Why holidays matter
In immersion and buried service, a pinhole concentrates the corrosive demand of the environment on a point of bare metal: a small anode facing a large protected area. Visual inspection does not find these defects reliably, which is why linings and buried coatings are tested electrically before they go into service.
How the detector works
A ground lead is clamped to bare substrate, and an electrode is moved over the coated surface. The coating acts as an insulator; where it is breached, current flows from the electrode to the substrate and the instrument signals. The method therefore needs a nonconductive coating on a conductive substrate. Coatings that conduct, such as zinc-rich primers, cannot be tested this way, and linings on concrete call for their own practice.
Low-voltage wet sponge vs high-voltage spark
| Method | Principle | Suits | Watch out for |
|---|---|---|---|
| Low-voltage wet sponge | A sponge wetted with water, with a wetting agent where the standard allows it, at a low DC voltage; water reaching the steel through a pinhole closes the circuit | Thinner films, within the thickness limit set by the cited standard | Finds only breaches that reach the substrate; wetting agent residue must be rinsed off before recoating |
| High-voltage spark | A brush, spring or rolling electrode at a voltage set from the film thickness; a spark jumps to the steel at a breach | Thick linings and pipeline coatings | Too low misses defects; too high can damage a marginal film. The voltage comes from the standard and the coating manufacturer |
The high-voltage method can also reveal areas thin enough to break down at the test voltage, which the wet sponge cannot.
When to use which
The specification decides, based on the coating type and its measured dry film thickness, the cited standard and the manufacturer's instructions, which may limit or exclude high voltage. Test once the film has cured as the data sheet requires: an uncured film can give false indications or be damaged. Mark every holiday, repair it, and retest the repair with the same method. For tank linings, the test is part of handover; our guide to selecting an internal tank lining shows where it fits among the other quality checks.
Standards
- NACE SP0188 (now maintained by AMPP): discontinuity (holiday) testing of new protective coatings on conductive substrates.
- ASTM D5162: discontinuity (holiday) testing of nonconductive protective coatings on metallic substrates, covering low- and high-voltage methods.
- ASTM G62: holiday detection in pipeline coatings.
- ASTM D4787: continuity verification of liquid or sheet linings applied to concrete substrates.
The specification should name the governing standard, the voltage basis and the repair-and-retest rule.
Common mistakes
- Setting the voltage without measured film thickness, or reusing a voltage from another job.
- A poor ground connection, which hides holidays.
- Moving the electrode too fast, or skipping edges, welds and corners, where holidays concentrate.
- Testing a wet or contaminated surface, which gives false indications.
- Repeated high-voltage passes over the same spot.
- Repairs that are never retested, and results that are never recorded.
Holiday testing is part of the quality records on Induscoat tank lining work.