Coating Disbondment Masks Pipe Potential
A 12-in. coal-tar-epoxy-coated pipeline is 10 years old. Rectifier output has risen from 12 A to 34 A over three years to maintain -1.05 V CSE at test stations. A close-interval survey with current interruption shows instant-off potentials near -0.95 V over most of the line, but a 500-ft section reads -0.71 V. A DCVG survey shows a large coating holiday at the start of that section. A coupon installed at a nearby test station reads -0.66 V instant-off, while the test station pipe reads -0.94 V. Soil resistivity is 1,200 ohm-cm.
1. Explain the discrepancy between the test-station reading and the coupon reading, and determine whether the 500-ft section is protected.
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The test station is reading the pipe potential at a location where the coating may still be intact and where the metallic path is relatively low-resistance. That reading can be more negative than the true potential at a coating holiday or disbonded area.
The coupon, which represents bare steel exposed to the soil, reads -0.66 V instant-off. That is less negative than -0.85 V and shows less than 100 mV polarization if the native potential is near -0.55 V to -0.60 V. The 500-ft section is therefore not demonstrated to be protected, despite the -0.94 V test-station value.
The DCVG holiday and rising current demand support coating disbondment or shielding in that section.
2. Evaluate whether increasing rectifier current is an appropriate response to the low section reading.
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Increasing current is not the first response. The section already shows a large holiday and a coupon that is less negative than the pipe reading. That pattern suggests the current is discharging locally at the holiday or is shielded from a disbonded area, not that the entire line lacks current.
Raising output further could overprotect adjacent coated pipe, increase coating damage, and still fail to polarize a shielded disbonded surface. It would also raise current demand without confirming that the low-potential area improves.
Current should be increased only after excavation confirms the coating condition and after determining whether the area is electrically shielded.
3. Recommend an investigation and repair sequence, and state how you would verify that the repaired section is protected.
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Sequence: review the close-interval and DCVG data; install additional coupons or reference electrodes at the holiday; excavate the holiday and inspect for disbondment and corrosion; remove defective coating and recoat to the specified system; and, if shielding is severe, consider supplemental anodes at the repaired section.
Verification: after repair, repeat instant-off potential measurements and coupon readings. The section should meet -0.85 V CSE instant-off or show at least 100 mV of polarization. Confirm that rectifier current has not simply shifted to the new coating holiday and that adjacent sections remain within acceptable potential limits.