Pipe coatings, polyethylene tape

Corrosion Protection. Although steel underground transmission lines are often protected by coal tar or fusion bonded epoxy coatings, a large volume of polyethylene tapes as well as some vinyl-backed tapes are used for corrosion protection. Pipes of large diameter, such as oil line pipes, are usually machine-wrapped in the field just prior to placement in the ground. The tapes are unusual in that the backing is from 12 to 15 mils thick and the butyl-based adhesive is approximately of the same thickness. For the pipeline industry the rolls are 18 in. wide and contain up to 800 feet of tape. There is a reduction currently in domestic pipeline construction but appreciable amounts of pipewrap are exported. 

These can run from several inches in diameter to a 24"/30" diameter and from hundreds of feet up to 29 miles in length. Smaller pipe can be bare, galvanized or poorly coated with coal tar mastic, as well as epoxy or polyethylene tape, which can be field or factory applied. 

Buried aluminum npelines are usually protected by saoificial anodes—zinc for coated lines and magnesium for uncoated lines. It is generally accepted that such coatings as exbuded polyethylene or a tape wrap should be applied to aluminum pipes for underground service. Because of the effectiveness and longevity of sacrificial anode systems and the need to avoid overprotection, impressed current (rectifier) systems generally are not used to protect aluminum pipelines. 

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