Alternating Current Corrosion Risk Arising from Alternating Current-Powered Rail Transit Systems on Cathodically Protected Buried Steel Pipelines and Its Measures Available to Purchase

Field tests were conducted to assess alternating current (AC) corrosion risk on a cathodically protected steel pipeline buried near an AC-powered rail transit system. AC corrosion risk was assessed with respect to the cathodic protection criteria that have been developed by the authors based on AC and direct current (DC) current densities measured using coupons. The average AC current density satisfied the criteria at all test stations in the test section where a distributed grounding system using distributed magnesium electrodes was installed for the mitigation of induced AC. However, the level of AC current density fluctuated greatly due to AC interference caused by the operation of the AC-powered rail transit system, and reached considerably high levels for short time periods during the passing of AC-powered trains where the pipeline was parallel to the AC-powered rail transit system. AC current density markedly decreased when the pipe was connected to a bare steel casing pipe through a solid-state DC decoupling device; thus, the risk of AC corrosion was eliminated completely even while AC-powered trains passed by.