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Title: Corrosion beneath disbonded pipeline coatings

Abstract

The relationship between coatings, cathodic protection (CP), and external corrosion of underground pipelines is described. Historically, this problem has been addressed by focusing on the corrosion and CP processes associated with holidays, e.g., coating disbondment and CP current flow within the disbonded region. These issues and those associated with disbonded areas distant from holidays are also discussed.

Authors:
;  [1]
  1. Cortest Columbus Technologies Inc., Dublin, OH (United States)
Publication Date:
OSTI Identifier:
460436
Resource Type:
Journal Article
Resource Relation:
Journal Name: Materials Performance; Journal Volume: 36; Journal Issue: 4; Other Information: PBD: Apr 1997
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING NOT INCLUDED IN OTHER CATEGORIES; CORROSION PROTECTION; PIPELINES; PROTECTIVE COATINGS; STEELS; CATHODIC PROTECTION; CURRENT DENSITY; DEFECTS; PERFORMANCE; DIFFUSION; EXPERIMENTAL DATA

Citation Formats

Beavers, J.A., and Thompson, N.G.. Corrosion beneath disbonded pipeline coatings. United States: N. p., 1997. Web.
Beavers, J.A., & Thompson, N.G.. Corrosion beneath disbonded pipeline coatings. United States.
Beavers, J.A., and Thompson, N.G.. 1997. "Corrosion beneath disbonded pipeline coatings". United States. doi:.
@article{osti_460436,
title = {Corrosion beneath disbonded pipeline coatings},
author = {Beavers, J.A. and Thompson, N.G.},
abstractNote = {The relationship between coatings, cathodic protection (CP), and external corrosion of underground pipelines is described. Historically, this problem has been addressed by focusing on the corrosion and CP processes associated with holidays, e.g., coating disbondment and CP current flow within the disbonded region. These issues and those associated with disbonded areas distant from holidays are also discussed.},
doi = {},
journal = {Materials Performance},
number = 4,
volume = 36,
place = {United States},
year = 1997,
month = 4
}
  • 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 cellmore » 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.« less
  • This paper describes the relationship between coatings, cathodic protection (CP), and external corrosion of underground pipelines. Historically, this problem has been addressed by focusing on the corrosion and CP processes associated with holidays, e.g., coating disbandment and CP current flow within the disbanded region. This paper addresses these issues but also considers corrosion associated with disbanded areas that are distant from holidays.
  • The penetration of cathodic protection potentials under disbonded pipeline coatings has been studied. A physical model based on laboratory data has been developed. In neutral-pH, low-conductivity trapped water, effective protection potentials were found to penetrate less than 10 cm (4 in.) into a simulated disbondment. The implications for general corrosion and stress corrosion cracking are discussed.
  • This article discusses the effectiveness of long-term cathodic protection (CP) to mitigate corrosion under a disbonded coating that contained an adjacent holiday and was placed in a high-resistivity water saturated soil environment. Parameters included environment resistivity and applied current density. The polarization of zinc ribbon specimens when subjected to CP in the same high-resistivity soils also was measured.
  • Disbonded, shielding coatings over pipeline steel can often lead to serious corrosion problems that cannot be readily controlled by conventional cathodic protection systems. Cathodic protection effectiveness in these cases is limited by the high resistance electrical path in the crevice formed between the steel surface and disbonded coating. This study investigates the hypothesis that application of pulsed cathodic protection may improve the penetration of protective cathodic protection beneath disbondments to provide more effective corrosion control. Polarization experiments beneath a simulated disbondment were conducted using conventional and pulsed cathodic protection. Current demand, potential, profiles, polarization, pH differences and effectiveness of eachmore » system were calculated. Results indicate that certain pulsed waveforms increase the distance in a crevice at which protective potentials can be achieved.« less