Evaluation of Underground Gas Pipeline External Corrosion Due to AC and DC Interference Currents

Adjunct to underground railway (which produce DC interference currents) and overhead power transmission lines (which produce AC interference currents), underground gas pipelines have been affected severely which have brought about corrosion costs namely direct and indirect costs. The effects of both types of stray currents on the UG gas pipelines located in a street in Tehran are studied. On-site inspections were performed and then additional laboratory studies were done to analyze and characterize the corrosion products. It is founded that the coating of the pipe has been disbonded in two points which are in ۱۰-meter distances from each other and the external pipe wall were locally corroded which has led to gas leakage. According to the standard criteria and the results of AC potential, AC interference current may not cause the corrosion in the points. However, the amounts of off- potentials were out of range according to the relevant standards and therefore DC stray current produced by Tehran underground railway has mainly led to the corrosion. Corrosion products were not uniformly covered the metal surface and mainly contains Fe۳O۴ and FeOOH as well as FeCl۲. Thus, it is recommended that additional evaluations need to be done according to BS EN ۵۰۱۶۲ and afterwards, protective measures must be taken to eliminate the side effects of the DC stray current.