d. The pipeline passes through a marshy area. The pie to either side of the marshy area is dry and well
aerated allowing for higher concentrations of oxygen at the pipe, whereas the part of the pipe in the marshy area
has a higher concentration of electrolyte (water/moisture).
7.
Stray Current Migration.
a. Stay current corrosion is that caused by direct current traveling through the earth from some source
external to the underground metallic structure. It then can be picked up by the structure at one point (creating a
"cathodic" condition), flow along the structure (example: pipeline) for a distance, and then discharge into the
environment to complete the circuit to the external source.
NOTE: The point of discharge from the metal structure is the anodic (anode) also referred to in the corrosion
field as a "holiday."
b. Stay currents tend to be most severe on long underground structures, such as pipelines, but can be a
problem with any underground structure if stray currents are present in the earth.
c. Sources of stray current can be either man-made (usually the more serious) or natural.
(1) Man-made sources of stray direct current include but are not limited to:
D-C electric motor.
D-C powered transit (trolley) systems.
D-C powered mining operations.
D-C welding operations.
High voltage direct current electric power transmission systems.
Cathodic protection systems on underground structures "other than the affected structure."
NOTE: Of the sources listed, all are dynamic sources except the last one which is typically static.
(2) A natural source of stray current is what is known as "telluric" or earth current of magnetic
origin. These are direct currents, of a variable nature, in the earth's crust which are a result of variations.
(a) Bacteriological corrosion:
1 Bacteria requires organic material as a food supply. This is normally available to at least
some degree in the earth.
16-5
QM5200
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