Abstract
A two-dimensional model was developed to predict the current and potential distribution of an underground coated pipe in high-resistivity soil under an impressed current, parallel anode, cathodic protection (CP) system. The model was designed to study the effect of discrete holidays of various sizes on coated pipe without having to assume that holidays simply reduce the efficiency of the protective coating. Full-scale experimental tests were conducted with a 0.508 m (20 in.) diameter pipe coated with 460 μm (18 mils) of fusion-bonded epoxy. The performance of a parallel anode CP system was measured in the presence and absence of a discrete longitudinal coating defect (2.4% of the pipe circumference) that exposed bare steel. All tests were conducted in 108,000 ohm-cm water. Good agreement was obtained between experimental results and modeling predictions. The results show that there can be a significant difference in the performance of a cathodic protection system when localized defects exist in the coating as compared to uniformly distributed holidays. This difference is expected to be most pronounced in high-resistivity soils with close anode to pipe spacing.
