Long-chain alkylthia-benzimidazoles as corrosion inhibitors for carbon steel in H 2SO 4 solution

Article abstract of DOI:10.1080/10426507.2012.717136

A series of long-chain N-arylundecanamides containing benzimidazole thioethers (2a-e) were synthesized and characterized by FT IR, ¹H NMR, and elemental analysis. The corrosion inhibitory properties of these compounds on cold rolled low carbon

Full text of DOI:10.1080/10426507.2012.717136

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Long-Chain Alkylthia-Benzimidazoles as
Corrosion Inhibitors for Carbon Steel in
H2SO4 Solution
A. Yıldırım a , S. Öztürk a & M. Çetin a
a Department of Chemistry , Uludağ University , Bursa , Turkey
Accepted author version posted online: 09 Aug 2012.Published
online: 05 Jul 2013.
To cite this article: A. Yıldırım , S. Öztürk & M. Çetin (2013) Long-Chain Alkylthia-Benzimidazoles as
Corrosion Inhibitors for Carbon Steel in H2SO4 Solution, Phosphorus, Sulfur, and Silicon and the Related
Elements, 188:7, 855-863, DOI: 10.1080/10426507.2012.717136
To link to this article: http://dx.doi.org/10.1080/10426507.2012.717136
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Phosphorus, Sulfur, and Silicon, 188:855–863, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.717136
LONG-CHAIN ALKYLTHIA-BENZIMIDAZOLES AS
CORROSION INHIBITORS FOR CARBON STEEL IN H2SO4
SOLUTION
¨
A. Yıldırım, S. Oztu¨rk, and M. C¸ etin
Department of Chemistry, Uludag˘ University, Bursa, Turkey
GRAPHICAL ABSTRACT
Abstract A series of long-chain N-arylundecanamides containing benzimidazole thioethers
(2a–e) were synthesized and characterized by FT IR, 1H NMR, and elemental analysis. The
corrosion inhibitory properties of these compounds on cold rolled low carbon steel metal
coupons were investigated by weight loss measurements in acidic media. Surface characteri-
zation studies of the metal coupons used were performed by contact angle measurements using
the sessile-drop method. In addition, the 3D image of the metal surface was measured using
an optical profilometer. The results obtained from these analyses showed that almost all of the
tested compounds between 25–150-ppm concentrations in acidic media are good to excellent
corrosion inhibitors with the values of percentage inhibition efficiency (η) = 77–99%.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.
Keywords Synthesis; long-chain alkylthia-benzimidazoles; corrosion inhibitors; acidic media
INTRODUCTION
Corrosion prevention of metallic materials is one of the tedious tasks for industrial
chemists.1 Certain heterocyclic compounds named as corrosion inhibitors are widely used
for this purpose.2–6 In general, inhibitors are organic compounds with certain functional
Received 16 May 2012; accepted 15 July 2012.
The authors are grateful to Uludag˘ University Scientific Research Projects Unit for providing financial
support under the project UAP(F)-2011/34 during this study.
Address correspondence to A. Yıldırım, Department of Chemistry, Uludag˘ University, Bursa, Turkey.
E-mail: yildirim@uludag.edu.tr
855

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A. YILDIRIM ET AL.
groups or heterocyclic moieties. These compounds are adsorbed on the metal surface to
form a protective layer against the corrosive medium. In the corrosion literature, there
are many papers related to inhibitors containing the benzimidazole ring.7–14 Compounds,
including this heterocyclic system, are suitable corrosion inhibitors for various metals in
different aggressive mediums. (1H-Benzo[d]imidazol-2-yl)methanethiol, also known as 2-
mercaptomethyl benzimidazole, has an aromatic π electrons cloud, five-membered diazole
ring, and thiol group that play important roles in corrosion inhibition.
In order to use organic compounds as corrosion inhibitors, some points must be con-
sidered. For instance, these compounds should be easily prepared and must be chemically
stable under the conditions of use. In addition, maximum protection should be obtained
at low amounts of corrosion inhibitors and these compounds should not be harmful to the
environment.1 Usually fatty acids are easily biodegradable compounds in nature. Therefore,
fatty acids and their derivatives are widely used as anti-corrosion agents.15
In this study, as a continuation of our work related to the development of high-
performance corrosion inhibitors, we have synthesized a few novel long-chain sulfur-
containing benzimidazoles. The inhibitory properties of the synthesized compounds were
investigated against the corrosion of cold-rolled, low-carbon steel in 1.5-M H2SO4 by
weight loss measurements, and further surface characterization studies were carried out.
Good to excellent results were achieved with low concentrations of inhibitors.
RESULTS AND DISCUSSION
11-Bromo-N-arylundecanamides (1a–e) were prepared as described in the litera-
ture.16 The synthetic pathway to the preparation of the benzimidazole derivatives (2a–e)
is shown in Scheme 1. (1H-Benzo[d]imidazol-2-yl)methanethiol and related bromo unde-
canamides (1a–e) were refluxed in acetone under nitrogen atmosphere along with potassium
carbonate and the catalytic amount of sodium iodide. After crystallization from suitable
solvents, compounds (2a-e) were obtained in high yields (Table 1).
Scheme 1 Synthesis of benzimidazole derivatives 2a–e.
The Fourier transform infrared (FT IR) and Proton Nuclear Magnetic Resonance
Spectroscopy (1H NMR) spectra confirm the structures of the synthesized compounds
(Table 2). The IR spectrum of (2a) shows the characteristic bands for NH stretching of
amide at 3293 cm−1, aromatic C H stretching at 3053 cm−1, and carbonyl stretching
1
(C O) frequency of amide at 1659 cm−1. The H NMR spectrum showed a singlet at
δ7.54 for the amide NH proton, signals for the three different methylene protons were

857

858
A. YILDIRIM ET AL.
Table 2 Spectral data of newly prepared compounds
Compound
Spectral data
2a
IR, ν˜/cm−1: 3293 ( NHCO), 3053 (aromatic
C
H), 1659 (amide C O)
1H NMR (CDCl3), δ: 7.54 (s, 1H, NHC O), 7.52 (s, br, 2H, ArH), 7.38-7.37 (m, 2H, ArH),
7.34-7.30 (m, 2H, ArH), 7.28-7.24 (m, 2H, ArH), 7.10 (t, J = 7.2 Hz, 1H, ArH), 3.98 (s, 2H,
CH2SCH2CN2), 2.48 (t, J = 7.6 Hz, 2H, CH2SCH2CN2), 2.37 (t, J = 7.6 Hz, 2H,
CH2CONH), 1.72 (quin, J = 8.0 Hz, 4H, 2CH2), 1.53 (quin, J = 7.6 Hz, 2H, CH2),
1.35-1.20 (m, 10H, 5CH2)
2b
2c
2d
2e
IR, ν˜/cm−1: 3368 ( NHCO), 3088 (aromatic
C H), 1678 (amide C O)
1H NMR (CDCl3), δ: 8.44 (t, J = 2.4 Hz, 1H, ArH), 8.01-7.99 (m, 1H, ArH), 7.95-7.93 (m,
1H, ArH), 7.57 (s, 1H, NHC O), 7.49-7.37 (m, 1H, ArH), 7.28-7.24 (m, 4H, ArH), 4.00 (s,
2H, CH2SCH2CN2), 2.46 (quin, J = 8.4 Hz, 4H, 2CH2), 1.74 (quin, J = 7.6 Hz, 4H, 2CH2),
1.52 (quin, J = 7.6 Hz, 2H, CH2-), 1.35-1.16 (m, 10H, 5CH2)
IR, ν˜/cm−1: 3288 ( NHCO), 3055 (aromatic
C H), 1658 (amide C O)
1H NMR (CDCl3), δ: 7.57 (s, br, 1H, NHC O), 7.45-7.37(m, 3H, ArH), 7.27-7.23 (m, 3H,
ArH), 6.87-6.83 (m, 2H, ArH), 3.97 (s, 2H, CH2SCH2CN2), 3.78 (s, 3H, -OCH3), 2.48 (t, J
= 7.6 Hz, 2H, CH2SCH2CN2), 2.35 (t, J = 7.6 Hz, 2H, CH2CONH), 1.72 (quin, J =
8.0 Hz, 2H, CH2), 1.52 (quin, J = 7.6 Hz, 2H, CH2), 1.36-1.19 (m, 12H, 6CH2)
IR, ν˜/cm−1: 3305 ( NHCO), 3052 (aromatic
C H), 1660 (amide C O)
1H NMR (CDCl3), δ: 7.41 (s, br, 1H, NHC O), 7.39-7.37 (m, 2H, ArH), 7.27-7.24 (m, 4H,
ArH), 7.12 (d, J = 8.4 Hz, 2H, ArH), 3.97 (s, 2H, CH2SCH2CN2), 2.48 (t, J = 7.6 Hz, 2H,
CH2SCH2CN2), 2.35 (t, J = 7.6 Hz, 2H, CH2CONH), 2.31 (s, 3H, CH3), 1.72 (quin,
J = 8.0 Hz, 2H, CH2), 1.53 (quin, J = 7.6 Hz, 2H, CH2), 1.35-1.20 (m, 12H, 6CH2)
IR, ν˜/cm−1: 3291 ( NHCO), 3062 (aromatic
C H), 1641 (amide C O)
1H NMR (CDCl3), δ: 7.57 (s, br, 1H, NHC O), 7.37-7.32 (m, 4H, ArH), 7.30-7.23 (m, 5H,
ArH), 4.46 (d, J = 5.6 Hz, 2H, PhCH2NH-), 3.97 (s, 2H, CH2SCH2CN2), 2.49 (t, J =
7.6 Hz, 2H, CH2SCH2CN2), 2.22 (t, J = 7.6 Hz, 2H, CH2CONH), 1.65 (quin, J = 8.0 Hz,
2H, CH2), 1.53 (quin, J = 7.6 Hz, 2H, CH2), 1.30-1.19 (m, 12H, 6CH2)
obtained as a singlet at δ3.98 for the ( CH2SCH2-benzimidazole), as a triplet at δ2.48 for
the ( CH2SCH2-benzimidazole), and as a triplet at δ 2.37 for the (–CH2CONHAr) protons
respectively.
The corrosion inhibition capabilities of the prepared compounds tested in 1.5-M
H2SO4 acidic medium are given as percentage inhibition efficiencies, η. The observed
results for (2a–e) inhibitors are given in Table 3. Percentage inhibition efficiencies were
calculated using the following equation:
η = [(Wo − W)/Wo] × 100,
Table 3 Corrosion inhibition efficiencies (η) for varying concentrations of tested benzimidazoles in 1.5 M H2SO4
for 5 h at room temperature (23 ◦C)∗
˙
Inhibition efficiencies (η)
Benzimidazoles
25 ppm
50 ppm 100 ppm
150 ppm
2a
2b
2c
2d
2e
85
77
85
90
86
92
78
91
90
92
98
94
98
97
98
99
96
99
98
99
∗Duplicate experiments were performed in each case and the mean value of the weight loss is reported.

ALKYLTHIA-BENZIMIDAZOLES AS CORROSION INHIBITORS
859
Figure 1 Possible orientation of inhibitor molecules 2a between the metal–acidic solution interfaces.
where η is the percentage inhibition efficiency, Wo is the weight loss of the coupon (metal
plate) in the absence of inhibitor, and W is the weight loss of the coupon in the same
environment in the presence of an inhibitor compound.17
As shown in Table 3, the tested compounds showed very good inhibitory activity.
Increase in inhibition efficiencies is parallel to the increase in concentration, which indi-
cates that these compounds are adsorption inhibitors. In corrosive media, compounds are
adsorbed on the metal surface through N, S, and O atoms and prevent corrosion.17 Pos-
sible orientation of inhibitor molecules (2a) between the metal–acidic solution interfaces
is shown in Figure 1. As can be seen, molecules are found in an upright position in the
metal surface. Such a position provides maximum protection. In addition, the following
statements can be made which are supported with Figure 2: Part of the molecules (2a)
Figure 2 Formation of secondary molecular layer.

860
A. YILDIRIM ET AL.
Figure 3 Interactions at low inhibitor concentrations.
adsorbed on the metal surface via S atom and benzimidazole ring, other molecules are
connecting on the metal surface still with S atom and π–π interactions between aromatic
rings also takes place. In addition, intermolecular H-bonds are formed via amide groups,
which increase the stability of the protective layer. Increase in inhibition activities due
to increase in inhibitor concentrations may indicate that a secondary molecular layer can
occur (bilayer) via H-bonds and Van der Waals forces.18–20
At low inhibitor concentrations, probably both ends of the molecule interact with
the metal surface to provide protection as shown in Figure 3. The horizontal orien-
tation of inhibitor molecules on the surface of the metal prevents the growth of the
secondary molecular layer. In addition, the presence of electron withdrawing –NO2
group in inhibitor (2b) decreases its effectiveness, which proves that the interaction via
amide end of the molecule is also possible. As a result, a slightly lower inhibition was
observed.
Typically, the chemical composition of the metal surface changes after the adsorption
of inhibitor molecules, which also changes the surface wettability. The characteristics of
the metal surface (hydrophilic or hydrophobic) can be determined indirectly by the water
contact angle (WCA) measurements. Static water contact angle measurements observed at
150-ppm inhibitor concentration are given in Table 4. The WCA measurements support η
values, which are calculated from the observed weight loss measurements given in Table 3.
Similar WCA and η values were observed for 2a, 2c, 2d, and 2e compounds. As can be
seen from the tables, the lowest η and WCA values were obtained for the inhibitor (2b).
These two values compliant with each other, supports the mechanism of inhibition that is
recommended above for this compound.
The 3D optical profilometer images of some metal coupon surfaces are given in
Figure 4. The photos were taken from the surface at 100× magnification. As seen from
images in Figure 4, the treatment of the metal coupon to acidic medium without inhibitor
leads to corrosion damages on the metal surface, while the treatment of the metal coupon
with inhibitor prevents the corrosion of the surface.
Table 4 Static water contact angle measurements on the metal surface that immersed in 1.5 M 100 mL H2SO4
solution containing one of the inhibitors 2a, 2b, 2c, 2d or 2e at 150 ppm concentration
Measured contact angles (Average values)
Inhibitors
Blank
2a
2b
2c
2d
2e
43.70 0.19 69.78 2.17 59.26 0.18 67.25 1.21 65.78 2.58
68.80 3.99

ALKYLTHIA-BENZIMIDAZOLES AS CORROSION INHIBITORS
861
Figure 4 3D optical profilometer images of the metal coupon surface.
CONCLUSIONS
The results of corrosion tests are quite satisfactory. The synthesized long-chain
alkylthia-benzimidazole derivatives exhibited the highest inhibition at 150-ppm inhibitor
concentration. In parallel with increase in inhibitor concentration, the observed adsorption
layer provides better protection. The results obtained from gravimetric tests and other mea-
surements indicate that these compounds are suitable anti-corrosion agents for carbon steel
in 1.5-M H2SO4.
EXPERIMENTAL
All reagents and solvents were purchased from either Merck or Fluka Chemie, and
¨
were used without further purification. Melting points were recorded by BUCHI melting
point B-540 apparatus. IR spectra were measured by Nicolet FT IR 6700 spectrometer.
1H NMR spectra were measured using Varian mercury plus spectrometer (400 MHz) in
deuterochloroform (CDCl3) using tetramethylsilane (TMS) as an internal standard.
(1H-Benzo[d]imidazol-2-yl)methanethiol was prepared according to the procedure
reported in the literature21 but with some modifications. A solution of o-phenylenediamine
(9.45 g, 87.4 mmol) and thioglycolic acid (19.8 g, 215 mmol) in 4-M HCl (100 mL) was
refluxed under nitrogen for 1.5–2 h. The green reaction mixture was then cooled to 0 ◦C in
an ice bath, maintaining the temperature of the mixture below 5 ◦C; the pH of the reaction
mixture was adjusted to 7.0 by slow addition of concentrated ammonia solution. After
precipitation from the solution was completed, the grey-white solid was collected, typically
by vacuum filtration and dried at room temperature overnight. The crude product was
crystallized from 50% EtOH with cautiously heating under nitrogen. The pink-grey needle
crystals were collected by vacuum filtration and dried in vacuum desiccator over CaCl2,
(8.5 g, 59%), mp 156–157 ◦C. This compound was used without further characterization.

862
A. YILDIRIM ET AL.
General Procedure for the Preparation Benzimidazole Derivatives (2a–e)
In a 100-mL round bottom flask fitted with a reflux condenser, acetone (30 mL),
(1H-benzo[d]imidazol-2-yl)methanethiol (1 g, 6.09 mmol), K2CO3 (1 g, 7.24 mmol),
and NaI (0.1 g, 0.67 mmol) were placed. The resulting mixture was heated in a wa-
ter bath for 10–15 min under nitrogen. Thereafter 11-bromo-N-benzyl or 11-bromo-N-
arylundecanamide (6.17 mmol) was added and flask was heated to reflux in a water bath
and refluxed for 5–6 h under nitrogen. After the amide was consumed (checked by TLC),
the acetone was removed by a rotary evaporator under reduced pressure and the residue
was dissolved in CHCl3. The chloroform solution was washed with water, dried with an-
hydrous Na2SO4, and concentrated under vacuum. The observed residue was purified by
crystallization or column chromatography.
Corrosion Tests Performed in 1.5-M H2SO4
Coupons were prepared according to the method previously described in the litera-
ture.17 The coupons are made from cold rolled low-carbon steel with the composition of
0.07% C, 0.35% Mn, 0.015% P, and 0.015% S, and were cut in rectangular shapes of 0.1 ×
2.2 × 5.0 cm in thickness, width, and length respectively. A solution of 1.5-M H2SO4 was
prepared from concentrated H2SO4 (98%) (Merck grade). Mass loss measurements were
performed as follows. Inhibition efficiencies of (2a–e) benzimidazoles were tested at 25,
50, 100, and 150 ppm concentration in 100 mL of 1.5-M H2SO4 solution. The synthesized
inhibitors were added into the acidic solution as dissolved in 10% acetone. However, some
solutions are not clear. Later the treatment solutions were poured into 150 mL of sealed
glass bottles and the coupons were suspended into these solutions without stirring and
kept for 5 h at room temperature (23◦C). Control tests were done in the same way without
inhibitors. After the corrosion test, the samples were treated as follows: coupons were taken
out and wiped with paper tissues. The coupons were gently polished with emery paper to
clean the rust as needed, rinsed with water then acetone, and dried in an oven to a constant
weight.
Contact Angle Measurements
The contact angles of the metal coupon surfaces were determined with the KSV
Attention Tetha (Hamburg, Germany) instrument. The contact angle of the surfaces was
measured with the Sessile Drop method by dropping one water drop. Ten separate photos
were taken from different parts of surfaces, and contact angle values were measured for
each drop. Measured contact angle values were obtained as the left contact angle, the angles
from the left contact point of the droplet with solid and right contact angle from the right
contact point. In addition, average contact angle values were obtained from the average of
two values. Contact angle values for the surfaces were the average of 10 measurements.
Optical Profilometer Images
In order to characterize the surface of metal coupons, Zeta-20 True Color 3D Optical
Profiler (Zeta Instruments, CA, USA) was used. Sample was mounted on sample holder
under the objective and the photos were taken from the surface at 100× magnification.

ALKYLTHIA-BENZIMIDAZOLES AS CORROSION INHIBITORS
863
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