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Investigation into Possible AC Corrosion from a Cathodic Protection Groundbed

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This case study involves an investigation into corrosion anomalies on an NPS 6 approximately 4 km long Yellow Jacket coated pipeline that interconnects two terminals. 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 which was unexpected as there are no high voltage powerlines in this area. A distribution powerline that parallels the pipeline for approximately 0.6 km was identified near the location with the elevated AC voltages.More surprisingly AC current densities of up to 1400 A/m2 and DC current densities of 2100 A/m2 were recorded on AC coupons in this area indicating a severe AC corrosion risk.Waveforms indicated that very little of the AC voltage was related to the fundamental 60 Hz powerline frequency. The majority of the AC voltage was 120 Hz frequency indicating that the AC is likely from an unfiltered single-phase rectifier. There are several CP rectifiers that influence this pipeline and one of these is associated with a ground bed installed in the area of concern.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.Keywords:AC Corrosion AC Interference AC Mitigation Cathodic Protection CP Ground Bed Rectifier Interference Unfiltered Rectifier Corrosion Investigation Pipeline.

Product Number: 51319-13188-SG
Author: Wolfgang Fieltsch
Publication Date: 2019
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AC Interference and Mitigation: Heartland Case Study

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