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Proof-of-Concept Development of Disbonded Coating Cathodic Protection Monitoring Coupon, 20-9216

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Principal Investigators
Oliver C. Moghissi
Darrell S. Dunn
Narasi Sridhar
(Pavan Shukla)
(Xihua He)

Inclusive Dates: 08/23/00 - 12/23/00

Background - To protect against external corrosion, pipelines and other buried steel structures are frequently coated and cathodically protected. The cathodic protection (CP) system is designed to protect the pipe where coating defects occur. Coating defects generally take the form of either exposed bare steel or a disbondment in which the coating remains intact but a crevice is formed between it and bare steel. CP systems effectively deliver current to defects directly exposed to soil or water and therefore mitigate corrosion. However, defects covered by disbonded coating are sometimes difficult to protect because the coating prevents the CP current from reaching deep inside the crevice (called cathodic protection shielding). An example of corrosion under disbonded coating is shown in the photograph of the pipe below. In addition, stress corrosion cracking and localized corrosion have predominantly been observed under disbonded coatings. Areas shielded from CP beneath disbonded coatings are not adequately assessed using present monitoring techniques. A need therefore exists to develop a method for determining when disbonded coatings are a problem and assessing the effectiveness of remedial measures (e.g., more CP). In addition, pipeline operators need to demonstrate CP effectiveness for regulatory compliance.

Approach - The overall objective was to develop a prototype device and preliminary data to demonstrate that monitoring beneath disbonded coatings is technically and practically achievable. The approach was to design and build a prototype disbonded coating coupon assembly and perform a limited number of laboratory tests to validate the concept.

Accomplishments - A prototype disbonded coating coupon was designed and assembled (see photograph below) that simulates a realistic pipeline coating disbondment. Measurements were taken demonstrating that using CP coupons to monitor beneath disbonded coatings is technically valid and practically achievable. Experimental results showed that a level of CP that protected an exposed defect failed to protect sufficiently underneath the adjacent disbondment in 30 kiloohm-centimeter water during a 30-day period. The results of this quick-look project have been incorporated in a multiclient proposal to the gas industry for funding in 2002.

Corrosion under disbonded spirally wrapped tape coating

Prototype disbonded coating coupon

Test results showing CP shielding

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