Premature corrosion of a substrate is usually due to a coating failure. A major cause is the presence of flaws in the finished coating, collectively referred to as porosity. Holiday test is used to detect holes, known as holidays or discontinuities, in a coating. Holiday testing allows the detection of even smallest coating flaws invisible to the naked eye
High voltage holiday test is normally used to determine the existences of discontinuities in coating films. 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. There are two types of high voltage holiday detectors- Continuous DC and Pulsed DC.