Holiday testing is a non-destructive test method applied on protective coatings to detect unacceptable discontinuities such as pinholes and voids. The test involves checking an electric circuit to see if current flows to complete the circuit. This testing is used to find coating film discontinuities that are not readily visible.
The testing is usually performed on tank interiors, chemical storage vessels and buried structures because of the importance of maintaining adequate coating protection in aggressive service environments.
Holiday testing is also known as continuity testing.
There are, essentially, three flaw detection methods:
UV Pinhole Detection
UV light can be used as a low cost, quick method of detecting pinholes in coatings. A base coat containing a UV fluorescing additive is applied. When the UV flashlight shines on the coating, areas where the base coat is not covered fluoresce, identifying the location of the pinhole.
Wet Sponge Technique
A low voltage is applied to a moist sponge. When the sponge moves over a coating flaw, liquid penetrates to the substrate and completes an electrical circuit, setting off the alarm. The wet sponge technique is suitable for measuring insulating coatings less than 500μm (20mils) on conductive substrates, and is ideal for powder coatings and other coatings where the user does not wish to damage the coating.
High Voltage Technique
The high voltage, or porosity technique, can be used to test coatings up to 25mm (1") thick and is ideal for inspecting pipelines and other protective coatings. Coatings on concrete can also be tested using this method.
A power supply generates a high voltage DC or pulsed DC to a probe. As the probe passes over a flaw, a spark at the contact point sets off the alarm.
This technique is suitable for locating the types of flaws described above, although care is required on thin coatings.
The high-voltage, or porosity technique, can be used to test coatings up to 1" (25mm) thick and is ideal for inspecting pipelines and other protective coatings. Coatings on concrete can also be tested using this method.
A power supply generates a high voltage DE or pulsed DC to a probe. As the probe passes over a flaw, a spark at the contact point sets off the alarm.
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