Indian Journal of Chemical Technology
Vol. 18, May 2011, pp. 244-253
Inhibition efficiency of two bipyrazole derivatives on steel corrosion in
hydrochloric acid media
K Tebbjia, A Aounitia, A Attayibatb, B Hammoutia*, H Ouddac, M Benkaddourd, S Radib & A Nahlee
a
LCAE-URAC18, Faculté des Sciences, Université Mohammed Premier, B.P. 4808, Oujda, Morocco
b
LCOMPN-URAC25, Faculté des Sciences, Université Mohammed Premier, Oujda, Morocco
c
Laboratoire des Procédés de Séparation, Faculté des Sciences, Université Ibn Tofail, Kenitra, Morocco
d
Laboratoire d’Analyse et Caractérisation des Matériaux, Faculté des Sciences, Université Mohammed Premier, Oujda, Morocco
e
Department of Chemistry, College of Sciences, University of Sharjah, Sharjah, PB 27272, United Arab Emirates
Received 4 January 2010; accepted 21 February 2011
The inhibitor effect of two isomers namely 2-(1',5,5'-trimethyl-1H,1'H-3,3'-bipyrazol-1-yl)ethanol (1-TBE) and 2(1',5,5'-trimethyl-1H,2'H-3,3'-bipyrazol-2-yl)ethanol (2-TBE) on the corrosion of mild steel in 1.0 M hydrochloric acid has
been investigated at 308 K using weight loss measurements and electrochemical techniques (impedance spectroscopy and
polarisation curves). Inhibition efficiency is dependent upon the pyrazole structure, with 1-TBE serving as a better inhibitor
than 2-TBE and its inhibition efficiency increases with the increase of concentration of inhibitor to attain 93% in the
presence of 10-3M. Polarisation curves indicate that 1-TBE and 2-TBE act essentially as cathodic inhibitors. Efficiency (E)
percent values obtained by various methods are reasonably good in agreement. EIS measurements show an increase of the
transfer resistance with the inhibitor concentration. The temperature effect on the corrosion behaviour of steel in 1.0 M HCl
without and with the inhibitor at 10-3M is studied in the temperature range 308-333 K, Some thermodynamic parameters
such as adsorption heat (∆H°), adsorption entropy (∆S°) and adsorption free energy (∆G°) have been calculated by
employing thermodynamic equations. Kinetic parameters such as apparent activation energy and pre-exponential factor have
also been calculated. Adsorption of 1-TBE on the mild steel surface in 1.0 M HCl follows the Langmuir isotherm model.
Keywords: Pyrazole, Steel, Hydrochloric acid, Corrosion inhibition, Adsorption
Corrosion problems have received a considerable
amount of attention because of their attack on
materials. The use of inhibitors is one of the most
practical methods for protection against corrosion.
Due to its prominent properties, hydrochloric acid is
widely used in industry, for example, acid pickling,
acid cleaning and acid descaling.
It is well-known that the presence of an organic
molecule in the medium inhibits corrosion of metals
by adsorbing at the metal-solution interface1,2. The
modes of adsorption are dependent on (i) the chemical
structure of the molecule, (ii) the chemical
composition of the solution, (iii) the nature of the
metal surface and (iv) the electrochemical potential of
the metal-solution interface3,4. The most important
aspect of inhibition, normally considered by corrosion
scientists is the relation between the molecular
structure and corrosion inhibition efficiency; such
effects were studied by many researchers5,6.
_____________
*Corresponding author (E-mail: [email protected])
Most of the well-known acid inhibitors are organic
compounds containing nitrogen, sulphur and/or
oxygen atoms. Nitrogen-containing compounds
function more effectively in HCl7,8, whereas sulphurcontaining compounds are sometimes preferred for
H2SO4.
The influence of nitrogen-containing organic
compounds, such as amines, heterocyclic compounds 9,10
and pyrazoles derivatives11-13 on the corrosion of
many metals in acidic solution has been investigated.
The inhibition of acidic corrosion of mild steel by
2,5-bis(n-pyridyl)-1,3,4-thiadiazoles has been studied
previously. In all acidic media, better performances
are seen in the case of 2,5-bis(3-pyridyl)-1,3,4thiadiazole14,15. The isomers of aminophenol inhibit
the corrosion of mild steel in 1.0 M HCl and
accelerate it in 0.5 M H2SO416. The inhibitive action
of two bipyrazolic isomers against the corrosion of
steel in 1.0 M HCl has been investigated17.
This investigation is aimed to study the influence
of the nitrogen-containing heterocyclic compounds,
TEBBJI et al.: INHIBITION EFFICIENCY OF BIPYRAZOLE DERVIATIVES ON STEEL CORROSION
such as: 2-(1',5,5'-trimethyl-1H,1'H-3,3'-bipyrazol-1yl)ethanol (1-TBE) and 2-(1',5,5'-trimethyl-1H,2'H3,3'-bipyrazol-2-yl)ethanol (2-TBE) on the corrosion
inhibition of steel in molar hydrochloric acid solution.
The behaviour of steel in 1.0 M HCl with and without
inhibitor
is
studied
using
gravimetric,
potentiodynamic and EIS measurements.
Experimental Procedure
Synthesis of the bipyrazolic compounds
The organic compounds tested as corrosion
inhibitors 1-TBE and 2-TBE were synthesized
according to the reported procedure18 and characterized
by infra-red, 1H nuclear magnetic resonance (NMR)
spectroscopic methods and microanalysis before use.
The molecular structures are shown in Scheme 1.
Scheme 1Chemical formulae of the compounds used.
Gravimetric, Rp, polarisation and EIS measurements
The aggressive solution (1.0 M HCl) was prepared
by dilution of analytical grade 37% HCl with doubledistilled water. Prior to all measurements, the steel
samples (0.09% P; 0.38% Si; 0.01% Al; 0.05% Mn;
0.21% C; 0.05% S and the remainder iron) were
polished with different emery paper up to 1200 grade,
washed thoroughly with double-distilled water,
degreased with AR grade ethanol, acetone and drying
at room temperature.
Gravimetric measurements were carried out in a
double walled glass cell equipped with a thermostatcooling condenser. The solution volume was 100 mL.
The steel specimens used had a rectangular form
(2.5 cm × 2 cm × 0.05 cm). The immersion time for
245
the weight loss was 6 h at 308 K. After the corrosion
test, the specimens of steel were carefully washed in
double-distilled water, dried and then weighed. The
rinse removed loose segments of the film of the
corroded samples. Duplicate experiments were
performed in each case and the mean value of the
weight loss is reported. Weight loss allowed us to
calculate the mean corrosion rate as expressed in
mg.cm-2 h-1.
Electrochemical measurements were carried out in
a conventional three-electrode electrolysis cylindrical
Pyrex glass cell. The working electrode (WE) in the
form of disc cut from steel has a geometric area of
1 cm2 and is embedded in polytetrafluoroethylene
(PTFE). A saturated calomel electrode (SCE) and a
disc platinum electrode were used respectively as
reference and auxiliary electrodes. The temperature
was thermostatically controlled at 308 ± 1 K. The WE
was abraded with silicon carbide paper (grade P1200),
degreased with AR grade ethanol and acetone, and
rinsed with double-distilled water before use.
Running on an IBM compatible personal computer,
the 352 Soft CorrTM III Software communicates with
EG&G Instruments potentiostat-galvanostat model
263A at a scan rate of 0.5 mV/s. Before recording the
cathodic polarisation curves, the steel electrode is
polarised at -800 mV for 10 min. For anodic curves,
the potential of the electrode is swept from its
corrosion potential after 30 min at free corrosion
potential, to more positive values. The test solution is
deaerated with pure nitrogen. Gas bubbling is
maintained through the experiments.
Near Ecorr, a scan through a potential range
performs polarisation resistance measurements. The
potential range is ±10 mV around Ecorr. The resulting
current is plotted versus potential. Polarisation
resistance (Rp) values are obtained from the current
potential plot. The scan rate was 0.05 mV/s.
The electrochemical impedance spectroscopy (EIS)
measurements were carried out with the
electrochemical system (Tacussel) which included a
digital potentiostat model Voltalab PGZ 100
computer at Ecorr after immersion in solution without
bubbling, the circular surface of steel exposing of 1
cm2 to the solution were used as working electrode.
After the determination of steady-state current at a
given potential, sine wave voltage (10 mV) peak to
peak, at frequencies between 100 kHz and 10 mHz
were superimposed on the rest potential. Computer
programs automatically controlled the measurements
INDIAN J. CHEM. TECHNOL., MAY 2011
246
performed at rest potentials after 30 min of exposure.
The impedance diagrams are given in the Nyquist
representation. Values of Rt and Cdl were obtained from
Nyquist plots.
In the case of polarization method the relation
determines the inhibition efficiency (EI):
EI % =
Results and Discussion
Weight loss tests
Gravimetric measurements of steel subjected to the
effects of 1.0 M HCl in the absence and presence of
various concentrations of isomers (n-TBE) were made
after 6 h of immersion at 35°C. The inhibition efficiency
(Ew, %) was calculated by the following relation:
Ew % =
Wcorr − Wcorr (inh)
× 100
Wcorr
… (1)
where Wcorr and Wcorr (inh) are the corrosion rates of
steel in the absence and presence of the organic
compound, respectively.
The corrosion rates and the inhibition efficiencies
calculated from weight loss measurements for different
concentrations of the two bipyrazolic isomers in 1.0 M
HCl are given in Table 1. As it can be seen from Table
1, inhibition efficiency (Ew) increases with increase in
inhibitor
concentration.
From
weight
loss
measurements, we can conclude that the inhibition
efficiency of 1-TBE is higher than 2-TBE.
Polarisation measurements
Figure 1 shows the potentiokinetic polarisation
curves of steel recorded in 1.0 M HCl in the absence
and presence of 10-3 M of 1-TBE and 2-TBE at 308 K.
In the cathodic domain, it is clear that the current
density decreases with addition of two isomers; this
indicates that these compounds are adsorbed on the
metal surface and hence inhibition occurs. At the
same concentration (10-3 M), the value of Icorr of steel
in the case of 1-TBE is smaller than with that of
2-TBE. The 1-TBE gives more inhibition efficiency
than isomer 2-TBE in 1.0 M HCl; this enhanced
efficiency is due to the position of (-CH2-CH2-OH)
group in the molecule of n-TBE. The polarisation
study confirms the excellent inhibiting character of
1-TBE obtained with weight loss measurements.
Polarization behaviour of steel in molar HCl at
various concentrations of 1-TBE is given in Fig. 2.
The extrapolation method for the polarization curves
was applied and the data for corrosion potential
(Ecorr), corrosion current density (Icorr), cathodic Tafel
slopes (βc) and percentage inhibition efficiency (EI %)
are given in Table 2.
I corr − I corr (inh)
× 100
I corr
… (2)
where Icorr and Icorr (inh) are the corrosion current
density values without and with the inhibitor,
respectively, determined by extrapolation of cathodic
Tafel lines to the corrosion potential.
As shown in Fig. 2 and Table 2, cathodic currentpotential curves give rise to parallel Tafel lines
indicating that the hydrogen evolution reaction is
activation controlled. Thus, the addition of two
isomers does not change the mechanism of the proton
discharge reaction on the metal surface.
The addition of two isomers causes a decrease of
the current density. The values of the corrosion
potential (Ecorr) and cathodic Tafel slop (βc) slightly
change when the inhibitor concentration increases
Table 1Gravimetric results of steel in acid without and with
addition of 1-TBE and 2-TBE
Inhibitor
Blank
1-TBE
2-TBE
Concentration (M) W (mg cm-2 h-1)
1
10-6
10-5
5×10-5
10-4
5×10-4
10-3
10-6
10-5
5×10-5
10-4
5×10-4
10-3
1.06
0.904
0.543
0.295
0.239
0.106
0.079
1.046
0.929
0.694
0.523
0.233
0.198
Ew %
15
49
72
77
90
93
1
12
35
51
78
81
Fig. 1Polarization curves of steel in 1 M HCl without and with
10-3M of 1-TBE and 2-TBE
TEBBJI et al.: INHIBITION EFFICIENCY OF BIPYRAZOLE DERVIATIVES ON STEEL CORROSION
247
Stern and Geary19 formulated the following
equation for corrosion current calculation:
I
corr
=
B
Rp
... (3)
where the constant B is
B =
Fig. 2Polarization curves of steel in 1.0 M HCl containing
various concentrations of 1-TBE
Table 2Electrochemical parameters of steel at various
concentrations of 1-TBE and 2-TBE studied in 1.0M HCl.
Corresponding corrosion inhibition efficiencies
Inhibitor Concentra- Ecorr
Icorr
βc
tion M mV vs mV dec-1 µA
SCE
cm-2
EI RP
(%) Ω cm2
ERp
%
Blank
11
46
69
71
85
89
3
8
34
44
75
78
12
48
66
73
82
91
1
11
31
45
71
77
1-TBE
2-TBE
1
10-6
10-5
5×10-5
10-4
5×10-4
10-3
10-6
10-5
5×10-5
10-4
5×10-4
10-3
-445
-446
-448
-451
-453
-457
-460
-440
-444
-445
-445
-454
-463
153
152
155
156
150
151
148
150
147
155
159
147
145
305
270
164
95
88
47
35
295
281
201
171
75
67
ba bc
2.303 (ba + bc)
… (4)
and Rp is the polarisation resistance determined
according to Eq. (5)20 from the slop of the polarisation
curve and ba and bc are Tafel slopes.
Rp =
S dE
di
… (5)
where S is the surface area of the electrode.
68
77
130
202
254
375
735
69
76
99
124
233
292
(Table 2). These results demonstrate that the
hydrogen reduction is inhibited and that the inhibition
efficiency increases with inhibitor concentration to
attain 89% and 78% at 10-3 M of 1-TBE and 2-TBE,
respectively.
The anodic curves indicate that the inhibition mode
of organic compounds depended upon electrode
potential (Figs 1 and 2). It seems that the presence of
the two inhibitors does not change the current versus
potential characteristics. These results indicated that
the two isomers tested act essentially as a cathodic
inhibitors.
The corresponding polarisation resistance (RP)
values of steel in 1.0 M HCl, in the absence and
presence of different concentrations of two inhibitors
(n-TBE), are also given in Table 2. RP was determined
by the slope of the potential versus current lines.
We remark that RP increases with increasing
inhibitor concentration. This in turn leads to a
decrease in Icorr values. The inhibition efficiency ERp
(%) is calculated as follows:
ER p % =
R'p − Rp
× 100
R'p
… (6)
RP and R'P are the polarisation resistance in absence
and in presence of the inhibitor, respectively.
From Table 2, we notice that the ERp (%) increased
with inhibitor concentration reaching maximum values
of 91% and 77% at 10-3 M for 1-TBE and 2-TBE,
respectively. This is in reasonably good agreement
with the value of inhibitor efficiency obtained from
weight loss measurements.
Effect of 1-TBE on the corrosion rate of steel at various
temperatures
To compare the anti-corrosion effectiveness of 1TBE, the concentration range of 1-TBE was varied
from 10-6 M to 10-3 M in 1.0 M HCl at different
temperatures. The immersion time for the weight loss
was 6 h. For practical purpose it is important to know
the ideal concentration levels at which minimal
corrosion occurs. The curves showing the values of
corrosion rate (W) and the concentrations (C) of 1TBE at different temperatures were drawn (Fig. 3).
The value of corrosion rate was calculated from the
following equation:
INDIAN J. CHEM. TECHNOL., MAY 2011
248
W =
m1 − m2
St
… (7)
where (m1) is the mass of the specimen before
corrosion, (m2) the mass of the specimen after
corrosion, (S) the area of the rectangular steel and (t)
the corrosion time.
Figure 3 indicates that the corrosion rate of steel
increased with increasing temperature. The results
also indicates that for a given temperature, the
corrosion rate of steel decreased with increasing
inhibitor concentration, this reduction is marked
between the following concentrations 10-6 M and
5×10-4 M, but when inhibitor concentration was above
5×10-4 M, the effect of inhibitor concentration on the
corrosion of steel was small, so the most suitable
range of inhibitor concentration was 5×10-4 to 10-3 M.
The values of inhibition efficiencies obtained from
the weight loss for different inhibitor concentrations
and at various temperatures in 1.0 M HCl are given in
Table 3 and Fig. 4. It is clear that inhibition efficiency
increased with increase in inhibitor concentration. The
maximum value of inhibition efficiency (Ew) obtained
for the inhibitor 1-TBE (10-3 M) is about 93% at 308 K.
The results showed that the inhibition efficiencies
decreased with the experimental temperature, which
indicated that the higher temperatures might cause
desorption of 1-TBE from the steel surface.
Table 3Inhibition efficiencies obtained from the corrosion rate
for different concentrations of 1-TBE in 1.0 M HCl at different
temperatures
Inhibition efficiency (Ew %)
Inhibitor
1-TBE, M
308 K
313 K
323 K
333 K
10-6
15
12
5
1
10-5
49
41
25
9
5×10-5
72
67
50
35
10-4
77
73
63
52
5×10-4
90
89
82
77
10-3
93
90
84
78
Table 4Some parameters of the linear regression between
ln (W) (corrosion rate) and 1/T
Concentration,
M
1-TBE
Pre-exponential
factor mg cm-2 h-1
Ea
kJ mol-1
Linear
regression
coefficient
Blank
10-6
10-5
5×10-5
10-4
5×10-4
10-3
1.2676 × 109
7.9289 × 109
1.2799 × 1012
3.6242 × 1012
7.8645 × 1012
1.0865 × 1013
1.1993 × 1014
53.564
58.662
72.969
77.036
79.737
82.687
89.445
0.9999
0.9999
0.9999
0.9998
0.9999
0.9995
0.9991
Apparent activation energy (Ea) and pre-exponential factor (A)
The adsorption process was well elucidated by
using a thermodynamic model; a kinetic model was
another useful tool to explain the mechanism of
corrosion inhibition for the inhibitor. The logarithm of
the corrosion rate (W) could be represented as a
straight line function of 1/T of steel in acid medium:
Fig. 3The relationship between corrosion rate and inhibitor
concentration of 1-TBE
Ea
... (8)
+ ln A
RT
where Ea represents the apparent activation energy, R
the gas constant, T the absolute temperature, A the
pre-exponential factor and W the corrosion rate.
The regression between ln W and 1/T was
calculated by computer, and Arrhenius plots of ln W
versus 1/T for the blank and different concentrations
of the 1-TBE isomer are shown in Fig. 5. All the
parameters were calculated and given in Table 4.
Some studies21-23 showed that compared with the
apparent activation energy in the absence of inhibitor,
higher values for Ea were found in the presence of
Fig. 4The relationship between inhibition efficiency (Ew %) and
inhibitor concentration of 1-TBE in 1.0 M HCl
ln (W) = −
TEBBJI et al.: INHIBITION EFFICIENCY OF BIPYRAZOLE DERVIATIVES ON STEEL CORROSION
249
Fig. 6The relationship between Ea and inhibitor concentration
of 1-TBE
Fig. 5Arrhenius plots related to the corrosion rate of steel for
various concentrations of 1-TBE in 1.0 M HCl
inhibitors. Other studies24,25 showed that in the
presence of inhibitor the apparent activation energy
was lower than that in the absence of inhibitor.
It was clear that (Table 4) the values of Ea in the
presence of the 1-TBE are higher than those in the
uninhibited acid solution (53.564 kJmol−1).
The decrease of inhibition efficiencies with
increase in experimental temperature and the increase
of Ea in the presence of the inhibitor indicates the
physical adsorption mechanism26,27.
In the present study, however, it could be found
that the apparent activation energy increasing in the
domain (range) of concentration of 1-TBE which is
varied between (0 to 5×10-5 M), then, decreased with
the increase in concentration of 1-TBE from 5×10-5 to
the 10-4 M, but all values of Ea in these range were
higher than that in the absence of 1-TBE, but when
inhibitor concentration of 1-TBE was above 10-4 M,
the apparent activation energy increases again. The
obtained results were similar to the earlier reported
results10.
As far as pre-exponential factor is concerned, with
an increase in concentration of 1-TBE, at first it
increases too, then it decreases and at a relative higher
concentration of 1-TBE (10-4 to10-3 M) one notes an
increase again pre-exponential factor. The variation in
pre-exponential factor, as a whole, is just like that of
the activation energy. From Eq. (8), it can be seen that
at a certain temperature, the value of the steel
corrosion rate is jointly decided by the apparent
activation energy and pre-exponential factor. As a
whole, the steel corrosion rate basically decreases
with an increase in concentration of 1-TBE.
For the present study, the values of A in the
presence of 1-TBE are higher than those of in the
absence of 1-TBE. Therefore, the decrease in the steel
corrosion rate is mostly decided by the apparent
activation energy. As can be seen from Table 4, it was
clear that both Ea and A increased in the presence of
1-TBE. Figure 6 clearly shows that there is a ‘‘peaklike’’ value for Ea in this study. That is to say, the
apparent activation energy acts as a function of
inhibitor concentration28.
Adsorption isotherm
Assuming the corrosion inhibition was caused by
the adsorption of 1-TBE, and the values of surface
coverage (θ) for different concentrations of 1-TBE in
1.0 M HCl were evaluated from weight loss
measurements using the Sekine and Hirakawa’s
method29:
θ=
Wo − W
Wo − Wm
... (9)
where Wm is the smallest corrosion rate.
In order to get a better understanding of the
electrochemical process on the metal surface,
adsorption characteristics were also studied for 1-TBE
compound at 303 K. This process is closely related to
the adsorption of the inhibitor molecules30,31 and
adsorption is known to depend on the chemical
structure32-34.
Adsorption isotherms are very important in
determining the mechanism of organic electrochemical
reactions. The most frequently used adsorption
isotherms are Langmuir, Temkin and Frumkin with the
general formula:
f (θ, x) exp(−2aθ) = KC
… (10)
INDIAN J. CHEM. TECHNOL., MAY 2011
250
In hydrochloric acid solution, the organic compound
follows the Langmuir adsorption isotherm. This is as
follows:
θ
… (11)
= KC
1- θ
between ln K and 1/T is shown in Fig. 8. Under the
experimental conditions, the adsorption heat could be
approximately regarded as the standard adsorption heat
(∆H°)35. The standard adsorption free energy (∆G°)
was obtained according to the following equation:
K=
Rearranging this equation gives:
C
θ
=
1
K
… (12)
+C
where C is the concentration of inhibitor, K is the
adsorptive equilibrium constant, and θ is the surface
coverage.
From the values of surface coverage, the linear
regressions between C/θ and C were calculated by the
computer, and the parameters (adsorption coefficients,
slopes, and linear correlation coefficients) are listed in
Table 5. Fig. 7 shows the relationship between C/θ and
C at various temperatures. These results show that all
the linear correlation coefficients (r) are almost equal to
1.000 and all the slopes are very close to 1.00, which
indicates the adsorption of inhibitor onto steel surface
accords with the Langmuir adsorption isotherm.
In addition, it could be found (Table 5) that the
adsorption coefficient (K) decreased with increasing
temperature. It is well known that K designates the
adsorption power of inhibitor onto the steel surface;
clearly, 1-TBE gave high values of K at lower
temperatures, indicating that it was adsorbed strongly
onto the steel surface. Thus, the inhibition efficiency
decreased with the increase in temperature as the result
of the improvement for the desorption of 1-TBE from
the steel surface.
1
∆G °
exp(−
)
55.5
RT
… (14)
According to the thermodynamic basic equation
∆G° = ∆H° −T∆S°, the standard adsorption entropy
Table 5Parameters of the linear regression between C/θ and
C of 1-TBE
Temperature, K Linear regression
coefficient (r)
308
313
323
333
0.9999
0.9999
0.9998
0.9995
K
Slope
71915.03
50145.67
31046.55
12026.86
1.07
1.09
1.15
1.17
Fig. 7The relationship between C/θ and C of 1-TBE
Thermodynamic parameters
The corrosion inhibition of 1-TBE isomer for steel
could be well explained by using thermodynamic model,
so, the adsorption heat, the adsorption free energy, and
the adsorption entropy were calculated to elucidate the
phenomenon for the inhibition action of 1-TBE.
According to the Van’t Hoff equation
ln ( K) = −
∆H°
RT
+ Constant
… (13)
where ∆H° and K are the adsorption heat and
adsorptive equilibrium constant, respectively.
To obtain the adsorption heat, the linear regression
between lnK and 1/T was dealt with, the relationship
Fig. 8The relationship between ln (K) and 1/T
TEBBJI et al.: INHIBITION EFFICIENCY OF BIPYRAZOLE DERVIATIVES ON STEEL CORROSION
∆S° could be calculated. All the calculated
thermodynamic parameters are listed in Table 6.
As for ∆G°, the negative values of ∆G° suggested
that the adsorption of 1-TBE onto the steel surface
was a spontaneous process. Furthermore, it could be
found that ∆G° became less negative with increase in
temperature (it was difficult for 1-TBE to adsorb onto
the steel surface), this phenomenon once again
indicated that the adsorption was unfavourable with
increasing experimental temperature as the result of the
desorption of inhibitor from the steel surface.
The negative values of ∆H° also show that the
adsorption of inhibitor is an exothermic process36, which
indicates that IEs decrease, with increasing the
temperature. Generally, an exothermic process signifies
either physisorption or chemisorption while endothermic
process is attributable unequivocally to chemisorption37.
In an exothermic process, physisorption is distinguished
from chemisorption by considering the absolute value of
a physisorption process is lower than 41.86 kJ mol−1
while the adsorption heat of a chemisorption process38
approaches 100 kJ mol−1. In the present case; the
standard adsorption heat -58.85 kJ mol-1 in Table 6
shows that a comprehensive adsorption (physical and
chemical adsorption) might occur.
The negative values of ∆S° might be explained in
the following way: before the adsorption of inhibitors
onto the steel surface, inhibitor molecules might freely
move in the bulk solution (the inhibitor molecules were
chaotic), but with the progress in the adsorption,
inhibitor molecules were orderly adsorbed onto the
steel surface, as a result, a decrease in entropy.
In order to confirming the result of the standard
adsorption heat (∆H° = 58.85 kJmol-1) found starting
from to the Van’t Hoff equation (13), we made call
also for its calculation the Gibbs-Helmholtz equation,
which is defined as follows:
∆H
 ∂( ∆G ° / T) 
=− 2


∂T

P
T
∆G°
∆H °
=
+A
T
T
251
… (16)
The variation of ∆G°/T with 1/T gives a straight line
with a slope that equals ∆H° (Fig. 9). It can be seen
from the figure that ∆G°/T decreases with 1/T in a
linear fashion. The values of ∆H° is negative (∆H° = 58.79 kJmol-1), reflecting the exothermic behaviour of
adsorption on the steel surface.
The value of the enthalpy of adsorption calculated
by the two methods such as Van’t Hoff and GibbsHelmholtz are in good agreement.
Electrochemical impedance spectroscopy (EIS)
The corrosion behaviour of steel, in acidic solution in
the absence and presence of two isomers was
investigated by the electrochemical impedance
spectroscopy (EIS) at 308 K after 30 min of immersion.
Impedance diagrams are obtained for frequency range
100 kHz - 10 mHz at the open circuit potential for steel
in 1.0 M HCl in the presence and absence of n-TBE. The
locus of the Nyquist plots was regarded as one part of a
semicircle. Nyquist plots of steel in inhibited and
uninhibited acidic solution containing various
concentrations of 1-TBE are shown in Fig. 10. The
O
… (15)
Fig. 9Relationship between ∆G°/T and the reverse of absolute
temperature
This equation can be arranged to give the following
equation.
Table 6The thermodynamic parameters of adsorption of 1-TBE
on the steel surface
Temperature, K ∆G°, kJ mol-1
308
313
323
333
-38.95
-38.64
-38.59
-37.16
∆H°, kJ mol-1 ∆S°, kJ mol-1 K-1
-58.85
-58.85
-58.85
-58.85
-64.61
-64.57
-62.72
-65.14
Fig. 10Nyquist diagrams for steel in 1M HCl containing
different concentrations of 1-TBE
INDIAN J. CHEM. TECHNOL., MAY 2011
252
Table 7Characteristic parameters evaluated from the impedance
diagram for steel in 1.0 M HCl at various concentrations of 1-TBE
and 2-TBE
Inhibitor
Blank
1-TBE
2-TBE
Conc. (M) Rt, Ω cm2 fmax, Hz
1
10-5
5×10-5
10-4
5×10-4
10-3
10-5
5×10-5
10-4
5×10-4
10-3
115
205
367
395
733
1107
131
171
222
432
523
15.82
12.5
7.937
7.937
5
4
15.83
12.5
11.161
7.143
7.143
Cdl, µF
cm-2
ERt, %
87.51
62.14
54.67
50.79
43.48
35.96
76.82
74.50
64.27
51.60
42.11
44
69
71
84
90
12
33
48
73
78
1
2π Cdl Rt
… (17)
EIS parameters were estimated as described
elsewhere40.
The percentage inhibition efficiency got from the
charge-transfer resistance is calculated by the following
relation:
E Rt % =
-1
-1
R t corr − R t corr(inh)
-1
R t corr
H3C
CH3
5
3
impedance diagrams obtained are not perfect semicircles
and this difference has been attributed to frequency
dispersion20.
The charge transfer resistance, Rt values are
calculated from the difference in impedance at lower and
higher frequencies, as suggested by Tsuru and
Haruyama39. To obtain the double capacitance (Cdl), the
frequency at which the imaginary component of the
impedance is maximum (-Zmax) is found and Cdl values
are obtained from the equation:
f (-Zmax) =
this group in 2-TBE provokes a steric gene to form a
Fe2+-coordinate. The 1-TBE appears to be good inhibitor
with a maximum efficiency of 90% at 10-3M.
N
H3C 1 N2
3
5
N
N 1
2
Fe2+
HO
Fe2+-1-TBE
Scheme 2Chemical formulae of the possible complex Fe2+-1-TBE
Conclusions
From this study, the following conclusions are
drawn:
(i) The two isomers inhibit the corrosion of steel in
1.0 M HCl, the better performance is seen in the
case of 1-TBE. The inhibition efficiency values
increase with the inhibitor concentration, but
decrease with the temperature.
(ii) Both inhibitors act as cathodic-type inhibitors.
(iii) The adsorption of 1-TBE on the steel surface from
1.0 M HCl obeys a Langmuir adsorption isotherm.
The adsorption process is a spontaneous and
exothermic process accompanied by a decrease in
entropy.
(iv) The inhibition efficiency of 1-TBE is temperaturedependent and the adsorption equilibrium constant
(K) decreased with increasing temperature
(v) The efficiencies obtained from weight loss,
potentiodynamic polarization and impedance
spectroscopy are in good agreement.
References
× 100
… (18)
where Rtcorr and Rtcorr(inh) are the charge transfer resistance
values without and with inhibitor, respectively. Rt is the
diameter of the loop.
The impedance parameters derived from these
investigations are given in Table 7. It is found that, as
the n-TBE inhibitors concentration increases, the Rt
values increase (the increase in 1-TBE being higher), but
the Cdl values tend to decrease. The decrease in the Cdl
values is due to the adsorption of inhibitor on the metal
surface. The difference in inhibition efficiency of n-TBE
may be attributed to the position of (-CH2-CH2-OH)
group in molecules tested. Furthermore, the presence of
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