External
The corrosion rate of Ductile Iron in sea water will normally vary between 2 and 20 mils per year depending on such factors as: whether the pipe is buried beneath the ocean floor, whether the sea water is continuously flowing or moving, depth beneath the surface (which influences oxygen concentration), whether the pipe is continuously submerged or at tidal levels, and a number of other factors. Polyethylene encasement has been used for pipe buried beneath the ocean floor. For applications where the 2 to 20 mil corrosion rate is unacceptable, or where polyethylene encasement cannot be used, contact your U.S. Pipe Sales Representative for specific recommendations.

Internal
As stated above, the corrosion rate of Ductile Iron in sea water is relatively low (between approximately 2 and 20 mils per year depending upon operating conditions). For extra protection in sea water, a 4 to 10 mil thick coating of epoxy is normally recommended for exposed iron in the joints (i.e., on the extreme spigot end of the pipe and in portions of the bell). Type V sulfate resistant cement, double thickness, in accordance with AWWA C104 Standard is also recommended for this type of service.

Double poly-wrap is recommended either with the first layer being the low-density poly with high-density over the outside or double wrapped with low-density.

Because Ductile Iron pipe uses a rubber-gasketed joint every 18 to 20 feet, it is normally considered electrically discontinuous and, as such, discourages pickup and discharge of stray electrical currents. Considering the many millions of feet of Ductile Iron pipe in the ground, reported instances of stray current corrosion are relatively rare.

However, stray current problems can occur in areas with a high current density in the soil. These areas can occur at or near cathodic protection anode beds, electric railroads and mine transportation equipment, industrial equipment, and some electrical grounding systems. 

Polyethylene encasement has been shown to be effective at protecting Ductile Iron pipelines at most levels of stray current encountered in the ground.  When Ductile Iron pipelines are exposed to the special high-density stray current areas described above, the pipeline should be rerouted or the anode bed relocated. If neither of these options is feasible, the Ductile Iron pipe in this area should be electrically bonded together, electrically isolated from adjacent pipe, polyethylene encased, and appropriate test leads and “current drain” installed.

The Ductile Iron Pipe Research Association (DIPRA) is available upon request to investigate installations in areas with potentially high-density stray currents.

Polyethylene Encasement is described in ANSI/AWWA C105/A21.5 Standard (American National Standard for Polyethylene Encasement for Ductile-Iron Pipe Systems).

Some recommendations regarding installation of Polyethylene Encasement are also given in ANSI/AWWA C600 (AWWA Standard for Installation of Ductile-Iron Water Mains and their Appurtenances).

Spiral winding is easier to do in a muddy trench and also does not allow the drilling mud to build-up underneath the poly-wrap causing it to balloon. All over-lapped edges of the poly-wrap should be taped except for the over-lapped poly along the length of the barrel, where the spiral winding is sufficient.

Actually, there is no "safe" pH range, when pH is considered in isolation. Other factors must be considered. For example, soils with a pH of 0.0 to 4.0 are acidic and serve well as a corrosive electrolyte. Soils with a near-neutral pH of 6.5 to 7.5 and low redox (oxidation-reduction) conditions are optimum for sulfate reduction by bacteria, which can cause corrosion. Soils with a pH of 8.5 to 14.0 are generally high in dissolved salts, yielding a corrosive soil with low resistivity.

In terms of the pH range of fluids inside the pipe, chemicals with a broad range of pH’s can attack iron, cement lining, asphalt seal coat, and/or rubber gaskets. In addition to pH, factors such as the specific chemical or chemicals being conveyed, concentration, temperature, flow rate, etc., are needed to determine the suitability of the pipe, lining, and gasket materials for a specific type of service.

Consult your U.S. Pipe Sales Representative for help with your specific application.

Certain soils can be identified as potentially corrosive without testing. These environments include, but are not limited to: coal, cinders, muck, peat, mine wastes, and landfill areas high in foreign materials. If your pipeline will run in the near vicinity of a known stray electrical current, this area should be considered potentially corrosive.

For installations in other areas, soil testing conducted in accordance with APPENDIX A of AWWA C105 Standard is recommended.

Evaluation of soil corrosivity for Ductile Iron pipe is described in detail in APPENDIX A of AWWA C105 Standard. The test method evaluates eight parameters: 1) resistivity, 2) pH, 3) redox (oxidation-reduction) potential, 4) sulfides, 5) moisture content, 6) soil description, 7) potential for stray current, and 8) experience with existing installations in the area.

In general, polyethylene encasement can remain effective at sustained temperatures up to around 180° F. Polyethylene encasement softens around 200° F and melts around 220° F to 230° F. Sustained temperatures above 180° F may eventually cause the polyethylene film to become brittle and crack. Stabilizing antioxidants can be added to the film during manufacturing to increase this temperature. As long as the polyethylene encasement continues to prevent direct contact of the pipe with the corrosive soil, it will remain an effective corrosion control system for Ductile Iron pipelines.
(Issue: Fall/Winter 1997)

The ANSI/AWWA C100 series are applicable to Ductile Iron pipe and fittings.  Below is a list of the Standards by title:

Further information may be found in the AWWA Manual M41, Ductile Iron Pipe and Fittings. 

These Standards and Manuals are available from the American Water Works Association, 6666 West Quincy Avenue, Denver, Colorado 80235, Telephone (800) 926-7337, Fax (303) 347-0804, or via e-mail at info@awwa.asn.au.