Current and Potential Distribution on a Coated Pipeline with Holidays Part II—Comparison of the Effects of Discrete and Distributed Holidays

The current and potential distribution on a pipe must be considered if cathodic corrosion protection will be provided with anodes placed in close proximity to the pipe. Correct design of a cathodic protection (CP) system will depend strongly on the types of holidays assumed to exist in pipe coating. Two models for the influence of holidays in coated pipes that encompass the extremes of holiday distribution are compared. The performance predicted by these models is compared to the experiment. The design constraints for a pipe with a single coating holiday are much more severe than those for a pipe with a uniform coating efficiency reduced in proportion to the holiday area. Therefore, the CP design must account for discrete holidays if localized damage to the coating is anticipated. An approach to CP design is suggested where a relationship will be established between the percentage of pipe that is expected to be bare and the “effective’’ coating efficiency that can be used to design the CP system.