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. One of the most obvious porosity issues would be pinholes - where the substrate is open to the elements through the pinhole. Pinholes are caused by air entrapment, which is then released from the surface, or by the entrapment of particulates (dust, sand etc.) which do not stay in place. Pinhole detection is therefore essential to monitor the integrity of the protective coating from its initial application, throughout its service life.
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. There are various techniques and equipment available for holiday detection.