On immersion service, a coating is only as good as its worst pinhole. A single discontinuity in a tank lining concentrates corrosion at that point, and the failure is disproportionate to the defect. Holiday detection finds those flaws before commissioning.

The two methods

Low voltage wet sponge testing. Used on thin film coatings, generally below about 500 microns. A wetted sponge carrying a low voltage is passed over the surface; where a discontinuity exists, current flows to the substrate and the instrument alarms. It is non destructive and suits linings and thin film systems.

High voltage spark testing. Used on thicker coatings, typically above 500 microns. A charged electrode passes over the surface and a spark jumps at any discontinuity. Voltage must be set according to the coating thickness, and this is where care is needed.

The risk of over voltage

Setting high voltage testing too high does not find more defects, it creates them. Excess voltage punches through sound coating, leaving a hole where none existed. Voltage should be calculated from the specified film thickness using the applicable standard or manufacturer guidance, and confirmed against actual measured thickness rather than nominal values. Testing a coating thinner than assumed at the calculated voltage is a common source of self inflicted damage.

When to test

After the coating has cured sufficiently, since testing a soft film damages it, and after dry film thickness has been verified, because voltage depends on that measurement. Testing before final coat completion is sometimes specified on multi coat linings to catch defects while repair is easy.

Repair discipline

Mark every detected holiday immediately and repair according to the manufacturer procedure: abrade the surrounding area, clean, and apply the specified repair material with correct overlap. Then retest the repaired area. Repairs that are not retested are simply undocumented.

Where it belongs in the programme

Tank internals, pipeline linings, secondary containment and any immersion or buried service. Combine it with the surface preparation and salt control described in our dehumidified blasting article, since immersion systems are the least forgiving of contamination.

MENA Industrial Supplies supplies high build epoxy linings suited to immersion service with data sheets stating film builds for inspection planning.