A field study was conducted by Corrosion Service and TransCanada Pipelines 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.
