CORROSION FAQS

Q: Can AC power lines cause corrosion of Ductile Iron pipe?

A: Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don't impose corrosion or safety concerns.

A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.

Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that can vary from a fraction of an ohm to several ohms but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints normally segment the pipe, restricting its electrically continuous length, and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare with only a standard 1-mil asphaltic coating and therefore are effectively grounded for their entire length, which further prevents magnetic induction on the pipeline.

During construction of Ductile Iron pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., "limit of approach" regulations governing construction equipment, grounding straps, chains attached to rubber-tired vehicles to provide a ground, grounding mats, etc., especially if safety concerns are heightened due to the use of joint bonding and dielectric coatings.
(Issue: Fall/Winter 2001)

Q: Are there any corrosion considerations regarding elevated temperatures?

A: Corrosion is the oxidation-reduction process by which metals are oxidized by oxygen in the presence of moisture. The Arrhenius equations show that reaction rates increase with temperature. The rule of thumb is that the rate of a reaction will double with every 18°F increase in temperature. Another factor is oxygen solubility. As temperature increases, the total solubility of oxygen in water decreases, and the rate of solution of oxygen increases. These lines cross at about 176°F, which is the temperature where corrosion is a maximum (available oxygen to fuel corrosion is at its maximum). For this reason, polyethylene encasement is recommended for any elevated temperature installation. The maximum operating temperature for linear low-density polyethylene is 180°F and 200°F for high-density cross-laminated polyethylene.
(Issue: Fall/Winter 2001)

Q: If the native soil is considered corrosive to Ductile Iron pipe, and select non-corrosive material is used for bedding and backfill around the pipe, do I still need to protect the pipe from corrosion?

A: Yes. The use of select, non-corrosive material (such as sand or limestone) for bedding and backfill is referred to as "trench improvement." It is recognized that trench improvement generally provides good structural support and helps delay the onset of corrosion activity. However, experience has shown that trench improvement does not provide long-term protection to the pipe, particularly in highly aggressive soil environments. Permeation of native soil and moisture into the select backfill over time tends to make the select material take on corrosive properties. Therefore, trench improvement should not be used as the only method of corrosion control. Polyethylene encasement remains the most effective method for corrosion prevention of Ductile Iron pipe.
(Issue: Spring/Summer 2001)

Q: What lining should I use for chilled water applications?

A: Cement-mortar linings. Cement-mortar linings have been successfully used to protect the interior of iron pipe and fittings since 1922. Cement linings prevent tuberculation by creating a high pH at the pipe wall and ultimately by providing a physical barrier to the water.
(Issue: Fall/Winter 2000)

Q: What coatings are recommended for Ductile Iron pipe in aboveground applications (outdoors, in pits and well housings, plants, etc)? How do I specify the coating (surface preparation, primers, finish coat, etc.)?

A: Unless otherwise specified, Ductile Iron pipe manufactured in accordance with ANSI/AWWA C151/A21.51 is supplied with an asphaltic coating approximately 1-mil thick. This coating, which is applied for aesthetic reasons, is used under normal conditions for both above and below ground applications. Typically, it's used in aboveground applications such as pump stations, bridge crossings, and pipe on supports installations. For special aboveground conditions, other types of coatings – epoxies, for example – are available. Installations that might require such coatings are corrosive wet wells, chemical environments, etc. Furthermore, some installations require the pipe to be primed for finish paint coats. The type of coating specified by the purchaser might depend on several criteria such as resistance to a given environment, temperature resistance, impact resistance, resistance to sunlight, gloss retention, appearance, compatibility to finish coats, etc.

Although Ductile Iron and carbon steel are both ferrous metals, there are inherent differences between the two that preclude the use of the same surface preparation and application of coatings. Attempts to apply steel surface preparation specifications to Ductile Iron is inappropriate and may actually result in damage to the pipe surface with subsequent reduced coating effectiveness and life expectancy.
Most coating manufacturers require some type of surface preparation prior to application as a condition of warranty. Since their recommendation for surface preparation will vary depending on the type of paint and the ultimate service environment, the coating manufacturer's technical data sheet should be consulted each time a special coating is used. Normally, recommendations given on coating manufacturer's technical data sheets are for carbon steel and might not apply to Ductile Iron pipe. Therefore, the pipe manufacturer should also be consulted regarding the type of coating, method of application, and type of surface preparation to be used.
(Issue: Fall/Winter 2000)

Q: What lining should I specify for sewer force mains when air pockets are unavoidable in the pipeline?

A: Ductile Iron pipe and fittings are normally furnished with a cement-mortar lining conforming to ANSI/AWWA C104/A21.4. Cement-mortar-lined Ductile Iron pipe can be used for certain wastewater applications such as non acid-producing gravity sewers and sanitary sewer force mains that unquestionably flow full.

Microbiologically-induced corrosion, which is sometimes referred to as hydrogen sulfide (H₂S) corrosion, can occur in gravity sewers. It occurs when bacteria in the anaerobic slime layer reduces existing sulfates to hydrogen sulfide. The H₂S is liberated into the crown area of the pipe above the flow where Thiobacillus bacteria further metabolize the H₂S into very low pH sulfuric acid. The sulfuric acid is corrosive to cement-mortar andiron.

This cannot occur in force mains where the pipe is flowing full. H₂S cannot be liberated and sulfuric acid will not be produced. For such applications, cement-mortar linings are more than adequate. However, if the force main is acid-producing and air pockets are unavoidable in areas such as high elevations, special linings should be considered in those sections. Contact the DIPRA member companies regarding the most suitable lining.
(Issue: Fall/Winter 1999)

Q: Can cement-mortar-lined Ductile Iron pipe be used for wastewater applications, or is it only suitable for potable water use?

A: Yes. Cement-mortar-lined Ductile Iron pipe can be used for certain wastewater applications. Ductile Iron pipe and fittings are normally furnished with a cement-mortar lining conforming to ANSI/AWWA C104/A21.4. While originally developed to prevent tuberculation in water mains, cement-mortar lining is also highly suitable for non-septic gravity sewers and sanitary sewer force mains. Long-term testing and experience of cement-mortar-lined iron pipe, both in the field and in the laboratory, have proven its effectiveness for these applications. In addition to the uses mentioned above, cement-mortar-lined Ductile Iron pipe can also be used for seawater applications. Special linings are often recommended for Ductile Iron pipe used to transport septic sewage where hydrogen sulfides create a corrosion-related problem. Please contact the DIPRA member companies regarding the most suitable lining for this condition or other special services.
(Issue: Spring/Summer 1999)

Q: Do underground AC electric cables present any possible corrosion or safety problems for Ductile Iron pipe in the vicinity?

A: No. Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, underground electrical cables normally do not impose corrosion or safety concerns for Ductile Iron pipelines. A consequence of underground electrical cables and buried pipelines sharing the same right-of-way is that AC voltages and currents can be induced on the pipelines by the expansion and contraction of magnetic fields. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the underground electrical cable, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating. Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that may vary from a fraction of an ohm to several ohms, but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints segment the pipe and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare and are therefore essentially grounded for their entire length which further prevents magnetic induction on the pipeline. If, for some reason, a bonded-joint Ductile Iron pipeline parallels an underground or overhead high-voltage AC power line, additional investigation may be warranted depending on the pipe coating (if any), length of parallelism, etc. AC voltages induced on a pipeline pose a shock hazard rather than a corrosion concern. Studies have concluded that AC current may cause corrosion at a rate that is only 1 percent or less than that of a similar electrical quantity of direct current. NACE RP0177-95, "Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems," considers 15 volts AC open circuit to constitute an anticipated shock hazard.
(Issue: Fall/Winter 1998)

Q: What methods are available that can help determine the in-place condition of Gray and Ductile Iron pipe?

A: The physical properties of Gray and Ductile Iron pipe do not change with time; therefore, corrosion (both external and internal) is the main factor that can affect the structural integrity of the pipe. Cement-mortar linings and special linings have effectively eliminated concerns with internal corrosion; however, older installations may not have been installed with these linings. The first source of information regarding the in-place condition of a pipeline is its break record. This record can supply information such as the number of breaks per unit length of pipeline, location of problem areas along the pipeline, and the type of failures (corrosion, structural, joint, etc.). A soil survey along the pipeline can determine if the soils are considered corrosive to Gray or Ductile Iron pipe, and can locate areas that may be more corrosive than others. From the soil survey and the break records data, a general assessment of the pipeline may be determined. This information may also help locate problem areas. Visual inspection is another method of assessing the condition of a pipeline. When utilizing this method, excavations are normally made along the pipeline at locations that are believed to be the most corrosive. The pipe is then inspected with a geologist's hammer, scratch awl, or other tool in order to identify any graphitization present. If graphitization is present, it is then removed and pit depths measured. Coupons can also be cut from the pipe and sandblasted in order to record possible pit depths on both the outside and inside diameters. Pipelines can also be internally inspected by remote control cameras. Non-destructive-evaluation (NDE) of Gray and Ductile Iron pipelines is a developing field offering potential in the future.
(Issue: Fall/Winter 1998)

Q: What is the expected life of buried Ductile Iron pipe?

A: Properly designed and installed Ductile Iron pipe systems could easily have a life expectancy of more than 100 years. Unlike other pipe materials, the physical properties of Ductile Iron pipe do not change with age. As long as Ductile Iron pipe is not subjected to loadings and pressures in excess of its original capabilities, the only factor that could shorten its life is corrosion. Internal corrosion has been effectively eliminated with the evolution of cement-mortar linings and other special linings. Also, not all soil environments are considered corrosive to iron pipe. This is evidenced by the fact that more than 500 utilities in the United States and Canada have had unprotected Cast Iron pipe that has provided service for 100 years or longer, and 12 utilities for more than 150 years. A common procedure used to determine if the soil is aggressive to iron pipe is the 10-point soil evaluation procedure outlined in Appendix A of the ANSI/AWWA C105/A21.5 Standard "Polyethylene Encasement for Ductile-Iron Pipe Systems." If the soil tests corrosive to Ductile Iron pipe, then corrosion protection is warranted. Polyethylene encasement is the corrosion protection method normally recommended by the Ductile Iron Pipe Research Association and the manufacturers of Ductile Iron pipe. If the soil is determined non-corrosive when tested in accordance with Appendix A of ANSI/AWWA C105/A21.5, or if it is determined corrosive and the pipe is encased with polyethylene in accordance with the standard, Ductile Iron pipe could have a life expectancy of more than 100 years. If Ductile Iron pipe is installed in aggressive environments without protection, its life expectancy would mainly be a function of that environment.
(Issue: Spring/Summer 1998)

Q: Does the thicker wall of ANSI/AWWA C110/A.21.10 fittings make them more corrosion resistant than ANSI/AWWA C153/A21.53 fittings?

A: No. The majority of soils found in North America are not considered corrosive to Ductile or Cast Iron. Therefore, in these soils, corrosion protection of any nature is not required. In soils that are considered corrosive to Ductile or Cast Iron, it has long since been proven that the use of sacrificial metal (i.e., additional wall thickness) for corrosion protection is neither reliable nor cost-effective. Additional sacrificial metal, at best, might increase the service life of the product a few years, which is hardly any consolation to the user. Appendix A to ANSI/AWWA C105/A21.5 standard details an accepted procedure to determine whether the soil is considered potentially corrosive to Ductile or Cast Iron products. Effective, economical protection of Ductile and Cast Iron products in corrosive-soil environments can be achieved by simply encasing them in polyethylene at the trench in accordance with ANSI/AWWA C105/A21.5. This standard includes an approved method of encasing fittings.
(Issue: Fall/Winter 1997)

Q: Over what temperature range is polyethylene encasement effective for corrosion protection of Ductile Iron pipe?

A: In general, polyethylene encasement can remain effective at sustained temperatures up to around 180° F. Polyethylene encasement softens around 200° F and melts around 220° F to 230° F. Sustained temperatures above 180° F may eventually cause the polyethylene film to become brittle and crack. Stabilizing antioxidants can be added to the film during manufacturing to increase this temperature. As long as the polyethylene encasement continues to prevent direct contact of the pipe with the corrosive soil, it will remain an effective corrosion control system for Ductile Iron pipelines.
(Issue: Fall/Winter 1997)

Q: Do overhead power lines paralleling Ductile Iron pipelines create concerns?

A: Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don't impose corrosion or safety concerns.

A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.

Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that may vary from a fraction of an ohm to several ohms, but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints segment the pipe and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare and are therefore essentially grounded for their entire length, which further prevents magnetic induction on the pipeline.

During construction of Ductile Iron pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., "limit of approach" regulations governing construction equipment, grounding straps, or chains attached to rubber tired vehicles to provide a ground, etc.
(Issue: Fall/Winter 1994)

Q: What is DIPRA's position on the use of controlled density backfill products for Ductile Iron pipe?

A: DIPRA's concern with controlled density backfill products is their potential to be considered corrosive to Ductile Iron pipe. Some literature published by the manufacturers of the material states that it is not corrosive because of its high pH and is therefore similar to concrete. However, the material usually contains a high percentage of fly ash and, unlike concrete, it is very porous.

Our laboratory has conducted tests on controlled density backfill products in accordance with Appendix A of the ANSI/AWWA C105/A21.5 Standard. The backfill materials did not test corrosive to Ductile Iron pipe after allowing them to dry out/hydrate; however, after allowing water to soak into the samples (not re-mixing) the backfill materials tested potentially corrosive. Therefore, if the porosity of the material allows the interface between the backfill material and the pipe to experience the addition of moisture in the field, it could cause the material to be considered corrosive.

Another concern is the possibility that the pipe is not completely encased by the material. This would occur if the pipe were placed in the trench, and controlled density backfill material was then poured around the pipe. In this case, the bottom of the pipe would not be encased in the controlled density backfill. This would also occur at the interface where the controlled density backfill material is terminated. In either event, accelerated pH differential corrosion cells may develop because of backfill material having a pH which would normally be much higher than the native soil.

Another point is the possible presence of cinders in the material. Without question, cinders are corrosive to iron pipe. Therefore, if cinders are encountered or suspected, DIPRA would not recommend use of the material for backfill.

As a result, DIPRA considers controlled density backfill material to be potentially corrosive to Ductile Iron pipe and recommends polyethylene encasement of the pipe in accordance with ANSI/AWWA C105/A21.5 whenever it is used. Care also should be taken to prevent the pipe from floating.
(Issue: Fall/Winter 1994)

Q: Is the standard 1-mil asphaltic coating applied to the outside of Ductile Iron pipe for corrosion protection?

A: No. The asphaltic coating is applied to the outside of Ductile Iron pipe in accordance with ANSI/AWWA C151/A21.51 to minimize atmospheric oxidation for aesthetic reasons. If soils are determined to be corrosive when tested in accordance with Appendix A of ANSI/AWWA C105/A21.5, DIPRA and its member companies recommend that polyethylene encasement in accordance with the AWWA C105 standard be installed for corrosion protection.
(Issue: Spring/Summer 1993)