(3) Active-passive concentration cell corrosion is commonly found on metals that depend on
a tightly adhering passive film, usually an oxide, for corrosion protection. The corrosive action starts as
an oxygen-concentration cell. For example, salt deposits forming on metal surfaces in the presence of
water containing oxygen can create an oxygen cell. The corrosive action of the low-oxygen cells will
break the passive film beneath the dirt particles, which in time will expose the active metal beneath the
film to corrosion action (Figure 5-9). An electrical potential develops between the large area of the
cathode (passive film) and the small area of the anode (active metal), which can produce rapid pitting of
the active metal.
Figure 5-9. Low-oxygen cell corrosion action
5-3. Causes. The corrosive action on pipelines, structures, and equipment conveying water,
petroleum, and gases is a problem of vast importance to the Army. Instead of maintaining a few feet of
pipe as we do in our homes, the Army maintains thousands of feet. A substantial saving is made if the
effect of corrosion on equipment is decreased. Corrosion may develop under a number of conditions,
among them are mill scale, cinder, dissimilarity of pipe surface, different soil condition, stray current,
bacteria, dezincification, graphitization, and hydrogen embrittlement.
a. Mill Scale. One cause of pipe corrosion is mill scale which is embedded in the walls of iron
pipe during its manufacture. The component parts of this corrosion condition are the
Cathode (cathodic area), which is the mill scale that causes the corrosion condition.
Anode (anodic area), which is the iron pipe where the corrosion takes place.
Conductor, which is the pipe that also serves as a metallic conductor passing currents
from the anodic area to the cathodic area via an external electrical circuit.
Electrolyte, which is the moist soil that acts as a nonmetallic conductor of current in the
external electrical circuit.
5-11
EN0562
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