Indian Journal of Chemical Technology
Vol. 11, January 2004, pp. 103-107
Effect of some nitrogen and sulphur based synthetic inhibitors on corrosion
inhibition of mild steel in acid solutions
M A Quraishi* & R Sardar
Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering & Technology,
Aligarh Muslim University, Aligarh 202 002, India
Received 10 September 2002; revised received 23 May 2003; accepted 26 June 2003
Three organic inhibitors namely, 5-mercapto-3-butyl-4-salicylidineimino-1,2,4-triazole (MBST), 5-mercapto-3-butyl-4benzylidineimino-1,2,4-triazole (MBBT) and 5-mercapto-3-butyl-4-cinnamylidineimino-1,2,4-triazole (MBCT) were
synthesized in the laboratory and their influence on the inhibition of corrosion of mild steel (MS) in aqueous solutions
containing 1N HCl and 1N H2SO4 was investigated by weight loss and potentiodynamic polarization techniques. The
inhibition efficiency (IE) of these compounds was found to vary with concentration, temperature and immersion time. Good
inhibition efficiency (IE) was evidenced in both acid solutions. The adsorption of these compounds on the steel surface for
both acids was found to obey Temkin’s adsorption isotherm. The potentiodynamic polarization data have shown, that,
compounds studied are mixed type inhibitors in both the acid solutions.
IPC Code: C23F11/10; C22C38/00
Most of the commercial inhibitor formulations are
found to include aldehydes and amines as essential
ingredients1,2. Turbina et al.3 have observed that
condensation products of carbonyls and amines which
are known as anils or Schiff’s bases give higher
inhibition efficiency than that for constituent
carbonyls and amines. Desai and co-workers4 have
studied few Schiff’s bases (derived from aromatic
aldehydes and amines) as corrosion inhibitors for mild
steel in HCl. They found that the inhibition efficiency
for the investigated Schiff’s bases is much greater
than that for corresponding amines and aldehydes.
These observations have actually led to the synthesis
of a few anils by condensing a few substituted
iminotriazoles and salicylaldehydes with a view to
evaluate their inhibiting properties on the corrosion of
mild steel. The inhibition efficiency for all the
inhibitors was found to be greater than that for the
corresponding amines and salicylaldehyde5.
The influence of 5-mercapto-3-butyl-4-salicylidineimino-1,2,4-triazole (MBST), 5-mercapto-3-butyl-4benzylidineimino-1,2,4-triazole (MBBT) and 5mercapto-3-butyl-4-cinnamylidineimino-1,2,4-triazole (MBCT) on corrosion of mild steel in 1N HCl and
1N H2SO4 is being reported here The choice of these
inhibitors is based on the considerations such as (a)
these can be synthesized conveniently from relatively
________
*For correspondence (E-mail: [email protected];
Fax: 0091+571+700528)
cheap raw materials, (b) the presence of lone pair of
electron on heteroatoms, and (c) π-electrons of the
benzene ring in these compounds are likely to induce
greater adsorption of the compounds on the surface
leading to higher efficiency.
Experimental Procedure
Inhibitors
The inhibitors were synthesized in the laboratory,
following the procedure described earlier6 and
compounds were characterized through their spectral
data and their purity was confirmed by thin layer
chromatography (TLC). Name, structural formula,
melting point and molecular weight of the
condensation products are given in Table 1.
Table 1—Name and molecular structures of the compounds used
N
H9C4
N
N
N
1. R = C6H4OH
2. R = C6H5
3. R = C6H5CH=CH
SH
CH
R
(5-mercapto-3-butyl-4-salicylidineimino-1,2,4-triazole MBST);
mol.wt = 276; melting point = 125°C
(5-mercapto-3-butyl-4- benzylidineimino-1,2,4-triazole (MBBT);
mol.wt = 260; melting point =110°C
(5-mercapto-3-butyl-4- cinnamylidineimino-1,2,4-triazole (MBCT);
mol.wt = 286; melting point =132°C
INDIAN J. CHEM. TECHNOL., JANUARY 2004
104
Electrolyte
The mild steel strips having composition, (wt %):
0.14% C, 0.35% Mn, 0.17% Si, 0.025% S, 0.03% P
and balance Fe were used for weight loss and
electrochemical studies. Strips were mechanically
polished with emery papers of 1/0, 2/0, 3/0 and 4/0
grade and degreased with trichloroethylene.
commercially available lacquer (lakme) with an
exposed area of 1.0 cm2 were used and the
experiments were carried out at temperature
(28±2°C). Equilibrium time leading to steady state of
the specimens was 30 min. Scan rate in
potentiodynamic
experiment
was
1mV/s.
Potentiodynamic polarization studies were carried out
using an EG & G Princeton Applied Research (PAR)
potentiostat/galvanostat (model 173), a universal
programmer (model 175) and an X-Y recorder (model
RE0089). A platinum foil was used as auxiliary
electrode and a saturated calomel electrode (SCE) was
used as reference electrode.
Weight loss studies
Results and Discussion
The aggressive solutions used were made of AR
grade 35% HCl and 98% H2SO4. Appropriate
concentrations of acids were prepared using
bidistilled water. The concentration range of inhibitor
employed was 100 to 500 ppm in both the acids.
Specimens
The mild steel strips of size [2.0×2.0×0.025] cm
were used for weight loss measurement studies.
Weight loss measurement studies were carried out at
28°C for 3 h in 1N HCl and 1N H2SO4 acid solutions.
The experiments were performed as per ASTM G 3172 method described previously7.
Weight loss studies
For potentiodynamic polarization studies, mild
steel strips of same composition, coated with
The values of percentage inhibition efficiency (IE)
and corrosion rate (CR) obtained from weight loss
method at different concentrations of these
compounds are given in Tables 2 a & b. It is seen that
all these compounds inhibit corrosion of mild steel in
both 1N HCl and 1N H2SO4 at all concentrations
under study. It was observed to increase progressively
with increase in concentration of the added inhibitor.
Table 2a—Corrosion parameters for mild steel in aqueous
solution of 1N H2SO4 in absence and presence of different
concentrations of various inhibitors from weight loss
measurements at 25°C for 3 h
Table 2b—Corrosion parameters for mild steel in aqueous
solution of 1N HCl in absence and presence of different
concentrations of various inhibitors from weight loss
measurements at 25°C for 3 h
Electrochemical polarization measurement
Inhibitor concentration
(ppm)
Weight loss
(mg)
IE
(%)
CR
(mmpy)
Inhibitor concentration
(ppm)
Weight loss
(mg)
IE
(%)
CR
(mmpy)
1N H2SO4
MBST
25
50
100
200
300
400
500
MBBT
25
50
100
200
300
400
500
MBCT
25
50
100
200
300
400
500
166.2
0.00
61.74
93.4
0.00
34.69
41.5
38.2
33.2
21.6
13.3
9.9
3.9
75.0
77.0
80.0
87.0
91.9
94.0
97.6
15.42
14.19
12.33
8.02
4.94
3.67
1.45
32.7
27.9
23.3
19.6
15.8
13.0
11.1
64.9
70.1
75.0
79.0
83.1
86.1
88.1
12.15
10.36
8.66
7.28
5.87
4.83
4.12
29.8
23.2
18.7
16.6
9.9
5.8
2.8
82.0
86.0
88.7
90.0
94.0
96.5
98.3
11.07
8.62
6.95
6.17
3.68
2.15
1.04
29.8
23.3
20.5
14.0
11.2
9.3
6.5
68.1
75.0
78.0
85.0
88.0
90.0
93.0
11.07
8.66
7.61
5.20
4.16
3.45
2.41
22.5
19.9
17.1
13.2
8.3
3.3
1.6
86.4
88.0
89.7
92.0
95.0
98.0
99.0
8.36
7.39
6.35
4.90
3.08
1.22
0.59
1N HCl
MBST
25
50
100
200
300
400
500
MBBT
25
50
100
200
300
400
500
MBCT
25
50
100
200
300
400
500
20.5
19.6
17.8
12.5
8.3
5.6
3.5
78.0
79.0
80.9
86.6
91.1
94.0
96.2
7.61
7.28
6.61
4.64
3.08
2.08
1.30
QURAISHI & SARDAR: EFFECT OF NITROGEN AND SULPHUR INHIBITORS ON CORROSION OF MILD STEEL
Maximum inhibition efficiency of each compound
was achieved at 500 ppm and a further increase in
concentration did not cause any appreciable change in
the performance of the inhibitor.
The variation of inhibition efficiency with increase
in acid concentration is shown in Figs 1a & 2a, it is
clear that the increase of acid concentration from 1 to
3N did not cause significant changes in inhibition
efficiency values, thereby, suggesting that all the
compounds are effective corrosion inhibitors in acid
solutions over this concentration range. The influence
of temperature on inhibition efficiency is shown in
Figs 1b & 2b. It is observed that inhibition efficiency
for all the compounds did not change significantly
with increase in temperature from 30-50°C. The effect
of immersion time on inhibition efficiency is shown
in Figs 1c & 2c. It is found that increase in immersion
time from 3 to 24 h did not show any significant
changes in inhibition efficiency values.
The values of activation energy (Ea) were
calculated using the Arrhenius equation8-9,
ln (r2 /r1 ) = −Ea ΔT / ( R ×T2 × T1)
105
where r1 and r2 are corrosion rates at temperature T1
and T2 respectively, ΔT is the difference in
temperature (T1 – T2).
The free energy of adsorption (ΔGads), at different
temperatures was calculated from the following
equation,
ΔGads = −RT ln (55.5K)
and K is given by
K = θ/C (1−θ)
where θ is degree of coverage on the metal surface, C
is the concentration of inhibitor in mol/L and K is
equilibrium constant. The values of Ea and ΔGads are
given in Table 3. The low and negative values of
ΔGads indicated the spontaneous adsorption of
inhibitors on the surface of mild steel. The negative
values of ΔGads also suggest the strong interaction of
the inhibitor molecules with the mild steel surface10-11.
Application of adsorption isotherm
A plot of θ versus log C gave straight lines in both
the acid solutions (Figs 3a and 3b). This showed that
Acid concentration (N)
Acid concentration (N)
Fig. 1a—Variation of inhibition efficiency with acid concentration
in 1N H2SO4 (1, MBST; 2, MBBT; 3, MBCT)
Fig. 2a—Variation of inhibition efficiency with acid concentration
in 1N HCl (1, MBST; 2, MBBT; 3, MBCT)
Solution temperature (°)
Solution temperature (°)
Fig. 1b—Variation of inhibition efficiency with solution
temperature in 1N H2SO4 (1, MBST; 2, MBBT; 3, MBCT)
Fig. 2b—Variation of inhibition efficiency with solution
temperature in 1N HCl (1, MBST; 2, MBBT; 3, MBCT)
INDIAN J. CHEM. TECHNOL., JANUARY 2004
106
Immersion time (h)
Fig. 1c—Variation of inhibition efficiency with immersion time in
1N H2SO4 (1, MBST; 2, MBBT; 3, MBCT)
Fig. 3a—Temkin’s adsorption isotherm plots for the adsorption of
various inhibitors in 1N H2SO4 (1, MBST; 2, MBBT; 3, MBCT)
Immersion time (h)
Fig. 2c—Variation of inhibition efficiency with immersion time in
1N HCl (1, MBST; 2, MBBT; 3, MBCT)
Table 3—Activation energy (Ea) and free energy of adsorption
(ΔGads) for mild steel in 1N H2SO4 and 1N HCl in the absence and
presence of various inhibitors
−ΔGads (KJ/mol)
30°C
40°C
50°C
Inhibitor concentration
(ppm)
Ea
(KJ/mol)
1N H2SO4
MBST
MBBT
MBCT
29.56
13.02
4.99
11.46
—
40.74
41.66
42.84
—
42.58
43.80
44.68
—
44.56
46.28
48.00
1N HCl
MBST
MBBT
MBCT
53.84
32.71
61.69
59.64
—
37.50
38.76
40.48
—
39.39
41.16
42.21
—
41.45
46.28
46.57
the adsorption of these compounds on the mild steel
in 1N HCl and 1N H2SO4 obeyed Temkin's adsorption
isotherm, supporting the hypothesis that corrosion
inhibition by these compounds results from
adsorption on the metal surface.
Potentiodynamic polarization studies
The polarization behaviour of mild steel in 1N HCl
and 1N H2SO4 in absence and presence of 500 ppm
inhibitor concentration is shown in Figs 4a and 4b.
Electro-chemical parameters such as corrosion current
Fig. 3b—Temkin's adsorption isotherm plots for the adsorption of
various inhibitors in 1N HCl (1, MBST; 2, MBBT; 3, MBCT; 4,
CPT)
density (Icorr) and corrosion potential (Ecorr) calculated
from Tafel plots are given in Table 4. It is observed
that presence of these compounds lowers Icorr values.
Maximum decrease in Icorr was observed for MBCT
indicating that MBCT is most effective corrosion
inhibitor among the studied condensation products.
Similar trend was observed by weight loss method. It
is also observed from the Table 4 that there is no
significant change in Ecorr values of inhibited and
uninhibited systems suggesting that the inhibitors are
mixed type in nature.
Mechanism of inhibition
Inhibition of corrosion of mild steel in acidic
solutions by these compounds can be explained on the
basis of adsorption. These compounds inhibit the
corrosion by controlling both the anodic and cathodic
reactions. In acidic solutions, these compounds exist
as protonated species. These protonated species may
be adsorbed on the cathodic sites of the mild steel and
decrease the evolution of hydrogen. The adsorption of
QURAISHI & SARDAR: EFFECT OF NITROGEN AND SULPHUR INHIBITORS ON CORROSION OF MILD STEEL
107
Table 4—Electrochemical polarization parameters for the
corrosion of mild steel in 1N H2SO4 and 1N HCl containing
optimum concentration of various inhibitors at 28±2°C
Fig. 4a—Potentiodynamic curves for mild steel in 1N H2SO4 in
the presence and absence of 500 ppm of various inhibitors (1, 1N
H2SO4; 2, MBST; 3, MBBT; 4, MBCT)
Fig. 4b—Potentiodynamic curves for mild steel in 1N HCl in the
presence and absence of 500 ppm of various inhibitors (1, 1N
HCl; 2, MBST; 3, MBBT; 4, MBCT)
these molecules on anodic sites occurs through πelectrons of aromatic rings and lone pair of electrons
of nitrogen and sulphur atoms which may decrease
anodic dissolution of mild steel12. Among the
compounds investigated in the present study, MBCT
has been found to give the best performance as
corrosion inhibitor. This can be explained on the basis
of the presence of an additional π-bond between
carbon atoms (−C = C−) in conjugation with aromatic
ring. These extensively delocalised π-electrons favour
greater adsorption on the metal surface as compared
to other compounds. The better inhibition efficiency
observed in 1N H2SO4 than in 1N HCl may be
attributed to the differences in adsorption of chloride
and sulphate ions. The chloride ions, which are
strongly adsorbed on the metal surface, leave less
space for the inhibitor molecules to adsorb. In
Inhibitor concentration
(ppm)
Ecorr
(mV)
Icorr
(mA cm-2)
IE
(%)
1N H2SO4
MBST
MBBT
MBCT
−550
−550
−560
−540
0.36
0.10
0.072
0.036
0.00
72.2
80.0
90.0
1N HCl
−596
0.35
0.00
MBST
MBBT
MBCT
−600
−586
−610
0.11
0.088
0.07
68.8
74.8
80.0
contrast, lower adsorption of sulphate ions on the
metal surface permits more space for the adsorption
of inhibitor molecules and enhances inhibition of
corrosion13.
Conclusions
Thus, from the above observations, it may be
concluded that, (i) all the triazole based compounds
acted as efficient corrosion inhibitors up to 3N HCl
and 3N H2SO4 acid solutions, (ii) all the compounds
inhibited corrosion of mild steel by adsorption on the
metal surface, (iii) adsorption of these triazole based
inhibitors on the mild steel surface in both the acid
solutions obeyed Temkin's adsorption isotherm, and
(iv) all the compounds examined acted as mixed-type
inhibitors in both 1N HCl and 1N H2SO4.
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