This is a case history of a large diameter pipeline with fusion bonded epoxy coating that experienced AC corrosion after being in operation within six (6) years while the existing 67-year-old large diameter pipeline with coal tar enamel coating experienced no AC corrosion. An AC study was completed after installation of the newer pipeline but showed no immediate threat. This paper presents a post mortem assessment after 17 years of what is required and the importance of not taking adequate data when AC load factors were not posing a threat condition. If it were not for the operator's due diligence to run an in-line- inspection device, this ongoing condition would have gone unnoticed. This paper provides the understanding of the need for complete information, data required for AC modeling, corrosion threats and mitigation. AC mitigation modeling was in already in place which could have been used effectively. The first AC modeling was developed by Dr. John Dabkowski for the Pipeline Research Council International1 (PRCI).

In high consequence areas (HCAs) multiple underground facilities run under the country’s urban and suburban environments as with these pipelines. Multiple utilities such as water lines, natural gas lines, and oil and gas pipelines all coexist to supply neighborhoods with fiber connections, sewage lines, electrical cable, and fuel to local homes and businesses. Below in figure 1 is a photo of AC mitigation being installed in backyards of homes where these type facilities are located.

As high population regions increase, pipeline companies need to install new lines in difficult right of ways (RWs) with (FBE) coating technologies that prove effective against galvanic or direct current (DC) corrosion. However, these same FBE coatings have opened the door to an unexpected corrosion threat - alternating current (AC) effects due to micro holidays. Normally these extremely small defects are not a problem regarding cathodic protection; however, they become a threat when AC and DC voltage and current densities become relatively high, when co-located near high current power transmission lines.

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