Pipelines are usually buried underground to protect them from damage, and keep them from interfering with the movement of traffic. Underground, the exterior of the pipe is exposed to conditions that can lead to corrosion. It should be noted that the interior of pipelines can also be attacked by corrosive products, such as hydrogen sulfides, CO2 and water, that might be contained in products transported in the pipeline.
History of Corrosion Protection
The earliest pipelines were buried without any external coatings. To prevent corrosion, pipeline manufacturers and operators began applying coatings to the exterior of pipe at the time it was being installed. This required the exterior surface of the pipe to be cleaned, usually by wire-brushing, and then application of a coating to the surface. These early pipe coatings were tape wraps and coal tars.
These coatings are susceptible to disbonding, however, and pipeline operators eventually determined that coatings alone would not provide complete corrosion protection.
Disbonding refers to a condition in which the pipe coating becomes separated from the exterior pipe wall. This allows water to come into contact with the pipe wall, setting up conditions that can lead to corrosion. Pipe coatings can also be damaged by equipment, and by rocks left in the backfill during installation, as well as by subsequent excavation activities.
To enhance protection, operators began installing cathodic protection (CP) systems. By taking periodic measurements of pipe-to-soil electrical potentials along the pipeline, CP systems can detect disbondment, and the operator can make necessary repairs to the pipe coating.
Today pipe coatings are applied by the pipe manufacturer at the time the pipe is manufactured. This has obvious advantages. The coating can be applied in a consistent manner, the exterior of the pipe is clean, and application conditions can be better controlled. Exterior pipe coatings still need to be applied during construction in the field at the girth welds when pipe sections are joined together.
The most widely used pipe coating today is Fusion Bonded Epoxy (FBE). FBE is applied to hot rotating pipe that is electrostatically charged. FBE is resistant to high temperature, can withstand high stress, and provides good protection against corrosion.
Coatings are also used to protect the interior surfaces of some pipe, but their use in that application is not pervasive in the oil and gas industries. Oil and gas pipeline operators prefer to treat their products through dewatering and the use of chemical additives to reduce corrosion, rather than using internal pipe coatings. Internal coatings may be used in pipelines were pretreatment is not feasible, such as gathering lines. Epoxy, cement, and ceramics can be used as internal coatings.
Pipe coatings protect pipelines from corrosion-related defects. They are just one element of the total package operators employ to ensure their pipelines are well protected. Cathodic protection and integrity assessments are also used to prevent corrosion, and to detect any defects that may result from corrosion.
Date of Revision: 12012011