A holiday is a leak or omission in the coated film, while a pinhole is usually a tiny hole in the coated film.These defects can lead to incomplete coatings, which reduce their protective effectiveness. Pinhole inspection is particularly important in cases where the coating is used for immersion service.Missed dot detection, pinhole detection, or spark testing can be used to find scratches, scratches, and pinholes in the coating film. Pinhole testing is particularly common when the coating is intended to be used for immersion service. Missed spot detection may need to be done after the penultimate or last coat of paint has been applied. Typically, this type of inspection is performed before the coating has dried sufficiently but has finally cured to ensure that the repair material has successfully bonded to the underlying coating.
Missed dot and pinhole Detectortype
Low-voltage wet sponge pinhole Detector
This type of Detector is used to find discontinuities in non-conductive coatings on conductive metal surfaces. It includes a Detector itself, a grounding cable, and a sponge electrode soaked in water. By moving the electrode across the surface, water allows a small current to reach the substrate through the pinhole. When the current reaches the substrate, the Detector emits an audible signal indicating the presence of a pinhole or discontinuity.
The pinhole test of the low-voltage wet sponge pinhole Detector can also be used to locate the conductivity of rivets, bolt threads, etc. Such tests should be followed by a visual inspection to confirm the presence of defective areas. The low voltage Detector is suitable for coatings up to 0.51 mm (20 mils) thick. The base unit consists of the Detector itself, the grounding cable, and the sponge electrode. The grounding cable is securely attached to the bare substrate and the sponge electrodes are soaked in saturated tap water. As the electrode moves over the entire surface, the water causes a tiny current to flow through the pinhole to the substrate. When the current reaches the substrate, the circuit is completed by the Detector and an audible signal is emitted to indicate the presence of pinholes or discontinuities. When the coating thickness exceeds 0.254 mm (10 mils), a non-foaming wetting agent (e.g., photo developer) can be added to the water to increase wetting performance. The compatibility of the wetting agent with the coating to be applied must be verified. If the coating system is found to exceed the 0.51 mm (20 mil) thickness limit, a high-voltage leak point detection device should be used. Pinhole testing has certain limitations. For example, in some cases, the Detector does not make a sound despite the pinhole not penetrating the substrate. Conversely, the Detector may mistakenly believe that there is a void because the coating itself has a metallic pigment or solvent that conducts electricity.
High Voltage Missing Coating Point Detector
This Detector operates on essentially the same principle as a low-voltage unit, but without the use of a sponge. It consists of a test cell, a grounding cable, and electrodes made of neoprene, rubber, brass, or steel. High-pressure leak point detection is frequently used in pipelines and other critical applications.
A high-voltage Detector consists of a test cell capable of producing different voltage outputs, a grounding cable, and electrodes made of neoprene, rubber, brass, or steel. High voltage units can be up to 20,000 volts and above. High-voltage leak spot detection is often required in critical applications such as pipelines. The electrode sparks from the coating through pinholes, missed spots, or omissions in the substrate and simultaneously triggers an audible and/or visual signal device in the inspection instrument. The rule of thumb for high voltage testing is 100 to 125 volts per inch. For example, a 1.02 mm (40 mil) coating will require a test voltage of 4000 to 5000 volts. Excessive test voltage may damage the coating film. Even if the test voltage is set correctly, sparks can penetrate thin, intact areas of the coating and create voids that must be repaired.
Attention!There are a few caveats that need to be taken when using any coating inspection instrument. For example, a Detector may make a sound because of the conductivity of the coating itself, when in fact there is no pinhole. In addition, excessive test voltages can damage the coating film and may even penetrate the coating and form voids that need to be repaired during the test.
