Corrosion inhibition by fatty acid triazoles for mild steel in formic acid

Abstract

5-Heptadec-8-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol (HPTT), 4-phenyl-5-undecyl-4H–[1,2,4] triazole-3-thiol (PUTT), and 5-dec-9-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol (DPTT) were synthesized and their influence on the inhibition of corrosion of mild steel in 20% formic acid was investigated by weight loss and potentiodynamic polarization techniques. The inhibition efficiency of these compounds was found to vary with their nature and concentration, temperature and immersion time. The values of activation energy and free energy of adsorption of the triazoles were calculated to investigate the mechanism of corrosion inhibition. Good inhibition efficiency (>90%) was found even at lower concentration (i.e., 25 ppm) in acid solution. The adsorption on mild steel surfaces was found to obey Temkin's adsorption isotherm. Potentiodynamic polarization results revealed that the compounds studied are mixed type inhibitors. Electrochemical impedance spectroscopy was also used to investigate the mechanism of the corrosion inhibition.

Full text

Corrosion inhibition by fatty acid triazoles for mild steel in formic acid
M.A. QURAISHI* and F.A. ANSARI
Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh
Muslim University, Aligarh – 202 002, (U.P.) India
(*author for correspondence, e-mail: maquraishi@rediffmail.com)
Received 25 August 2002; accepted in revised form 28 January 2003
Key words: fatty acid triazoles, formic acid, inhibition, mild steel
Abstract
5-Heptadec-8-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol (HPTT), 4-phenyl-5-undecyl-4H–[1,2,4] triazole-3-thiol
(PUTT), and 5-dec-9-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol (DPTT) were synthesized and their influence on
the inhibition of corrosion of mild steel in 20% formic acid was investigated by weight loss and potentiodynamic
polarization techniques. The inhibition efficiency of these compounds was found to vary with their nature and
concentration, temperature and immersion time. The values of activation energy and free energy of adsorption of
the triazoles were calculated to investigate the mechanism of corrosion inhibition. Good inhibition efficiency
(>90%) was found even at lower concentration (i.e., 25 ppm) in acid solution. The adsorption on mild steel surfaces
was found to obey Temkin’s adsorption isotherm. Potentiodynamic polarization results revealed that the
compounds studied are mixed type inhibitors. Electrochemical impedance spectroscopy was also used to investigate
the mechanism of the corrosion inhibition.
1. Introduction
A variety of organic compounds containing heteroatoms
such as O, N, S and multiple bonds in their molecule are
of particular interest as they give better inhibition
efficiency than those containing N or S alone [1–5].
Most research on corrosion inhibition of metals has
been done in mineral acids. Despite the importance of
organic acids in industry, few corrosion studies involv-
ing these acids [6–9] have been made. However, at high
temperatures, these acids can dissociate, forming more
aggressive ions that can cause faster corrosion than
might otherwise be expected. In the fabrication of
reaction vessels, storage tanks, etc. mild steel is used in
the manufacture and use of formic acid. Formic acid is
the most corrosive of the common organic acids. In
previous work we have studied the effect of a few fatty
acid derivatives on the corrosion inhibition of mild steel
in acidic solution [10–12]. In the present investigation we
report the influence of three fatty acid triazoles: namely,
5-heptadec-8-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol
(HPTT), 4-phenyl-5-undecyl-4H–[1,2,4] triazole-3-thiol
(PUTT), 5-dec-9-enyl-4-phenyl-4H–[1,2,4] triazole-3-
thiol (DPTT) on the corrosion inhibition of mild steel
in 20% formic acid.
These fatty acid triazoles were chosen because they
are more environmentally benign, have low toxicity and
are more cost effective than petroleum based products.
Further, they possess nonbonding electron pairs on the
nitrogen atoms additional to the p-electrons of the
phenyl and triazole rings and readily polarizable sulfur
atoms that tend to induce greater adsorption of the
compounds on the metal surface leading to higher
efficiency.
2. Experimental details
Weight loss experiments were performed with cold
rolled mild steel strips of size 2 cm ·2.5 cm ·0.25 cm
having composition (wt %): 0.14% C, 0.35% Mn,
0.17% Si, 0.025% S, 0.03% P, balance Fe as per
standard method [13]. Formic acid (Merck) of AR grade
was used for preparing solutions. Double distilled water
was used to prepare 20% solutions. The triazoles of
fatty acid were synthesized as described by Kittur et al.
[14] and characterized by their infrared spectra and the
purities of the compounds were checked by thin layer
chromatography. The names and molecular structure of
the compounds are given in Table 1.
Potentiodynamic polarization studies were carried out
using an EG&G PAR (model 173) potentiostat/galva-
nostat, a model 175 Universal programmer and a model
RE0089 X–Y recorder. A platinum foil was used as the
auxiliary electrode, a saturated calomel electrode as the
reference electrode and mild steel as the working
electrode. All the experiments were carried out at a
constant temperature of 26 ± 2 C and a scan rate of
Journal of Applied Electrochemistry 33: 233–238, 2003. 233
2003 Kluwer Academic Publishers. Printed in the Netherlands.

1mVs
1
at o.c.p. The polarization curves were ob-
tained after immersion of the electrode in the solution
until a steady state was reached.
Impedance measurements were performed for the
mild steel in 20% formic acid at 26 ± 2 C in the
absence and presence of 100 and 500 ppm of DPTT at
E
corr
with the a.c. voltage amplitude 5 mV in the
frequency range 5 Hz–100 kHz. A time interval of a
few minutes was given for the open circuit potential
(o.c.p.) to read a steady value. All the measurements
were carried out with an EG&G PAR (model 273A)
potentiostat/galvanostat, and an EG&G PAR (model
5301A) lock-in amplifier, using an IBM computer.
3. Results and discussion
3.1. Weight loss
Figure 1(a) show the variation of inhibition efficiency
with inhibitor concentration. The inhibition efficiency
was obtained from weight loss measurements at different
triazole concentrations at 30 C. The percentage inhibi-
tion efficiency (e
IE
) and surface coverage (h) of each
concentration were calculated using the following equa-
tions:
eIE ¼r0r
r0
100 ð1Þ
h¼r0r
r0
ð2Þ
where r
0
and rare the corrosion rates in the absence and
presence of inhibitors, respectively. The inhibition effi-
ciency for all the compounds increases with increase in
concentration. The maximum e
IE
of each compound
was achieved at 500 ppm. Schmitt [15] and Quraishi
et al. [16] reported that a mixture of nitrogen and sulfur
containing compounds are better inhibitors than either
type alone. The compounds studied contain both
nitrogen and sulphur atoms; hence they exhibit good
performance (e
IE
>94%) on the corrosion of mild steel
in 20% formic acid, even at low concentration.
The variation of inhibition efficiency with increase in
acid concentration is shown in Figure 1(b). It is clear
that these compounds are good corrosion inhibitors,
providing greater than 90% inhibition efficiency over the
acid range from 10% to 30%.
The variation of inhibition efficiency of all the three
fatty acid triazoles with immersion time is shown in
Figure 1(c). Increase in immersion time from 24 to 96 h
show no significant change in inhibition efficiency.
The influence of temperature at maximum concentra-
tion (i.e., 500 ppm) on e
IE
is shown in Figure 1(d)
.
The
inhibition efficiency decreases slightly with increase in
temperature from 30 to 50 C, which may be attributed
to desorption of the inhibitor molecules from the metal
surface at higher temperature.
The values of activation energy (E
a
) were calculated
using the Arrhenius equation [17, 18]:
ln r2
r1

¼EaDT
RT1T2
ð3Þ
where r
1
and r
2
are corrosion rates at temperature T
1
and T
2
, respectively, DT¼T
2
T
1
. The free energy of
adsorption (DG
ads
) at different temperatures was calcu-
lated from the equation [19].
DGads ¼RT ln ð55:5KÞð4Þ
Table 1. Name and structure of fatty acid triazoles used
S. No. Structure Designation and abbreviation
1. 5-heptadec-8-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol (HPTT)
2. 4-Phenyl-5-udecyul-4H–[1,2,4] triazole-3-thiol (PUTT)
3. 5-Dec-9-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol (DPTT)
234

where K¼h/C(1 h), his the degree of surface
coverage on the metal surface, Cis the concentration
of inhibitor (in mol l
1
) and Kis the equilibrium
constant. The values of E
a
and DG
ads
are given in
Table 2. E
a
values for inhibited systems are higher than
those of uninhibited systems, indicating that all the
inhibitors are more effective at room temperature [20].
The low and negative values of free energy of adsorption
(DG
ads
) indicate spontaneous adsorption of the inhibitor
on the mild steel surface [21]. The values of DG
ads
for all
the compounds are O40 kJ mol
1
indicating physical
adsorption of the inhibitor molecules [22], and the
negative values also suggest a strong interaction of the
inhibitor molecules on the mild steel surface [23].
3.2. Adsorption isotherm
The mechanism of corrosion inhibition may be ex-
plained on the basis of adsorption behaviour [4]. The
degrees of surface coverage (h) for different inhibitor
concentrations were evaluated by the weight-loss meth-
od. Data were tested graphically by fitting to various
isotherms. A plot of hagainst log Cwas linear (Fig-
ure 2) suggesting that the adsorption of the com-
pounds on to the mild steel surface follows the
Temkin adsorption isotherm. The Temkin isotherm
equation is
bC¼expðahÞ1
1exp½að1hÞ ð5Þ
where b¼(1/55.5)[exp (DG
ads
/RT)], G
ads
is the free
energy of adsorption, hthe surface coverage, Cthe
Fig. 1. Variation of inhibition efficiency with: (a) inhibitor concentration, (b) acid concentration, (c) immersion time and (d) solution
temperature, in 20% formic acid. (1) HPTT; (2) PUTT; (3) DPTT.
Table 2. Activation energy (E
a
) and free energy of adsorption (DG
ads
)
for mild steel in 20% formic acid in the absence and presence of the
inhibitor of 500 ppm of various inhibitors
System E
a
/kJ mol
)1
)DG
ads
/kJ mol
)1
30 C40C50C
20% Formic acid 51.28 – – –
HPTT 70.48 36.85 37.28 37.99
PUTT 71.32 36.32 38.87 39.25
DPTT 61.44 38.79 39.62 40.42
235

concentration of inhibitor, athe molecular interaction
constant, and for a>0 attraction and for a<0=>
repulsion.
3.3. Potentiodynamic polarization
Potentiodynamic anodic and cathodic polarization
scans were carried out in 20% formic acid for different
fatty acid triazoles at 26 ± 2 C. The various electro-
chemical parameters calculated from Tafel plots are
given in Table 3. The lower corrosion current density
(I
corr
) values in the presence of the triazoles, without
causing significant changes in corrosion potential (E
corr
),
b
a
(anodic Tafel slope) and b
c
(cathodic Tafel slope),
suggest that they are mixed type inhibitors (Figure 3).
The maximum decrease in I
corr
was observed for DPTT
(5-dec-9-enyl-4-phenyl-4H–[1,2,4] triazole-3-thiol).
3.4. Electrochemical impedance
The electrical equivalent circuit for the system is shown
in Figure 4. Impedance diagrams obtained for the
frequency range 5 Hz–100 kHz at E
corr
for mild steel
in 20% formic acid are shown in Figure 5. The
impedance diagrams are not perfect semicircles and this
difference has been attributed to frequency dispersion
[24]. The values of R
t
and C
dl
were obtained using the
Nyquist and Bode plots, respectively [25]. Percentage e
IE
was calculated using the following:
eIE ¼ð1=Rt0Þð1=RtiÞ
ð1=Rt0Þ100 ð6Þ
where R
t0
and R
ti
are the charge transfer resistance
without and with inhibitor, respectively, and are given in
Table 4. Values of R
t
increase with increase in inhibitor
concentration (DPTT) and this, in turn, leads to an
increase in e
IE
. The addition of DPTT to 20% formic
acid lowers the C
dl
values, suggesting that the inhibition
can be attributed to surface adsorption [26].
3.5. Mechanism of corrosion inhibition
The corrosion of mild steel in nonaqueous and aqueous
solution may be considered in the following steps [9]:
Fig. 2. Temkin’s adsorption isotherm plot for the adsorption of
various inhibitors in 20% formic acid, on the surface of mild steel.
(1) HPTT; (2) PUTT; (3) DPTT.
Table 3. Electrochemical polarization parameters for the corrosion of mild steel in 20% formic acid in the absence and presence of 500 ppm of
various inhibitors
System E
corr
/mV
b
a
/mV decade
)1
b
c
/mV decade
)1
I
corr
/mA cm
)2
e
IE
/%
20% Formic acid )416 68 104 0.350 –
HPTT )390 60 120 0.160 54.28
PUTT )402 70 120 0.062 82.28
DPTT )420 64 112 0.046 86.85
Fig. 3. Electrochemical polarization curves for the corrosion of mild
steel in 20% formic acid in the absence and presence of 500 ppm
concentration of various inhibitors. (1) 20% formic acid; (2) HPTT; (3)
PUTT; (4) DPTT.
Fig. 4. Electrical equivalent circuit for the system (R
W
uncompensated
resistance, R
p
polarization resistance and C
dl
double layer capaci-
tance).
236

Fe þHCOO! ½ FeðHCOOÞads þeð7Þ
½FeðHCOOÞads! ½ FeðHCOOÞþþeð8Þ
½FeðHCOOÞþþHþ+
(FeþþHCOOH ð9Þ
The evolution of hydrogen occurs as the cathodic
reaction by the following mechanism:
MþHCOOH þe! MHads þHCOOð10Þ
MHads þMHads! H2þMð11Þ
The adsorption of formate ions on the surface of iron is a
prerequisite for the anodic dissolution to occur; thus the
rate of corrosion should depend on the concentration of
formate ion in the solution. The conductance of formic
acid solution gradually increases in the concentration
range 5%–20%. As a result, the extent of adsorption of
formate ion, as well as the rate of Step 7 increases and,
consequently, the rate of corrosion will also increase.
The triazoles inhibit the corrosion by controlling both
the anodic and cathodic reactions. In acidic solutions
these compounds exist as protonated species. These
protonated species adsorb on the cathodic sites of the
mild steel and decrease the evolution of hydrogen. The
adsorption on anodic sites occurs through the p-elec-
trons of aromatic rings and the lone pair of electrons of
nitrogen and sulfur atoms which decrease the anodic
dissolution. Among the compounds investigated, DPTT
has been found to give the best performance as
corrosion inhibitor. This can be explained on the basis
of the presence of polar groups, as well as through the
p-electrons of the double bond. This leads to greater
surface coverage, thereby giving higher inhibition effi-
ciency. HPTT, containing more than 10 carbon atoms in
the side chain and an internal double bond at position 8,
showed lowest inhibition efficiency, because compounds
containing more than 10 carbon atoms have decreased
solubility and increased steric hindrance to adsorption
[27].
4. Conclusions
Fatty acid triazoles show excellent performance as
corrosion inhibitors in formic acid media. All the
triazoles acted as efficient corrosion inhibitors over a
wide acid range, that is, 10% to 30% formic acid
solutions. They inhibit corrosion of mild steel in formic
acid by an adsorption mechanism, which follows the
Temkin adsorption isotherm.
References
1. S. Muralidharan and S.V.K. Iyer, Anti-Corros Met. and Mater. 44
(1997) 100.
2. M.A. Quraishi, M.A.W. Khan and M. Ajmal, Anti-Corros. Met.
and Mater. 43 (1996) 5.
3. B. Hammouti, A. Aouniti, M. Taleb, M. Bright and S. Kertit,
Corrosion 51 (1995) 411.
4. N. Al-Andis, E. Khamis, A. Al-Mayouf and H. Aboul-Enein,
Corros. Prev. and Cont. 42 (1995) 13.
5. B.A. Abd-El-Nabey, E. Khammis, M.Sh. Ramadan and A. El-
Gindy, Corrosion 52 (1996) 671.
6. E. Heitz, ‘Corrosion of Metals in Organic Solvents’ (Plenum Press,
New York, NY, 1974), p. 226.
7. I. Sekine, H. Ohkawa and T. Hank, Corros. Sci. 22 (1982) 1113.
8. I. Sekine and A. Chinda, Corrosion 40 (1984) 95.
9. M.M. Singh and A. Gupta, Mat. Chem. Phys. 46 (1996) 15.
10. M.A. Quraishi, D. Jamal and M.T. Saeed, J. Am. Oil Chemist’s
Soc.77 (2000) 265.
11. M. Ajmal, D. Jamal and M.A. Quraishi, Anti-Corros. Met. and
Mater. 47 (2000) 77.
12. M.A. Quraishi and D. Jamal, J. Appl. Electrochem., submitted.
13. ASTM (American Society for testing and Materials), ‘Metal
Corrosion, Erosion and Wear’, Annual Book of ASTM Standards
(1987) 0.3.02, G1-72.
14. M.I.H. Kittur and C.S. Mahajanshetti, J. Oil Tech. Assoc. (India)
16 (1984) 49.
15. G. Schmitt, Brit. Corros. J.19 (1984) 165.
16. M.A. Quraishi, M.A.W. Khan, M. Ajmal, S. Muralidharan and
S.V. Iyer, J. Appl. Electrochem. 26 (1996) 1253.
17. M. Schorr and J. Yahalom, Corros. Sci.12 (1972) 867.
Fig. 5. (a) Nyquist plot and (b) Bode plot for mild steel in the absence
and presence of various concentrations of DPTT. (1) 20% formic acid;
(2) 100 ppm and (3) 500 ppm.
Table 4. Electrochemical impedance parameters for the corrosion of
mild steel in 20% formic acid containing different concentration of
DPTT at room temperature
Concentration
/ppm
R
t
/Wcm
2
C
dl
/lFcm
2
e
IE
/%
20% Formic acid 75.00 1862.09 –
DPTT
100 700.00 295.80 89.23
500 778.59 258.22 90.15
237

18. R.T. Vashi and V.A. Champaneri, Ind. J. Chem. Tech.4(1997)
180.
19. J. Radosevic, M. Kliskic, L.J. Aljinovic and S. Vuko, Proceedings
of the 8th European Symposium on ‘Corrosion Inhibition’, Ann.
Univ. Ferrara, Italy (1995), p. 817.
20. I.N. Putilova, S.A. Balezin and U.P. Baranik, ‘Metal Corrosion
Inhibitors’ (Pergamon Press, New York, NY: 1960), p. 31.
21. G.K. Gomma and M.H. Wahadan, Ind. J. Chem. Tech.2(1995)
107.
22. S. Brinic, Z. Grubac, R. Babic and M. Metikos-Hukovic, ‘Study of
the thiourea adsorption on iron in acid solution’, Proceedings of
8th European Symposium on ‘Corrosion Inhibition’, Ann Univ,
Ferrara, Italy 1(1995), pp. 197–205.
23. M. Elachouri, M.S. Hajji, M. Salem, S. Kertit, J. Aride, R.
Coudert and E. Essassi, Corrosion 52 (1996) 103.
24. M.A. Quraishi, M.A.W. Khan, M. Ajmal, S. Muralidharan and S.
Angappan, Portg. Electrochim. Acta 13 (1995) 63.
25. S.T. Hirozawa, Proceedings of the 8th European Symposium on
‘Corrosion Inhibition’, Ann. Univ. Ferrara, Italy, 1(1995) p. 25.
26. N.C. Subramaniyam and S. Mayanna, Corros. Sci.25 (1985)
163.
27. P. Li, T.C. Tan and J.Y. Lee, Corrosion 53 (1997) 186.
238