Coating failure in which CP current at a holiday produces hydroxide, and at very negative potentials hydrogen, at the steel surface, and the high pH attacks the bond so the coating lifts outward from the flaw. Overprotection speeds it up. Coatings are rated with cathodic disbondment tests that hold a drilled holiday at a fixed negative potential for days or weeks and measure the disbonded radius; FBE and three-layer systems score well.
Cathodic disbondment around every holiday near the groundbed, that's what -1.5 volts off does to coal tar.
Also heard as
- CD
- cathodic disbonding
Related terms
Overprotection
So much cathodic current that potentials get too negative, risking coating damage and hydrogen problems.
FBE
Fusion-bonded epoxy: a powder coating fused onto heated pipe, the standard external line pipe coating.
Holiday
Pinhole, void or bare spot in a coating that leaves the metal underneath exposed.
SCC
Stress corrosion cracking: brittle cracking caused by tensile stress acting together with a specific corrosive environment.