Page 421 - Corrosion Engineering Principles and Practice

390 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 391

Water
Water constitutes the essential electrolyte that supports electro-
chemical corrosion reactions in water saturated or unsaturated soils.
The groundwater level is important in this respect. It fluctuates
from area to area, with water moving from the water table to higher
soil, against the direction of gravity. Saturated water flow is depen-
dent on pore size and distribution, texture, structure, and organic
matter. Water movement in soil can occur by gravity, capillary
action, osmotic pressure (from dissolved species), and/or electro-
static interaction with soil particles. The water-holding capacity of a
soil is strongly dependent on its texture. Coarse sands retain very
little water, while fine clay soils store water to a high degree.
Degree of aeration
Oxygen concentration typically decreases with increasing soil
depth. In neutral or alkaline soils, oxygen concentration has an
important effect on corrosion rate as a result of its participation in
the cathodic reaction. However, in the presence of certain microbes
(such as sulfate-reducing bacteria), corrosion rates can be very
high, even under anaerobic conditions. Oxygen transport is more
rapid in coarse-textured, dry soils than in fine, waterlogged tex-
tures. Excavation can obviously increase the degree of aeration in
soils. It is generally accepted that corrosion rates in disturbed soil
with greater oxygen availability are significantly higher than in
undisturbed soil.

pH
Soil pH typically varies between 5 and 8. In this range, pH is generally
not considered to be the dominant variable affecting corrosion rates.
More acidic soils produced by mineral leaching, decomposition of
acidic plants (e.g., coniferous tree needles), industrial wastes, acid
rain, and certain forms of microbiological activity represent a serious
corrosion risk to common construction materials such as steel, cast
iron, and zinc coatings. On the other hand, alkaline soils tend to have
high sodium, potassium, magnesium, and calcium contents, with the
latter two elements forming possibly protective calcareous deposits
on buried structures.

Soil resistivity
Resistivity has historically been used as an indicator of soil corrosivity.
Since ionic current flow is associated with soil corrosion reactions,
high soil resistivity will usually slow down corrosion reactions. Soil
resistivity generally decreases with increasing water content and the
concentration of ionic species. Soil resistivity is by no means the only
parameter affecting the risk of corrosion damage and a high soil
resistivity alone will not guarantee the absence of corrosion. Soil
resistivity variations along the length of a pipeline, for example, may