The cathodic disbondment test: how does it work and what does it tell you?
In the Test explained series we look at one test at a time: why you do it, how it works and what you can do with the result. This time: the cathodic disbondment test (CD test) to ISO 15711. What does cathodic protection do to the adhesion of your coating?
Why do we do this test?
At sea, steel is often cathodically protected. A sacrificial anode or an impressed current system lowers the potential of the steel, so that it hardly rusts any more.
That potential can affect the adhesion of the coating. This is called cathodic disbondment. The CD test shows whether a coating is susceptible to it.
What can you do with the result?
- Choosing the right coating. You choose a coating that suits cathodically protected steel.
- Demonstrating compliance. You demonstrate that your coating system complies with ISO 12944.
- Objective comparison. You can compare coating systems or suppliers objectively.
Note: the CD test is one of three tests that ISO 12944-9 requires for the splash zone. The cyclic test and immersion in seawater are also part of it.
Which standards do we use?
A standard is an internationally agreed method. That way everyone tests in the same way and results can be compared one to one.
- ISO 12944-9: coatings for offshore applications. This standard sets out which tests a coating system must pass, including the CD test.
- ISO 15711: the CD test itself. This standard describes how the test is carried out: artificial seawater, the applied potential and the measurement of the disbondment. ISO 15711 prescribes a 10 mm hole.
Note the difference: for ISO 12944-9 we drill a 6 mm hole, not the 10 mm from ISO 15711. Does your specification ask for something else? Then we follow it.
How do we carry out the test?
Test conditions: three panels of 150 × 75 mm, a hole of Ø 6 mm, artificial seawater, a potential of −1050 mV and a test duration of 4200 hours.
- 1. Preparation. We measure the coating thickness and drill a 6 mm hole in each panel, down to the steel. This is the deliberate damage.
- 2. Sealing the edges. We carefully seal the edges and the back. Only at the hole does the seawater reach the steel.
- 3. In the tank. A potentiostat (an instrument that keeps the potential constant) holds the steel at −1050 mV relative to a reference electrode for 4200 hours.
- 4. Finishing. Afterwards, we determine how far the coating has detached from the hole.
How do we assess the result?
How: we cut into the coating from the hole and lever it up with a knife. Whatever comes away easily has disbonded.
What we look at: how far the coating around the hole has detached from the steel. We call this the disbondment.
Reading: the distance of the disbondment, measured from the hole, in millimetres. Per panel, for all three panels.
Assessment: we compare the result with the requirement from ISO 12944-9 or your specification: pass or fail.
The most common mistakes with this test
A CD test takes almost six months. A mistake often only shows at the end, and by then that time is lost.
- Edges that are not properly sealed. Current leaks away through the edges. Less load then reaches the hole, and the coating appears better than it is. How we do it: we give the edges extra sealing and check them before the start.
- Water that is not refreshed often enough. Metal particles dissolve in the seawater. They disturb the electrochemical reaction, and with it the result. How we do it: we refresh the artificial seawater at fixed intervals.
- A hole of the wrong size. If you use the 10 mm from ISO 15711, you get a result that cannot be compared with the requirement from ISO 12944-9. How we do it: we drill 6 mm, unless your specification asks for something else.
How do we know the test is right?
Over 4200 hours, a small deviation adds up considerably. That is why we monitor the set-up throughout the test.
- Potential and current. The potentiostat displays the potential and the current. We read them regularly and record them.
- The seawater. We monitor the artificial seawater and refresh it in good time.
- Operation and assessment. Determining the disbondment requires craftsmanship. Our specialists are trained for this and demonstrably competent.
Quality you can demonstrate
M2LAB is accredited to ISO/IEC 17025. What does that mean? An independent body, in the Netherlands the Dutch Accreditation Council (Raad voor Accreditatie, RvA), has established that we carry out our tests competently, impartially and reproducibly. It is not a one-off stamp: the RvA returns regularly to assess this again.
The most important proof: round robin testing
In a round robin, several laboratories receive the same material and carry out the same test. The results are then compared. If our result differs, we find out why and adjust our method. This demonstrates that our results match those of other labs.
What else we do
- Calibration. We regularly compare our measuring equipment with recognised references.
- Fixed method. Every test follows a defined, proven method in accordance with the standard.
- Qualified people. Only demonstrably trained staff carry out tests and assessments.
- Measurement uncertainty. We know how accurate our result is and take that into account.
- Audits. We carry out internal audits. We investigate errors and complaints down to the root cause.
For you, this means: a test report you can build on, even if a discussion arises later.
Can your coating withstand the influence of cathodic protection?
We test to the applicable standards and explain the result clearly.
M2LAB investigates the service life of coated products: corrosion, sunlight, chemicals, mechanical stress and visual appearance.
