Ductile Iron Pipe ~ The Water Industry’s Most Widely-Used Pipe
The Rust Bullet Application
Q: What is Ductile Iron Pipe?
A: Ductile iron (also referred to as ductile cast iron, spheroidal graphite iron or nodular cast iron is much more flexible and elastic. In ductile iron the graphite, created when molten cast iron solidifies, are spherical nodules rather than flakes (as in grey iron). These nodules inhibit the creation of cracks and provide enhanced ductility (manipulation without fracture). Ductile Iron was initially developed for use in the gas industry but has since been used extensively in the water industry.
Q: What is the expected life of buried Ductile Iron pipe?
A: Properly designed and installed Ductile Iron pipe systems could easily have a life expectancy of more than 100 years. Unlike other pipe materials, the physical properties of Ductile Iron pipe do not change with age. As long as Ductile Iron pipe is not subjected to loadings and pressures in excess of its original capabilities, the only factor that could shorten its life is corrosion. Internal corrosion has been effectively eliminated with the evolution of cement-mortar linings and other special linings. Also, not all soil environments are considered corrosive to iron pipe. This is evidenced by the fact that more than 500 utilities in the United States and Canada have had unprotected Cast Iron pipe that has provided service for 100 years or longer, and 12 utilities for more than 150 years. A common procedure used to determine if the soil is aggressive to iron pipe is the 10-point soil evaluation procedure outlined in Appendix A of the ANSI/AWWA C105/A21.5 Standard “Polyethylene Encasement for Ductile-Iron Pipe Systems.” If the soil tests corrosive to Ductile Iron pipe, then corrosion protection is warranted. If Ductile Iron pipe is installed in aggressive environments without protection, its life expectancy would mainly be a function of that environment.
Q: What coatings are recommended for Ductile Iron pipe in aboveground applications (outdoors, in pits and well housings, plants, etc)? How do I specify the coating (surface preparation, primers, finish coat, etc.)?
A: Unless otherwise specified, Ductile Iron pipe manufactured in accordance with ANSI/AWWA C151/A21.51 is supplied with an asphaltic coating approximately 1-mil thick. Asphaltic coating is not compatible with most top coats. This coating, which is applied for aesthetic reasons, is used under normal conditions for both above and below ground applications. Typically, it’s used in aboveground applications such as pump stations, bridge crossings, and pipe on supports installations. For special aboveground conditions, other types of coatings – epoxies, for example – are available. Installations that might require such coatings are corrosive wet wells, chemical environments, etc. Furthermore, some installations require the pipe to be primed for finish paint coats. The type of coating specified by the purchaser might depend on several criteria such as resistance to a given environment, temperature resistance, impact resistance, resistance to sunlight, gloss retention, appearance, compatibility to finish coats, etc.
Although Ductile Iron and carbon steel are both ferrous metals (ferrous indicates the presence of iron), there are inherent differences between the two that preclude the use of the same surface preparation and application of coatings. Attempts to apply steel surface preparation specifications to Ductile Iron is inappropriate and may actually result in damage to the pipe surface with subsequent reduced coating effectiveness and life expectancy. Most coating manufacturers require some type of surface preparation prior to application as a condition of warranty. Since their recommendation for surface preparation will vary depending on the type of paint and the ultimate service environment, the coating manufacturer’s technical data sheet should be consulted each time a special coating is used. Normally, recommendations given on coating manufacturer’s technical data sheets are for carbon steel and might not apply to Ductile Iron pipe. Therefore, the pipe manufacturer should also be consulted regarding the type of coating, method of application, and type of surface preparation to be used.
Rust Bullet Standard Formula will have no issue with superior adhesion to Ductile Iron and carbon steel. The surface preparation for both will be as outlined in the Application Guidelines for Rust Bullet Standard Formula. Additionally as a surface-tolerant, immersion-grade, moisture-cured urethane, Rust Bullet Standard Formula is essentially a universal primer compatible with all major topcoats, including, solvent or water-based asphaltic coatings, epoxies, acrylics, polyurethanes and moisture cured urethane topcoats. Due to this compatibility, Rust Bullet is extremely well suited for most applications including situations in which the end use may not be known. Advantages such as abrasion resistance, moisture resistance, chemical resistance, and easy repair set Rust Bullet Formulas as a high-performance coating suitable for both immersion and non-immersion services.
Although Rust Bullet is similar to Phenolic Alkyd Primers in that it is a fast-drying, lead and chromate free corrosion control primer. PHENOLIC ALKYD PRIMERS ARE NOT RECOMMENDED FOR IMMERSION and require up to 30 DAYS OF CURING BEFORE TOPCOATING with certain coatings. This primer is compliant with NSF Standard 61 (Potable Water) AS AN EXTERIOR SURFACE COATING ONLY. Rust Bullet will provide superior performance for immersion service and may be topcoated in 24 to 48 hours. Rust Bullet passes the EPA Standards for Potable Water (both primary and secondary) passing not only the 30-day standard test, but an additional 120-test also met and passes the same standard.
Epoxy Primers are a high-solids, chemical and corrosion-resistant coating for protection against abrasion, moisture, corrosive fumes, chemical attach and immersion. Unlike Rust Bullet Formulas, EPOXY PRIMERS, SHOULD AFTER 60 DAYS OF CURING, be uniformly scarified by brush-blasting with a fine abrasive before topcoating. This primer is compliant with the ANSI/NSF Standard 61 for potable water contact for pipe, fitting, and valves WHEN COMBINED WITH APPROVED TOPCOATS. Rust Bullet Formulas do not need a topcoat to be compliant with the standards for potable water.