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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe 2 Ductile Iron and Corrosion DUCTILE IRON PIPE Ductile iron pipe (DIP) is a ferrous-based alloy fabricated into seamless pipe using a centrifugal casting process in which the mold is rotated while the molten metal (1399°C) is poured into it from bell to spigot. For water and sewage distribution applications, the specifications for DIP are based on mechanical properties rather than on chemical composition. Typically, the grade of DIP for these applications is 60-42-10, indicating a minimum required tensile strength of 60,000 pounds per square inch (psi) (414 megapascal [MPa]), a minimum required yield strength of 42,000 psi (289 MPa), and a minimum required elongation of 10 percent. While the nominal composition in mass fraction for unalloyed ductile iron is 3.1 to 3.9 percent carbon, 2.1 to 2.8 percent silicon, and less than 1 percent of other elements (chromium, nickel, molybdenum, and others) 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) are used to transform the as-cast microstructure into a softer material with uniformly distributed graphite nodules, which give the ductile iron the desired combination of strength, ductility, and resistance to fracture for use in water transmission pipelines. The annealing heat treatment also produces an adherent, silicon-rich oxide layer of ~5 mils (0.125 millimeters [mm]) on the surface of the pipe that is subsequently coated with ~1 mil (0.025 mm) of asphaltic coating, which prevents atmospheric corrosion and
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe maintains the aesthetic appearance of the pipe prior to burial. For water-bearing applications, the pipe is also internally lined with cement mortar. Ductile iron contains approximately 3.5 percent graphite by weight. The final microstructure of ductile iron consists of a uniform distribution of graphite nodules within a ferritic iron matrix. When corrosion occurs, the carbon present remains an integral part of the corrosion by-products that adhere firmly to the noncorroded metal substrate. These graphitic by-products are not as strong as the original iron matrix but do have some mechanical strength and can provide a barrier against further corrosion. Left undisturbed, the residual carbon can slow or even stop the corrosion process in many soil environments. The spherical graphite structure of DIP promotes more uniform corrosion over the surface of the metal as opposed to more localized attacks.1 The electrical discontinuity (isolation) of the individual ductile iron pipeline sections (without cathodic protection [CP]) is considered important by some for avoiding the creation of corrosion cells over extended distances, as well as for preventing stray current accumulations. Others recommend that joints be bonded to allow corrosion monitoring and interference mitigation. Of course, such electrical isolation is neither possible nor desirable when CP is used for the protection of the pipeline as it is for the subject of this study. With respect to DIP metallurgy for transmission applications, DIP manufacturers have stated that alloying elements, such as nickel, improve the corrosion resistance but may degrade the mechanical properties of DIP.2 However, alloying additions also significantly increase the cost of the pipe. Based on experience and according to the available literature, changes in the chemical composition and microstructure to improve the corrosion resistance of DIP within the guidelines of the performance specifications from the American National Standards Institute (ANSI) and the American Water Works Association (AWWA) have not been explored and are not considered an engineering alternative at this time. After the casting and annealing process, DIP has an exterior surface that looks “peened.” The peened surface has an irregular profile of approximately 5 to 15 mils (0.127 to 0.381 mm), depending on the manufacturer and casting process. The standard length of DIP in the United States is between 18 and 20 feet. A minimum yield strength of 42,000 psi (290 MPa) is required for DIP. The casting allowance is between 0.05 and 0.09 inch depending on pipe size, and the service allowance is 0.08 inch regardless of pipe size. 1 Troy Stroud and James Voget, “Corrosion Control Measures for Ductile Iron Pipe,” 46th Annual Appalachian Underground Corrosion Short Course (Morgantown, W.Va.: West Virginia University, 2001). 2 Gene Oliver, “Memo on Ductile Iron Pipe Metallurgy for the NAS Committee,” September 8, 2008.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe Currently, U.S. manufacturers cast pipe in standard pressure classes of 150, 200, 250, 300, and 350 psi, with a minimum thickness of 0.25 inch. The wall thickness required for a section of pipe is determined on the basis of hoop stress induced in the pipe wall by the internal pressure that the pipe is to carry, or on the basis of external loads, and owners and engineers can specify wall thickness different from the standard thickness. Table 2-1, which is derived from application of the conventional formula and has been presented in various forms in AWWA publications, shows the wall thicknesses of pipe manufactured in the United States today. Of relevance to this study is the fact that, over time, as manufacturing processes 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 Nominal Pipe Size (in.) Outside Diameter (in.) Pressure Class 150 200 250 300 350 Nominal Wall Thickness (in.) 3 3.96 — — — — 0.25* 4 4.80 — — — — 0.25* 6 6.90 — — — — 0.25* 8 9.05 — — — — 0.25* 10 11.10 — — — — 0.26 12 13.20 — — — — 0.28 14 15.30 — — 0.28 0.30 0.31 16 17.40 — — 0.30 0.32 0.34 18 19.50 — — 0.31 0.34 0.36 20 21.60 — — 0.33 0.36 0.38 24 25.80 — 0.33 0.37 0.40 0.43 30 32.00 0.34 0.38 0.42 0.45 0.49 36 38.30 0.38 0.42 0.47 0.51 0.56 42 44.50 0.41 0.47 0.52 0.57 0.63 48 50.80 0.46 0.52 0.58 0.64 0.70 54 57.56 0.51 0.58 0.65 0.72 0.79 60 61.61 0.54 0.61 0.68 0.76 0.83 64 65.67 0.56 0.64 0.72 0.80 0.87 NOTE: An asterisk in the last column refers to the minimum casting thickness for that size pipe. The pipe wall thickness is in excess of the requirements for a 350 pressure class rating. The dash indicates that pipe is not manufactured in that class. SOURCE: Adapted from standard pressure classes as given in AWWA C150 and C151. (American Water Works Association, ANSI/AWWA Standard C150/A21.50-02 (2002): “American National Standard for Thickness Design of Ductile-Iron Pipe.” American Water Works Association, ANSI/AWWA C151/A21.51-09 (2002): “Ductile-Iron Pipe, Centrifugally Cast, for Water.”)
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe class3 (Class 50) would be 0.390 inch thick; if manufactured after 1992 for the lowest pressure class thickness (150 pressure class), the pipe would be 0.340 inch thick—a reduction in thickness of 13 percent. This demonstrable reduction in the pipe thickness will decrease the time required for corrosion to penetrate the entire thickness of the pipe. CORROSION MECHANISMS External corrosion of DIP can occur in soils through a number of mechanisms, described in the following subsections.4 Uniform Corrosion Uniform corrosion is defined as “corrosion that proceeds at about the same rate over a metal surface.”5 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. It is should be noted that monitoring (discussed later in this report) can be used to evaluate the service condition of materials undergoing uniform corrosion, as this form of corrosion generally happens at a somewhat steady pace. Failures can be avoided through preventive maintenance based on what is learned through monitoring and inspection. Pitting Corrosion Pitting corrosion, which is one of the more frequently seen forms of corrosion on DIP, is initially confined to a point or small area that takes the form of cavities.6 These pits can penetrate deep in the metal and, in DIP, can cause failure due to perforation. Pits are usually initially small in diameter and isolated, and occur in areas that are more anodic with respect to the rest of the metal surface. Soils with relatively high concentrations of chloride, nitrate, or sulfate can cause pitting. Once a pit is initiated, 3 Pipes manufactured prior to 1992 were manufactured to a “thickness” class rather than to the “pressure class” in use currently. 4 L. Veleva, “Soils,” in Corrosion Tests and Standards, R. Baboian, ed. (West Conshohocken, Pa.: ASTM International, 2005); P. Roberge, Corrosion Basics, 2nd ed. (Houston, Tex.: NACE International, 2006); S. Corcoran, “Effects of Metallurgical Variables on Dealloying Corrosion,” in Corrosion: Fundamentals, Testing and Protection, ASM Handbook, Vol. 13A (Materials Park, Ohio: ASM International, 2003). 5 American Society of Testing and Materials (ASTM), ASTM Standard G 15-08, ASTM Book of Standards, Vol. 03.02 (West Conshohocken, Pa.: ASTM International, 2008), p. 57. 6 ASTM Standard G 15-08.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe an active corrosion cell is established in which rapid dissolution of metal in the pit produces an excess of positive metal ions that hydrolyze to form hydrogen ions and cause the environment in the pit to become more acidic. Once initiated, these pits can become autocatalytic, continuing even if the initial source of corrosion is removed. Pitting can also be associated with local inhomogeneities on the metal surface, mechanical or chemical defects in films or coatings, galvanic effects, or biological activity. Pit propagation rates are not generally linear and, all other things being equal, unless autocatalytic, tend to decrease as the pit gets deeper. Unlike some other forms of corrosion, pitting is not easily detected during corrosion monitoring or inspection. Thus, due to the difficulties in detection and the unique nature of the corrosion mechanism, pitting corrosion can be particularly damaging, leading to failure of the pipe with little or no warning. Crevice Corrosion Crevice corrosion, a particular form of pitting, is the localized corrosion of a metal surface at or adjacent to an area that is shielded from full exposure to the environment.7 Crevice corrosion is initiated at unbonded or disbonded coatings, gaskets, bell and spigot joints, surface deposits, and other crevice geometries. Oxidation occurs within the crevice, while reduction reactions (decreases in oxidation state) 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.8 The potential difference between dissimilar metals in electrical and electrolytic contact causes electron flow between them. Attack of the more noble metal is usually decreased and corrosion of the less noble (more active) metal is usually increased. For example, buried ductile iron can corrode at an accelerated rate when coupled to bare copper bonding straps or copper services. The extent of this corrosion depends not only on the magnitude of the potential difference of the metals, but also on their polarizability (i.e., the amount of current required to change the potential of the metals). This factor is dependent on the nature of the metal surfaces, the soil environment (i.e., conductive soil can contribute to galvanic corrosion, as it will determine the influence of the ohmic potential drop), their configuration (i.e., the distance between the dissimilar metals), and the area ratio of the different materials’ surface area. One example of galvanic corrosion of DIP in soils is the result of the corrosion 7 ASTM Standard G 15-08. 8 ASTM Standard G 15-08.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe layer formed on iron pipe that has been buried for a relatively long time. This older pipe section is typically cathodic to a newer pipe, even when the composition of the older and newer pipe is identical. 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.9 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. However, this scale is brittle and vulnerable to impact; small pieces may crack and fall off the DIP surface. Areas covered with the oxide scale are cathodic to areas of bare metal, so galvanic corrosion of the bare metal can occur in the soil environment. This effect can be particularly damaging as the relative surface area ratios—that is, large cathode to small anode—are favorable to aggressive galvanic attack. Graphitic Corrosion Graphitic corrosion is a form of selective leaching (the removal of one element from a metal or alloy by corrosion) unique to iron structures containing graphite.10 The graphite is cathodic to the iron in the pipe, and therefore the iron corrodes galvanically leaving a graphite network. The nature of this graphite network is different in various cast irons. In ductile iron, the graphite precipitates as discrete spheroids and thus results in embedded nodules of graphite in a continuous iron corrosion matrix. Graphitic corrosion occurs over time, often in water with varying chloride levels, to yield a structure that is weaker and more brittle than the original structure. Although little dimensional change occurs, the strength and other metallic properties—such as hardness and electrical and thermal properties—of buried pipe can be significantly altered. However, it has been shown that graphitized pipe can have enough mechanical strength to withstand minimum pressure requirements in service.11 These pipes may ultimately fail under mechanical stresses such as water hammer12 and frost heave.13 9 Roberge, Corrosion Basics; Veleva, “Soils,” in Corrosion Tests and Standards. 10 Corcoran, “Effects of Metallurgical Variables on Dealloying Corrosion,” in Corrosion: Fundamentals, Testing and Protection. 11 T. Spence, “Corrosion of Cast Irons,” in Corrosion: Materials, ASM Handbook, Vol. 13B (Materials Park, Ohio: ASM International, 2005). 12 Water hammer consists of pressure variations caused by sudden changes in flow rates from closing of valves too quickly, pumps starting or stopping too quickly, or a sudden drop or increase in water flow, and so on. 13 Frost heave consists of pressure from the soil on the pipe wall caused by the increased volume of a soil when it freezes. Other physical soil pressures can include the swelling of clays, frost going out of the ground, and excavation or physical activity near the pipe.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe Graphitized iron pipe sounds dull when struck with a metal object, is soft (like a pencil), and can be gouged with a knife or screwdriver. To evaluate the damage fully, the graphite layer must be removed by abrasive blasting or tested by nondestructive evaluation methods (e.g., ultrasound or remote eddy field current). Recent papers summarizing an assessment of pipe material by both the city of Ottawa14 and the city of Calgary15 confirm that the pipe needs to be abrasively blasted (to remove surface debris) in order to identify the actual metal loss. This is so because the smooth surface appearance of graphitized cast iron, and in some cases ductile iron, may lead a casual observer to believe that the pipe has no corrosion damage and is still structurally sound. Microbiological Activity Microbes can play an important role in the corrosion of metals owing to the chemical activities associated with their metabolism, growth, and reproduction.16 This phenomenon is referred to as microbiologically influenced corrosion (MIC). Generally, microbes influence corrosion by changing the chemistry of the electrolyte at the metal surface, forming a biofilm. In buried soil environments, because of local chemistry, moisture, and surface characteristics, the moist soil itself is able to localize organisms near the metal surface and change the chemistry of the pore water in the soil.17 Biological influences on this chemistry can be divided into four general categories: (1) production of organic or inorganic acids as metabolic by-products, (2) production of sulfides under anaerobic conditions, (3) introduction of new redox reactions, and (4) production of oxygen or chemical concentration cells. In soils, the most important of these categories is the production of sulfides by sulfate reducing bacteria (SRB).18 SRB are anaerobic and live in poorly drained, wet soils with little or no oxygen that contain sulfate ions, organic compounds, and minerals. Conditions for MIC due to SRB can vary but are typically in the range of a pH of 6 to 8 and temperatures from 20°C to 30°C. As temperatures increase, SRB metabolism also increases. During metabolism of the bacteria, oxygen is extracted from the sulfate ions, and 14 L.B. Carroll, P. Eng, and J. Luffman, “Pipe Material and Soil Examination Techniques Used in the City of Ottawa Drinking Water System Maintenance and Planning Program,” presented at the NACE International Northern Area Eastern Conference, Ottawa, Ontario, October 2003. 15 G. Kozhushner, R. 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. 16 Veleva, “Soils,” in Corrosion Tests and Standards. 17 S. Dexter, “Microbiological Effects,” in Corrosion Tests and Standards, R. Baboian, ed. (West Conshohocken, Pa.: ASTM International, 2005). 18 Veleva, “Soils,” in Corrosion Tests and Standards.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe this reaction converts the soluble sulfates to sulfides and promotes corrosion of the metal surface. MIC of metals by SRB is a well-recognized problem in DIP; the corrosion rate of DIP due to MIC can be over an order of magnitude greater than that in comparable sterile soils.19 Stray Current Stray currents—or interference in buried structures—are currents flowing through earth from a source not related to the structure. Sources of these currents include direct current (dc) equipment and systems such as electric railway and streetcar systems, grounded dc power supplies, electric welders, CP systems, electroplating plants, and electric transmission systems.20 When these stray currents are discharged from the structure, such as a metal pipe, electrolytic corrosion may occur. 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. Severe corrosion of the pipe can occur at this location. SOIL CORROSIVITY Because of the complex content and characteristics of soils, a wide range of factors are important in soil corrosivity.21 These include the following: Moisture content, Resistivity, Permeability, Chloride ion content, Sulfide ion content, Sulfate ion content, Presence of corrosion-activating bacteria, Oxygen content, pH, and Total hardness of soil moisture. 19 Veleva, “Soils,” in Corrosion Tests and Standards. 20 Veleva, “Soils,” in Corrosion Tests and Standards. 21 J. Bushman and T. Mehalick, “Statistical Analysis of Soil Characteristics to Predict Mean Time to Corrosion Failure of Underground Metallic Structures,” in Effects of Soil Characteristics on Corrosion, V. Chaker and D. Palmer, eds. (West Conshohocken, Pa.: ATM International, 1989).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe Soil Moisture Soil corrosivity is strongly dependent on the amount of water retained by a soil and is a result of equilibrium between capillary and gravitational forces. The impact of moisture changes over time can also influence corrosion. For example, a sandy soil in a dry area may not be very corrosive. However, if there is infrequent moisture (from rain) and the soil contains chlorides, there can be highly aggressive, albeit transient, conditions of high corrosivity. During drying, these chlorides can become concentrated on the surface, making the local conditions even more aggressive. If pitting is initiated, this wet/dry process can lead to very intense corrosion, especially if the pits are small and become self-sustaining. Relative Acidity/Alkalinity, pH Most soils range from pH 4 to 8, and in this range soils are considered to be less corrosive.22 When lower than 4 and higher than 8.5, the pH can be a considerable factor in corrosivity. While natural environmental processes (e.g., mineral leaching, decomposition of plants, acid rain and snow, and some types of microbiological activity) can produce acidity in soils,23 soils with pH levels in the extremes above this range are rarely found unless other contamination has occurred. A neutral pH is the most favorable for SRB that would contribute to MIC, as described earlier in this chapter. Resistivity Electrical resistivity is an indication of the ability of an environment to carry corrosion current. A soil’s resistivity is a function of moisture and the concentration of current-carrying soluble ions and is typically measured in ohm-cm, either in situ or by sampling from the actual environment. Soil resistivities can range from less than 1,000 ohm-cm in soils with high water and ion contents to more than 100,000 ohm-cm in dry sand or gravel.24 22 Veleva, “Soils,” in Corrosion Tests and Standards. 23 Veleva, “Soils,” in Corrosion Tests and Standards. 24 D. Palmer, “Environmental Characteristics Controlling the Soil Corrosion of Ferrous Piping,” in Effects of Soil Characteristics on Corrosion, V. Chaker and D. Palmer, eds. (West Conshohocken, Pa.: ASTM International, 1989); V. Chaker, “Corrosion Testing in Soils—Past, Present and Future,” in Corrosion Testing and Evaluation, R. Baboian and S. Dean, eds. (West Conshohocken, Pa.: ASTM International, 1990); D. Palmer, “Field Soil Corrosivity Testing,” in Corrosion Testing and Evaluation, R. Baboian and S. Dean, eds. (West Conshohocken, Pa.: ASTM International, 1990); E. Escalante, ed., Underground Corrosion (West Conshohocken, Pa.: ASTM International, 1981).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe Models have been created that link soil resistivity to corrosion rate, and rating scales are commonly used for this purpose. However, other variables can influence soil corrosivity; the relationship between resistivity and corrosivity is more fully explored in the section that follows. 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).25 In their analysis of corrosion rates on iron piping used for water distribution, Kozhushner and colleagues report that soil resistivity is the most important factor, along with the type of pipe and wall thickness and the presence of copper services and conclude that soil resistivity below 2,000 ohm-cm is most corrosive to iron pipe.26 Alternatively, DIP manufacturers, through the Ductile Iron Pipe Research Association (DIPRA), have developed a 10-point system to evaluate other environmental conditions in addition to soil resistivity that contribute to ductile iron corrosion. This 10-point system uses soil resistivity, pH, redox potential, sulfides, and moisture to assess the corrosivity of a soil. This soil assessment method, although not a part of the ANSI/AWWA C105/A21.5-05 Standard, “Polyethylene Encasement for Ductile-Iron Pipe Systems,” is included with that standard as Appendix A.27 Under this soil corrosivity scale, a total of 10 points or more indicates that the soil is corrosive to as-manufactured DIP, and additional corrosion protection measures are recommended. When the effects of all other factors are eliminated, soil would have a resistivity of 1,500 ohm-cm or less to be considered corrosive to DIP. Some utilities and corrosion consultants have developed their own methods for soil classification based on an expansion of the 10-point system. Some have developed a 25-point system28 that assesses soil corrosivity as a function of the 25 Bureau of Reclamation, U.S. Department of the Interior, Technical Memorandum 8140-CC-2004-1, “Corrosion Considerations for Buried Metallic Water Pipe,” Washington, D.C., July 2004. 26 G. Kozhushner, R. Brander, and B. Ng, “Use of Pipe Recovery Data and the Hydroscope® NDT Inspection Tool for Condition Assessment of Buried Water Mains.” 27 American Water Works Association, ANSI/AWWA Standard C105/A21.5-05 (2005): “Polyethylene Encasement for Ductile-Iron Pipe Systems, Denver, Colo.” 28 W. Spickelmire, “Corrosion Considerations for Ductile Iron Pipe—A Consultant’s Perspective,” Materials Performance 41(7):16 (2002).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe pipeline type so as to assign a risk factor and allow designers to determine what level of corrosion control measures is appropriate. Another method is the proprietary Design Decision Model (DDM™) employed by Corrpro and DIPRA. This model expanded the 10-point soil evaluation system with the inclusion of field and laboratory data. The DDM™ risk model considers both the likelihood and the consequence of pipe failure due to external corrosion.29 The Bureau of Reclamation uses a 10-percentile probability to sense resistivity. This means that if resistivity measurements are performed for 100 separate soil samples, the 10th-lowest measured value is the soil resistivity used as the design criterion, which results in a conservative approach to determining corrosion control requirements for pipeline projects. Reclamation justifies this approach by stating: While all these factors are important in development of pipeline corrosion strategy, these tests can be cost intensive for long pipe alignments and very much subject to judgment or interpretation as to which point values should be assigned.30 It was noted by Reclamation during the committee’s first meeting31 and in Technical Memorandum 8140-CC-2004-1 that the designers of pipeline projects have the liberty to consider other factors but that the soil resistivity criteria in Table 2 of the Technical Memorandum (see Figure 1-1 in this report) are the absolute minimum. However, it should be noted that, according to Reclamation’s procedures, the designer is allowed to identify and treat areas differently as the circumstances dictate. For instance, if a pipeline is 20 miles long and 1 mile of soil is identified as highly corrosive and 19 miles are identified as moderately corrosive, the 1-mile section may be isolated and treated differently from the remaining 19 miles. 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. 29 D.H. Kroon, D. Lindemuth, S. Sampson, and T. Vincenzo, “Corrosion Protection of Ductile Iron Pipe,” Corrosion 2004 Conference, Paper No. 46, Houston, Tex., 2004. 30 Bureau of Reclamation, U.S. Department of the Interior, Technical Memorandum 8140-CC-2004-1, July 2004. 31 Bureau of Reclamation Technical Service Center Staff, U.S. Department of the Interior, “Corrosion Considerations for Buried Ductile Iron Pipe,” presentation to the committee, Washington, D.C., July 28, 2008.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe There is no consensus on what corrosion protection methods would best protect DIP in specific environmental conditions. When corrosion protection for DIP is required or recommended, the following methods are commonly used either individually or in combination: Polyethylene encasement, Cathodic protection, Bonded dielectric coatings. Currently Available Corrosion Protection Methods This subsection reviews corrosion control methods used for DIP. It identifies which are available in the United States and which are available only outside the United States. 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) for corrosion protection. PE was introduced in 1958 and has evolved into two main products: linear, low-density polyethylene film (8 mils thick) and high-density, cross-laminated polyethylene film (4 mils thick). Both types of film are available in tube or sheet form. AWWA adopted the first standard for PE, ANSI/AWWA Standard C105, in 1972, and other standards for PE are available in the United States, Japan, Great Britain, and Australia.33 According to the ANSI/AWWA standard, PE is to be fitted to the pipe contour in a snug, but not tight, encasement with limited space between 32 David H. Kroon, Dale Lindemuth, Sheri Sampson, and Terry Vincenzo, Advanced Corrosion Protection Solutions for Ductile Iron Pipe (Medina, Ohio: Corrpro Companies, Inc., 2004). 33 American Water Works Association Standard ANSI/AWWA C105 (2005) “Polyethylene Encasement for Ductile-Iron Pipe Systems”; International Standards Organization Standard ISO 8180 (August 15, 2006): “Ductile Iron Pipelines—Polyethylene Sleeving for Site Application,” 2nd ed.; ASTM International Standard ASTM A 674-05 (2005): “Standard Practice for Polyethylene Encasement for Ductile Iron Pipe for Water or Other Liquids”; British Standard BS 6076 (1996): “Specification for Polymeric Film for Use as a Protective Sleeving for Buried Iron Pipes and Fittings (for Site and Factory Application)”; Japan Ductile Iron Pipe Association Standard JDPA Z 2005-1989, “Polyethylene Sleeve for Ductile Iron Pipes and Fittings”; Australian Standard AS 3680 (1989): “Polyethylene Sleeving for Ductile Iron Pipelines.”
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe the pipe and the encasement.34 In 1993, the standard was revised to allow the use of either an 8-mil low-density polyethylene film or a 4-mil high-density cross-laminated polyethylene film. A recommendation added that in wet conditions the PE should be taped every 2 feet around the pipe. In 2000, ANSI/AWWA Standard C105 was again revised to replace low-density polyethylene with linear, low-density encasement material, and the soil resistivity ranges were modified, resulting in more conservative evaluation procedure values. A paragraph was also added to the standard acknowledging that other corrosion control methods may be required in “uniquely severe” environments. ANSI/AWWA Standard C105 does not recommend any particular method of PE installation; however, the tube form is favored for most installations because it speeds field installation while minimizing accidental contamination by means of foreign material that becomes lodged under the PE.35 The shape of the tube also helps limit entry of oxygen under the film at unsealed edges when compared to the flat-sheet type of PE. The flat-sheet type is reportedly more useful for the encasement of irregularly shaped appurtenances such as valves and tees. On the basis of testing that it conducted, DIPRA recommends either 4-mil or 8-mil material and states that the newer 4-mil, high-density, cross-laminated film and the 8-mil, linear low-density film are more resistant to damage than is the older 8-mil, low-density PE.36 Like ANSI/AWWA Standard C105, DIPRA does not recommend either a preferred specific film thickness or installation method. Polyethylene, like any other material, can degrade over time, although the degradation is usually minimal. The Bureau of Reclamation conducted a 7-year test on underground polyethylene sheeting that was being used as canal liner material; this test indicated that the polyethylene film demonstrated limited loss of tensile strength and elongation.37 Reclamation also conducted accelerated testing (estimated to be 5 to 10 times faster than field burial conditions) which indicated that polyethylene is highly resistant to bacteriological deterioration. 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. (1967). 36 DIPRA, “Polyethylene Encasement Effective, Economical Protection for Ductile Iron Pipe in Corrosive Environments,” DIPRA Brochure POLYTECH/5-07/5M, January 1992, rev., May 2007. 37 U.S. Department of the Interior, Bureau of Reclamation, Laboratory and Field Investigation of Plastic Films, Rept. No. ChE-82 (Washington, D.C., September 1968); Harry Smith, “Corrosion Prevention with Loose Polyethylene Encasement,” Water and Sewage Works, May 1972. 38 DIPRA, “Polyethylene Encasement Effective.”
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe At a location where some of the oldest polyethylene-encased pipe is buried (LaFourche Parish, Louisiana), DIPRA in 2008 indicated that the polyethylene encasement “is not suffering from significant deterioration, while in service.”39 DIPRA did acknowledge that the PE material can change during burial, but believed that the material would be flexible enough to provide adequate protection to the pipe. The purpose of the PE is to develop a physical barrier between the pipe and its surrounding environment. It is therefore important during installation that (1) no soil becomes trapped between the pipe and polyethylene; (2) the polyethylene is snug, but not tight, to allow it to bridge irregularities without stretching; (3) overlaps and ends are secured with adhesive tape or plastic tie straps; (4) any damaged areas that occur during installation are repaired prior to final backfilling; and (5) service lines of dissimilar metals are wrapped with polyethylene or a suitable dielectric tape for a minimum of 3 feet clear distances.40 PE is reported to possess good dielectric insulating characteristics, which protect the pipe from low-level stray direct current.41 In theory, intact PE is a passive corrosion protection system that may prevent direct contact with the soil and limit the access of oxygen to the pipe surface; PE is also said to minimize the electrolyte available to support corrosion.42 Typically the weight of the backfill around the pipe and the small space between the PE and the pipe are assumed to reduce any significant exchange of groundwater from the backfill to the area between the pipe and the PE. Initially there may be groundwater trapped under the wrap that can exhibit the same characteristics as the surrounding soil. If that water is corrosive and has a high oxygen content, the initial corrosion rate can be relatively high. This initial corrosion cell activity should decrease over time as the oxygen is depleted. 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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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe protect the pipe without the formation of concentration cells at holidays (tears or defects in loose or bonded coating).44 Although PE is a standard form of corrosion control for DIP in less corrosive environments, there is an ongoing debate about the benefits of PE in more corrosive soils.45 Advantages of PE reported by DIPRA include low cost, ease of installation, no needed maintenance or monitoring, and ease in repairing damage to the PE prior to burial.46 DIPRA states that “the number of documented failures of polyethylene encased pipelines—the vast majority of which are the result of improper installation—is insignificant compared to the miles of Cast and Ductile Iron pipe that are afforded excellent protection with this method of corrosion prevention.”47 Based on 20 years of experience and field research, Smith48 reported that there had been no failure of DIP with PE. Historical data from DIPRA indicate minimum corrosive attack of DIP with PE installed in the United States,49 but DIPRA does not recommend PE as the sole protection method in areas where high-density stray currents may be present.50 In contrast to the results reported by DIPRA, other reports describe failures of DIP with PE. A 1975 study conducted by Hawn and Davis for the city of Calgary, Alberta, concluded that PE was not protective of pipes, and fittings could be severely corroded.51 Experience and problems with PE have been documented in other Canadian cities, many of which have aggressive soils.52 In a paper presented 44 DIPRA, “Polyethylene Encasement Effective.” 45 C.W. Crabtree, M.R. Breslin, J.A. 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 Conference Proceedings (Denver, Colo., 2001); Kevin Garrity, “Corrosion Control Design Considerations for a New Well Water Line,” Corrosion Conference Paper 408, New Orleans (1989); B. Rajani and Y. Kleiner, “Protecting Ductile Iron Water Mains: What Protection Method Works Best for What Soil Condition,” Journal of the American Water Works Association 95(11):110-125 (2003); Michael Szeliga, “An Independent Evaluation of the Effectiveness of Polyethylene Encasement as a Corrosion Control Measure for Ductile Iron Pipe,” paper presented at ASCE Pipelines Conference, Atlanta, Ga., 2008. 46 Stroud and Voget, “Corrosion Control Measures for Ductile Iron Pipe.” 47 Troy Stroud, “Polyethylene Encasement Versus Cathodic Protection: A View on Corrosion Protection,” Ductile Iron Pipe News (Spring/Summer 1998). 48 W.H. Smith, “Corrosion Prevention with Loose Polyethylene Encasement,” Water and Sewage Works (May 1997). 49 Stroud and Voget, “Corrosion Control Measures for Ductile Iron Pipe.” 50 DIPRA, “Polyethylene Encasement Effective.” 51 D.E. Hawn and J.R. Davis, Special Corrosion Investigation, Report for the City of Calgary, Water Transmission and Distribution System (Edmonton, Alberta: Caproco Corrosion Prevention Ltd., 1975). 52 Caproco Corrosion Prevention Ltd., Underground Corrosion of Water Pipes in Canadian Cities, Case: The City of Calgary, CANMET Contract Report No. OSQ81-00096 (Edmonton, Alberta, May
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe at the 2001 AWWA Infrastructure Conference, the city of Calgary compared its experience with bare DIP and DIP with PE, noting: “From the two studies, we can say roughly that [PE] offered us about a 30% average reduction in corrosion rate and consequently in corrosion break rate, where no (uninsulated) copper services are involved.”53 Through field evaluations,54 others have identified cases where DIP with PE has failed prematurely. PE can be damaged during installation if proper care is not taken by the contractor, resulting in rips or tears. These defects and holidays may create holes through which environmental water can reach the DIP and may lead to accelerated corrosive attack of the pipe in the vicinity of these defects. Corrosion under intact PE and in MIC conditions has also been reported.55 Specific cases are discussed in more detail in Chapter 3. Cathodic Protection Cathodic protection is another common method for the reduction of corrosion rates. The two most common types of CP are passive (galvanic) and active (impressed current) systems. CP as a method of corrosion control requires the bonding of DIP joints for electrical continuity. Since DIP is typically installed with push-on or mechanical joints, suitably sized insulated copper wires56 or straps are installed across each joint and secured to each side of the pipeline joint by exothermic welding or pin-brazing techniques. For galvanic anode systems, a sacrificial metal (e.g., magnesium or zinc) is attached to the pipe at regular intervals. These prepackaged galvanic anodes are sized and spaced along the pipeline according to design-specified current requirements and pipeline attenuation characteristics. Bare galvanic ribbon anode systems are also used to provide CP protection. In an active system, an impressed current is applied to the pipeline using a dc power source (typically a rectifier) and impressed current anodes (high-silicon cast iron, graphite, and so on) at intervals according to designed current requirements 1985); J.A. Jakobs and F.W. 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):22-26 (2001); Michael Szeliga. “Ductile Iron Pipeline Failures,” Materials Performance 44(5):26-30 (May 2005); Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective.” 56 DIPRA, “Guidelines for the Electrical Bonding of Ductile Iron Pipe Joints, Thermite Weld Method” (rev. October 1999).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe and pipeline attenuation characteristics. These can be electrically remote ground beds or a distributed-type ground bed located next to the pipeline. In both systems, the dc current produced from the source (galvanic anode or impressed anodes) passes through the soil to the pipeline being protected. A properly designed CP system can reduce corrosion rates to a desired level, thereby extending the useful life of the pipeline. Polyethylene Encasement with Cathodic Protection In corrosive soils, the combination of CP and PE may be a viable method of corrosion control. The use of PE may reduce the annual operating cost of the active CP system or extend the life of the galvanic anode CP system by reducing the demand on the CP system, allowing the two systems to complement each other.57 Theoretically, CP should only be required in those areas where the PE is damaged and direct contact of the DIP with the corrosive soils occurs. However, one concern is that PE may shield the CP system from actively mitigating corrosion under the intact PE.58 Trapped corrosive materials (organic clays, microbial waters, or soils) can migrate along the loose PE away from damaged areas and create an environment suitable for corrosion and/or MIC. The effectiveness of the CP may be reduced as a function of distance from a holiday or damaged area in the PE. If MIC activity is occurring or corrosive groundwater has traveled, under the PE outside that area of CP influence, the CP system may be ineffective. If these active corrosion cells are electrically shielded from the CP current by the PE, the corrosion can continue, which is why National Association of Corrosion Engineers (NACE) Standard SP0169 advises against the use of materials or construction practices that create electrical shielding on the pipeline.59 Similarly, in two separate decisions, the U.S. Department of Transportation’s Office of Pipeline Safety ruled against the use of PE with CP for DIP and cast iron in oil and gas lines 57 G. Bell and A. Romer, “Making ‘Baggies’ Work for Ductile Iron Pipe,” ASCE Pipelines 2004 Conference, San Diego, California, August, 2004; D. Lindemuth and D. Kroon, “Cathodic Protection of Pipe Encapsulated in Polyethylene Film,” NACE Corrosion, Paper 07040 (2007); M. Schiff and B. McCollum, “Impressed Current Cathodic Protection of Polyethylene-Encased Ductile Iron Pipe,” presented at NACE Corrosion 93, Houston, Tex.; J. Schramuk and V. Rash. “Case History: Cathodic Protection for a New Ductile Iron Water Transmission Main,” Materials Performance 44(10):20-24 (2005). 58 John H. Fitzgerald III, “Cathodic Protection of Miscellaneous Underground Structures,” in Proceedings of Seventeenth Annual Appalachian Underground Corrosion Short Course (Morgantown: West Virginia University, 1972); Kevin Garrity, “Corrosion Control Design Considerations for a New Well Water Line.” 59 NACE Standard SP0169, Control of External Corrosion on Underground or Submerged Metallic Piping Systems (NACE International, Cleveland, Ohio, 2007).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe because of concerns about electrical shielding and MIC corrosion.60 One European DIP manufacturer states: “Polyethylene sleeving is not considered an adequate coating for ductile iron pipes subject to cathodic protection.”61 One of the concerns regarding CP shielding is that problems cannot be detected by means of routine CP-monitoring techniques. If the coating is shielding the current, CP potentials measured along the pipeline will not indicate a problem until a leak occurs. Several authors summarize these problems in regard to disbonded coatings, which are electrically similar to shielding with intact PE.62 One author concluded, “Electronic (smart) pigging surveys are the best, and only reasonably sure, method for determining corrosion on pipeline surfaces under disbonded coatings.”63 The ongoing debate about the suitability of DIP with PE and CP in highly corrosive soils is focused on the following: (1) whether moisture and oxygen can be present under undamaged PE, (2) whether PE can be installed without sustaining damage, (3) whether all damaged PE can be identified and repaired before burial, (4) whether electrical shielding can restrict the CP current from reaching the pipe surface in all areas, and (5) whether CP with PE is effective if these conditions occur. Detailed cases of the use of DIP with PE and CP, including examples both of successes and of corrosion of DIP with PE and CP, are presented in Chapter 3. Bonded Dielectric Coatings and Cathodic Protection Historically, the use of bonded dielectric coatings on DIP has received mixed support.64 The primary disagreements center on whether DIP can be economically 60 Office of the Secretary of Transportation ruling: Federal Register 36, no. 126 (June 1971): 12297; ruling by Office of the Pipeline Safety: Federal Register 36, no. 166 (August 1971): 16949. 61 St. Gobain Web site, available at www.saint-gobain-pipelines.co.uk. Accessed September 2008. 62 Spicklemire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; F.M. Song, D.W. Kirk, J.W. Graydon, D.E. Cormack, and D. Wong, “Corrosion Under Disbonded Coatings Having Cathodic Protection,” Materials Performance 42(9):24-26 (2003); Douglas Moore, “Phorgotten Phenomena: Cathodic Shielding Can Be a Major Problem After a Coating Fails,” Materials Performance 39(4):44-45 (2000). 63 Duane Tracy, “Disbonded Coatings Influence CP, Pipe Line Risk Assessment,” Pipe Line and Gas Industry 80(2):27-31 (1997). 64 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; Stroud and Voget, “Corrosion Control Measures for Ductile Iron Pipe”; A.M. Horton, “Special Protective Coatings and Linings for Ductile Iron Pipe,” Proceedings from Second International Conference, Pipeline Division and ASCE-TCLEE (Reston, Va.: ASCE-TCLEE, 1995), pp. 745-756; Brander, “Water Pipe Materials in Calgary, 1970-2000”; D. Lieu and M. Szeliga, “Protecting Underground Assets with State-of-the-Art Corrosion Control,” Materials Performance 41(7):24 (2002); “Surface Preparation of Ductile Iron Pipe to Receive Special Coatings,” Ductile Iron Pipe News (Fall/Winter 1993), p. 17; Shiwei Guan, “Corrosion Protection by Coatings for Water and Wastewater Pipelines,” paper
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe coated and whether the coating will perform satisfactorily. The majority of the reported problems have related to application of the coating. The selection of the correct type of coating and surface preparation are important as, unlike steel pipe which is smooth, the “peened” surface of DIP with scale requires a modification to the specified surface preparation, and the adherence of some coatings has been reported to be poor regardless of the level of surface preparation.65 The National Association of Pipe Fabricators, in its standard for surface preparation for DIP receiving special external coatings, also specifically notes that the standards developed for steel are not applicable to DIP.66 Some of the factors that U.S. pipe manufacturers cite as limiting the feasibility of a bonded coating for DIP include the following: (1) pipe (substrate) damage, including blisters and slivers, caused by abrasive blasting; (2) abrasive blast surface preparation negating the protective effects afforded by the asphaltic shop coating and the annealing oxide layer; (3) difficulty in meeting holiday test requirements because of the rough external surface and as-cast peen pattern; (4) a limited number of knowledgeable coating applicators; (5) susceptibility of the coating to 45damage during shipment and installation; and (6) adverse impact of the coating on joint configurations and joint tolerances (i.e., field cuts, push-on joints, and restrained joints). These issues are reportedly the reason that North American DIP manufacturers announced in August 2002 that they would no longer provide pipe to be used with bonded coatings.67 This continues to be the case, and DIPRA notes that none of its member companies will provide pipe with bonded coatings.68 Some DIP manufacturers discourage end users from using such coatings or even from obtaining pipe without the asphaltic coating (which is taken as an indication of intent to apply a bonded dielectric coating).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. February 14, 2006). 67 Jose Villalobos, “Successful Coating Applications,” V&A Consulting Engineers Infrastructure Preservation News 1 (June 2003), available at http://www.vaengr.com/VANewsletter/June2003/Interest-June2003.html. Accessed December 28, 2008. 68 L. Gregg Horn, DIPRA, Letter to John Keys, Commissioner of the Bureau of Reclamation (November 16, 2004). 69 Mike Woodcock, “Review of the Bureau of Reclamation’s Technical Memorandum,” presentation to the committee, July 29, 2008. 70 Rajani and Kleiner, “Protecting Ductile Iron Water Mains.”
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe fied for DIP include coal tar epoxy, coal tar enamel, extruded polyolefin, sprayed polyolefin, polyurethane, hot- and cold-applied tapes, and cement coatings.71 Wax-petrolatum tapes have also been reported to have been successfully used on DIP and fittings.72 Extruded polyethylene-type coatings have been adapted for bell-and-spigot-type DIP joints and have been successfully used since 1975.73 Tape coatings have been used since the mid-1970s, with use of polyurethane coating beginning in 1988. According to one report, a bonded thermoplastic coating for DIP was used in Seattle, Washington,74 and this type of coating has also been used successfully in Europe. Liquid epoxy, fusion-bonded epoxy, and thermoplastic-type coatings have been used successfully for ductile- and cast-iron fittings. Brush- and spray-applied coatings, tape, or heat-shrink sleeves have historically been used for pipe joint coatings. A wider variety of bonded dielectric coatings have been used overseas than in North America to protect buried DIP. In Europe, the primary external coating for corrosion control in use since the 1960s has been metallic zinc spray with a bitumen or epoxy top coat. The two methods of zinc application (metallic zinc or zinc-rich coatings) are covered by International Organization for Standardization (ISO) 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. 71 A.M. Horton, “Special Protective Coatings and Linings for Ductile Iron Pipe,” pp. 745-756 in Advances in Underground Pipeline Engineering II, Bellevue, Wash.: American Society of Civil Engineers (1995); Rajani and Kleiner, “Protecting Ductile Iron Water Mains”; Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; Guan, “Corrosion Protection by Coatings for Water and Wastewater Pipelines.” 72 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; Bell and Romer, “Making ‘Baggies’ Work for Ductile Iron Pipe.” 73 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; Brander, “Water Pipe Materials in Calgary, 1970-2000.” 74 J.R. Pimentel, “Bonded Thermoplastic Coating for Ductile Iron Pipe,” Materials Performance 40(7):36 (2001). 75 ISO Standard 8179, Ductile Iron Pipe—External Zinc Coating (Geneva, Switzerland: ISO, rev. 2007). 76 J.E. Drew, Pipe Materials Selection Manual (Swindon, U.K.: WRc, 1995), p. 128. 77 Trevor Padley, Saint Gobain Pipelines plc, Derbyshire, England, communication with the committee, 2008; Saint Gobain Web site information, Section 5, Pipe Coatings, available at www.saint-gobain-pipelines.co.uk. Accessed September 2008.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe As-Manufactured DIP with Cathodic Protection Since bonded coatings on DIP are not available in the United States, several utilities and corrosion engineers have elected to install bare DIP with CP instead of with PE to minimize the problems with electrical shielding. The city of Seattle, Washington, installed a 40,000-foot section of bare DIP with an impressed current CP system next to a transit authority.78 Russell Corrosion Consultants reported that it is installing more than 15 miles of bare ductile iron pipeline with CP on 18 different projects in the northeastern United States.79 The Washington Suburban Sanitary Commission in the Washington, D.C., area has reported that, because of concerns about shielding from PE, it is installing several thousand feet of bare (as-manufactured) 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 enhancements (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. These are discussed in more detail in Appendix D of this report. Selection of a Corrosion Control Method In selecting a method for corrosion control for DIP, the designer and owner should consider all factors, including soil condition, capital costs of construction, operation and maintenance of the CP system, consequences of failure, and cost of repair. 78 Les Nelson, Seattle Public Utilities, Seattle, Washington, communication with the committee, September 2008. 79 Michael Szeliga, Russell Corrosion Consultants, communication with the committee, September 2008. 80 Dick Newell, Washington Suburban Sanitary Commission, communication with the committee, September 2008.