CORROSION
The
corrosion of metals is essentially the reverse of the
recovery of metals from their ores. The reduction of
aluminum oxide to metallic aluminum is a good example.
(a) Metal Refining
Aluminum Ore + Energy - - - - - -Aluminum Metal
(b) Corrosion
Aluminum Metal - - - - - - - -- - - Aluminum Ore + Expenditure
of Energy
Man
expends much energy, money and technology to obtain
metals in a relatively pure state. Metals do not want
to exist in a pure state and will take the path of least
resistance to accomplish this objective. The latter
part of this statement can be considered as corrosion.
This is why only a few metals are found in a metallic
state and why these metals exhibit good corrosion resistance.
Gold, silver, platinum and copper are good examples.
To
excape the pure state, metals will react with practically
any chemical environment. The length of time required
for substantial or complete corrosion to occur depends
on several factors:
- The
chemical activity of the metal involved.
- The
type, concentration and temperature of the chemical
environment.
- Proximity
to other metals.
The
general problem of corrosion is extremely complex and
many areas are not completely understood even by corrosion
engineers with expertise in this field.
Some
appreciation for the mechanisms involved in corrosion
of metals can be gained by reviewing the electrochemistry
involved in a simple battery or plating bath. A battery,
for example, can be as fundamental as taking a breaker,
filling it with chemical solution (electrolyte) and
suspending a strip of copper on one side and a strip
of iron on the other side. A simple experiment and observation
will reveal two types of phenomena are occurring.
- A
voltmeter connected between the two plates will
show that a voltage can be read.
- Metal
will leave one plate and be deposited on the other
plate.
These
phenomena are a mixed blessing:
- With
proper selection of materials and controls they form
the basis for the manufacture of batteries and the
electroplating industry.
- On
the other hand they are the basic mechanism for galvanic
corrosion of metals.
The
vulnerability of exposed metallic components in a buried
cable can be easily visualized with a little imagination.
Take for example a buried type URD cable. The concentric
wires are exposed to a chemical warehouse, the contents
of which will vary from one geographical location to
another. Some possibilities are:
- Acid
soils
- Alkaline
soils
- Chemical
solutions (water plus dissolved chemicals)
- Industrial
waste and fills
In
this type of environment the concentric wires become
one plate, a buried pipeline a second plate and with
a wet soil in between - an ideal set-up for electrochemical
reaction resulting in corrosion of the concentric wires.
Some
costly and complex schemes are available to minimize
corrosion. The simplest and most feasible with cables
is to provide a covering or jacket so that metal components
are not exposed to detrimental chemical environments.
This, too, adds cost.
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