corrosion cell circuit is often restored to some
degree with the coating providing some measure
of resistance to the flow of current.
degree with the coating providing some measure
of resistance to the flow of current.
4.2. Characteristics of specific coatings.- The type
of coating and the nature of the damage or
deterioration it suffers will bear strongly on the
nature of the metal corrosion which may occur.
Damage to coal-tar enamel, a widely used coating
for buried metal, is most often of the mechanical
type caused by rocks during backfilling. This
damage usually produces holes in the coating at
a few distinct locations, the remainder of the
surface being fully protected. The enamel itself
undergoes virtually no degradation over the years
and offers effectively complete electrical
installation of the metal. The result is that
corrosion is usually concentrated at a very few
points of damage which can be repaired or
cathodically protected at nominal cost. Concrete
or cement mortar, another widely used coating,
protects by virtue of the alkaline environment it
creates. This is usually effective as long as the
coating is of good quality and free of wide cracks
or spalls. Mortar is also a very durable coating but
one which at present is considered to provide little
or no electrical resistance in a corrosion cell
circuit. The CA-50 coal--tar paint, asphalt, red
lead and aluminum, vinyl resin, and many thin film
coatings suffer much more repaid deterioration of
the coating materials; and gradually, defects in
the coating become generally distributed over the
surface. As deterioration proceeds, the degree of
electrical resistance usually provided decreases.
Hot-dip zinc (galvanizing) acts as the anode of a
corrosion cell and, thus, protects the base metal
at points where there are small breaks in the
coating. When the zinc has been entirely
consumed, the base metal is exposed for
corrosion.
of coating and the nature of the damage or
deterioration it suffers will bear strongly on the
nature of the metal corrosion which may occur.
Damage to coal-tar enamel, a widely used coating
for buried metal, is most often of the mechanical
type caused by rocks during backfilling. This
damage usually produces holes in the coating at
a few distinct locations, the remainder of the
surface being fully protected. The enamel itself
undergoes virtually no degradation over the years
and offers effectively complete electrical
installation of the metal. The result is that
corrosion is usually concentrated at a very few
points of damage which can be repaired or
cathodically protected at nominal cost. Concrete
or cement mortar, another widely used coating,
protects by virtue of the alkaline environment it
creates. This is usually effective as long as the
coating is of good quality and free of wide cracks
or spalls. Mortar is also a very durable coating but
one which at present is considered to provide little
or no electrical resistance in a corrosion cell
circuit. The CA-50 coal--tar paint, asphalt, red
lead and aluminum, vinyl resin, and many thin film
coatings suffer much more repaid deterioration of
the coating materials; and gradually, defects in
the coating become generally distributed over the
surface. As deterioration proceeds, the degree of
electrical resistance usually provided decreases.
Hot-dip zinc (galvanizing) acts as the anode of a
corrosion cell and, thus, protects the base metal
at points where there are small breaks in the
coating. When the zinc has been entirely
consumed, the base metal is exposed for
corrosion.
4.3. Effect of defective coatings on corrosion. -
Corrosion of metal protected by defective coat-
ings progresses at locations of the defects; that is,
where the coating is actually gone from the
surface as a result of blistering, cracking, peeling,
or mechanical damage. This process produces
the pitting type of corrosion. Under normal
corrosive conditions such as a very localized
galvanic cell, the penetration of the metal at the
defect may be little, if any, faster than if the
Corrosion of metal protected by defective coat-
ings progresses at locations of the defects; that is,
where the coating is actually gone from the
surface as a result of blistering, cracking, peeling,
or mechanical damage. This process produces
the pitting type of corrosion. Under normal
corrosive conditions such as a very localized
galvanic cell, the penetration of the metal at the
defect may be little, if any, faster than if the
coating were not present over the rest of the
surface. However, if sizeable stray currents are
operative or a galvanic cell is producing a
significant voltage, more rapid consumption of the
metal localized at the defects may be expected. In
the light of these facts, it may be asked why
coatings are applied. In most instances, coatings
provide excellent corrosion prevention in
themselves, sufficiently effective that cathodic
protection is not usually needed on Reclamation
structures. Coatings for bur-led structures are
very durable and corrosion voltages are very
rarely high enough to promote accelerated failure.
Further, should detrimental corrosion occur, an
insulating type of coating cuts cathodic protection
costs drastically (to perhaps 10 percent of the
cost of protecting bare pipe).
surface. However, if sizeable stray currents are
operative or a galvanic cell is producing a
significant voltage, more rapid consumption of the
metal localized at the defects may be expected. In
the light of these facts, it may be asked why
coatings are applied. In most instances, coatings
provide excellent corrosion prevention in
themselves, sufficiently effective that cathodic
protection is not usually needed on Reclamation
structures. Coatings for bur-led structures are
very durable and corrosion voltages are very
rarely high enough to promote accelerated failure.
Further, should detrimental corrosion occur, an
insulating type of coating cuts cathodic protection
costs drastically (to perhaps 10 percent of the
cost of protecting bare pipe).
4.4. Compatibility of protective coatings and
cathodic protection.- Coatings and cathodic
protection complement each other and, where
possible, should be used as a combination to
achieve the best economy and protection.
However, the coating must be compatible with
cathodic protection. Certain materials, notably
phenolic resin and aluminum pigment, deteriorate
rapidly in the alkaline environment which cathodic
protection creates where the structure is being
protected. Coal-tar enamel and vinyl resins are
relatively unaffected. Both high stray current
voltages and excessive cathodic protection
voltages may "blow off' coatings; that is, cause
disbonding and rupture of the coatings. All
coatings are susceptible, but high adhesion
decreases the vulnerability to this effect. Since
the cost of cathodic protection is a function of
coating resistance, the better electrical insulator
cathodic protection.- Coatings and cathodic
protection complement each other and, where
possible, should be used as a combination to
achieve the best economy and protection.
However, the coating must be compatible with
cathodic protection. Certain materials, notably
phenolic resin and aluminum pigment, deteriorate
rapidly in the alkaline environment which cathodic
protection creates where the structure is being
protected. Coal-tar enamel and vinyl resins are
relatively unaffected. Both high stray current
voltages and excessive cathodic protection
voltages may "blow off' coatings; that is, cause
disbonding and rupture of the coatings. All
coatings are susceptible, but high adhesion
decreases the vulnerability to this effect. Since
the cost of cathodic protection is a function of
coating resistance, the better electrical insulator
the coating is, the lower the cost. Coal-tar enamel
and plastic tape coatings offer the greatest
advantage from this standpoint. The preceding
information should be considered in evaluating
the condition of a coating where a corrosion
problem exists. It may be found that providing
protection may best be accomplished by restoring
the continuity of an existing coating, and the
condition of an existing coating will always be a
factor in evaluating the desirability of installing
cathodic protection. Reclamation's Paint Manual
should be referred to for a discussion of the
characteristics of various
advantage from this standpoint. The preceding
information should be considered in evaluating
the condition of a coating where a corrosion
problem exists. It may be found that providing
protection may best be accomplished by restoring
the continuity of an existing coating, and the
condition of an existing coating will always be a
factor in evaluating the desirability of installing
cathodic protection. Reclamation's Paint Manual
should be referred to for a discussion of the
characteristics of various
(FIST 4- 5)
6