Review of the Bureau of Reclamation's Corrosion Prevention Standards for Ductile Iron Pipe (2009) / Chapter Skim
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2 Ductile Iron and Corrosion
2 Ductile Iron and Corrosion
Pages 22-42
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From page 22... ...
with the balance iron, it is the shape and distribution of the nodular carbon within the iron matrix that imparts the desired mechanical properties. Heat treatments and chemical additives (e.g., magnesium)
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From page 23... ...
TroyStroud and James Voget, "Corrosion Control Measures for Ductile Iron Pipe," 46th Annual Appalachian Underground Corrosion Short Course (Morgantown, W.Va.: West Virginia University, 2001)
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From page 24... ...
Of relevance to this study is the fact that, over time, as manufacturing pro cesses have improved and control of metallurgy has become more consistent, DIP manufacturers have been able to produce pipe with increasingly thinner walls. For example, a 30-inch pipe manufactured before 1992 for the lowest thickness TABLE 2-1 Wall Thickness of Ductile Iron Pipe, by Pressure Class Pressure Class 150 200 250 300 350 Nominal Pipe Outside Size (in.)
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From page 25... ...
CORROSION MECHANISMS External corrosion of DIP can occur in soils through a number of mechanisms, described in the following subsections. Uniform Corrosion Uniform corrosion is defined as "corrosion that proceeds at about the same rate over a metal surface." It occurs when there are no anomalies on the surface or in the soil. This mechanism is not generally considered as serious as other mechanisms because corrosion rates are predictable and the pipe wall thickness can be specified for adequate strength even in the presence of corrosion.
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From page 26... ...
occur on the metal surface external to the crevice. Galvanic Corrosion Buried metal systems that contain dissimilar metals are susceptible to galvanic corrosion, which is the accelerated corrosion of a metal that occurs when it is in electrical contact with a more noble metal in the soil. The potential difference between dissimilar metals in electrical and electrolytic contact causes electron flow between them.
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From page 27... ...
Therefore, when a section of DIP is replaced, the newer iron in contact with the older, passivated iron surface can locally corrode galvanically in the soil environment. Another example of galvanic corrosion is the effect of surface oxide films formed on DIP during fabrication. The oxide layer, or "scale," provides some corrosion protection to the metal if it remains adherent.
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From page 28... ...
Brander, and B Ng, "Use of Pipe Recovery Data and the Hydroscope® NDT Inspection Tool for Condition Assessment of Buried Water Mains," presented at the 2001 American Water Works Association Infrastructure Conference, Denver, Colo.
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From page 29... ...
This type of corrosion could result from the interaction between an electric railway and a buried iron pipe. Current can enter the pipe from the railway positive feeder and travel to a location of discharge in close proximity to a negative return.
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From page 30... ...
; E Escalante, ed., Underground Corrosion (West Conshohocken, Pa.: ASTM International, 1981)
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DEFINITION OF HIGHLY CORROSIVE SOILS Part of the charge to the committee is to answer the following question: Does polyethylene encasement with cathodic protection work on ductile iron pipe installed in highly corrosive soils? In the committee's review of the charge and the literature, it became clear that the pipeline community does not have a common definition for "highly corrosive soils." The Bureau of Reclamation, in Table 2, "Corrosion Prevention Criteria and Minimum Requirements," of its Technical Memorandum 8140-CC-2004-1, defines "highly corrosive soils" as any soil with a soil resistivity of 2,000 ohm-cm or less (see Figure 1-1 in this report)
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From page 32... ...
METHODS OF CORROSION PROTECTION FOR DUCTILE IRON PIPE Corrosion of buried and submerged metallic substrates is a naturally occurring phenomenon. As previously discussed, common causes of corrosion on buried DIP include low-resistivity soil, soil chemistry, anaerobic bacteria, the presence of dissimilar metals, and stray currents.
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From page 33... ...
As noted in Chapter 1, "Introduction," this report does not discuss economic aspects of these corrosion control methods. Polyethylene Encasement The Ductile Iron Pipe Research Association and Corrpro have reported that the majority of soils found in North America are not considered corrosive to ductile iron.32 Where the soils are considered aggressive, DIPRA recommends the addition of polyethylene encasement (PE)
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Similarly, a DIPRA brochure on polyethylene encasement states that polyethylene encasement "doesn't deteriorate underground."38 34 ANSI/AWWA Standard C105/A21-5-05 "Polyethylene Encasement for Ductile-Iron Pipe Systems," AWWA, Denver, Colo. 35 Cast Iron Pipe Research Association, "Protection of Cast Iron Pipe by Encasement in Polyethylene Tube," Chicago, Ill.
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From page 35... ...
However, in very corrosive soils, with high or fluctuating groundwater, the replenishment of oxygen and electrolytes may support the corrosion process and allow it to continue. Some case studies have suggested that PE might not provide enough protection in continuously saturated soils, although it might be used in conjunction with a CP system.43 Others maintain that pinholes do not significantly diminish the ability of the PE to provide protection and that unlike bonded dielectric coatings, PE can 39 DIPRA, "Polyethylene Encasement Effective"; Cast Iron Pipe Research Association, "Protection of Cast Iron Pipe by Encasement in Polyethylene Tube." 40 ANSI/AWWA C105/A21.99, "Polyethylene Encasement for Ductile Iron Pipe Systems." 41 DIPRA, "Polyethylene Encasement Effective." 42 DIPRA," Polyethylene Encasement Effective." 43 Ian Lisk, "The Use of Coatings and Polyethylene for Corrosion Protection," Water Online, January 14, 1997, available at: http://www.wateronline.com/article.mvc/The-Use-of-Coatings-and-Polyethylene-for-Corr-0001, Accessed December 28, 2008.
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From page 36... ...
Terrazan et al., "Assessing Polyethylene Encased Ductile Iron Pipeline," Advances and Experiences with Trenchless Pipeline Projects, Pipelines Conference 2007; Roy Brander, "Water Pipe Materials in Calgary, 1970-2000," in AWWA 2001 Infrastructure Confer ence Proceedings (Denver, Colo., 2001) ; Kevin Garrity, "Corrosion Control Design Considerations for a New Well Water Line," Corrosion Conference Paper 408, New Orleans (1989)
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From page 37... ...
Hewes, "Underground Corrosion of Water Pipes in Calgary, Canada," Materials Performance (May 1987) : 42-49; Rajani and Kleiner, "Protecting Ductile Iron Water Mains." 53 Brander, "Water Pipe Materials in Calgary, 1970-2000." 54 Michael Szeliga and Debra Simpson, "Evaluating Ductile Iron Pipe Corrosion"; Spickelmire, "Corrosion Control Considerations for Ductile Iron Pipe -- A Consultant's Perspective." 55 Michael Szeliga and Debra Simpson, "Corrosion of Ductile Iron Pipe: Case Histories," Materials Performance, 40(77)
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McCol lum, "Impressed Current Cathodic Protection of Polyethylene-Encased Ductile Iron Pipe," presented at NACE Corrosion 93, Houston, Tex.; J Schramuk and V
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From page 39... ...
. 64 Spickelmire, "Corrosion Control Considerations for Ductile Iron Pipe -- A Consultant's Perspec tive"; Stroud and Voget, "Corrosion Control Measures for Ductile Iron Pipe"; A.M.
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From page 40... ...
.69 In spite of the objections from U.S.-based manufacturers, bonded dielectric coatings for DIP that were used successfully for many years in the United States prior to 2002 are used outside the United States.70 Coatings that have been speci presented at Appalachian Underground Corrosion Short Course, Water and Wastewater Program, Morgantown, W.Va., 2001. 65 Guan, "Corrosion Protection by Coatings for Water and Wastewater Pipelines." 66 National Association of Pipe Fabricators, Inc., NAPF-500-03: Surface Preparation Standard for Ductile Iron Pipe and Fittings in Exposed Locations Receiving Special External Coatings and/or Special Internal Linings (Edmond, Okla., rev.
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From page 41... ...
Standard 8179.75 This zinc prime coat with a finish coat is used by most European iron pipe manufacturers as well as by the Water Research Centre for mildly and moderately corrosive soils. In more-corrosive soils, the metallic zinc coating and bitumen or epoxy finish coats may be supplemented by PE or bonded dielectric coatings.76 Other protective coating systems employed by European pipe manufacturers include but are not limited to extruded polyethylene, polyurethane, zinc-aluminum metallic spray with epoxy top coat, tape, and reinforced cementitious coatings.77 A summary of the use of bonded coatings is presented in Chapter 5 of this report.
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From page 42... ...
DIP with CP.80 Other Methods of Corrosion Control Other corrosion control methods that may be considered include but are not limited to specifying additional pipe wall thickness in the design of the pipe system to account for a calculated corrosion rate for the life of the system, soil enhance ments (e.g., controlled low-strength material) , anti-MIC PE, microperforated PE with CP, use of resistance probes or perforated plastic monitoring pipes, and pipeline monitoring and repair.
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