The inside and outside of a pipe’s surface is typically coated with a protective layer. Exterior pipe coatings often protect the pipe from environmental and/or chemical corrosion, as well as temperature changes. Sometimes pipe coatings are used to weigh down the pipe.
Pipe Coatings and Linings
Prior to the application of pipe coatings, the pipe must be thoroughly cleaned and any internal and/or external markings must be removed. Special equipment is used to handle the uncoated pipes to reduce the chances of incurring additional damages. Bare pipes are carefully inspected and completely free of foreign materials, like dust and grease, before the coating process can begin. Pipes that do not meet the cleanliness requirements are either recleaned or scrapped.
Bare pipes that pass the cleanliness inspection are then primed and coated. Coatings and linings that are applied to pipe surfaces are measured in millage units. Each millage unit is equivalent to 1000th of an inch. Coating materials must be packaged, stored and handled in accordance to the manufacturer’s preferred conditions in order to prevent damage and contamination. Any material that is damaged or contaminated must be eliminated.
Manufacturers inspect and test coated pipes to make sure the specified quality measures are met. Every step in the process requires meticulous inspection.
Types of Pipe Coatings
The outside diameter of a steel pipe can be coated in several ways. Galvanized pipes, for example are immersed into molten zinc. The zinc coating of galvanized pipes help prevent corrosion caused by the atmosphere, as well as drilling and cutting.
Fusion bonded epoxy, commonly called FBE coatings, is applied to pipes used for piping connections, such as pipeline construction and concrete reinforcing bars. This type of coating material is typically applied in powder form. The powder coating is then melted, using heat. Once the coating is cooled, it hardens and will remain that way despite further heating. Fusion bonded epoxy coated steel pipes are highly resistant to chemical reactions.
Coal tar epoxy coating is applied to steel pipes used for underground systems, such as water treatment facilities and pipelines. Coal tar epoxy is sprayed onto the steel pipe in timed intervals until the coating reaches the specified thickness. Steel pipes coated with coal tar epoxy is particularly resistant to damages caused by natural conditions like moisture, extreme temperatures and soil bacteria. Coal tar epoxy coating is among the most effective corrosion protectors.
Polyethylene/polyolefin pipe coating on steel pipes provides excellent protection from highly corrosive soil. Polyethylene coated steel pipes are more resistant to acid and alkaline media exposure. Polyethylene coated steel pipes are also highly impermeable, thus resistant to damaging water conditions.
Concrete weight coating is applied to steel pipes to provide negative buoyancy and mechanical protection from wet environments. Prior to concrete weight coating, a non-slip adhesive system is applied to the surface of the steel pipe. During the coating application, reinforcing wire and a plastic outer wrap is added to the outer layer. Concrete coatings must be cured until they achieve compressive strengths. A polyethylene coating is then applied to the concrete coating to create an outer wrap.
There are two categories of tape coating products: hot applied tapes and cold applied tapes. Hot applied tapes are manufactured with materials saturated with either coal tar or asphalt bitumen. Cold applied tapes are manufactured with a plastic wrap and adhesive material that can withstand application during extreme cold temperatures. These tape coatings are used to protect field joints, special piping and fittings from corrosion. Cold applied tapes can be applied by hand or with a wrapping machine.
Types of Pipe Linings
Pipe linings vary depending on the pipe’s intended use. The inside diameter of a pipe can be lined with epoxy to ensure anti-corrosion protection in mid to high temperatures. Primer is used to form a bond to the steel and to the outer coating. A rubber sheath can be inserted into the steel pipe to protect the inner lining from abrasive media. Cement linings are inserted to protect the inner lining for use in transporting water.