Indian Journal of Chemical Technology
Vol. 12, July 2005, pp. 472-476
Corrosion inhibition of carbon steel by adipic acid – Zn2+ system
G Ruba Helen Florencea, A Noreen Anthonya, J Wilson Sahayarajb, A John Amalrajb & Susai Rajendranb*
a
Department of Chemistry, Holy Cross College, Tiruchirappalli 620 002, India
Corrosion Research Centre, Department of Chemistry, GTN Arts College, Dindigul 624 005, India
b
Received 22 July 2004; revised received 28 March 2005; accepted 25 April 2005
The inhibition efficiency (IE) of adipic acid (AA)-Zn2+ system in controlling corrosion of carbon steel immersed in
well water has been evaluated by weight-loss method. The formulation consisting of 50 ppm of AA and 50 ppm of Zn2+ has
95% IE. At lower pH value(pH=6) IE decreases and in alkaline medium (pH=8) IE increases. Polarization study reveals that
AA- Zn2+ system functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal
surface. FTIR spectra reveal that the protective film consists of Fe2+-AA complex and Zn(OH)2.
Keywords: Carbon steel, corrosion inhibition, adipic acid, well water, zinc ion
IPC Code: C23F11/00
Several carboxylates such as sodium salicylate1,
sodium cinnamate2, anthranilate3, and adipate4 have
been used as inhibitors. These inhibitors are described
by the formula R(COO-)n where R can be alkyl or aryl
and n is usually 1 or 2 but can be 3-6. A very wide
range of such chemicals have been shown to be
effective inhibitors of the corrosion of mild steel. The
chief requirement seems to be that the R.COOH acid
should have a pKa value of at least 4. Thus, in the
straight chain monocarboxylic acids, the sodium salt
of formic H.COOH acid with pKa=3.75 is not
inhibitive whereas the higher members of the series,
beginning with acetate, pKa=4.76 are inhibitive.
Similarly, with the dicarboxylates (CH2)n (COO-)2
oxalate with n=0 and pKa =1.23 and malonate with n
=1 and pKa =2.54 are non-inhibitive whereas higher
members of the series ie., succinate (pKa=4.17)
azelate (pKa=4.54) and sebacate (pKa=4.55) are very
good inhibitors1. It has also been shown1 that for aryl
carboxylates ortho-substituted benzoates are less
effective than meta- or para-substituted compounds.
This effect is also presumably associated with the
lower pKa value of the ortho-compounds since there is
little difference in efficiency between ortho- and parasubstituted cinnamates for which the pKa values are
similar. This observation on the acid strength is
probably the only reliable statement that can be made
________________
*For correspondence (E-mail:[email protected])
in predicting whether a carboxylate will be an
inhibitor and even this prediction may relate only to
mild steel. The inhibitive properties of carboxylates to
other metals and alloys are, so far, impossible to
predict. Benzoate is a good inhibitor of corrosion of
mild steel but not of cast iron or zinc1 whereas the
structurally related cinnamate is effective for these
other metals. A ring structure for the inhibitor
molecule is not essential for the protection of cast iron
since some aliphatic dicarboxylates have this
property2. Generally, the substituted cinnamates are
better than substituted benzoates as inhibitors.
Experience with benzoate suggests that carboxylates
are ‘safe’ inhibitors in the sense that they are less
likely to promote localized attack than some other
anodic inhibitors in the presence of excess chloride or
sulphate. Reviews of carboxylates as corrosion
inhibitors have appeared from time to time1, 3-6, 7.
More detailed studies of particular carboxylates
have also been published. Some examples included
sodium salicylate8, sodium cinnamate9, sodium
phenyl acetate10, anthranilate11, thiodivaleric12,
adipate13. Corrosion of tin in citric acid solution and
effect of some inorganic anions have been studied14.
Synergistic effect of succinic acid and Zn2+ in
controlling corrosion of carbon steel has been
reported15. The present work is undertaken: (i) to
evaluate the inhibition efficiency of adipic acid (AA)
as its sodium salt in controlling corrosion of carbon
FLORENCE et al.: CORROSION INHIBITION OF CARBON STEEL
steel in well water, in the absence and presence of
Zn2+ (ii) to study the various pH values on the IE of
the AA- Zn2+ system, (iii) to analyze the protective
film by FTIR (iv) to make use of polarization study
and AC impedance spectra to know the mechanistic
aspects of corrosion inhibition, and (v) to propose a
suitable mechanism of corrosion inhibition based on
the results from the above studies.
Experimental Procedure
Preparation of the specimens
Carbon steel specimens (S 0.026, P 0.06, Mn 0.4, C
0.1% and the rest iron) of the dimensions 1.0×4.0×0.2
cm were polished to a mirror finish and degreased
with trichloroethylene, and used for the weight-loss
method and surface examination studies. AR grade
chemicals were used in the present study.
Weight-loss method
The characteristics of the well water used in the
present study have been shown in Table 1.
Carbon steel specimens in triplicate were immersed
in 100 mL of the solutions containing various
concentrations of the inhibitor (sodium salt of adipic
acid) in the presence and absence of Zn2+ (as
ZnSO4.7H2O) for one day. The weight of the specimens before and after immersion were determined
using a Shimadzu AY62 model balance. The
corrosion products were cleansed with Clarke’s
solution16. The corrosion inhibition efficiency (IE)
was then calculated using the equation
IE =100[1-(w2/w1)]%
where,
w1= corrosion rate(mdd) in absence of inhibitor
w2= corrosion rate(mdd) in presence of inhibitor.
Surface examination studies
The carbon steel specimens were immersed in
various test solutions for a period of one day. After
one day, the specimens were taken out, washed with
distilled water and then dried. The nature of the film
formed on the surface of metal specimens was
analyzed by the following surface analysis techniques.
FTIR spectra
The film was carefully removed with sharp edged
glass rod, mixed thoroughly with KBr and made into
pellets, and the FTIR spectra were recorded on a
Perkin-Elmer 1600 spectrophotometer.
473
Potentiostatic polarization study
This study was carried out using EG and G
electrochemical impedance analyzer model 6310. A
three-electrode cell assembly was used. Carbon steel
was used as working electrode, platinum was used as
counter
electrode
and
saturated
calomel
electrode(SCE) was used as reference electrode.
Corrosion potential, corrosion current and Tafel
slopes were calculated.
AC impedance measurements
EG and G electrochemical impedance analyzer
model 6310 has introduced a very effective approach
to AC impedance measurements. The cell set-up was
the same as that used for polarization measurements.
A time interval of 5 to 10 min was given for the
system to attain a steady state open circuit potential.
Then over this steady state potential, AC potential of
10 mV was superimposed. The real part(z’) and
imaginary part(z”) of the cell impedance were
measured in ohms for various frequencies. The Rt
(charge transfer resistance) and Cdl (double layer
capacitance) values were calculated.
Results and Discussion
Weight-loss study
The corrosion inhibition efficiencies of adipic acid
(AA)-Zn2+ systems are given in Table 2. It is found
Table 1⎯Physico- chemical parameters of well water
Parameters
Value
pH
Conductivity
TDS
Chloride
Sulphate
Total hardness
8.38
3111 μmhos/cm
2010 ppm
665 ppm
14 ppm
1100 ppm
Table 2⎯Inhibition efficiency (IE%) of various AA-Zn2+
systems, when carbon steel is immersed in well water for one
day
Inhibition efficiency (IE%)
AA
ppm
0
5
0
50
100
150
200
250
62
63
75
76
77
3
85
86
86
87
87
Zn2+(ppm)
10
25
5
90
91
91
92
92
8
93
95
96
96
96
50
22
95
96
97
98
98
474
INDIAN J. CHEM. TECHNOL., JULY 2005
that the IE increases as the concentration of AA
increases. As the concentration of Zn2+ increases, IE
also increases. A synergistic effect exists between
AA and Zn2+ . For example, 50 ppm of AA has 62%
IE; 50ppm of Zn2+ has 22% IE. However, the
formulation consisting of 50 ppm of AA and 50 ppm
of Zn2+ has 95% IE. That is mixture of inhibitors
shows better inhibition efficiency than the individual
inhibitors.
Table 3⎯Influence of pH on the inhibition efficiency of the
adipic acid (AA)(50 ppm) and AA (50 ppm)-Zn2+ (5 ppm)
systems
Immersion period: one day
Sl. No.
pH
1
2
3
4
6
8
9
11
Influence of pH on the IE of AA- Zn2+ system
The influence of pH (addition of H2SO4 or addition
of NaOH) on the IE of AA and AA-Zn2+ systems is
given in Table 3. It is found that when pH=6(acidic),
the IE decreases. This is due to the fact that in acidic
medium the protective film is broken by the acid;
moreover in acid medium adipic acid exists in
unionized form. So, it cannot easily coordinate with
Fe2+ to form Fe2+-AA complex on the metal surface.
When pH is increased, IE slightly increases for AA
system. This is due to the fact that ionization of AA is
increased by the presence of OH- in solution, and
hence coordination of AA with Fe2+ is enhanced.
When pH is increased IE slightly decreases for the
AA- Zn2+ system. This is due to the fact that when
NaOH is added to the AA- Zn2+ system Zn(OH)2 is
precipitated in the bulk of the solution. Zn2+ is not free
to transport AA towards the metal surface. Hence, a
decrease in the IE is noticed. However, when more
NaOH is added (at pH 11) sodium zincate is formed
which is in solubilized form. Now, Zn2+ is free
transport AA towards metal surface to form Fe2+-AA
complex on the metal surface and hence an increase in
IE is noticed.
Surface analysis
The protective film formed on the surface of the
metal in the presence of AA-Zn2+ system has been
analyzed by FTIR spectroscopy.
Analysis of FTIR spectra
FTIR spectra of pure adipic acid (AA) and film
formed on the surface of metal after immersion in
well water containing 50 ppm of AA and 50 ppm of
Zn2+ are shown in Fig. 1a and Fig. 1b respectively.
The C=O stretching frequency of carboxyl group
shifts from 1719 cm-1 (Fig. 1a) to 1700 cm-1 (Fig. 1b).
This indicates that the oxygen atom of carboxyl group
has coordinated with Fe2+-AA complex on the anodic
sites of the metal surface. The peak at 1350 cm-1 is
due to Zn(OH)2 formed on the cathodic sites17,18.
Inhibitor system
AA
AA-Zn2+
IE (%)
51
70
62
85
66
75
68
88
Table 4⎯Corrosion parameters of carbon steel immersed in
well water in the presence and absence of inhibitor
obtained by polarization method
AA
ppm
Zn2+
ppm
Ecorr
mV vs SCE
Icorr
A/cm2
ba
mV
bc
mV
0
50
0
50
-511
-477
2.6×10-8
0.16×10-8
32.00
53.00
32.50
53.00
Fig. 1⎯FTIR Spectra: (a) Pure adipic acid (KBr), (b) Film formed
on carbon steel after immersion in well water containing 50 ppm
of adipic acid and 50 ppm of Zn2+
Analysis of potentiostatic polarization curves
The potentiostatic polarization curves of carbon
steel immersed in various test solutions are shown in
Fig. 2. The corrosion parameters are given in Table 4.
When carbon steel is immersed in well water, the
corrosion potential is –511 mV versus saturated
calomel electrode (SCE). The formulation consisting
of 50 ppm of AA and 50 ppm of Zn2+ shifts the
corrosion potential to –477 mV versus SCE. This
suggests that the anodic reaction is controlled
predominantly, since more AA is transported to the
anodic sites in the presence of Zn2+. The Tafel
slopes(ba and bc) are equal (53 mV). These results
suggest that the AA-Zn2+ formulation functions as a
mixed inhibitor. The corrosion current for well water
is 2.6×10-8 A/cm2. The corrosion current for
formulation consisting of AA(50 ppm)-Zn(50 ppm)
has decreased to 0.16×10-8 A/cm2.
FLORENCE et al.: CORROSION INHIBITION OF CARBON STEEL
475
Fig. 2⎯Polarization curves of carbon steel immersed in various
test solution: (a) Well water, (b) Well water containing 50 ppm of
adipic acid and 50 ppm of Zn2+
Analysis of AC impedance spectra
The AC impedance spectra of carbon steel
immersed in various test solutions are shown in
Fig. 3. The AC impedance parameters, namely,
charge transfer resistance(Rt) and double layer capacitance (Cdl) are given in Table 5. Well water has Rt
value of 0.04×103 Ωcm2 and Cdl value of 9.999×10-5
μFcm-2. When AA and Zn2+ are added to well water,
Rt value increases tremendously from 0.04×103 to
363.3×103 Ωcm2. The Cdl decreases from 9.999×10-5
to 0.0044×10-5 μFcm-2. This suggests that a protective
film is formed on the surface of the metal. This
accounts for the very high IE of AA-Zn2+ system.
Mechanism of corrosion inhibition
Weight loss study reveals that the formulation
consisting of 50 ppm of adipic acid(AA) and 50 ppm
of Zn2+ has 95% IE. Polarization study reveals that
this formulation functions as a mixed inhibitor. AC
impedance spectra reveal that a protective film is
formed on the metal surface. FTIR spectra reveal that
the protective film consists of Fe2+-AA complex and
Zn(OH)2. In order to explain the above facts, the
following mechanism of corrosion inhibition is
proposed.
¾ When the formulation consisting of 50 ppm of
AA and 50 ppm of Zn2+ is prepared, there is
formation of Zn2+-AA complex in solution.
¾ When carbon steel is immersed in this solution,
Zn2+-AA complex diffuses towards the metal
surface.
¾ On the metal surface, Zn2+-AA complex is
converted into Fe2+-AA complex on the anodic
sites.
Fig. 3⎯AC impedance spectra of carbon steel immersed in
various test solutions: (a) Well water, (b) Well water containing
50 ppm of adipic acid and 50 ppm of Zn2+
Table 5⎯Impedance parameters of carbon steel in well water
in the presence and absence of inhibitor obtained by
AC impedance method
AA
ppm
Zn2+
ppm
Rt
Ωcm2
Cdl
μFcm-2
0
50
0
50
0.04×103
363.3×103
9.999×10-5
0.0044×10-5
Zn2+-AA + Fe2+
Fe2+-AA + Zn2+
¾ The released Zn2+ combines with OH- to form
Zn(OH)2 on the cathodic sites.
Zn2+ + OH-
Zn(OH)2
¾ Thus, the protective film consists of Fe2+-AA
complex and Zn(OH)2.
Conclusion
The present
conclusions:
•
study
leads
to
the
following
The inhibition efficiency(IE) of adipic acid
(AA)-Zn2+ system in controlling corrosion of
INDIAN J. CHEM. TECHNOL., JULY 2005
476
•
•
•
•
•
carbon steel immersed in well water has been
evaluated by weight-loss method.
The formulation consisting of 50 ppm of AA
and 50 ppm of Zn2+ has 95% IE.
At lower pH value (pH=6) IE decreases and
in alkaline medium (pH=8) IE increases.
AC Impedance spectra reveal that a
protective film is formed on the metal
surface.
FTIR spectra reveal that the protective film
consists of Fe2+-AA complex and Zn(OH)2.
Polarization study reveals that AA-Zn2+
system functions as a mixed inhibitor.
Acknowledgement
The authors are thankful to their management,
Prof. E. Kalman(Hungary), Prof. F. Zucchi (Italy) and
UGC, India, for their support and encouragement.
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