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Findings from a Three-year AC Corrosion Field Study

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A field study was conducted to determine the influence of the AC current density and of the coating holiday size on the rate of AC corrosion.This field study involved burying steel coupons of three different sizes (i.e. 1 cm2 6 cm2 and 10 cm2) applying cathodic protection to an industry standard and varying AC current densities (i.e. 20 A/m2 50 A/m2 and 100 A/m2) for a three-year period. Four sets of twelve coupons each were installed for statistical relevance. Each set contained three coupons with no AC current applied (i.e. placebos) one for each size. A special power supply cabinet was designed to provide uninterrupted DC and AC current with each coupon being energized by a separate module.At the end of the test scheduled for July 2018 the coupons will be retrieved cleaned photographed and the corrosion rate measured.This paper covers the results of the study as well as the various findings from three years of data collection including the effect of AC currents on the protection level of the coupons in clayish soils accuracy of recording DC potentials under AC interference changes in the spread impedance due to calcareous deposits and effects of very high AC voltages on the calcareous deposits.Keywords: AC corrosion AC current density holiday size DC current density polarized OFF potential AC coupons AC/DC power supply cabinet.

Product Number: 51319-12907-SG
Author: Sorin Segall
Publication Date: 2019
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Picture for Investigation into Possible AC Corrosion from a Cathodic Protection Groundbed
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Investigation into Possible AC Corrosion from a Cathodic Protection Groundbed

Product Number: 51319-13188-SG
Author: Wolfgang Fieltsch
Publication Date: 2019
$20.00

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.
Picture for Looking Back and Understanding a Case History of AC Corrosion
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51318-10603-Looking Back and Understanding a Case History of AC Corrosion

Product Number: 51318-10603-SG
Author: Joe Pikas
Publication Date: 2018
$20.00

A case history of a large diameter pipeline with fusion bonded epoxy coating that experienced AC corrosion within six (6) years while a similar 67-year-old pipeline with coal tar enamel coating experienced none.
Picture for 51318-10911- Field Examination of AC Accelerated Corrosion Featured in Monitored Locations
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51318-10911-Field Examination of AC Accelerated Corrosion Featured in Monitored Locations

Product Number: 51318-10911-SG
Author: Shane Finneran, P.E. / Dr. Dave Norfleet / Lyle Buckingham
Publication Date: 2018
$20.00

AC current density (rather than voltage alone) should always be the factor considered when assessing susceptibility to AC corrosion. Also presented are analyses used in assessing the metal loss features and the comparison of site-specific data.
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