Cathodic Protection to Mitigate External Corrosion of Underground Steel Pipe Beneath Disbonded Coating

A study was conducted to determine the feasibility of cathodically protecting the steel surface beneath a disbonded coating with a holiday using an external power source. A laboratory cell was used to simulate the field condition of a steel pipe buried in a soil saturated with ground water that had resistivity of 3,050 Ω-cm to 4,400 Ω-cm. The local pipe potential and solution pH within the crevice of the disbonded coating during cathodic protection were measured over time. The relative advantages of using a pulsed current (PC) vs those of a direct current (DC) were examined. Results indicated applied cell voltage was an important factor for proper protection of a coated steel pipe in high-resistivity soils. A low cell voltage would not permit sufficient polarization of the steel surface within the crevice. A high voltage induced excessive hydrogen evolution at the crevice opening, which blocked penetration of cathodic current into the crevice. The optimal cell voltage was: (a) PC of a frequency of 1 Hz and a duty cycle 0.5 at a controlled voltage setting (Von) = 15 V to 20 V and (b) DC at Von = 15 V to 20 V. Both DC and PC protected the steel pipe adequately in the test soil. The mechanism of cathodic protection in the high-resistivity environment was identified as: (a) polarization to a “corrosion immunity” potential for the steel surface in the vicinity of crevice opening, (b) oxygen depletion for the solution in the interior of the crevice, and (c) a pH increase in the crevice and passivation of the steel in the alkaline environment.