New Schiff base cationic surfactants as corrosion inhibitors for carbon steel in acidic medium: Weight loss, electrochemical and SEM characterization techniques

Article abstract of DOI:10.1007/s11164-015-2219-7

Two new Schiff base cationic surfactants were synthesized and their chemical structures confirmed using FTIR spectroscopy. The tensioactive and thermodynamic characteristics of their solutions were determined using surface tension/log concentration plots.

Full text of DOI:10.1007/s11164-015-2219-7

Res Chem Intermed
DOI 10.1007/s11164-015-2219-7
New Schiff base cationic surfactants as corrosion
inhibitors for carbon steel in acidic medium: weight
loss, electrochemical and SEM characterization
techniques
Ahmed I. Adawy1 Mohamed A. Abbas2
•
•
Khaled Zakaria3
Received: 2 July 2015 / Accepted: 7 August 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Two new Schiff base cationic surfactants were synthesized and their
chemical structures confirmed using FTIR spectroscopy. The tensioactive and
thermodynamic characteristics of their solutions were determined using surface
tension/log concentration plots. The protection performance of the synthesized
compounds against carbon steel corrosion in 1 M HCl solution was evaluated using
weight loss and electrochemical techniques. The compounds were found to be
excellent corrosion inhibitors, and their efficiency increased with increasing inhi-
bitor concentration. The mode of adsorption obeyed the Langmuir adsorption iso-
therm. Polarization curves showed that the two compounds were mixed–type
inhibitors. The temperature dependence of the corrosion rate in the absence and
presence of these compounds was studied in a temperature range of 303 to 333 K
using the weight loss data. Activation parameters Ea, DHoa, and DSao were determined
and discussed. In addition, the presence of a protective film on the carbon steel
surface was confirmed using scanning electron microscopy (SEM).
&
Khaled Zakaria
khaled209eg@yahoo.com
Ahmed I. Adawy
chem_ahmedadawy@yahoo.ca
Mohamed A. Abbas
mohamedabbas1966@yahoo.com
1
2
3
Petrochemicals Department, Egyptian Petroleum Research Institute, Nasr City, Cairo 11727,
Egypt
Petroleum Applications Department, Egyptian Petroleum Research Institute, Nasr City,
Cairo 11727, Egypt
Analysis and Evaluation Department, Egyptian Petroleum Research Institute, Nasr City,
Cairo 11727, Egypt
123

A. I. Adawy et al.
Keywords Schiff bases Á Carbon steel Á Acid corrosion Á Acid inhibition Á
Polarization Á SEM
Introduction
Carbon steel is considered a vital structural material, and is one of the most common
and popular construction materials across a number of industries; it has played an
important role in national economic development, and accounts for approximately
85 % of annual steel production worldwide [1, 2]. Acidic solutions are widely used
for industrial cleaning and oil well acidification and in petrochemical processes,
especially hydrochloric acid, which is generally used in the pickling process of
metal and alloys, and these aggressive solutions may come into contact with the
carbon steel surface [3–5]. In all of these many applications, carbon steel is prone to
suffer the effects of this aggressiveness, leading to serious corrosion problems.
Numerous methods have been used to overcome corrosion issues, the most effective
and economic of which is the use of corrosion inhibitors for the protection of metals,
especially in acidic media [6, 7]. Organic compounds containing heteroatoms such
as nitrogen, oxygen or sulphur and multiple bonds are well recognized as efficient
corrosion inhibitors for metals and alloys that allowing greater adsorption on the
metal surface [8, 9].
Surface-active compounds (surfactants) are a very important class of organic
compounds that can be used as corrosion inhibitors [10–12]. They have many
advantages, including low toxicity, safety, easy adsorption on the steel surface, low
cost, ease of production, and high inhibition efficiency [13–19]. A surfactant
compound is one that tends to accumulate at a surface or interface due to its high
surface activity resulting from the adsorption of the surfactant molecules at the
interface between bulk phases, forming aggregations called micelles, such as air and
water, oil and water or electrode and solution [20–22]. The concentration at which
surfactants aggregate is called the critical micelle concentration (CMC). An
excellent surfactant inhibitor is one that aggregates or adsorbs at low concentrations.
Surfactants with low CMC values are desirable because they adsorb at low
concentrations; consequently, the CMC is a key indicator in determining the
effectiveness of surfactants as corrosion inhibitors.
Among the large number of synthetic and naturally occurring heterocyclic
organic compounds, Schiff bases have attracted the greatest interest. The presence
of an azomethine group (–C=N–) in Schiff bases enhances their adsorption ability
and corrosion inhibition efficiency [23]. Studies have shown that adsorption
depends mainly on certain physicochemical characteristics of the inhibitors, such as
electron density of the donor atoms, p-orbital character, and electronic structure of
the molecules acting as reaction centres for the adsorption process [24, 25]. Such
criteria are present in Schiff base molecules, allowing them to be easily adsorbed on
the metal surface by the formation of a coordinate covalent bond (chemical
adsorption) or electrostatic interaction between the metal and the inhibitor (physical
adsorption).
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New Schiff base cationic surfactants as corrosion…
Experimental
Chemicals
The raw chemicals and solvents used for the synthesis of Schiff base cationic
surfactant compounds were obtained from Sigma-Aldrich; materials were of
analytical grade and were used without further purification. All physicochemical
properties were determined in bi-distilled water.
Synthesis of Schiff base compounds
Schiff base compounds were synthesized according to the Schiff base reaction, as
follows: In a single-neck flask, 0.5 mol of N, N-dimethyl ethylene diamine was
condensed with 0.5 mol of benzaldehyde in the presence of 200 mL ethanol as a
solvent. The reaction mixture was refluxed for 6 h and left overnight until the
product was precipitated. The solid product was recrystallized from ethanol and
dried under a vacuum at 40 °C to obtain the Schiff bases.
Synthesis of Schiff base cationic surfactants (I and II)
In a single-neck flask, the synthesized Schiff base compounds (1 mol) were refluxed
separately with 1 mol of (dodecyl, hexadecyl) bromide individually for 90 h in the
presence of ethyl alcohol as solvent (Scheme 1 showing the chemical formulas of
the two prepared compounds I and II). The solid product was recrystallized from
ethanol and dried under a vacuum at 40 °C to obtain the Schiff base cationic
surfactants [(N, benzylidene amino) ethylene- dodecyl dimethyl ammonium
bromide, compound I] and (N, benzylidene amino) ethylene-hexadecyl dimethyl
ammonium bromide, compound II]. The chemical structure of the synthesized
compounds was confirmed by Fourier transform infrared (FTIR) spectra using the
ATI Mattson InfinityTM Series benchtop spectrometer (model 961), controlled by
WinFIRSTTM v2.01 software.
Tensiometry measurements
The tensioactive properties of the two synthesized compounds were identified by
observing their behaviour in aqueous solutions. Surface tension was measured using
¨
a du Nou¨y tensiometer (KRUSS K6) at different concentrations ranging from
4 9 10-2 to 1.9 9 10-5 M/L at 35 °C, controlled by a thermostatic water bath. The
curves of surface tension versus log surfactant concentration (c * log C) were
plotted and their CMC values calculated from the break points. At the same time,
the surface tension corresponding to CMC, cCMC, was obtained on the curves of
c * log C.
123

A. I. Adawy et al.
CH3
CH3
O
NH2CH2CH2N
CH3
CH3
CH
NCH2CH2N
CH3 CH2 Br
CH3 CH2 Br
11
15
CH3
Br
NCH2CH2N
CH3
Br
CH2 CH3
CH
15
CH2 CH3
CH
NCH2CH2N
11
CH3
CH3
Compound II, n =15
Compound I, n =11
Compound I: (N,benzylidene amino) ethylene- dodecyl dimethyl ammonium bromide and
Compound II: (N,benzylidene amino) ethylene- hexadecyl dimethyl ammonium bromide.
Scheme 1 The synthetic route of the two new Schiff base cationic surfactants
Corrosion tests
Metal sample preparation
All experiments were carried out on carbon steel of the following composition (wt%):
0.19 % C, 0.05 % Si, 0.94 % Mn, 0.009 % P, 0.004 % S, 0.014 % Ni, 0.009 % Cr,
0.034 % Al, 0.016 % V, 0.003 % Ti, 0.022 % Cu, and the rest Fe. Rectangular carbon
steel coupons having dimensions of length 5.6, width 2.7, and thickness 0.5 cm with an
exposed total surface area of 38.54 cm2 were used for gravimetric measurements. For
electrochemical experiments, the carbon steel cylinder, embedded in epoxy resin with
a surface area of 1 cm2 exposed to the electrolyte, was used as working electrode.
Before running all experiments, the surface of the carbon steel was abraded with emery
paper at a series of grades (320–600–800–1000–1200) and then washed with bi-
distilled water and acetone.
Corrosive medium
The corrosive medium, 1 M HCl, was prepared by dilution of analytical-grade HCl
37 % with bi-distilled water. The concentration of the synthesized compounds used
ranged from 100 to 1200 ppm. All solutions were prepared using bi-distilled water.
123

New Schiff base cationic surfactants as corrosion…
Gravimetric measurements
In order to perform gravimetric experiments, the coupons were dried and accurately
weighed, and were immersed in 100 ml of 1 M HCl solution with and without the
addition of different concentrations of the synthesized compounds. The effect of
temperature was studied by carrying out the experiments at 303, 313, 323 and
333 K.
Electrochemical measurements
Electrochemical experiments were carried out using a VoltaLab 40 PGZ301
potentiostat in a conventional electrolytic cell with a three-electrode arrangement:
saturated calomel reference electrode (SCE), a platinum wire as a counter electrode,
and the carbon steel as working electrode (WE). All measurements were conducted
at 303 K. Potentiodynamic polarization curves were obtained by changing the
electrode potential automatically (from -800 to -300 mV vs. SCE) at open circuit
potential with a scan rate of 2 mV s-1
.
Electrochemical impedance spectroscopy (EIS) measurements were carried out
using a potentiostat (VoltaLab PGZ-301) attached to the Zsimpwin software
program. Impedance spectra were obtained in a frequency range of 100 kHz to
50 mHz using 10 steps per frequency decade at the corrosion potential after 30 min
of immersion in test solutions without and with different concentrations of the
prepared inhibitor. An AC signal with 10 mV peak-to-peak amplitude was used to
perturb the system. The EIS diagrams are given as Nyquist representations. All
experiments were performed at 303 K under atmospheric conditions without
stirring.
Scanning electron microscopy (SEM)
It is necessary to ascertain the morphological changes occurring on the carbon steel
surface due to the corrosion process before and after the addition of inhibitors. For
this purpose, SEM was used. The specimens were first immersed in 1 M HCl in the
absence and presence of optimal concentrations (0.01 M) of the tested compounds
for 48 h at room temperature. They were then extracted from the test solutions,
cleaned with bi-distilled water and acetone, and dried with cool air. SEM images
were acquired using a JEOL model JSM-53000 scanning electron microscope.
Results and discussion
Chemical structure confirmation
The chemical structure of the new synthesized Schiff base cationic surfactants was
confirmed by FTIR spectroscopy. Figure 1 shows the characteristic bands of the two
synthesized compounds. FTIR spectra showed absorption bands at
1643.8–1646.2 cm-1 (imine group, CH=N), 2853.5 and 2852.2 cm-1 (CH2), and
123

A. I. Adawy et al.
Fig. 1 FTIR spectra of the synthesized Schiff base cationic compounds
2924.3 and 2921.3 cm-1 (N?) for surfactants I and II, respectively. The data
confirmed the expected functional groups in the synthesized cationic surfactants.
Surface activity parameters
The surface tension of aqueous solutions of the synthesized Schiff base surfactants
was measured at 35 °C with a surface tension technique and the results are depicted
in Fig. 2. It is clear that surface tension decreases with increasing concentration of
the surfactants due to increased migration of the surfactant molecules from the bulk
to the interface of the solution [26]. Of note, the reduction in surface tension of
surfactants at identical concentrations is greatly affected by the hydrophobic chain
length of the respective compound under investigation. Hence, compound I contains
a shorter hydrophobic chain (n = 11) and exhibits less reduction in surface tension
123

New Schiff base cationic surfactants as corrosion…
80
Compound I
Compound II
60
40
20
0
-5
-4
-3
-2
-1
0
log C, M/L
Fig. 2 Surface tension (c) versus log C at various concentrations of the synthesized Schiff base
compounds
than compound II (n = 15), suggesting a higher tendency of longer hydrophobic
chains towards adsorption at the air/water interface, hence increasing number of
hydrophobic chains, leading to increased repulsion between the aqueous medium
and the hydrophobic chains due to differences in their polarity.
The CMC values of the two surfactants were determined by the intercept of the
pre- and post-micellar trends at the break points of two straight lines fitted to the
interval of concentration characterized by a linear decrease in surface tension (c)
and to the concentration region of nearly constant surface tension (Table 1). CMC is
considered an important parameter because, at this concentration, most of the
physical and chemical characteristics of the surfactant solutions present abrupt
variations. It is evident that CMC values decrease as the length of the hydrophobic
chains increases; the value is lower for compound II (3.6 9 10-3 M/L) and higher
for compound I (3.9 9 10-3 M/L) [27]. However, this increase in hydrophobic
chain length results in decreased solubility, leading to a concentration of the
surfactant molecules at the surface and formation of micelles.
Table 1 Tensioactive and thermodynamic characteristics of the synthesized Schiff base cationic sur-
°C
factants (I and II) at 35
Inhibitor CMC 9 103 pCMC
Pc20 9 105 Cmax 9 1011
(M/L)
Amin
(nm2)
DG8mic
(kJ/mol)
DG8ads
(kJ/mol)
(M/L)
(mN/m)
(mol/cm2)
I
3.9
3.6
41.8
43.0
2
1
7.37
7.88
2.25
2.11
-14.2
-14.4
-67.3
-72.7
II
123

A. I. Adawy et al.
The surface tension of the surfactant solution at the CMC determines its
effectiveness (pCMC = co - cCMC). Hence, increasing the hydrophobic chain length
of the two surfactants increases the reduction in surface tension at the interface,
while the efficiency (PC20), i.e., the concentration of surfactant required to produce a
20 mN m-1 reduction in surface tension, is decreased. At this value, the surfactant
concentration is close to the minimum needed to produce maximum adsorption at
the interface.
Another important factor determining the surface activity of surfactant molecules
at the interface is the maximum surface excess, Cmax, which can be evaluated by
applying the Gibbs adsorption isotherm, as given in the following equation [28]:
ꢀ
ꢁ
1
dc
Cmax ¼ À
;
ð1Þ
2:303nRT d log c
T
where c is the equilibrium surface tension in dyne/cm, Cmax is the saturation
adsorption amount in mol/1000 m2, R is the gas constant (8.314 mol-1 K-1), T is
the absolute temperature, C is the surfactant concentration, and (dc/dlnC) is the
slope in the surface tension isotherm when the concentration is near the CMC. The
value of n expresses the number of solute species and is equal to 2 for conventional
ionic surfactants.
The values of maximum surface excess at the interface are increased by
increasing the hydrophobic chain of the two compound molecules, and this can be
attributed to the increased hydrophobicity of the molecules that increases the
tendency towards adsorption of surfactant molecules, and consequently Cmax is
increased, indicating higher surface concentration and increased number of
surfactant molecules at the surface.
The values of the minimum surface area occupied by the synthesized Schiff base
cationic surfactants at the interface, Amin (nm2), when the surface adsorption was
saturated were determined using the following equation:
106
Amin
¼
;
ð2Þ
NCmax
where N is Avogadro’s number (6.023 9 10 23), Cmax is maximum surface excess in
mol cm-2, Amin in nm2.
Basic information about the orientation mode of the molecules at the interface
can be provided by determining the minimum surface area (Amin). An increase in
adsorbed molecules at the interface leads to an increase in maximum surface excess
values; accordingly, the area available for each molecule will decrease. As a result,
a dense compacting layer of surfactant molecules will be formed at the interface.
The values of CMC, pCMC, PC20, Cmax and Amin are presented in Table 1. Briefly,
it is clear that compound II achieves the given values of surface tension at a lower
concentration than compound I. Consequently, it can be concluded that compound II
is more surface-active than compound I, revealing much better surface properties.
Generally, CMC values are dependent on the structure of the compound and its
quaternization by alkyl halide according to its alkyl chain length.
123

New Schiff base cationic surfactants as corrosion…
Thermodynamics of micellization and adsorption
The standard free energies of adsorption and micellization (DGoads and DGmo ic) at
35 °C were used to study the thermodynamic behaviours of the synthesized Schiff
base cationic surfactants in their solutions. The thermodynamic free energy of
adsorption is calculated from the following equation [29, 30]:
DGoads ¼ DGomic À ð0:6023 Â pCMC Â AminÞ;
ð3Þ
where DGomic = RT ln CMC, pCMC = co - cCMC, and co is the surface tension of
water and cCMC is the surface tension of surfactant solution at the same temperature.
Data pertaining to the thermodynamic parameters of micellization and adsorption
are listed in Table 1, from which it is clear that the values of the standard free
energies of adsorption and micellization of the synthesized Schiff base cationic
surfactants are always negative. This phenomenon indicates that the adsorption and
micellization processes both occurred spontaneously. In addition, the standard free
energies of adsorption of the two compound molecules at the air/water interface are
more negative than those of the micellization, indicating that the adsorption process
is more favourable than the mobilization. Moreover, the large difference between
DGoads and DGmo ic values can be attributed to the fact that the adsorbed molecules at
the interface are highly compacted, and consequently the water molecules do not
interact significantly with these molecules.
Weight loss measurements
The weight loss method, given its simplicity and the reliability of its measurements,
is the most commonly used non-electrochemical technique for evaluating the
inhibition efficiency of any inhibitor [31, 32]. Table 2 shows the values of corrosion
rate Cr (mg cm-2 h-1), surface coverage (O), and inhibition efficiency gw (%)
–
obtained from weight loss measurements for carbon steel after 6 h of immersion in
1 M HCl solution with and without the addition of different concentrations of the
two synthesized compounds at temperatures of 303, 313, 323 and 333 K. The
corrosion rate (Cr) was calculated from the following equation [33]:
DW
At
Cr ¼
;
ð4Þ
where W is the weight loss of the carbon steel coupons (mg), A is the total surface
area of the carbon steel coupon (cm2), and t is the immersion time (6 h). The
inhibition efficiency was obtained from the calculated corrosion rates as follows:
Cro À Cr
gWLð%Þ ¼
 100;
ð5Þ
Cro
where Cor and Cr are the corrosion rates of the carbon steel coupons in the absence
and presence of the inhibitor, respectively.
123

A. I. Adawy et al.
Table 2 Corrosion parameters obtained from weight loss data of carbon steel in 1 M HCl with and
without various concentrations of Schiff base derivatives at different temperatures, with an immersion
time of 6 h
T (K) Conc.
(ppm)
Compound I
Compound II
DW (mg) Cr
g
w (%) DW (mg) Cr
gw (%)
(mg cm-2 h-1
)
(mg cm-2 h-1
)
303
313
323
333
1 M HCl
93.0
45.4
0.4022
0.1963
0.0917
0.0768
0.0680
0.0567
0.5233
0.2920
0.1460
0.1188
0.1052
0.0942
0.6660
0.3869
0.2071
0.1811
0.1592
0.1339
0.7006
0.4302
0.2746
0.2543
0.2340
0.2067
–
93
0.4022
0.0639
0.0446
0.0346
0.0302
0.0209
0.5233
0.1162
0.0853
0.0675
0.0502
0.0403
0.6660
0.1798
0.1325
0.1079
0.0892
0.0686
0.7006
0.2179
0.1786
0.1548
0.1254
0.0925
–
100
51.2
77.2
80.9
83.1
85.9
–
14.79
10.32
8.00
84.1
88.9
91.4
92.5
94.8
–
300
21.2
600
17.8
900
15.7
6.98
1200
1 M HCl
100
13.1
4.84
121.0
67.52
33.76
27.47
24.32
21.78
154.0
89.47
47.89
41.89
36.81
30.95
162.0
99.47
63.50
58.81
54.12
47.79
121
44.2
72.1
77.3
79.9
82.0
–
26.86
19.72
15.61
11.62
9.32
77.8
83.7
87.1
90.4
92.3
–
300
600
900
1200
1 M HCl
100
154
41.9
68.9
72.8
76.1
80.0
–
41.58
30.65
24.95
20.64
15.86
162
73.0
80.1
83.8
86.6
89.7
–
300
600
900
1200
1 M HCl
100
38.6
60.8
63.7
66.6
70.5
50.38
41.31
35.80
29.0
68.9
74.5
77.9
82.1
86.8
300
600
900
1200
21.38
Influence of inhibitor concentration
The corrosion rate (Cr) and inhibition efficiency data obtained from weight loss
measurements are depicted in Table 2. It is clear that the weight loss values are
decreased, leading to suppression of the corrosion rate with increasing inhibitor
concentration, and that both compounds are highly effective against corrosion in
acidic solution, suggesting adsorption of these compounds onto the carbon steel
surface from acidic solutions [34].
The high protective performance of the synthesized inhibitors is due to the
presence of the phenyl and azomethine group (–C=N–) of Schiff base moiety in
their chemical structure, as well as the presence of protonated amine species, which
123

New Schiff base cationic surfactants as corrosion…
result in increased adsorption on the metal surface [35]. Consequently, these
compounds exhibit excellent protective performance as corrosion inhibitors for
carbon steel in an acidic medium, which is attributable to the presence of high
electron density from the benzene ring and the azomethine group. Indeed, these
groups are adsorbed onto the positive centre on the carbon steel surface, while the
alkyl chains of alkyl halides are arranged on the metal surface, preventing contact
between the metal surface and the aggressive solution, resulting in a reduction in the
rate of carbon steel dissolution [36].
Figure 3 shows the effect of compounds I and II concentrations on the inhibition
efficiencies of the carbon steel in 1 M HCl solution at different temperatures. In a
comparison of protection efficiency between compounds I and II in acidic solution,
the latter displays higher inhibition efficiency, which can be attributed to an increase
in the side chain attached to the tested inhibitors during the quaternization reaction.
Hence, inhibition efficiency is higher in the hexadecyl bromide derivative than the
dodecyl bromide derivative.
Effect of temperature
Weight loss measurements were used to study the effect of temperature on
inhibition efficiency percentage at 303–333 K for different concentrations of the
synthesized Schiff base compounds. From Table 2 and Fig. 3, we note that the
corrosion rates of carbon steel in both free and inhibited acid media increased with
increasing temperature for both inhibitors. Inhibition efficiency was found to
decrease as temperature increased. This phenomenon can be explained on the basis
that an increase in temperature leads to a shift in equilibrium constant towards
desorption of the inhibitor molecules at the surface of carbon steel, resulting in
physical adsorption of the synthesized inhibitors onto the metal surface [37, 38].
100
80
60
40
20
120
100
80
303 K
313 K
323 K
333 K
303 K
313 K
323 K
333 K
Compound I
Compound II
60
40
20
100
300
600
900
1200
100
300
600
900
1200
C, ppm
C, ppm
Fig. 3 Inhibition efficiency dependence on concentration of Schiff base compounds in 1 M HCl solution
at different temperatures
123

A. I. Adawy et al.
Adsorption isotherm
The application of adsorption isotherms is widely used for deducing basic
information about interactions between inhibitors and carbon steel surface in acidic
media. The adsorption capacity of the inhibitor on the carbon steel surface is
significantly related to its degree of protection efficiency, which is dependent on the
electronic characteristics of the inhibitor, the nature of the metal surface,
temperature, stress effects, and the varying degrees of surface-site activity [39,
40]. Accordingly, the mode of adsorption as well as the adsorption isotherm can
yield important information about the interaction between the inhibitor and the
metal surface.
–
The degree of surface coverage (O) for different concentrations of the tested
inhibitors in the studied temperature range (303–333 K) was evaluated by weight
–
loss data and is presented in Table 2. Various models were used to fit the O values,
including Langmuir, Temkin, Frumkin and Flory–Huggins. The linear relationships
–
of C/O versus C presented in Figs. 4a and 5a suggest that the best fit was obtained
with the Langmuir adsorption isotherm, and is described by the following equation
[41]:
50
(a)
40
30
303 K
313 K
323 K
20
333 K
10
0
0
5
10
15
20
25
30
C x 104, M /L
9
8.5
8
-30
(b)
(c)
-31
-32
-33
-34
-35
2.9
3
3.1
103 T-1 K-1
3.2
3.3
3.4
300
310
320
330
340
Temperature (K)
Fig. 4 Curves fitting the adsorption process of Schiff base compound I on carbon steel surface in 1 M
HCl solution; a Langmuir adsorption isotherm, b Van’t Hoff plot, and c variation of DGoads against 1/T
123

New Schiff base cationic surfactants as corrosion…
40
(a)
30
20
10
0
303 K
313 K
323 K
333 K
0
5
10
15
20
25
30
C x 104, M /L
-32
10.5
9.5
(b)
(c)
-34
-36
-38
8.5
300 305 310 315 320 325 330 335
Temperature (K)
2.9
3
3.1
3.2
3.3
3.4
103 T-1 K-1
Fig. 5 Curves fitting the adsorption process of Schiff base compound II on carbon steel surface in 1 M
HCl solution; a Langmuir adsorption isotherm, b Van’t Hoff plot, and c variation of DGoads against 1/T
Cinh
1
¼
þ Cinh
;
ð6Þ
h
Kads
where Cinh is the inhibitor concentration, Kads is the adsorption equilibrium constant,
–
and O is the degree of surface coverage with the value of inhibition efficiency. The
degree of surface coverage of each inhibitor and the inhibition efficiency at a given
concentration can be determined using the following equations, respectively [33]:
Wo À Wi
h ¼
ð7Þ
Wo
Wo À Wi
gWLð%Þ ¼
 100;
ð8Þ
Wo
where Wo and Wi indicate weight loss in the absence and presence of inhibitors,
respectively. Table 3 illustrates the strong correlation of the Langmuir adsorption
isotherm obtained for compounds I and II. Both linear correlation coefficients and
slope values are very close to unity, indicating adsorption of both compounds on the
carbon steel surface according to the Langmuir adsorption isotherm. A slight
123

A. I. Adawy et al.
Table 3 Different
thermodynamic parameters of
Inhibitor
T (K)
Slope
R2
Kads
–
linear regression between Cinh/O
and Cinh in 1 M HCl for the
prepared compounds I and II at
various temperatures
Compound I
303
313
323
333
303
313
323
333
1.1153
1.1499
1.1818
1.3497
0.9356
0.9521
0.9795
1.1319
0.9996
0.9997
0.9989
0.9983
0.9982
0.9977
0.9970
0.9965
6363
5012
4269
3924
Compound II
17,262
11,823
9946
7559
deviation from unity is noted in slope values, indicating the presence of a repulsive
force in the adsorption layer [42].
Equilibrium constant and standard free energy of adsorption (Kads
and DGoads
)
It is well recognized that the adsorption equilibrium constant (Kads) is related to the
standard free energy of adsorption (DGoads) deduced from the following equation
[43]:
ꢀ
ꢁ
1
DGoads
RT
Kads
¼
exp À
;
ð9Þ
55:5
where R (8.314) is the universal gas constant, T is the absolute temperature, and 55.5
is the molar concentration of water in solution expressed in mol/L. The values of
Kads obtained for the two compounds at different temperatures are listed in Table 3.
The relatively high values of the adsorption equilibrium constant reflect the higher
adsorption capacity of these molecules on the carbon steel surface in 1 M HCl. It
can also be seen from Table 3 that the adsorption equilibrium constant (Kads
)
decreases with an increase in experimental temperature, which likely indicates
desorption of some adsorbed inhibitor molecules from the metal surface at higher
temperatures, confirming the physical adsorption of the inhibitors.
With respect to DGoads values (Table 4), these values were shown to be negative
for the tested compounds, and the values are consistent with the spontaneity of the
adsorption process and the stability of the adsorbed layer on the carbon steel
surface. As a general rule, DGoads values of -20 or higher are in agreement with an
electrostatic interaction between charged molecules and charged metal surface
(physisorption), while values of -40 or lower are associated with charge or transfer
from the inhibitor molecules to the metal surface to form a coordinate covalent bond
(chemisorption) [44]. In the present work, the mean value of DGoads among the
various temperatures was -32.9 for compound I and -35.2 for compound II. This
implies that the adsorption of synthesized inhibitors on carbon steel surface is
neither typical physisorption nor typical chemisorption but is a complex mixed type
123

New Schiff base cationic surfactants as corrosion…
Table 4 Standard thermodynamic parameters of the adsorption of compounds I and II on carbon steel
surface in 1 M HCl
Inhibitor
T (K)
DGoads (kJmol-1
)
DHoads (kJmol-1
)
DSoads (Jmol-1 K-1
)
Compound I
303
313
323
333
303
313
323
333
-32.18
-32.26
-33.23
-34.03
-34.70
-34.86
-35.50
-35.84
-12.2
-12.2
-12.2
-12.2
-22.2
-22.2
-22.2
-22.2
65.2
65.2
65.2
65.2
41.1
41.1
41.1
41.1
Compound II
in which adsorption of inhibitor molecules on the carbon steel surface in 1 M HCl
solution involves both physisorption and chemisorption.
Also, Table 4 clearly shows that temperature has an influence on the values of
DGoads, as it can be concluded that DGaods became more negative with increased
temperature, indicating that the adsorption of compounds I and II on carbon steel
was favourable to the experimental temperature. Furthermore, from an adsorption
point of view, compound II was more stable than compound I, as it had a higher
DGoads value than compound I in 1 M HCl solution.
Standard enthalpy and entropy of adsorption (DHoads and DSaods
)
The sequel to the thermodynamic parameters that are important in studying
adsorption of organic inhibitors on metal surface, the heat of adsorption DHoads is
calculated using the Van ’t Hoff equation [45]:
DHaods
RT
ln Kads ¼ À
þ A
ð10Þ
A straight line was obtained when ln Kads was plotted against 1/T, as shown in
Figs. 4b and 5b. The slope of these straight lines is equal to (-DHoads/2.303 R). The
heat of adsorption (DHoads) can be approximately regarded as the standard heat of
adsorption under experimental conditions [46]. From the basic thermodynamic
equation, the standard adsorption entropy (DSoads) can be deduced as follows:
DHoads À DGoads
DSoads
¼
ð11Þ
T
A plot of DGoads versus T gives a straight line showing good dependence of DGaods on
T, confirming the good correlation among thermodynamic parameters as shown in
Figs. 4c and 5c. All thermodynamic parameters obtained are listed in Table 4. It is
clear that values of DHoads are negative, suggesting that the adsorption of inhibitors is
an exothermic process and inhibition efficiency decreases with an increase in
temperature [47]. This behaviour can be interpreted on the basis that more des-
orption of the adsorbed molecules from the carbon steel occurred at higher
123

A. I. Adawy et al.
temperatures. It is evident from Table 4 that the values of DSoads are negative, and
this is an exception, as adsorption is an exothermic process always accompanied by
a decrease of entropy, suggesting that there is an association rather than dissociation
in the rate-determining step.
Kinetic parameters referring to energy, enthalpy and entropy of activation
(Ea, DHoa and DSoa)
In addition to thermodynamic parameters, the kinetic model is another important
tool for elucidating the mechanism of corrosion inhibition and for further clarifying
the inhibitory properties of the inhibitor at different experimental temperatures.
It is generally accepted that the activation parameters for the corrosion process
can be determined from the Arrhenius equation [48]:
Ea
ln Cr ¼ À
þ ln A;
ð12Þ
RT
where Ea represents the activation energy, R is the gas constant, A is the pre-
exponential factor, and Cr is the corrosion rate obtained from weight loss mea-
surements. Figure 6 shows Arrhenius plots for the corrosion rate of carbon steel in
1 M HCl at different temperatures. The slope of linear regression between ln Cr and
1/T is (-Ea/R), and hence the values of activation energy (Ea) were calculated for
carbon steel in acidic solution at different concentrations without and with an
inhibitor.
There is another formulation of the Arrhenius equation, as [49]:
ꢀ
ꢁ
ꢀ
ꢁ
RT
Nh
DSoa
ÀDHao
Cr ¼
exp
exp
;
ð13Þ
R
RT
where h is Plank’s constant, N is Avogadro’s number, DSoa is the entropy of acti-
vation, and DHoa is the enthalpy of activation. Figure 7 shows a plot of ln
0
-1
-2
-3
-4
0
-1
-2
-3
-4
-5
blank
blank
100 ppm
300 ppm
600 ppm
900 ppm
1200 ppm
100 ppm
300 ppm
600 ppm
900 ppm
1200 ppm
Compound I
Compound II
2.9
3
3.1
103 T-1 K-1
3.2
3.3
3.4
2.9
3
3.1
3.2
3.3
3.4
103 T-1 K-1
Fig. 6 Arrhenius plots for carbon steel corrosion in 1 M HCl in the absence and presence of different
concentrations of Schiff base compounds
123

New Schiff base cationic surfactants as corrosion…
-2
-4
-5
Compound I
-6
Compound II
-6
-7
-8
blank
100 ppm
300 ppm
-8
blank
100 ppm
300 ppm
600 ppm
900 ppm
1200 ppm
600 ppm
900 ppm
1200 ppm
-10
-12
-9
-10
2.9
3
3.1
3.2
3.3
3.4
2.9
3
3.1
3.2
3.3
3.4
103 T-1 K-1
103 T-1 K-1
Fig. 7 Transition state plots for carbon steel corrosion in 1 M HCl in the absence and presence of
different concentrations of Schiff base compounds
(Cr/T) against 1/T for compounds I and II. Straight lines are obtained with a slope of
(-DHoa/R) and an intercept of (ln R/Nh ?DSao/R), from which the values of DHoa and
DSoa are calculated. The important activation parameters for carbon steel in 1 M HCl
without and with inhibitors at various concentrations are collected and listed in
Table 5. From Table 5, it is clear that Ea values are higher in the presence than in
the absence of inhibitors, indicating higher activation energies for the metal dis-
solution reaction; hence the process is activation-controlled. The increase in
apparent activation energy with inhibitor concentration therefore indicates a more
efficient inhibiting effect that can be attributed to the thickening of the electric
double layer [50]. In addition, Ea and DHoa seem to vary in the same manner. The
positive signs of the enthalpies (DHoa) reflect the endothermic nature of the disso-
lution process of carbon steel, suggesting that process is slow in the presence of
inhibitors [51]. With regard to the values of activation entropies (DSoa), large neg-
ative values imply that the activated complex in the rate-determining step represents
an association rather than a dissociation step, meaning that a decrease in disordering
takes place going from reactants to the activated complex [52]. However, the value
of DSoa increases gradually with increasing concentrations of compounds I and II.
This behaviour, which can be attributed to the adsorption of organic inhibitor
molecules from the acidic solution, can be regarded as a quasi-substitution process
between the inhibitor in the aqueous phase and water molecules at the electrode
surface [53]. Accordingly, the adsorption of organic inhibitor is followed by des-
orption of water molecules from the metal surface. Consequently, the increase in
entropy of activation can be attributed to the increasing solvent entropy [54].
Polarization curves
The potentiodynamic polarization method is considered the most important
electrochemical tool for the study of various electrochemical phenomena that
occur on metal surface. The polarization curves of carbon steel in the absence and
123

A. I. Adawy et al.
Table 5 Activation parameters Ea, DHoa and DSao for carbon steel in 1 M HCl in the absence and presence
of compounds I and II at different temperatures
Inhibitor
Conc. (ppm)
Ea (kJmol-1
)
DHoa (kJmol-1
)
DSoa (Jmol-1 K-1
)
Compound I
1 M HCl
100
15.85
22.03
30.35
33.51
34.34
35.10
15.85
34.21
38.25
41.28
40.32
41.50
15.85
21.91
30.27
33.36
34.28
35.15
13.23
31.59
35.64
38.67
37.70
38.89
-200.0
-185.7
-164.6
-156.1
-154.2
-152.7
-208.6
-162.9
-152.6
-144.7
-149.5
-148.2
300
600
900
1200
1 M HCl
100
Compound II
300
600
900
1200
presence of different concentrations of the synthesized inhibitors (I and II) at 303 K
are depicted in Fig. 8. The important related electrochemical parameters, such as
corrosion current density (Icorr, mA/cm2), anodic Tafel slope (ba, mV/dec), and
cathodic Tafel slope (bc, mV/dec) were obtained by extrapolation of Tafel lines at
the corrosion potential (Ecorr, mV) and are presented in Table 6. The surface
–
coverage (O) and inhibition efficiency (gTafel, %) were calculated from the following
equations [55]:
(a)
(b)
1
0
100 ppm
300 ppm
1
0
1MHCl
1MHCl
600 ppm
-1
-2
-3
-4
-5
900 ppm
100 ppm
300 ppm
-1
-2
-3
-4
1200 ppm
600 ppm
900 ppm
1200 ppm
-800
-700
-600
-500
-400
-800
-700
-600
-500
-400
-300
E (mV)
E (mV)
Fig. 8 Potentiodynamic polarization curves (E–log I relationship) of carbon steel in 1 M HCl solution in
the absence and presence of different concentrations of the inhibitors: a compound I and b compound II
123

New Schiff base cationic surfactants as corrosion…
Table 6 Polarization parameters and corresponding inhibition efficiencies for the corrosion of carbon
steel in 1 M HCl containing different concentrations of Schiff base derivatives at 303 K
Inhibitor
Conc.
(ppm)
Ecorr.
(mV)
Icorr. (mA/
cm2)
ba (mV/
dec)
bc (mV/
dec)
Rp (9X
gTafel
(%)
cm2)
1 M HCl
–
100
-605
-575
-576
-566
-575
-575
-603
-605
-603
-602
-601
0.3532
0.1800
0.0680
0.0476
0.0452
0.0414
0.0436
0.0354
0.0349
0.0340
0.0280
115
108
121
139
115
138
125
137
132
133
130
-125
-123
-109
-158
-120
-140
-129
-138
-145
-142
-133
78
163
–
Compound I
49
300
476
80.7
87.4
87.3
88.4
87.5
90.1
90.1
90.4
92.1
600
766
900
784
1200
937
Compound II 100
798
300
600
1080
1090
1110
1240
900
1200
ðicorrÞHCl À ðicorrÞinh
ðicorrÞHCl
h ¼
ð14Þ
ð15Þ
ðicorrÞHCl À ðicorrÞinh
ðicorrÞHCl
gTafelð%Þ ¼
and (icorr inh
presence of the inhibitor, respectively.
 100;
where (icorr HCl
)
)
are the corrosion current densities in the absence and
The inhibitory action of these compounds obtained from plotting of the
polarization curves was used to find the kinetic parameters of corrosion and to
compare them with the kinetic parameters from the solution without inhibitor. In
addition, we calculated the efficiency of the two inhibitors so that we could precisely
determine which gave the best result in a given medium and at what concentration.
After the results were analyzed, the following conclusions were drawn:
1. Table 6 shows that the corrosion current densities (icorr) decreased with
increasing concentrations of compounds I and II, indicating that these
compounds acted as inhibitors and that their inhibition efficiencies increased.
The degree of inhibition was dependent on the concentration and type of
inhibitor present (the length of the hydrophobic chain).
2. The corrosion potential (Ecorr) can be seen to shift slightly towards more
negative and positive potential in the presence of an inhibitor, suggesting that
the mechanism of inhibitor adsorption over the metal surface obeys the
adsorption phenomenon that hinders both anodic and cathodic reactions. The
value of Ecorr displacement was less than 85 mV, indicating that these were
mixed-type inhibitors with predominant control of the anodic reaction in the
presence of compound I resulting in a shift of corrosion potential in a more
noble direction compared with that obtained in the presence of an inhibitor [56].
123

A. I. Adawy et al.
This can be explained on the basis that inhibitor adsorption on the metal surface and
the adsorption process is enhanced with increasing inhibitor concentration.
3. Careful analysis of Fig. 8 shows that both cathodic and anodic reactions of
carbon steel electrode corrosion are suppressed with inhibitor concentrations as
a result of the ability of these inhibitors to control both reactions. In addition,
cathodic current–potential curves give rise to parallel Tafel lines, indicating that
oxygen reduction reaction is activation-controlled and that the addition of the
Schiff base compounds does not modify the mechanism of this process.
Generally speaking, the inhibitor is first adsorbed onto the carbon steel surface
and then impedes by merely blocking the active sites of the steel surface; as a
result, the surface area available for H? ions is decreased, while the actual
reaction mechanism remains unaffected [57].
4. The order of decreased inhibition efficiency of the synthesized compounds is
Compound II [ Compound I, in agreement with other techniques.
Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) was used to examine the corrosion
inhibition property of Schiff base compounds (I and II) on carbon steel, including
surface properties, electrode kinetics and mechanistic information. The Nyquist
plots of carbon steel obtained at open-circuit potential in 1 M HCl solution in the
absence and presence of various concentrations of Schiff base derivatives
(compounds I and II) are shown in the Fig. 9. It is obvious from the Nyquist
plots that the electrochemical response of carbon steel samples appeared in the form
of depressed semicircle capacitive loops, and this can be attributed to the formation
of a protective capacitive layer at the metal–acid interface.
(a)
(b)
300
200
100
0
400
300
200
100
0
900 ppm
1200 ppm
120
0 ppm
600 ppm
300 pp
900 ppm
100 ppm
300 ppm
6
m
00 ppm
100ppm
l
HC
1 M
1MHCl
0
200 400 600 800 1000 1200
0
200
400
600
800 1000
2
2
Z'(ohm.cm )
Z'(ohm.cm )
Fig. 9 Nyquist representation plots for carbon steel in 1 M HCl in the absence and presence of different
concentrations of the inhibitors at 303 K: a compound I and b compound II
123

New Schiff base cationic surfactants as corrosion…
It can also be noted from Fig. 9 that the impedance diagrams are larger than in
the blank solution upon the addition of inhibitors to the corrosive solution, and the
size of the impedance diagram (semicircle loops) increases as the concentration
rises; hence the protection efficiency increases due to the adsorption of inhibitor
molecules to the metal surface [58]. However, higher impedance against corrosion
reactions was obtained for compound II than compound I. The corrosion reaction at
the metal substrate/solution interface is related to the charge transfer resistance that
is indicated as a measure of electron transfer across the surface and is proportional
to the rate of corrosion. It is generally accepted that the depressed semicircle in the
Nyquist representation is attributable to the frequency dispersion as well as
inhomogeneities, roughness of the metal surface, and mass transport process [59].
Consequently, pure double-layer capacitors are better described by a transfer
function with constant phase elements (CPE) to compensate for non-ideal capacitive
response of the interface and to obtain a more accurate fit of the experimental data
set [60]. Its impedance function is given by [61]:
ZCPE ¼ YoÀ1ðjxÞÀn
;
ð16Þ
where Yo is a proportional factor, x is the angular frequency, and a deviation
parameter (-1 B n B ? 1) has the meaning of a phase shift. While n = 0, the CPE
represents a pure resistor, for n = -1 an inductor and for n = ?1 a pure capacitor.
The values of the double-layer capacitance (Q) and inhibition efficiency (gE) are
calculated as follows:
À
Á
nÀ1
Q ¼ Yo xnm
ð17Þ
ð18Þ
RCt À RoCt
gEISð%Þ ¼
 100;
RCt
where xnm is the angular frequency at the maximum value of the imaginary part of
the impedance spectrum, and Roct and Rct are the charge transfer resistance in the
absence and presence of Schiff base inhibitors, respectively.
The electrochemical equivalent circuit model used to verify technical details of
impedance spectra is shown in Fig. 10, and consists of Rs solution resistance, Rct
charge transfer resistance and CPE constant phase elements for the double layer.
The important electrochemical impedance parameters are listed in Table 7. In
Table 7, it is noted that Rct values increased with inhibitor concentration while Cdl
values decreased. The increase in charger transfer resistance values can be attributed
to the adsorption of the Schiff base compounds at the metal–acid interface, which
effectively blocked the movement of charges across the interface, and hence
corrosion inhibition was accomplished. A large charge transfer resistance is
associated with a slower-corroding system, and this fact was well confirmed by the
decrease in Cdl values, which may be attributed to the increase in the thickness of
the protective layer, decrease in local dielectric constant of the film, or the
occurrence of both simultaneously. This may be further explained with the help of
the well-known Helmholtz model, according to the following equation [62]:
123

A. I. Adawy et al.
Fig. 10 Electrochemical
equivalent circuit used to fit the
EIS data for carbon steel in 1 M
HCl
Table 7 Electrochemical impedance parameters for the corrosion of carbon steel in the absence and
presence of different concentrations of Schiff base derivatives at 303 K
Inhibitor
Conc.
(ppm)
Rs (X cm2) Q (l Fcm-2
)
n
Rct (X cm2) Cdl (l Fcm-2
)
gz (%)
1 M HCl
–
5.4
6142
300
120
90
0.82
0.82
0.82
0.78
0.74
0.58
0.82
56
130
509
829
977
990
756
293
147
65
43
34
12
60
49
38
13
11
–
Compound I 100
4.17
2.69
5.27
8.1
57
300
600
900
88.9
93.2
94.3
94.3
92.5
94.9
95.1
95.1
96.0
80
1200
12.1
78
Compound II 100
3.7
9.8
6.1
6.3
9.8
104
84
300
600
900
0.82 1093
0.82 1151
0.62 1149
0.72 1421
66
59
1200
35
eoeA
Cdl
¼
;
ð19Þ
d
where eo is the permittivity of free space (8.854 9 10-14 F cm-1), e is the dielectric
constant of the protective layer, A is the electrode area, and d is the thickness of the
protective layer. Accordingly, the fractional replacement of water molecules by
inhibitor molecules on the carbon steel surface may change these values, also
leading to a decrease in the number of active sites.
In addition, the change in the interface surface condition of the carbon steel
electrode could be effectively deduced from the variation in the value of the phase shift
(n) that can be used as an indicator for prediction of the dissolution mechanism and is
related to the heterogeneity or roughness of the surface [63]. Table 7clearly showsthat
the values of the phase shift (n) did not vary significantly, thus confirming that the
charge transfer controlled the dissolution mechanism of carbon steel in 1 M HCl
without and with inhibitors. Upon addition of Schiff base derivatives in the corrosive
solution, n values (ranging from 0.58 to 0.82) remain constant or decrease compared to
those obtained in inhibitor-free 1 M HCl (0.82). This indicates that there is an increase
in surface inhomogeneity as a result of inhibitor adsorption. Accordingly, ideal
capacitor behaviour was not shown (n \ 1), which may have resulted from the
123

New Schiff base cationic surfactants as corrosion…
irregular surfaces of the electrode or arbitrary distribution of current on the electrode
surface, causing frequency dispersion [64, 65].
Surface morphological studies by SEM
The SEM micrographs of the carbon steel surface are shown in Fig. 11. The surface of
carbon steel after immersion in uninhibited 1 M HCl for 48 h shows an aggressive
attack of the corrosive medium, as illustrated in Fig. 11a. Moreover, the corrosion
products appear very uneven, with cube-shaped morphology, and there is a roughness
on the metal surface layer. On the other hand, as shown in Fig. 11b, in the presence of
the inhibitors individually, much less damage is observed on the carbon steel surface,
with a more compact and pronounced layer, in the presence of compound II. These
results confirm the inhibitory ability with adsorbed film on the carbon steel surface,
which does not exist in Fig. 11a. Accordingly, it may be concluded that the adsorption
on film can efficiently retard the corrosion of carbon steel.
Corrosion inhibition mechanism
The inhibitory action of the synthesized Schiff base cationic surfactants (SBCS) on
the corrosion of carbon steel is dependent on the extent of its adsorption on the
metal surface. The adsorption of SBCS molecules on the metal surface can be
attributed to the presence of loosely bound electrons, lone pairs of electrons, p-
electron clouds and aromatic (e.g., benzene) ring systems. As we know, most
organic inhibitors possess at least one functional group, regarded as the reaction
centre or anchoring group. The strength of adsorption depends on the charge on this
anchoring group [rather on the hetero atom (i.e., atoms other than carbon, including
nitrogen and sulphur) present in the anchoring group]. The structure of the rest of
the molecule influences the charge density on the anchoring group [66].
On the other hand, iron has a coordination affinity for ligands possessing
heteroatoms due to its empty d-orbitals. In general, the potential at which there is no
charge on the metal is known as the zero-charge potential (ZCP) [67, 68]. The
charge on a metal surface in a given medium can be determined from the corrosion
potential (Ecorr) and zero-charge potential. When the difference (Ecorr - ZCP) is
negative, the metal is negatively charged and adsorption of cations is favoured.
When (Ecorr - ZCP) is positive, the metal is positively charged and adsorption of
anions is favoured. The ZCP of iron is -530 mV versus SCE in HCl. In the present
system, the obtained value of Ecorr in 1 M HCl was -605 mV versus SCE. The steel
surface charges negative in 1 M HCl solution because the value of
Ecorr - ZCP = 0 \ 0. Consequently, adsorption occurs as a result of electrostatic
forces between the electric charge on the metal and the ionic charges or dipoles on
the inhibitor molecules. Also, in an acid solution, owing to the presence of neutral
atoms in the (SBCS) molecules, it may become protonated as follows:
xþ
SBCS þ xH $ ½SBCSHxŠ
ð20Þ
123

A. I. Adawy et al.
Fig. 11 SEM micrographs of carbon steel surfaces after immersion for 48 h: a uninhibited 1 M HCl, and
in the presence of b Compound I and c Compound II
123

New Schiff base cationic surfactants as corrosion…
The proposed electrostatic interactions between the protonated inhibitor molecules
and the negatively charged metal surface facilitate adsorption due to the synergy
created between them. Moreover, the adsorption of the protonated molecules can
also occur on the carbon steel surface by direct interaction of the unshared lone pairs
of electrons on the N atom with the vacant d-orbitals of Fe, leading to the formation
of metal inhibitor complexes with Fe2? ions resulting from the anodic dissolution of
steel, according to the following equations:
Fe ! Fe2þ þ 2eÀ
½SBCSHxŠxþþFe2þ $ ½SBCSHxÀFeŠ
ð21Þ
ð22Þ
ðxþ2Þþ
Accordingly, these complexes may adsorb onto the steel surface through the van der
Waals force, enhancing the inhibition of corrosion.
Conclusions
On the basis of the experimental results in the present work, the following
conclusions can be deduced:
1. The synthesized Schiff base compounds were found to be efficient corrosion
inhibitors for carbon steel in 1 M HCl, and their adsorption followed the
Langmuir adsorption model.
2. Polarization studies indicated that these compounds are mixed-type inhibitors
affecting both cathodic hydrogen reduction reactions and anodic metal
dissolution, and that corrosion efficiency increased with inhibitor concentration.
3. The adsorption of these inhibitors involves both physical and chemical
processes based on the values of (DHoads and DGaods).
4. The experimental data of the impedance diagrams (EIS) were fitted with one
equivalent structure model.
5. The formation of a protective inhibitor film on the carbon steel surface after
48 h of immersion in the presence of optimum inhibitor concentration in 1 M
HCl was confirmed by SEM studies.
Acknowledgments The authors are extremely grateful for financial support from the Egyptian
Petroleum Research Institute (EPRI) fund.
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