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9
Cathodic Protection
A tremendous investment exists worldwide in underground metallic members such as pipelines, storage facilities, well casings, structure supports,
communication cables, etc. When these members are in direct contact with
the soil and unprotected, they are subject to corrosion.
Attempts have been made to overcome this problem using various types
of coatings applied to the surface. Although such coatings can be effective,
invariably small holes or holidays are usually present. These holidays provide access to the uncovered base metal and permit corrosion.
Subsequent repairs can be extremely costly. In addition, undetected failure
(or leakage) can cause accidents or lead to safety hazards, such as causing
roadways to collapse as a result of an underground water leak, or to environmental hazards resulting from oil leaks.
By cathodic protection, bare metals and holidays in coated metals can be
protected from corrosion.
9.1 Background
Cathodic protection is a major factor in metals’ corrosion control. When an
external electric current is applied, the corrosion rate can be reduced to practically zero. Under these conditions, the metal can remain indefinitely in a
corrosive environment without deterioration.
In practice, cathodic protection can be utilized with such metals as steel,
copper, brass, lead, and aluminum against corrosion in all soils and almost
all aqueous media. Although it cannot be used above the waterline (because
the impressed electric current cannot reach outside the electrolyte), it can
effectively be used to eliminate corrosion fatigue, intergranular corrosion,
stress corrosion cracking, dezincification of brass, or pitting of stainless steels
in seawater, or steel in soil.
In 1824, Sir Humphry David reported that by coupling iron or zinc to copper, the copper could be protected against corrosion. The British Admiralty
had blocks of iron attached to the hulls of copper-sheathed vessels to provide
corrosion protection. Unfortunately, cathodically protected copper is subject
to fouling by marine life, which reduced the speed of vessels under sail and
forced the admiralty to discontinue the practice. However, the corrosion rate
9
Cathodic Protection
A tremendous investment exists worldwide in underground metallic members such as pipelines, storage facilities, well casings, structure supports,
communication cables, etc. When these members are in direct contact with
the soil and unprotected, they are subject to corrosion.
Attempts have been made to overcome this problem using various types
of coatings applied to the surface. Although such coatings can be effective,
invariably small holes or holidays are usually present. These holidays provide access to the uncovered base metal and permit corrosion.
Subsequent repairs can be extremely costly. In addition, undetected failure
(or leakage) can cause accidents or lead to safety hazards, such as causing
roadways to collapse as a result of an underground water leak, or to environmental hazards resulting from oil leaks.
By cathodic protection, bare metals and holidays in coated metals can be
protected from corrosion.
9.1 Background
Cathodic protection is a major factor in metals’ corrosion control. When an
external electric current is applied, the corrosion rate can be reduced to practically zero. Under these conditions, the metal can remain indefinitely in a
corrosive environment without deterioration.
In practice, cathodic protection can be utilized with such metals as steel,
copper, brass, lead, and aluminum against corrosion in all soils and almost
all aqueous media. Although it cannot be used above the waterline (because
the impressed electric current cannot reach outside the electrolyte), it can
effectively be used to eliminate corrosion fatigue, intergranular corrosion,
stress corrosion cracking, dezincification of brass, or pitting of stainless steels
in seawater, or steel in soil.
In 1824, Sir Humphry David reported that by coupling iron or zinc to copper, the copper could be protected against corrosion. The British Admiralty
had blocks of iron attached to the hulls of copper-sheathed vessels to provide
corrosion protection. Unfortunately, cathodically protected copper is subject
to fouling by marine life, which reduced the speed of vessels under sail and
forced the admiralty to discontinue the practice. However, the corrosion rate