UFC 3-570-06
JANUARY 31 2003
and the electrolyte is in contact with both, current flows, resulting in oxidation and
reduction reactions (corrosion and protection). The area of the pipeline or tank, which is
the anode, corrodes.
Since the ground tends to consist of horizontal layers of dissimilar soils,
pipelines that traverse several layers of soil tend to be affected by this type of corrosion
frequently. Water and oil well casings are prime examples of this type of
electrochemical corrosion cell. Other examples are pipelines that go through areas of
generally different materials such as rock, gravel, sand, loam, clay, or different
combinations of these materials.
There are over 50 general types of soil that have been characterized for
corrosion properties. Each of the different types of soils has different soil resistivity
values. In areas where the soil resistivity values vary greatly in relatively short
distances, dissimilar environment corrosion cells are formed. These types of
electrochemical corrosion cells are most serious when the anode is relatively small, soil
resistivity is the lowest and the electrical potential difference is the greatest. Examples
of corrosive soils are Merced (alkali) silt loam, Montezuma (alkali) clay adobe, muck,
and Fargo clay loam.
Figure 2-3. Concentration Cell Caused by Different Environments
Anodic Area on Pipeline
Cathodic Area on Pipeline
Cathodic Area on Pipeline
Current Flow in the Earth
2-2.2.2
Oxygen Concentration. Pipelines or tanks that are exposed to an
electrolyte with a low oxygen concentration are generally anodic to the same material
exposed to an electrolyte with a high oxygen content. This is most severe when a
pipeline or tank is placed on the bottom of the excavation, then backfill is placed around
the remaining part of the structure. The backfill contains a relatively high amount of
oxygen during the excavation and backfill operation. This can also occur when the
metal is exposed to areas that have different levels of oxygen content.
2-7
Previous Page
