Field investigation and laboratory analyses were performed reviewing six (6) external metal loss features identified in proximity to HVAC transmission lines and an existing AC coupon test station (CTS) monitoring location. The objective of the assessment was to identify the likely mechanism(s) contributing to the external metal loss features, and asses the variations from the monitored CTS data.
Analysis of the six metal loss features identified evidence indicating that AC corrosion was the most likely mechanism contributing to the metal loss. Additionally, data collected during the site excavations indicated AC current density values near the individual features was significantly greater than reported by the coupon test station as close as 150 feet away.
Common industry feedback is that when induced AC potential is less than ”a few volts”, generally cited as 2 VAC or less, that susceptibility to AC corrosion is assumed low. While previous literature has consistently stated the need for considering AC current density, rather than AC voltage alone, there is a noted lack of published evidence of AC corrosion occurring at lower induced potentials, therefore this inaccurate guideline persists. The case study presented has shown that this assumption is not valid, and AC current density should always be the factor considered when assessing susceptibility to AC corrosion. Additionally, this paper presents analyses used in assessing the metal loss features, and the comparison of the site-specific data, as measured from the field and laboratory evaluations to the longterm monitoring data from the nearby CTS.
Following a recent in-line inspection (ILI) run in 2016, Six (6) locations were identified with metal loss greater than 65% reported along the subject pipeline (Line 1) within an approximately 0.5-mile long section. Of the six (6) indications, five (5) were found to be within a 600-foot segment of Line 1.
Line 1 was constructed in the 1990s, and is primarily coated with coal tar, with several short replacement segments coated with a fusion-bonded epoxy (FBE). The segment of Line 1 containing the metal loss features is comprised of 12-inch nominal diameter by 0.219-inch wall thickness, API 5L X42 line pipe, and is collocated with one 345 kV and one 69 kV HVAC transmission line referred to as ”TL 1” and ”TL 2” respectively. Four (4) other power lines were identified near the dig sites. The voltage ratings for these lines is unknown but they are assumed to be lower voltage distribution lines based on the tower design configuration.