Synthesis of quaternary, long-chain N-alkyl amides and their corrosion inhibition in acidic media

Article abstract of DOI:10.1007/s11743-014-1566-5

New cationic surfactants were synthesized by the quaternization of a number of straight-chain amide derivatives with triethylamine or pyridine. The corrosion inhibition tests of the surface-active compounds were performed at room temperature for 24 h on carbon steel coupons in acidic media using the gravimetric method. The acidic media used were 1.5 M HCl and 1.5 M H ₂SO₄. Almost all of the synthesized cationic surfactants showed efficient inhibition of corrosion in the test. To establish the inhibition efficiencies of the inhibitors, surface characterization studies (contact angle measurements, SEM analysis and optical profilometer images) of the metal coupons used were performed.

Full text of DOI:10.1007/s11743-014-1566-5

J Surfact Deterg
DOI 10.1007/s11743-014-1566-5
ORIGINAL ARTICLE
Synthesis of Quaternary, Long-Chain N-Alkyl Amides and Their
Corrosion Inhibition in Acidic Media
¨
•
•
•
Serkan Oztu¨rk Ayhan Yıldırım Mehmet C¸ etin
Mustafa Tavaslı
Received: 8 September 2013 / Accepted: 3 January 2014
Ó AOCS 2014
Abstract New cationic surfactants were synthesized by
the quaternization of a number of straight-chain amide
derivatives with triethylamine or pyridine. The corrosion
inhibition tests of the surface-active compounds were
performed at room temperature for 24 h on carbon steel
coupons in acidic media using the gravimetric method. The
acidic media used were 1.5 M HCl and 1.5 M H2SO4.
Almost all of the synthesized cationic surfactants showed
efficient inhibition of corrosion in the test. To establish the
inhibition efficiencies of the inhibitors, surface character-
ization studies (contact angle measurements, SEM analysis
and optical profilometer images) of the metal coupons used
were performed.
and alloys via an electrochemical reaction. This metallic
degradation causes serious economical disadvantages in
several industrial fields throughout the world [2]. In acidic
cleaning methods, the use of organic corrosion inhibitors is
one of the most practical methods for protection against
metallic corrosion. Organic corrosion inhibitors are com-
monly used to decrease corrosion on metallic materials.
Small amounts of these compounds are added into corrosive
electrolytes to form a thin passive film on the surface of the
metal substrate. This passive and protective layer that forms
after adsorption of the inhibitor onto the metal surface, which
blocks the active sites and retards the access of the corrosive
species to the metal and thereby decreases the corrosion rate
[3, 4]. The adsorption of inhibitors takes place through het-
eroatoms, such as nitrogen, oxygen, phosphorus and sulfur,
and p-electrons in triple bonds, conjugated double bonds or
aromatic rings. The inhibition efficiency should increase in
the order O \ N \ S \ P [5].
Keywords Cationic surfactants Á Acidic medium Á
Carbon steel Á Corrosion inhibitor Á Surface studies Á
Scanning electron microscopy
The use of amide-based organic compounds as corrosion
inhibitors is one of the most important uses of these com-
pounds in industry. Some research into the prevention of
corrosion of metals and alloys in acidic media by amide-
based organic compounds that contain long alkyl chains has
been reported [6–13]. In addition, cationic-based surfactants
have become important for use as corrosion inhibitors.
Quaternary ammonium surfactants have been reported to be
excellent corrosion inhibitors for iron and steel in acidic
media [14–30]. Particularly, studies have indicated that
cationic surfactants containing an amide functional group
are important as inhibitor additives in hydrochloric and
sulfuric acids [31–33]. The synergistic effect between
organic cations and halide anions is considered to be an
important factor in the inhibition of metal corrosion [34–37].
In the present study, a new series of organic cationic
surfactants containing long-chain N-alkyl amide functional
Introduction
Acid solutions are widely used in industry. One important
application is to remove undesirable substances and corro-
sion products in metallic materials. Hydrochloric and sul-
furic acids are the most common acids used in these acidic
cleaning methods [1]. Metallic corrosion that can occur
during acidic cleaning processes is the degradation of metals
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-014-1566-5) contains supplementary
material, which is available to authorized users.
¨
S. Oztu¨rk (&) Á A. Yıldırım Á M. C¸ etin Á M. Tavaslı
Department of Chemistry, Faculty of Science and Arts,
˘
Uludag University, 16059 Bursa, Turkey
e-mail: serkanozturk@uludag.edu.tr
123

J Surfact Deterg
groups were synthesized. These compounds were charac-
terized using FT-IR and NMR. Corrosion inhibitory prop-
erties of these compounds were investigated in 1.5 M HCl
and 1.5 M H2SO4 solutions. Sample metals were treated
with an acidic medium for a period of 24 h at room tem-
perature, and the inhibition efficiencies were determined by
the gravimetric method. In addition, contact angle mea-
surements using the sessile drop method, which involves
dropping 1 drop of water onto the metal surface and taking
images of the metal surface using an optical profilometer.
Scanning electron microscopy (SEM) analysis was also
done to support the inhibition results.
in 20 mL of chloroform. Triethylamine (1.2 equivalents)
was added to the solution and the flask was cooled to 0 °C
in an ice bath. From the dropping funnel, 1 equivalent of
the corresponding x-bromoalkanoyl chloride was added
dropwise over 30 min, and the reaction mixture was stirred
overnight at room temperature. After the completion of the
reaction, the reaction mixture was washed with dilute HCl
solution and then with water. The organic phase was dried
over sodium sulfate, and the solvent was removed with a
rotary evaporator. The residue was crystallized from THF/
petroleum ether.
General Procedure for the Synthesis of Alkylamino-N,N,N-
Triethyl-Oxoalkan-1-Ammonium Bromides (2a-l)
Experimental Procedures
To a 100-mL round-bottom flask fitted with a reflux con-
denser (protected by a calcium chloride guard-tube), 1
equivalent of the corresponding x-bromo-N-alkyl-alkana-
mide and 4 equivalents of triethylamine were added with
15 mL of acetonitrile as the solvent, and the reaction
mixture was refluxed overnight. After completion of the
reaction (as observed by TLC), the solvent was removed
under vacuum, and the residue was crystallized using a
mixture of diethyl ether/acetone (2:1).
Materials and Instrumentation
All the reagents and solvents were purchased from either
Merck (Darmstadt, Germany) or Fluka Chemie (Switzer-
land) and used without further purification. Melting points
¨
were recorded using a BUCHI melting point B-540 appa-
ratus. FT-IR spectra were measured using a Thermo
1
Nicolet 6700 FT-IR spectrometer. H-NMR spectra were
measured using a Varian mercury plus spectrometer
(400 MHz) in CDCl3 using TMS as the internal standard.
13C-NMR spectra were measured using a Varian 400 NMR
spectrometer (100 MHz) in CDCl3 using TMS as the
internal standard. The elemental analyses were performed
using an LECO CHNS-932 elemental analyzer.
General Procedure for the Synthesis of 1-Substituted
Alkylamino Oxoalkyl Pyridinium Bromides (3a-l)
To a 100-mL round-bottom flask fitted with a reflux con-
denser (protected by a calcium chloride guard-tube), 1
equivalent of the corresponding x-bromo-N-alkyl-alkana-
mide and 4 equivalents of pyridine were added with 15 mL
of acetonitrile as the solvent, and the reaction mixture was
refluxed overnight. After completion of the reaction (as
observed by TLC), the solvent was removed under vacuum,
and the residue was crystallized using a mixture of diethyl
ether/acetone (2:1).
Synthesis of Compounds
Synthesis of x-Bromoalkanoyl Chlorides
The syntheses of 5-bromopentanoyl chloride, 6-bromo-
hexanoyl chloride and 11-bromoundecanoyl chloride were
performed using the following procedure: one equivalent of
the corresponding bromoalkanoic acid was placed in a
100-mL round-bottom flask fitted with a reflux condenser
(protected by a calcium chloride guard-tube). Five equiva-
lents of SOCl2 were added to the flask, and the reaction
mixture was stirred overnight at room temperature without
solvent. At the end of the reaction, excess SOCl2 was
removed under reduced pressure, and the observed oily
product was used without further purification in the next step.
Corrosion Tests Performed in Acidic Media
Preparations of Coupons and Acidic Solutions
Gravimetric measurements were performed using coupons
made from cold-rolled low-carbon steel and possessed DIN
EN 10130 technical conditions [38] with a composition of
0.07 % C, 0.35 % Mn, 0.015 % P and 0.015 % S. The
coupons were cut into a rectangular shape of 0.1 9
2.2 9 5.0 cm in thickness, width and length, respectively.
The following cleaning procedure was carried out before
the immersion test: first, the coupons were immersed in
15 % HCl, polished lightly with paper tissue and washed
twice with deionized water. They were then immersed in
acetone and finally dried. Solutions of 1.5 M HCl and
General Procedure for the Synthesis of
x-Bromo-N-Alkyl-Alkanamides (1a-l)
In a 100-mL two-necked flask fitted with a reflux con-
denser (protected by a calcium chloride guard-tube), 1
equivalent of the corresponding alkylamine was dissolved
123

J Surfact Deterg
SEM
1.5 M H2SO4 were prepared from concentrated HCl
(37 %) and concentrated H2SO4 (95–97 %) (Merck grade),
respectively.
The carbon steel specimens (of a size of 0.1 cm 9 2.2 cm
9 5.0 cm) were abraded with emery paper and then were
washed with distilled water and acetone. After immersion
in 1.5 M HCl with and without the addition of 25 or
50 ppm of the prepared inhibitors at room temperature for
24 h, the steel specimens were cleaned with distilled water
and dried in a vacuum desiccator, and then the surface was
investigated using a Carl Zeiss EVO 40 (SEM).
Weight Loss Measurements
Inhibition efficiencies of derivatives (2a-l) and (3a-l) were
tested at 25, 50 and 100 ppm in 100 mL 1.5 M HCl and
1.5 M H2SO4 solutions, respectively. The corrosion inhibi-
tors (2a-l) and (3a-h) were directly dissolved in the acid
solutions, while compounds (3i-l) were added to the acid
solutions as solutions in 10 % acetone. Afterwards, the
treatment solutions were poured into 150-mL sealed glass
bottles, and the coupons were suspended in these solutions
without stirring for 24 h at room temperature. Control tests
were performed in the same way without the inhibitors. After
the corrosion test, the coupons were removed and wiped with
paper tissues. Then, they were gently abraded with emery
paper to clean the rust as needed, rinsed with water then
acetone and dried to a constant weight in an oven.
Results and Discussion
The synthesis of the corrosion inhibitors was performed in
three steps. In the first step, the carboxylic acid derivatives
reacted with thionyl chloride to obtain the acid chlorides. In
the second step, the acid chlorides were treated with the
corresponding amines to synthesis the amide derivatives
(1a-l). Finally, the amide derivatives were refluxed with
triethylamine in acetonitrile to obtain the cationic ammo-
nium bromide derivatives (2a-l) (Scheme 1). The cationic
pyridinium bromide derivatives (3a-l) were obtained via the
treatment of the same amide derivatives with pyridine in the
final step (Scheme 2). The characterization data of newly
prepared cationic surfactants are given in Table 1.
Contact Angle Measurements
Contact angles of the metal stripe surfaces were determined
using a KSV Attention Theta (Hamburg, Germany)
instrument. After the inhibition test in acidic medium at
room temperature for 24 h, the metal coupons were kept in
a vacuum desiccator to prevent possible corrosion of the
metal coupons in the air before performing the contact
angle measurements. The contact angles of the surfaces
were measured using the sessile drop method by dropping
1 water drop from a syringe onto the metal stripe surface.
The volume of the water contained by the syringe was 10
lL. Ten separate photos were taken from different parts of
the surfaces, and the contact angle values were measured
for each drop. The measured contact angle values were
obtained as the left contact angle, the angles from the left
contact point of the droplet with the solid and the right
contact angle from the right contact point. In addition, the
average contact angle values were obtained from the
average of two values. The contact angle values for the
surfaces were the average of 10 measurements.
The structures of all of the compounds were confirmed
1
by FT-IR, H-NMR and 13C-NMR spectroscopic methods.
1
The FT-IR, H-NMR and 13C-NMR spectra of compound
2a are given in Fig. 1. In addition, the spectral data of 1a,
2a and 3a compounds are shown in Table 2. The FT-IR
spectra of (1a) showed absorption bands at 3,322 cm-1
,
due to the amide –NH– group, and 1,631 cm-1, due to the
amide C=O group. Similar adsorption bands were obtained
for all of the synthesized compounds. Additionally, the FT-
IR spectra of (3a) showed absorption bands at 3,405 cm-1
,
due to the amide –NH– group; 3,062 cm-1, due to the
pyridine=C–H stretching; and 1,635 cm-1, due to the
amide C=O group. Compounds (3b-l) showed adsorption
bands similar to 3a. The 1H NMR spectrum of (1a) showed
a triplet at d = 5.45 ppm for the HNC=O proton, a triplet
at d = 3.42 ppm for the CH2CH2Br protons, a quartet at
d = 3.23 ppm for the methylene protons of the
CH2CH2NHC=O group, a triplet at d = 2.20 ppm for the
O=CCH2CH2 and a quintet at d = 1.49 ppm for the
methylene protons of the CH2CH2CH2NHC=O group. In
Optical Profilometer Images
To characterize the surface of the metal stripes, a Zeta-20
True Color 3D Optical Profiler (Zeta Instruments, CA,
USA) was used. The metal coupons that were stored in a
vacuum desiccator after the inhibition test in the acidic
medium were mounted on a sample holder under the
objective lens of the Optical Profiler, and 3D photos were
taken from the 1009 magnified surface via the operating
program on the computer.
1
contrast, the H-NMR spectrum of (2a) showed a triplet at
6.50 ppm for the HNC=O proton, a quartet at 3.38 ppm for
the methylene protons of the CH2CH2NHC=O group, a
quartet at 3.14 ppm for the –N?(CH2CH3)3 protons, a
triplet at 2.48 ppm for the O=CCH2CH2 protons and a
triplet at 1.38 ppm for the methyl protons of
–N?(CH2CH3)3 group. The peak attributed to the
123

J Surfact Deterg
Scheme 1 Synthetic pathways
for the preparation of 1a-l and
2a-l
O
N
O
O
NH2
m
l
SOC 2
Br
Br
r
B
l
C
H
O
n
m
n
,
C
n
Δ
Et3N
H l
C 3
H
1a-l
m
n
1a, 2a
2
2
2
2
3
3
3
3
8
8
8
8
8
10
12
14
8
10
12
14
8
Δ
N
,
N
H
C
3C
1b, 2b
1c, 2c
1d, 2d
1e, 2e
1f, 2f
1g, 2g
1h, 2h
1i, 2i
O
N
N
n
m
Br
H
2a-l
10
12
14
1j, 2j
1k, 2k
1l, 2l
(10 %) was added to facilitate solubility of the inhibitors
(3i-l) in acidic solutions and to the reference solutions to
eliminate the solvent factor. The abilities of the synthesized
compounds to inhibit corrosion are represented as the
percent inhibition efficiencies (IE %). The inhibition
properties of the triethylammonium and pyridinium sur-
factants are given in Tables 3 and 4.
O
O
N
Br
N
N
N
n
m
H
H
,
N
Δ
n
H
m
C
3C
Br
1a-l
3a-l
m
8
n
3a
2
2
2
2
3
3
3
3
8
8
8
8
3b
3c
3d
3e
3f
3g
3h
3i
1
0
12
14
8
As seen in Tables 3 and 4, all of the synthesized cationic
surfactants exhibited excellent corrosion inhibition effi-
ciency in 1.5 M HCl solution over 24 h at room tempera-
ture. In contrast, all of the cationic surfactants displayed
lower inhibition efficiency in 1.5 M H2SO4 than in 1.5 M
HCl solution. In particular, compounds 2c, 2d, 2e, 3e and
3i-l showed less inhibition efficiency in 1.5 M H2SO4
solution. The reason for this difference in efficiency
depends on the different adsorption abilities of the chloride
(Cl-) and sulfate (SO42-) anions on the metal surface. In
1
0
12
14
8
1
0
12
3j
3k
3l
14
Scheme 2 Synthetic pathways for the preparation of 3a-l
CH2CH2Br protons at 3.42 ppm found in the 1H-NMR
spectrum of (1a) was not found in that of (2a). This dif-
ference is evidence that the nucleophilic substitution
reaction between 1a and triethylamine was successful. This
the adsorption mechanism of cationic surfactants, the
2-
first adsorb on the metal surface to
anions Cl- and SO4
create dipoles in acidic solutions. Then, the cationic
inhibitor may adsorb on the negatively charged metal
surface through electrostatic interactions. It is well known
that Cl- ions have a stronger tendency to adsorb onto metal
1
observation is also valid for compounds 2b-l. In the H-
NMR spectrum of (3a), the protons of pyridine were
observed as a doublet at 9.55 ppm, as a triplet at 8.46 ppm
and as a triplet at 8.09 ppm. The methylene protons of
CH2CH2N?Py and O=C–CH2 were observed at 5.08 and
2.46 ppm, respectively, both as a triplet peaks. The quartet
peak at 3.19 ppm is attributed to the protons of
CH2CH2NHC=O. The triplet peak at 5.08 ppm attributed
to the methylene protons of CH2CH2N?Py group showed
that the nucleophilic substitution reaction between 1a and
pyridine was successful.
2-
surfaces than SO4 ions [39], and, in addition, the lower
2-
interference of SO4 ions with adsorbed cationic surfac-
tants may lead to a lower adsorption of the cationic
inhibitor onto the metal surface [40]. Therefore, the
adsorption of N,N,N-triethylammonium and pyridinium
surfactants onto the steel surface in HCl solution is stronger
than that in H2SO4 solution, which leads to higher inhibi-
tion performance in HCl than in H2SO4.
The synthesized cationic surfactant molecules have a
positively charged tetraalkyl ammonium or pyridinium
functionality as the hydrophilic head group. The adsorption
of the cationic compounds onto to the dipoles that formed
from the adsorption of anions of the acids to the metal surface
occurred via the electrostatic interaction of this hydrophilic
head groups with the dipole. Moreover, the pyridinium ring
Corrosion Tests in Acidic Media
Corrosion tests in 1.5 M HCl and 1.5 M H2SO4 acidic
media were performed using weight loss measurements. In
the corrosion tests corrosion inhibitors (2a-l) and (3a-h)
were directly dissolved in the acid solutions. Acetone
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J Surfact Deterg
Table 1 Characterization data of newly prepared compounds
Compound
Formula
MW
(g/mol)
wi (calc.)/% - wi (found)/%
Yield (%)
M.p. (oC)
C
H
N
a
2a
2b
2c
2d
2e
2f
C21H45N2OBr
C23H49N2OBr
C25H53N2OBr
421.51
449.56
477.61
505.67
435.53
463.59
491.64
519.70
505.67
533.72
561.78
589.83
399.41
427.47
455.52
483.58
413.44
441.50
469.55
497.60
483.58
511.63
539.69
567.74
59.84–59.75
61.45–61.39
62.87–62.78
64.13–64.08
60.67–60.56
62.18–62.09
63.52–63.45
64.71–64.63
64.13–64.01
65.26–65.10
66.28–66.11
67.20–67.08
60.14–60.05
61.82–61.87
63.28–63.21
64.58–64.50
61.01–60.90
62.57–62.50
63.95–64.01
65.17–65.09
64.58–64.69
65.73–65.83
66.77–66.65
67.70–67.80
10.76–10.79
10.99–11.02
11.19–11.15
11.36–11.33
10.88–10.91
11.09–11.14
11.28–11.24
11.44–11.41
11.36–11.28
11.52–11.55
11.66–11.57
11.79–11.83
8.83–8.81
6.65–6.61
6.23–6.18
5.87–5.90
5.54–5.56
6.43–6.39
6.04–5.98
5.70–5.74
5.39–5.42
5.54–5.58
5.25–5.18
4.99–5.06
4.75–4.67
7.01–7.04
6.55–6.49
6.15–6.18
5.79–5.81
6.78–6.72
6.35–6.37
5.97–6.00
5.63–5.59
5.79–5.70
5.48–5.40
5.19–5.24
4.93–4.97
84
75
75
50
96
65
71
77
73
81
61
82
49
48
81
63
91
45
72
79
85
63
91
83
–
a
–
111–113
122–124
C
C
27H57N2OBr
a
22H47N2OBr
–
C24H51N2OBr
89–90
2g
2h
2i
C26H55N2OBr
C28H59N2OBr
C27H57N2OBr
C29H61N2OBr
C31H65N2OBr
C33H69N2OBr
C20H35N2OBr
124–125
134–135
83–84
2j
78–79
2k
2l
82–83
73–74
a
3a
3b
3c
3d
3e
3f
–
C22H39N2OBr
9.20–9.17
73–74
C24H43N2OBr
C26H47N2OBr
C21H37N2OBr
C23H41N2OBr
C25H45N2OBr
C27H49N2OBr
C26H47N2OBr
C28H51N2OBr
C30H55N2OBr
C32H59N2OBr
9.51–9.48
78–79
9.80–9.82
82–83
9.02–9.06
113–114
118–119
123–124
126–127
84–86
9.36–9.33
3g
3g
3i
9.66–9.60
9.93–9.92
9.80–9.71
3j
10.05–9.96
10.27–10.20
10.47–10.35
94–95
3k
3l
93–95
96–97
a
These compounds were obtained as oily products
has multiple pi-electrons in its aromatic ring, which can also
be adsorbed onto the metal surface through donor–acceptor
interactions between the pi-electrons of the pyridinium ring
and the vacant d-orbitals of Fe [41].
metal surface (hydrophilic or hydrophobic) can be deter-
mined indirectly by the water contact angle (WCA) mea-
surements. The photographs taken from the water contact
angle (WCA) measurements of some cationic surfactants
measured after the corrosion tests in 1.5 M HCl medium
are shown in Fig. 2. The measured contact angle values of
the metal surfaces were given on each photograph. As seen
in Fig. 2, the measured contact angle values of the metal
surfaces containing inhibitors are higher than those for the
metal surface without inhibitors. These results indicated
that cationic surface-active agents increased the hydro-
phobicity of the metal surface, which increased the value of
the measured contact angle for the water droplet. Thus, the
treatment with inhibitor decreased the wettability and
increased the hydrophobic character of the metal surface.
The enhancement of hydrophobicity of the metal surface
was occurred through the long alkyl chains appearing in the
structure of the cationic surfactants.
The hydrophobic portion of the synthesized cationic
surfactants that consists of long alkyl chains also plays an
important role in the corrosion inhibition of the inhibitor
molecules. The length of the chain is very important for
this activity. In general, longer hydrocarbon chains provide
better corrosion protection, but sometimes the solubility of
the inhibitors with longer chains is lower, which leads to a
reduction in their inhibitory activity.
Surface Characterization Studies
Surface characterization studies (water contact angle
measurement, optical profilometer and SEM images) were
performed to support and prove the inhibition results.
Surface characterization studies are important to establish
the prevention of corrosion and to see the effectiveness of
the corrosion inhibitors that formed hydrophobic films on
metal surface via adsorption. The characteristic of the
The 3D optical profilometer images of the metal coupon
surfaces that contained inhibitors 2a, 2b, 2h, 3c and 3e
after testing in 1.5 M HCl are shown in Fig. 3. The photos
were taken of a 1009-magnified surface. In Fig. 3, image a
123

J Surfact Deterg
1
Fig. 1 FT-IR, H-NMR and
13C-NMR spectra of compound
2a
shows that the immersion of the metal coupon in 1.5 M
HCl without any inhibitor led to corrosion. In contrast,
after the immersion of the metal coupons in 1.5 M HCl
with inhibitors, no damages were observed (Fig. 3b–f).
Scanning electron microscope images (Figs. 4, 5) were
also recorded to establish the interaction of the organic
molecules with the metal surface. The SEM image of
Fig. 4a, which was taken from the 3,0009 magnified sur-
face, show the feature of the steel surface after immersion
in 1.5 M H2SO4 at room temperature for 24 h in the
absence of inhibitor. Figure 4b and c, which were also
taken from the 3,0009 magnified surface, show the fea-
tures of the steel surface after immersion in 1.5 M H2SO4
at room temperature for 24 h with 25 and 50 ppm of the
inhibitor 3b. As seen in the SEM images, in the absence of
an inhibitor, the metal surface is covered with more cor-
rosion products than the metal surfaces with the inhibitor.
The deformation of the metal surface treated with 25 ppm
of inhibitor 3b was slightly greater than that of the metal
surface treated with 50 ppm of the inhibitor.
The SEM images of the metal surfaces with and without
50 ppm of inhibitors 2c, 2l, 3b and 3f obtained after testing
in 1.5 M HCl for 24 h are shown in Fig. 5. The photos
were taken from the 3,0009-magnified surface. In the
images shown in Fig. 5, the differences in the unevenness
between the metal surfaces with or without one of the
inhibitors is clearly shown. The coupons that were
immersed in HCl in the presence of an inhibitor have a
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J Surfact Deterg
Table 2 Spectral data of 1a, 2a and 3a compounds
Table 4 Corrosion inhibition efficiencies (IE %) for varying con-
centrations of the pyridinium surfactants in an acidic medium for 24 h
at room temperature
Compound Spectral data
1a
FT-IR, v/cm-1: 3,322 (–NHCO), 2,922 (aliphatic C–H),
1,631 (amide C=O)
1H NMR (400 MHz, CDCl3), d (ppm): 5.45 (t, 1H,
–NHC=O), 3.42 (t, 2H, –CH2Br), 3.23 (q, 2H,
–CH2NHC=O), 2.20 (t, 2H, O=CCH2CH2), 0.88
(t, 3H, –CH3)
Cationic
surfactants
Corrosion inhibition efficiency (IE %)a
25 ppm
50 ppm
100 ppm
1.5 M 1.5 M
HCl
1.5 M 1.5 M
1.5 M 1.5 M
H2SO4
H2SO4 HCl
H2SO4 HCl
13C NMR (100 MHz, CDCl3), d (ppm): 173.20
(–NHCOCH2), 39.60 –(CH2NHCO), 35.38
(–NHCOCH2), 33.25 (–CH2CH2Br), 14.02 (–CH3)
FT-IR, v/cm-1: 3,268 (–NHCO), 2,923 (aliphatic C–H),
1,648 (amide C=O)
1H NMR (400 MHz, CDCl3), d (ppm): 6.50 (t, 1H,
–NHC=O), 3.38 (q, 2H, –CH2NHC=O), 3.14 (q, 6H,
-N?(CH2CH3)3), 2.48 (t, 2H, O=CCH2CH2), 1.38
(t, 9H, -N?(CH2CH3)3), 0.88 (t, 3H, –CH3)
13C NMR (100 MHz, CDCl3), d (ppm): 165.68
(–NHCOCH2), 57.30 (–CH2N?), 53.23 (3C,
N?(CH2CH3)3), 39.67 (–CH2NHCO), 34.74
(–NHCOCH2), 14.09 (–CH3), 7.92 (3C,
N?(CH2CH3)3)
FT-IR, v/cm-1: 3,405 (–NHCO), 3,062 (pyridine = C–
H), 2,922 (aliphatic C–H), 1,635 (amide C=O)
1H NMR (400 MHz, CDCl3), d (ppm): 9.55 (d, 2H, Py–
H), 8.46 (t, 1H, Py–H), 8.09 (t, 2H, Py–H), 6.90 (t, 1H,
–NHC=O), 5.08 (t, 2H, –CH2N?Py), 3.19 (q, 2H,
-CH2NH), 2.46 (t, 2H, O=C–CH2), 0.87 (t, 3H,
–CH3)
13C NMR (100 MHz, CDCl3), d (ppm): 173.11
(–NHCOCH2), 145.53 (4-PyC), 144.83 (2C, 2-PyC),
128.50 (2C, 3-PyC), 61.14 (–CH2Py?), 39.64
(CH2NHCO), 34.97 (–NHCOCH2), 14.07 (–CH3)
3a
3b
3c
3d
3e
3f
96.67
96.37
96.14
78.34
93.21
96.03
95.96
90.10
93.27
92.76
92.98
74.03
10.51
58.28
78.11
73.73
2.34
97.21
96.37
96.09
88.52
95.49
96.22
94.77
91.54
93.36
92.98
93.44
75.04
85.11
93.23
82.28
76.38
13.35
82.97
83.46
65.07
20.61
20.98
15.54
12.28
97.52
96.01
96.36
90.82
96.18
95.96
95.90
91.25
93.81
94.12
93.77
77.19
93.47
93.95
87.60
79.44
33.95
86.59
86.44
69.71
69.44
26.87
22.19
15.83
2a
34.80
80.25
65.17
8.32
3g
3h
3ib
3jb
3kb
3lb
a
15.03
17.24
15.90
3a
Mean of three measured values
b
10 % acetone was added to solubilize the inhibitors in the acidic
medium
smooth surface, while those immersed in HCl without an
inhibitor have a rough surface covered with corrosion
products. This observation indicated that the inhibitor
molecules hinder the corrosion caused by the HCl solution
by forming an adsorbed layer on the steel surface and
thereby avoiding the corrosion damages.
Table 3 Corrosion inhibition efficiencies (IE %) for varying con-
centrations of the N,N,N-triethylammonium surfactants in acidic
medium for 24 h at room temperature
Corrosion Inhibition Mechanism
The corrosion inhibition mechanism can be explained
based on the adsorption of the long chain cationic mole-
cules onto the metal. Inhibitor molecules are adsorbed onto
the dipoles to build a monomolecular layer below and at
the critical micelle concentration. The dipoles are observed
via interaction of chloride or sulfate ions and the positively
charged metal surface. The hydrophilic quaternary
ammonium head group of the surfactant molecules is
adsorbed onto the dipole via electrostatic attraction. On the
other hand the hydrophobic tail of the surfactant molecule
is oriented perpendicularly to the metal surface. Thus
bilayer or multimolecular layers designated as protective
layers are formed which behave as a barrier between the
metal and the corrosive medium [32].
Cationic
surfactants
Corrosion Inhibition Efficiency (IE %)a
25 ppm
50 ppm
100 ppm
1.5 M 1.5 M
HCl
1.5 M 1.5 M
1.5 M 1.5 M
H2SO4
H2SO4 HCl
H2SO4 HCl
2a
2b
2c
2d
2e
2f
2g
2h
2i
96.34
95.73
93.95
90.21
97.03
90.97
92.21
89.44
90.84
93.49
89.44
91.90
88.35
26.61
18.25
20.90
16.34
29.63
80.75
78.62
49.77
84.07
83.09
84.63
96.24
95.83
94.69
92.77
96.63
91.19
91.24
88.54
92.06
93.32
89.16
92.23
97.26
65.86
26.21
29.46
30.42
65.18
84.15
82.19
78.44
87.64
86.03
86.05
96.44
96.14
94.91
93.50
96.95
92.07
91.05
88.31
92.26
93.32
88.06
92.89
98.22
85.57
51.78
40.56
74.24
86.77
85.06
87.08
90.35
88.85
87.89
87.73
As shown in Tables 1 and 2 the inhibition efficiencies of
surfactants tested in 1.5 M HCl were not changed so much
with increasing the inhibitor concentration and the results
were almost the same. As a result of these similar effi-
ciencies at different concentrations, we can confirm that the
2j
2k
2l
a
Mean of three measured values
123

J Surfact Deterg
Fig. 2 Photographs of contact angle measurements of some metal surfaces immersed in a 1.5 M HCl medium for 24 h
Fig. 3 3D optical profilometer
images of the metal coupon
surfaces after testing in 1.5 M
HCl, a without inhibitor, b with
50 ppm concentration of
inhibitor (2a), c with 50 ppm
concentration of inhibitor (2b),
d with 100 ppm concentration
of inhibitor (2h), e with
100 ppm concentration of
inhibitor (3c), f with 100 ppm
concentration of inhibitor (3e)
surfactants have reached the critical micelle concentrations
(CMC) and are in the form of micelles at these concen-
trations. Particularly, for the compounds 2e, 2g, 2h, 2j, 2k,
3f and 3h the highest inhibition values were obtained at the
CMC of the inhibitors. As it can be seen from the evaluated
results, there is no increase in inhibition efficiency above
the CMC. These results also proved us that the cationic
surfactants were in the form of micelles at the metal-HCl
123

J Surfact Deterg
Fig. 4 SEM images of the metal coupon surfaces immersed in 1.5 M H2SO4 for 24 h. a Without inhibitor 3,0009, b 25 ppm (3b) 3,0009,
c 50 ppm (3b) 3,0009
Fig. 5 SEM images of the metal coupon surfaces immersed in 1.5 M HCl for 24 h. a Without inhibitor 3,0009, b 50 ppm (2c) 3,0009,
c 50 ppm (2l) 3,0009, d 50 ppm (3b) 3,0009, e 50 ppm (3f) 3,0009
solution interface. On the other hand, the inhibition effi-
ciencies of compounds tested in 1.5 M H2SO4 increased
with an increase in inhibitor concentration. The possible
non-polar interactions between the long alkyl chains of the
molecules resulting from Van der Waals forces increase
when the concentration of the inhibitor increased. The non-
polar interactions between the long alkyl chains of the
molecules provide a better protective layer against the
corrosive media and this leads to an enhancement in the
inhibitory activity.
Conclusion
It may be concluded that almost all of the synthesized cat-
ionic surface-active compounds exhibited fairly good cor-
rosion inhibitory efficiencies in both HCl and H2SO4 media.
The corrosion prevention capabilities of the cationic sur-
factants were more effective in HCl than in H2SO4. Addi-
tionally, the surface characterization studies (contact angle
measurements, optical profilometer and SEM images) sup-
port the results obtained by the weight loss measurements.
123

J Surfact Deterg
˘
Acknowledgments The authors are grateful to the Uludag Univer-
18. Tam-Chang SW, Biebuyck HA, Whitesides GM, Jeon N, Nuzzo
RG (1995) Self-assembled monolayers on gold generated from
alkanethiols with the structure RNHCOCH2SH. Langmuir
11:4371–4382
sity Scientific Research Projects Unit for providing financial support
under the project no. 2011/34 during this study. This study is a part of
PhD thesis of the first author.
19. Qiu LG, Xie AJ, Shen YH (2005) Understanding the effect of the
spacer length on adsorption of gemini surfactants onto steel
surface in acid medium. Appl Surf Sci 246:1–5
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˘
Ayhan Yıldırım received his Ph.D. in organic chemistry from Uludag
University in 2009. His research field is the synthesis and character-
ization of new organic compounds (oleochemicals, long chain
heterocycles via the 1,3-dipolar cycloaddition reactions) suitable for
industrial applications. He works at the Uludag University chemistry
department as a research and teaching assistant in the organic
chemistry laboratory.
˘
˘
Mehmet C¸ etin is a Professor at the Uludag University Department of
Chemistry. His primary research field is physics and chemistry of
fatty acids and their derivatives. He is also interested in the subject of
corrosion and its prevention with organic corrosion inhibitors.
˘
Mustafa Tavaslı is a Professor at the Uludag University Department
of Chemistry. His research fields are organic and organometallic dyes
for high tech applications (organic light emitting diodes, OLEDs and
organic photovoltaics, OPVs) and investigate their thermal, photo-
physical and electrochemical properties.
¨
Serkan Oztu¨rk is a Ph.D. student at Uludag University. He is
˘
continuing his studies under the supervision of Professor Mustafa
˘
Tavaslı and now works in the Uludag University chemistry depart-
ment as a research and teaching assistant in the organic chemistry
laboratory.
123