Pipeline Safety Stakeholder Communications

Quick Facts:

• SCC is a form of corrosion that produces a marked loss of pipeline strength with little metal loss.
• The combined influence of pipeline stress due to its pressurized contents and a corrosive medium can occasionally result in the formation of interlinking crack clusters that can grow until the affected pipe fails.
• Improved technologies have led to better prevention, monitoring, detection, and mitigation of pipeline SCC – even for old pipelines.
• New pipeline Integrity Management regulations will promote early identification and repair of SCC.
• Refer to other Fact Sheets for specific discussion of issues related to corrosion in general, external corrosion, internal corrosion, and selective seam corrosion.

What is stress corrosion cracking and why does it occur?

SCC in pipelines is a type of environmentally assisted cracking. SCC results from the formation of cracks due to various factors in combination with the environment surrounding the pipeline that together reduces the pressure-carrying capability of the pipe. When a pipeline under higher pressures (stress) comes into contact with water due to coating failure, the minerals, ions, and gases in the water at the pipe surface create corrosion that attacks the pipe. SCC tends to propagate as crack clusters or “colonies” as pipeline stress opens cracks that are subject to corrosion, which are then corroded further, weakening the pipeline metal by further cracking. There are 2 types of SCC of concern to the pipeline industry:

• High pH SCC – typically occurs within 15 miles downstream of pipeline compressor or pump stations where temperatures of the transported medium may exceed 100 °F. Along with the elevated temperatures, a groundwater/carbonate/bicarbonate solution (high pH) would have to be in contact with the pipe through a defect in the pipeline coating.
• Near-neutral pH SCC – typically occurs in pipe where a combination of inadequate cathodic protection (corrosion protection) and coating defects exist which is then exposed to a groundwater solution containing dissolved carbon dioxide. The source of the carbon dioxide is typically the decay of organic matter in the soil.

What are the risks from stress corrosion cracking?

SCC results in clusters or colonies of cracks on the external surface of the affected pipeline. The cracks vary in depth and length. Over time, the cracks may increase in depth and length, and link together to form longer cracks. At some point these cracks may reach a critical depth and length combination that can result in a pipeline failure unless the SCC is repaired, the affected pipeline section is replaced, or the operating pressure of the pipeline is reduced.

Pipeline SCC creates weaknesses at points in the pipe, which in turn makes the pipe more susceptible to third party damage, overpressure events, etc. (i.e., SCC doesn't necessarily need to cause the leak or rupture itself to increase risk).

Pipeline Failure Rates from Stress Corrosion Cracking

Due to higher pipeline temperatures, the majority of pipeline incidents due to SCC are found on natural gas pipelines lines in comparison to the occurrence rate for hazardous liquid pipelines.

The failure rate due to SCC is low, approximately 1 percent of the incidents reported to OPS, however, it is a risk that must be addressed as it can manifest itself wherever the right combination of factors exist.

What is being done to prevent/mitigate Stress Corrosion Cracking?

• Research into the causes and prevention of SCC are ongoing within the industry. It has been found that cleaning and preparing pipe surfaces through shot-peening and then applying special coatings (fusion bonded epoxy) will protect the pipeline from the occurrence of SCC.
• Operators use cathodic protection systems to protect pipe from corrosion. Annual surveys of the effectiveness of cathodic protection reveal areas where coating may be disbonded from the pipeline, allowing corrosion solutions to come in contact with the pipeline surface and potentially creating conditions conducive to SCC.
• The Office of Pipeline Safety has implemented new Pipeline Integrity Management (“IM”) regulations that require all pipeline operators to inspect and assess all of their pipelines that could affect areas of high consequence such as populated areas and environmentally sensitive areas. The operators are required to inspect and assess their pipelines for integrity issues, such as SCC, and repair or replace affected pipe. Inspection techniques may include:
• Inline inspection tools – “smart pigs”
• Hydrostatic pressure tests
• Direct assessment
• By implementing the requirements of the regulation and through responsible maintenance programs, pipeline operators continuously inspect their pipelines for indications of SCC.

Stress Corrosion Cracking: What more can be done

• Public : Be aware of pipelines located near you. Be observant for signs of pipeline damage, leakage, or security concerns. Report any concerns you have regarding pipeline safety to the pipeline operator immediately. Always respect the pipeline right-of-way. Do not dig or build on a pipeline right-of-way without first contacting the pipeline operator or your state one-call center.
• Industry : Pipeline operators and industry stakeholders can continue to develop and implement improved SCC detection and prevention technologies. Operators must continue to implement SCC protection effectively and strengthen pipeline integrity management programs. Operators must mitigate the effects of SCC when it is detected.
• Regulators : OPS and state regulators must continue to inspect pipeline operators to ensure they effectively implement required integrity management programs, including protection for SCC, to ensure that risks to pipelines are identified and mitigated at the earliest possible time. Better coordination is needed between local permitting agencies and pipeline operators to facilitate expeditious granting of permits when public safety is potentially threatened.

Stress Corrosion Cracking: Where can I learn more?

• PHMSA Community Liaison Services

Date of Revision: 07232014