Holiday detection on Painted or Coated Surface

A coating must be applied as a continuous film to perform its intended function. Early failure will occur if there is a discontinuity—a holiday, as it is commonly called—such as a pinhole, void, crack, thin spot, foreign inclusion, or contaminant in the coating film. Many of these defects are not readily visible, but they can be located using holiday detection equipment.

Discontinuity:

A void, crack, thin spot, foreign material inclusion, or contamination in the coating film that significantly lowers the electrical resistance of the coating. May also be identified as a holiday or pinhole.

Holiday

Defects characterized by a film having areas of insufficient thickness, even to the point where parts of the surface may remain uncoated.

Holiday Detector: A device that locates discontinuities in a coating film applied to a conductive substrate.

Pinhole

The formation of minute hole in the wet film or drying due the burst of gases or solvents.

Holiday Detector is a portable quality control inspection instrument used to electrically locate defects in protective coatings applied to a conductive substrate. This electrical testing will locate the number of discontinuities in a coating film. The detector converts low voltage direct current (rechargeable batteries) into regulated high voltage pulses.

  The machine is used to detect pinholes, voids and areas with thin paint films. The machine gives a light or signal when pinhole or weak point is detected. This is a destructive test method for detecting pinholes, voids and thin spots in paint film because during pin hole is detection, the gap previously filled by mud, dust or thin film of paint will be pulled up and pin hole will be created on that area.

This standard is intended for use only with new coatings applied to conductive substrates. Inspecting a coating previously exposed to an immersion condition could result in damage to the coating or could produce an erroneous detection of discontinuities due to permeation or moisture absorption of the coating.

To prevent damage to a coating film if a high-voltage spark tester is being used, the total film thickness and dielectric strength of the coating system shall be considered in selecting the appropriate voltage for detection of discontinuities.

 The coating manufacturer shall be consulted to obtain the following information, which can affect the accuracy of the tests described in this standard to determine discontinuities:

a.  The length of time required to adequately dry or cure the applied coating film prior to testing. Solvents retained in an uncured coating film may form an electrically conductive path through the film to the substrate.

b.  Whether the coating contains electrically conductive fillers or pigments that may affect the normal dielectric properties.

This versatile unit can be used to inspect protective coatings applied to pipelines, tanks, pilings or any similar conductive surface.

Need For Holiday Detection

Protective coating failure can result in corrosion, or other deterioration, of the underlying material or substrate. Possible problems include the formation of corrosion product such as rust or pits and chemical attack. The resulting repairs of the coating and the inability to use the affected pipes, equipments and vessels can be very expensive.

Often, failure occurs due to the presence of flaws in the finished coating. Typical flaws are pinholes (a very narrow hole running from the coating surface to the substrate), holidays (small uncoated areas), inclusions (objects trapped in the coating, e.g. grit from blast cleaning), air bubbles, cracks and thin spots. Therefore, it is good practice to inspect a coating for defects and flaws, following established guidelines or procedures.

Working of Holiday Detector

Holiday detectors are instruments that use electricity to locate film discontinuities. Most coatings are poor electrical conductors, and so they act as insulators. On the other hand, a metallic substrate such as steel is conductive to electricity.

A holiday detector consists of a power source, a ground wire, a probing electrode, and an indicator.

To holiday test, Spring electrode will be connected on the coated pipe. Ground cable will be connected to the uncoated part of the pipe. When the power source is on, the current flows through the spring electrode and ground cable to complete the circuit. In case of holiday or pinhole on the coated pipe, the current circuit will be completed, an audible sound or visual sound is activated on the holiday detector.

 

Some coatings, especially zinc-rich primers, are excellent electrical conductors because of the conductive nature of the zinc particles, and so holiday testing cannot be performed on them. However, if a topcoat of a non-conductive coating such as an epoxy or urethane is applied over the conductive primer, the holiday test would be valid. In this case, the holiday test would not distinguish between a discontinuity that reaches only to the primer and one that reaches to the substrate.

 

If you are ordering Elcometer 266, you will be provided with broom type probe along with the kit. If you want to use Rolling Spring type, we must order it separately. For the usage of Rolling spring, we need to order other parts also:

a) Phosphor Bronze Rolling Spring Holder

b) Probe Extension Piece of desired length.

Note: Please refer User Guide of Elcometer 266 for more information