What Does Holiday Test Mean?
A holiday test is an inspection method used to detect discontinuities in painted/coated surfaces using specialized tools and equipment. These tools, called holiday detectors, are portable devices that are swept across the coated surface.
Holidays tests work on the concept of electrical conductivity. Metal substrates are excellent conductors of electricity, and therefore allow current to flow through them. On the other hand, many coatings are poor conductors of electricity and resist the flow of electricity. Using this principle, holiday tests use instruments to locate flaws in anticorrosive paints and coatings.
During holiday testing, a ground wire and probing electrode are attached to the same power source. The ground wire is clamped to the specimen being tested while the probe is swept across the surface of the metal substrate. If the probe comes into contact with a coating discontinuity, the exposed metal completes the electrical circuit between the electrode and the grounding wire, resulting in a flow of electricity. This electricity shows up on an indicator, alerting the equipment operator of the defect.
While holiday tests are effective, they do possess limitations. Because these tests depend on the coating being non-conductive, they are not effective on conductive metallic coatings, such as zinc-rich primers.
A holiday test is also known as a continuity test.
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Corrosionpedia Explains Holiday Test
In the coating industry, a holiday is a general term for various types of coating discontinuities, such as:
- Runs and sags
These defects affect the coating’s thickness, thus revealing enough of the underlying metal to complete the circuit between the electrode and the grounding wire.
Types of Holiday Testing Devices
There are two main types of holiday testing devices: low voltage and high voltage holiday detectors.
Low Voltage Holiday Detectors
Low voltage holiday detection, also known as the wet sponge technique, is used to test relatively thinly coated substrates. They are effective for coating thicknesses of 500 microns or less. This detection method is suitable for identifying pinhole defects only.
During low voltage holiday detection, the electrode is connected to a damp sponge, which is swept over the tested surface. The ground wire, which is connected to the same power source as the electrode, is clamped to an uncoated part of the substrate.
The pinholes act as capillary tubes that pull moisture from the damp sponge down to the bare metal surface. Because water is a conductor of electricity, the circuit is completed, and the indicator alerts the operator of the defect.
High Voltage Holiday Detectors
High-voltage holiday detectors, as their name implies, operate using higher voltages than their low-voltage counterparts. This higher voltage allows them to detect holidays in thicker coatings (greater than 500 microns). The electrodes of these testing devices are composed of copper wires or carbon-embedded runners as opposed to sponges.
Like low-voltage detectors, the high-voltage electrodes are swept over the coated surface. The presence of holidays completes the electrical circuit, which triggers the alarm on the indicator. These devices also feature adjustable voltages that can be set depending on the coating’s thickness. The thicker the coating, the higher the voltage needs to be to detect the defect.
Caution needs to be exercised when testing holidays with high-voltage devices because excessive voltages can damage the coating. Furthermore, high voltages can cause a current to flow even through properly coated areas, resulting in false readings.