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
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.
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.
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.
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).
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.
[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