Cathodic Disbondment Testing
How laboratories measure a coating's resistance to disbonding under cathodic protection — test cells, intentional holidays, potentials, temperatures and how results are read.
Key takeaways
- Cathodic disbondment (CD) tests drill a small holiday through a coating, apply a cathodic potential in an electrolyte, and measure how far the coating loses adhesion around the defect.
- Results are reported as a disbonded radius or distance in millimetres; smaller is better, and limits are set by the governing product or project standard.
- Temperature, potential, electrolyte, holiday size and duration all change the result, so values are only comparable within the same method.
- CD testing is a laboratory qualification and quality-control tool for buried, submerged and offshore coatings — it is not a field inspection test.
Buried pipelines, tank bottoms, ship hulls and offshore structures usually combine a coating with cathodic protection (CP). The coating does most of the work; CP protects steel at defects. But the electrochemistry at those defects produces hydroxide and hydrogen, which can attack the coating bond and cause it to peel back from the holiday — a failure mode known as cathodic disbondment.
Cathodic disbondment testing simulates that process under accelerated conditions so that coating systems can be compared, qualified and controlled before they go into service.
How the test works
The basic setup is similar across standards:
- Prepare the specimen. A coated pipe section or panel, produced and cured as in production, is inspected for holidays and film thickness.
- Drill an intentional holiday. A small hole of specified diameter, typically a few millimetres, is drilled through the coating to expose bare steel.
- Attach a test cell. A cylinder is sealed over the holiday and filled with electrolyte, commonly a sodium chloride solution, or the specimen is immersed in it.
- Apply potential. The steel is polarized cathodically using a sacrificial anode or a power supply with a counter electrode, at a set potential measured against a reference electrode.
- Hold for the test period. The cell is kept at the specified temperature for anywhere from about a day to several weeks or months.
- Evaluate. After rinsing and drying, the coating is cut radially from the holiday and lifted with a knife; the extent of disbonded coating is measured.
The disbonded area is usually reported as an average radius or distance from the edge of the original holiday, in millimetres. Some methods report the disbonded area instead.
Test standards
Several families of standards are used, and specifications normally name both a method and its conditions:
- ASTM G8 — cathodic disbonding of pipeline coatings, generally at ambient temperature over an extended period.
- ASTM G42 — cathodic disbonding of pipeline coatings subjected to elevated temperatures.
- ASTM G95 — the attached cell method, in which the electrolyte cell is fixed to the surface of a coated pipe rather than immersing the specimen.
- ISO 15711 — resistance to cathodic disbonding of coatings exposed to seawater, used for marine and offshore systems.
- Pipeline coating product standards such as CSA Z245.20 for fusion-bonded epoxy and parts of ISO 21809 include CD tests with defined temperatures, potentials, durations and acceptance limits for qualification and production testing.
Offshore coating qualification under ISO 12944-9 also includes cathodic disbonding for systems intended for immersion with CP.
| Variable | Typical range or practice | Effect on result |
|---|---|---|
| Temperature | Ambient up to elevated service temperatures | Higher temperature accelerates disbondment |
| Potential | More negative than normal CP levels | More negative potential increases disbondment |
| Duration | 24 hours to 28 days or longer | Longer tests give larger, more discriminating results |
| Electrolyte | Often sodium chloride solution; seawater for marine | Composition affects reactions at the holiday |
| Holiday diameter | A few millimetres, as specified | Changes current and the starting area |
Interpreting results
A small disbonded radius indicates good resistance; a large radius, or coating that lifts easily far from the holiday, indicates poor resistance. Acceptance criteria come from the product standard or the project specification and differ by test temperature and duration, so a value from a short, hot test cannot be judged against a limit for a long ambient test.
Inspectors also note the character of the disbonded area: whether the steel beneath is bright or darkened, whether blisters formed around the holiday, and whether the coating lifted at the steel interface or within a primer layer. These observations help identify whether surface preparation, coating chemistry or cure was the weak point.
CD tests are deliberately severe. A system that shows some disbondment in the lab is not necessarily unsuitable for service; the purpose is to rank coatings and catch production problems under consistent conditions.
What affects CD resistance
- Surface preparation. A clean, angular surface profile and low soluble salts improve resistance; surface pretreatments are used on some pipeline coatings for the same reason.
- Coating chemistry and cure. Fusion-bonded epoxy and multilayer polyolefin systems are formulated and qualified specifically for CD resistance; under-cured coatings perform poorly.
- Service temperature. Resistance generally falls as temperature rises, which is why hot pipelines need systems qualified at elevated temperature.
- CP levels. Over-protection — excessively negative potentials — increases hydrogen and hydroxide generation and worsens disbondment in service.
When comparing data sheets, look beyond the number. Check the standard, temperature, potential, holiday size and duration behind each CD value before deciding one coating outperforms another.
Where CD testing is used
CD testing is central to qualifying pipeline coatings, girth weld field joint coatings and repair materials. It is also used for ship hulls, ballast tanks, offshore jackets and other marine coatings, buried tank exteriors and immersed water infrastructure protected by CP. Pipeline coating plants commonly run CD tests on production samples at a set frequency as part of their quality plan, alongside adhesion, cure and holiday testing.
Frequently asked questions
Can cathodic disbondment be tested in the field?
Not in the standardized sense. CD tests require controlled potential, temperature and duration in a laboratory or plant. In the field, inspectors assess disbondment around existing defects during excavations or failure investigations.
Is a smaller disbonded radius always better?
Within the same test method and conditions, yes. Comparing radii from different methods, temperatures or durations is not meaningful.
Why is an intentional holiday drilled?
Disbondment starts where steel is exposed to the electrolyte under CP. The drilled holiday provides a reproducible defect so results can be compared.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
