Inhibition
of Corrosion of Copper by 4-Amino-3-Phenyl-5-Mercapto-1, 2, 4-Triazole in 3.5% Sodium
Chloride Solution.
M. Yadav1*, Sushil Kumar1, Abu
Nasar2 and Sunil Kumar2
1Department of Applied
Chemistry, ISM University, Dhanbad, India
2Department of Chemistry, S.N. PG College, Azamgarh,
India
*Corresponding Author E-mail: yadav_drmahendra@yahoo.co.in
ABSTRACT:
In the present investigation 4-amino-3-phenyl-5-mercapto-1,
2, 4-triazole has been synthesized and studied as inhibitor for the corrosion
of copper in 3.5 % NaCl solution. The inhibition efficiency of the compound
have been evaluated by weight loss and electrochemical methods (Impedance
spectroscopy and polarization curves). The surface study was done by using SEM
techniques. The inhibitor appears to inhibit corrosion process through
formation of protective film which was found to consist of Cu(I)-inhibitor
complex, cuprous chloride, CuCl or CuCl2- complex ion or
both on the surface.
KEYWORDS: Copper; Corrosion inhibition; 3.5% NaCl solution
1. INTRODUCTION:
Due to its high electrical and thermal conductivity and
good mechanical workability, copper is a material widely used in pipelines for
domestic and industrial water utilities, including seawater, heat conductors,
and heat exchangers1. In spite of the relatively noble potential of
copper, its corrosion takes place at a significant rate in sea water and
chloride environment2. It is generally accepted that anodic
dissolution of copper in chloride environments is influenced by the chloride
ions concentration. At chloride concentrations lower than 1 M, the dissolution
of copper occurs through formation of CuCl, which is not protective enough and
is converted to the soluble 
by reacting with excess chloride
ions, on the other hand, at concentrations higher than 1 M, higher cuprous
complexes such as 
and 
are formed, in addition to the
ones with fewer chlorides, such as CuCl and 
3.
It is generally believed that corrosion inhibitors
effectively eliminate the undesirable destructive effects of aggressive media
and prevent copper, dissolution. Organic compounds containing polar groups
including nitrogen, sulphur, and oxygen and heterocyclic compounds with polar
functional groups and/or conjugated double bonds, such as mercaptotriazole4,
imidazoles5, cysteine6, semicarbazides7,
thiosemicarbazide 8 and benzotriazole 9 have been
reported to inhibit copper corrosion. The inhibiting action of these compounds
is usually attributed to their interactions with the copper surface via their
adsorption. Polar functional groups are usually regarded as being a reaction
center by establishing the adsorption process10.
In the present investigation, experiments have been
performed to assess the inhibitive action of
4-amino-3-phenyl-5-mercapto-1,2,4-triazole in 3.5% NaCl solution at 25oC.
2. MATERIALS AND METHODS:
Copper specimens taken for experiments were supplied by
M/s Good Fellow Metals Ltd England (99.99% pure Cu). Sodium chloride (NaCl,
Merck 99%), thiocarbohydrazide (Aldrich, 98%) benzoic acid (Merk, 99%) and
absolute ethanol (C2H5OH, Merk, 99.9%) were used as
received. The samples for the weight loss and electrochemical polarization
studies were of the size 3 cm × 2 cm × 0.1 cm and 1 cm × 1 cm × 0.1 cm,
respectively. The samples were polished successively with 1/0 4/0 grade emery
papers, washed with benzene followed by hot soap solution and finally with
distilled water. They were degreased by immersing in acetone for 1-2 min, dried
and stored in vacuum desiccators. The weight loss experiments were carried out
in 500 ml corning glass beakers with lid containing 300 ml of 3.5% NaCl
solution in static condition. The inhibition efficiencies were evaluated after
a period of 120 h. using 20, 50, 100 and 150 ppm of compound.
% IE = θ × 100
Where, θ is the fraction of surface area covered
by inhibitor, and θ= (a-b)/a, where a is weight loss of the sample in
absence of inhibitor and b is weight loss of sample in presence of inhibitor.
After removing the specimens from the electrolytes, they were washed
thoroughly with distilled water, dried and then weighed. Mean of weight loss
values of three identical experiments were used to calculate the inhibition
efficiencies of the inhibitors. The electrochemical experiments were performed
using a VoltaLab-10 electrochemical analyser containing Voltamaster 4.0
software. For potentiodynamic polarization experiments, the potential was
scanned from -600 to 500 mV at a scan rate of 1 mV/s. Electrochemical Impedance
Spectroscopy (EIS) measurements were performed between 100 kHz and 0.05 Hz
frequency range.
The inhibitor
4-amino-3-phenyl-5-mercapto-1,2,4-triazole was prepared by refluxing a mixture
of thiocarbohydrazide and benzoic acid in 1:1 ratio in the presence of ethanol
for 4 hours. The resulting clear solution was allowed to stand overnight in a
capped round bottom flask during which time the product crystallized as a
coarse off-white crust on the sides of the flask. The solid was collected by
suction filtration and air dried to remove residual acid.
For
calculating %IE by electrochemical polarization method we use the formula-
%
IE =
Where, I0 = Corrosion current in absence of
inhibitor
Iinh = Corrosion current in presence of
inhibitor
% IE by impedance measurements were calculated by using
the formula
Where, Rct is the charge transfer resistance
of the metal in absence of inhibitor and Rct(Inh) is the charge
transfer resistance in presence of inhibitor.
3. RESULTS AND DISCUSSION:
3.1
Weight loss
studies:
The inhibition efficiency values of the inhibitor at
various concentrations at 25oC calculated by weight loss and
polarization techniques have been mentioned in table I. It is evident from the
data in the table that inhibition efficiencies (IEs) of the inhibitor increases
with increase in concentration and becomes more or less constant at 150 ppm.
Considering the potential dependent adsorption of the molecule, the effectiveness
of the inhibitor can be correlated with the structure and size of inhibitor
molecule.
Table 1: Percentage inhibition efficiency (% IE) values
calculated by weight loss and polarization techniques for
4-amino-3-phenyl-5-mercapto-1,2,4-triazole.
|
Concentration (ppm)
|
% IE by weight
loss method
|
% IE by
polarization method
|
|
20
|
72.24
|
72.42
|
|
50
|
84.36
|
84.92
|
|
100
|
90.42
|
91.56
|
|
150
|
91.18
|
92.12
|
Most of the organic compounds and metal complexes used
as inhibitors have been found to inhibit corrosion process following the
mechanism of adsorption. Assuming that this mechanism is valid for present
molecules as well, IE of the inhibitor can be explained in term of the number
of active centres for the adsorption, delocalized electron density and the
projected surface area covered as a result of their adsorption. The inhibitor
consist of nitrogen and sulphur as active centres and delocalized
π-electron density at phenyl and triazole ring causing a high % IE for the
inhibitor. It may be noted that there does not exist any direct correlation
between magnitude in increase in IE values and the number of expected sites of
adsorption and size. This may be due to the fact that the number of active
centres actually involved in adsorption may be different than the number of
active centres present in the molecules owing to their geometry.
3.2
Electrochemicalc
polarization studies:
The electrochemical polarization behaviour of copper
was studied in 3.5% NaCl solution containing different concentrations of the
synthesized inhibitor. Fig.1 represent the electrochemical polarization
behaviour of copper in 3.5% NaCl solution at 25oC in absence and
presence of different concentrations of the inhibitor at 25oC. As
reported earlier the anodic polarisation curve in absence of inhibitor exhibit
Tafel region at lower applied potential extending to a peak current density (Ipeak)
due to the dissolution of copper into Cu+, a region of decreasing
current until a minimum (Imin ) is reached due to formation of CuCl
and a region of sudden increase in current density leading to a limiting value
(Ilim.) as a result of formation of soluble CuCl2-.
The nature of polarisation curve in presence of the inhibitor resembles the
curves in its absence with slight gradual shift towards lower current density
at all the concentrations.
Table 2: Corrosion parameters obtained from
potentiodynamic polarisation curves shown in fig. 1 for copper electrode in
3.5% NaCl solution in the absence and presence of inhibitor.
|
Concentration (ppm)
|
Parameters
|
|
Ecorr (mV)
|
Icorr (µA cm-2)
|
bc (mV dec-1)
|
ba (mV dec-1)
|
Kcorr (mpy)
|
% IE
|
|
0
|
-220
|
14.00
|
90
|
50
|
3.24
|
-
|
|
20
|
-230
|
3.08
|
98
|
60
|
0.71
|
72.42
|
|
50
|
-240
|
2.10
|
115
|
68
|
0.48
|
84.92
|
|
100
|
-250
|
1.26
|
125
|
70
|
0.29
|
91.56
|
|
150
|
-260
|
1.12
|
130
|
72
|
0.26
|
92.12
|
Thus, the inhibitor may be considered to inhibit
corrosion of copper by blanketing a part of the electrode surface due to formation
of protective film of Cu(I)-inhibitor complex and it polarizes the anode
without affecting the basic mechanism of corrosion.
The decrease in Icorr, Ipeak, and
Imin values in presence of inhibitor is mainly due to the decrease
in the chloride ion attack on the copper surface due to the adsorption of the
inhibitors. The negative shift in the Ecorr in presence of inhibitor
on increasing the concentration of the inhibitor is due to the decrease in the
rate of cathodic reaction. Moreover, the increase in the cathodic and anodic
Tafel slopes (βc and βa) are related to the
decrease in both the cathodic and anodic currents. This indicates that the the
inhibitor is good corrosion inhibitor for copper in seawater and its inhibition
efficiency increases on increasing their concentrations. At higher
concentrations, the Tafel region almost vanishes completely and peak current
density disappears in presence of the inhibitor. Therefore, the inhibitor may
be considered to inhibit the corrosion process both through chemical adsorption
via formation of complex at the surface of the copper.
Fig. 1 shows that addition of inhibitor significantly
decreases the cathodic and anodic currents, with the corrosion potential (Ecorr.)
values slightly shifted in the negative direction. Corrosion parameters such as
Ecorr., Icorr., cathodic slope (bc),
anodic slope (ba) and Kcorr. obtained from Fig. 1 are given
in Table:2.
Fig. 1: Electrochemical polarisation of Cu
in 3.5% NaCl solution in presence of the 1-0ppm, 2-20ppm, 3-50ppm, 4-100ppm, 5-150ppm
of the inhibitor at 250C.
The decrease in corrosion current (Icorr.),
peak current (Ipeak), minimum current (Imin) and rate of
corrosion(Kcorr ) values are mainly due to the decrease in the
chloride ions attack on the copper surface, which causes the decrease in Cu
dissolution by absorption of the inhibitor molecules. Furthermore the increase
in anodic and cathodic Tafel slopes (ba and bc
) values are related to the decrease in the anodic and cathodic currents, which
in turn limits the electro dissolution of copper.
3.3
Impedance study:
To get further information concerning the inhibition
process and to confirm the potentiodynamic polarization experiments,
electrochemical impedance spectroscopic investigations of Cu in absence and
presence of inhibitors in 3.5% NaCl solution were carried out. Electrochemical
impedance is a powerful tool in the investigation of the corrosion and
adsorption phenomenon. The impedance data of Cu, recorded in presence of 20, 50
and 100 ppm of the inhibitor in 3.5% NaCl solution at 25oC as
Nyquist plots are shown in fig. 2. The Nyquist plots show depressed circular
shape with their centres below the real axis. This behaviour is typical for
solid metal electrodes that show frequency dispersion of the impedance data.
Table 3: Equivalent circuit parameters and
inhibition efficiency for Cu in 3.5% NaCl solution in absence and presence of
the inhibitor at 25oC
|
Compound
|
Concentration
of inhibitor
|
Rs (W cm2)
|
Rct (kW cm2)
|
Cdl (µF
cm-2)
|
%IE
|
|
|
0
|
2.5
|
0.44
|
19.72
|
-
|
|
|
20ppm
|
3.2
|
1.58
|
16.24
|
72.15
|
|
|
50 ppm
|
3.1
|
3.02
|
14.52
|
85.30
|
|
|
100 ppm
|
3.4
|
6.10
|
12.64
|
92.78
|
Fig. 2: Nyquist plot for Cu in 3.5% NaCl solution in
presence
For a simple equivalent circuit consisting of parallel
combination of a capacitor Cdl, and a charge transfer resister Rct,
in series with a solution resister Rs, the electrode impedance (Z)
in this case is represented by the mathematical formula
Where, a denotes
an empirical parameter (0 £ a £ 1) and f
is the frequency in Hz.
The impedance spectra obtained experimentally were
analysed using software provided with the electrochemical analyser.
The impedance data of the copper electrode in presence
of 20, 50 and 100 ppm of the inhibitor was analysed using the equivalent
circuit shown in fig. 3. The calculated equivalent circuit parameters for Cu in
3.5% NaCl solution at 25oC in presence of 20, 50 and 100 ppm of the
inhibitor is presented in Table 3. From the data in Table 3, it is clear that
the value of Rct increases on increasing the concentration of the
inhibitor, indicating that the corrosion rate decreases in presence of the
inhibitor. It is also clear that the value of Cdl decreases on
addition of inhibitor, indicating a decrease in the local dielectric constant
and/or an increase in the thickness of the electrical double layer, suggesting
that the inhibitor molecule function by formation of the protective layer at
the metal surface.
In order to confirm the potentiodynamic results, the
corrosion inhibition efficiency (IEs) in presence of 20, 50 and 100 ppm
concentration of the inhibitor in 3.5% NaCl solution at 25oC was
also calculated from the corresponding electrochemical impedance data according
to
Where, Rct is the charge transfer resistance
of the metal in absence of inhibitor and Rct(Inh) is the charge
transfer resistance in presence of inhibitor.
The inhibition efficiencies calculated from impedance
data are in good agreement with those obtained from electrochemical
polarization and weight loss measurement.
Fig. 3: Equivalent circuit model used in the fitting of
the impedance data of Cu in 3.5%NaCl solution at 25oC
Where, Rs = Sodium resistance; Rct
= Charge transfer resistance; Cdl = Constant phase element of the
double layer; Cad = Constant phase element of the adsorbed layer; Rad
= Adsorbed layer resistance
3.4
SEM study:
Figures 4(a, b, c) show the micrographs for copper in
3.5%NaCl solution in absence and presence of 150 ppm of the inhibitor at 200X
magnification. On comparing these micrographs, it appears that in the presence
of inhibitor the surface of the test material has improved remarkably with
respect to its smoothness. Smoothening of the surface would have been caused by
the deposition of inhibitor molecules on it and thus, the surface is fully
covered
(a)
(b)
(c)
Fig.4: SEM image of copper (a) Polished sample (b) Exposed
to 3.5% NaCl solution (c) In presence of 150 ppm of the inhibitor.
4. CONCLUSIONS:
(i). 4-amino-3-phenyl-5-mercapto-1, 2, 4-triazole act as
efficient corrosion inhibitor for copper in 3.5% NaCl solution.
(ii) The inhibitor act as mixed inhibitors.
(iii) EIS measurements show that charge transfer
resistance increases in presence of inhibitors.
(iv) It is suggested from the results obtained from SEM
experiments that the copper corrosion is inhibited by the formation of a protective
layer of Cu(I)-inhibitor complex on the copper surface.
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