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 corrosion.

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 300F. 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 CORROSION PROTECTION

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 Coatings

Asphalt base coatings are the most common type of protective coatings used. They are produced from petroleum residue and natural sources. Asphalt base coatings can take considerable 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 product.

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.

Paint Coatings

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 recommended 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 condensation.

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 electrolysis. 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 Coatings

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

Concrete coatings have been used with success when properly applied to pipelines to be laid in highly corrosive soils, such as areas containing