Comparison of the synergistic effect of counterions on the inhibition of mild steel corrosion in acid solution: Electrochemical, gravimetric and thermodynamic studies

Article abstract of DOI:10.1039/c4ra12286k

Quinoline quaternary ammonium salts 1-benzylquinoline bromide (1) and 1-benzylquinoline chloride (2) have been synthesized and then developed as corrosion inhibitors in acidic HCl solution. Electrochemical impedance, potentiodynamic polarization and gravimetric measurements, as well as SEM, indicate that both can efficiently protect the mild steel from corrosion. The fact that 1 shows better performance than 2 under the same conditions could be attributed to the cooperative effect of counterions on the corrosion process. The results of thermodynamic and electrochemical studies were in agreement with the observed inhibition efficiency, indicating that counterions can affect the interactions between organic cations and the metal surface, and influence the stability of the protective film, which leads to the detected differences in their inhibition behavior.

Full text of DOI:10.1039/c4ra12286k

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Comparison of the synergistic effect of counterions
on the inhibition of mild steel corrosion in acid
solution: electrochemical, gravimetric and
thermodynamic studies†
Cite this: RSC Adv., 2015, 5, 4716
a
a
b
Li Bai, La-Jun Feng, Hong-Yan Wang, Yong-Bin Lu,c Xiao-Wei Leid
*
and Fang-Lin Baic
Quinoline quaternary ammonium salts 1-benzylquinoline bromide (1) and 1-benzylquinoline chloride (2)
have been synthesized and then developed as corrosion inhibitors in acidic HCl solution.
Electrochemical impedance, potentiodynamic polarization and gravimetric measurements, as well as
SEM, indicate that both can efficiently protect the mild steel from corrosion. The fact that 1 shows better
performance than 2 under the same conditions could be attributed to the cooperative effect of
counterions on the corrosion process. The results of thermodynamic and electrochemical studies were
in agreement with the observed inhibition efficiency, indicating that counterions can affect the
interactions between organic cations and the metal surface, and influence the stability of the protective
film, which leads to the detected differences in their inhibition behavior.
Received 13th October 2014
Accepted 4th December 2014
DOI: 10.1039/c4ra12286k
www.rsc.org/advances
double bonds, and aromatic rings in their molecular struc-
tures.5–9 In some recently published reports, quinoline and its
derivatives have been highlighted as useful inhibitors.10–12
Among them, quaternary ammonium salt can act as the pre-
vailing organic inhibitor, achieving excellent inhibition effects
based on its pronounced water-solubility.13–15
1 Introduction
Iron-containing mild steel has been widely employed in the
petroleum industry and machinery. Due to the prevalent
applications of acidic solution in oil well acidication and
enhanced oil recovery techniques, the severe corrosion of
materials causes serious economic and safety problems.1 It is
generally acknowledged that organic inhibitors represent an
important and practical category for the prevention of metal
corrosion, coating the metal surface with a physical barrier to
restrict the exposure of steel to acidic media.2–4 Up to now,
various organic compounds have been reported as effective,
inexpensive and less polluting corrosion inhibitors for mild
steel in acidic media. Typically, N-heterocyclic inhibitors act via
adsorption on the metal surface, by donating electrons to the
metal from the nitrogen heteroatoms, triple or conjugated
With regard to corrosion efficiency, the synergistic effect
represents an effective method to improve the inhibitive action
of an inhibitor in the presence of another species in a corrosive
medium. Typically, much attention has been paid to the
synergism between organic compounds and halide salts (e.g.
NaX) added into the inhibitive solution.16–22 It is usually
considered that the halide ions can create oriented dipoles to
change the charge on the corroding electrode surface, which
accordingly facilitates the adsorption of inhibitor cations.23 The
existing data in most cases indicate that the synergistic effꢀect of
the halide ions proceeds in the order Iꢀ > Brꢀ > Cl .16–22
Jeyaprabha explained that the cooperative effect of ions
depends on their adsorption on the metal surface.24 Addition-
ally, the synergistic effect of halide salts in improving the
corrosion inhibition behavior of quaternary ammonium salts is
also well documented.14,25,26 The results highlight the important
features of the additives that determine the interactions
between the inhibitor and the metallic surface to some extent.
Considering that halide salts can synergistically affect the
interactions between the inhibitor and the metallic surface, the
questions of whether and how counterions in inhibitors inu-
ence the adsorption of organic inhibitors, and lead to the
detected inhibitory effect, have not received a considerable
aSchool of Materials Science and Engineering, Xi’an University of Technology, Xi’an
710048, P.R. China
bSchool of Chemistry and Chemical Engineering, Shaanxi Normol University, Xi’an
710119, P. R. China. E-mail: hongyan-wang@snnu.edu.cn; Tel: +46 0729428391
cResearch Institute of Yanchang Petroleum (Group) CO. LTD, Xi’an 710075, P.R. China
dState Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University,
Xi’an 710049, P.R. China
† Electronic supplementary information (ESI) available: Electrical equivalent
circuit used for modeling the metal–solution interface; variation of the
corrosion rate for mild steel with different concentrations of inhibitor; 1H NMR
spectra of 1 and 2; electrochemical parameters from Tafel polarization curves;
EIS parameters for the corrosion of mild steel with different concentrations of
inhibitors; EDX and XPS spectra of the surface with the addition of 1 or 2. See
DOI: 10.1039/c4ra12286k
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amount of attention. Meanwhile, the corresponding mecha- solvent afforded a pink powder. Aer the powder was dissolved
nism is still unclear. If it can be explained theoretically, in CH2Cl2, the solution was poured into diethyl ether, and a
changing the counterion in inhibitors could be another useful white precipitate was collected by ltration, which was then
means of producing efficient inhibitors without complicated washed with ethanol three times. The pure products of 1 and 2
syntheses.
Keeping the above in mind, we have presented two kinds of
quinoline quaternary ammonium salts, 1-benzylquinoline 9.14 (d, J ¼ 8 Hz, 1H), 8.32 (d, J ¼ 8.4 Hz, 2H), 8.07 (m, 2H), 7.91
bromide (1) and 1-benzylquinoline chloride (2), synthesized by (m, 1H), 7.39 (m, 3H), 7.29 (m, 2H), 6.24 (s, 2H); ESI-MS: m/z ¼
the quaternization of quinoline and benzyl bromide or benzyl 220.1 (M+).
were lily-white powders.
2.2.1 1. 1H NMR (D2O, 400 MHz) d: 9.31 (d, J ¼ 5.6 Hz, 1H),
chloride, respectively, and developed these as corrosion inhib-
2.2.2 2. 1H NMR (D2O, 400 MHz) d: 9.31 (d, J ¼ 5.6 Hz, 1H),
itors in acidic HCl solution. The improved corrosion inhibition 9.13 (d, J ¼ 8 Hz, 1H), 8.31 (d, J ¼ 8.4 Hz, 2H), 8.06 (d, J ¼ 5.2 Hz,
performance displayed in the presence of 1 highlights the 2H), 7.91 (m, 1H), 7.39 (m, 3H), 7.30 (m, 2H), 6.24 (s, 2H); ESI-
cooperative effect of counterions on the corrosion process. MS: m/z ¼ 220.1 (M+).
Here, using a combination of electrochemical and thermody-
namic studies, it is hoped that persuasive evidence that sheds
light on the happenings at the metal surface is afforded. Our
ndings on the cooperative effect of counterions indicate that
2.3 Corrosion tests performed in acidic medium
2.3.1 Specimens and preparation of the acidic solution.
The specimens were samples of the usual mild steel widely used
in the oil gas and industry. The chemical composition of the test
material is shown in Table 1, and was analyzed using a direct
reading spectrometer (Baird Spectrovac 2000) on the basis of
GB/T 4336-2002. In order to ensure reproducibility, three
parallel specimens were tested during the experiments. Addi-
tionally, a solution of 1.0 M HCl prepared from concentrated
HCl (37%) (Merck grade) was used as the acidic medium to
check the activity of the tested inhibitors.
counterions can affect the interactions between organic cations
and the metal surface and inuence the stability of the
protective lm, leading to the different inhibition results.
2 Experimental methods
2.1 Materials and instrumentation
All reactions and operations were carried out under a dry N2
atmosphere with standard Schlenk techniques. All solvents
were used directly without further drying and distillation.
Quinoline, benzyl bromide and benzyl chloride were purchased
from Aldrich and used as received. 1H NMR spectra were
obtained using a Bruker-400 spectrometer with tetramethylsi-
lane (1H) as an internal standard. Mass spectra were recorded
with Trio-2000 GC-MS spectrometers.
2.3.2 Electrochemical measurements. Electrochemical
experiments were carried out in a conventional three-electrode
cell with a platinum counter electrode (CE) and a saturated
calomel electrode (SCE, saturated KCl aqueous solution)
coupled to a ne Luggin capillary as the reference electrode. To
minimize the ohmic contribution, the Luggin capillary was kept
close to the working electrode. The working electrode (WE)
comprised the tested steel, embedded in a PVC holder using
epoxy resin, leaving one part of the surface of area 1.0 cm ꢁ 1.0
cm exposed. The exposed surface was abraded with emery paper
up to #800 progressively, then degreased with acetone, rinsed
with absolute alcohol and dried with a cold air stream. Before
measurement, the electrode was immersed in 1.0 M HCl solu-
tion with different inhibitor concentrations at open circuit
potential (OCP) at 25 ꢂC for 2 h to attain a stable state. All
electrochemical measurements were carried out using a CS350
advanced electrochemical system (Princeton Applied Research).
Each experiment was repeated at least three times to check the
reproducibility, and acceptable reproducibility was obtained.
The potential of potentiodynamic polarization curves was
measured, ranging from ꢀ100 mV to +100 mV versus OCP with a
scan rate of 0.5 mV sꢀ1. The current responses were recorded
and analyzed as Tafel plots. Corrosion characteristics, such as
corrosion potential (Ecorr), corrosion current (Icorr) and anodic
(ba)/cathodic (bc) Tafel slopes were obtained from the soware
installed in the instruments. The inhibition efficiency (hP) was
dened and calculated from Icorr values as follows:29
2.2 Synthesis and characterization
The structures of 1 and 2 are shown in Fig. 1. Compounds 1 and
2 were prepared according to the general procedures known
from the literature,27,28 except that ethanol was used as the
solvent instead. Keeping the ratio of quinoline to benzyl
bromide at 1 : 1.1 ought to have provided the best conditions
for the synthesis of 1. However, for 2, 1 mol quinoline was
reacted with 1.3–1.5 mol benzene chloride, otherwise a sticky
and dark red by-product was obtained instead. The mixture was
reuxed until the reagents had completely reacted, as moni-
tored by TLC (SiO2; CH3COOC2H5–CH2Cl2, 30 : 70 v/v). The
color of the reaction mixture changed to dark red, and the
obtained precipitate was ltered and collected. The crude
products were puried using ash column chromatography
with CH3COOC2H5–CH2Cl2 as the eluent. Removal of the
À
Á
Icorr ꢀ IcorrðinhÞ
hP ¼
ꢁ 100
(1)
Icorr
Fig. 1 The structures of 1 and 2.
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Table 1 The chemical composition of the test material (wt%)
Elements
C
Si
Mn
P
S
Cr
Mo
Ni
Ti
0.036
Cu
Components
0.27
0.26
1.41
0.014
0.003
0.089
0.080
0.049
0.030
where Icorr and Icorr(inh) represent the corrosion current density
without and with the inhibitor, respectively.
3 Results and discussion
3.1 Potentiodynamic polarization studies
Electrochemical impedance spectroscopy (EIS) was carried
out at OCP in the frequency range of 100 kHz to 10 mHz using a
10 mV peak-to-peak voltage excitation. The impedance data
were tted using Z-view soware. The inhibition efficiency (hi)
was determined on the basis of the equation:29
The inhibition mechanism for the competitive substitution of
water coatings on surfaces by inhibitor molecules is well
known.31 Nucleophilic centers with heteroatoms and delo-
calized p-electrons in 1 and 2 are readily available for sharing
and donating electrons to the empty orbital of the electrophilic
metal to form a coordinate covalent chemical bond. Addition-
ally, the water solubility of 1 and 2 can enhance substitution
with water molecules.
The evaluation of inhibitors 1 and 2 in 1.0 M HCl solution
was carried out using the potentiodynamic polarization tech-
nique. The results are simultaneously related to the effect of the
inhibitors’ concentrations on both the anodic and cathodic
curves of mild steel (Fig. 2). The electrochemical corrosion
parameters, such as the free corrosion potential (Ecorr), corro-
sion current density (Icorr), slope of the cathodic branch (bc) and
slope of the anodic branch (ba), associated with the different
À
Á
R
tðinhÞ ꢀ Rtð0Þ
hi ¼
ꢁ 100
(2)
RtðinhÞ
where Rt(0) and Rt(inh) are the charge transfer resistance values
for mild steel in the absence and presence of inhibitors,
respectively.
2.3.3 Weight loss tests. The surfaces of specimens with a
size of 40 mm ꢁ 10 mm ꢁ 3 mm used for weight loss tests
were mechanically abraded using emery paper up to #800
progressively, then degreased with acetone, rinsed with
absolute alcohol, weighed using a balance (with a precision
of 0.1 mg), and nally mounted in the acid solution. Inhibi-
tion efficiencies of the synthesized derivatives were tested at conditions were determined and are listed in Table S1.† As
5.0 ꢁ 10ꢀ5 M, 1.0 ꢁ 10ꢀ4 M, 2.0 ꢁ 10ꢀ4 M and 5.0 ꢁ 10ꢀ4 M,
reected in the plots, the presence of inhibitor 1 or 2 causes a
visible decrease in the corrosion rate. The distinct feature is that
the Tafel curves bodily shi both the cathodic and anodic
processes to lower currents, but to different extents, relative to
the blank curve. Therefore, 1 and 2 can affect both the anodic
dissolution of the metal and cathodic evolution of hydrogen to
some extent.32 Compared with the blank experiment, the
respectively, in 500 ml of 1.0 M HCl solution. The treatment
solution was poured into sealed glass bottles, then the
specimens were suspendeꢂd in theseꢂ solutions without stir-
ꢂ
ring and kept at 60 C, 75 C and 90 C for 4 h. Control tests
were done in the same way without inhibitors. Aer the
corrosion test, the corroded specimens were wiped with
paper tissues, and the scale was gently and carefully cleaned,
rinsed with water then absolute alcohol, dried in its natural
state, and weighed again using the balance (with a precision
of 0.1 mg) to calculate the corrosion rate.
The corrosion rate (k) and inhibitor effectiveness (hw) were
calculated by means of the following equations:30
ðW0 ꢀ W1Þ
k ¼
ꢁ 100
(3)
At
À
Á
k
ð0Þ ꢀ k
ðinhÞ
hw
¼
ꢁ 100
(4)
k
ð0Þ
where k is in g mꢀ2 hꢀ1; A is the specimen area (in m2); W0 is the
original weight of the specimen, and W1 is the specimen weight
(in g) aer the immersion period, t is the immersion period (in
h), and k(0) and k(inh) are the corrosion rates without and with an
inhibitor, respectively.
2.3.4 SEM and XPS measurements. SEM (Philips XL-20)
was utilized to investigate the micromorphology of the corro-
sion scale and XPS (Kratos AXIS ULTRA) was used to investigate
Brꢀ and Clꢀ on the surface. Before SEM observation, the
samples were cleaned using acetone in order to remove the
surface contamination.
Fig. 2 The effects of 1 (a) and 2 (b) on the polarization behavior of mild
steel in 1.0 M HCl at 25 ꢂC.
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cathodic branches of the polarization curves recorded for both
compounds are shied more signicantly towards a reduction
in the current, in line with higher bc values.33 Besides, the values
of corrosion potential move in the positive direction with the
addition of each compound. Meanwhile, with the increasing
concentration of the inhibitors, the cathodic Tafel slope (bc)
slightly changes and is approximately independent of the
inhibitors’ concentrations. This phenomenon indicates that the
addition of 1 and 2 does not change the mechanism of the
cathodic reactions, and the corrosion is rather inhibited by the
blocking of the iron surface by a simple adsorption process.34
For the anodic dissolution reaction of the metal, within the
scanned region, the anodic branches reveal a clearly inhibited
region with the increasing concentration of the inhibitors,
suggesting that the inhibitors can efficiently form the protective
adsorption lm and retard the anodic dissolution reaction.
Furthermore, the polarization curves with different concen-
trations of inhibitors were used to determine the protection
efficiency (hp). The data collected in Table S1† demonstrate the
dependence of hp on the concentration of the inhibitor.
Generally, the inhibition efficiency increases with the inhibitor
concentration in the cases of both 1 and 2. The increase in hp at
higher concentrations is not as much as at low concentrations
of the inhibitor. For instance, hp can increase from 0 to 93.11%
in the presence of 2.0 ꢁ 10ꢀ4 M 1, while increasing the
concentration of 1 to 1.0 ꢁ 10ꢀ3 M can simply allow the value of
hp to reach up to 94.47%. This means that the dynamic balance
between the adsorption and desorption of the inhibitor results
in a coverage of molecules that approaches saturation, so that
higher concentrations of inhibitor cannot lead to a much
greater protection efficiency.35 The potentiodynamic polariza-
tion curves of 1 and 2 shown in Fig. 2 display similar cathodic
and anodic lines, indicating the same inhibition mechanism.
However, carefully observing the data in Table S1,† it is not
difficult to nd out that, in each case, the decrease of Icorr is
more distinct in the presence of 1 than with 2 under the same
conditions, implying that it provides better protection.
Fig. 3 The EIS behavior of mild steel containing different concentra-
tions of 1 (a) and 2 (b) in 1.0 M HCl at 25 ꢂC.
affect the corrosion mechanism for the dissolution of mild steel
in HCl.40
For the analysis of the impedance spectra, the convincing
tting results of the high frequency capacitive loops of
compound 1 for an appropriate equivalent circuit are shown in
Fig. S1,† which consists of the parallel combination of a
capacitor Cdl and a charge transfer resistor Rt in series with a
solution resistor Rs. Similar tting results were obtained for 2 as
well. The pure electric models can verify or rule out mechanistic
models and enable the calculation of numerical values corre-
sponding to the physical and/or chemical properties of the
electrochemical system under investigation.41 The proposed
circuit allows the identication of both the solution resistance
and charge transfer resistance. The capacitance value is affected
by imperfections of the surface, which can be simulated via a
constant phase element (CPE).42 The CPE is determined by a
component Qdl and a coefficient a, where a quanties different
physical phenomena, like surface inhomogeneousness result-
ing from surface roughness, inhibitor adsorption, porous layer
formation, etc. In this case, the capacitance is calculated by the
following mathematical formulation:43
3.2 Electrochemical impedance spectroscopy (EIS)
Nyquist diagrams for the mild steel in 1.0 M HCl at 25 ꢂC with 1
and 2 were produced using electrochemical impedance spec-
troscopy, and are shown in Fig. 3. Clearly, for both 1 and 2, every
impedance spectrum mainly consists of a large capacitive loop
at high frequencies. The large capacitive loop indicates that the
corrosion of the test steel in HCl solution is mainly controlled
by a charge transfer process and the formation of a protective
layer on the metal surface. Deviations from the ideal semicircle
are generally attributed to the frequency dispersion, as well as
inhomogeneities, the roughness of the metal surface and the
mass transport process.36–38 Evidently, across all tested
concentrations, the diameter of the capacitive loop with the
addition of inhibitors 1 or 2 is bigger than that in their absence,
and the changes in the impedance response are more
pronounced with the increasing concentration of inhibitors.39
Simultaneously, similar shapes are maintained in all the
diagrams, indicating that the presence of inhibitors cannot
aꢀ1
Cdl ¼ Qdl ꢁ (2pfmax
)
(5)
where fmax represents the frequency at which the imaginary
component of the impedance is maximal on the Nyquist plot.
The electrochemical parameters of Rt, Cdl and Rs are calculated
by soware and presented in Table S2.†
Upon inspection of the data in Table S2,† it can be seen that
the value of Cdl decreases prominently, while the Rt values
increase with the concentration of inhibitors 1 or 2, suggesting
that the inhibitor molecules function by adsorption at the
metal–solution interface.44 A larger charge transfer resistance is
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associated with a system corroding more slowly. In contrast, the corrosion product layer dissolution, therefore the corrosion
better protection provided by an inhibitor can be associated product layers nally become looser and porous, and cannot
with a decrease in the capacitance of the metal. Typically, at 5.0 supply the matrix with effective protection, but increase the
ꢁ 10ꢀ4 M of each inhibitor, the presence of 1 gives a Rt value of contact area of the corrosion reactions. In contrast, in the
655.9 U cm2 and Cdl value of 90.612 mF cmꢀ2, while 2 gives presence of an inhibitor, although the thermal motion of the
respective values of 406.4 U cm2 and 111.66 mF cmꢀ2. Under any inhibitor molecule is accelerated with an increase in tempera-
given conditions, Rt(1) > Rt(2), while Cdl(1) < Cdl(2), implying ture, leading to the desorption of the inhibitor to some extent,
improved inhibition by 1 than by 2. The values of inhibition the reliable interactions between the inhibitor molecules and
efficiency (hi) were calculated from Rt in the absence and pres- empty orbitals of the electrophilic metal guarantee the steady
ence of an inhibitor. In both cases, generally, hi increases with supply of a compact and dense lm. Therefore, some powerful
the concentration of the inhibitor, and follows the order: hi(1)> inhibitors still display highly protective effects at high temper-
hi(2), in accordance with the results from the potentiodynamic atures.45–47 The variation of the corrosion rate (k) with the
polarization. However, the fact that the values are not in perfect concentrations of the inhibitors in HCl is shown in Fig. S2(a)
agreement with each other might be due to the different testing and (b)† and Table 2. Clearly, the steel is more rapidly corroded
modes.
with an increase in temperature in the absence of an inhibitor.
Furthermore, the electrochemical impedance spectra in the And the corrosion rate decreases while the inhibition efficiency
presence of 1 and 2 at 90 ꢂC were also measured (Table S3 and (hw) increases with the inhibitor concentration.
Fig. S4†). At higher temperatures, the steel was corroded more
It was found in the presence of 2.0 ꢁ 10ꢀ4 M inhibitor, that
ꢂ
seriously, in accordance with the much lower Rt values. With the the results exhibited maximum hw values of 93.76% at 60 C,
concentration, the inhibition efficiency displayed a similar 97.36% at 75 ꢂC and 96.75% at 90 ꢂC for 1 and 92.54% at 60 ꢂC,
trend to that measured at room temperature, but 1 still showed 88.24% at 75 ꢂC and 93.92% at 90 ꢂC for 2. Noticeably, when the
much better performance than 2 under the same conditions.
concentration of the inhibitor was less than 1.0 ꢁ 10ꢀ4 M, k
decreased sharply with an increase in concentration, however, a
further increase caused no appreciable change in performance.
It is considered that the adsorption of inhibitors on the metal
surface is an exothermic process, therefore, aer the adsorption
of inhibitors to the point of saturation, the excess molecules
desorb from the metal surface due to intermolecular repulsion.
Additionally, the fact that hw slightly increases or remains
almost the same with an increase in temperature is explained by
the likely specic interactions between the iron surface and the
inhibitor.48 The increase of hw with temperature can lead to a
change in the nature of the adsorption mode: at lower
temperatures, the inhibitor is being physically adsorbed, while
chemisorption is favoured as the temperature increases.49
Table 2 affords a visible comparison of the performances of 1
and 2 as well. Distinctly, at 90 ꢂC, with the addition of the
inhibitors at a concentration of 5.0 ꢁ 10ꢀ4 M, the corrosion
rates of the measured materials decreased from 127.6654 g mꢀ2
hꢀ1 to 3.5846 g mꢀ2 hꢀ1, with a hw value of 97.19% for 1, while k
was reduced to 6.4129 g mꢀ2 hꢀ1 for 2, with a hw value of
94.97%. Clearly, the materials in the presence of 1 are generally
3.3 Corrosion inhibition by 1 and 2 based on weight loss
tests
Concentration dependent experiments of 1 and 2 (0, 5.0 ꢁ 10ꢀ5
M, 1.0 ꢁ 10ꢀ4 M, 2.0 ꢁ 10ꢀ4 M and 5.0 ꢁ 10ꢀ4 M respectively) in
1.0 M HCl were performed using the weight loss method at 60
ꢂC, 75 ꢂC and 90 ꢂC for 4 h. In general, the temperature plays an
important role in acidic corrosion, which seriously affects the
formation rate of the corrosion product layer. At low tempera-
tures, though, without an inhibitor, the protective layer is
hardly formed on the steel surface, because the low deposition
rate of the corrosion product, lower dissolution rate of the
matrix and lower mass transfer rate can lead to a lower corro-
sion rate. Instead, the presence of an inhibitor benets the
formation of a protective lm, which efficiently prevents the
exposure of the metal surface to the acidic medium. With the
increase in temperature, in the absence of an inhibitor, owing
to the negative temperature gradient of the corrosion product, it
quickly deposits onto the surface of the metal. However, this
process is accompanied by the acceleration of mass transfer and
Table 2 Corrosion rate (k) for mild steel as well as the inhibition efficiency (hw) with different concentrations of inhibitors 1 and 2 in 1.0 M HCl at
60 ꢂC, 75 ꢂC, and 90 ꢂ
C
60
75
1
90
1
T (ꢂC)
1
2
2
2
Concentration of
inhibitors
k
k
k
k
k
k (g mꢀ2 hꢀ1) hw (%) (g mꢀ2 hꢀ1) hw (%) (g mꢀ2 hꢀ1) hw (%) (g mꢀ2 hꢀ1) hw (%) (g mꢀ2 hꢀ1) hw (%) (g mꢀ2 hꢀ1) hw (%)
0
8.3824
0.8365
0.5882
0.5299
0.3493
0
8.3824
0
27.8309
1.3971
0.9927
0.7353
0.6802
0
27.8309
6.1949
4.3382
3.2721
1.5441
0
127.6654
9.1177
5.3768
4.1452
3.5846
0
127.6654
34.8796
12.0315
7.7621
0
5.0 ꢁ 10ꢀ5
1.0 ꢁ 10ꢀ4
2.0 ꢁ 10ꢀ4
5.0 ꢁ 10ꢀ4
90.02 0.8640
92.98 0.6802
93.67 0.6250
95.83 0.4963
89.69
91.89
92.54
94.08
94.98
96.43
97.36
97.56
77.74
84.41
88.24
94.45
92.86
95.79
96.75
97.19
72.68
90.58
93.92
94.97
6.4129
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corroded more slowly, in line with the results based on the counterions in the corrosion process is dominant. It is well
electrochemical analysis.
known that the corrosion inhibition exhibited by organic
molecules is through adsorption on the corroding metal. The
adsorption does not always involve direct interactions between
the organic molecules and the metal surface, but in some cases
takes place via already adsorbed anions in the system.50,51
Halide ions have been shown to inhibit the corrosion of some
metals in strong acids. Inhibition performance depends on
ionic size and charge, the electrostatic eld set up by the
negative charge of the anion on the adsorption site and the
nature of the halide. More stabilization of the adsorbed halide
ions by means of interactions with the heterocyclic cation leads
to greater surface coverage protection efficiency. The observa-
tions in the experiments are in line with the well documented
functions of halide ions, accordingly the counterion can display
a cooperative effect in the inhibition process. Moreover, from
the observed differences in the protective action, it can be
concluded that if halide ions are treated as counterions induced
into the inhibitory medium, they can still have a profound
inuence on the adsorption process.53–55
3.4 SEM analysis of the tested materials in the presence or
absence of 1 and 2
The effect of inhibitors 1 and 2 on the surface micro-
morphology of the tested mild steel was explored using SEM
images of the corroded steel surface in the absence or presence
of inhibitors. It can be seen from Fig. 4(a) that the steel surface
before corrosion shows some abrading scratches. Typical SEM
photographs of the surfaces aer immersion in 1.0 M HCl at 90
ꢂC in the absence and presence of 2.0 ꢁ 10ꢀ4 M 1 or 2 for 4 h are
shown in Fig. 4(b–d), respectively. Close examination of the
SEM images reveals that the specimen surface was damaged,
accompanied by an aggressive attack of the corroding medium
on the steel surface. The corrosion scale appears too uneven,
with the corrosion products arranged layer upon layer. Whereas
in the presence of 1 and 2 the steel displayed a smooth surface
with a few small notches, which means that both inhibitors can
inhibit the dissolution of iron, thereby reducing the rate of
corrosion and affording better protection against corrosion.
Less damage to the mild steel surface can be seen with 1 than
with 2, which reveals that 1 facilitates the better protection from
acidic corrosion.
3.5 The effects of temperature and the activation parameters
of 1 and 2 based on weight loss tests
In terms of molecular structure, as the same organic cation
and different inorganic anions, Brꢀ or Clꢀ, can dissociate in
aqueous solution from 1 and 2, the differences in inhibition
behavior should be ascribed to the dissociation of quaternary
ammonium salts or the effect of the counterions. In general, 1H
NMR (Proton Nuclear Magnetic Resonance), a popular method
for determining the structure of organic compounds, can reect
the electronic atmosphere of protons via the interactions
between the nucleus and magnetic eld. In 1H NMR, no
differences were detected between 1 and 2 (Fig. S3†), which
means that both 1 and 2 can dissociate completely in the
aqueous solution. Therefore, the inuence of halide
To further elucidate their inhibition properties, thermodynamic
parameters are very useful for shedding light on the adsorption
behavior of inhibitors. We were interested in exploring the
activation energy of the corrosion process with the tested
inhibitors, which has been accomplished by investigating the
temperature dependence of the corrosion rates. The effect of
temperature on the inhibited acid–metal reaction is highly
complex, because many changes occur on the metal surface,
such as rapid etching, desorption of the inhibitor and the
decomposition and/or rearrangement of the inhibitor itself.
Based on the study on weight loss, the corrosion rates were
found to increase with the temperature for both inhibited and
uninhibited acid solutions while they decreased with an
increase in inhibitor concentration at a given temperature. It is
well known that the temperature dependence effect reects the
activation energy. For the acidic corrosion of steel, the
temperature dependence of the corrosion rate (k) can be
expressed by the Arrhenius equation, where the natural loga-
1
T
56
rithm of k is a linear function with
:
ðꢀEaÞ
ln k ¼
þ ln A
(6)
RT
where k is the corrosion rate, Ea is the activation energy, T is
the absolute temperature and R is the universal gas constant.
Based on gravimetric measurements, Fig. 5(a) and (b) graphi-
1
T
cally represent the regression of ln(k) with an increase in for
mild steel in the absence and presence of different concentra-
tions of the synthesized inhibitors 1 and 2, respectively. The
straight lines indicate the linear relationship between ln(k) and
Fig. 4 SEM images of the steel surface (a) before corrosion, (b) after
immersion in 1.0 M HCl, (c) after immersion in 2.0 ꢁ 10ꢀ4 M 1, and (d)
after immersion in 2.0 ꢁ 10ꢀ4 M 2 at 90 ꢂC for 4 h.
1
. The Ea values at different temperatures were calculated from
T
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concentration of the inhibitor greatly affect the activation
energy for the corrosion process. In contrast, with the addition
of 2, the Ea values initially increase with the concentration of the
inhibitor and then decrease. This can be interpreted as physical
adsorption occurring in the rst stage, at a lower concentra-
tion.60,61 At higher concentrations, the chemisorption of inhib-
itors is the prevailing mechanism, with the decrease in
activation energy, in line with the reported literature: an
unchanged or lowered activation energy is related to the exis-
tence of chemisorption.58,62,63
An alternative formulation of the Arrhenius equation is:64
ꢀ ꢁ
ꢂ
ꢀ
ꢁ
ꢀ
ꢁꢃ
k
R
DS*
DH*
ln
¼ ln
þ
ꢀ
(7)
T
NAh
R
RT
where h is Planck’s constant, NA is Avogadro’s number, R is the
universal gas constant, DH* is the enthalpy of the activation and
ꢀ ꢁ
k
T
1
T
DS* is the entropy of the activation. Plotting ln
against
ꢀ
ꢁ
ꢀDH*
gave straight lines with a slope
and an intercept of
RT
ꢀ
ꢁ
ꢀ
ꢁ
Fig. 5 Arrhenius plots of ln(k) versus 1/T for mild steel in 1.0 M HCl
without and with different concentrations of the inhibitors 1 (a) and 2
(b).
R
DS*
ln
þ
, from which the values of DH* and DS*
NAh
R
(shown in Fig. 6) have been calculated and listed in Table 3.
Upon inspection of these data, it seemed that Ea and DH* varied
in the same fashion.
ꢀEa
the slope, equal to
of each straight line, which are listed in
The presence of inhibitor 1 or 2 does not inuence the
positive values of DH*, indicating that the addition of the
inhibitors cannot affect the corroding nature of the tested
R
Table 3. Generally, in the presence of 1, the activation energy is
lower in the presence of inhibitors than in their absence, and a
more obvious decrease in the activation energy is accompanied materials.65 Nonetheless, the DH* values for the dissolution
by a more efficient inhibitory effect with the concentration of reaction of the tested steel in 1.0 M HCl in the presence of the
the synthesized inhibitors. The decrease in Ea with the inhibitors were generally less than those in the absence of the
concentration of 1 is typical of chemisorption. This was attrib- inhibitors. Simultaneously, in the presence of inhibitors, larger
uted by Hoar and Holliday57 to the slow rate of inhibitor
negative values of the entropy of the activation DS* imply that
the disorder on the surface of the mild steel is decreased.66,67
This might be a result of the adsorption of organic inhibitor
molecules from the acidic solution, which can be regarded as a
adsorption, with a resultant closer approach to equilibrium,
during the experiments at a higher temperature. However, Riggs
and Hurd58 explained that the decrease in the activation energy
of corrosion at higher levels of inhibition arises from a shi in quasi-substitution process between the organic compounds in
the net corrosion reaction from that on the uncovered part on the aqueous phase and water molecules at the electrode
the metal surface to the covered part. Schmid and Huang59 surface.31 In this situation, the adsorption of organic inhibitors
found that organic molecules inhibit both the anodic and was accompanied by the desorption of water molecules from
cathodic partial reactions on the electrode surface and a parallel
reaction takes place on the covered area, but that the reaction
rate on the covered area is substantially less than on the
uncovered area. Generally, one can say that the nature and the
the surface. Compared with inhibitor 1, although the DS* values
also varied from positive to negative with the concentration of 2,
the more slight trend reveals a weaker association between
Table 3 Values of the activation thermodynamic parameters for mild steel in 1.0 M HCl in the absence and presence of different concentrations
of the inhibitors 1 and 2
In the presence of 1
In the presence of 2
Concentration (M)
Ea (kJ molꢀ1
)
DH* (kJ molꢀ1
)
DS* (kJ molꢀ1 Kꢀ1
)
Ea (kJ molꢀ1
)
DH* (kJ molꢀ1
)
DS* (kJ molꢀ1 Kꢀ1
)
0
91.02
79.32
73.53
68.20
77.48
ꢀ88.09
ꢀ76.79
ꢀ70.61
ꢀ65.44
ꢀ74.59
16.16
91.02
123.92
96.58
84.72
85.53
ꢀ88.09
ꢀ121.04
ꢀ93.65
ꢀ81.86
ꢀ82.71
16.16
96.89
13.49
5.0 ꢁ 10ꢀ5
1.0 ꢁ 10ꢀ4
2.0 ꢁ 10ꢀ4
5.0 ꢁ 10ꢀ4
ꢀ37.24
ꢀ59.53
ꢀ75.68
ꢀ51.69
ꢀ22.67
ꢀ23.48
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Fig. 7 Flory–Huggins adsorption isotherms for 1 (a) and 2 (b) in 1.0 M
HCl at 60 ꢂC, 75 ꢂC and 90 ꢂC.
Fig. 6 Arrhenius plots of ln(k/T) versus 1/T for mild steel in 1.0 M HCl
without and with different concentrations of the inhibitors 1 (a) and 2
(b).
with the higher inhibition efficiency, provide evidence of the
stronger binding power of the organic cations on the steel
surface in cooperation with Brꢀ.68
organic molecules and the metal surface, which is in line with
the lower inhibition efficiency.
Furthermore, the adsorption equilibrium constant (Kads) is
related to the standard adsorption free energy (DG0ads), as shown
in the following equation:69
3.6 Thermodynamic parameters of 1 and 2 based on weight
loss tests
ꢀDG0ads ¼ RT ln(55.5Kads
)
(9)
Since the corrosion of the mild steel is inhibited by the
adsorption of organic cations, the adsorption models should be
provided so as to clarify the basic information on the interac-
tions between the inhibitors and the metal surface. The weight
loss data, obtained using the weight loss tests, coꢂrresponding to
different concentrations of 1 or 2 in the 60–90 C range, were
used to study the adsorption behavior of the inhibitors and to
choose the most suitable adsorption isotherm. Attempts were
made to t the experimental data with various isotherms, such
as the Langmuir, Terkin, Frumkin and Flory–Huggins
isotherms. It was found that the Flory–Huggins isotherm
provides the best description of the adsorption behavior of the
tested inhibitors, and has the following form:26
where R is the gas constant, T the absolute temperature, and the
value 55.5 is the concentration of water in the solution. The
DG0ads values are also presented in Table 4. Generally, values of
DG0ads up to ꢀ20 kJ molꢀ1or less negative are consistent with
electrostatic interactions occurring between the charged mole-
cules and the charged metal (physisorption), while those
around ꢀ40 kJ molꢀ1 or more negative are assumed to involve
the sharing or transferral of electrons from the inhibitor
molecules to the metal surface to form a coordinate type of
bond (chemisorption).70 The large negative values of DG0ads in
both 1 and 2 indicate that the organic species can adsorb on the
mild steel surface spontaneously with typical chemisorption,
which ensures that the adsorptive layer is highly stable. In
addition, the more negative values in the presence of 1 than
those in the presence of 2 are also consistent with the results of
the protection efficiency, implying that 1 can form a more stable
protective lm coating on the surface of steel.
ꢀ ꢁ
q
log
¼ logðcKadsÞ þ c logð1 ꢀ qÞ
(8)
c
where c is the concentration of the inhibitor, Kads is the
adsorption equilibrium constant, and q is the surface coverage,
hw
expressed by
. According to the Flory–Huggins model, plots
100
ꢀ ꢁ
q
of log
against log(1 ꢀ q) gave straight lines with a slope of c
3.7 Explanation for the cooperative effect of cations and
mechanism for adsorption
c
and an intercept of log(cKads), as shown in Fig. 7, from which
the values of Kads have been calculated and listed in Table 4. It is As is apparent from the above, the counterions induced into a
generally known that Kads denotes the strength between the corrosion medium can display a clear cooperative effect in the
adsorbate and adsorbent. The higher Kads values of 1, in line inhibition process in the presence of quaternary ammonium
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Table 4 Thermodynamic parameters calculated from the Flory–Huggins adsorption isotherms for 1 and 2 in 1.0 M HCl at 60 ꢂC, 75 ꢂC and 90 ꢂC
In the presence of 1
Kads (Mꢀ1
In the presence of 2
Kads (Mꢀ1
Temperature (ꢂC)
)
DG0ads (kJ molꢀ1
)
)
DG0ads (kJ molꢀ1
)
60
75
90
3.03 ꢁ 106
3.33 ꢁ 107
3.40 ꢁ 106
ꢀ52.44
ꢀ61.83
ꢀ57.51
7.35 ꢁ 106
9.09 ꢁ 104
1.84 ꢁ 105
ꢀ54.89
ꢀ44.66
ꢀ48.71
salts 1 and 2. The co-adsorption mechanism is available for the
interpretation of the behavior of ionic inhibitors. In fact, the co-
adsorption mechanism may constitute either competitive or
cooperative adsorption. In competitive adsorption, the anion
and the inhibitor cation are adsorbed at different sites on the
metal surface. In cooperative adsorption, the anion is adsorbed
on the metal surface and the cation is adsorbed on the anion
layer. Sometimes both competitive and cooperative mecha-
nisms may occur simultaneously. Based on the above results,
cooperative adsorption seems much more dominant. In the
beginning, the positively charged mild steel surface prevents
organic cations from becoming adsorbed onto the metal
surface. However, in the presence of HCl, chloride ions can
undergo the expected specic adsorption, followed by the mild
steel surface becoming charged. Being specically adsorbed,
they create an excess negative charge towards the solution and
favor the electrostatic adsorption of the cations.71 Thus, by
means of electrostatic attraction, organic cations gain access to
the mild steel–solution interface, consequently leading to an
increase in surface coverage and inhibition efficiency. The
subsequent chemical adsorption occurs by the transfer of
electrons from the inhibitor molecules to the unoccupied d-
orbital of iron atoms (chemical adsorption) at the mild steel–
solution interface.66 Clearly, the presence of 2 cannot lead to a
change in the formation of the system but each component.
However, the addition of 1, accompanied by the introduction of
a Brꢀ ion, can cause Clꢀ, initially adsorbed on the anode’s metal
surface, to be displaced by Brꢀ. This is because the attachment
of Brꢀ to the surface of the metal is much more favored, due to
advantages in its degree of hydration and electronegativity.72
Besides, the effect of halide ions depends on their ionic size and
charge, the electrostatic eld set up by the negative charge of
the anion on the adsorption site and the nature and concen-
tration of the halide ions.46,52 The observed trend for the
increase in the protective action is in the order of Brꢀ > Clꢀ,
because electronegativity (Brꢀ ¼ 2.8, Clꢀ ¼ 3.0) results in lower
repulsion between ions, which causes Brꢀ to be more disposed
towards adsorption than chloride ions.73 Being specically
adsorbed, bromide can create an excess negative charge
towards the solution and allow the adsorption of more organic
cations by coulombic attraction, thus leading to a denser lm
and better protection of the steel from exposure to the acidic
medium (Fig. 8). Additionally, the composition of the steel
surface aer corrosion with inhibitors 1 and 2 was investigated
by energy-dispersive X-ray (EDX) spectroscopy. In an EDX
spectrum, each element present gives a unique set of peaks and
the intensities of these peaks can be used to determine the ratio
Fig. 8 The proposed mechanisms for the inhibition behavior of 1 (a)
and 2 (b).
of the elements in the sample. As shown in Fig. S5,† the pres-
ence of C and N conrms that the corrosion inhibitors can cover
the surface of steel as a protective lm to prevent acidic corro-
sion. X-ray photoelectron spectroscopy (XPS) is a surface
sensitive method used to deduce more detailed information
about surfaces, and was also employed to obtain direct infor-
mation on the effects of the counterions. A signal at around
198–202 eV ascribed to Cl 2p1/2 and Cl 2p3/2, shown in Fig. 9,
conrms HCl can result in the chloride ions expected and
specic adsorption on the steel surface both with the addition
Fig. 9 The XPS fine scan spectra showing the Cl 2p and Br 3d regions
of the steel surface with: 2.0 ꢁ 10ꢀ4 M 1 (a) and 2 (b) after immersion at
90 ꢂC for 4 h.
4724 | RSC Adv., 2015, 5, 4716–4726
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of 1 and 2.74 More interestingly, the signal at 69 eV matching the
binding energy of Br 3d3/2 mixed with Br 3d5/2 indicates the
presence of Brꢀ aer the immersion of steel in the HCl solution
containing 1, however, in the same region,74 there were no
signals detected with the addition of 2. Therefore, it can be
interpreted that the counterion Brꢀ in 1 can adsorb on the steel
7 I. B. Obot, N. O. Obi-Egbedi and S. A. Umoren, Corros. Sci.,
2009, 51, 276–282.
8 F. Bentiss, M. Bouanis and B. Mernari, J. Appl. Electrochem.,
2002, 32, 671–678.
9 Q. Qu, S. Jiang and W. Bai, Electrochim. Acta, 2007, 52, 6811–
6820.
surface and display a co-adsorption effect between the coun- 10 S. M. Li, H. R. Zhang and J. H. Liu, Trans. Nonferrous Met.
terions and the organic cations.
Soc. China, 2007, 17, 318–325.
11 S. V. Lamaka, M. L. Zheludkevich and K. A. Yasakau,
Electrochim. Acta, 2007, 52, 7231–7247.
12 G. Achary, H. P. Sachin and Y. A. Naik, Mater. Chem. Phys.,
2008, 107, 44–50.
13 A. Popova, M. Christov and A. Vasilev, Corros. Sci., 2011, 53,
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15 X. Wang, H. Yang and F. Wang, Corros. Sci., 2012, 55, 145–
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16 P. C. Okafar and Y. Zheng, Corros. Sci., 2009, 51, 850–859.
17 S. A. Umoren and E. E. Ebenso, Mater. Chem. Phys., 2007,
106, 387–393.
18 M. Bouklah, B. Hammouti and A. Aouniti, Appl. Surf. Sci.,
2006, 252, 6236–6242.
19 S. A. Umoren, O. Ogbobe and I. O. Igwe, Corros. Sci., 2008, 50,
1998–2006.
20 S. A. Umoren, Y. Li and F. H. Wang, Corros. Sci., 2010, 52,
1777–1786.
21 L. G. Qiu, Y. Wu and Y. M. Wang, Corros. Sci., 2008, 50, 576–
582.
22 E. E. Oguzie, Y. Li and F. H. Wang, Electrochim. Acta, 2007,
52, 6966–6988.
23 E. E. Oguzie, Y. Li and F. H. Wang, J. Colloid Interface Sci.,
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24 C. Jeyaprabha, S. Sathiyanarayanan and G. Venkatachari,
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4 Conclusions
Two kinds of quinoline quaternary ammonium salt, 1 and 2,
have been synthesized and characterized. Several methods have
been employed to evaluate and compare the effectiveness of 1
and 2 under the same conditions. The detected differences in
inhibition behavior highlight the cooperative effect of the
counterions. Several interesting results are summarized as
follows:
(1) According to the results of potentiodynamic polarization,
electrochemical impedance, gravimetric, and SEM studies, both
inhibitors 1 and 2 displayed good corrosion inhibition effects
for mild steel in acidic HCl solution, but a better performance
was observed in the presence of 1.
(2) The higher inhibition efficiency of 1 compared to 2 dis-
played under the same conditions highlights that the coun-
terion can display a synergistic effect in the inhibition process.
(3) The fact that the thermodynamic parameters and acti-
vation parameters are in agreement with the observed inhibi-
tion efficiency provides evidence that the counterions can affect
the interactions between organic cations and the metal surface,
as well as the stability of the protective lm.
(4) The counterions can synergistically improve the inhibi-
tion performance of the inhibitor, in the order of Brꢀ > Clꢀ,
which can be explained by the co-adsorption effect between the
counterions and organic cations.
Acknowledgements
26 A. Khamis, M. M. Saleh and M. I. Awad, Corros. Sci., 2013, 66,
343–349.
27 F. Diaba, C. L. Houerou and M. Grignon-Dubois, J. Org.
Chem., 2000, 65, 907–910.
28 F. M. Moghaddam, Z. Mirjafary and H. Saeidian,
Tetrahedron, 2010, 66, 3678–3681.
The Applied Basic Research Funds (2011D-4603-0102, 2011A-
4208) of China National Petroleum Corporation (CNPC),
National Support Fund (2011BAE25B04) are gratefully
acknowledged.
29 Z. Tao, S. Zhang and W. Li, Corros. Sci., 2009, 51, 2588–2595.
30 X. H. Li, S. D. Deng and G. N. Mu, Corros. Sci., 2008, 50, 420–
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