Inhibition of mild steel corrosion in 5 % HCl solution by 5-(2-hydroxyphenyl)-1,2,4-triazole-3-thione

Article abstract of DOI:10.1007/s11164-012-0797-1

A new corrosion inhibitor, namely 5-(2-hydroxyphenyl)-1,2,4-triazole-3- thione (5-HTT), has been synthesized and its influence on corrosion inhibition of mild steel in 5 % HCl solution has been studied using weight loss method and electrochemical measurements. Potentiodynamic polarization measurements clearly reveal that the investigated inhibitor is of mixed type, and it inhibits the corrosion of the steel by blocking the active site of the metal. Changes in impedance parameters were indicative of adsorption of 5-HTT on the metal surface, leading to the formation of protective films. The degree of the surface coverage of the adsorbed inhibitors was determined by weight loss measurements, and it was found that the adsorption of these inhibitors on the mild steel surface obeys the Langmuir adsorption isotherm. The effect of the temperature on the corrosion behavior with addition of 5 × 10^-4 M of the inhibitor was studied in the temperature range 30-60 °C. The reactivity of this compound was analyzed through theoretical calculations based on density functional theory to explain the different efficiency of these compounds as a corrosion inhibitor.

Full text of DOI:10.1007/s11164-012-0797-1

Res Chem Intermed
DOI 10.1007/s11164-012-0797-1
Inhibition of mild steel corrosion in 5 % HCl solution
by 5-(2-hydroxyphenyl)-1,2,4-triazole-3-thione
•
H. B. Ouici O. Benali Y. Harek L. Larabi
•
•
•
•
B. Hammouti A. Guendouzi
Received: 22 June 2012 / Accepted: 29 August 2012
ꢀ Springer Science+Business Media B.V. 2012
Abstract A new corrosion inhibitor, namely 5-(2-hydroxyphenyl)-1,2,4-triazole-3-
thione (5-HTT), has been synthesized and its influence on corrosion inhibition of mild
steel in 5 % HCl solution has been studied using weight loss method and electro-
chemical measurements. Potentiodynamic polarization measurements clearly reveal
that the investigated inhibitor is of mixed type, and it inhibits the corrosion of the steel
by blocking the active site of the metal. Changes in impedance parameters were
indicative of adsorption of 5-HTT on the metal surface, leading to the formation of
protective films. The degree of the surface coverage of the adsorbed inhibitors was
determined by weight loss measurements, and it was found that the adsorption of these
inhibitors on the mild steel surface obeys the Langmuir adsorption isotherm. The
effect of the temperature on the corrosion behavior with addition of 5 9 10-4 M of
the inhibitor was studied in the temperature range 30–60 ꢁC. The reactivity of this
compound was analyzed through theoretical calculations based on density functional
theory to explain the different efficiency of these compounds as a corrosion inhibitor.
H. B. Ouici ꢀ A. Guendouzi
´ ´ ´
Departement de Chimie, Faculte des Sciences et de la Technologie, Universite Tahar Moulay,
¨
Saıda, Algeria
O. Benali (&)
´
´ ´
Departement de Biologie, Faculte des Sciences et de la Technologie, Universite Tahar Moulay,
¨
Saıda, Algeria
e-mail: benaliomar@hotmail.com
Y. Harek ꢀ L. Larabi
´ ´ ´
¨
Departement de Chimie, Faculte des Sciences, Universite Abou Bakr Belkaıd, Tlemcen, Algeria
B. Hammouti
Laboratoire de Chimie Appliquee et Environnement, Faculte des Sciences, Universite Mohamed
Premier, Oujda, Morocco
´
´
´
123

H. B. Ouici et al.
Keywords Mild steel ꢀ 5-(2-Hydroxyphenyl)-1,2,4-triazole-3-thione (5-HTT) ꢀ
Inhibition corrosion ꢀ 5 % HCl ꢀ Adsorption
Introduction
Acid solutions are widely used for industrial cleaning, oil well acidification, and
pickling [1]. The corrosion of steel in such environments and its inhibition
constitute a complex problem of process. Generally, using organic inhibitors to
mitigate corrosion of mild steel in acidic media is highly cost effective [2, 3].
Recently, the inhibition of steel in acid solutions by different types of organic
inhibitors has been extensively studied [4–10]. A perusal of the literature on acid
corrosion inhibitors reveals that most organic substances employed as corrosion
inhibitors can adsorb on the metal surface through heteroatoms, such as nitrogen,
oxygen, sulfur, and phosphorus, multiple bonds or aromatic rings and block the
active sites decreasing the corrosion rate [11–13].
Moreover, nitrogen-containing heterocyclic substances such as triazole-type
compounds are considered to be effective corrosion inhibitors in acidic media [14–
19]. In view of the excellent performance of triazole-type organic compounds, the
present authors have synthesized a triazole-based inhibitor bearing phenyl, mercapto
and hydroxyl groups in the same molecule, with the objective of evaluating its
corrosion inhibition properties on mild steel in 5 % HCl solution. The thermody-
namic parameters for the inhibiting process of the synthesized triazole-type inhibitor
were calculated and discussed.
Experimental
Materials
Mild steel, composed of (wt%): C B 0.1 %, Si B 0.03 %, Mn B 0.2 %, P B 0.02 %,
Cr B 0.05 %, Ni B 0.05 %, Al B 0.03 %, and the remainder iron, was used as the
working electrode for all studies.
The acid solutions were made from AR grade HCl. Appropriate concentration of
acid was prepared by using double-distilled water.
Inhibitor
1,2,4-triazole derivative has been synthesized in the laboratory as shown in Fig. 1 via
aromatic carboxylic acid hydrazide intermediate [20, 21]. The aromatic carboxylic
acid is first esterified in the presence of concentrated H2SO4 and absolute ethanol. The
corresponding acid ester is then treated with 80 % hydrazine hydrate in the presence of
absolute ethanol as solvent that resulted in the formation of corresponding carboxylic
acid hydrazide in equimolar proportions. The resulting hydrazide is treated with potas-
sium thiocyanate in acidic medium which resulted in the formation of thiosemicar-
bazide and the yields were quantitative. Cyclization of the thiosemicarbazide in the
123

Inhibition of mild steel corrosion
O
O
O
H
N
N
OMe
NH2
N
NH2
NH2-NH2
EtOH
KSCN
N
H
N
H
NaOH
SH
N
H
OH
S
OH
OH
H2O, HCl
OH
Fig. 1 Scheme of synthesis of 5-(2-hydroxyphenyl)-1,2.4-triazole-3-thione (5-HTT)
presence of NaOH resulted in the formation of the corresponding carboxylic acid-
1,2,4-triazole-3-thiol derivative [22]. The obtained compound was purified and
analyzed by IR and NMR spectroscopies before use.
Weight loss measurements
For the weight loss measurements, the experiments were carried out in solution of
5 % HCl acid (uninhibited and inhibited) on mild steel. Sheets with dimensions
15 mm 9 11.8 mm 9 1.8 mm were used. They were polished successively with
different grades of emery paper up 1,000 grade. Each run was carried out in a glass
vessel containing 100 ml of the test solution. A clean weight mild steel sample was
completely immersed at an inclined position in the vessel. After 1 h of immersion in
5 % HCl with and without addition of inhibitor at different concentrations, the
specimen was withdrawn, rinsed with distilled water, washed with ethanol, dried,
and weighed. The weight loss was used to calculate the corrosion rate in milligrams
per square centimeter per hour.
Electrochemical measurements
Electrochemical experiments were carried out in a glass cell (CEC/TH-Radiometer)
with a capacity of 500 ml. A platinum electrode and a saturated calomel electrode
(SCE) were used as a counter electrode and a reference electrode. The working
electrode was in the form of a disk cut from mild steel under investigation and was
embedded in a Teflon rod with an exposed area of 0.5 cm2.
Electrochemical impedance spectroscopy (EIS), potentiodynamic and linear
polarization were conducted in an electrochemical measurement system (VoltaLab40)
which comprises a PGZ 301 potentiostat, a personal computer, and VoltaMaster 4 and
Zview software.
The potentiodynamic current–potential curves were recorded by changing the
electrode potential automatically from -750 to -350 mV with a scanning rate of
0.5 mV s-1. The polarization resistance (Rp) measurements were performed by
applying a controlled potential scan over a small range, typically 20 mV with
respect to Ecorr. The resulting current is linearly plotted versus potential, the slope of
this plot at Ecorr being the Rp. All experiments were carried out in freshly prepared
solution at constant temperatures, 30, 30, 40, and 60 0.1 ꢁC using a thermostat.
The a.c. impedance measurements were performed at corrosion potentials (Ecorr
)
over a frequency range of 10 kHz–40 mHz, with a signal amplitude perturbation of
10 mV. Nyquist plots were obtained.
123

H. B. Ouici et al.
Theoretical calculations
All calculations were done by GAUSSIAN 09W software [23], using the B3LYP
functional [24] and a 6-31G* basis set [24]. The B3LYP, a version of DFT method,
uses Becke’s three-parameter functional (B3) and includes a mixture of HF with
DFT exchange terms associated with the gradient corrected correlation functional of
Lee, Yang, and Parr (LYP) [24].
Results and discussion
Weight loss measurements
Values of the inhibition efficiency and corrosion rate obtained from the weight loss
measurements of mild steel for different concentrations of 5-HTT in 5 % HCl at
30 ꢁC after 1 h of immersion are given in Table 1. The inhibition efficiency is
defined as follows:
Wcorr ꢁ Wc0orr
P % ¼
ꢂ 100
ð1Þ
Wcorr
where Wcorr and Wc0orr are the corrosion rates of steel due to the dissolution in 5 % HCl
in the absence and the presence of definite concentration of inhibitor, respectively.
It is obvious from Table 1 that the 5-HTT inhibits the corrosion of mild steel in
5 % HCl solution at all concentrations used in this study, and that the corrosion rate
(W) is seen to decrease continuously with increasing additive concentration at
30 ꢁC. Indeed, corrosion rate values of mild steel decrease when the inhibitor
concentration increases while P (%) values of 5-HTT increase with the increase of
the concentration; the maximum P (%) of 97 % is achieved at 5.10-4 M. The
inhibition of corrosion of mild steel by 5-HTT can be explained in terms of
adsorption on the metal surface. This compound can be adsorbed on the metal
surface by the interaction between lone pairs of electrons of nitrogen and sulfur
atoms of the inhibitor and the metal surface. This process is facilitated by the
presence of vacant orbitals of low energy in iron atom, as observed in the transition
group metals [4, 10].
Table 1 Corrosion rate of mild
Conc.
(M)
Wcorr
(mg cm-2 h-1
P
(%)
steel and inhibition efficiency
for different concentrations of
5-HTT for the corrosion of mild
steel in 5 % HCl
)
Blank
5.50
2.03
0.75
0.39
0.23
0.18
–
5-HTT
1 9 10-4
2 9 10-4
3 9 10-4
4 9 10-4
5 9 10-4
63.10
86.36
92.90
95.82
96.73
123

Inhibition of mild steel corrosion
Polarization measurements
Potentiodynamic anodic and cathodic polarization scans were carried out at 30
0.1 ꢁC in 5 % HCl with different concentrations of 5-HTT. Anodic and cathodic
polarization curves in the absence and in the presence of inhibitors at different
concentrations after 1 h of immersion and at 30 ꢁC are shown in Fig. 2.
Table 2 gives the values of kinetic corrosion parameters as the corrosion
potential Ecorr, corrosion current density Icorr, Tafel slope bc, and inhibition
efficiency for the corrosion of mild steel in 5 % HCl with different concentrations of
5-HTT.
The corrosion current densities were estimated by Tafel extrapolation of the
cathodic curves to the open circuit corrosion potential. The inhibition efficiency was
then calculated using the expression:
Ic0orr ꢁ Icorr
P % ¼
ꢂ 100
ð2Þ
Ic0orr
where Ic0orr is the corrosion current density in uninhibited acid and Icorr is the cor-
rosion current density in inhibited acid.
Table 2 shows that an increase in inhibitor concentration is resulted in increased
inhibition efficiency. It is evident from the results that:
•
•
•
Icorr values decrease considerably in the presence of inhibitor. Thus, P (%)
increases with inhibitor concentration, reaching the values 96.02 at 5 9 10-4 M.
The values of bc and Ecorr remained almost unchanged on increasing the
concentration of 5-HTT.
In the anodic range (Fig. 1), 5-HTT influenced the anodic reactions for an
overvoltage higher than Ecorr This result indicated that 5-HTT exhibits both
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
Blank
5-HTT 10-4M
5-HTT 2.10-4M
5-HTT 3.10-4M
5-HTT 4.10-4M
5-HTT 5.10-4M
-700
-600
-500
-400
-300
E (mV vs CSE)
Fig. 2 Potentiodynamic polarization curves for mild steel in 5 % HCl containing different concentrations
of 5-HTT at 30 ꢁC
123

H. B. Ouici et al.
Table 2 Electrochemical parameters and the corresponding inhibition efficiencies for the corrosion of
mild steel in 5 % HCl containing different concentrations of 5-HTT at 30 ꢁC
Conc.
(M)
Ecorr (mV/
SCE)
Icorr
(mA cm-2
bc (mV dec-1
)
Rp
(X cm2)
PI (%)
PR (%)
)
Blank
5HTT 10-4
2.10-4
-450
-464
-480
-468
-478
-469
6.28
0.75
0.45
0.40
0.36
0.25
180
153
168
178
156
172
4.08
14.52
40.05
43.50
49.20
72.05
–
–
88.06 71.90
92.83 89.81
93.63 90.62
94.27 91.70
96.02 94.33
3.10-4
4.10-4
5.10-4
Table 3 Impedance parameters and inhibition efficiency for the corrosion of mild steel in 5 % HCl
containing different of different concentrations of 5-HTT at 30 ꢁC
Conc. (Mol/l)
Q (sn X-1 cm-2
)
n
Rt (X cm2)
Cdl (l F cm-2
)
P (%)
Blank
10-4
2 9 10-4
3 9 10-4
4 9 10-4
5 9 10-4
4.2 9 10-4
2.6 9 10-4
2.3 9 10-4
2.7 9 10-4
2.2 9 10-4
1.8 9 10-4
0.83
0.81
0.81
0.75
0.77
0.78
3.60
12.86
39.4
111.06
68.27
73.30
61.19
56.12
51.43
–
72.00
90.86
91.65
92.33
94.50
43.11
46.93
65.46
cathodic and anodic inhibition effects. This suggested a mixed-type control and
5-HTT mainly acts as a mixed-type inhibitor in 5 % HCl, the same phenomenon
has been reported by Bentiss et al. [25] with 3,5-diphenyl-4H-1,2,4-triazole in
1 M HCl.
Linear polarization technique was performed in 5 % HCl with various concen-
trations of 5-HTT. The corresponding Rp values of mild steel in the absence and in the
presence of different concentrations are also given in same table (Table 2). It is
apparent that Rp increases with increasing inhibitor concentration. The inhibition
percentage (P %) calculated from Rp values are also presented in Table 2. We remark
that P % increases with increasing concentration of inhibitor and attains 94.33 % for
the optimal concentration. The inhibition efficiency of corrosion of cold rolled steel is
calculated by Rp as follows:
R0P ꢁ RP
P % ¼
ꢂ 100
ð3Þ
RP0
where RPand R0P are the Rp values without and with inhibitor, respectively.
AC impedance studies
Nyquist plots of mild steel in 5 % HCl in the presence and absence of additive are
given in Fig. 3. These curves have obtained after 1 h of immersion in the
123

Inhibition of mild steel corrosion
50
Blank
5-HTT 10-4
M
40
30
20
10
0
5-HTT 2.10-4
5-HTT 3.10-4
5-HTT 4.10-4
5-HTT 5.10-4
M
M
M
M
0
20
40
60
80
ZRe(Ω.cm2)
Fig. 3 Nyquist plots for mild steel 5 % HCl solution in the absence and in the presence of different
concentrations of 5-HTT at 30 ꢁC
corresponding solution. All the plots display a single capacitive loop. Impedance
parameters derived from the Nyquist plots, percent inhibition efficiencies, P (%) and
the equivalent circuit diagram are given in Table 2 and Fig. 4, respectively. The
circuit consists of a constant phase element (CPE) Q, in parallel with a resistor Rt.
The use of CPE-type impedance has been extensively described [26–28]
ꢁ1
n
ZCPE ¼ ½QðjwÞ ꢃ
ð4Þ
The above equation provides information about the degree of non-ideality in
capacitance behavior. Its value makes it possible to differentiate between the
behavior of an ideal capacitor (n = 1) and of a CPE (n \ 1).
The percent inhibition efficiency is calculated by charge transfer resistance
obtained from Nyquist plots, according to the equation:
R0t ꢁ Rt
P % ¼
ꢂ 100
ð5Þ
Rt0
where Rt and R0t are the charge transfer resistance values without and with inhibitor,
respectively.
Considering that a CPE may be considered as a parallel combination of a pure
capacitor and a resistor that is inversely proportional to the angular frequency, the
value of capacitance, Ci, can thus be calculated for a parallel circuit composed of a
CPE (Q) and a resistor (Rt), according to the following formula [29, 30]:
n
ðC RtÞ
Q ¼
ð6Þ
Rt
123

H. B. Ouici et al.
Table 4 Some parameters of the linear regression between C/h and C at different temperatures
Temperature (K)
1/K
Slope
R2
DGads (kJ/mol)
303
313
323
2.71 9 10-5
3.51 9 10-5
5.09 9 10-5
0.980
0.994
0.996
0.999
0.998
0.999
-36.61
-37.14
-37.33
Table 5 Values of corrosion rates and inhibition efficiency for different concentrations of 5-HTT at
various concentrations
Temperature (K)
303
Conc. (Mol/l)
W (mg cm-2 h)
Ew (%)
Blank
10-4
5.50
2.03
0.75
0.39
0.23
0.18
9.33
4.03
1.32
0.88
0.82
0.49
14.15
9.74
3.53
1.94
1.73
1.21
24.90
27.40
22.90
9.28
3.40
2.71
–
63.10
86.36
92.90
95.82
96.73
–
2.10-4
3.10-4
4.10-4
5.10-4
Blank
10-4
2.10-4
3.10-4
4.10-4
5.10-4
Blank
10-4
2.10-4
3.10-4
4.10-4
5.10-4
Blank
10-4
2.10-4
3.10-4
4.10-4
5.10-4
313
323
333
56.81
85.85
90.57
91.21
94.75
–
31.17
75.05
86.29
87.77
91.45
–
-10.04
08.03
62.73
86.34
89.11
The impedance spectra of mild steel in 5 % HCl with and without inhibitor were
analyzed by using the circuit in Fig. 4, and the double layer capacitance (Cdl) was
calculated in terms of Eq. (6). Values of elements of the circuit corresponding to
different corrosion systems, including values of Cdl, are listed in Table 3. As can be
seen from this table, the increase in resistance, in the presence of 5-HTT compared
to HCl alone, is related to the corrosion protection effect of the molecules. The
value of Cdl decreases in the presence of these inhibitors, suggesting that the 5-HTT
molecules function by adsorption at the metal solution/interface [31]. It is important
123

Inhibition of mild steel corrosion
Fig. 4 The equivalent circuit of
the impedance spectra obtained
for 5-HTT
100
90
80
weight loss
Icorr
Rp
Rt
70
60
50
0.0000 0.0001
0.0002
0.0003
0.0004
0.0005
0.0006
Conc. (mol/L)
Fig. 5 Variation of P % evaluated by different methods
to point out that, in the presence of 5-HTT, n decreases with the increase of the
concentration and reaches a value of 0.78. This shows an increase of the surface
inhomogeneity as a result of the inhibitor’s adsorption.
Globally, the inhibition efficiency measured for different methods was estimated at
about 90 % at very low concentrations (2 9 10-4 M), and the inhibition efficiency
obtained from weight loss and electrochemical measurements are in good agreement
(Fig. 5).
Effect of immersion time on corrosion of mild steel
In order to assess the stability of inhibitive behavior of inhibitors on a time scale,
weight loss measurements were performed in 5 % HCl in absence and presence of
5-HTT at 5 9 10-4 M for different immersion time at temperature 303 K (Fig. 6).
Inhibition efficiency was plotted against immersion time as seen from Fig. 7.
From the Fig. 7, we can see that the inhibition efficiency of 5-HTT increased
with immersion time and stabilized after 240 min. The increase in the inhibition
efficiency may be due to the adsorption of 5-HTT to form an adherent film on the
metal surface and suggests that the coverage of the metal surface with this film
decreases the double layer thickness. The augmentation in the inhibition efficiency
values may be due to the gradual replacement of water molecules by the adsorption
of the 5-HTT molecules on the metal surface, decreasing the extent of dissolution
reaction [6, 10, 32].
123

H. B. Ouici et al.
10
8
Blank
5x10-4 M 5-HTT
6
4
2
0
-2
0
200
400
600
800 1000 1200 1400 1600
Immer. Time (min)
Fig. 6 Corrosion rate of mild steel with and without 5 9 10-4 M of 5-HTT at different immersion time
100
99
98
97
96
95
0
400
800
1200
1600
Immersion time (min)
Fig. 7 Variation of P % versus the immersion time
The inhibition of corrosion of mild steel by 5-HTT can be explained in terms of
adsorption on the metal surface. This compound can be adsorbed on the metal
surface by the interaction between lone pairs of electrons of nitrogen and sulfur
atoms of the inhibitor and the metal surface or via interaction of 5-HTT with already
adsorbed chloride ions [6, 33]. This process is facilitated by the presence of vacant
orbitals of low energy in iron atom, as observed in the transition group metals [10].
Ashassi-Sorkhabi and Nabavi-Amri [34] proposed the following mechanism
123

Inhibition of mild steel corrosion
involving two adsorbed intermediates to account for the retardation of Fe anodic
dissolution in the presence of an inhibitor:
Fe þ H2O ꢀ Fe H2Oads
ð7aÞ
ð7bÞ
ð7cÞ
Fe H2Oads þ MꢁFe OHꢁads þ Hþ þ M
Fe H2Oads þ M ꢀ Fe Mads þ H2O
rds
Fe OHꢁadsꢁ! Fe OHads þ e
ð7dÞ
Fe Mads ꢀ Fe Mþads þ e
Fe OHads þ FeMþadsꢁFe Mads þ FeOHþ
Fe OHþ þ HþꢁFe2þ þ H2O
ð7eÞ
ð7fÞ
ð7gÞ
where M represents the inhibitor species.
Considering the inhomogeneous nature of metallic surfaces resulting from the
existence of lattice defects and dislocations, a corroding metal surface is generally
characterized by multiple adsorption sites having activation energies and heats of
adsorption. Inhibitor molecules may thus be adsorbed more readily at surface active
sites having suitable adsorption enthalpies. According to the detailed mechanism
above, displacement of some adsorbed water molecules on the metal surface by
inhibitor species to yield the adsorbed intermediate FeMads (Eq. 7c) reduces the
amount of the species FeOHꢁads available for the rate-determining steps and
consequently retards Fe anodic dissolution [35].
Adsorption isotherms
The inhibitor efficiency depends on the type and number of active sites at metal
surface, the charge density, the molecular size of the inhibitor, the metal–inhibitor
interaction, and the metallic complex formation. The adsorption isotherm can give
information on the metal–inhibitor interaction. The adsorptionisotherm can be derived
from the curve surface coverage against inhibitor concentration. Surface coverage was
estimated as in Eq. 1. The h values for different inhibitor concentrations are tested by
fitting to various isotherms. By far the best fit was obtained with the Langmiur
isotherm. According to this isotherm h is related to concentration inhibitor C via:
C
1
¼
þ C
ð7Þ
h
K
ꢀ
ꢁ
1
DG0ads
RT
with K ¼
exp ꢁ
ð8Þ
55; 5
where K is the adsorptive equilibrium constant and DG0ads the free energy of adsorption.
C
h
It was found that Fig. 8 (plot of vs. C) gives straight lines with slope equal or
nearly equal to 1.00 for three temperatures (303, 313, and 323 K) and intercepts
provide the calculation of the values of K are given in Table 4. The obtained results
indicate that the adsorption of compound under consideration on mild steel/acidic
solution interface follows the Langmuir adsorption isotherm.
123

H. B. Ouici et al.
0.00055
0.00050
0.00045
0.00040
0.00035
0.00030
0.00025
0.00020
T= 303 K
T= 313 K
T= 323 K
0.00020 0.00025 0.00030 0.00035 0.00040 0.00045 0.00050 0.00055
Conc. (Mol/L)
C
h
Fig. 8 Dependence of on the concentration of 5-HTT in 5 % HCl at different temperatures
Inspection of the data of Table 4 shows the following adsorption characteristics
of 5-HTT in 5 % HCl:
•
•
Values of slopes are nearly to the unity indicating that each molecule of the
inhibitor is attached to one active site of the steel surface.
It could be found that Kads decreased with increasing temperature. This is as a
result of the improvement for the desorption of 5-HTT from the steel surface
suggesting that the inhibitor is physically adsorbed on the metal surface [36].
All values of negatives DGads show that the adsorption of 5-HTT is a
spontaneous process under the experimental conditions described and also the
strong interaction between inhibitor molecules and the metal surface [6, 37].
It generally accepted that the values of DGads up to -20 kJ mol-1, the types of
adsorption were regarded as physisorption, the inhibition acts due to the
electrostatic interactions between the charged molecules and the charged metal,
while the values around -40 kJ mol-1 or smaller, were seen as chemisorption,
which is due to the charge sharing or a transfer from the inhibitor molecules to
the metal surface to form a covalent bond [38, 39]. The values of DGads in our
measurements range from -36 to -37 kJ mol-1 (in Table 4), it is suggested
that the adsorption of these triazole derivatives involves two types of interaction,
chemisorption and physisorption.
•
•
Effect of temperature on corrosion of mild steel
To investigate the mechanism of inhibition and to calculate the activation energies of
the corrosion process, weight loss measurements were taken at various temperatures
123

Inhibition of mild steel corrosion
in the absence and the presence of different concentrations of 5-HTT. Corresponding
data are given in Table 2.
From this table, we can see that, in the studied temperature range (30–60 ꢁC), the
corrosion rate increases with increasing temperature both in uninhibited and
inhibited solutions and the values of the inhibition efficiency of 5-HTT decrease
with increasing concentration. Thus, the corrosion rate of mild steel increases more
rapidly with temperature in the absence of the inhibitor. For the lower concentration
(10-4) at temperatures 303, 313, and 323 K, the inhibitor stimulates the corrosion of
mild steel at 333 K.
The activation parameters for the corrosion process were calculated from an
Arrhenius type plot according to the following equation:
Ea
ln Vcorr ¼ ꢁ
þ log A
ð9Þ
R T
where Ea is the apparent activation energy, A the pre-exponential factor, R the
universal gas constant, and T the absolute temperature.
The variations of the logarithm of the corrosion rate of mild steel (ln Vcorr) in 5 %
HCl containing various concentrations of 5-HTT used with reciprocal of the
absolute temperature are presented in Fig. 9. Straight lines with coefficients of
correlation (c.c.) up to to 0.98 are obtained.
The calculated values of Ea and pre-exponential factor A are recorded in Table 6.
Table 6 shows that the values of Ea for inhibited solution are higher than that for
the uninhibited solution. The values of A in the presence of MMI are higher than
that in uninhibited solution. It is clear from Eq. (9) that the higher Ea and the lower
A lead to the lower corrosion rate. In general, the effect of Ea on mild steel corrosion
4
2
0
Blank
5-HTT 10-4 M
5-HTT 2.10-4
5-HTT 3.10-4
5-HTT 4.10-4
5-HTT 5.10-4
M
M
M
M
-2
-4
0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330
1/T (K-1)
Fig. 9 ln Vcorr versus 1/T for cold-rolled steel dissolution in 5 % HCl in the presence and the absence of
different concentrations of 5-HTT
123

H. B. Ouici et al.
Table 6 The values of
Conc. (Mol/l) Pre-exponential factor (mg cm-2 h) Ea (kJ/mol)
activation parameters for steel in
5 % HCl in the absence and the
presence of different
Blank
10-4
7.63 9 107
4.95 9 1012
7.78 9 1015
1.60 9 1014
9.60 9 1011
1.57 9 1012
41.45
72.08
93.70
85.13
72.75
74.91
concentrations of 5-HTT
2.10-4
3.10-4
4.10-4
5.10-4
is larger than that of A on mild steel corrosion. In the present study, values of Ea and
A vary in a similar manner and, therefore, the combined effect of Ea and A results in
an increase of corrosion rate with temperature. The same behavior has been reported
by Ahamad et al. [40]. It is noted that, in our previous work with 2-mercapto
1-methyl imidazole in the same medium and metal [41], we have noted the same
phenomenon.
Theoretical calculations
In order to compare these results with those obtained in our previous work for the
same mild steel and the same medium [41], we have performed the experiments in
the same conditions. From Table 1 (weight loss measurement) and results given
with 2-mercapto 1-methyl imidazole (MMI) [41], we can see that the value of the
inhibition efficiency is 94 % for MMI at 5 9 10-3 M and 97 % for 5-HTT at
5 9 10-4 M (Table 5).
It is clear that the inhibition efficiency depends on many factors [42], including
the number of adsorption centers, mode of interaction with metal surface, molecular
size and structure [37], and in order to give further insight into the experimental
results, different quantum parameters, such as chemical potential (PC), molecular
volume (V), highest occupied molecular orbital (EHOMO) and lowest unoccupied
molecular orbital (ELUMO) energies, LUMO–HOMO energy gap (DELUMO–HOMO
)
and dipole moment (l) of the two molecules, are calculated. Table 7 gives the
corresponding data.
It is known that the value of EHOMO is often associated with the electron-donating
ability of the inhibitor molecule, higher values of EHOMO is an indication of the
greater ease of donating electrons to the unoccupied d orbital of metal. The value of
ELUMO is related to the ability of the molecule to accept electrons, lower values of
ELUMO shows the inhibitor molecule would accept electrons. Consequently, the
value of the DE gap provides a measure for the stability of the formed complex on
Table 7 Calculated quantum chemical parameters of 5-HTT and MMI derivatives
IE (%)
CP
V (Aꢁ3)
EHOMO (eV)
ELUMO (eV)
DE (eV)
l (D)
5-HTT
MMI
94
97
-3.80
-2.73
161.29
102.67
-6.03
-5.77
-1.57
4.46
6.08
4.74
3.31
0.31
123

Inhibition of mild steel corrosion
Fig. 10 Schematic representation of HOMO and LUMO molecular orbital of 5-HTT and MMI
the metal surface. The lower the value of DE, the higher the stability for the formed
complex. The value of DE for 5-HTT and MMI are 4.46 and 6.08 eV, respectively.
These results yield to a band gap difference of 1.62 eV in favor of 5-HTT and
indicate that 5-HTT can easily transfer an electron from the HOMO level to the vacant
d-orbitals of the iron surface. Furthermore, less negative HOMO energy and the
smaller energy gap (ELUMO–EHOMO) are reflected in a stronger adsorption and greater
inhibition efficiency [43]. Moreover, increasing values of EHOMO and l may facilitate
adsorption by influencing the transport process through the adsorbed layer [44]. On the
other hand, frontier orbital theory may be used to predict the adsorption centers of
the inhibitor molecule [8]. The HOMO and LUMO electronic density distributions of
the two molecules are plotted in Fig. 10. It is observed that, for the two molecules, the
highest values of the HOMO density were found in the vicinity of the sulfur atom,
clearly indicating that the nucleophilic center is sulfur atom. Thus, the bond with the
metal from sulfur will be easily formed, rather than N or C atoms.
Moreover, increasing values of EHOMO, dipole moment (l), PC and V may
facilitate adsorption by influencing the transport process through the adsorbed layer
[44], and facilitate electrostatic interaction between the electric field due to the
metal charge and the electric moment of these compounds [45].
Conclusions
It can be concluded as follows:
•
The -(2-hydroxyphenyl)-1,2,4-triazole-3-thione (5-HTT) was found to perform
in 5 % HCl.
123

H. B. Ouici et al.
•
•
•
•
•
•
Polarization studies showed that the compounds under investigation were mixed
type inhibitors.
The inhibition efficiency of 5-HTT increased with the concentration and reached
a maximum at 5 9 10-4 M.
The weight loss, EIS, polarization curves, and linear polarization were in good
agreement.
Adsorption of inhibitor molecules on the mild steel surface from 5 % HCl
followed the Langmuir isotherm.
P % of 5-HTT decreased with increasing temperature and their addition led to
increase of the activation corrosion energy.
Theoretical calculation results confirm the higher inhibition efficiency of 5-HTT
relatively to MMI.
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