of equipment are buried in the ground. Some
examples of localized corrosion are discussed in
the following paragraphs.
. Corrosion due to mill scale. The mill scale
embedded in the walls of iron pipe during its
manufacture is one cause of pipe corrosion. It
actually becomes the cathodic area, the iron pipe
the anodic area, and the moist soil the electrolyte,
as shown in figure 7-13. Current leaves the iron
pipe wall and passes through the electrolytic soil
to the mill scale. This electrochemical action
causes severe pitting of the pipe metal at the
anodic areas. Continued action of this type will
eventually weaken the pipe to the extent of failure.
l Corrosion due to cinders. Another type of
corrosion occurs when iron pipe is laid in a cinder-
fill in direct contact with the cinders. The cinders
and the iron pipe make up the dissimilar metals.
The pipe forms the anodic area, the cinders form
the cathodic area, and the highly ionized soil
serves as the electrolyte. The current leaves the
pipe through the soil to the cinders and returns
to the pipe. Severe corrosion occurs at the points
where the current leaves the pipe.
. Corrosion due to dissimilarity of pipe
surface. This type of galvanic corrosion occurs
when there are bright or polished surfaces on some
areas of the pipe walls in contact with suitable
electrolytic soil. These bright surfaces become
anodic to the remaining pipe surfaces. In highly
ionized soil, the polished surfaces corrode at an
accelerated rate, thus weakening the pipe at that
. Corrosion due to different soil conditions.
This is a general corrosion problem, especially
prevalent in highly alkaline areas. Corrosion cur-
rents leave the pipe wall and pass into compact
soils and enter the pipe wall from light sandy soils.
The intensity of the corrosion currents and the
resulting rate of corrosion at the anodic areas of
the pipe are directly proportional to the con-
ductivity of the soil.
l Corrosion due to stray currents. Direct
current circuits that pass in and out of an elec-
trolyte usually cause stray currents, many of which
are a direct cause of corrosion. Corrosion does
not occur at the point where the current enters
the structure, because it is catholically protected.
However, at the section where the current leaves
the structure, severe stray current corrosion
occurs. Over a period of a year, this type of cor-
rosion has been known to displace as much as 20
pounds of pipe wall for every ampere of current.
. Corrosion due to bacteria. Biological
corrosion is another distinct type of corrosion
caused by electrolytic or galvanic cell action.
It is the deterioration of metals by corrosion
processes that occurs as either a direct or an
indirect result of the metabolic activity of certain
minute bacteria, particularly in water or soil
envirnments. These organisms that cause
bacterial corrosion are bacteria, slime, and fungi.
Figure 7-13.Pipe with corroding (anode) and noncorroding (cathode) areas.