INVESTIGATION INTO POSSIBLE AC CORROSION FROM A CATHODIC PROTECTION GROUNDBED

摘要

This case study involves an investigation into corrosion anomalies on a 4 km long NPS 6 liquid pipeline that interconnects two facilities in an urban setting. An in-line inspection (ILI) in 2017 identified 70 external corrosion anomalies, with two of them exceeding 55% wall loss. An additional five corrosion anomalies had been previously repaired. Recent annual cathodic protection (CP) surveys, and a close-interval potential survey conducted in 2015, all indicate good cathodic protection levels along the line. AC voltages as high as 6 V were measured along the pipeline and were initially thought to be related to a 0.6 km parallel with a high voltage powerline. AC coupons were installed and AC current densities of up to 1400 A/m~2 and DC current densities of 2100 A/m~2 were recorded in this area, indicating a severe AC corrosion risk. Waveforms indicated that most of the AC voltage was 120 Hz frequency, and that very little of the AC voltage was related to the fundamental 60 Hz powerline frequency. This paper discusses the testing performed to confirm the source of the elevated AC current densities and quantify the associated corrosion risk. The mitigation strategy employed to address this risk is provided.