Corrosion In Soil

CorrosionScience,Vol. 38, No. 12,pp. 2083-2090,1996
Copyright 0 1996Else&r Science Ltd
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AERATION CELL CORROSION OF CARBON STEEL IN SOIL:
1h’ SITU MONITORING
CELL CURRENT AND POTENTIAL
E. LEVLIN
Water Resources Engineering, Royal Institute of Technology, S-100 44 Stockholm, SwedenAbstract-Aeration
cell corrosion in soil has been studied in situ in the soil at a test site in Goteborg, Sweden. The
aeration cell consisted of two carbon steel sheets, the anodic, buried in a lump of clay, and the cathodic, buried in the
surrounding filling material. Two aeration cells were used, one with a cathodic sheet of the same size as the anodic
sheets, and an other with a cathodic sheet 10times larger. The anode-cathode area ratio of the cells were 1:l and
1:10. Both corrosion current and potential was measured during the 2.67 y test period. As a reference, the corrosion
potential of sheets with no connection to any aeration cells were measured. The corrosion of the anodic sheets in
clay can be calculated from the cell current to 31.7 urn/y (2.59 pA/cmz) for the cell with arearatio 10 and 5.0 urn/y
(0.47 pA/cm*) with area ratio 1. The difference in cell current between the cells increased with time; from about three
times larger in the beginning to about eight times larger. This 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 cellcurrent will be 10 with no corrosion of the cathodic sheets and 1.82 with the same corrosion rates on
both anodic and cathodic sheets. Copyright 0 1996 Elsevier Science Ltd

INTRODUCTION
Aeration cells have 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 foroxygen, for example sand, and one part in a soil with poor
permeability, for example water saturated clay. The part in the soil with good permeability
has a high concentration of oxygen and becomes the cathode and the part in the soil with
poor permeability, has a low oxygen concentration and becomes the anode. The corrosion
of an aeration cell has been studied in the laboratory’,’ where thecorrosion current was
measured between two cast iron samples connected through an external cable, one sample
buried in sand and the other in clay. The maximum corrosion 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 laboratorystudy the aeration cells in this project were created and
monitored in situ in the soil. In situ studies of corrosion in soil have been made, for example,
at the U.S. National Bureau of Standards,3 where polarization techniques were used to
study corrosion of samples buried in the ground outside the laboratory. The samples was
connected to the laboratory by cables. Polarization measurementswas taken at 40 occasions
during the 16 month test period. The corrosion of the steel samples seemed to have a
logarithmic time relationship. In an aeration cell one of the samples are buried in a well
aerated soil and the other in a poorly aerated soil. The difference in corrosion between well
Manuscript received 5 February 1996.
2083

E. Levlin

2084

and poorly aerated soils has alsobeen studied at the U.S. National Bureau of Standards.4*5
The exponent for the time dependence of both mass loss and pitting is small in soil with good
aeration and high in poorly aerated soils. Due to the good oxygen supply in good aerated
soil, the corrosion rate is high in the beginning, but with time the rate decreases. Dissolved
ferric ions are oxidized and precipitated close to the...