structure, severe, stray-current corrosion occurs. Over a period of a year, this type of corrosion is known
to displace as much as 20 pounds of iron-pipe wall for every ampere of current.
f. Bacteria.
(1) Another distinct corrosion that results from the electrolytic or galvanic cell action of
minute organisms is bacteria (microbiological corrosion). Microbiological corrosive action is the
deterioration of metal by a corrosion process that occurs as either a direct or indirect result of the activity
of certain bacteria, particularly in water or soil environments. Organisms that cause microbiological
corrosion are bacteria, slime, and fungi.
(2) The microbiological corrosive action that occurs in the soil is due to the physical and
chemical changes in soil by the action of bacterial-type organisms. Some bacterial-type organisms are
responsible for the production of active galvanic cells. These cells are produced by variations of the
oxygen content in the soil (differential aeration) or by the reduction of the hydrogen film over the
cathodic areas (depolarization). Bacterial-type organisms are mostly found in highly water-logged,
sulfate-bearing, blue-clay soils. The bacterial concentration as well as the corrosion rate varies
considerably with the different seasons of the year. Cast iron and steel pipe are corroded mostly by the
sulfides produced by the bacteria.
g. Dezincification. Dezincification is a selective corrosion that occurs in copper and zinc
alloys. When alloys of this kind (brasses) are exposed to dezincification corrosion, the zinc will dissolve
out of the alloy, leaving only copper. Since most pipe fittings are made of brass, dezincification attacks
and weakens these brass fittings to the point of failure. In this case, the zinc ions go into solution,
leaving the copper. The solution may be impure water or oil that acts as an electrolyte.
h. Graphitization. Graphitization, or graphite softening, is a peculiar disintegration form that
attacks gray cast iron. Cast iron is an alloy made of iron and carbon, the carbon being in the form of
graphite. When cast iron with such a composition is subjected to graphitization, the graphite pipe may
last for many years if it is not subjected to any mechanical forces or sudden pressures.
i. Hydrogen Embrittlement. Hydrogen embrittlement is a term applied to metal that becomes
brittle due to hydrogen action on its surface. When hydrogen forms on the surface of steel, the hydrogen
action may form blisters or actually embrittle the metal. It has been demonstrated that hydrogen, which
is liberated near the surface of steel in an electrolyte environment, will diffuse into the metal quite
rapidly. This hydrogen, picked up by the steel in an atomic state, causes the steel to become brittle.
When atomic hydrogen production on the metal's surface stops, the hydrogen leaves the metal in a few
days and the metal again regains its original ductility. Carbon steels have shown that they are affected
by hydrogen embrittlement according to the hardness in the steel. The harder the metal, the greater the
susceptibility to hydrogen embrittlement. Hydrogen embrittlement in carbon steel is also increased by
the presence of stress.
5-13
EN0562
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