The current leaves the iron-pipe wall, passes through the electrolyte soil to the mill scale, and returns to
the iron pipe. This electrochemical action causes severe metal pitting at the anodic areas. Continued
action of this type will eventually weaken the pipe and cause it to fail.
b. Cinder. Another type of corrosion occurs when iron pipe is laid in a cinder fill and is in
direct contact with the cinders. The component parts of cinder corrosion are the-
Dissimilar metals, which are the cinders and the iron pipe that also serve as conductors.
Anode, which is the iron-pipe wall where the corrosion takes place.
Cathode, which is the cinders that cause the corrosion.
Electrolyte, which is the highly ionized soil that serves as the external electrical circuit or
environment for the corrosion process.
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. Galvanic corrosion wears away the pipe at an
accelerated rate because of the nonpolarizing effect of the cinders and the highly ionized soil
contamination of the cinders.
c. Dissimilarity of Pipe Surface. This galvanic corrosion occurs when there are bright or
polished surfaces on some areas of iron-pipe walls, and the dissimilar pipe surfaces are in contact with
suitable electrolytic soil. A pipe wrench can produce bright surfaces, such as scars and scratches, on the
pipe when assembling it. The threads on both ends of a coupling may expose polished surfaces that
corrode easily. Corrosion in the threads will eventually cause the perforation of the iron-pipe wall.
d. Different Soil Condition. This is a general corrosion problem that is especially prevalent in
high-alkaline areas. Corrosion currents enter or pass through an iron-pipe wall from compact soils.
Corrosion currents also enter or pass through the iron-pipe wall from light sandy soils. The intensity of
the corrosion currents and the resulting corrosion rate at the pipe's anodic areas are directly proportional
to the soil's conductivity. Earth current meters are used to determine the location of the anodic and
cathodic areas and the extent to which a corrosion current exists. This meter determines if the pipe
requires protection.
e. Stray Current. Stray currents, many of which are direct causes of corrosion, are usually
direct-current circuits that pass in and out of an electrolyte. This condition poses the greatest problem in
the vicinity of electrical-transportation systems, electrified coal mines, or manufacturing plants where
the direct-current distribution system requires a ground as a complete or partial circuit return. If a
metallic structure, such as a tank or pipeline, is laid in such an area, a large galvanic cell is created,
making a perfect setup for corrosion. Corrosion does not occur at the point where the current enters the
structure because it is cathodically protected. However, at the section where the current leaves the
EN0562
5-12
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