About Holiday Detection

Holiday detection or Pinhole detection is an important testing method used in coating industry. A 100% testing carrying out in all the coated surface can reduce the risk of corrosion happening due to holidays.

Lets see what is a Holiday. Holidays are an exposed area or discontinuity within the coating film where part of the substrate remains uncoated. On a day to day coating activities some few examples of holidays are Craters, Pinholes, Fisheyes, Cracks, Missed areas etc. These holidays / tiny voids / gaps can lead to premature coating failure as they expose substrate to the atmosphere. The presence of holiday completes the corrosion cell by allowing an electrolyte to make contact with the surface. Means it leads to corrosion.

Mainly there are two methods of detection as Low Voltage and High Voltage Holiday detectors. Before going into detail, we can have some idea about how this works. Holiday detectors operate by forming an electric circuit when an exposed portion of substrate is detected. These detectors works only on non-conductive coatings that are applied to a conductive substrate. For easy understanding, we can put an example. Epoxy coatings applied to a steel can be tested for holidays, while a zinc-rich coatings applied to same steel could not (zinc is a conductive metal in the coating). Holiday testing can be done on both steel as well as concrete coatings.

The type of holiday detector (Low / High) used is based on the total thickness of the coating system. As a standard practice, coating below 500 microns (20 mils) are checked with Low voltage and above that are done with High voltage detectors.

Low voltage holiday detectors are sometimes called Wet sponge holiday detector. They are using the water (tape water) to find the holidays. Surfactants or Wetting agents can be used to increase the sensitivity of test. The inspectors can met with a phenomenon called 'Telegraphing', which is chances of getting false readings because of oversaturating the sponge with water. If any holidays are found it has to repaired before proceeding to next stage.

High voltage detectors are sometimes called Spark testers. It uses a high voltage and care should be taken while operating these detectors. Commonly two types of high voltage detectors are used, Constant current (DC) and Pulsed direct current (DC) models.

The standards referring holiday detectors are NACE SP0188 and ASTM D5162.

This is a brief about holiday detection. Try to carryout these testing for getting a better coating life.