AN EXPERIMENTAL STUDY ON METHIONINE AS CORROSION
INHIBITOR
Muzaffer ÖZCANa, İlyas DEHRİb,c
a
Department of Science and Technology Education, Cukurova University, 01330 Adana, Turkey
b
Department of Chemistry, Cukurova University, 01330 Adana, Turkey
c
Department of Chemistry, Korkutata University, 80000 Osmaniye, Turkey
ABSTRACT: The inhibition effect of methionine (Met) on the corrosion of mild steel in 0.5 M HCl solution
was investigated using AC impedance spectroscopy and linear polarization resistance measurement (LPR)
techniques at 25±1ºC. The experimental results showed that this compound inhibits the corrosion to some extent
and its inhibition efficiency increased with its concentration. The fractional coverage of the metal surface (θ) was
determined using ac impedance results and it was found that the adsorption of Met on mild steel surface follows
Langmuir adsorption isotherm.
Key Words: Corrosion; Inhibitor; Methionine; Impedance; CPE.
KOROZYON İNHİBİTÖRÜ OLARAK METİYONİN ÜZERİNE DENEYSEL BİR
ÇALIŞMA
ÖZ: Yumuşak çeliğin 0.5 M HCl çözeltisi içindeki korozyonuna metiyoninin inhibitör etkisi 25±1ºC sıcaklıkta
alternatif akım empedans spektroskopisi ve lineer polarizasyon direnci ölçümü yöntemleri kullanılarak
araştırılmıştır. Deneysel sonuçlar bu bileşiğin korozyonu bir noktaya kadar önlediğini ve etkinliğinin
konsantrasyonuyla arttığını göstermiştir. Yüzey kaplanma kesri (θ) alternatif akım empedans spektroskopisi
sonuçları kullanılarak belirlenmiş ve metiyoninin yumuşak çelik yüzeyine adsorpsiyonunun Langmuir
adsorpsiyon izotermine uyduğu belirlenmiştir.
Anahtar Kelimeler: Korozyon; İnhibitor; Metiyonin; Empedans; CPE.
1. INTRODUCTION
Organic compounds containing heteroatoms such as sulfur, nitrogen and oxygen act as
corrosion inhibitors. It is a common fact that there are a large number of organic compounds;
however, we are restricted in choosing a proper inhibitor for a particular system because of
the specific properties of the inhibitors and of the corrosion systems (1).
The limitation of the use of a large number of organic and inorganic inhibitors due to
their possible toxic effects and the realization of their inadequate efficiencies in some cases,
have recently been supporting the claims putting forward the requirement of searching for
new and more effective substances in order to use as corrosion inhibitors (2). Under the light
of these environmental facts, researchers tended to look for ways that were more satisfactory
and these resulting surveys have come with the conclusion of the use of green corrosion
inhibitors (2-4).
The aim of this study is to investigate the inhibition effect of Met using
electrochemical techniques.
2. EXPERIMENTAL DETAILS
1
The working electrode was cut from a mild steel rod (C,0.11; Mn,0.45; Si,0.25;
S,0.050; P,0.040 and remainder iron) and inserted to polyester resin leaving only 0.196 cm2
of the surface area exposed to the corrosive solution. The electrolyte was 0.5M HCl solution
without and with various concentrations (0.5, 1, 5 and 10 mM) of Met. Prior to each
experiment, the working electrode was mechanically polished with a series of emery papers
down to 1200 grade to a mirror finish, degreased in acetone, rinsed with bi-distilled water and
dried with soft paper then immediately inserted into the glass cell containing 100 mL of
solution. All the experiments were done at 25±1°C in solutions open to the atmosphere under
unstirred conditions after 30 min of exposure time.
AC impedance spectroscopy measurements were performed in 100kHz-10mHz
frequency range using a CHI 604A AC electrochemical analyzer. Impedance data were
obtained at the corrosion potential (Ecorr) of the working electrode measured against a
Ag/AgCl reference electrode using a 5mV rms sinusoidal perturbation. A platinum counter
electrode with 1cm2 surface area was also used. The impedance parameters were calculated
by fitting the experimental results to an equivalent circuit using ZView software from
Scribner Associates, Inc.
Polarization resistance measurements were carried out at -10mV to +10mV vs. Ecorr at
a scan rate of 1mVs-1.
3. RESULTS AND DISCUSSION
Fig.1 shows typical set of complex plane plots of mild steel in 0.5M HCl solution in
the absence and in the presence of various concentrations of Met.
1. Blank
2. 0.5 mM Met
3. 1 mM Met
4. 5 mM Met
5. 10 mM Met
Zimag(Ω.cm2)
─ Fit Results
▪ Experimental Results
Zreal(Ω.cm2)
Fig. 1. Complex plane plots for mild steel in 0.5M HCl solution in the absence and in the presence of various
concentrations of Met.
All of the impedance spectra obtained in the absence and in the presence of inhibitors
consist of one depressed capacitive loop corresponding to one time constant in Bode plots. In
order to obtain accurate results the analysis of complex plane plots was done by fitting the
2
experimental results to the equivalent circuit given in Fig. 2, which has been used previously
to model the mild steel/acid interface (5-8). It is obvious from Fig. 1 that the fit results
generally present the similar shape with the ones of experimental results with an average error
of about 2.6% in all the cases. This clearly indicates that the equivalent circuit used to model
the systems under investigation is the most appropriate one.
Fig. 2. Equivalent circuit model represents the metal/solution interface. CPE constant phase element, Rct charge
transfer resistance, Rs solution resistance.
The impedance parameters derived from the complex plane plots are given in Table 1.
The Rp values were also measured at working electrode conditions using the polarization
resistance measurement technique and are given in the same table.
Rct(Ωcm2)b
Rs(Ωcm2)b
Rp(Ωcm2)a
-Ecorr(V)
Ci(mM)
Mol.
Table 1
Impedance parameters derived from complex plane plots in 0.5M HCl solution in the absence
and in the presence of various concentrations of Met.
CPEdl
Yo(x106snΩ-1cm-2)
Blank
Met.
Cdl(x106sΩ-1cm-2)c
IE%d
n(0-1)
-
0.536
71.4
11.0
63.4
60.0
0.889
30.4
-
0.5
0.531
149.3
9.0
126.2
45.4
0.892
24.2
50
1
0.530
196.1
8.8
172.0
38.5
0.891
21.1
63
5
0.527
357.1
8.8
381.8
25.4
0.893
14.8
83
10
0.522
384.6
8.9
419.5
25.3
0.896
15.1
85
a:Rp from polarization resistance measurements. b:Rs and Rct from complex plane plots. c:Yo values were
converted into Cdl as described in Ref. 9. d:Values of percent inhibition efficiency (IE%), were calculated using
Rct from complex plane plots.
Table 1 shows that the addition of the inhibitors into the corrosive solution causes to
an increase in the charge transfer resistance and a decrease in the double layer capacitance.
The decrease in Cdl, which may result from a decrease in local dielectric constant and/or an
increase in the thickness of the electrical double layer (10,11), shows that the inhibitor was
adsorbed on the mild steel surface. Among the most frequently used adsorption isotherms, the
Langmuir adsorption isotherm exhibited the best fit to the experimental data (Fig. 3). The
Langmuir adsorption isotherm can be expressed as (12):
C 1
 C
θ K
(1)
3
where θ is fractional coverage of the metal surface, C is the inhibitor concentration in the
electrolyte and K is the equilibrium constant for the adsorption/desorption process. The value
of ∆G°ads was found as -29.15kJ/mol for Met. The high negative value of ∆G°ads indicates that
this compound is strongly adsorbed on the mild steel surface.
12
10
C/θ
8
6
4
2
0
0
2
4
6
8
10
12
C(mM)
Fig. 4. Langmuir adsorption isotherm plot for mild steel in 0.5 M HCl solution in the presence of various
concentrations of Met.
4. CONCLUSIONS
AC impedance spectroscopy and polarization resistance measurement results indicated
that Met inhibits the corrosion of mild steel in 0.5M HCl solution. AC impedance results were
interpreted using an equivalent circuit in which a constant phase element (CPE) was used in
place of a double layer capacitance (Cdl) in order to give more accurate fit to the experimental
results. Langmuir adsorption isotherm exhibited the best fit to the experimental data with
standard free energy of adsorption, ∆G°ads, of -29.15kJ/mol for Met.
Acknowledgements: The experimental studies of this work were carried out at Physical
Chemistry Research Laboratory of Chemistry Department, Çukurova University.
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