External Corrosion Direct Assessment for Unique Threats to Underground Pipelines
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External Corrosion Direct Assessment for Unique Threats to Underground Pipelines

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      Final Report
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      External corrosion direct assessment process (ECDA) implemented in accordance with the NACE Recommended Practice RP0502-02 relies on above ground DA techniques to prioritize locations at risk for corrosion. Two special cases warrant special consideration. 1. In situations when the ECDA process has to be conducted in areas with AC and DC (static and dynamic) stray current activity, the stray currents (SC) will likely greatly affect any voltage or current-based measurements used in the indirect assessment step of ECDA. The ECDA potential/voltage measurement techniques are subject to IR-drop errors (observed to reach magnitudes in excess of 5V) due to the flow of both CP current and stray current in the soil. These IR-drops must be considered before this type of data can be accurately related to existing protection criteria. Given that the traditional ECDA methods, such as CIS, DCVG, and PCM, rely on the measurements directly impacted by the SC, the principal problem is to establish the conditions under which the ECDA techniques are compromised to such extent that the survey results become invalid. The accuracy and reliability of the common ECDA tools under varying conditions of stray (static and dynamic DC and AC) requires an assessment; it is expected that the limitations are dependent on both the level of stray current activity and the conditions of the external coating on the buried pipeline. 2. As a further complication, some corrosion-related concerns are not specifically addressed by the ECDA process in its present form. The pipelines have been documented to suffer from AC corrosion; further, a recent research project (cofunded by DOT and PRCI) investigated the risks of excessively high CP potentials to both external coating and linepipe steels. Whereas the threats from these adverse influences have been acknowledged, they are not currently being explicitly addressed in the indirect assessment step of the ECDA process (no procedure to account for AC corrosion threat exists). Although neither NACE International nor the Code of Federal Regulations presently offer any quantitative criteria for either AC corrosion or high CP-related risks, there are published data which tentatively set the upper limits on the AC discharge current density and CP potentials/CP current density.
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