Microbiological corrosive action in the soil is due
to physical and chemical changes in the soil caused
by the presence of these organisms. Some bacteria
are responsible for the production of active
galvanic cells. These bacteria are mostly found in
highly waterlogged, sulfate-bearing, blue clay
soils. The bacteria concentration, as well as the
corrosion rate, varies considerably with the dif-
ferent seasons of the year. Cast-iron and steel
pipes are corroded mostly by sulfide production.
Compositional corrosion alters the composi-
tion of metals. Some of the specific types of
compositional corrosion are discussed in the
. Dezincification. This is a selective type of
corrosion that occurs in copper and zinc alloys.
When alloys of this kind (brasses) are exposed to
this type of corrosion, the zinc dissolves out of
the alloy and leaves only the copper.
. Graphitization. Another type of composi-
tional corrosion is graphitization or graphitic
softening. It is a peculiar form of disintegration
that attacks grey 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
iron dissolves out and leaves only the graphite.
This action leaves cast-iron pipes and other similar
equipment weakened mechanically. However,
after graphitization corrosion occurs, the graphite
pipe may last for many years if it is not subjected
to any mechanical forces or sudden pressures. The
action of this type of corrosion is similar to
. Hydrogen embrittlement. This is a term
applied to metal that becomes brittle because of
the action of some form of corrosion that causes
the formation of hydrogen on its surface. When
hydrogen forms on the surface of steel, the ac-
tion of the hydrogen may form blisters or actually
embrittle the metal. Hydrogen liberated near the
surface of steel in an electrolyte will diffuse into
the metal quite rapidly. The hydrogen picked up
by the steel is in an atomic state and causes the
steel to become brittle.
When the production of atomic hydrogen
on the surface of the metal stops, the hydrogen
leaves the metal in a few days and the metal again
regains its original ductility.
Stress Fatigue of Metals
Corrosion affects metals that are under stress.
The action caused by stresses on a pipeline or
structure is due to the shifting of the various rocks
and soils of the earth. Usually a complete pipeline
is not under stress; certain sections are under stress
while adjacent sections are not. Because of these
pressures and strains, localized electrochemical
action takes place. The section of the pipe or
structure under stress becomes anodic, whereas
the unstressed sections become cathodic. In this
way, the pipe under stress begins to corrode and
weaken because of the action of corrosion.
Corrosion Caused by Nonelectrolytes
Nonelectrolytes are materials that will not
conduct electricity. These materials include
nonelectrolytic vapors, liquids, and bacterial
organisms. Since they do not conduct electricity,
they do not, in themselves, cause corrosion.
NONELECTROLYTE GASES AND
VAPORS. Nonelectrolytic gases and vapors
usually must be subjected to high temperatures
before corrosive action can take place. Hydrogen
sulfide causes scaling of iron at temperatures from
1400° to 2000°F. High-chromium alloy steels
resist this type of corrosion best. The only remedy
for this type of corrosion is to keep the gases away
from the metal or use a metal that can resist
High-carbon steels are attacked by hydrogen
at temperatures above 750°F. This hydrogen com-
bines with the carbon grains in the steel and causes
the metal to weaken at the grain boundaries
between the iron and carbon.
Oxygen will combine directly with most metals
at high temperatures. The temperature at which
oxygen will combine with the metals depends
mostly upon the type of metal. In the process of
cutting iron with an oxyacetylene torch, the
oxygen combines with the iron.
NONELECTROLYTIC FLUIDS. Non-
electrolytic fluids include such liquids as pure
water, lubricating oils, fuel oils, and alcohols.
These fluids do not cause corrosion, but corro-
sion does occur in storage tanks that contain these
liquids and in pipelines that carry them. The
corrosion is not caused by the nonelectrolyte
liquids, but by the foreign products in them. For
example, if impure water is introduced into an oil
pipeline, the water will cause the inside of the