When do you have stray direct current (DC) interference on a pipeline? What tasks are required to ensure that an effective level of cathodic protection is being maintained? What are some of the methods utilized to mitigate stray DC interference?
With new pipelines being built and existing pipelines aging, maintaining a pipeline’s cathodic protection is a dynamic task that never ends. Safety, coordination, troubleshooting, various testing methods and construction practices are some of major parts associated with mitigating stray DC interference. As corrosion engineers we must work in all of these areas to ensure that a pipeline maintains an effective level of cathodic protection and any stray DC interference has been identified and addressed.
This paper discusses the many concerns related to stray DC interference. Including safety, testing, documentation and lessons learned. Also addressed are DC powered transit systems and other unordinary sources of DC interference. The topics discussed are practical and the lessons learned are first-hand accounts. The purpose of this paper is to share what has been learned and some of the company policies that have been taken from many years of pipeline operation.
Sustainability is the capacity to endure. The root purpose of any corrosion control / cathodic protection system is to mitigate corrosion. By preserving a pipe or other metallic structure and not letting it corrode away to rust, it is being allowed to endure. Therefore, mitigating corrosion leads to sustainability.
Cathodic protection is most commonly achieved by galvanic or impressed current means. A galvanic type cathodic protection system is comprised of sacrificial anodes (aluminum, magnesium, zinc); where one metal corrodes in order to provide protective currents for the other metal. An impressed current type system utilizes external power to force impressed current anodes (cast iron, graphite, mixed metal oxide) to corrode in order to provide the protective currents. The external power is provided by a rectifier. A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.
This is where the fun begins. For the purpose of this paper, when we are discussing current it shall be understood to be direct current (DC), unless otherwise specified.
Pipelines utilize rectifiers in order to achieve cathodic protection and DC transit systems utilize them to power their trains. Both uses are common practice and effective. However, issues may arise for nearby pipeline facilities. A pipeline’s impressed current cathodic protection (ICCP) system may, inadvertently, direct its current onto a foreign pipeline. Where the foreign pipeline receives current is commonly referred to as the cathode area or pick-up area. In this area corrosion generally does not occur. Where the foreign pipeline loses current is commonly referred to as the anode area or discharge area. This is the area where generally corrosion does occur.