PR-208-004-R01 Cathodic Protection Under Disbonded Coatings

Abstract

The objective of this research was to: - To quantify the relative ability of disbonded pipeline coatings to transmit cathodic-protection current and provide corrosion control to the steel beneath, - To provide a clearer understanding of the current paths that control cathodic protection to the steel under disbonded coatings, - To identify coatings that may be less restrictive to cathodic protection current, - To determine cathodic protection system designs and procedures to maximize the effectiveness of cathodic protection to the steel surface under disbonded coatings. Laboratory experiments were conducted on six, representative, pipeline anticorrosion coatings applied to steel plates with and without intentional disbondments. The six coatings studied were coal-tar enamel, asphalt enamel, coal-tar epoxy, fusion-bond epoxy, coal-tar urethane, and polyethylene tape. The 30-inch-long (76 cm) coated-steel plates were exposed to two electrolytes, a 1,000- and 8,000-ohm-cm soil leachate, and encompassed several conditions; a grit-blasted surface with no holiday in the coating, a mill-scaled surface with no holiday but with a disbondment underneath the coating, and a grit-blasted surface with a disbondment and a holiday of either 1/8" (0.3 cm) or 1" (2.54 cm) width.