- Virtually all hazardous liquid and natural gas transmission pipe in service today is made from steel. This steel – when not otherwise protected and exposed to oxygen and/or water – can corrode. Corrosion can result in small holes in the pipe, or loss of pressure-carrying capacity.
- Corrosion is the electro-chemical reaction of a metallic material with its environment. Pipe environments include soil, water, air, and even the contents of the pipe itself.
- In all electrolytes (the ground, rain water, river or sea water, moisture in the air or transported product), metal atoms from the pipe go into solution as electrically charged ions. The movement of the ions causes a flow of electrical current from the metal pipe to the electrolyte (ground or water). This process causes loss of metal from the metal surface, and is commonly recognized as rust.
- Cathodic Protection (CP) systems help prevent corrosion from occurring on the exterior of pipes, by substituting a new source of electrons, commonly referred to as either a “sacrificial anode” or “impressed current anode”. Both systems operate by imparting a direct current onto the buried pipeline, using devices called rectifiers. As long as the current is sufficient, corrosion is prevented, or at least mitigated and held in check.
- In most cases, coatings on the exterior of a pipe are used in conjunction with CP. Coatings have a high dielectric strength, which prevents the flow of electrons to the pipe’s surroundings, thus interrupting the electro-chemical reaction of the metal with its environment.
Where is cathodic protection used?
CP protects buried pipelines, ship hulls, underground tanks, offshore platforms and any other metal surfaces which may come in contact with the ground or with water. It is not used to prevent atmospheric corrosion, or corrosion that may occur inside the pipe due to its contents. Other corrosion-control measures are used in these cases.
What are the regulatory requirements for cathodic protection on a pipeline?
- Gas pipelines installed after July 31, 1971, and hazardous liquid interstate pipelines installed after March 31, 1970, must be properly coated and have CP. Effective dates for other categories of pipelines apply.
- CP is required on any pipeline installed before these dates if the pipeline is coated, or where areas of active corrosion are present if the line is bare or ineffectively coated.
- Performance of CP Systems must be monitored regularly with tests performed at least once per year. Records must be maintained for the life of the pipeline.
- Where CP systems utilize rectifiers, each rectifier must be checked six times a year, with a maximum interval between checks of 2 ½ months.
- Each pipeline must have sufficient test points for electrical measurement to determine CP adequacy. Test points should be shown on CP system maps.
- Operators must maintain records or maps of their CP systems. Records of all tests, surveys, or inspections required by the regulations must be maintained.
- Pipelines that are found to have deficient CP must be remediated in a timely manner (usually within 12 to 18 months after discovery).
- More stringent requirements apply to gas pipelines operating under an alternative (higher) MAOP.
- More detailed information on the regulatory requirements pertaining to cathodic protection can be found in 49 CFR 192 Subpart I and 49 CFR 195 Subpart H.
Date of Revision: 10242017