What causes corrosion on Iron and
Steel?
Principles of Corrosion.
Corrosion may take place by direct
chemical attack or by electrochemical (galvanic) attack; the latter
is by far the most common mechanism. When two dissimilar metals that
are in electrical contact are connected by an electrolyte, an
electromotive potential is developed, and a current flows.
The magnitude of the current depends on
the conductivity of the electrolyte, the presence of high resistance
“passivating” films on the electrode surfaces, the relative areas
of electrodes, and the strength of the potential difference. The
metal that serves as the anode undergoes oxidation and goes into
solution (corrodes).
When different metals are ranked
according to their tendency to go into solution, the galvanic series,
or electromotive series, is obtained. Metals at the bottom will
corrode when in contact with those at the top; the greater the
separation, the greater the attack is likely to be.
Table 4-14 is such a ranking, based on
tests by the International Nickel Company, in which the electrolyte
was seawater.
The nature of the electrolyte may
affect the order to some extent. It also should be recognized that
very subtle differences in the nature of the metal may result in the
formation of anode-cathode galvanic cells: slight differences in
composition of the electrolyte at different locations on the metal
surface, minor segregation of impurities in the metal, variations in
the degree of cold deformation undergone by the metal, etc.
It is possible for anode-cathode
couples to exist very close to each other on a metal surface. The
electrolyte is a solution of ions; a film of condensed moisture will
serve.
Corrosion Prevention.
An understanding of the mechanism of
corrosion suggests possible ways of minimizing corrosion effects.
Some of these include:
(1) avoidance of metal combinations
that are not compatible,
(2) electrical insulation between
dissimilar metals that have to be used together,
(3) use of a sacrificial anode placed
in contact with a structure to be protected (this is an expensive
technique but can be justified in order to protect such structures as
buried pipelines and ship hulls),
(4) use of an impressed emf from an
external power source to buck out the corrosion current (called
cathodic protection),
(5) avoiding the presence of an
electrolyte—especially those with high conductivities, and
(6) application of a protective coating
to either the anode or the cathode.
0 comments:
Post a Comment