Cathodic protection to mitigate external corrosion of underground steel pipe beneath disbonded coating
- Clarkson Univ., Potsdam, NY (United States). Dept. of Chemical Engineering
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 [Omega]-cm to 4,400 [Omega]-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 (V[sub on]) = 15 V to 20 V and (b) DC at V[sub on] = 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.
- OSTI ID:
- 6898822
- Journal Information:
- Corrosion (Houston); (United States), Journal Name: Corrosion (Houston); (United States) Vol. 50:10; ISSN 0010-9312; ISSN CORRAK
- Country of Publication:
- United States
- Language:
- English
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