pipe to corrode. The water collects on the inside
of the pipe because the pipe is usually cooler than
the oil. In a storage tank, the water will settle to
the bottom of the tank because water is heavier
than oil, and will cause the bottom to corrode.
Hydrogen sulfide and sulphur dioxide may also
be introduced into the pipeline to add to the cor-
rosiveness of the water that collects on the metal.
The only way to prevent corrosion from this
source is either to coat the inside of the pipeline
and tanks with a protective film or to remove the
water from them.
Bacterial organisms may also cause
microbiological corrosion. Colonies of bacteria
that live close to the metal surface in stationary
slimy deposits produce corrosive substances such
as carbon dioxide, hydrogen sulphide, ammonia,
and organic and inorganic acids. These corroding
substances are found only in the locality of the
colony and may be undetected in the surrounding
water or soil. Bacteria that cause corrosion in this
way need to produce only small amounts of cor-
rosive products for localized attack. However,
colonies of bacteria that do not produce corrosive
products may act as a protective film around the
metal, causing unequal distribution of electrical
potential, which gives rise to local anodes and
cathodes. In this way, the production of local cells
will cause increased corrosive action.
Biological corrosion is extremely difficult to
control, since the organisms are very resistant to
normal methods of sterilization. Probably the
most logical method to reduce microbiological
corrosion is by the use of some barrier coating
between the environment and the metal.
Corrosion Caused by Electrolytes
An electrolyte is any substance that conducts
electricity. It conducts electricity because it con-
tains ions that carry electrical charges, either
negative or positive, that move in electrical fields.
Some of the more important electrolytes are
discussed in the following paragraphs.
ATMOSPHERIC CONDITIONS. Cor-
rosion due to atmospheric conditions is caused
mainly by the water in the atmosphere. Pure water
is a nonelectrolyte, but because water is a universal
solvent, it is not found to be pure very often. Rain
water is often considered to be pure, but this is
not true. As rain falls to the ground, it dissolves
gases out of the atmosphere and becomes impure.
For this reason, any water vapor in the
atmosphere is also impure. If a piece of metal is
exposed to atmospheric air, and the metal is cooler
than the air, water vapor from the air will collect
on the surface of the metal. The layer of water
on the metal maybe so thin that it cannot be seen;
but there is enough of it, if impure, to start cor-
rosion. In this case, when the gases dissolve into
the water, the water becomes an electrolyte. When
metal is exposed to an electrolyte, galvanic cells
are produced on the surface of the metal, since
there are impurities in it. Each one of these cells
starts to act on the metal, causing corrosion by
WATER AND WATER SOLUTIONS. If
metal is exposed to water or water solutions, cor-
rosion is likely to occur if the water or metal is
impure. If the water or metal is pure, corrosion
probably will not occur; however, these conditions
seldom exist in nature. Impurities in the water and
metal produce galvanic cells that cause corrosion.
CHEMICAL AGENTS. Chemical agents
such as acids and salts also cause corrosion. When
these agents are present in the environment, direct
chemical attack on metal is the result. For
example, if a piece of zinc is exposed to
hydrochloric acid, a definite chemical reaction
takes place. The zinc and hydrochloric acid com-
bine, producing zinc chloride and hydrogen. This
action continues until the zinc is completely
dissolved or the acid is too weak to act on the zinc.
Corrosion causes the zinc to dissolve.
Another example that may be used to illustrate
corrosion through the use of a chemical agent is
to place aluminum in a lye solution. The lye will
pit (corrode) the aluminum as long as chemical
action continues between the aluminum and lye.
MATERIALS LEAST LIKELY
TO BE AFFECTED BY
SCALE AND CORROSION
Whenever installing various types of plumb-
ing equipment in areas where corrosion is active,
you should select equipment made of materials
least affected by corrosion. To prevent elec-
trochemical action in plumbing equipment, the
equipment should be made of materials that are
not affected by electrolysis. Plastic materials such
as polyethylene polyester and polyvinyl chloride
are not acted upon by corrosion. Glass is another
material that is not acted on by corrosion. (This