Corrosion control of ductile iron pipe with polyethylene encasement

In 1952, a bolt study on polyethylene encasement was started at the Everglades City, Florida test site. It involved burying a section of a six-inch cast iron mechanical joint pipe. Polyethylene was used to protect the bolts of the mechanical joint, as well as a portion of the pipe. After 18 years of exposure, the pipe was exhumed and brought back to the laboratory for analysis. The bolts were corrosion free, as was the pipe under the polyethylene encasement (See Figure 2).

Published results have confirmed the effectiveness of polyethylene encasement as a corrosion control measure. Statistical data, obtained from DIPRA’s research on corrosion, indicates that, in soils considered “non-corrosive” (less than 10 points), unprotected iron pipe would have a mean time-to-penetration of a 0.25-inch wall of over 370 years. In those same soils, the analysis indicates that properly installed polyethylene encased pipe would have an infinite mean time-to-penetration. With soils considered “corrosive” to iron pipe (more than 10 points), the mean time-to-penetration of ductile iron pipe protected with polyethylene encasement would be some 550 years.

Installation

Polyethylene encasement is easily installed in the field during pipeline construction. Most utilities and contractors prefer Method A of installation as outlined in the ANSI/AWWA C105/A21.5 Standard. This involves cutting a piece of polyethylene tube approximately one foot longer than the pipe on each end and overlapping the joints. The polyethylene encasement tube diameter is slightly larger than the pipe diameter to allow for joints and appurtenances.

In order to minimize the initial moisture that can accumulate between the encasement and the pipe, the excess is taken up along the barrel of the pipe and folded over the top. The fold can be held down using tape, string or tie wraps, which will leave a snug fit. Proper installation is a key to the success of any corrosion control measure and this is also the case with polyethylene encasement.

Small rips or tears in the polyethylene tube can be easily repaired with tape. If larger tears occur, they should be repaired by cutting a piece of polyethylene and securing it over the tear. It is also a good practice to use a sling to move the pipe instead of a chain, to avoid damage to the polyethylene encasement.

Corrosive soils also affect valves, fittings, hydrant barrels and other appurtenances. It is important to protect these infrastructure components as well. When direct tapping is necessary, the preferred method is to wrap two or three layers of polyethylene tape around the pipe and to tap directly through the tape and polyethylene encasement. The copper service should also be encased with polyethylene for a minimum of three feet perpendicular from the pipe.

Polyethylene encasement investigations

In addition to research at its test sites, DIPRA has been investigating polyethylene encased iron pipe installations in situ for over 50 years. This is typically done at the invitation of a utility that selects the area where the inspection will be conducted. Pipe is carefully exposed and a portion of the polyethylene encasement is removed and taken to the DIPRA laboratory for material properties analysis. The pipe is cleaned and then inspected for any corrosion pitting and/or graphitization. A soil sample is also procured and tested for its corrosive properties.

The oldest known installation of polyethylene encasement in an operating system is in Lafourche Parish, Louisiana. In 2013, this installation attained 55 years of service and is still performing very well (See Figure 4).

Moving forward with V-Bio

Polyethylene encasement has been helping utilities protect their iron pipe for over 55 years. This form of protection is inexpensive and easily installed in the field. It yields to soil stresses, does not deteriorate underground and most importantly it is a passive corrosion control measure. Once it is installed there is no need for further monitoring.

In 2013, DIPRA introduced V-Bio polyethylene encasement. This consists of three layers of co-extruded film that are fused into one. It features an inside surface infused with a biocide and a volatile corrosion inhibitor (VCI). The biocide will mitigate the potential influence of anaerobic bacteria through microbiologically influenced corrosion. The VCI will control galvanic corrosion. It meets all requirements of ANSI/AWWA C105/A21.5 Standard for polyethylene encasement.

V-Bio is the next step in a proven and successful method of corrosion control, protecting against corrosion without consumption or degradation of either the biocide or the corrosion inhibitor.

Normand De Agostinis, P.Eng., is with the Ductile Iron Pipe Research Association. This article appeared in ES&E’s March/April 2015 issue.