Corrosion control of ductile iron pipe with polyethylene encasement

By Normand De Agostinis

In the United States prior to the 1950s, and in Canada prior to the 1970s, corrosion and corrosion control in the water works industry was not fully appreciated. As a result, infrastructure tended to be installed without any corrosion protection, even though it may have been warranted. Today, utilities have grown accustomed to protecting their infrastructure when necessary. Research and education have helped engineers and utilities to understand, evaluate and protect iron pipe and appurtenances from corrosive environments.

Research

One of the mandates of the Ductile Iron Pipe Research Association (DIPRA) has been to conduct research on the corrosion of ductile iron pipe. In Canada, DIPRA has assisted SaskWater at their test site in Riverhurst, Saskatchewan. It was commissioned in 2002 to evaluate ductile iron pipe in soils with high sulfate content.

Research is typically carried out at DIPRA test sites. Specimens in these test sites are four-to-five-foot sections of production pipe purchased from ductile iron pipe manufacturers. Pipe samples are capped on each end to ensure that the effects of corrosion are strictly external. Specimens are exhumed over time and shipped back to the DIPRA research laboratory in Birmingham, Alabama, for examination and data collection. During this evaluation, the pipe is cleaned, weighed and pit depths are measured.

Soil corrosivity

Most soils are not corrosive to cast or ductile iron pipe and appurtenances. Some cast iron pipes have lasted for over 300 years. One example is a pipeline commissioned by France’s King Louis XIV to bring water to the Versailles fountains.

In North America, DIPRA awards commemorative plaques to utilities that have had at least 100 continuous years of service from their cast iron water pipes. There are presently at least 630 members of the “Cast Iron Pipe Century Club.” There is also a Cast Iron Pipe Sesquicentury Club plaque that honours those utilities that have attained at least 150 years of continuous service. There are presently over 24 members of this club, including Montreal, Quebec City, Halifax, Nova Scotia and Port Hope, Ontario.

Soil testing

In order to help identify corrosive soils, the 10-point soil evaluation system was developed in the mid 1960s. It analyzes resistivity, redox-potential, pH, sulfides and moisture. This system assigns points to each parameter, based on a sliding scale, which are then totaled. It was developed exclusively for cast and ductile iron pipe and was not intended for other materials.

A soil with a point total of 10 or more is considered aggressive to cast or ductile iron pipe. There are a number of soil environments known to be corrosive to iron pipe and appurtenances. Some of these include coal, cinders, mine wastes and landfills. When these environments are encountered, soil evaluations are not necessary and infrastructure must automatically be protected.

Polyethylene encasement research

Since its original use in a research study in 1951, polyethylene encasement protection has been the iron pipe industry’s first line of defense against corrosion. It prevents pipe from being in direct contact with the corrosive environment. Although polyethylene encasement is not a watertight system, this does not diminish its protective properties. Even though the moisture between the pipe and the polyethylene encasement contains oxygen, research has shown it is eventually depleted, causing oxidation to stop.

This process leaves a uniform stagnant environment around the pipe, which results in a protective environment. The weight of the soil surrounding the polyethylene encased pipe typically helps to prevent any significant replenishment of groundwater between the polyethylene encasement and the pipe. During installation, care should be taken to ensure that the opportunity for groundwater to flow under the wrap is reduced.