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07118 Chemistry and Corrosion Rates under Pipeline Disbonded Coatings With or Without Cathodic Protection

Picture for 07118 Chemistry and Corrosion Rates under Pipeline Disbonded Coatings With or Without Cathodic Prote
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The transport of dissolved O2, and that of dissolved carbon dioxide (CO2) if present, into the disbonded region through holiday can be key to determining the disbonded corrosion rate. The evolution of chemistry in the disbonded region and the transient behavior of corrosion potential and rate in the disbonded region are investigated and reported in this work.

Product Number: 51300-07118-SG
ISBN: 07118 2007 CP
Author: Fengmei Song and Narasi Sridhar
Publication Date: 2007
Industry: Cathodic Protection
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Underground steel pipelines are protected by coatings and cathodic protection (CP). Pipeline corrosion occurs when the coating is disbonded away from a defect (or holiday). The corrosion rate varies spatially along the disbondment (or crevice). In the presence of dissolved oxygen (O2) in soil ground water, a differential O2 concentration cell may develop in the disbonded region because O2 diffuses more readily into the disbonded region through holiday than through the disbonded coating. CP can decrease or eliminate the O2 concentration cell depending on the potential applied at the holiday. Since pipeline coatings are usually ionically non-conductive, CP is unable to protect the steel surface under the disbonded region. The transport of dissolved O2, and that of dissolved carbon dioxide (CO2) if present, into the disbonded region through holiday can be key to determining the disbonded corrosion rate. The evolution of chemistry in the disbonded region and the transient behavior of corrosion potential and rate in the disbonded region are investigated and reported in this work.

Underground steel pipelines are protected by coatings and cathodic protection (CP). Pipeline corrosion occurs when the coating is disbonded away from a defect (or holiday). The corrosion rate varies spatially along the disbondment (or crevice). In the presence of dissolved oxygen (O2) in soil ground water, a differential O2 concentration cell may develop in the disbonded region because O2 diffuses more readily into the disbonded region through holiday than through the disbonded coating. CP can decrease or eliminate the O2 concentration cell depending on the potential applied at the holiday. Since pipeline coatings are usually ionically non-conductive, CP is unable to protect the steel surface under the disbonded region. The transport of dissolved O2, and that of dissolved carbon dioxide (CO2) if present, into the disbonded region through holiday can be key to determining the disbonded corrosion rate. The evolution of chemistry in the disbonded region and the transient behavior of corrosion potential and rate in the disbonded region are investigated and reported in this work.

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Product Number: 51300-07036-SG
ISBN: 07036 2007 CP
Author: Ernest Klechka, Steven F. Daily, and Kevin C. Garrity
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This paper reviews the concerns of applying excessive levels of cathodic protection current to pipelines and the need for establishing an upper potential limit. Coating disbondment, hydrogen induced stress cracks, stress corrosion cracking, hard spots and the problems associated with measurement of a true polarized pipe-to-electrolyte potential are addressed.
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Based on some articles that have been published in Materials Performance (MP) and the NACE Courses 9 and Standards for Cathodic Protection (CP) practitioners, this author has concluded that present day CP practitioners, do not have an adequate understanding of some basic scientific principles.
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