With little or no fanfare, the Ductile Iron Pipe Research Association (DIPRA) announced in August 2002 that its members—the eight leading manufacturers in North America—will no longer honor a warranty for ductile iron pipe with any exterior dielectric coating other than polyethylene encasement. At a stroke, this decision has effected a substantial, not to say dramatic change in the way water and wastewater utilities, among others, will build, rehabilitate, and protect their facilities in the future.

Polyethylene (PE) encasement—wrapping pipe at the time of installation in a sleeving of 8-mil (0.008-in) polyethylene, in tube or sheet form—is considered the most economical of all the corrosion protection methods currently in general use. Manufacturers and installers alike favor the method for its ease and economy of field application and the passive (and therefore presumably “maintenance-free”) nature of its protection. They have long taken the position that PE encasement alone is a viable corrosion protection system for ductile- and gray-iron pipe.

The decision to offer warranty protection only for PE-encased pipe goes a long step further in that direction. In effect, end-users of these materials—primarily water and wastewater utilities—are now required to accept DIPRA’s view that (1) the alternative corrosion protection technologies, including bonded coatings and cathodic protection (CP), are prohibitively costly, seldom cost-effective, and “in most cases . . . also unnecessary due to the availability of alternative methods of corrosion control,” (2002 edition, p. 2) and (2) that ductile iron pipe “needs no additional protection in most soils…(except) in certain soil environments.” (2000 edition p. 5)

In fact, DIPRA does concede that the PE encasement “also has limitations.” The organization’s leading publication on the subject, Polyethylene Encasement: Effective, Economical Protection for Ductile Iron Pipe In Corrosive Environments, states:

“..it [PE encasement] is not universally applicable for all ductile iron pipelines where corrosion protection is warranted. There are instances where it is not feasible to install polyethylene encasement due to unusual construction conditions. Additionally, in certain high-density stray current environments and in ‘uniquely severe environment[s]’ . . . the sleeving alone might not provide the degree of protection needed. In such cases, DIPRA sometimes recommends alternative methods of corrosion protection.” (2000 edition, pp. 14-15)

“Uniquely Severe Environments”?

As corrosion engineers, our concern is to protect and preserve various elements of the public and private urban and intercity civil infrastructure. One key question, therefore, is what we mean by a “corrosive” or (in DIPRA’s terminology) “uniquely severe” environment—or in other words, in what percentage of instances is an “alternative method of corrosion protection” likely to be required?

DIPRA’s general document on the subject lists a number of soil environments in which additional corrosion protection is warranted. Their list includes soils contaminated by coal mine wastes, cinders, refuse, or salts; environments such as swamps, peat bogs, and other wet, low-lying areas; expansive clays, alkali soils, heterogeneous soils, and those with low resistivity, anaerobic bacteria, or differential aeration around the pipe. Dissimilar metals and external stray direct currents may also necessitate additional corrosion protection.

To test for corrosivity, DIPRA recommends the “10-Point Soil Test Evaluation,” described in ANSI/AWWA Standard C105 A21.5-88 and introduced by the organization (then known as the Cast Iron Pipe Research Association, or CIPRA) in 1964. A soil sample, taken at pipe depth, is awarded a varying number of points according to five separate criteria: soil resistivity, pH, presence of sulfides, moisture, and oxidation-reduction (redox) potential. For example, a soil resistivity of 2,100 to 2,500 ohm-cm is worth two points, a pH between 2 and 4 rates three points, trace sulfides add another two points, and so on. If the sample is assigned a total of ten or more points, then the soil is considered corrosive to ductile iron pipe and protective measures are required (e.g., polyethylene encasement for the pipe, fittings, valves, and other appurtenances.)

DIPRA places great reliance on this test, stating confidently, “In more than 20 years of field application by DIPRA engineers and qualified personnel in utilities throughout the nation, there has been no instance where the soil evaluation procedure has proved inadequate or faulty in predicting where corrosion protection is needed." (2002 edition, p.4)

Overlooked: Cl^- Concentration and Stray Current

Useful as they are, however, the “10 Point Test” criteria leave two critically important factors out of account: (1) chloride concentration and (2) stray current, either of which can also cause corrosion of buried ductile iron pipe.

Typically, a soil chloride concentration from 100 to 500 parts per million (100 to 500 ppm) is considered at the threshold of being corrosive to metal structures.¹ In addition, the influence of stray current which causes a shift in the polarized pipe-to-earth potential greater than ±50 mV versus a copper-copper sulfate reference electrode may result in corrosion of a buried metallic structure.²

When these conditions apply, an alternative to polyethylene encasement should be considered: specifically, using sacrificial anodes or an impressed current system to cathodically protect a pipeline. Sacrificial anodes, which are very effective at distributing the current along the pipeline, may be used for pipelines with small current requirements (less than 1 Amp) and in low-resistivity soils. Impressed current systems may be used for pipelines with larger current requirements (greater than 1 Amp) and in low-resistivity soils.

Both cathodic protection systems require maintenance, including an annual survey by a certified corrosion engineer. In many cases, a dielectric coating is recommended in addition to cathodic protection. A typical dielectric coating may be coal tar epoxy, polyurethane, or other bonded coatings. These are recommended for several reasons. First, no coating or encasement is ever applied perfectly; there is always a possibility of defects. In addition, other “foreign” pipelines may be installed at some future time; these may interfere with the cathodic protection system, and cause an area of accelerated corrosion to develop. Finally, the soil moisture, chemical content or temperature may change over time, e.g., as a result of seasonal changes, increased rainfall or contamination.

V&A Recommendation

ANSI/AWWA C105 A21.5-88 establishes a minimum 8-mil thickness for the polyethylene encasement. An alternative to polyethylene encasement is dielectrically-bonded tape wrap, which is used to prevent contact between pipeline and the surrounding air, soil, or water. Some manufacturers of high density polyethylene (HDPE) or polyolefin tape wraps include the Protecto Wrap Company (Denver, CO) and Tapecoat (Evanston, IL). They have built a sound reputation for producing high quality waterproofing systems for the oil, gas and chemical industries as well as water and wastewater systems. They offer a variety of products that can be field or factory-applied, in hot or cold weather, in a variety of soils, air, or water.

Polyethylene encasement alone may not provide an adequate soil/pipe barrier for several reasons. The PE sheet or sleeve may be punctured during installation. It may trap moisture, since it is not closely adhered to the pipe surface, which will begin the oxidation of the metal. And soil chloride concentration may be dangerously high, as AWWA C105/A21.5 Appendix A does not include chloride concentration among its criteria.

For clients considering ductile iron pipes (DIP) for new or replacement installations, V&A strongly recommends that the soils along the proposed alignment be evaluated for corrosion potential. Based on the results—i.e., the soil chemical properties—and the potential for stray current exposures, V&A recommends that the owner and/or engineer consider one of the following four corrosion control strategies, which are presented in order of increasing severity (corrosivity) of the soil environment:

1. Install DIP with PE encasement, with no bonds across the pipe joints, no bonded coating, and no test stations along the alignment.

2. Install DIP with the manufacturers’ applied shop coating, PE encasement, bonds across the pipe joints, and test stations along the alignment to allow for the monitoring and testing of the pipeline. Bonding across the pipe joints during installation allows for the option of installing a cathodic protection system in the future in case of any unforeseen changes along the alignment.

3. Install DIP with a bonded coating, bonds across the pipe joints, and test stations along the alignment to allow for the monitoring and testing of the pipeline. As with Option 2, bonding across the pipe joints during installation will enable installation of a cathodic protection system in case of future need.

4. Install DIP with a bonded coating, joint bonds across the pipe joints, test stations, and cathodic protection. The joint bonds are required for cathodic protection and the test stations allow for the monitoring and testing of the pipeline.

The first two options should be considered in high-resistivity, high pH soils that have low concentrations of sulfides, chlorides and bicarbonates. Option 2 is more expensive than Option 1, since it takes more man-hours to install joint bonds. Options 3 and 4 should be considered in low-resistivity soils with either low pH or high concentrations of sulfides, chlorides, or bicarbonates.

No specific values are suggested here for soil resistivity, pH or for concentrations of sulfides, chlorides, and bicarbonates; it is usually necessary to look at all soil characteristics and then make the appropriate recommendation. The choice of option will depend on, among other things, the project budget, any applicable standard requirements for wastewater or water pipelines, and also on the possibility of failure in the future and how much risk the parties are willing to assume if they decide not to protect the pipe.

We also recommend that the owner’s corrosion engineer investigate soils aggressively, in particular taking into account the chloride concentration and any possibilities of stray current, in order to limit the corrosion of the pipeline. If pipe manufacturers will not supply pipe with a bonded coating or allow for a third party application of bonded coating, to the marketplace, other materials should be considered, such as steel, PVC, fiberglass, or concrete pressure pipe.

Although the initial cost may be higher, the combination of a dielectrically bonded coating with cathodic protection will greatly reduce the risk of failure and provide additional insurance that will prolong the service life of the pipe, especially with a large-diameter or long-length pipe. If a failure in the pipe were to occur at some time in the future, the owner would potentially have to foot the bill for the cost of repairing the pipe.

That cost would be affected by (i.e., include) such unpredictable factors as pipe diameter and depth, mobilization costs, size of crew, and the equipment and repair materials required for the job. It would also include costs that are harder to quantify but are equally or more damaging, such as loss of service to system users, delays and inconvenience to the public (including motorists and pedestrians), and loss of public confidence in the service provider.

The cost of the additional recommended protection is quite modest when measured against the direct costs of excavating and repairing the pipeline in the event of a failure—particularly if the failure were to occur in an environmentally sensitive or urban area. To take a typical example, the yearly power requirement of an impressed current for 6,500 feet of a 12-inch cement mortar coated pipe is comparable to that for continuously lighting a 100-watt bulb; the yearly power needed for 15,000 feet of a 66-inch cement mortar coated pipe is comparable to continuously running one 3-hp lawn mower for the same period.

Ultimately, the success of polyethylene encasement should depend on the concentrations of water-soluble chlorides and sulfates, soil pH, soil resistivity, and the presence of stray currents along a pipeline alignment. In the meantime, DIPRA will continue to recommend polyethylene encasement for buried ductile iron pipe applications with the 10 Point guideline.

For more information
Two DIPRA publications, both available online, offer additional information and guidance on this topic:

Polyethylene Encasement: Effective, Economical Protection for Ductile Iron Pipe In Corrosive Environments, Copyright © 2000, 1995, 1992 by Ductile Iron Pipe Research Association; available online at http://www.dipra.org/pdf/polyethyleneEncasement.pdf

“Polyethylene Encasement versus Cathodic Protection: A View on Corrosion Protection,” By Troy F. Stroud, P.E., President, Ductile Iron Pipe Research Association; available online at http://www.dipra.org/pdf/PEvsCP.pdf.

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¹ Schiff, M.J., “What is a corrosive soil?”, Proceedings of the Western States Corrosion Seminar, California State Polytechnic State University, Pomona (1993); Romanoff, M., “Underground Corrosion”, National Bureau of Standards Circular 579, 1957, p. 53.

² This observation is drawn from V&A Consulting Engineers’ own extensive field experience.