Field Investigation of the Effect of Stray Current Generated by DC Traction Systems on Cathodically Protected Gas Distribution Pipelines in the City of Tehran

Stray current from DC Traction systems (metro) can affect buried metallic structures from corrosion point of view. In order to investigate these effects on cathodically protected gas distribution pipelines, the metro lines and the gas pipeline maps were superimposed on the urban map of city of Tehran in order to prepare a comprehensive locational map. 213 intersections and almost 100150 meters of parallel sections of pipelines were found with respect to the railway right of way, which were specified for field measurements. The potential measurements were carried out in specific time periods of the day and the data was collected for one hour at each test point. In order to thoroughly investigate the effects of dynamic stray current on the pipelines, 24-hour measurements pipe-to-soil potentials were carried out. The results showed that dynamic stray current from DC traction appears as potential fluctuations on pipeline applied cathodic potentials. The fluctuations faded away during the period of metro shut down systems between 11:30 pm to 4:30 am. These fluctuations can cause the pipeline potential to exit the safe potential zone and can result in corrosion of the zones, where the DC stray current leaves the pipeline and enters the soil. In this study, a maximum potential shift of 100 mv in the pipe-to-soil potential was considered as a commencement criterion for effective dynamic stray current that requires further investigations. Results showed that a potential fluctuation range between 30 mV to 3 V exists in measured points on pipelines, which can exceed the proposed criterion and can cause damage. Corrosion rates influenced by stray currents were also calculated using coupons at a specific region. Results showed that coupons connected to the pipeline had a corrosion rate of 4.4 mpy (with cathodic protection and under influence of stray currents) which was about 1.5 times more than that of free corrosion rate of 2.6 mpy (for unconnected coupons to the pipelines, i.e. without cathodic protection).