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Understanding Holiday Detectors

5 Rules of Thumb for using Holiday Detectors on Steel Surface Coatings

Recently we had a customer who was unsure if the holiday detection equipment they had on site would work for their project. This is an important consideration, because if you use a high voltage detector on a thin coating, you will destroy it. And if you use a low voltage on a high-build coating, you wont get the correct results

First off, what’s a HOLIDAY?

Holidays are tiny pinhole sized exposures in the coating. They often form from solvent entrapment.  When coating bubbles burst they expose the steel. They may be small, but they create a corrosion cell, which will compromise the integrity of the coating system and will become the cause of premature coating failure.

A Holiday Detector uses electricity grounded through to the underlying steel substrate. You run the tool over the coated surface and it will find where these little pinholes are hiding by completing the electric circuit and sounding an alarm.

So here are 5 rules of thumb to help you on your projects.

  1. For steel coatings with a DFT less than 20mils or 500 microns, you’d use a Low Voltage detector. These emit direct current between 5-90 volts.
  2. For coatings thicker than 20 mils or 500 microns, the high voltage method is recommended. These emit thousands to tens of thousands of volts.
  3. As mentioned, the electricity generated by these detectors is conducted via the steel substrate. However, a zinc rich primer is also an excellent conductor and gives false results. If your project uses a zinc primer, the detector is reliable only if the top-coat is non-conductive.
  4. Retained solvents can also give false results when testing for continuity issues and so it’s vital to make sure the coating system is fully cured before running any form of holiday detection.

And finally… a note on safety

5. It’s important before conducting holiday testing in areas where flammable vapors may be present, to thoroughly check (and if necessary, purge) the atmosphere to prevent the holiday detector from becoming an ignition source.

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