A new green ionic liquid-based corrosion inhibitor for steel in acidic environments

Article abstract of DOI:10.3390/molecules200611131

This work examines the use of new hydrophobic ionic liquid derivatives, namely octadecylammonium tosylate (ODA-TS) and oleylammonium tosylate (OA-TS) for corrosion protection of steel in 1 M hydrochloric acid solution. Their chemical structures were deter

Full text of DOI:10.3390/molecules200611131

Molecules 2015, 20, 11131-11153; doi:10.3390/molecules200611131
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
A New Green Ionic Liquid-Based Corrosion Inhibitor for Steel
in Acidic Environments
Ayman M. Atta 1,2, Gamal A. El-Mahdy 1,3,*, Hamad A. Al-Lohedan 1
and Abdel Rahman O. Ezzat 1
1
Surfactants Research Chair, Department of Chemistry, College of Science, King Saud University,
P.O. Box 2455, Riyadh 11451, Saudi Arabia; E-Mails: aatta@ksu.edu.sa (A.M.A.);
hlohedan@ksu.edu.sa (H.A.A.-L.); ao_ezzat@yahoo.com (A.R.O.E.)
2
Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo 11727, Egypt
3
Department of Chemistry, Faculty of Science, Helwan University, Helwan 11795, Egypt
* Author to whom correspondence should be addressed; E-Mail: Gamalmah2000@yahoo.com;
Tel.: +96-611-467-5998; Fax: +96-611-467-5992.
Academic Editor: Jean Jacques Vanden Eynde
Received: 16 April 2015 / Accepted: 9 June 2015 / Published: 17 June 2015
Abstract: This work examines the use of new hydrophobic ionic liquid derivatives,
namely octadecylammonium tosylate (ODA-TS) and oleylammonium tosylate (OA-TS) for
corrosion protection of steel in 1 M hydrochloric acid solution. Their chemical structures
were determined from NMR analyses. The surface activity characteristics of the prepared
ODA-TS and OA-TS were evaluated from conductance, surface tension and contact angle
measurements. The data indicate the presence of a double bond in the chemical structure
of OA-TS modified its surface activity parameters. Potentiodynamic polarization,
electrochemical impedance spectroscopy (EIS) measurements, scanning electron microscope
(SEM), Energy dispersive X-rays (EDX) analysis and contact angle measurements
were utilized to investigate the corrosion protection performance of ODA-TS and OA-TS
on steel in acidic solution. The OA-TS and ODA-TS compounds showed good protection
performance in acidic chloride solution due to formation of an inhibitive film on the
steel surface.
Keywords: steel; aggregation; contact angle; hydrophobic ionic liquids; potentiodynamic
test; EIS

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1. Introduction
Carbon steel is a material commonly used for the production and transportation of crude oil in the
oil industry and natural gas due to its excellent mechanical properties [1–4]. Several problems occur
during transportation of crude oil in the pipelines, where the migrating ions come into contact with
metal due to the breakdown of the oil-aqueous emulsion, which stimulates the corrosion process [5]
Furthermore, the corrosion is enhanced by the presence of trace water and salts in the oil and the acidic
media which are used in descaling and oil well acidification [6,7]. In a strong acid medium, the corrosion
processes produce structural damage to the steel. There are several types of corrosion inhibitors which
are widely used to control the corrosion problem of low carbon steel upon exposure to acidic solutions,
which vary from organic macromolecules to nanocomposites [8–15]. These compounds are adsorbed
onto the metallic surface, blocking the active corrosion sites. The applicability of these materials as
corrosion inhibitors for metals in acidic media has been recognized for a long time. However, most of
these materials are heavily toxic and environmentally hazardous [16], therefore, attempts have been
carried out to search for eco-friendly treatment materials for metals in acid solutions. Recently, ionic
liquid (IL)-based products have been developed to solve this problem [17]. ILs are organic salts with
negligible vapor pressure that have a melting point below 100 °C, which makes them less hazardous
inhibitors and eco-friendlier metal corrosion inhibitors [17]. Moreover, they have a large number of
advantageous physicochemical properties such as non-flammability and high ionic conductivity, as well
as excellent thermal and chemical stability [18–20]. Ionic liquids (ILs) are organic salts containing both
organic and inorganic components with various functional groups. Most of these salts are based on
imidazolium and pyridinium species as cations, while typical anions are sulfonium, phosphonium,
Al2Cl7, tetrafluoroborate, and bis(trifluoromethane-sulfonyl)imide [17]. ILs are generally considered
to be efficient corrosion inhibitors for various metals and alloys due to their high activity in acidic
media [21–25]. There are several works reporting the quaternization of alkyl amines with different
organic cations to produce quaternary ammonium organic salts like ILs [25–29]. The aim of this
work was to prepare ionic liquids to protect steel from corrosion in an acidic environment by reacting
octadecyl or oleylamine with p-toluenesulfonic acid. To our knowledge no information is available about
the corrosion inhibition characteristics of these materials, so electrochemical and surface characteristic
techniques were used to investigate the inhibition effects of such materials in 1 M HCl aqueous solution.
2. Results and Discussion
2.1. Preparation of Octadecylammonium Tosylate (ODA-TS) and Oleylammonium Tosylate (OA-TS)
The preparation of ILs from ODA or OA can be performed by the interaction of an amino group
free electron pair with the H+ of PTSA with the formation of a new quaternized amine bond. This type
of reaction proceeds effectively in the absence of solvent. The reaction scheme was presented in
Scheme 1.

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+
RNH
-
+
CH3
RNH2
3O3S
SO3H
H3C
PTSA
R = C18H37 ODA
R= CH3(CH2)7CH=CH(CH2)8- OA
Scheme 1. Reaction scheme of OA-TS and ODA-TS.
1
The molecular structures of the compounds were characterized by NMR. The H-NMR and
1
13C-NMR spectra of ODA-TS and OA-TS are represented in Figures 1 and 2. In the H-NMR spectra
(Figure 1) the peaks appearing at chemical shifts (δ) of 7.709 ppm (doublet, 2H, J = 8.1 Hz), and
7.24 ppm (doublet of doublets, 2H, J = 8.4, 0.6 Hz, HW) represent the phenyl protons, indicating the
incorporation of PTSA as anion in the chemical structures of ODA-TS and OA-TS. Moreover the
appearance of peaks at 2.892 (triplet, 2H, J = 6.9 Hz) and 2.374 ppm (singlet, 3H) represent the
+
formation of CH2-N and NH3 moieties, respectively, confirming the formation of the cation part of
ODA-TS and OA-TS. The peaks at 1.628 ppm (multiplet, 2H, J = 7.2 Hz), 1.290 ppm (triplet, 30H,
J = 6.6 Hz), and 0.902 ppm (triplet, 3H, J = 6.6 Hz) represent the (CH2)16 and CH3 groups,
respectively, confirming the presence of the alkyl chains of ODA and OA without degradation. New
peaks at 5.1 ppm in the spectrum of OA-TS (Figure 1a) which represent –CH=CH- protons conform
13
the presence of unoxidized vinyl groups. The C-NMR spectra (Figure 2) showed the appearance of
peaks at δ = 143.63, 141.86, 129.99 and 127.09 ppm which confirm the presence of phenyl and C-N
carbons, respectively. The new peak at 132.23 ppm (Figure 2a) confirms the presence of –CH=CH- in
the chemical structure of unoxidized OA-TS. The peaks at 40.92, 33.23, 30.95, 30.69, 30.39, 28.72,
27.60, 23.89, 21.49 and 14.61 ppm in the spectra of both OA-TS and ODA-TS indicate the presence of
CH2 and CH3 carbons that are not fragmented during the synthesis procedure.
Figure 1. 1H-NMR spectra of (a) OA-TS and (b) ODA-TS.

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Figure 2. 13C-NMR spectra of (a) OA-TS and (b) ODA-TS.
2.2. Surface Properties of OA-TS and ODA-TS
The solubility of OA-TS and ODA-TS in water depends on the temperature of the solution. It was
observed that at the concentrations of OA-TS and ODA-TS below 1.3 mmol·dm−3 they are completely
soluble in water at room temperature. However, at above these concentrations the OA-TS and ODA-TS
are initially insoluble, and there is often a temperature at which the solubility suddenly increases very
dramatically. Accordingly, it is necessary to determine the Krafft temperature, TK, at which the solubility
of the OA-TS and ODA-TS increased. The TK was determined from conductivity measurements as
described in the Experimental section. The relation between conductivity of OA-TS or ODA-TS and
the solubility temperature is represented in Figure 3.
Figure 3. Relation between conductance of ODA-TS and OA-TS and solubility temperature
at concentration of 1 mmol·dm−3.

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The values of TK for OA-TS and ODA-TS without any salts were 33 and 38 °C, which indicated
that OA-TS is more soluble than ODA-TS in water. The solubility of OA-TS and ODA-TS in water is
limited below TK due to the hydrophobicity of ODA and OA. Micelles or aggregations of ODA-TS and
OA-TS begin to form at TK. The aggregation and adsorption parameters of liquid crystalline species can
be determined from the relation between dynamic surface tension (γ; mN·m−1) and liquid crystalline
material concentrations (lnc, mol·L−1). The relation between the γ and −lnc of microgels at 25 °C is
illustrated in Figure 4. The relation between the γ and ageing time at different microgel concentrations
were selected as representative and plotted in Figure 5.
Figure 4. Adsorption isotherms of OA-TS and ODA-TS at temperature of 25 °C.
Figure 5. Relation between surface tension and ageing time of different aqueous solutions
of OA-TS at 25 °C.
The optical microscope photos of OA-TS and ODA-TS at different concentrations ae presented
in Figure 6. The surface tension data were listed in Table 1. The time dependent surface tension
measurement relationship (Figure 6) reveals that OA-TS and ODA-TS are adsorbed very fast at the
air/water interface at high concentration, reaching surface tension equilibrium in only a few seconds.
The time required to equilibrate the γ measurements increases with dilution of OA-TS and ODA-TS.
The micellization and adsorption of OA-TS and ODA-TS are based on the critical micelle concentrations
(cmc; mol·L−1), which were determined from the surface balance method as presented in Figure 4. The
cmc values of the prepared OA-TS and ODA-TS were measured in water at 25 °C and are listed in
Table 1. The corresponding surface tension at cmc defined as γcmc was determined and also listed in

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Table 1. Data of surface tension measurements, Table 1 and Figures 4 and 5, indicate that the prepared
OA-TS and ODA-TS reduced the surface tension of water from 72.2 to 25.1 mN·m−1. The appearance
of one adsorption curve (Figure 4) indicates the purity of OA-TS and ODA-TS. This means that the
prepared OA-TS and ODA-TS behave like an amphiphile when solubilized in water and they are pure
compounds. Careful inspection of the data indicated that the cmc values were reduced with the
saturation of the alkyl group (ODA) which indicated that the hydrophobic moiety of ILs controls their
aggregation in water. Moreover the presence of an isolated vinyl group in the alkyl hydrophobic chain
(OA-TS) reduces γcmc more than ODA-TS which aggregated at a lower cmc than OA-TS. This means
that the presence of a vinyl group in the structure of OA-TS increases the solubility of the surfactant in
water [30]. The aggregation and micellization of OA-TS and ODA-TS can be observed by optical
microscope (Figure 6). It was observed that OA-TS and ODA-TS formed aggregates above the cmc.
A complete understanding of the aqueous behavior of OA-TS and ODA-TS requires knowledge of the
entire self-assembly spectrum. It is well established that surfactant molecules have different micelle or
aggregate shapes in the aqueous solutions (formation of micelles, liquid crystal phases, bilayers or
vesicles, etc.). References [31,32] described the importance and existence of liquid crystalline phases.
In the present work, OA-TS and ODA-TS form lyotropic liquid crystal structures at concentrations above
the cmc as presented in Figure 6.
Figure 6. Optical microscope photographs of aqueous solutions of (a) ODA-TS
(1 × 10−4 mmol/L); (b) ODA-TS (3 × 10−4 mmol/L); (c) OA-TS (1 × 10−4 mmol/L) and
(d) OA-TS (3 × 10−4 mmol/L).
Table 1. Surface properties of the ILs at the air/water interface.
−ΔG kJ·mol−1
Amin
nm2/molecule
Compounds Cac mol/L γcac mN/m (−∂ γ/∂ lnc) Γmax × 10 10 mol/cm2
ΔGagg
20.39
18.73
ΔGads
21.55
19.58
ODA-TS
OA-TS
0.0002
0.0004
31.3
25.1
8.41
3.51
5.50
0.047
0.030
13.14

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The optical microscopy photos (Figure 6) indicated that the ODA-TS formed nematic phase micelles
that changed to rod-like structures with increasing ODA-TS concentration (Figure 6a,b). The OA-TS
(Figure 6c,d) formed micelles having hexagonal and cubic forms as the OA-TS concentration increased.
The presence of double bond at the center of hydrophobic part of OA-TS oriented the micelles from an
elongate micelle arrangement into hexagonal arrays with increasing OA-TS concentration.
The magnitude of the hydrophobic effect and interactions can be estimated in terms of the standard free
energy of transfer of the hydrophobic chain from bulk hydrocarbon to water. The aggregation processes
of surfactants depend on the thermodynamic parameters of aggregation (enthalpy ΔHmic, entropy ΔSmic
and free energy ΔGmic). The values of ΔGmic in the water can be estimated [33] as ΔGmic = RT ln cmc.
The ΔGmic was calculated and is listed in Table 1. The low value (below zero) of ΔGmic indicated that
the micellization process occurred spontaneously. Moreover, it was observed that the ΔGmic values are
less negative with the increasing hydrophobicity of ODA-TS by saturation of the OA-TS double bond,
which indicated the increment of molecule aggregation with increase of hydrophobicity. This can be
explained on the basis of steric bulk of the structure that leads to steric inhibition of aggregation [32].
The adsorption of OA-TS and ODA-TS at the air/water interface is the alternative mechanism for
aggregation or micellization processes which illustrate the surface activities of molecules in water and
interface. The concentration of OA-TS and ODA-TS at the air/water interface was measured from
surface excess concentration Гmax. It was calculated from Equation (1):
Гmax = 1/RT × (−∂γ/∂ lnc)T
(1)
where (−∂γ/∂ lnc) is the slope of the straight lines of γ vs. lnc at T (constant temperature) and R gas
T
constant (in J·mol−1·K−1). The Гmax values were used to calculate the minimum area Amin at the air/water
interface. From the surface excess concentration, the area per molecule at the interface is calculated
using the equation:
Amin = 1016/N Гmax
(2)
where N is Avogadro’s number. The data of Гmax, Amin, and (−∂γ/∂ lnc) of OA-TS and ODA-TS were
determined and are listed in Table 1. It was reported that the interaction between amphiphilic materials
at the interface is very important for stabilization of particles at interfaces [34] The data listed in
Table 1 indicates that the increment of Гmax causes an increment of adsorbed molecules at the interface
between air and water, which also means a lower surface tension. The lowest value of Amin obtained is
0.030 nm2/molecule, suggesting adsorption of OA-TS which is oriented away from the liquid in a more
tilted position. However, a low Amin data suggest complete surface coverage with the formation of
flexible OA-TS chains at interface. The free energy of the microgel adsorption at air/water interface
(ΔGad) can be calculated from Equation (3):
ΔGad = RT ln cmc − 0.6023 πcmc Amin
(3)
where πcmc = surface tension of water (72.1 mN·m−1) − γcmc. The ΔGad values were calculated and
are listed in Table 1. All ΔGad values are more negative than ΔGmic, indicating that the adsorption of
microgels at the air/water interface is associated with a decrease in the free energy of the system.
This may be attributed to the effect of steric factors on inhibition of aggregation more than its effect on
the adsorption process.

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2.3. Contact Angle and Surface Free Energy Measurements
One of the interesting criteria used to study the formation of thin films on the surface of steel is
contact angle measurements. Moreover, the surface degradation of steel in 1 M HCl can be evaluated
from the surface free energy values which can be determined from surface tension and contact angle
measurements in the presence and absence of corrosion inhibitors [24,35]. It is well known that the
surface degradation of the steel creates corrosion products and an increase in surface roughness.
The surface degradation of steel can be measured in 1 M HCl as a blank and in different concentrations
of OA-TS and ODA-TS molecules by measuring water contact angle values as described in the
Experimental section. The surface free energy values (E) were calculated from the work of adhesion
(WA) and contact angles (θ) according to the Young equations [35]:
WA = γ (1 + cosθ)
(4)
(5)
WA = 2(γ × E)1/2exp [−β (γ − E)2]
where γ is the surface tension of aqueous 1 M HCl solution in the absence and presence of OA-TS and
ODA-TS molecules, while the value of β is 0.0001247 ± 0.000010 (mJ/m2)−2 [24]. The relation between
the contact angle values and the concentrations of OA-TS and ODA-TS is presented in Figure 7. The
relation between contact angle and ageing time is illustrated in Figure 8. The data (Figure 8) indicate
that ODA-TS and OA-TS in 1M HCl reached equilibrium (stable contact angle) during 5 and 20 s,
respectively. It means that ODA-TS forms thin layers on the surface of steel in short time more than
OA-TS. The increment of ODA-TS hydrophobicity more than OA-TS (as described in previous section)
is responsible for its short time adsorption at steel surface in 1M HCl.
Figure 7. Relation between contact angle measurements and concentrations of OA-TS and
ODA-TS in 1 M HCl aqueous solutions at steel surfaces.

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Figure 8. Relation between contact angle measurements and at ageing time at steel surfaces
of different concentrations of OA-TS and ODA-TS in 1 M HCl aqueous solutions.
The surface free energy values obtained at different concentrations of OA-TS and ODA-TS are listed
in Table 2. The data indicated the contact angle decreased with and the surface free energy increased
after the sample exposure to HCl solution. These are responsible for a decrease in the water contact
angle value and an increase in surface free energy. Moreover, it is clear from Figure 7 and Table 2 that
the lowest contact angle and the highest surface free energy values were obtained for the samples
immersed in HCl solution without OA-TS and ODA-TS. The data show that an increment of OA-TS
and ODA-TS concentrations caused increase in surface free energy and an increase in contact angle. It
can be concluded that addition of OA-TS and ODA-TS to the HCl solution caused lower steel surface
degradation due to the formation of adsorbed thin film layers resulting in a lower surface roughness.
Moreover, the adsorption of OA-TS and ODA-TS on the steel surface makes it more hydrophobic due
to the presence of hydrophobic octadecyl and oleyl tails. These are responsible for an increase in
contact angle and a decrease in surface free energy. These results can be proved from polarization test
and EIS to evaluate the corrosion inhibition efficiencies of the prepared OA-TS and ODA-TS inhibitors.
Table 2. Surface free energy values obtained for the steel samples exposed to different
concentrations of OA-TS and ODA-TS in 1 M HCl solution.
Surface Free Energy (mJ/m2)
Concentrations (mmol/L)
ODA-TS
5.1
OA-TS
5.1
0
0.272
2.84
8.9
6.5
10.6
12.4
14.2
16.1
7.6
5.70
8.5
11.36
22.73
10.8
12.2

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2.4. Monitoring the Open Circuit Potential (EOCP
)
The evolution of the open circuit potential (EOCP) with time for steel in 1 M HCl solutions without
and with OA-TS and ODA-TS is illustrated in Figure 9a,b, respectively.
Figure 9. The change of open circuit potential of steel electrode as a function of immersion
time in 1 M HCl solution and containing various concentrations of (a) OA-TS and
(b) ODA-TS.
The plots show clear modifications in the EOCP-time behavior due to the presence of the inhibitor. It
is clear that an anodic displacement of EOCP is observed with addition of OA-TS and ODA-TS. It is
evident that there are potential shifts in the anodic (noble) direction in the presence of OA-TS and
ODA-TS and these are more pronounced at high concentration. The inhibited solution exhibited higher
positive EOCP values when compared with those obtained in blank solution, indicating the formation of
the protective film due to the adsorption of the inhibitor on the steel surface. The addition of OA-TS
and ODA-TS to 1 M HCl solution shifts the Eocp towards more negative values during the initial stage
of monitoring then it increases continuously to positive values with the increasing immersion time and
eventually attains a steady state during the last stage of monitoring. The negative shift in Eocp values
in the presence of inhibitor can be explained with the excessive dissolution of iron during the initial
immersion. The continuous shift to noble values can be attributed to the formation of a protective and
inhibitive film, which increases with increasing the inhibitor concentration and immersion time.

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2.5. Polarization Measurements
The electrochemical potentiodynamic polarization (EP) curves of steel in 1 M HCl solution in the
presence and absence of OA-TS and ODA-TS with different inhibitor concentrations are shown in
Figure 10a,b, respectively. It is clear that both anodic and cathodic currents were decreased in the
presence of inhibitor and the decrease is more pronounced at higher inhibitor concentrations. The
adsorption of inhibitor on steel surface decreases the cathodic hydrogen reaction and also hinders the
acid attack on the steel electrode. In addition, the inhibitor also suppresses the anodic reaction and
causes a reduction in the anodic current experienced during the polarization. Generally, the adsorption
of OA-TS and ODA-TS compounds on active reaction sites of the steel surface inhibit both cathodic
hydrogen evolution and anodic iron dissolution. Therefore, the OA-TS and ODA-TS compounds
behaved as mixed-type inhibitors [36]. The inhibitory action of corrosion inhibitors can be explained on
the basis of adosption of OA-TS and ODA-TS compounds on the accessible reaction surface through
a geometric blocking effect. The values of electrochemical parameters, i.e., corrosion potential (Ecorr),
corrosion current density (Icorr), cathodic Tafel slopes (bc) anodic Tafel slopes (ba), were calculated from
the polarization curves and are listed in Tables 3 and 4 for OA-TS and ODA-TS, respectively. It is
established that the dissolution reaction is the transfer of Fe ions from the surface into the solution
and the reduction of hydrogen ions is the cathodic reaction during corrosion of steel in acidic solutions.
The addition of inhibitor with different concentrations altered the cathodic Tafel slopes (ba and bc) and
confirms the effect of inhibitor on reduction of cathodic and anodic reactions [37]. The inhibition
efficiency (IE%) is evaluated from icorr values using the relationship [38–40]:
IE% = [1 − (Icorr2/Icorr1)] × 100
(6)
where Icorr1 and Icorr2 are corrosion current densities in the absence and presence of inhibitor, respectively.
The values of IE% with different inhibitor concentrations are listed in Tables 3 and 4 for OA-TS and
ODA-TS, respectively. IE% increases with inhibitor concentrations. Increasing the inhibitor concentration
resulted in an increase in the surface coverage of steel surface due to an increase in the amount
adsorbed of the inhibitor on steel surface and leads to high corrosion inhibition.
(b)
(a)
Figure 10. Polarization plots of steel electrode obtained in 1 M HCl solution and
containing various concentrations of (a) OA-TS and (b) ODA-TS.

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Table 3. Inhibition efficiency values of steel electrode in 1 M HCl containing various
concentrations of the OA-TS calculated by polarization and EIS methods.
Polarization Method
Concentrations
EIS Method
Cdl
ba
bc
Ecorr
Icorr
μA/cm2
839
C.R.
mm/year
9.7324
0.2204
0.1392
0.1160
Rct
(mmol/L)
IE%
IE%
(mV) (mV)
(V)
Ohm
(μF/cm2)
Blank
0.023
0.114
0.341
69
46
56
53
120
202
125
156
−0.3955
−0.3242
−0.3784
−0.3775
____ 1.80
334
____
91.6
94.8
96.6
73
91.1
94.7
96.3
21.6
35
105
44
103
31
54
101
Table 4. Inhibition efficiency values of steel electrode in 1 M HCl containing various
concentrations of the ODA-TS calculated by polarization and EIS methods.
Polarization Method
EIS Method
Concentrations
(mmol/L)
ba
bc
Ecorr
Icorr
μA/cm2
839
C.R.
mm/year
9.7324
0.8468
0.5104
0.3596
Rct
Ohm
1.80
65.7
80.7
90.3
Cdl
(μF/cm2)
334
IE%
IE%
(mV) (mV)
(V)
Blank
0.023
0.114
0.341
69
40
42
48
120
206
153
116
−0.3955
−0.3235
−0.3369
−0.3606
____
97.7
98.5
98.8
____
97.2
97.7
98.0
19
95
12
93
10
91
2.6. Electrochemical Impedance Spectroscopy (EIS)
The effect of the OA-TS and ODA-TS concentrations on the impedance behavior of steel in
1 M HCl solution is presented as Nyquist plots in Figure 11a,b, respectively. The impedance diagrams
show a capacitive loop, whose size increased with increasing inhibitor concentration. The capacitive loop
was related to charge transfer in the corrosion process [41]. The electrical equivalent circuit employed
for fitting the data composed of solution resistance (Rs), charge transfer resistance (Rct) and double layer
capacitance (Cdl) as shown in Figure 12. The impedance parameters as (Rct) and (Cdl) are calculated
and are listed in Tables 3 and 4 for OA-TS and ODA-TS, respectively. The increase in the thickness of
the electrical double layer and/or decrease in local dielectric constant resulted from the decrease in
the double capacitance and increase of charge transfer resistance indicating that the inhibitor acts by
adsorption at the steel/solution interface [42]. The gradual replacement of water molecules by
adsorption of the inhibitor molecules on the steel surface causes a change in the values of Rct and Cdl
and decreases the extent of acidic dissolution of the steel [43]. The data presented in Tables 3 and 4
indicated that that the inhibitor had strongly adsorbed to the surface of steel. The inhibition efficiency
(%IE) is calculated from Rct using the following relation:
IE% = [1 − (Rct1/Rct2)] × 100
(7)
where Rct1 and Rct2 are the charge transfer resistances in the absence and in the presence of the
inhibitors, respectively. The percentage inhibition efficiency (IE%) was calculated from Equation (2)
and listed in Tables 3 and 4 for OA-TS and ODA-TS, respectively. The maximum inhibition efficiency
(98%) was achieved at an inhibitor concentration of 150 ppm. A strong adsorption of inhibitor on steel
surface indicates a more surface coverage by the inhibitor and accompanies by an increase in the

Molecules 2015, 20
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charge-transfer resistance (Rct) values [44]. The inhibition efficiencies calculated from EIS are in good
agreement with those obtained from potentiodynamic polarization.
(a)
Blank
(b)
Blank
ure 10. Nyqusit plots of steel electrode obtained in 1M HCl solution and con
centrations of: a) ODA-TS and b) OA-TS (Square and circle symbols indicate e
data, respectively).
Figure 11. Nyqusit plots of steel electrode obtained in 1 M HCl solution and containing
various concentrations of: (a) ODA-TS and (b) OA-TS (square and circle symbols indicate
experimental and fit data, respectively).
Cdl
Rs
Rct
Figure 12. Equivalent circuit employed for fitting EIS data.

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2.7. SEM and EDX Measurements
To confirm the formation of the films on the steel surface, the SEM technique was used to
characterize the iron surface. Figure 13a–d,f presents the morphology of the polished bare steel in 1 M
HCl aqueous solution and in 1 M HCl containing OA-TS and ODA-TS, respectively. Figure 13a shows
the steel surface after polishing, which appeared as a smooth surface with some scratches. Inspection of
Figure 13b reveals that the steel after a 5 h immersion in uninhibited 1 M HCl shows aggressive attack
damage as a great deal of deep cavities were found. Figure 13c, present the images of the iron sheet
covered by OA-TS and ODA-TS films after immersion in 100 ppm of aqueous solutions.
Figure 13. SEM micrographs of (a) polished steel bare; (b) steel bare immersed in
1 M HCl; (c) steel coated with 100 ppm of OA-TS; (d) steel coated with 100 ppm of
OA-TS after immersion in 1 M HCl; (e) steel coated with 100 ppm of ODA-TS before
immersion in 1 M HCl and (f) steel coated with 100 ppm of ODA-TS after immersion
in 1 M HCl.
However, under the same corrosion circumstances, the surface of film-modified steel sheet
(Figure 13d,f) was a smooth and cleaner surface and only a few small pits are observed on the steel
surface. Inspection of the morphologies implied that the presence of the OA-TS and ODA-TS films
can efficiently protect the steel from corrosion. Moreover, the formation of self-assembled films at the
surface of steel increases the interaction between OA-TS and ODA-TS and the steel surface, resulting
in a decrease in the contact between the steel and the aggressive medium. These results are in good
agreement with the results obtained by EIS and potentiodynamic polarization measurements. This

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behavior suggested that both compounds mitigate steel corrosion rates with complete inhibition all
over the steel surface. EDX is an interesting technique used to analyze the protective film formed
on a polished steel surface. It is used to confirm the composition of the formed protective film. The
instrument does not give the corresponding peaks for N, and O, but confirmed the corresponding peaks
for metals. In this respect, the strength of the Fe peak in the absence and presence of the inhibitor gives
an idea about the thickness of the protective film. The EDX spectra of carbon steel samples were
presented in Figure 14a. The data indicate that the strength of the iron band decreased when steel was
immersed in the OA-TS and ODA-TS inhibitors (Figure 14c). The EDX data confirm the formation of
a strongly adherent protective film of self-assembled OA-TS and ODA-TS adsorbed on the steel surface,
which inhibits the corrosion of steel [45].
Figure 14. EDX analyses of (a) steel; (b) steel coated with 100 ppm of ODA-TS and
(c) steel coated with 100 ppm of OA-TS after immersion in 1 M HCl aqueous solutions.
2.8. Mechanism of Corrosion Inhibition
The presence of OA-TS and ODA-TS in 1 M HCl blocked the local cathodes and anodes by
adsorbing on the steel surface. It is established that the adsorption of the inhibitor on the steel surface
displaces water molecules and other species, forming an adsorbed and inhibitive film at the steel surface.
The higher the surface coverage of steel the slower the corrosion process of steel as evident from the

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extremely large reduction in anodic and cathodic current density. The adsorption of the inhibitor on the
steel surface in acidic chloride solution is mainly due to a geometric blocking effect. Therefore, the
protection efficiency (IE) equals the coverage of the steel surface with the adsorbed inhibitor species.
The experimental results were fitted to series of adsorption isotherms, i.e., Langmuir, Temkin,
Bockris-Swinkels, Flory-Huggins and Frumkin. The adsorption behavior of both inhibitors obeys
Langmuir adsorption isotherm, which is given by [46]:
C(inh)/θ = 1/Kads + C(inh)
(8)
where C(inh) is inhibitor concentration and Kads is the equilibrium constant for the adsorption process.
A plot of C/θ vs. C (Figure 15) gives a straight line with an average correlation coefficient of 0.9999
and 1 for OA-TS and ODA-TS as shown in Figure 14a,b, respectively. The near unity slope suggests
that the adsorptions of both inhibitors on steel obey a Langmuir adsorption isotherm. From the values
of the adsorption constant, Kads, the standard free energy of adsorption (ΔG°ads) for the inhibitor is
determined using the following equation [47,48]:
ΔG° = −RT ln (55.5 Kads)
(9)
ΔG° was calculated for OA-TS and ODA-TS and found to be −38.456 kJ·mol−1 and −38.474 kJ·mol−1,
respectively. The high value of ΔG° provides evidence of strong interactions between the inhibitor
and the steel surface [49–52]. It was established that when the standard free energy of adsorption is
around −20 kJ·mol−1 or lower a process of physical adsorption may occur predominantly through
electrostatic interactions between the charged inhibitor and the charged exposed surface [53–57]. A
chemical adsorption takes place [43–47] when the standard free energy of adsorption (ΔG°ads) values
are around −40 kJ·mol−1 or higher. The negative values of the standard free energy of adsorption
(ΔG°ads) indicate a spontaneous adsorption of the inhibitor on the steel surface by displacement of
water molecules from the steel surface by the inhibitor and formation of a protective film [57]. The
oxidation reactions of iron are significantly inhibited as a result of spontaneous adsorption of the
inhibitor molecules on the iron surface and form a protective layer [48,58,59]. The adsorption of the
inhibitor at cathodic sites of steel reduces H2 gas evolution [49,50,53]. In addition, the high molecular
weight and large size of the inhibitor molecules extremely affect the protection efficiency of the
inhibitors [60]. The ODA-TS and OA-TS molecules are ionized in aqueous solution to alkylammmonium
cations and tosylate ions, as illustrated in the Scheme 1. These molecules can be adsorbed physically on
the steel surfaces through an electrostatic interaction mechanism between ODA and OA ammonium
cations and the steel surface that possesses negative charges produced from reaction with HCl to produce
(FeCl−) in the anodic reaction [61]. The adsorption of alkylammonium cations at the steel surface
forms an adsorbed monomolecular or bimolecular layer by forming a complex on the steel surface.
The adsorbed layers protect the steel surfaces from Cl− attack to prevent the steel oxidation reaction at
the anode. Moreover, the protonated alkyl ammonium cations of ODA and OA adsorb at cathodic sites
to compete with H+ to prevent hydrogen evolution. Moreover, the tosylate anion can interacts with H+
produced by the HCl to prevent the hydrogen evolution. The presence of a double bond in the structure
of OA can form an interaction between the π electrons and the unoccupied d-orbital of iron to produce
a new center of adsorption action, which cannot be obtained for ODA cations.

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(a)
(b)
Figure 15. Langmuir adsorption plot of steel in 1 M HCl solution containing different
concentrations of (a) OA-TS and (b) ODA-TS.
3. Experimental Section
3.1. Materials
Oleylamine (OA), p-toluenesulfonic acid monohydrate (TSA; 99.5%), and octadecylamine (ODA;
97%) were purchased from Aldrich Chemicals Co. (St Louis, MO, USA) and used with no further
purification. Hydrochloric acid (37 wt %) was obtained from Merck (Darmstadt, Germany). Tests were
performed on steel specimens with weight percentage composition as follows: 0.14% C, 0.57% Mn,
0.21% P, 0.15% S, 0.37% Si, 0.06% V, 0.03% Ni, 0.03% Cr and Fe is the balance. The aggressive acid
environment was 1 M HCl. The surface of the working electrode was prepared by grinding with
different grades of emery papers, then rinsing with distilled water and ultrasonic degreasing in ethanol
and eventual drying at room temperature.
3.2. Synthesis Procedure
Equimolar amounts of ODA or OA and p-toluenesulfonic acid were mixed in a flask equipped with
a magnetic stirrer, thermometer and reflux condenser. The reaction mixture was heated for 24 h under
nitrogen atmosphere at temperature 145 °C. Finally, the solid octadecylammonium tosylate (ODA-TS)
and oleyl ammonium tosylate (OA-TS) products were obtained after cooling to room temperature. The

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melting temperatures of OA-TS and ODA-TS salts are 56 and 85 °C, respectively. The products were
purified by heating at 45 °C under reduced pressure (at 10 mm·Hg) by using vacuum rotary evaporator
for 24 h. The yields of products ODA-TS and OA-TS are 99.5 wt % and 99 wt %. The C, H, N and S %
determined for ODA-TS are 67.92%, 10.63%, 3.17% and 7.24%, respectively. The C, H, N and S %
detected from elemental analysis for OA-TS are 68.24%, 10.21%, 3.19% and 7.26%, respectively.
3.3. Characterization
1H- and 13C-NMR spectra of the prepared polymers were recorded on a 400 MHz Avance DRX-400
spectrometer (Bruker, Baden-Württemberg, Germany). A drop shape analyzer model DSA-100 (Krüss
GmbH, Hamburg, Germany) was used to perform the surface tension measurements of IL solutions and
measure the contact angles. Krafft Temperature TK determined from the relation between conductivity
and temperature as reported before [62]. The conductance was determined using an AB30 conductivity
meter (Fisher Scientific, Waltham, MA, USA). A scanning electron microscope (SEM, Model JSM-5400,
JEOL, Tokyo, Japan) was used to determine the microstructure of IL films formed on the steel surfaces.
The chemical analyses of the steel surfaces were determined using an energy dispersive X-ray analysis
(EDX) accessory which was attached to the SEM-JEOL 5400. The analyses were carried out after
immersion of steel working electrodes (0.5 cm2) in the inhibitor solutions for 6 h and drying with
nitrogen at room temperature. An optical microscope (BX51, Olympus, Tokyo, Japan) equipped with a
digital camera (XD200, Flovel Co., Ltd., Tokyo, Japan) was used to determine the micelle morphology.
3.4. Electrochemical Measurements
Electrochemical measurements were performed on a Solartron 1470E system (Potentiostat/Galvanostat)
with a Solartron 1455A frequency response analyzer to perform all polarization and EIS measurements
(Solartron, Farnborough, UK). Polarization curves were recorded at a constant sweep rate of 1 mV/s.
EIS was in the frequency range of 10 kHz to 10 mHz. A three-electrode cell assembly was employed to
perform electrochemical experiments. Platinum sheet was used as a counter electrode and saturated
calomel electrode (SCE) as the reference electrode. The electrochemical experiments data were collected
and analyzed by using CorrView, Corr-Ware software. The analysis of impedance data and selection of
an equivalent circuits to fit the experimental results were conducted using the Zplot and ZView software.
4. Conclusions
The main conclusion of the present study may be summarized in the following points:
(1) OA-TS and ODA-TS can act as an effective corrosion inhibitor for steel in 1 M hydrochloric acid.
(2) OA-TS and ODA-TS act as a mixed type inhibitor retarding the anodic and cathodic reactions.
(3) The electrochemical results indicated that the protection efficiency of the inhibitor is highly
dependent upon concentration and attains a maximum value (98%) at an inhibitor concentration of
150 ppm.
(4) EIS data indicated that the charge transfer resistances increase with the addition of inhibitor and
the corrosion process is mainly controlled by charge transfer reactions.
(5) SEM and EDX techniques revealed the formation of protective films on the steel surface.

Molecules 2015, 20
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Acknowledgments
The authors extend their appreciation to the Deanship of Scientific Research at King Saud
University for funding this work through research group no RGP-VPP-235.
Author Contributions
Ayman M. Atta suggested the research work and discussed the data, Gamal A. El-Mahdy discussed
the corrosion data, Hamad A. Al-lohedan discussed and supported the work and Abdel Rahman O. Ezzat
finalized the experimental work.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds OA-TS and ODA-TS are available from the authors.
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/4.0/).