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 60 Hz AC current densities (i.e., 20 A/m2, 50 A/m2, and 100 A/m2) for a three-year period. Four sets of 12 coupons each were installed for statistical relevance. Each set contained three coupons with no AC current applied (i.e., controls); 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, the coupons were 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 and changes in the spread impedance due to calcareous deposits.
At the beginning of 2013, the AC corrosion was already recognized as a significant threat to pipeline integrity; however, no NACE standard criteria were available. Instead, Prinz general guidelines related to the AC current density (ACCD) were typically used. 1 These rules, based on laboratory tests, introduced a “grey area” for AC current densities between 20 A/m2 and 100 A/m2, where “AC corrosion was not predictable”.
This “grey area” was a concern because the cost of AC mitigation depends primarily on what the target current density is. Under some conditions, trying to mitigate the AC current density to the lower threshold of 20 A/m2 was prohibitively expensive. Some companies used 30 A/m2 as the lower threshold and some organizations used a target of 50 A/m2 based on the statement that “only current densities above 50 A/m2 are serious”.1
Furthermore, selecting the size of the holiday or the size of the coupon has a major impact on the calculated or measured AC current density. The typical size used in the industry was 1 cm2, but subsequent research indicated that maximum corrosion rates occur on a 6.45 cm2 holiday. 2