Pinhole & Holiday Detection
After a protective coating has been applied, it is important to ensure there are no defects or discontinuities present that expose the substrate beneath. Small areas of thin or missing coating, called 'pinholes' or 'holidays', can become foci for corrosion and drastically reduce the life of a protective coating system. A pinhole or holiday detector is a non-destructive instrument used for detecting discontinuities in a coating system including holidays, pinholes, and cracks. Other names include porosity detector, continuity tester, holiday tester, spark tester, and jeep meter.
DeFelsko offers two types of porosity/pinhole detectors: low voltage (wet sponge tester) and high voltage (spark tester). Low voltage detectors, like the PosiTest LPD, are typically used on paint systems less than 500 µm (20 mils) thick. High voltage holiday testers, like the PosiTest HHD operate at voltages up to 35,000 V and are typically used on coatings at least 200 µm (8 mils) thick.
What is a Holiday?
A holiday—pinhole, flaw, void, crack—is a discontinuity within a coating system where a small area of thin or missing coating exposes the substrate to the environment.
Why is Holiday Detection necessary?
Due to a variety of potential errors in the application of a new protective coating, flaws—or holidays—can form. They can be invisible to the naked eye and are near-impossible to detect without proper instrumentation.
Holiday detection is a necessary step in identifying these areas of thin or missing coating. Porosity detectors are often used in applications where corrosion is difficult to monitor, or in aggressive service environments such as buried pipelines where performance of the protective coating is critical.
How are Holidays Detected?
Outlined by standards organizations like AMPP (NACE), ASTM, and ISO, a holiday detector uses a charged electrode passed over a coating. When a holiday is encountered, current flows from the electrode into the conductive substrate below. The current returns to the detector through a ground wire, completing the circuit and triggering an alarm, alerting the inspector to the defect.
What is a Wet Sponge Tester?
A wet sponge tester is another name for Low voltage Pinhole Detectors like the PosiTest LPD. By using a wet sponge to pass current through a grounded part, flaws and pinholes in the coating can be identified. They are typically used on paint systems less than 500 µm (20 mils) thick.
What is a Holiday Detector?
A holiday detector (porosity detector, continuity detector/tester, holiday tester) like the PosiTest HHD, is a non-destructive inspection instrument used for detecting discontinuities in a coating system, including holiday, pinholes, and cracks.
What are the Types of Holiday Detectors?
There are three main types of Pinhole/Holiday Detectors:
Low voltage Pinhole Detectors, such as the PosiTest LPD use a wet sponge to detect pinholes in coatings less than 500 microns (20 mils).
High voltage Pulse Holiday Detectors, such as the PosiTest HHD use pulse DC and either a spring or brush electrode to detect holidays in coatings greater than 200 µm (8 mils) thick. By using multiple pulses of high voltage every second, the unit is able to:
- Use a trailing ground lead instead of a direct electrical connection to the substrate
- Operate on damp or slightly conductive surfaces
- Feature much longer battery life
High voltage Continuous Holiday Detectors operate at the same voltage range as High voltage Pulse detectors, but do not feature the benefits of pulse technology.
What type of Holiday Tester should be used for my application?
The correct instrument will depend on the thickness of the coating being inspected and the specified standard. Both the PosiTest LPD and the PosiTest HHD work on non-conductive coatings over a conductive substrate (such as steel, aluminum, other metals, and concrete).
The PosiTest LPD has a voltage output of 9–90 V and is suitable for coatings less than 500 microns (20 mils).
The PosiTest HHD has a voltage output of 5–35,000 V and is suitable for coatings greater than 200 µm (8 mils).
What are the Types of Holiday Detectors?
There are three main types of Pinhole/Holiday Detectors:
Low voltage Pinhole Detectors, such as the PosiTest LPD use a wet sponge to detect pinholes in coatings less than 500 microns (20 mils).
High voltage Pulse Holiday Detectors, such as the PosiTest HHD use pulse DC and either a spring or brush electrode to detect holidays in coatings greater than 200 µm (8 mils) thick. By using multiple pulses of high voltage every second, the unit is able to:
- Use a trailing ground lead instead of a direct electrical connection to the substrate
- Operate on damp or slightly conductive surfaces
- Feature much longer battery life
High voltage Continuous Holiday Detectors operate at the same voltage range as High voltage Pulse detectors, but do not feature the benefits of pulse technology.
What type of Holiday Tester should be used for my application?
The correct instrument will depend on the thickness of the coating being inspected and the specified standard. Both the PosiTest LPD and the PosiTest HHD work on non-conductive coatings over a conductive substrate (such as steel, aluminum, other metals, and concrete).
The PosiTest LPD has a voltage output of 9–90 V and is suitable for coatings less than 500 microns (20 mils).
The PosiTest HHD has a voltage output of 5–35,000 V and is suitable for coatings greater than 200 µm (8 mils).
How are Holidays Formed?
There are many possible causes of porosity or holidays in a coating system. Some of the most common include:
Under-coating — If an insufficient build of coating is applied over a blasted surface, peaks in the substrate can protrude through the coating and remain exposed to the environment. Use a coating thickness gage, such as the PosiTector 6000 to ensure the proper coating thickness.
Over-coating — Due to the way in which a coating cures, with the outermost area drying and shrinking above the wet area below, cracks can form if too much coating is applied.
Improper coating technique – Poor spraying or brushing technique can leave voids or gaps in the coating.
Non-ideal viscosity — A coating with a viscosity that is too low can sag or curtain, resulting in areas of very low coating thickness. A coating with a viscosity that is too high may fail to cover the entire surface, leaving voids.
Introduction of too much air during paint mixing — Coatings with trapped gasses can cause pinholes (bubbling).
Surface preparation — Ensuring that the surface profile meets job specifications can reduce the likelihood that an error in a coating formulation/application will create a holiday.
Surface contamination — Grease, dirt, or other contaminants such as salt can repel a coating and prevent it from adhering to the contaminated area. Ensure the surface is adequately cleaned and free of salt contamination (PosiTector SST Soluble Salt Tester).
Environmental factors — Application of a coating under less-than-ideal environmental conditions, particularly when the dew point is within 3°C (5°F) of the substrate temperature, increases the chances of holidays forming. Use a PosiTector DPM Dew Point Meter to ensure proper environmental conditions.