necessary materials and equipment required for
galvanic cathodic protection of underground pipes
and fittings must be considered. First you must
understand what corrosion is and how it occurs.
TYPES OF CORROSION
Man has had corrosion problems to contend
with ever since he started making articles out of
metal. For thousands of years, the only fact
known about corrosion was that it would affect
some metals more than others. For example, iron,
one of the most abundant and useful metals, cor-
rodes very much; whereas metals such as gold,
platinum, and silver corrode very little. Later, men
began to study corrosion to find out what caused
it. As might be expected, many theories were
proposed to explain corrosion and its causes.
Among the many theories, the electrochemical
theory is most generally accepted as an explana-
tion of corrosion.
The electrochemical theory of corrosion is best
explained by the action that takes place in a
galvanic cell. A galvanic cell can be produced by
placing two dissimilar metals in a suitable elec-
trolyte, as shown in figure 7-12. The resulting
electrochemical reaction develops a potential
difference between these metals. This causes one
metal to be negative or anodic and the other metal
to be positive or cathodic. In a dry cell battery,
the zinc case is the anode and the carbon rod the
cathode. Now, when an external electrical circuit
is completed, current flows from the zinc case into
the electrolyte, taking with it particles of zinc. This
is an example of galvanic corrosion of the zinc
case. It is this electrochemical action that
illustrates the electrochemical theory.
Corrosion may be divided into several types,
such as uniform corrosion, localized corrosion,
and compositional corrosion. Each type will be
explained in the following paragraphs.
Uniform corrosion is caused by direct
chemical attack. An example of this type of cor-
rosion is zinc exposed to hydrochloric acid. If you
examine the surface of zinc in a solution of
hydrochloric acid, you will find that the entire
surface is corroding. Furthermore, if the zinc is
left in the acid long enough, it will be dissolved
by the acid.
Localized corrosion is caused by the elec-
trolytic action of a galvanic cell. A local galvanic
action is set up when there is a difference of
potential between the areas on a metallic surface
that is an electrolyte. Localized corrosion may be
in the form of pits, pockets, or cavities due to the
deterioration or destruction of metal.
Localized corrosion may develop under a
number of various conditions when different types
Figure 7-12.Galvanic cell showing internal galvanic action.