How it happens
At a holiday, cathodic protection current reaches bare steel. The cathodic reaction there — reduction of dissolved oxygen, and of water at more negative potentials — produces hydroxyl ions, and hydrogen evolves at the most negative potentials. The pH at the steel-coating interface rises sharply. The alkaline solution undercuts the bond at the edge of the defect, and the disbonded front advances beneath the coating. Whether water under a disbonded coating still receives CP current, or is shielded from it, depends on the coating type, so disbondment can end up as a corrosion site rather than a protected one.
What drives it
- Potential. More negative potentials produce more hydroxyl ions and more disbondment, which is why overprotection damages coatings.
- Temperature. Higher operating temperatures accelerate disbondment; test standards include elevated-temperature variants for this reason.
- Surface preparation. Soluble salts, poor cleanliness, an inadequate profile or a missing pretreatment weaken the interface the alkali attacks.
- Coating chemistry and application. Cure, film thickness and, on pipelines, the bond at field joints and at the mill coating overlap.
How it is tested
The principle is shared: an intentional holiday is drilled through the coating, the specimen is exposed to an electrolyte and held cathodic for a set time, then the coating around the holiday is lifted and the disbonded radius or area is measured.
| Standard | Scope |
|---|---|
| CSA Z245.20 | Plant-applied external coatings for steel pipe, including fusion bond epoxy; cathodic disbondment is one of its coating property tests |
| ISO 15711 | Resistance to cathodic disbonding of paint coatings exposed to seawater |
| ASTM G8 | Cathodic disbonding of pipeline coatings |
| ASTM G95 | Cathodic disbondment of pipeline coatings by the attached cell method |
Potential, electrolyte, temperature, duration and acceptance criteria are set by the standard and the project specification. Results are only comparable when those conditions match, so a data sheet value quoted without its test conditions says little.
Where it matters
Buried and submerged pipelines, tank bottoms, marine structures and any coated steel under CP. On pipelines the weak points are field joints, where surface preparation and application happen outdoors; see pipeline coatings in Canada for the CSA Z245 family and field-joint practice. Holiday detection before burial limits the number of starting points.