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ELECTROCHEMICAL STUDIES ON THE CORROSION
OF SOME Fe-Si INJECTION ANODES
Adrian Cristian Manea1, Mihai Buda
1, Teodor Vişan
1, Iosif Lingvay
2 and Liviu Ancăş
3
1Department of Applied Physical Chemistry and Electrochemistry,
University POLITEHNICA of Bucharest, Romania 2ICPE C.A., 313, Splaiul Unirii, Bucharest, Romania 3S.C.P.T.G.N. TRANSGAZ , 6, Unirii street, Mediaş, Romania
Abstract. Investigations on Fe-Si alloys with three different Si contents (11.5% to 15.2%)
were performed in Na2SO4 and Na2SO4-NaCl aqueous solutions at 25 0C. OCP-time, cyclic
voltammetry, i-E potentiodynamic curves as well as EIS curves (Nyquist and Bode diagrams)
were analysed. Corrosion potentials and currents in both types of solutions were estimated.
Introduction
The corrosion behaviour of Fe-Si alloys is a topic of considerable interest with
practical applications and economic impact in various fields, being known for their chemical
stability in acid media. This justifies the use of Fe-Si as material for manufacturing of pumps
or valves in chemical industry [1], as well as of injection anodes (auxiliary electrodes) for
cathodic protection systems in natural gas transportation [2]. The formation of passive layers
was demonstrated [3] by immersing Fe-Si samples with 8-20 %(at) Si in 1 N sulphuric acid;
in the same work was shown that the passivity of alloy with a content below 14% Si is
controlled by the iron oxide film increasing, whereas in the case of higher silicon content the
control is due to formation of silicon dioxide. Early, Brusic and MacInnes [4] have shown that
in alkaline media the passivating film contains a mixture of oxides and hydroxides of Si and
Fe.
In the present paper, the investigation on corrosion behaviour of Fe-Si electrodes with
three silicon concentrations in the range 11.5-15.2 %(wt) is reported. Both Na2SO4 and
Na2SO4-NaCl aqueous solutions were selected as aggresive media, in order to simulate the
stability of buried injection anodes in salted soils. It is obvious that the efficient use of such
auxiliary electrodes and, therefore, the economicity of cathodic protection systems is
determined mostly on the quality of such anodes, especially on their electrical resistance
(called in the specialty literature as "dispersion resistance") [5,6]. Besides the soil resistivity
(or more generally, the resistivity of environment), the keeping of dimensions of these anodes
depends on the stability of Fe-Si alloy and on the amplitude of protection current and life
time, as well.
Experimental Part
Three samples of cast-iron with silicon content of 11.54 %(wt), 12.59 %(wt) and
15.24 %(wt), respectively, provided from Vlahita Foundry, Harghita county, Romania, were
used for investigations. In the following, their corresponding notations will be sample II,
sample IV and sample VII, respectively. They were in a shape of rods with electrical isolated
cylindrical part, exposing to electrolyte the bottom surface area of 2.70-3.15 cm2, only. Prior
to each electrochemical test, the exposed metal surface was abraded gradually by emery
paper, until a bright surface is reached, was cleaned carefully and washed with distilled water.
The auxiliary electrode was a platinum gauze; all electrode potentials were measured with
respect to saturated calomel electrode (SCE), used as reference electrode.
The test solutions were 0.5 M and 0.1 M Na2SO4 aqueous solutions, prepared from
analytical grade sodium sulphate in distilled water. Chloride ion was also introduced (0.2 M
and 0.5 M NaCl concentration) in order to simulate a more aggresive soil. During performed
experiments the temperature was kept constant (25 0C) and solutions were not desaerated.
The measurements were carried out using a computer controlled Zahner IM 6
electrochemical system. The behaviour of Fe-Si anodes was assessed on the basis of four
electrochemical techniques: variation of open-circuit potentials (OCP) in time, cyclic
voltammetry (CV), potentiodyamic polarization curves (i-E) and electrochemical impedance
spectroscopy (EIS).
Results and Discussion
Firstly, we present comparatively the results obtained using Fe-Si electrodes (samples
II, IV and VII) in Na2SO4 solutions (Figures 1-8).
0 300 600 900 1200 1500 1800
-0.6
-0.5
-0.4
-0.3
-0.2
E, V
/EC
S
t, s
VII
IIIV
Fig. 1a. The variation of OCP in time for all three Fe-Si samples in 0.5 M Na2SO4.
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0
10
20
30
40
50
60
70
80
90
100
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
II, VII
i, m
A
E, V/ECS
IV
i, m
A
E, V/ECS
II
VII
Fig. 1b. CV curves (scan rate 10 mV/s) in 0.5 M Na2SO4.
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1E-4
1E-3
0.01
0.1
1
10
100
i, m
A
E, V/ECS
IV
VII
II
Fig. 1c. The semilogarithmic polarization curves (i-E potentiodynamic curves,
scan rate 3 mV/s) in 0.5 M Na2SO4.
0 100 200 300 400
0
100
200
300
400
E = -0.600 V/SCE
E = -0.420 V/SCE
E = -0.360 V/SCE
E = +0.500 V/SCE
-Zim
,
Zre,
Fig. 2a. EIS - Nyquist curves for sample II in 0.5 M Na2SO4
at four electrode potentials (indicated in Figure).
10-1
100
101
102
103
104
105
100
101
102
0
-10
-20
-30
-40
-50
-60 E = -0.600 V/SCE
E = -0.420 V/SCE
E = -0.360 V/SCE
E = +0.500 V/SCE
|Z|,
f, Hz
j, deg
Fig. 2b. EIS - Bode curves both in log/Z/-log frequency (f) and in degree of phase angle (φ)-
-log frequency (f) coordinates for sample II in 0.5 M Na2SO4.
0 50 100 150 200 250 300 350 400
0
50
100
150
200
250
300
350
400
0 1 2 3 4 5 6 7 8
0
1
2
3
4
5
6
7
8
E = -0.6 V/SCE
E = -0.2 V/SCE
-Zim
,
Zre,
-Z
im,
Zre,
Fig. 3. The Nyquist curves for sample IV in 0.5 M Na2SO4
at two electrode potentials (-0.6 V/SCE in insertion).
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
E = -0.6 V/SCE
E = -0.2 V/SCE
E = +0.2 V/SCE
E = +0.6 V/SCE
-Zim
,
Zre,
Fig. 4. The Nyquist curves for sample VII in 0.5 M Na2SO4.
0 300 600 900 1200
-0.7
-0.6
-0.5
-0.4
E, V
/SC
E
t, s
II
IV
VII
Fig. 5a. The variation of OCP in time for all three Fe-Si samples in 0.1 M Na2SO4.
Fig. 5b. CV curves (scan rate 10 mV/s) in 0.1 M Na2SO4
(samples II and VII in insertion).
-0.7 0.0 0.7 1.4
1E-3
0.01
0.1
1
10
100
i, m
A
E, V/SCE
IV
IIVII
Fig. 5c. i-E potentiodynamic curves, scan rate 3 mV/s, in 0.1 M Na2SO4.
0 500 1000 1500 2000 2500
0
500
1000
1500
2000
2500
E = -0.6 V/SCE
E= -0.4 V/SCE
E = -0.32 V/SCE
E = +0.8 V/SCE
-Zim
,
Zre,
Fig. 6. EIS - Nyquist curves for sample II in 0.1 M Na2SO4.
0 100 200 300 400 500 600 700
0
100
200
300
400
500
600
700
E = -0.6 V/SCE
E = -0.2 V/SCE
E = +0.8 V/SCE-Zim
,
Zre,
Fig. 7. EIS - Nyquist curves for sample IV in 0.1 M Na2SO4.
0 500 1000 1500 2000 2500 3000
0
500
1000
1500
2000
2500
3000 E = -0.6 V/SCE
E = -0.35 V/SCE
E = -0.2 V/SCE
E = +0.8 V/SCE
-Zim
,
Zre,
Fig. 8. EIS - Nyquist curves for sample VII in 0.1 M Na2SO4.
From OCP-time curves (Figs. 1a and 5a) one can notice a drift of potential towards
more negative potentials. After a time period of 3-5 minutes, the open-circuit potential of
sample VII shifts to more positive values, showing the passive properties of this alloy. By
contrast, the samples II and IV, for which OCP potentials remain quite constant or even move
slowly towards more negative potential values, cannot be considered as stable materials.
It was shown by cyclic voltammetry (Figs. 1b and 5b) the occurence of an anodic
maximum current followed by a sudden decrease up until oxygen evolution occurs at about
+ 1.3 V, as can be seen from the Figures inserts. The shape of cyclovoltammograms indicates
an irreversible dissolution process of Fe-Si alloys, consisting probably in the selective
dissolution of Fe and formation of iron oxides.
The same process of continuous dissolution and passivation is clearly seen on the
anodic branches of potentiodynamic curves (Figs 1c and 5c) as plateaux. Due to the silicon
content of alloys and also of inhomogeneity, large stains on the corroded surface and even
pitting attacks were noticed.
Better performances of sample VII were confirmed by EIS measurements; for this
alloy, as can be seen from Figs.2-4 and 6-8, the diameter of semicircles is larger, thus
showing the lowest corrosion current (the diameter in Nyquist diagrams is a measure of
polarization resistance, Rp, which in turn is inversely proportional to corrosion current).
The obtained results in the presence of chloride ions are presented in Figures 9-
0 300 600 900 1200 1500 1800
-0.7
-0.6
-0.5
-0.4
-0.3
E, V
/SC
E
t, s
VII
II
IV
Fig. 9a. The variation of OCP in time for all samples in 0.1 M Na2SO4 + 0.2 M NaCl.
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1E-3
0.01
0.1
1
i, m
A
E, V/SCE
II
VII
Fig. 9b. i-E poytentiodynamic curves, scan rate 3 mV/s, in 0.1 M Na2SO4 + 0.2 M NaCl.
0 200 400 600 800 1000 1200
0
200
400
600
800
1000
1200
E = -0.6 V/SCE
E = -0.45 V/SCE
E = -0.29 V/SCE
E = +0.8 V/SCE
-Zim
,
Zre,
Fig. 10a. EIS - Nyquist curves for sample II in 0.1 M Na2SO4 + 0.2 M NaCl.
10-1
100
101
102
103
104
105
1
10
100
1000
0
-10
-20
-30
-40
-50
-60 E = -0.6 V/SCE
E = -0.45 V/SCE
E = -0.29 V/SCE
E = +0.8 V/SCE
|Z|,
f, Hz
j, grd
Fig. 10b. EIS - Bode curves for sample II in 0.1 M Na2SO4 + 0.2 M NaCl.
0 500 1000 1500 2000 2500 3000 3500 4000
0
500
1000
1500
2000
2500
3000
3500
4000
E = -0.6 V/SCE
E = -0.2 V/SCE
E = +0.3 V/SCE
E = +0.8 V/SCE
-Zim
,
Zre,
Fig. 11. EIS - Nyquist curves for sample VII in 0.1 M Na2SO4 + 0.2 M NaCl.
0 300 600 900 1200 1500 1800
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
E, V
/EC
S
t, s
VII
II
IV
Fig. 12a. The variation of OCP in time for all samples in 0.1 M Na2SO4 + 0.5 M NaCl.
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
i, m
A
E, V/ECS
II
VII
Fig. 12b. i-E poytentiodynamic curves, scan rate 3 mV/s, in 0.1 M Na2SO4 + 0.5 M NaCl.
0 100 200 300 400 500 600
0
100
200
300
400
500
600
E = -0.6 V/ECS
E = -0.2 V/ECS
E = 0.2 V/ECS
E = 0.7 V/ECS
-Zim
,
Zre,
Fig. 13. EIS - Nyquist curves for sample II in 0.1 M Na2SO4 + 0.5 M NaCl.
0 300 600 900 1200 1500
0
300
600
900
1200
1500
E = -0.6 V/ECS
E = -0.2 V/ECS
E = 0.2 V/ECS
E = 0.7 V/ECS
-Zim
,
Zre,
Fig. 14. EIS - Nyquist curves for sample VII in 0.1 M Na2SO4 + 0.5 M NaCl.
By adding NaCl in the 0.1 M Na2SO4 solutions sample IV shows a markedly
increased corrosion, sample II has an intermediate behaviour, whereas sample VII seems to
be only slightly affected or even unaffected. Table 1 shows comparatively the corrosion
potentials and currents in both kinds of solutions.
Table 1. The values of corrosion potentials and corrosion currents estimated by
drawing Tafel slopes for both cathodic and anodic branches
of i-E potentiodynamic curves (surface area of electrodes: 2.7-3.1 cm2).
Electrode II Electrode IV Electrode VII
icorr, μA Ecorr,
V/SCE icorr, μA
Ecorr,
V/SCE icorr, μA
Ecorr,
V/SCE
0.1 M Na2SO4 19 -0.51 87 -0.70 33 -0.61
0.5 M Na2SO4 35 -0.52 97 -0.60 3.4 -0.18
0.1 M Na2SO4 + 0.2 M NaCl 13 -0.51 large - 9.8 -0.44
0.1 M Na2SO4 + 0.5 M NaCl 44 -0.51 large - 9.9 -0.48
The particular behaviour of sample IV is surely due to its inhomogeneity, proving that
this is an important parameter in obtaining a good corrosion resistance.
Our results confirm that silicon containing cast-iron cannot be used in chloride media,
because Cl- ion (and F
-, as well) is a strong depolarizer of corrosion process represented by
dissolution of iron. However, this kind of injection anodes, having a silicon content of 11-15
%(wt), exhibits a corrosion rate 20 times less than iron wastes and is recommended in those
areas where the chloride content is below 0.1 g Cl-
/ kg soil and soil resistivity is below
20 Ω·m.
R E F E R E N C E S
1. G.H. Kelsall, R.A. Williams, J. Electrochem. Soc.,138, 931 (1991).
2. C. Lingvay, F. Stoian, "Conception and design of the injection anodes for complex active
corrosion protection systems" in: Study and control in the perspective of sustainable
development of urban distribution grids, 2-nd Int.Conf., June 19-21, 2003, Miercurea-Ciuc,
Romania, p.151-159.
3. Y. Omurtag, M. Doruk, Corros.Sci., 10, 225 (1969).
4. V. Brusic, R.D. MacInnes, J. Aboaf, Passivity of Metals (R.P. Frankenthal, J. Kruger eds.),
The Electrochemical Society, Pennington, N.J., 1978 p.170.
5. I. Lingvay, G. Rus, C. Lingvay, L. Ancăş, Protectia anticoroziva a conductelor metalice
subterane, Electra, Bucharest, 2002.
6. I. Lingvay, C. Lingvay, M. Paraian, Electrosecuritatea si controlul coroziunii structurilor
metalice, Electra, Bucharest, 2003.
Acknowledgements
This work was supported by the Ministry of Education, Research and Youth,
RELANSIN - National Research Programme.
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