A 2018 field investigation on an NPS 6, approximately 4 km long, liquid pipeline, identified a rectifier as the likely source of the elevated DC and AC current densities, which resulted in AC corrosion anomalies detected during in-line inspections (ILI). The findings of this case study, presented in the NACE Corrosion 2019 Paper No. 13188, indicated that the 120 Hz rectifier ripple may have contributed to the accelerated AC corrosion at this location.
This follow-up paper is Part A of a two-part further investigation into corrosion rates on cathodically protected structures due to AC frequencies other than the fundamental power frequencies of 50 Hz and 60 Hz. Part A involves additional field testing on the subject pipeline, and commissioning of the recommended AC mitigation and monitoring systems. Part B of this investigation will be presented in a separate paper (NACE Corrosion 2020 Paper No. 14916) and will discuss the results of laboratory testing to determine corrosion rates of cathodically protected steel in simulated soil conditions at various AC frequencies.
A 2018 AC corrosion investigation on an NPS 6, approximately 4 km long, polyethylene coated pipeline determined that the accelerated corrosion anomalies detected during in-line inspections (ILI) were due to AC corrosion. The AC and DC current densities on the AC coupons adjacent to these anomalies were orders of magnitude above the limits recommended in NACE(1) SP214241. It was also determined that the line was severely cathodically over-protected, and that most of the AC voltage measured on the line was due to 120 Hz AC ripple from a foreign rectifier. The findings of the initial 2018 investigation were presented in NACE Corrosion 2019 Paper No. 131882.
Several recommendations were made in the subject paper to mitigate and monitor the AC corrosion risks and to reduce the level of over-protection on the line. At the time of this paper, the majority of the recommendations have either been implemented or are in progress.
