External corrosion where every CP reading passes: the shielded disbondment call
A 20 in (508 mm) crude transmission line, API 5L X65, 9.5 mm wall, was built in 2011 with plant-applied 3-layer PE and heat-shrink field joints. Operating temperature is 40 C. Close-interval CP surveys for the last three years have read between -0.95 V and -1.10 V versus Cu/CuSO4 (instant-off), comfortably beyond the -0.85 V criterion. In 2024 a high-resolution MFL run flagged external metal loss up to 38% of wall in a 260 m section crossing a poorly drained clay floodplain. Two bell holes found external corrosion beneath coating that looks intact at grade but has disbonded in sheets up to 400 mm across. No coating holidays were found at the corroded pits, and soil resistivity is 1,800 ohm-cm. The operator asks why the CP record and the ILI result disagree.
1. Explain how 38% wall loss developed at a location where every CP reading satisfies the -0.85 V criterion. Address what the survey actually measures and what it cannot measure.
Read the worked answer
The survey measures the potential of the pipeline at points where the steel is in electrolytic contact with the soil through the reference electrode path - effectively at holidays, or wherever the coating is permeable. It is not a measurement of the steel under a disbonded coating.
Key reasoning:
- Coating acts as a membrane. Once coating disbonds but stays unbroken, the trapped electrolyte in the crevice is stagnant and resistive. Cathodic protection current cannot migrate into it, so the steel there is not polarized to -0.85 V no matter what the surface reading shows.
- The reading is dominated by the holiday. The -0.95 to -1.10 V values reflect the small holiday area carrying all the current. Potential is not uniform over the pipe surface.
- The crevice chemistry makes it worse. Oxygen depletion, chloride concentration, and possibly microbial activity (clay floodplain, poor drainage) produce an aggressive cell. A rate consistent with 38% of 9.5 mm wall in about 13 years is roughly 0.25 mm/y - entirely normal for an unprotected, shielded cell.
- Overprotection is also a symptom, not a cure. Potentials more negative than about -1.1 to -1.2 V CSE increase cathodic disbondment and hydrogen evolution at holidays, enlarging the shielded area over time.
The correct conclusion is that the CP criterion was met at the pipe surface as measured, but the disbonded region was never a protected surface.
2. List the field and laboratory data you would collect at the bell holes, and state what each measurement distinguishes between: CP shielding, cathodic disbondment driven by overprotection, or stray/AC interference.
Read the worked answer
At the excavation
- Map the disbondment boundary and measure its area; record whether it is a tight or loose disbond. Loose, water-filled blisters point to coating failure rather than CP.
- Inspect corrosion product colour and morphology under the coating. Black magnetite/siderite films suggest some cathodic current reached the steel; bright orange rust with no blackening indicates an unprotected, shielded cell.
- pH of the trapped electrolyte under the disbonded sheet. Alkaline (pH > 9) suggests CP current ingress; near-neutral or acidic (pH 5-7) indicates shielding and a classic near-neutral SCC/corrosion environment.
- Holiday detection over the disbonded area using a low-voltage wet-sponge method, not just high-voltage spark testing, which can miss tight disbonds.
- Soil sampling for chloride, sulphate, redox potential, resistivity, and bacteria to NACE TM0106/TM0212 if microbial corrosion is suspected.
Electrical
- Current-interrupted (instant-off) potentials, DCVG/ACVG, and current span measurements to estimate current distribution; a well-protected section showing no current pick-up is consistent with shielding.
- AC voltage and AC current density if an AC transmission line or DC transit system is nearby (NACE SP0177, ISO 18086). AC current density above roughly 30 A/m2 is a concern, above 100 A/m2 severe - this would point to interference rather than simple shielding.
Laboratory
- DSC (Tg, residual exotherm), peel adhesion, cathodic disbondment, and FTIR to confirm the coating is the specified product and to quantify degradation.
3. Recommend a remediation plan for the 260 m section, including how to treat the 38% wall loss and how to avoid repeating the failure.
Read the worked answer
Integrity first
- Assess the 38% feature with RSTRENG or ASME B31G for remaining strength; at 38% a composite repair sleeve or a cut-out/replacement is normally required rather than continued operation with a derating alone. Any feature interacting with a weld or another feature must be re-evaluated on a combined basis.
Coating and CP
- Strip the coating over the corroded and disbonded area, surface-prepare to Sa 2.5 (NACE No. 2 / SSPC-SP 10), and recoat with a cathodic-disbondment-resistant product compatible with the existing 3LPE system and qualified for the operating temperature. Field joints must use a qualified, temperature-matched system per ISO 21809-3.
- Do not solve this with more CP current. The failure mode is shielding; raising the potential only drives more disbondment. Cap the protection at the criterion (about -0.85 to -0.95 V instant-off), confirm with the 100 mV polarization criterion, and eliminate overprotection.
- Improve current distribution with additional or deep groundbeds if the current is concentrated, and confirm with an interrupted survey.
Prevention
- Re-run DCVG/ACVG and CIS over the whole floodplain crossing; disbondment is likely to extend beyond the 260 m flagged.
- Shorten the ILI interval in this section and record the root cause in the integrity management plan.
- Review the AC/DC interference picture and the coating application records for the 2011 construction campaign.