08/24/2026
📝 Field Notes from RCS
💡 CP Shielding: When the Coating Protects the Corrosion
In case you missed our newsletter post, here's the key takeaways 👇
👉 A disbonded but unbroken coating can shield CP current from the steel underneath. The dielectric strength that makes a coating good is the same property that makes a disbonded one dangerous. It blocks protective current from reaching the bare, wet steel in the crevice.
👉 Polyethylene tape is the classic offender. Its known in-service failure is losing adhesion, from water migration and soil stress, while staying otherwise intact. That's the exact recipe for a shield.
👉 A passing off-potential can be blind to shielded steel. The instant-off is honest about the steel CP can reach. It says nothing about steel screened out of the circuit. A high on-potential like −1.82 V is mostly IR drop and isn't the pipe's true potential, so don't misread it as over-protection.
👉 The routine tools miss it. DCVG and ACVG need current crossing the coating, and a shield blocks that. A coupon models a holiday, so it reads protected. Low current demand on a tape line can be a shield at work, not a sign of health.
👉 What finds it: direct examination at every bell hole, in-line inspection for metal loss across miles you'll never dig, and pattern recognition by coating type, age, and soil.
👉 You can't CP your way out. More current can't reach the shielded steel. It only over-protects the steel CP could already reach. The missing oxygen didn't stop the corrosion, and the coating stops the cure.
👉 The fix is mechanical: excavate, remove all disbonded coating back to sound adhesion, assess metal loss, prepare the surface properly, recoat with a non-shielding system, verify the repair, and treat the find as a flag for the rest of that coating vintage.
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