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Summary BACKGROUND OF THE STUDY Cast iron is a material that was used for years to fabricate pipe for applications such as water transmission and distribution. About 50 years ago ductile iron pipe (DIP) was introduced as a more economical and better-performing product. DIP is similar to cast iron pipe (CIP) in that it contains several percent carbon distrib- uted as graphite in an iron matrix. However, by the addition of small amounts of magnesium and the controlled annealing of DIP, the graphite becomes distributed as spherical nodules in DIP, whereas it is distributed in the form of flakes in tra- ditional CIP. This microstructure in DIP results in mechanical properties that are superior to those of traditional cast iron. As with iron or steel pipes, DIP is subject to corrosion, the rate of which depends on the environment in which the pipe is placed. Corrosion mitigation protocols that depend on the corrosivity of the soil are employed to slow the cor- rosion process to an acceptable rate for the application. A popular, economical, and often effective corrosion mitigation method for DIP in buried applications is the use of polyethylene encasement (PE), consisting of a sheet of polyethylene wrapped around the pipe or a tube of polyethylene sheeting slipped over the pipe at the time of installation. Another protective method is to use bonded dielectric coatings consisting of various polymers applied as coatings on prepared surfaces or to wrap tapes with adhesives on the same prepared surfaces. Bonded dielectric coatings are commonly used on steel pipes but are less commonly used on DIP. Cathodic protection (CP) can also be used to protect metal pipes, including DIP. CP
Corrosion Prevention Standards for Ductile Iron Pipe electrochemically protects the metal structure either by imposing a direct current (dc) electrical potential to the pipeline or by connecting the pipeline to a sacrificial anode made of a more electrochemically active metal. The decision to use corrosion mitigation systems and the choice of the system to use for buried DIP depend on the corrosivity of the soils in which the pipeline is buried. Many methods of assessing the corrosivity of soils have been developed. All use the resistivity of the soil either as the defining parameter or as one param- eter in conjunction with others, such as the presence or concentration of specific chemical species that foster corrosion. Soils with low resistivity are more corrosive than soils with high resistivity. The Bureau of Reclamation (commonly called Reclamation or BOR) of the U.S. Department of the Interior has from its inception fostered in the western United States water transmission projects designed to provide water for agricultural and municipal uses in areas otherwise deficient in local water supplies. In doing so, Rec- lamation has specified the types of materials appropriate for particular applications and the type of corrosion mitigation systems appropriate for each pipeline material depending on the corrosivity of the soils. As technology has been developed further and as experience has been gathered, Reclamation has changed the specifications for corrosion mitigation systems from time to time, most recently in 2004. The Bureau of Reclamation has chosen a relatively simple means of classifying the corrosivity of soils for pipelines installed under its control. For any section of pipeline to be designed, the in situ soil resistivity is measured at multiple posi- tions along the length of the pipeline. A computer program develops a layered model of true resistivities and forms an ordered probability plot developed from these resistivities; the resistivity at the 10th percentile of probability is selected. In other words, 10 percent of the true resistivities derived from the in situ resistivity measurements have lower resistivity values than this characteristic resistivity, and 90 percent have higher resistivity values. If this characteristic resistivity is greater than or equal to 3,000 ohm-cm, the soil is considered to be in the least-corrosive category. If it is between 2,000 and 3,000 ohm-cm, the soils are considered to be of moderate corrosivity. If the characteristic resistivity is less than or equal to 2,000 ohm-cm, the soils are considered to be highly corrosive. When Reclamation issued modified corrosion protection requirements in its technical memorandum, TM 8140-CC-2004-1, polyethylene encasement (PE) was specified as the corrosion mitigation method for DIP in soils of low corrosivity. For DIP used in moderately corrosive soils, PE and CP were specified. For highly corrosive soils, Reclamation specified that bonded dielectric coatings and CP were needed for adequate corrosion protection of DIP. It is this latter specification that âBureau of Reclamation, U.S. Department of the Interior, Technical Memorandum 8140-CC-2004-1, âCorrosion Considerations for Buried Metallic Water Pipe,â Washington, D.C., July 2004.
Summary has been and continues to be contested by some as an overly stringent requirement that necessitates the modification of the pipe from its as-manufactured state and thereby adds unnecessary cost to a pipeline system. In particular, the Ductile Iron Pipe Research Association (DIPRA), representing North American manufacturers of DIP, has conducted research both at test sites and in controlled installations; DIPRA interprets this research as showing that PE with CP provides adequate protection to DIP even in highly corrosive soils. In the past, bonded dielectric coatings on DIP were available, and a limited amount of such pipe was installed and is in use. However, the current specification of bonded dielectric coatings for DIP is difficult to meet, because in recent years the U.S. manufacturers of DIP have discouraged the application of bonded dielec- tric coatings on their product and will not supply DIP with such coatings, citing the difficulty of preparing the surface for coating application and the expense of applying such coatings to a relatively rough surface. Considering the research data available and the lack of DIP with bonded dielectric coatings, DIPRA and supporters of its position have vigorously petitioned the Bureau of Reclamation and the U.S. Department of the Interior to reconsider the decision to specify bonded dielectric coatings for DIP used in highly corrosive soils and to allow PE as at least part of the corrosion mitigation system. DIPRA also believes that Reclamationâs specification for this form of corrosion protection in highly corrosive soils has an impact on the use of DIP not only for applications specified by Reclamation but also for much broader applications, because the Rec- lamation specifications are used by many others designing pipeline systems. By contrast, the Bureau of Reclamation and those supporting its standards contend that the specifications are needed to ensure the reliability of Reclamationâs pipeline systems, particularly since most of these systems are large-diameter, non- redundant water transmission systems in which a failure affects large numbers of users and therefore has major economic and public health consequences. Nev- ertheless, recognizing the persistent criticism of its specifications, the Bureau of Reclamation has requested that the National Research Council (NRC) form a study committee to make recommendations concerning corrosion protection for DIP in highly corrosive soils. In response to Reclamationâs request, the NRC established the Committee on the Review of the Bureau of Reclamationâs Corrosion Prevention Standards for Ductile Iron Pipe. This committee of experts was convened to study corrosion protection of DIP in highly corrosive soils. Specifically, the charge to the committee was that it provide advice to Reclamation concerning the following: ⢠Does polyethylene encasement with cathodic protection work on ductile iron pipe installed in highly corrosive soils? ⢠Will polyethylene encasement and cathodic protection reliably provide a minimum service life of 50 years?
Corrosion Prevention Standards for Ductile Iron Pipe ⢠If the committee answers either of the above questions in the negative, the committee will provide recommendations for alternative standards that would provide a service life of 50 years. The committee met twice, in July 2008 and September 2008, and conferred many times in teleconference. To meet the charge, the committee heard the findings of experts, studied available data, and sought information from users concerning their experiences with the reliability of pipeline systems. The committee reviewed and discussed the information and data that were presented to it and sought out by the committee in order to come to the findings, conclusions, and recommenda- tions outlined in this report. The information and data were sorted on the basis of their relevance to and reliability in predicting the first failures to occur in a pipeline system. Specifically, the first failures are expected to be represented by corrosion behavior at the tails of the distribution, where the corrosion is fastest, and not by average behavior. The more relevant and reliable data (i.e., those that included the full distributions as opposed to those that included only averages) were therefore used as the primary basis for the findings, conclusions, and recommendations presented here. FINDINGS, CONCLUSIONS, AND RECOMMENDATIONS At the committeeâs first meeting, it sought clarification from the Bureau of Reclamation regarding some of the language in the charge to the committee. Writ- ten clarification provided after the meeting was as follows: Project Scope Question A: Does polyethylene encasement with cathodic protection work on ductile iron pipe installed in highly corrosive soils? Reclamation Clarification: This question is intended to seek the committeeâs technical assessment of the ability of this corrosion mitigation system to protect DIP from corrosion in highly corrosive soils. This question is not intended to solicit a response on how effective this system is, only if the committee does or does not believe the system is a technically sound approach to corrosion mitigation in highly corrosive soils. Project Scope Question B: Will polyethylene encasement and cathodic protection reliably provide a minimum service life of 50 years? Reclamation Clarification: This question is intended to seek the committeeâs technical assessment of the ability of DIP, installed in severely corrosive soils with CP and PE, to delay or reduce corrosion induced failures to a level that would allow the pipeline to pro- vide reliable service for the minimum 50-year service life, which Reclamation requires. The question of what Reclamation considers reliable service is discussed below in our responses to some additional questions posed by the committee. âMichael Gabaldon, Bureau of Reclamation, letter to Emily Ann Meyer, National Research Coun- cil, re: National Academies Review of the Bureau of Reclamationâs Corrosion Prevention Standards
Summary Regarding the first question, on whether PE with CP works on DIP in highly corrosive soils, the committee finds that if manufactured and installed correctly, polyethylene encasement with cathodic protection provides a betterment to bare and as-manufactured ductile iron pipe without cathodic protection in highly corrosive soils. Regarding the second question, on whether PE and CP reliably provide a mini- mum service life of 50 years, the committee asked Reclamation for a definition of what level of reliability it seeks for the 50-year service life. The response was that, ideally Reclamation would prefer that its pipeline systems be designed in such a way that no failures would take a system out of service during a service life of 50 years. However, it is recognized that no engineering system can be designed to ensure absolutely that no failures will occur, and Reclamation advised the committee that the level of reliability experienced by the extensive natural gas pipeline systems in the United States is a reasonable benchmark to strive for in its water pipeline systems. Reclamation calculated a benchmark failure rate from natural gas pipeline data from the Office of Pipeline Safety in the U.S. Department of Transportation. The committee also studied the data sets available from the Office of Pipeline Safety and calculated rates of failure for these pipelines. The committee then studied the available research data on the corrosion rates of DIP protected by PE and by PE with CP. It went on to find and evaluate known failures on water pipelines using these corrosion mitigation systems. The available data on known failures of DIP with PE and CP in highly corrosive soils are sparse, since relatively little DIPâperhaps in the hundreds of milesâhas been installed in highly corrosive soils with this corrosion mitigation system in place. Although DIP with CP and PE should work in theory and has been successful in many instances, the practical application of these technologies has not always been successful. Moreover, in order to provide a full analysis of the potential reliability of this system, it was necessary for the committee to focus on the failures rather than on the successes. The committee finds that the limited data available and the scientific under- standing of corrosion mechanisms show that ductile iron pipe with polyethylene encasement and cathodic protection is not likely to provide a reliable 50-year service life in highly corrosive soils (<2,000 ohm-cm). In part, this finding reflects the high standard set by the highly engineered and maintained gas pipeline systems and in part it reflects a meager but margin- ally significant set of data demonstrating probabilities of failure higher than the benchmark rate. In view of that finding, it was necessary for the committee to address the for Ductile Iron PipeâA Response to the Committeeâs Request for Clarification on Project Scope, August 21, 2008.
Corrosion Prevention Standards for Ductile Iron Pipe request by the Bureau of Reclamation concerning recommendations for alternative standards that would provide a service life of 50 years. To carry out that task, the committee studied available and conceptual means of providing adequate corro- sion mitigation that would meet Reclamationâs benchmark. In doing so, it devoted considerable effort to analyzing the reliability experience for DIP with bonded dielectric coatings and CP in highly corrosive soils, since that system is the current stated specification of Reclamation. After considerable study and deliberation, the committee finds that using the performance of bonded dielectric coatings on steel pipe with cathodic pro- tection as a benchmark for reliability, and based on available information, it is unable to identify any corrosion control method for DIP that would provide reliable 50-year service in highly corrosive soils. The committee considered other corrosion mitigation methods such as anti- MIC (microbiologically influenced corrosion) PE, microperforated PE, zinc coat- ings with epoxy top coat, controlled low strength material backfill, and the building in of a corrosion allowance. The committee finds that these other corrosion mitigation methods show promise, but the evidence is too limited to make any recommendations for their use at this time. This finding does not mean that any of these corrosion mitigation systems will not meet the required benchmark, but rather that the data are insufficient to draw a conclusion in either the affirmative or the negative. The majority of the data were on bonded dielectric coatings with CP on DIP, and although the committee is aware of no failures for systems in place, the length of time during which these systems have been in place is generally less than 50 years, and the number of miles of such pipeline again is in the hundreds. Therefore, while the use of bonded dielectric coatings with cathodic protection (CP) on ductile iron pipe (DIP) appears to be more effective than the use of polyethylene encasement with CP on DIP, the committee finds that it has insufficient evidence to assure that bonded dielectric coating with cathodic protection will meet the expected level of reliability. The committee recognizes that the case for using bonded dielectric coatings with CP appears to be supported in part by analogy with such coatings on steel pipe. Steel pipe, DIP, and cast iron pipe are generally acknowledged to corrode at roughly the same rate. Bonded dielectric coatings with CP are used on the steel pipe analyzed to set the benchmark standard for gas pipelines. Therefore, simple logic would suggest that bonded dielectric coatings on DIP should provide the same level âCommittee member Alberto A. Sagüés, while agreeing with the committeeâs conclusions and recommendations, disagreed with the interpretation of the data. His dissenting statement appears in Appendix B.
Summary of protection of DIP against external corrosion. However, some of the committee members thought that there could be no assurance that bonded coatings on DIP would function analogously with those on steel pipe. A primary concern is that the cast surface of DIP is rougher than that of steel, reflecting the texture on the mold surfaces for DIP. As a result, the surface prepa- ration of DIP is quite different from that of steel. Additionally, the nature of pipe section connections for DIP involves a bell on one end of the pipe, imparting sig- nificant diameter fluctuations along the pipeline not seen in the welded steel pipe used for natural gas pipelines. This diameter variability poses different challenges for a complete bonded dielectric coating on DIP because the spiral wrapping speci- fied for tape is reportedly difficult to achieve on conically shaped objects, bends, and other irregular shapes. There is also a drawback to CP when it is used with coatings. Whether they are PE or bonded dielectric, coatings provide their own level of corrosion protection and limit the current needed for adequate corrosion protection compared to that needed for bare pipe protected only with CP; both are advantageous characteristics. However, if the coatings are damaged as a result of tears, breaks, or other defects in the dielectric layer, reactants for corrosion can enter the area between the pipe and the coating. CP will protect the pipe in the vicinity of the damage, but the needed current does not protect at distances from the damage where corrosion may eventu- ally appear, a phenomenon known as current shielding. This is particularly acute with PE as there is no intended bonding between the pipe and the PE. Despite these shortcomings in surface preparation and in ensuring adequate cathodic protection (CP), the committee finds that bonded dielectric coatings with CP may provide superior protection to ductile iron pipe when compared to the protection provided by polyethylene encasement with CP. The committee recommends several areas in which additional studies could clarify these findings and provide data to improve the reliability of water-carrying pipe systems. ⢠The committee recommends that to improve the reliability of existing pipelines, modern testing systems such as intelligent in-line inspection (âsmart piggingâ) should be studied and introduced in order to monitor more closely the corrosion of pipes and permit the repair of pipe systems prior to failure. The adoption of such pipeline monitoring systems that send a remotely con- âL. GreggHorn, Product Advisory: Tape Coat (Birmingham, Ala.: Ductile Iron Pipe Research Asso- ciation, 1990).
Corrosion Prevention Standards for Ductile Iron Pipe trolled measuring device down the pipe to collect data such as pipeline wall thick- ness is not common in the water industry, but if adopted they could enhance the reliability of these pipelines, particularly where failure would have large negative impacts. The method of random digging to diagnose the corrosion behavior of pipelines is inadequate to predict the state of corrosion for the total pipeline and thus to examine fastest corrosion rates at the tail of the distribution. Studies should be initiated to evaluate some of the promising corrosion control systems considered in this study for water pipelines in order to determine whether the adoption of any of these systems would meet the desired level of pipeline reliability. There is evidence that better monitoring has led to enhanced reliability on the gas pipeline system. It is expected that a similar asset management program for critical water pipelines would enhance the reliability of these systems as well. ⢠The committee recommends that data on pipeline reliability be assem- bled for all types of pipe specified by the Bureau of Reclamation in Table 2, entitled âCorrosion Protection Criteria and Minimum Requirements,â in Technical Memorandum 8140-CC-2004-1 along with the specified cor- rosion protection applied in the various soil types. The Bureau of Reclamation and the committee had to rely on an expansive, but not entirely analogous, data set from the U.S. Department of Transportation on gas pipelines to set a benchmark for achievable pipeline reliability. The committee evaluated these data in many ways, but it believed that more data should have been available on the reliability of water pipelines of the type that Reclamation specifies. While the challenge to Reclamationâs specifications concerned only DIP in highly corrosive soils, there are other categories of pipes (steel, pretensioned concrete, and reinforced concrete) and corrosion mitigation methods (including mortar/coal tar epoxy, mortar, concrete, and concrete/coal tar/epoxy; the methods vary depending on the pipe material) that are not challenged, and implicit is the assumption that these pipe types and corrosion mitigation methods meet Reclamationâs expecta- tions for reliability. Therefore, had such reliability data been available on this large body of installed pipelines, a better-justified reliability benchmark reflecting Reclamationâs expectations could have been calculated. Such information would also allow the Bureau of Reclamation to gain a better understanding of what level of reliability it will accept and at what cost. The committee believes that the questions on adequate corrosion protection for DIP in highly corrosive soils and the appropriateness of Reclamationâs specifi- cations can be answered with greater certainty only after more data are generated. Greater experience is needed with the use of DIP in highly corrosive soils protected both by PE with CP and by bonded dielectric coatings with CP. Only with such
Summary experience and the data on reliability from that experience can recommendations with more robust statistics be forthcoming. Unfortunately, the prospects for more data other than those from existing pipelines in the United States are not encour- aging. While some users appear reluctant to use DIP with PE and CP in highly corrosive soils, others may find it difficult to obtain DIP with bonded dielectric coatings owing to its unavailability from U.S. manufacturers.
