Buried ancillary steelwork can combine a coating with cathodic protection. At a coating defect, the exposed metal and nearby coating interface experience an electrochemical condition set by the protection system and electrolyte. The coating assessment should represent that boundary. [1]
ISO 15711 addresses sea-water exposure; a buried component needs a separately justified electrolyte and protection boundary for its actual soil service. [1] First confirm that cathodic protection actually applies to the component. An above-ground tank without that system should not inherit a buried-structure test conclusion merely because both are steel near a transformer.
Define the defect environment
| Boundary | Evidence to specify | Why it matters |
|---|---|---|
| Electrical condition | Relevant potential/reference and operating history | Defines the applied cathodic exposure |
| Electrolyte | Soil or test liquid conditions | Controls the local electrochemical environment |
| Coating defect | Size, shape and preparation | Sets exposed-metal/interface geometry |
| Coating system | Material, cure and substrate preparation | Defines the adhesion/barrier premise |
Inspect the interface after a defined exposure
Use the applicable qualified test to assess loss of adhesion around the defect. Retain the method for determining the affected boundary and its uncertainty. The result should distinguish damaged coating area from the intentionally exposed defect.
A laboratory condition can be deliberately severe, but that severity needs a stated relation to the service environment. No numerical life prediction follows from exposure time alone.
Keep paint execution and supervision tied to the actual product and coated region; a finished appearance does not recover missing process evidence. [2]
Separate the intentional defect from disbondment
Assumptions under one declared accelerated-test condition: a circular intentional defect is 5 mm diameter and the affected outer diameter is 15 mm. Excluding the intentional opening, annular affected area is π(15² − 5²)/4 = 157.1 mm². The defined radial advance from its edge is (15 − 5)/2 = 5 mm. Against an assumed project radial-advance criterion of at most 4 mm, it fails by 1 mm. Reject that specimen under the specified exposure and evaluate the protection/coating interface. Preserve the actual potential/reference, electrolyte, temperature and duration; neither this geometric result nor the sea-water method title establishes buried field life. [1]
References
- ISO, ISO 15711:2003, Paints and varnishes — Determination of resistance to cathodic disbonding of coatings exposed to sea water.
- ISO, ISO 12944-7:2017, Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 7: Execution and supervision of paint work.