is why hot-water tanks are lined with glass.) Other
materials used for the manufacture of pipe that
resists corrosion are vitrified clay, cement, fiber,
asbestos, and rubber. Glass fibers reinforced with
epoxy or polyester resins are also resistant to
Dielectric bushings may be installed to stop
electrolytic action in plumbing systems or
wherever dissimilar metals are used. These
bushings are made of nylon and are usually
colored. They withstand pressures to 100 psi and
temperatures up to 300°F. The bushings are
usually placed in pipe systems as recommended
by the manufacturer. Some metals least likely to
be affected by corrosion are copper, brass, Monel,
and stainless steel.
COATINGS AND WRAPPINGS FOR
Coatings and wrappings are commonly used
to combat corrosion on exterior piping systems.
There are many different types of coatings such
as asphalts, coal tars, plastics, mastics, greases,
and cements. These coatings are considered to be
insulating materials, but each is not effective in
all environments. Each one was developed for a
certain type of corrosive environment.
Asphalt base coatings are the most common
type of protective coatings used. They are pro-
duced from petroleum residue and natural
sources. Asphalt base coatings can take con-
siderable abrasion, impact, and temperature
changes wit bout creating a corrosive condition.
However, they absorb a considerable amount of
water and dissolve easily into a form of petroleum
Coal Tar Coatings
Coal tar coatings are commonly used on
pipelines. They possess continuity, hardness,
adhesion, and corrosion resistance. Coal tar
coatings are less expensive than asphalt coatings.
They do not have a very good impact resistance,
and a wide temperature change often causes the
surface to crack.
Some of the most important paint coatings are
coal tar, asphalt, rubber, and vinyl.
Coal tar paints have the outstanding
characteristics of low permeability and resistance
to electrolytic reaction. They are not affected by
the action of water. These paints are recom-
mended for piers, marine installations, flood
control structures, sewage disposal plants, and
industrial concrete pipelines.
Asphalt paints are weather resistant and
durable against industrial fumes, condensation,
and sunlight action. Because of their resistance
against water solvency, they are used on steel
tanks and concrete reservoirs.
Rubber base paints are very resistant to acids,
alkalies, salts, alcohols, petroleum products, and
inorganic oils. The resistance of these products
makes them ideal for use on the inside of metallic
and concrete storage tanks. If these structures are
submerged in water or are under ground, a special
form of this paint should be used because of
Vinyl paint is one of the many synthetic resin
base paints. These paints dry to a film that is
tough, abrasionproof, and highly resistant to elec-
trolysis. They are odorless, tasteless, nontoxic,
and nonflammable. The film is especially
resistant to oils, fats, waxes, alcohols, petroleums,
solvents, formic acid, organic acids, ammonium
hydroxides, and phenols. Because of these
characteristics, vinyl paint is very applicable for
tanks, pipelines, wellheads, offshore drilling rigs,
pipe used in oil industries, railroad hopper cars,
dairy and brewery equipment, storage tanks, and
concrete exposed to corrosive environments.
Grease is another material used to form a
protective coating on structures. It is usually made
from a petroleum base and resembles paraffin or
wax, Grease can be applied either hot or cold.
However, it must be protected by some type of
wrapping to keep the grease from being displaced
or absorbed by the backfill soil when it is applied
to underground surfaces.
Concrete coatings have been used with success
when properly applied to pipelines to be laid in
highly corrosive soils, such as areas containing