In areas where severely aggressive soils are encountered, the use of a polyethylene tube or sheet encasement has been proven to provide highly effective, economical protection. The protection against corrosion provided by loose polyethylene is different in several ways and should not be confused with coatings applied directly to the barrel of the pipe. The most significant difference is its ability to protect without creation of concentration cells at holidays. Also, since the encasement is applied when the pipe is actually put in the ground, damage due to shipping, handling, etc., is minimized.

As water may be present in the soil around the pipe, water may also be present between the pipe and wrap. Water inside the polyethylene tubing initially bears some characteristics of the soil environment, and corrosion may start. But within a short period of time initial oxidation depletes the oxygen supply in the water, and other electrochemical corrosion reactions also quickly progress to completion. At this point a state of chemical equilibrium is reached and there is no corrosion.

Since the first field installation of polyethylene wrap on gray iron pipe in 1958, installations have been made in severely corrosive soils throughout the United States. The success of the polyethylene encasement procedure developed in the United States has been adopted by several other countries, and an International Standard for Polyethylene Sleeving (ISO-8180) has been developed.

Research by the Ductile Iron Pipe Research Association (DIPRA) at several severely corrosive test sites has verified that polyethylene encasement provides a high degree of protection and results in minimal and generally insignificant exterior surface corrosion of either ductile or gray iron pipe thus protected. These findings have been confirmed by the results of hundreds of investigations of field installations. See dipra.org for more information.

Field tests have also indicated that the dielectric capability of polyethylene provides shielding for ductile iron pipe against stray current at most levels encountered in the field.

Because polyethylene encasement is a passive method of protecting ductile iron pipe in aggressive soils, it can effect greater reliability and savings than cathodic protection systems, which require continual monitoring, maintenance and other operating expenses, as well as trained personnel. Cathodic protection systems can also cause collateral corrosion in some cases to nearby unprotected ferrous structures.

For protection in areas of severely aggressive soils, AWWA C105 covers materials and installation procedures for polyethylene encasement of underground installations of ductile iron piping for water and other liquids.

Polyethylene wrap in tube or sheet form for piping encasement is manufactured of virgin polyethylene material conforming to the requirements of ANSI/ASTM Standard Specification D1248. The specified minimum thickness for linear low-density polyethylene film is 0.008 inches (8 mils). The specified minimum thickness for high-density, cross-laminated polyethylene film is 0.004 inches (4 mils).

Material, required markings, and installation methods are all in accordance with the requirements of AWWA C105. This standard and more detailed publications by DIPRA regarding loose polyethylene encasement are available from AMERICAN.