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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe Appendix D Other Considerations for Corrosion Control In this appendix, the committee presents some of the other considerations for corrosion control that they discussed in the development of the recommendations. CORROSION ALLOWANCE The use of a corrosion allowance (manufacturing pipe with an excess wall thickness to extend the time to failure) in determining the thickness of the pipe by increasing the wall thickness of the ductile iron pipe (DIP) to allow for metal loss is generally not cost-effective or reliable, especially when pitting is the primary failure mode for DIP.1 The method for providing corrosion allowance involves (1) determining the required thickness of pipe to maintain adequate strength based on operating pressure and depth of cover, (2) determining the rate of corrosion based on soil condition, (3) identifying the desired life of the pipe system, and (4) based on these factors, determining the thickness of the pipe system to be specified. For example, a 30-inch-diameter pipe system with a working pressure of 300 pounds per square inch (psi) normally needs a wall thickness of 0.45 inch, assuming a soil environment that would cause a rate of corrosion of 20 mils per year (mpy). To design the pipe for a 50-year service life, the pipe thickness would be 0.45 inch 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).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe plus 0.020 inch per year over 50 years, giving a total design pipe wall thickness of 1.45 inches. This thickness would achieve the desired service life without loss of the required design thickness of the pipe system. However, as was noted in Chapter 2 of this report, such extremely thick pipe is not routinely manufactured. ENVIRONMENTAL CONTROLS (SOIL ENHANCEMENTS) Specialized trench backfill in corrosive soils could be used to reduce the corrosivity at the pipe surface. This may provide only short-term protection to DIP due to leaching of the soil into the backfill. It has been observed, in many instances, that the less-corrosive backfilling material eventually takes on the characteristics of the surrounding soil.2 This method can be as simple as using clean sand backfill, which will reduce the corrosion rate for a period. However, in areas with fluctuating water tables, the clean sand could become corrosive in a relatively short period of time. An alternative method to consider in reducing corrosion rates is to change the environment in the trench with the use of controlled low-strength material (CLSM) that is placed in the pipeline trench after the pipe is installed.3 The physical and chemical nature of this material tends to create a controlled environment of nonaggressive conditions and may inhibit the movement of chlorides and sulfates into the trench. The installer may also consider a chemical-enhanced backfill to counter the naturally occurring corrosive materials. ANTI-MIC POLYETHYLENE ENCASEMENT In an attempt to minimize concerns about microbiologically influenced corrosion (MIC) under polyethylene encasement (PE), one manufacturer has developed a PE with anti-MIC additives. These anti-MIC polyethylene-encasement materials appear to hold some promise and are now available for commercial distribution.4 The anti-MIC PE was used recently (2007-2008) on a project in Minnesota.5 Additional independent testing and long-term evaluations still need to be completed to confirm the influence of MIC under PE on both water and wastewater lines. 2 American Water Works Association, AWWA Manual M41, Ductile-Iron Pipe and Fittings (Denver, Colo., 1996), p. 173. 3 Kevin Folliard, David Trejo, Scott Sabol, and Dov Leshchinsky, Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction, NCHRP Report 597 (Washington, D.C.: Transportation Research Board, 2008). 4 George Ash, Fulton Enterprises, Birmingham, Ala., communication with the committee, 2008. 5 William Spickelmire, “Corrosion Control 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 MICROPERFORATED POLYETHYLENE ENCASEMENT WITH CATHODIC PROTECTION One PE manufacturer is experimenting with micro-perforated PE (similar to the perforated rock shield concept) on cathodically protected pipe samples in an effort to minimize cathodic protection (CP) shielding.6 This concept will increase the current needed for CP but should prevent shielding of any area of the pipe from the protective current. Preliminary testing has been started in the Florida Everglades to determine electrical shielding, polarization, and current densities for microperforated PE pipe samples. Initial unpublished test data and field reports of the microperforated PE appear to be favorable, with calcareous deposits appearing on the pipe surface next to the perforations.7 The initial current requirements for the microperforated polyethylene-encased pipe samples are reported to be less than for bare pipe but more than for polyethylene-encased pipe samples. Additional independent testing and long-term evaluations are needed to validate the effectiveness of microperforated PE in controlling corrosion and to determine whether it minimizes electrical shielding successfully. PIPELINE MONITORING AND REPAIR Passive Monitoring Systems The committee reviewed several passive monitoring methods to determine more accurately the level of corrosion protection under PE. These include the use of resistance probes, reference electrodes, and perforated plastic monitoring pipes placed inside and outside the PE.8 Although these passive monitoring techniques have only been used in a few locations, they show promise and are discussed in more detail below. 6 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective.” 7 Ash, communication with the committee, 2008. 8 M. Schiff and B. McCollum, “Impressed Current Cathodic Protection of Polyethylene-Encased Ductile Iron Pipe,” Paper 583 at Corrosion 93, New Orleans, La.; Graham Bell, Clifford Moore, and Scott Williams, “Development and Application of Ductile Iron Pipe Electrical Resistance Probes for Monitoring Underground External Pipeline Corrosion,” NACE International Corrosion 2007 Paper 07335, Dallas, Tex.; Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; A.M. Horton, D. Lindemuth, and G. Ash, “Corrosion Control Performance Monitoring of Ductile Iron Pipe in a Severely Corrosive Tidal Muck,” Materials Performance 45(5):50-54 (2006).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe Perforated Plastic Monitoring Pipes and Permanent Reference Electrodes Perforated plastic monitoring pipes allow potential measurements to be made inside (under) the polyethylene encasement and outside the polyethylene encasement for comparison purposes. This is done by inserting and pulling a portable reference electrode through the plastic monitoring pipes and recording potential measurements as the reference electrode is moved.9 The theory is based on cathodic protection monitoring techniques first used for ground-level storage tanks, where perforated plastic monitoring pipes with portable reference electrodes were used to determine cathodic protection levels under the tank bottom. An example of this type of basic monitoring technique is shown in Figure D-1. Other types of passive monitoring techniques include the placement of permanent reference electrodes inside and outside the polyethylene encasement.10 The theory of the permanent reference electrodes is to compare the potentials made between similar types of reference electrodes (one on each side of the PE wrap). The resistance probes are described in more detail in the following subsection. Electrical Resistance Corrosion Probes An electrical resistance (ER) probe is a device that serves as a corrosion surrogate for the pipe wall; it monitors the accumulation of corrosion damage on a thin strip of metal by measuring the change in resistance of the thin strip as a function time. As the strip of metal thins due to corrosion, the resistance of the strip increases and the corrosion rate in mils per year can be calculated based on the length of time. The intent of the resistance probes is to measure the rate of corrosion in order to provide a snapshot of the protection levels being provided at that location on the pipeline with an above-grade type of measurement without having to excavate the pipeline. Typically probes are provided in pairs for cathodically protected pipelines; one probe is connected to the cathodically protected pipeline and one is not, as it is allowed to freely corrode to provide a reference for the normal corrosion rate in that soil. A pair of probes (four probes total) on DIP with PE are often placed both inside (under) and outside the encasement. One pair is placed inside and one pair is placed outside to compare the difference in corrosion rates under and outside the PE with and without CP. This monitoring 9 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective.” 10 Horton et al., “Corrosion Control Performance Monitoring of Ductile Iron Pipe in a Severely Corrosive Tidal Muck”; Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective.”
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe FIGURE D-1 Typical Type T Test Station with plastic monitoring pipe. SOURCE: Courtesy of William Spickelmire, RUSTNOT Corrosion Control Services, Inc. technique has been tried on several different projects.11 A few of the larger projects are discussed below in more detail. Southwest Pipeline Project The Southwest Pipeline Project in North Dakota was installed in the 1980s with approximately 400,000 lineal feet (lf) of DIP with PE and CP. Commercially available steel probes (Figure D-2) were installed at eight locations along the pipeline. A total of 20 probes were installed at these eight separate locations.12 Ten probes were placed under the PE wrap, 8 on the spigot next to the bell joint, and 2 at the midpoint of the pipe length. Eight were placed in the imported sand backfill and 2 in native-soil backfill next to the pipe trench. The probes have a lead wire to allow them to be bonded to the cathodically protected pipeline. The probes were measured at regular intervals from 1982 to 1992. 11 Schiff and McCollum, “Impressed Current Cathodic Protection of Polyethylene-Encased Ductile Iron Pipe”; Horton et al., “Corrosion Control Performance Monitoring of Ductile Iron Pipe in a Severely Corrosive Tidal Muck”; Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective”; Graham E.C. Bell, Schiff Associates, “Measurements of Performance of Corrosion Control Mechanisms on DIP,” presentation to the committee, Washington, D.C., July 29, 2008. 12 Graham E.C. Bell, Schiff Associates, “Measurements of Performance of Corrosion Control Mechanisms on DIP,” presentation to the committee, Washington, D.C., July 29, 2008.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe FIGURE D-2 Rohrback Cosasco Model 620 Corrosometer Probe. SOURCE: Courtesy of Rohrback Cosaco Systems. In August 1989 some of the probes were grounded to the pipeline and protected with a galvanic anode. In July 1991 the pipeline was cathodically protected with an impressed-current CP system. An example of the type of steel resistance probes used on the Southwest Pipeline is shown in Figure D-2. The results of the Southwest Pipeline’s use of CP were as follows: After the application of CP, the corrosion rate of all probes bonded to the pipeline, both inside and outside the PE wrap, showed low corrosion rates, averaging 0.0189 mpy. The corrosion rates of DIP at undamaged encasement were low and governed by soil corrosivity, dissimilar metal, environmental corrosion cells, and so on. A clean sand backfill reduces corrosion rates below the rate in native-soil backfill. There is still an area of controversy regarding shielding of polyethylene-encased DIP from CP currents; it is recommended that probes be provided under encasement for other projects. To avoid technical issues, the probes should be made of the same material as the pipeline. According to Graham Bell in his presentation to the committee, the actual corrosion rates measured by the electrical resistance probes are most useful for comparison purposes only and “may not be an accurate measurement of the true pipe corrosion rate.” According to a presentation to the committee that was based on recent testing by the speaker, the probes that were placed on the DIP under the PE with CP
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe have continued to show corrosion rates of less than 0.1 mpy.13 The probe data are reported to show that the total accumulated corrosion damage for more than 20 years is less than 2.0 mils for DIP with PE and CP. Despite the fact that the small steel surface area of the steel-type resistance probes is anodic to the DIP, the testing indicated that CP has overcome the galvanic cell corrosion due to the difference between the steel (anode) and ductile iron (cathode) in addition to the influence of the soil corrosivity. As mentioned in the presentation to the committee, on certain isolated portions of the Southwest Pipeline Project, cathodic protection current requirements were higher than for the majority of the pipeline sections. Potentials measured at the ground surface were also lower (below National Association of Corrosion Engineers [NACE] protection criteria) in these isolated sections. Schiff Associates14 participated in an investigation to determine the reasons for these high current requirements and low potentials in one of these isolated portions of the pipeline. They concluded that the increased current requirements were due to the use of gravel backfill in those wet areas, which caused damage to the PE and exposed more pipe surface, increasing the CP current requirements. The rectifier was replaced with a larger sized rectifier to provide the additional current required. Schiff Associates reported that a field investigation of the DIP with PE and CP in that area did not show any corrosion on the pipe. Mid-Dakota Rural Water Service Project The Mid-Dakota Pipeline Project in South Dakota was installed between 1996 and 2002 with approximately 262,000 lf of DIP with PE and CP. Commercially available ductile iron probes were installed along the pipeline. In an effort to ascertain the distance that cathodic protection may reach back under intact polyethylene encasement, a section of the polyethylene encasement was intentionally damaged and four resistance probes were placed at varying distances from the damaged PE location. Figure D-3 shows a schematic of this installation used to determine electrical shielding that was installed at Station 1529+04. Figure D-4 shows that the probes were performing relatively well from installation in 1997 to May 2003 (2,100 days after installation), at which time Probe 3 failed and the readings on Probes 1, 2, and 4 increased rapidly. The site was excavated in June 2004 and the probes examined. One probe (Probe 3) was sent back to the original manufacturer, whose testing indicates that the probe was not manufactured correctly and not sealed properly; Probe 3 failed due to a manufacturing defect. Probe 3 was removed and two new probes were installed (Probes 5 and 6). A photo of the failed Probe 3 is shown in Figure D-5. 13 Schiff and McCollum, “Impressed Current Cathodic Protection of Polyethylene-Encased Ductile Iron Pipe.” 14 Graham Bell, “Measurement of Performance of Corrosion Control Mechanisms on DIP.”
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe FIGURE D-3 Installation detail of probes at Station 1529+04 Mid-Dakota Rural Water Service Project, South Dakota. SOURCE: Schiff Associates, report to Bartlett & West Engineers, Revised Summary and Conclusions; Excavation of MDRWS Station 1529+04, June 30, 2004. Courtesy of Schiff Associates, Claremont, California. FIGURE D-4 Graph of probe readings at Station 1529+04 Mid-Dakota Rural Water Service (MDRWS) Project. SOURCE: Courtesy of Schiff Associates, Claremont, California.
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe FIGURE D-5 From Mid-Dakota Rural Water Service Project, South Dakota: failed Probe No. 3, following removal, showing manufacturing defect. SOURCE: Courtesy of Schiff Associates, Claremont, California. During the excavation, the DIP surface was also inspected for corrosion. No significant corrosion damage was found on the pipe surface. But this probe reliability problem calls into question whether probes of this same type are providing accurate corrosion rates or are suffering from faulty manufacturing techniques. In an effort to minimize some of the problems seen with high-profile (bulky) and steel-type probes, one manufacturer has worked recently to develop an improved DIP ER probe.15 An example of this low-profile-type probe for DIP is shown in Figure D-6. Laboratory Evaluation of Electrical Resistance Probes In a paper presented at NACE Corrosion 2007, Bell and colleagues16 reported on their development and testing of various low-profile probe configurations, including these: Bare steel and coated steel, DIP with oxide removed (bare), DIP with oxide intact, and DIP with oxide and asphaltic material. 15 Schiff Associates Report to Bartlett & West Engineers, Investigation of High CP Current Requirements South West Pipeline Project near Taylor, ND (Claremont, Calif., May 25, 2006). 16 Bell et al., “Development and Application of Ductile Iron Electrical Resistance Probes for Monitoring Underground External Pipeline Corrosion.”
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe FIGURE D-6 Thin flat ductile iron pipe probe. SOURCE: Courtesy of Schiff Associates, Claremont, California. Probes were tested under controlled conditions that included the following: Simulated sandy soil with 500 parts per million Cl-water, Room temperature, and Air bubbling. The conclusions indicate that the low-profile ductile probes showed very good results and are a good surrogate for assessing the corrosion activity and rates on buried DIP. Field evaluation of the low-profile probes was performed in November 2005. Although the data are limited and relatively short term, the indications are that the corrosion rates for steel in the native-soil backfill material are approximately 2.5 mpy, and the corrosion rates under the PE with and without CP are below the limit of detection for the instrument for the 98-day time period investigated.17 WEB Water Project Three DIP ER probes were installed on August 5, 2005, at one location on the WEB (Walworth, Edmunds, and Brown) Water Project in South Dakota. These were the new prototypical ductile-iron-type flat probes specifically designed to be used on polyethylene-encased ductile iron pipe. The thin flat profile (Figure D-6) of the probe is designed to help prevent “tenting” of the PE, which may lead to false indications of corrosion on the ER probes and not accurately reflect the actual pipeline condition.18 17 Graham Bell et al., “Development and Application of Ductile Iron Pipe Electrical Resistance Probes for Monitoring Underground External Pipeline Corrosion.” 18 Schiff Associates, Report to WEB Development Association, Electrical Resistance Probes Installed at Test Station 1459+35 (Claremont, Calif., May 11, 2006)
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe While all three of these passive monitoring techniques (plastic monitoring pipes, permanent reference electrodes, and resistance probes) show promise, there has been limited use of them in the industry. Additional installations, time, and comparisons to actual pipe corrosion rates need to be completed to verify their accuracy in determining actual corrosion rates and protection levels on buried pipelines. As they only show a snapshot of possible conditions at a specific location, they should be combined with other monitoring methods (above-grade potential surveys, excavations, smart pigging, and so on) to confirm that they accurately reflect the actual conditions for the entire pipeline. Active Monitoring and Repair In addition to the passive monitoring alternatives recommended to avoid a failure in 50 years, several active pipeline monitoring and repair protection measures can be employed on DIP pipelines. These active-monitoring and repair-type measures are currently used or are being considered by the Pipeline and Hazardous Materials Safety Administration of the U.S. Department of Transportation to ensure the safe transportation of gas and oil products. Use of these measures has reduced the rate of transmission pipeline failures by alerting owners and operators of the presence of corrosion pitting, wall thinning, and other incipient failure mechanisms. Repairs can then be scheduled and made before a major rupture occurs. An article in the AWWA Journal in 1999 pointed out that as more-sophisticated leak and pipe corrosion evaluation techniques become available, owners and utilities will be able to perform more evaluations of pipe condition and investigations of why failures are occurring.19 These techniques include the following: Close-interval surveys using the most-up-to-date techniques for assessing external corrosion and correct functioning of the CP systems. These surveys should follow the External Corrosion Direct Assessment methodology developed by the Gas Technology Institute.20 The guide rates existing technologies for assessing external corrosion in cased and noncased crossings, pipes shielded by coatings, and segments with stray currents or interference from other pipelines. There is also a phase-sensitive technology under development for pipelines with CP that may reliably detect coating disbondment from aboveground. Providing access for intelligent internal in-line inspection devices. This technique involves such modifications as including launching and receiving 19 Jon Makar and Nathalie Chagnon, “Inspecting Systems for Leaks, Pits, and Corrosion,” AWWA Journal 91(7):26-46 (1999). 20 T.A. Bubenik and D.D. Mooney, Development of External Corrosion Direct Assessment Methodology (Columbus, Ohio: Battelle Memorial Institute, 2002).
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Review of the Bureau of Reclamation’s Corrosion Prevention Standards for Ductile Iron Pipe arrangements, and valves that will permit the devices to pass. These new “smart pigs” can be used while a pipeline is in operation. They provide highly accurate location and severity information on external or internal corrosion as well as on other damage like dents or gouges. Some devices measure electrical currents traveling in pipelines, thus assessing the status of CP or stray currents from other sources. Magnetic-flux-leakage sensors can also detect coating disbondment. Conducting potential measurements. Corrosion occurring under loose-bonded intact PE or disbonded bonded dielectric coatings can electrically shield active corrosion from testing techniques for surface pipeline potential made along the ground surface. These techniques will locate areas of damaged polyethylene or coating defects where contact with the soil is imminent, but they have small likelihood of detecting corrosion that may be occurring under intact polyethylene or disbonded coatings. As the Colorado Springs, Colorado, initial in-line inspection data indicated, although only 2 percent of the tested sections had major corrosion damage (with 26 to 50 percent remaining wall life), the pipeline was suffering enough corrosion leaks and damage that it could not be counted on to provide reliable service and therefore was replaced.21 Use of remote field technology, as described in a 2002 paper by Calgary and Hydroscope.22 This type of technology was evaluated initially under American Water Works Association Research Foundation Project No. 90601 for Nondestructive Testing of Water Mains for Physical Integrity.23 The use of these intelligent in-line inspections (smart pigging) methods, such as employed previously by Colorado Springs and Calgary in their pipeline condition assessments, are a more reliable method of determining actual pipe conditions than are just random digs.24 21 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective.” 22 W. Hartman, K. Karlson, and R. Brander, “Waterline Restoration Based on Condition Assessment—A Case Study,” Distribution and Operations Conference, Nashville, Tenn., September 2002. 23 R. Jackson, C. Pitt, and R. Skabo, Nondestructive Testing of Water Mains for Physical Integrity (Denver, Colo.: AWWA Research Foundation, 1992). 24 Spickelmire, “Corrosion Control Considerations for Ductile Iron Pipe—A Consultant’s Perspective.”