C. The EIS plots of carbon steel
1
5

ACCEPTED MANUSCRIPT
in 15% HCl solution in the absence and presence of various concentrations of
inhibitors are shown in Figs. 7-9.
Nyquist plots of carbon steel immersed in 15% HCl solution without and with
different concentrations of inhibitors HA-1, HA-2 and HA-3 are presented in Figs. 7a,
8
a and 9a, respectively. The impedance spectra of all samples exhibit a single
depressed capacitive loop, which indicates that the corrosion of carbon steel is mainly
controlled by double layer behavior and charge transfer process [45]. Thus, the
corrosion inhibition mechanism is almost the same in the presence of studied
inhibitors [46]. The diameter of capacitive loop increases significantly when the
Schiff base compounds are added into the aggressive solution, which proves that HA-
1
, HA-2 and HA-3 inhibitors can significantly inhibits the corrosion of carbon steel in
1
5% HCl solution [47]. The charge transfer resistance values (R ) were determined
ct
from the difference between impedance values at the higher and lower frequencies
48]. In order to obtain a more accurate and representative fit, the double layer
[
capacitance (C ) was replaced by the constant phase element (CPE). According to the
dl
EIS data, the inhibition efficiency (P%) and the double-layer capacitance (C ) were
dl
calculated from charge transfer resistance (R ) and CPE parameters (Y and n) using
ct
0
the following equations [3]:
1
1
− n
n
(4)
C = (Y R
)
dl
0
ct
Rct − R
ct(0)
(5)
P% =
×100
Rct
where Y is CPE constant and n is CPE exponent (represents the deviation from the
0
ideal behavior and it lies between 0 and 1). Rct(0) and R are the charge transfer
ct
resistance values without and with inhibitor, respectively.
1
6

ACCEPTED MANUSCRIPT
The corresponding phase angle and Bode impedance magnitude plots recorded
for carbon steel electrode in 15% HCl solution with addition of various inhibitors
dose at its Ecorr are shown in Figs. 7b, c , 8b,c and 9b,c. As seen from Figs. 7b, 8b and
9
b, the Bode plots refer to the existence of an equivalent circuit containing a single
constant phase element in the interface of steel/acid. In addition, at low frequencies in
Bode plot the increase of absolute impedance confirms the higher protection with the
increasing of inhibitors concentration, which is related to the adsorption of HA-1,
HA-2 and HA-3 compounds on the surface of carbon steel [49]. In the phase angle
plots (Figs. 7c, 8c and 9c), increasing the concentration of HA-1, HA-2 and HA-3
inhibitors in 15% HCl results in more negative values of phase angle indicating
superior inhibitive performance due to more inhibitor molecules adsorbed on carbon
steel surface at higher concentrations The maximum values in phase angle plots in the
presence of investigated inhibitors approach towards -90°, which indicates that there
is solely one time constant related to the electrical double layer formation at the
surface- solution interface [50]. The linear region of the log(f)–log ǀZǀ plots should
have a slope (α) of unity for the ideal capacitors. The values of α are calculated from
the log (f)–log ǀZǀ plots and are given in Table 4. As shown in Table 4, α values range
from -0.68 to -0.78, -0.70 to -0.81 and -0.73 to -0.85 for HA-1, HA-2 and HA-3,
respectively, this could be related to the non-ideal structure of the metal/solution
interface. In this case, a differential capacitance should be formed at the interface
rather than an ideal capacitor.
Figure 10 a and b shows the comparison of the simulated spectrum and the
experimental EIS data, using the Z-View impedance fitting software for carbon steel
samples immersed in 15% HCl without and with of 1.0 mM of HA-1 inhibitor. A
good fit was obtained with the model used for all experimental data. The equivalent
1
7

ACCEPTED MANUSCRIPT
circuit used in the present study is shown in Fig. 10 a and b (Inset)). In this equivalent
circuit, R , R and CPE are the solution resistance, the charge transfer resistance and
s
ct
the constant phase element (instead of C ), respectively. The values of impedance
dl
parameters are determined by using the equivalent circuit software Z-view and listed
in Table 4.
As observed in Table 4, the C values decrease while R values increase with
dl
ct
2
increasing inhibitors concentration. The highest R (130.7, 151.5 and 296.6 ꢁ cm for
ct
HA-1, HA-2 and HA-3, respectively) have been found at optimum concentration 1.0
mM. This behavior was the result of an increase in the surface coverage by the
inhibitor molecules, indicating that the exposed area decreased. The R obtained in
ct
the presence of the HA-3 is bigger than that in the presence of the other compounds
HA-1 and HA-2. The decrease in C values may be considered in term of Helmholtz
dl
model [51]:
A
C = ε0ε
dl
(6)
d
where ε , ε, d and A are the permittivity of air, the local dielectric constant, the
0
thickness of the film and the surface area of the electrode, respectively.
In fact, the decrease in the C can result from a decrease in local dielectric
dl
constant and/or an increase in the thickness of the electrical double layer, suggests
that the inhibitor molecules act by adsorption at the steel substrate/solution interface
[
52]. The addition of the synthesized inhibitors provides lower C values, probably
dl
as a consequence of substitution of water molecules by the synthesized Schiff base
-2
compounds at the electrode surface. The value of C is decreased from 128.4 µF cm
dl
-2
in the blank solution to 12.1 8.5 and 5.3 µF cm by addition of 1.0 mM of HA-1, HA-
,
2
and HA-3, respectively, as a result of the effective adsorption of the studied
inhibitors.
1
8

ACCEPTED MANUSCRIPT
As listed in Table 4, the values of inhibition efficiency (P%) increase with the
concentration of these Schiff base inhibitors up to 90.51 % for HA-1, 91.81 % for
HA-2 and 95.82% for HA-3, when concentration reaches 1.0 mM. These results again
confirm that, the investigated Schiff bases compounds exhibit efficient inhibitive
performance for carbon steel in the chosen strong acid solution (15% HCl) and P%
values follows the order: HA-3 > HA-2 > HA-1. This result is in good agreement
with obtained data from the potentiodynamic polarization studies that, 1.0 mM of the
inhibitors (HA-1, HA-2 and HA-3) was the optimum concentration and HA-3 is a
better corrosion inhibitor in 15% HCl solution.
<
<
<
<Figure 7>>
<Figure 8>>
<Figure 9>>
<
<Figure 10>>
<Table 4>>
<
3
.4. Effect of immersion times
It is well known that, the inhibition efficiency ability of organic inhibitor
molecules depends on immersion time in the aggressive media. LPR corrosion rate
measurements was employed to investigate the effect of exposure time on the
corrosion behavior of carbon steel in 15% HCl containing 1.0 mM of HA1-, HA-2
and HA-3, respectively. Figure 11a shows the variation of corrosion rate (CR) results
with immersion time in 15% HCl in the absence and presence of 1.0 mM of HA1-,
HA-2 and HA-3 at 45 °C. Inspection of the figure reveals that, the corrosion rate
increases with increasing immersion time from 1 to 24 h in 15%HCl solution free
1
9

ACCEPTED MANUSCRIPT
inhibitors. However, in the presence of 1.0 mM of the under testing Schiff base
inhibitors the corrosion rate decrease until 2 h of exposure and after that remain stable
(from 2 to 24 h). Figure 11b show that inhibition efficiency of the investigated
inhibitors was increased with increasing immersion time until 2 h which may be
attributed to the formation of strong inhibitor layer which prevents the attack of
corrosive media on the metal surface. However, the results show a stabilization of the
inhibition efficiency from 2 h to 24 h of immersion. According to this study, it was
found that the studied Schiff base compounds are very effective inhibitors for carbon
steel in 15%HCl solution because after an immersion time of 24 h, the power of
protection already achieved 88.7, 92.5 and 94.4% for HA1-, HA-2 and HA-3,
respectively. Notwithstanding, these compounds can provide good protection to the
carbon steel substrate even after 24 h exposure to this highly corrosive media
(15%HCl). The results of LPR corrosion rate measurements also indicated that the
inhibition efficiency (P%) of the inhibitor HA-3 was greater than that of the other
inhibitor.
<<Figure 11>>
3
.5. Adsorption isotherm
The adsorption of organic inhibitor on the carbon steel surface has been
widely investigated by classical adsorption isotherms. The adsorption isotherm could
provide useful information for the mechanism of inhibiting effect [43, 53]. The type
of the adsorption isotherm is determined from the inhibitor concentration and the
surface coverage.
The experimental data obtained from EIS measurements at 45 ºC were
supplied on different adsorption isotherm equations including Frumkin, Langmuir,
2
0

ACCEPTED MANUSCRIPT
Temkin, Freundlich and Flory–Huggins isotherms and found to fit the Langmuir
isotherm (Fig. 12) which may be formulated as:
Cinh
1
=
+ Cinh
(7)
θ
Kads
where C , K and θ are the inhibitor concentration, the adsorption–desorption
inh
ads
equilibrium constant and the surface coverage, respectively. A linear relation can be
2
found between C /θ and C with regression coefficients (R ) 0.9988, 0.9996 and
inh
inh
0
.9993 for inhibitors HA-1, HA-2 and HA-3 respectively. The Kads values were
,
determined from the intercepts of the lines in Fig. 12. The value of Kads was found to
5
5
5
-1
be 5.31×10 , 8.64×10 and 12.40×10 mol l for HA-1, HA-2 and HA-3 respectively.
,
Kads value represents the binding force of the inhibitor molecules adsorb onto the
metal surface, and the large value of Kads reflects strong adsorption ability of the
inhibitor molecules onto the metal surface [43, 54]. The investigated Schiff base
inhibitors give larger Kads values at 45 °C, imply more efficient adsorption and hence
better inhibition efficiency. In addition, HA-3 inhibitor had larger Kads values at the
studied temperature; the results meant that the adsorption ability of this inhibitor onto
steel surface was stronger than that of HA-1 and HA-2 compounds. The Kads value is
o
possess of the standard free energy of adsorption (∆G ) by the following equation
ads
[1-4]:
ο
ads
1
∆G
(8)
K =
exp(−
)
ads
5
5.5
RT
where 55.5 is the concentration of water in solution expressed in mol/L,
R is
molar gas constant,
T
is the corresponding temperature.
o
The calculated values of ∆G for the investigated compounds HA-1, HA-2
ads
-1
and HA-3 were -45.43, -46.74 and -47.71 kJ mol , respectively. The obtained
negative values indicated that, the inhibitors strongly adsorbed onto the metal surface
2
1

ACCEPTED MANUSCRIPT
o
ads
-1
at 45 °C. In general, when the absolute value of ∆G is lower than 20 kJ mol
indicates physisorption, while it is related to chemisorption when the value is 40 kJ
-1
o
ads
mol or larger [55]. In our study, it is noted that the absolute values of ∆G for
-1
carbon steel were larger than 40 kJ mol at the studied temperature, indicating that
the adsorption mechanism of inhibitors onto steel surface were chemical adsorption at
4
5 °C.
<<Figure 12>>
3
.6 . Surface morphological observation and analysis
3
.6.1. SEM analysis
The surface morphologies of carbon steel surfaces exposed and unexposed to
5% HCl solutions in the absence and presence of 1.0 mM of HA-1 and HA-3 after
4 h of immersion were observed by the SEM examination presented in Fig. 13. It can
1
2
be seen from Fig. 13a that the steel specimen before immersion looks like smooth and
appears some abrading scratches on the surface. After immersion 15% HCl solution
with no inhibitor, displayed a very rough metal surface covered with thick porous
oxide layer due to severe corrosion (Fig. 13b). In presence of the inhibitors (Fig. 13c,
d), much less damage and smooth surfaces are observed. Among HA-1 and HA-3,
the presence of HA-3 as corrosion inhibitor results into comparatively cleaner and
smother surface. This elucidates a good adsorption and corrosion inhibition
potentiality of HA-3 compare to HA-1. These results also approved that, the
formation of the adsorbed layer of inhibitor provided the protective property against
corrosion, resulting in excellent corrosion inhibition.
<<Figure 13>>
2
2

ACCEPTED MANUSCRIPT
3
.6.2. Energy dispersive analysis of X-Rays (EDX) Studies
In order to determine the elements present on the carbon steel surface after 24
h of exposure in the uninhibited and inhibited 15%HCl, the EDX spectra were used. A
comparable elemental distribution of blank carbon steel, with and without studied
Schiff base inhibitors analyzed by (% weight) is shown in Table 5. Before the
corrosion test, the characteristic peaks are related to the metals which are present in
the carbon steel alloy (Table 5). In the absence of inhibitors, the data exhibits the
distinctive peaks which are regarding to Fe, Mn, P, S, Ni, Cr, Cl and oxygen elements.
This indicated that the passive film contained Fe O and/or FeCl . The EDX spectrum
2
3
3
of carbon steel immersed in the 15% HCl containing 1.0 mM of HA-1 and HA-3
inhibitors shows the additional peaks, demonstrating the existence of C and N (owing
to the carbon and nitrogen atoms of Schiff base inhibitors). These finding shows that
the carbon and nitrogen atoms covered the steel surface. In addition, the intensity of
O signals is reduced from 34.4% to 12.4% and 11.7% and the intensity of Fe signal is
increased from 43.45% to 71.54% and 72.41% (Table 5) in the presence of 1.0 mM of
HA-1 and HA-3 inhibitors, respectively. This result demonstrated that, the adsorption
of inhibitor molecule on carbon steel surface leads to a decrease of metal oxide layer.
<<Table 5>>
2
.7. Quantum Chemical Studies
In order to confirm the correlation between and inhibition efficiency of the
investigated Schiff base inhibitors and electronic structure quantum chemical
calculations were performed. In this study, the relationship between quantum
chemical indices and inhibition efficiency was inspected. The distribution of lowest
unoccupied molecular orbitals ( LUMO
E
) and highest occupied molecular orbitals
(EHOMO) of HA-1, HA-2 and HA-3 models are shown in Fig. 14. It is obvious from
2
3

ACCEPTED MANUSCRIPT
this figure that the HOMO and LUMO orbitals are distributed on oxygen, nitrogen
atoms and phenyl ring. Table 6 shows the values of some quantum chemical
parameters including ELUMO
,
EHOMO, the total electronic energy of the molecules (Ee),
EHOMO), dipole moment ( ), core-core repulsion (C-CR),
the energy gap (ꢀ ELUMO
E
=
-
µ
absolute hardness (η), ionization potential (IP) and heat of formation (Hf).
EHOMO and ELUMO are significant parameters for the prediction of the reactivity
of inhibitor species. The EHOMO is often associated with the electron donating ability
of a molecule [56-58]. According to Eddy and Ebenso [59], increasing values of
EHOMO facilitate the adsorption of the inhibitor. It is evident that from Table 6, the
EHOMO for the investigated imines decreases in the order; (HA-3> HA-2>HA-1),
which is in agreement with the experimental results of inhibition efficiency (
addition, the lower the ELUMO, as the ꢀ decreased and the % improved [60].
Inspection of the data in Table 6 reveals ꢀ value obeys the order of HA-3< HA-2<
HA-1, which is in completely accordance with the order of inhibition efficiency ( %)
of HA-3 > HA-2 > HA-1. Thus, there is a good correlation between ꢀ and %.
) is a suitable index for the prediction of the trend of a
is related to the distribution of electron in a molecule
P%). In
E
P
E
P
E
P
The dipole moment (
µ
corrosion inhibition process.
µ
and is measure of polarity in a bond [58]. The large value of dipole moment probably
increases the inhibitor adsorption through electronic force [61]. In Table 6, HA-3 has
higher ꢀ value that that of the other, which suggests that the better inhibitive behavior
of HA-3 would be arisen from intermolecular electrostatic force to some extent. The
dipole moment of the investigated inhibitors are 3.922, 2.23 and 7.01 Debye for HA-
1
, HA-2 and HA-3, respectively, which is higher than that of H O (µ = 1.88 Debye).
2
Accordingly, the adsorption of Schiff base molecules can be considered as a quasi
2
4

ACCEPTED MANUSCRIPT
substitution method between the inhibitor molecule and water molecules at the
electrode surface.
Absolute hardness (η) is important parameter to measure the reactivity and
molecular stability. A hard molecule has a large energy gap and vice versa [62]. In
our present study HA-3 with low hardness value 3.34 eV compared with other
compounds have a low energy gap and highest inhibition efficiency. The core-core
repulsion energy (C-CR) is a quantum chemical index that may have excellent
correlation with inhibition efficiency. El Ashry et al. [63] reported that, the inhibition
efficiency of some Schiff bases increased with decreasing value of core-core
repulsion energy. Similarly, the inhibition efficiencies of the investigated inhibitors
were found to increase with decreasing values of C-CR.
The electronic energy of the molecule (Ee), heat of formation (H ), and
f
ionization potential were also calculated. These indices have been found to be
useful in expecting the direction of a corrosion inhibition reaction for some systems
[64]. From the results in Table 6, it is evident that, the ionization potentials (IP)
increased in the same order of inhibition efficiency (HA-3 > HA-2 > HA-1). This is
because IP is directly related with the EHOMO. High electronic energy of the molecule
indicates high stability and chemical inertness and small value indicates high
reactivity of the atoms and molecules [65]. The low Ee (-21721.9 eV) of HA-3
enumerates its better inhibition efficiency. It was obvious that HA-2 had a heat of
-1
formation (H ) (21062. kJ mol ) than did of the other compounds.
f
In order to ascertain the adsorption sites of inhibitor atomic charges in the
molecule are considered. Based on the MOPAC calculations using AM1 method, the
atomic charge distributions of the HA-1 compound were calculated. Net atomic
charge distribution on monomer and dimer of HA-1 molecule and are given in Fig.
2
5

ACCEPTED MANUSCRIPT
1
5. From the Figure, it could be observed that the oxygen and nitrogen atoms play an
important role in the adsorption process of molecule on carbon steel surface. The
results reveal that, the most negative charge in the studied compound is resident in the
nitrogen and oxygen atoms. As seen from Fig. 15, all non-hydrogen atomic charge
distributions of the dimer structure are decreased according to the secondary dipole-
dipole interaction forces. The charges of nitrogen are -0.154 and -0.118 for monomer
and dimer HA-1 molecule, respectively and oxygen charges are -0.249 and -0.231 for
monomer and dimer HA-1 molecule, respectively. It was found that, the charge
decreased in the case of dimer, this due to the monomer molecule consumed a part
from charge to form a dimer. This explains why the HA-1 compound has the less
adsorption and subsequently lower inhibition efficiency. This observation is
consistent with the data obtained from experimental investigations.
<
<Figure 14>>
< Figure 15>>
<Table 6>>
<
<
3
.8. Inhibition mechanism and potential of zero charge (PZC)
In general, the mechanism of corrosion inhibition in acid solution is the
adsorption of the inhibitor onto the metal surface. The adsorption process of organic
compounds onto the metal surface is influenced by the type of aggressive media, the
nature and charge of the metal and the charge or dipole moment of the inhibitor
molecule. Generally, due to the complex nature of adsorption, it is not possible to
foresee a single adsorption mechanism [66]. The charge on the metal surface can be
determined from the position of corrosion potential ( corr) in regard to the respective
E
2
6

ACCEPTED MANUSCRIPT
zero charge potential ( PZC
E
). Antropove [67] suggests a special scale of potential, the
initial value of which is EPZC. Then the potential ɸ on this scale will be the difference
between Ecorr and EPZC [3]:
ɸ= Ecorr
Ecorr of the metal in a given electrolyte, the metal surface is negatively
charged, then the adsorption of cations will be most likely. For EPZC Ecorr, the metal
–
EPZC
(9)
If the EPZC
>
<
surface is positively charged, the adsorption of anions is most likely. EIS
measurements were used to gain more information about the surface charge of the
carbon steel. The EPZC of the steel electrode in the investigated acid solution
containing 1.0 mM of inhibitors HA-1, HA-2 and HA-3 was found to be -0.591, -
0
.601 and -0.599 V (SCE), respectively. From Table 3, it was observed that, the Ecorr
of carbon steel in 15% HCl solution with addition of 1.0 mM of HA-1, HA-2 and HA-
are -0.514, -0.545 and -0.555 V (SCE), respectively. It can be said that ɸ potential
3
is positive at the open circuit potential after 1 h of exposure time (ɸ=+0.077, +0.056
and +0.044 V (SCE) for HA-1, HA-2 and HA-3, respectively). From the above result,
−
Cl ions in aqueous HCl solution first adsorbed at the steel/acid interface at the Ecorr
through electrostatic attraction force due to the excess positive charge at this interface.
After this first adsorption step, the carbon steel surface becomes negatively charged.
Among the studied Schiff base inhibitors HA-1, HA-2 and HA-3 molecules exist as
protonated through -OH and nitrogen atoms (-C=N-) in HCl solution and it is in
equilibrium with the corresponding molecular form. Hence, the positively charged of
-
cationic forms of molecule has been formed an electrostatic bond with the Cl ions
already adsorbed on carbon steel surface. Accordingly, the adsorption of HA-2
molecule on steel surface can be proposed as in Fig. 16.
2
7

ACCEPTED MANUSCRIPT
In addition to the physical adsorption, these molecules are also adsorbed
through the chemisorption mechanism that may be formed between the lone pair
electrons of the unprotonated 4-bromo-2-{(Z)-[(3, 5-dimethylphenyl) imino]
methyl}phenol, 1-{(Z)-[(4,6 dimethylpyridin-2-yl)imino]methyl}naphthalen-2-ol and
1
-{(Z)-[(2-methoxy-4-nitrophenyl)imino]methyl}naphthalen-2-ol and the unoccupied
d-orbital in the Fe surface to form a coordinate type of a bond which enhanced the
combination strength between the inhibitor molecule and metal surface. [Fe(II)–HA-
1
] complex was characterized by UV/vis spectrophotomertic method as shown in Fig.
. From UV/vis spectra obtained, one can observe that, HA-1 compound exhibits two
3
maxima at about 379 and 443 nm. While the Fe(II) ions do not show any absorption
maxima. But the [Fe(II)–HA-1] complex does exhibit one absorption maximum at
about 480 nm. The shift in band maximum from 443 nm to 480 nm, suggesting the
interaction between organic molecule and Fe(II) ions in the solution. Abboud et al.
[68] has reported that change in position of the absorbance maximum and change in
absorbance value indicate the formation of a complex between two species in
solution. The stoichiometries of the titled inhibitors were measured by applying a
continuous variation method [8] (Fig. 17). The results suggested the possible
formation of 1:2 complex (Fe (II): Schiff base) as in Fig. 18.
Among the Schiff base investigated inhibitors, the corrosion inhibition
efficiency follows the order: HA-3 > HA-2> HA-1. The higher protection ability even
at low concentrations were probably due to the presence of aromatic rings and –OH, –
C=N- groups. Lower values of P% of HA-1 inhibitor compared to those of HA-3 and
HA-2 inhibitors may be due to the presence of that compound in the dimer form (as
discussed in Section 3.7 by using the quantum chemically calculated charge density
values). Otherwise, the highest inhibition efficiency of HA-2 and HA-3 inhibitors
2
8