ABSTRACT
Direct Current (DC) Cathodic Protection (CP) coupons are widely used in the pipeline industry to monitor the CP potential and estimate current densities at holidays. Various DC coupon shapes include flat metal surface embedded in an insulating plane and short cylinders. Of these, flat coupons, including rectangular and disk shapes, are most widely used as they are assumed to be representative of holidays at the pipe surface. However, embedded flat coupons are prone to current density measurement errors due to edge effects. Because the holidays on the pipeline are surrounded by coated surfaces, the current density distribution on the coupons is expected to be different than on the holidays. Specifically, a holiday covered by a coating will have a lesser edge effect due to coating conductance and thickness compared to the coupon surface, which is surrounded by insulating material. The size of the coupon with respect to an actual holiday can influence interpretation of the coupon data. In addition, different metallurgy and acclimation periods between the coupon and holiday could lead to different native potentials, causing differences in coupon and holiday current and potential distributions. This paper investigates edge, size, and native potential effects on the CP coupon current density and identifies a methodology to interpret the coupon electric current and potential data at the holiday surface. A model was developed to study the effects on the CP coupon current density and interpretation of the CP coupon data for a given holiday. The model results indicate that the most dominant factor affecting coupon is the size, followed by corrosion potential. The size of the coupon relative to the holiday will cause its current density and off-potential to be different from the holiday. The difference in the native (corrosion) potentials of the coupon and holiday causes a significant difference in the current densities but only a marginal difference in the off-potential values, provided surface areas of the two are the same.
