For a buried pipeline under dynamic DC stray current interference, field experiments of corrosion coupons were carried out at two selected test stations by burying coupons of different bare areas at two different depths. Twenty-four hour on/off potential and coupon current density monitoring were also carried out for each coupon which leaded to a new method for the evaluation of dynamic stray current interference and related corrosion possibility. Corrosion rate of the stray current affected pipeline was estimated via coupon current monitoring and was compared to weight loss measurements.
With urbanization and infrastructure development, city metro system is becoming more and more widespread. Electric traction systems are the biggest and best known sources of dynamic stray current due to the longitudinal resistance of the rails as current returning circuit and the insufficient insulation between the rails and the ground. The expansion of the subway poses a threat to corrosion control of underground pipelines. Due to the low electrical resistance, underground pipelines tend to pick up stray currents from subway systems, carrying them as an alternative route, and subsequently discharge them into the earth, going back to their original source. Current pick-up areas of the buried structure will be electrochemically protected, while the current discharge areas can be subjected to corrosion . Many corrosion cases of buried pipeline caused by stray current interference have been reported in the past decades [2-6]. In addition to China, some other countries in the world, such as the United States, Canada, Russia, Britain, Italy, and so on, also encountered the corrosion problems produced by metro stray current [7-11]. Not to mention the fact that in case of failure, it can lead to pollution of the environment and subsequently create hazards to human life.
Dynamic nature of the stray current generated by rail transit system is due to continuous track to earth potential changes in the railway. In most cases, there is alternating current flowing in and out at the same area of the buried pipeline which is accordance with the field measured potentials varying with time. These fluctuations are affected by transit vehicle acceleration and deceleration, and the number and location of vehicles on the system, among other influencing factors. So far, considerable studies concerning dynamic stray current interference have mainly focused on field tests of potential or current fluctuation and interference mitigation methods for buried pipelines [12-21]. There is lack of research on corrosion law and mechanism on DC dynamic stray current interference which may not be the same as steady DC stray current corrosion due to its dynamic nature. The quantitative relationship between corrosion rate and dynamic stray current parameters hasn't been obtained, which is important to the evaluation of corrosion risk. Meanwhile there haven't been agreed criteria in the world for corrosion risk evaluation under dynamic DC stray current interference.