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.