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U.S. Department
of Transportation

Pipeline Safety Stakeholder Communications

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Fact Sheet: External Corrosion

Quick Facts:

  • Corrosion is the natural process where materials made from metal deteriorate through an electrochemical reaction known as oxidation (rusting). Corrosion on the exterior surface of pipeline systems is referred to as external corrosion. External corrosion on pipeline systems can be prevented through material selection, protective coatings, and cathodic protection systems.
  • Data show that from 1998-2017 approximately 8% of pipeline incidents were caused by external corrosion.
  • Improved technologies continue to lead to better prevention, monitoring, detection, and mitigation of external pipeline corrosion, for both newly installed and existing pipelines.
  • Federal pipeline integrity management regulations promote early identification of external corrosion issues on pipe or pipeline equipment by requiring periodic inspections, testing, and assessments, and timely repair or replacement where necessary.
  • Federal pipeline integrity management regulations promote early identification of external corrosion issues on pipe or pipeline equipment by requiring periodic inspections, testing, and assessments, and timely repair or replacement where necessary.
  • Refer to other fact sheets on PHMSA’s Stakeholder Communications website for specific discussions of corrosion, internal corrosion, selective seam corrosion, stress corrosion cracking, microbiologically-influenced corrosion, and stray current interference corrosion.

What is external corrosion and why does it occur?

Corrosion is the deterioration of a steel pipeline that results from an electrochemical reaction with its immediate surroundings. This reaction causes the iron in the steel pipe or other pipeline appurtenances to oxidize (rust). Corrosion results in metal loss in the pipe. Over time and if left unmitigated, corrosion can cause the steel to lose its strength and possibly render it unable to contain the fluid in the pipeline at its operating pressure.

External corrosion occurs due to environmental conditions on the outside of the pipe. It is the natural interaction between the exterior surface of the pipe and the soil, air, or water surrounding it. Many highly localized factors can contribute to the aggressiveness and persistence of external corrosion. Because pipelines are extremely long-serving and critical infrastructure, it is paramount for pipeline operators to maintain the physical integrity of pipelines. Fortunately, there are effective methods for preventing and mitigating external corrosion damage to pipelines, including many that are very technologically advanced.

Typically, the exterior surface of the pipe is coated to prevent the surrounding soil, air, or water from contacting the steel pipe, thus preventing external corrosion. External corrosion can also be halted “electrically” on both bare pipelines as well as on coated pipelines where coating damage or coating flaws exist. This is known as “cathodic protection”. When these protective measures are effective, or when sufficient mitigative efforts are sustained, steel pipelines can last indefinitely.

What are the risks from external corrosion?

External corrosion can result in gradual and usually localized metal loss on the exterior surface of pipeline systems, resulting in reduction of the wall thickness of the pipe or other equipment. This metal loss can occur relatively evenly over an area of the pipe’s exterior surface (sometimes referred to as “general corrosion”) or in isolated spots on the surface (“localized corrosion” or “pitting”). The loss of material from corrosion can eventually result in “pinhole” leakage, or a crack, split, or rupture of the pipeline unless the corrosion is repaired, the affected pipe section is replaced, or the operating pressure of the pipeline is reduced.

Left untreated, external corrosion can weaken the pipe where the corrosion occurs, and make the pipe more susceptible to overpressure events, earth movement, and other external stresses. Thus, external corrosion can sometimes also increase the risk of other types of pipeline failures.

Pipeline failure rates from external corrosion

Transmission and gathering pipelines. Historically, external corrosion accounts for just under 40% of all pipeline incidents caused by corrosion. For example, during the 5-year period of 2013-2017, approximately 8% of reported incidents on gas transmission, gas gathering, and hazardous liquid pipelines were caused by external corrosion.

Natural gas distribution pipelines. Over the same 5-year period (2013-2017) less than 1% of reported incidents on natural gas distribution pipelines were caused by external corrosion. Natural gas distribution system mains and service lines operate at much lower pressures and are typically made of non-corrosive materials (like plastic). Even if a gas distribution line is made of steel, the likelihood of a pipe rupture is low because of the lower operating pressures, and most external corrosion failures would result in leaks.

What is being done to prevent/mitigate external corrosion?

  • Modern manufacturing processes for steel pipe and protective external coatings are subject to rigorous fabrication, inspection, and quality control standards to reduce the occurrence of pipe and coating defects that can lead to external corrosion.
  • The steel pipe used in modern pipeline systems is typically now coated under strictly controlled conditions in the pipe factory rather than at the construction site.
  • In the field, pipeline operators also use cathodic protection systems (CPS) to prevent external corrosion. One type of CPS imposes a very low-voltage direct electrical current on the pipeline to oppose the natural corrosion-inducing currents at locations where pipe coating flaws exist or where pipe coating is damaged. Another type of CPS uses a “sacrificial” anode to stop the electrochemical reaction between the pipe metal and the surrounding environment to prevent external corrosion. Both types of CPS must be routinely tested to maintain their effectiveness.
  • Federal pipeline safety regulations also require pipeline operators to develop and implement integrity management programs and continually inspect and assess the integrity of pipelines that could affect areas of high consequence, such as populated areas or environmentally sensitive areas. Operators are required to periodically inspect and assess their pipelines for external corrosion and other integrity issues, and repair or replace affected pipe. Inspections are typically performed using one or more types of specialty inline inspection (ILI) tools, hydrostatic pressure testing, or a process called “direct assessment”.

External corrosion: What more can be done?

  • Public : Be aware of pipelines located near you. Always respect the pipeline right-of-way and be observant for signs of pipeline damage, leakage, or security concerns. Remember that your own safety may be involved. Know the phone numbers in your area and call the pipeline operator and local public safety officials immediately to report any pipeline safety concerns.
    • Be observant of changes to soils and vegetation around pipelines. These could be signs of a leak. Report these and possible earth movement or other conditions that could impact the pipeline to the pipeline operator immediately.
    • Note any suspicious activity, especially at aboveground pipeline facilities such as valve or pumping stations; report such activity to local public safety officials and the pipeline operator immediately.
    • Do not dig or build on a pipeline right-of-way. Always call to have underground facilities located and marked before you dig (dial 811).

  • Industry : Pipeline operators should follow current regulations, guidelines, and standards to ensure the integrity and security of their pipeline facilities. This includes evaluating all potential threats that may impact the integrity of their pipelines.
  • Regulators : PHMSA must continue to work closely and in cooperation with other organizations to ensure pipeline integrity management requirements address evolving concerns. Federal and state pipeline safety regulators must continue to inspect operators’ integrity management programs to ensure they are effectively identifying and assessing potential threats, including external corrosion, and are implementing appropriate activities in a timely manner.

External corrosion: Where can I learn more?

Date of Revision: 09242018