Oral presentation at Eurocorr 2000, September 10-14 2000, London UK

 

 

CORROSION IN AN URBAN SOIL PROFILE - AERATION CELL EXPERIMENT

IN SITU IN THE SOIL

 

 

E Levlin* and T-G Vinka**

*Water Resources Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden

**Swedish Corrosion Institute, Roslagsvägen 101 hus 25, S-104 05 Stockholm, Sweden

 

 

 

ABSTRACT

 

Aeration cell corrosion has been studied in situ in the soil. The aeration cells consisted of two carbon steel sheets. The anode was buried in a lump of clay, and the cathode was buried in the surrounding filling material. Two types of aeration cells were used, one with a cathode of the same size as the anode, and an other with a cathode being 10 times larger. The cell current and the corrosion potential was measured for a period of 2.685 year and the corrosion of the anode was calculated from the cell current and determined by the weight loss. The corrosion potentials of sheet without connection to aeration cells were also measured. Weight loss measurement showed a higher corrosion rate on the anodes in the clay than on cathodes in the filling material. Sheet exposed without connection to any aeration cell had a corrosion rate of about 13 mm/year in both clay and filling material. The pitting corrosion was much higher on unconnected sheets in the filling material compared to sheets in the clay. Connecting the sheets together to aeration cells made the pitting corrosion of the anode to increase and of the cathode to decrease.

 

 

INTRODUCTION

 

Aeration cells has been assumed to create corrosion problems to metallic constructions buried in the soil. An aeration cell is created when one part of a construction lies in a soil with good permeability for oxygen and one part in a soil with poor permeability. The oxygen permeability is good in sand above the groundwater level and the part in the soil with god permeability has a high concentration of oxygen and becomes cathode. The part in the soil with poor permeability, such as water saturated clay, has a low oxygen concentration and becomes anode. Reduction of oxygen on the cathode causes oxidation of metal and thus corrosion on the anode.

 

The corrosion of an aeration cell has previous been studied by laboratory experiments [1,2] where the cell current was measured between two cast iron samples connected through an external cable, one sample buried in sand and the other in clay. The object was to study the influence of soil acidification on corrosion in soil. To simulate the influence of acidification, two cells were used there one was sprayed with water acidified with sulphuric acid and the other with water without acid. However, the effect of acidification on the corrosion rate was small. Other effects such as the position of the groundwater level had a larger influence on the cell current. The maximum current was achieved then the groundwater level was regulated to just below the aeration cell. This was a result of good oxygen supply through the sand combined with high conductivity of the soil. As a reference to the laboratory study the aeration cells in this project was created and monitored in situ in the soil. Since it is difficult to find two similar test places, with different degree of acidification, the influence of acidification could not be studied in this project.

 

The work is part of a larger project made at a test site in Göteborg, Sweden, studying groundwater and soil properties in an urban environment and their effects on the corrosion of soil buried constructions. The other studies has been made on characterising of the soil at the test site, groundwater fluctuation and quality and corrosion of samples of carbon, steel and zinc [3,4,5] buried at the test site together with the aeration cells. The soil consists of a about 1 meter thick layer of filling material of sand with clay and gravel, deposited on a peat bog.

 

 

EXPERIMENTAL METHOD

 

Ten aeration cells consisting of two cold rolled carbon steel SS 13 16-32 sheets were buried at a depth of 1 m and connected together with a cable. The composition of the steel in per cent of weight is 0.063 Al, 0.044 C, 0.006 Cr, 0.022 Cu, 0.222 Mn, 0.003 Mo, 0.004 N, 0.015 Ni, 0.007 P, 0.007 S, 0.013 Si, 0.002 Ti and 0.008 V. The anodic sheets was buried in a lump of clay, and the cathodic was buried in the surrounding filling material. Table I shows the composition of clay and filling material at the depth of 1 meter the underlying peat layer and table II. shows the clay divided into fractions after grain size in mm. Since the groundwater level is constantly at a depth of about 2 meter, the soil around the cathodes is not water saturated..

 

 

Table I. Composition of clay and filling material [3].

 

 

Cl mg/g

SO4 mg/g

pH (H2O)

CaCO3 mg/g

Org. Cont, mg/g*

Clay

185

185

8.1

-

40

Filling material

13

81

7.6

23

Q

*Organic content measured as weight loss while burning a sample at 900 ºC.

 

 

Table II. Clay divided into fractions after grain size in mm.

 

 

Clay

Fine silt

Silt

Coarse silt

Sand

Grain size, mm

< 0.002

0.002 – 0.006

0.006 – 0.02

0.02 – 0,06

> 0.06

Fraction

30 %

11 %

13 %

27 %

19 %

 

 

Figure 1 shows a sketch of two pits with aeration cells, including connections to the logger. Pit 1 with four aeration cells was excavated after 396 days (1.085 years) and pit 2 with six aeration cells was excavated after 980 days (2.685 years). Two types of aeration cells was used, one with a large cathodic sheet, 10 cm times 100 cm (2000cm²), and one with a small cathodic sheet, 10 cm times 10 cm (200cm²). The anodic sheets in the clay was of the same size, 10 cm times 10 cm giving the anode-cathode area ratios 1:1 and 1:10. The distance between the sheets were about 50 cm, with the boundary between clay and filling at a distance of about 30 cm from the sheets in clay. In both pits there were equal number of aeration cells with small and with large anodes. The cell currents passing between cathode and anode were measured manually.

 

 

 

Figure 1. Sketch of the aeration cells buried in two pits at a depth of 1 m and the connections to an automatic potential measuring device with 8 channels.

 

 

Two aeration cells in pit 2 were connected by cables through an eight channel logger to an automatic potential measuring device above ground. At the logger the cables were connected to each other and the potentials were measured against a buried copper sulphate reference electrode. Channel 7 on the logger was connected to a aeration cell with a 10x10 cm cathode and channel 8 to a cell with a 10x100 cm cathode. Daily mean values were calculated from the automatically measured potentials, which were recorded every second or every sixth hour. For reference also sheets without connection to any aeration cells were exposed to the soil in the filling material as well as in the clay. The six remaining channels of the logger were used for measuring the potential of one 10x100 cm sheet (channel 5) and one 10x10 cm sheet (channel 2) in the filling material, a 10x10 cm sheet in the clay (channel 4), a 10x10 cm zinc sheet (channel 6) and 10x10 cm sheets at a depth of 0.5 m (channel 1) and at 1.5 m (channel 3). . Due to failure of the automatic potential measuring device, values for the aeration cells are missing for some periods. During these period manually measured values are used for the none cell sheets, channel 1 to 6.

 

 

EXPERIMENTAL RESULTS

 

Cell currents

 

Figure 2 shows cell current versus time for the aeration cells [6]. The cell current was larger for the cell with the 10x100 cm cathode than for the cell with the 10x10 cm cathode. The difference in cell current between the cells increased with time; from about three times larger in the beginning to about eight. The average cell current was 517.5 mA for the cell with a large cathode and 94.36 mA for the other cell. Integration of the cell currents over the 2.67 year test period gives 42161 As for the cell with a 100x10 cm cathode and 6683 As for the cell with a 10x10 cm cathode. Assuming no cathodic reaction in clay and dissolution of divalent ferric ions, the corrosion of the anodic sheets in the clay can be calculated to 31.7 mm/year for the cell with the large cathode and 5.0 mm/year for the other cell. Since the anodes in the clay are of equal size the measured cell current will be proportional to the corrosion of the anodes.

 

 

Figure 2. Cell current versus time for the aeration cells [6].

 

 

Corrosion potential

 

Figure 3 shows corrosion potential of the carbon steel sheets connected to the automatic measuring device. The numbers at the lines corresponds to the numbers of the channels of the logger in figure 1. The highest corrosion potential were measured for the sheet at the depth of 0.5 m (channel 1) and the lowest for the sheet embedded in clay (channel 4). The corrosion potential is controlled by diffusion of oxygen through the soil, which is shown with that the corrosion potential decreases with increasing depth (channel 1, 0.5 m, channel 2, 1m and channel 3, 1.5 m). Oxygen transport is smallest to the sheet in clay (channel 4), which is shown by this sheet having the lowest potential. The two sheets with different size at 1 m 10 x 10 cm (channel 1) and 100x10 cm (channel 5) have no difference in corrosion potential. The corrosion potential of the aeration cells are mixed potentials and lies between the corrosion potential for the sheet in filling and the sheet in clay. The potential of the aeration cells are a mixed potential and connection of the cathode, sheet in filling, with the anode, sheet in clay, makes the potential of the cathode to decrease and the potential of the anode to increase. The potentials for the cell with a 100x10 cm cathode (channel 5) are higher than the potentials for the cell with a 10x10 cm cathode (channel 2). The potentials are increasing with time with the highest increase for the sheets in filling material.

 

 

 

Figure 3. Corrosion potential of carbon steel sheets measured against a copper sulphate reference electrode. The numbers at the lines corresponds to the channels of the logger in figure 1.

 

 

Weight loss and maximum pitting depth

After the end of the exposure time weight loss and maximum pitting depth was measured on all sheets. Table III shows the weight loss and maximum pitting depth for the sheets of the aeration cells. The weight loss was measured by removing the corrosion products with Clarkes solution (concentrated HCl with 20 g Sb2O3/l and 60 g SnCl2 · 2H2O/l). The sheets were weighted during the etching and the weight was extrapolated back to etching time zero, which is the weight loss due to corrosion. The pitting rate was determined with a microscope. Table IV shows the average general corrosion rate calculated from weight loss and maximum pit growth rate for both coupled sheets of the aeration cells, but also for uncoupled sheets exposed together with the aeration cells in both filling material and clay and quotas coupled/uncoupled, anode/cathode and pit 2/pit 1.

 

 

Tabell III. Weight loss, pitting rate and pitting factor for the sheets in the aeration cells. The numbers of the sheets corresponds to the numbers of the sheets in figure 1.

 

Sheet in pit 1

Cathode

Anode

Cathode

Anode

Cathode

Anode

Cathode

Anode

1.085 year exposure

10

11

12

13

5

14

6

15

Weight loss, g

0.93

1.99

1.84

3.26

14.89

9.52

13.39

9.79

Weight loss, g/m²

46.5

99.5

92.0

163

74.5

476

67.0

490

Weight loss, mm

5.9

12.7

11.7

20.7

9.5

60.6

8.5

62.3

Weight loss, mm/year

5.5

11.7

10.8

19.1

8.7

55.8

7.8

57.4

Pitting mm

196

107

226

127

531

239

496

232

Pitting mm/år

181

98,3

208

117

490

220

457

214

Pitting factor

33

8.4

19

6.1

56

3.9

58

3.7

 

Sheet in pit 2

Cath.

An.

Cath.

An.

Cath.

An.

Cath.

An.

Cath.

An.

Cath.

An.

2.685 year exposure

2

1

4

5

6

7

11

3

13

8

14

9

Weight loss, g

2.72

15.0

1.78

13.7

2.60

13.1

33.3

26.3

33.3

21.9

36.3

16.9

Weight loss, g/m²

136

749

89.0

683

130

656

167

1317

167

1097

182

843

Weight loss, mm

17.3

95.3

11.3

86.9

16.6

83.5

21.2

167

21.2

140

23.1

107

Weight loss, mm/year

6.46

35.5

4.21

32.4

6.17

31.1

7.90

62.4

7.9

52.0

8.61

39.9

Pitting mm

662

675

401

712

624

681

1173

808

1055

747

998

516

Pitting mm/year

247

252

149

265

232

254

437

301

393

278

372

192

Pitting factor

38.2

7.09

35.5

8.19

37.6

8.17

55.3

4.83

49.8

5.35

43.2

4.81

 

 

Tabell IV. Average general corrosion rate calculated from weight loss and maximum pit growth rate for coupled and uncoupled sheets and the quotas coupled/uncoupled, anode/cathode and pit 2/pit 1.

 

Pit 1:

1,085 year

Cathode (sheet in fillning)

Anode (sheet in cley)

Quota anode/cathode

General

Pitting

General

Pitting

General

Pitting

Small

cathode

Uncoupled sheets

15.7 mm/year

573 mm/year

15.1 mm/year

137 mm/year

0,96

0.24

Coupled cells

8.13 mm/year

194 mm/year

15.4 mm/year

108 mm/year

1,86

0.56

Quota coupled/uncoupled

0.53

0.34

1

0.79

 

 

Large

cathode

Uncoupled sheets

17.5 mm/year

834 mm/year

15.1 mm/year

137 mm/year

0.88

0.16

Coupled cells

8.29 mm/year

473 mm/year

56.6 mm/year

217 mm/year

6.9

0.46

Quota coupled/uncoupled

0.49

0.57

3.8

1.58

 

 

 

Pit 2:

2,685 year

Cathode (sheet in fillning)

Anode (sheet in cley)

Quota anode/cathode

General

Pitting

General

Pitting

General

Pitting

Small

cathode

Uncoupled sheets

12.8 mm/year

517 mm/year

13.5 mm/year

69.7 mm/year

1.05

0.13

Coupled cells

5.61 mm/year

209 mm/year

33.0 mm/year

257 mm/year

5.88

1.23

Quota coupled/uncoupled

0,44

0.40

2.44

3.69

 

 

Large

cathode

Uncoupled sheets

14.7 mm/year

617 mm/year

13.5 mm/year

69.7 mm/year

0.92

0.11

Coupled cells

8.14 mm/year

401 mm/year

51.4 mm/year

257 mm/year

6.31

0.54

Quota coupled/uncoupled

0.55

0.65

3.81

3.69

 

 

 

Quota

pit 2/pit 1

Cathode (sheet in fillning)

Anode (sheet in cley)

General

Pitting

General

Pitting

Small

cathode

Uncoupled sheets

0,82

0.90

0.90

0.51

Coupled cells

0,69

1.08

2.2

2.38

Large

cathode

Uncoupled sheets

0,86

0.74

0.90

0.51

Coupled cells

0,98

0.85

0.90

1.18

 

 

Weight loss measurement showed a corrosion rate on the anode in the clay of 33 mm/year with a 10x10 cm cathode and 51.4 mm/year with a 100x10 cm cathode and a corrosion rate on cathode in the filling material of 5.6 mm/year for the 10x10 cm cathode and 8.2 mm/year for 100x10 cm cathode. Sheet exposed without connection had a corrosion rate of about 13 mm/year in both clay and filling material. For sheets without connection the corrosion was almost the same in clay and in filling material (about 13 mm/year). The pitting corrosion was much higher on unconnected sheets in the filling material (about 600 mm/year) compared to sheets in the clay (70 mm/year). Connecting the sheets together to aeration cells made the pitting corrosion of the anode to increase (257 mm/year) and of the cathode to decrease (300 mm/year).

 

 

DISCUSSION

 

Figure 4 shows a comparison of between general corrosion of unconnected sheets measured by weight loss (about 13 mm/year), compared to current densities of the anodes and cathodes of the aeration cells. The potentials of the sheets of the aeration cells were measured by disconnecting the cell and registrating the potential immediately after disconnection. The cell current measured before disconnection and the potentials for unconnected sheets measured at the same occasion are used in the diagram. The corrosion of the anodes is the general corrosion increased with the contribution from the cell current and the corrosion of the cathodes is the general corrosion decreased with the contribution from the cell current. Only for the anode connected to a ten times larger cathode is the corrosion caused by the cell current larger than the general corrosion for an uncoupled sheet in clay. The anodes of the aeration cells are anodically polarized and have a higher corrosion potential than the uncoupled sheet in clay and the cathodes are cathodically polarized and have a lower corrosion potential than the uncoupled sheet in filling. The anode connected to the 100x10 cm cathode is more anodically polarized than the anode connected to the 10x10 cm cathode and has a larger current density and a higher corrosion potential. The 10x10 cm cathode is more cathodically polarized than the 100x10 cm cathode and has a larger current density and a lower corrosion potential.

 

 

 

Figure 4. Corrosion potential versus current density for the sheets of the aeration cells and corrosion potential for uncoupled sheets versus corrosion measured as weight loss and calculated to current density.

 

 

The obtained results can be explained by assuming that the rate of corrosion of the sheets in clay is limited by diffusion of oxygen through the water saturated clay and in the filling material by precipitation of corrosion products on the surface. The cathodic reaction of the sheets in the filling material occurs on the magnetite layer deposited close to the metal surface [5]. The increased difference in cell current with time between the aeration cells; from about three times larger in the beginning to about eight can be explained by deposition of corrosion products on the cathodic sheet, causing a larger part of the anodic dissolution to be transferred to the anodic sheet. The difference in cell current will be 10 with no corrosion of the cathodic sheets and 1.82 with the same corrosion rates on both anodic and cathodic sheets [6]. Deposition of corrosion products on the sheets in filling can also explain why pitting corrosion is higher on unconnected sheets than on connected. The corrosion of the sheets in the filling material is higher on spots there the layer of corrosion products are weaken which gives pitting corrosion. Connecting the sheet to an aeration cells moves the anodic reaction to the sheet in the clay which are not protected by corrosion products and the pitting corrosion of the cathode decreases. However, pitting corrosion resulting in pit holes in pipelines is by experience preferentially occurring in water logged soil there oxygen transport is limited [1]. Water saturated soils has a good electric conductivity and the corrosion current can be transported long distances which makes it possible for the cathodic reaction on a large surface to supply one growing hole. The penetration rate will be proportional to the amount of corrosion current supplied to the hole. Aeration cells is therefore the best explanation to pitting in water logged soils, since the cathodic reaction taking place in good aerated parts of the soils can supply corrosion pits growing in poor aerated parts.

 

 

ACKNOWLEDGEMENT

 

This work is a part of a larger project on corrosion and geochemical studies at a test site in Göteborg financed by the Swedish Council for Building Research. Gratitude's are expressed to the City of Göteborg for assistance with excavator and to the partners of the larger project; Malin Årebäck and her successor Malin Norin at the Department of Geology, Chalmers University of Technology, for doing all the daily work at the test site. Grant to support participation in Eurocorr 2000 has been given by Civilingenjörsförbundets miljöfond.

 

 

REFERENCES

 

[1]     Levlin E, ‘Corrosion of water pipe systems due to acidification of soil and groundwater’, Dep. of Applied Electrochemistry and Corrosion Science, Royal Institute of Technology (Stockholm) 1992

[2]     Levlin E, British Corrosion Journal 1 (1991) 36 - 66

[3]     Norin M, ‘Groundwater and soil properties in an urban environment and their effects on the corrosion of soil buried constructions of carbon, steel and zinc’, Dep. of Geology, Chalmers University of Technology (Göteborg) 1998

[4]     Årebäck M, ‘Hydrogeological investigations and corrosion monitoring in an urban soil profile in Göteborg, Sweden’ Dep. of Geology, Chalmers University of Technology (Göteborg) 1994

[5]     Årebäck M, Vinka T-G, Future Groundwater Resources at Risk, IHAS Publ. 222 (1994) 379-386

[6]     Levlin E, Corrosion Science 12 (1996) 2083-2090

[7]     Stratmann M, Hoffman K, Müller J, Werkstoffe und Korrosion (1991) 467 – 472