UFC 3-570-06
JANUARY 31 2003
(become a cathode) and another part of the structure to discharge current (become an
anode).
Stray current corrosion occurs where the current from the external source
leaves the metal structure and enters back into the electrolyte, normally near the external
power source cathode. The external power source is the driving force, or the voltage, of
the cell. Stray current corrosion is different from natural corrosion because it is caused by
an externally induced electrical current and is basically independent of such
environmental factors as concentration cells, resistivity, pH and galvanic cells. The
amount of current (corrosion) depends on the external power source, and the resistance
of the path through the metallic structure compared to the resistance of the path between
the external source's anode and cathode.
Figure 2-12. Stray Current Corrosion Cell Caused by External Anode and Cathode
Metallic connection or External DC power source
Cathodic Area on Tank
Anodic Area on Tank
TANK
Anode
Cathode
Current Flow in the Earth
Current Flow in the Earth
An example of stray current corrosion is caused by impressed current
cathodic protection systems, where a "foreign" electrically continuous structure passes
near the protected structures anodes and then crosses the protected structure
(cathode). This corrosion is usually found after failures in the foreign structure occur.
Stray current corrosion is the most severe form of corrosion because the metallic
structure is forced to become an anode and the amount of current translates directly
into metal loss. If the amount of current leaving a structure to enter the electrolyte can
be measured, this can be directly translated into metallic weight loss. Different metals
have specific amounts of weight loss when exposed to current discharge. This weight
loss is normally measured in pounds (or kilograms) of metal lost due to a current of one
amp for a period of one year (one amp-year). For example, if a stray current of just two
amps were present on a steel pipeline, the result would be a loss of 18.2 kilograms
2-15
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