An impressed current cathodic protection (ICCP) system was recently installed to provide corrosion control protection for approximately 2 miles (3.2 kilometers) of underground, direct buried ductile iron piping at the Savannah River Plant (SRP). The piping system serves well water pumps and delivers the water from two newly installed water wells to the main plant area for potable and domestic water use.
The route of the piping traverses a variety of soil conditions and for a significant length, is installed in a power line right-of-way with direct buried copper cable counterpoise. The conditions presented a unique challenge for the designers of the corrosion control system, especially considering the safety precautions required.
Extensive detailed testing was performed along the proposed route of the pipeline. This was required to gather the engineering data necessary to establish soil characteristics and the specific requirements for such a corrosion control system. The results of the testing, the design options which were evaluated, the problems encountered during construction, and the effectiveness of the installed system are discussed.
In 1985, a corrosion ion engineering study was performed along the proposed right‑of-way of a new ductile iron water line which was to be installed at the United States Department of Energy, Savannah River Plant, located near Aiken, South Carolina. The pipeline was to provide water transmission from two newly installed water wells to the main plant area for potable and domestic water use.
The first segment of the piping is a 10 inch (25.4 centimeter) nominal diameter section installed between the two wells. This section of ductile iron pipe was installed approximately 3.5 feet (1.1 meters) deep and spans approximately 585 feet (178.3 meters). At this point the 10 inch (25.4 centimeter) pipe connects with a 14 inch (35.56 centimeter) diameter pipe from the discharge side of the second well. The 14 inch (35.56 centimeter) pipeline then extends approximately 8,300 linear feet (2.53 kilometers) to the main plant area and ties in through various connections with existing potable and domestic water lines.
Throughout this route the pipeline traverses and crosses several structures including direct buried instrumentation and control cables, copper counterpoise cables, reinforced concrete storm drains and grade level railroad tracks. Throughout the majority of its length the pipeline is installed in a common right-of-way with a 13.8 KV and a 115 KV overhead AC transmission line. Figure 1 shows the general routing of the pipeline under investigation.
The pipeline is constructed of bell and spigot type joints (Figure 2), which very often render one section of pipe electrically discontinuous from adjacent sections. Original pipeline specifications called for the pipeline to be installed in a polyethylene encasement which was intended to act as a corrosion control barrier between the pipeline and the soil environment.
In order to gather the data necessary to analyze the requirements for corrosion control, a detailed site survey was performed. The survey consisted of evaluating soil conditions along the proposed route of the pipeline to determine soil resistivity, chemical analysis, and the presence of, or potential for, stray current activity which may adversely affect the proposed pipeline.
Of particular concern to the investigators was the presence of a low lying area along the right-of-way containing considerable groundwater under swampy conditions. The presence of this low lying area with standing water posed a significant potential for corrosion of the ductile iron waterline as a result of exposure to non-homogeneous soil conditions. In addition, the stagnant condition of the water was expected to be conducive to corrosion through microbiological attack.
At the termination end of this pipeline, approximately 700 feet (213.5 meter) of the line was directly buried beneath a standing coal pile for the steam generator facility located in this area. This also posed considerable concern as a result of the potential for drastic variations in soil resistivity as well as contributing factors from acidic soil conditions due to the coal pile run-off.
During the investigation soil resistivity measurements were made at 5, 10, 15, and 20 foot (1.5, 3.1, 4.6, 6.1 meters) depths on 300 foot (91.5 meters) centers along the pipeline right-of-way. These measurements were obtained using the Wenner Four Electrode Method in accordance with ASTM Standard G57-78. In order to ensure the least amount of influence from the existing overhead transmission lines as well as the direct buried counterpoise, all measurements were obtained adjacent to the pipeline right-of-way and perpendicular to all direct buried and overhead cabling.
In addition to the in situ resistivity measurements, 20 soil samples were obtained along the pipeline right-of-way at a depth of 4 feet (1.22 meters). A water sample was also taken from the swamp. These samples were analyzed for resistivity through a calibrated soil box. pH, chlorides and sulfates were also measured. Similar tests were also performed on samples of the pipeline backfill material obtained from a storage area on the opposite side of the plant.
The results of the survey indicated that soil conditions were only moderately corrosive throughout the majority of the pipeline right-of-way. However, isolated sections existed which were considered to be very corrosive with regard to underground ferrous structures. Table 1 shows the range of soil resistivities and the average resistivity at the four depths measured during the survey. Table 2 shows the pH values observed on soil samples obtained along the pipeline right-of-way. As anticipated, soil conditions at pipe depth in the area of the coal storage pile were extremely acidic and corrosive to underground metallic structures.
The results of chloride and sulfate analysis did not indicate concentration significant enough to contribute to corrosion. There also was no evidence of stray current activity along the route which might adversely affect the pipeline.
As a result of the concern over the possibility of concentrated corrosion in the isolated areas where a corrosive environment was encountered as well as areas where the corrosion characteristics of the soil changed dramatically, the designers considered the need for cathodic protection as a means of corrosion control. To facilitate the performance of cathodic protection the line was rendered electrically continuous through the installation of #4 AWG copper stranded bonding jumpers exothermically welded to either side of each bell and spigot pipe joint (See Figure 6). This procedure was also recommended to facilitate electrical continuity throughout the pipeline length to support analysis of corrosion patterns and to evaluate the degree of success being afforded by cathodic protection.
With the firm recommendations for cathodic protection, concern was expressed by mechanical and piping engineers associated with the project, that ductile iron piping systems do not normally experience corrosion and that the pipeline was being installed in an 8 mil thick polyethylene encasement thereby further restricting the potential for corrosion.
Previous studies by the United States Department of Interior, Bureau of Reclamation1, concluded that ductile cast iron had a greater pitting rate than either steel or gray cast iron. In fact, the corrosion rate of ductile cast iron was about 2 to 3 times greater than that of carbon steel. Research conducted by Romanoff as published in the National Bureau of Standards, Circular 5792 concluded that ductile cast iron and carbon steel corrode at nearly the same rate. This research coupled with past experience supported the decision for implementing corrosion control.
When considering cast iron a common type of corrosion, known as graphitization, often occurs as a result of de-alloying caused by selective dissolution of the iron from cast iron. This typically weakens the overall pipe wall contributing to mechanical failures which are often misdiagnosed as being structural rather than corrosion related. In ductile iron very often the corrosion mode is that of pitting attack driven by galvanic effects (graphitization). When properly protected, carbon steel, cast iron and ductile iron all provide a long service life. It was, however, the position of the designers that the use of polyethylene encasement, a loose wrap, did not provide all of the normally expected characteristics of a protective coating. These characteristics include a high dielectric strength, a permanent bond to the substrate, resistance to deformation, resistance to disbondment and resistance to chemical attack.
Past experience has shown that water can migrate underneath the polyethylene wrap at areas where the wrap has been torn or damaged. This presence of moisture underneath the polyethylene wrap contributes to localized corrosion. It was also felt that the presence of a loose film may impede the effectiveness of cathodic protection. For these reasons, a coal tar enamel coating, in accordance with AWWA C2033, was selected in conjunction with an impressed current cathodic protection system for corrosion protection. A sacrificial anode system was ruled out as being technically and economically infeasible due to the high variability of soils, the current required for protection and the amount of anode material necessary to generate the required current. In order to optimize cathodic protection current the pipeline was electrically isolated at the tie in points through the use of dielectric insulating flange kits. Test stations were installed on 1,000 foot (0.3 kilometer) intervals to facilitate testing and evaluating of the effectiveness of the cathodic protection system.
Cathodic protection design considerations were based upon an assumed coating efficiency of 97% and a design current density of 2 milliamperes per square foot (21.52 milliamperes per square meter). To ensure a uniform distribution of current, to facilitate a low resistance groundbed and to minimize the potential for stray current interference the anode groundbed was designed to be installed in the swampy area known as Tim's Branch Swamp, in the pipeline right-of-way.
The swamp is located approximately 3,000 feet (0.9 kilometers) downstream of the newly installed wells and approximately 6,000 feet (1.8 kilometers) upstream of the termination point of the well water line. The resistivity of the water in the swamp is approximately 15,000 ohm-cm and the surrounding soil ranges from 18,000 to 25,000 ohm-cm.
The type of system selected was a semi-remote surface groundbed consisting of 15, 3 inch (7.62 centimeter) diameter by 60 inch (152.4 centimeter) long resin-treated graphite anodes pre-packaged in 8 inch (20.32 centimeter) diameter by 84 inch (213.4 centimeter) long canisters. The anodes were to be installed to a 20 foot (6.1 meter) depth on 10 foot (3 meter) centers in a vertical configuration. A deep anode groundbed approach was considered. However, this was rejected due to the inaccessibility to large drilling equipment in the area chosen and the potential for stray current interference resulting from current distributed remote by virtue of the high surface soil resistivities encountered.
Because of the need to provide electrical continuity throughout the piping system and as a result of the close proximity to overhead AC transmission lines consideration was given to the potential for induced AC problems on the pipeline. Consideration was given to the possible shock hazards that may be experienced by personnel coming into contact with the pipeline or the associated test stations and the possibility for arcing resulting in damage to the structure at points where the pipeline crosses directly over or under bare copper counterpoise. For safety reasons, electrolytic grounding cells were specified at each point where the pipeline crossed over or under the grounding system and across each electrical isolation device.
During the installation of the cathodic protection groundbed in the vicinity of Tim's Branch Swamp, difficulties were encountered in drilling through the muck to install the anodes vertically. As such, a deviation from the plans and specifications was approved to permit the horizontal installation of pre-packaged canister anodes where the strata would not permit vertical installation.
In order to compensate for the increased resistance related to horizontal anode installation, the total number of anodes was increased to 20 from the previously specified 15. It was felt, by the designers, that increasing the number of anodes would result in approximately the same overall groundbed resistance as the vertically installed 15 anodes.
The anodes were installed horizontally, at an approximate depth of 5 feet (1.53 meters) on 10 foot (3.05 meters) centers and wherever possible perpendicular to the anode header cable rather than end to end, so as to further reduce the resistive coupling between anodes. Figure 9 shows the general arrangement of the completed installation.
Following installation, the system was placed into operation and testing performed to determine the optimum output in order to achieve satisfaction of NACE recognized criteria. A close interval electrical survey was conducted along the entire length of the pipeline and potential measurements were obtained on 2.5 foot (0.76 meters) spacings with the rectifier cycling on and off. The results of the survey indicated that in several short sections throughout the length of the line, initial protected levels did not meet NACE criteria.
Further investigation determined a probable cause as that of coating damage during the pipeline installation. The close interval survey also detected an open joint in the vicinity of the water wells where grading contractors reconstructing a drainage ditch had inadvertently broken a bonding jumper across a mechanical joint. After the bonding jumper was repaired and the system remained operative for twenty four hours, retesting was concentrated in those areas of low potentials. Polarization well in excess of 100 millivolts was achieved in all cases.
Further testing and analysis performed during the system commissioning revealed that all existing insulators were effective in electrically isolating the pipeline network from the termination points. Stray current testing performed on nearby electrically isolated structures did not reveal any detrimental effects resulting from stray DC currents. AC potential levels along the pipeline were also determined to be less than 1 volt AC to ground in all cases under investigation.
In most cases, instant off potentials obtained along the line were equal to or more negative than -0.85 volts with respect to a copper-copper sulfate reference electrode. The final operating output of the rectifier was selected at 30 volts, 2.25 amperes DC. Based on calculated surface areas, this protective current equates to 2 milliamperes per square foot of bare surface area and 3.5% bare area. This value is considered to be reasonable based upon the quality of coating and installation procedures and compares very favorably to the design parameters.
Soil conditions in the right-of-way for the new ductile iron well water line at the Savannah River Plant were found to be detrimental. Isolated pockets of extremely corrosive soil and an extreme variation in soil electrical and chemical characteristics existed throughout the pipeline right-of-way. Initially the line was to be constructed of ductile iron and wrapped in a polyethylene encasement. However, based on previous experience with loose polyethylene encasement, corrosion engineers questioned its performance as a corrosion control barrier and its compatibility with cathodic protection. A coal tar enamel coating was specified in lieu of polyethylene encasement along with cathodic protection. The impressed current cathodic protection system installed is providing protection for the entire line well within design parameters with an anticipated system life expectancy in excess of 30 years.