What are Holidays in Coating?
Holidays in coating are, when coating any steel substrate there are some missing areas left un-intentionally or inadequately coated or there are some inclusions that are found when testing with the holiday detector such as, discontinuities, pinholes and holidays. These are coating terms or coating defects.
Pinholes or holidays are so minute that they cannot be seen with naked eyes, hence testing machines are required.
These pinholes go all the way to the substrate, they draw moisture causing gradual corrosion.
When any holidays are detected the machine will alarm and a beep sound will be heard.
How to Detect Pinholes or Holidays?
There are two types of holiday detectors such as Low Voltage Pinhole Detector also known as Wet Sponge Detector and the other one is the High Voltage Detector.
Holiday Testing Machine – Holiday Testing Voltage
Low Voltage or Wet Sponge Holiday Detector
The dual voltage detector can be set to 9V for testing thin coatings up to 300 microns (12 mils) thick.
The 90V, for coatings up to or less than 500 microns (20mils)
And the triple voltage detector can also test at 67.5V
Low Voltage – Holiday Detector – The Recommended Ones
When coming to Elcometer pinhole detector the highly recommended one is Elcometer 270 which provides both visual and audible alarms when any defect is found.
It is supplied in either dual or triple voltage
The next one is Tinker-Rasor Low Voltage Wet Sponge Holiday-Pinhole Detector
And the other one is PosiTest LPD Low voltage Pinhole Detector
High Voltage Holiday Detector
The high voltage continuous DC (Direct Current) is typically used to test the pinholes or porosity of non-conductive coatings.
It uses high voltage greater than 800 volts of electricity to discover discontinuities of a thickness that is between 500 microns (20 mils) applied over conductive substrates.
The voltage is set based on coating thickness since this is a destructive test. This tester is also commonly known as Spark tester as it gives spark when discontinuities are found when testing.
High Voltage – Holiday Detector – The Recommended Ones
Highly recommended holiday detectors equipped with the latest technology making them both efficient and easy to use are
– Elcometer 236 High Voltage Holiday Detector
– Elcometer 266 High Voltage Holiday Detector
– Elcometer 280 Pulsed DC Holiday Detector
– Spy High Voltage Holiday Detector
Holiday Detector Test – How do you perform a holiday test?
Holiday detectors will only work on non-conductive coatings over a conductive substrate.
Refer to the following Industry Standards:
– ASTM D5162
– ASTM D4787
– NACE SP0188
Low Voltage – Procedure
The ground cable is attached directly to the steel substrate to be tested.
The sponge is saturated with distilled water or as per manufacturer recommendations.
Before applying the sponge to the substrate, compress the sponge to get rid of the excess water.
Before beginning the testing, check or test that the machine is running okay by touching the wet sponge probe onto the substrate.
Pass the sponge over the substrate of the dry coating at approximate rate of 0.3 m/s (1 ft/s) using a single pass by beginning from top to bottom.
The wet sponge is the inspection probe that passes over the coated surface. The excess water gets drawn down to the pinhole and creates currents. This current goes down to the equipment and creates a beep indicating a pinhole.
Remember this works only on thin coatings between 500 microns (20 mils) applied over conductive substrates.
When any holidays are found mark the area with chalk or an approved marker.
These marked areas shall be repaired as per the project specification and re-inspected.
Low voltage holiday testing is carried out on thin coatings, powder coatings and other applications.
Low voltage holiday detectors are used where you do not wish to damage the coatings.
A low voltage detector not greater than 90 volts is applied on a damp sponge.
The machine consists of a battery and the battery produces current and is connected to a large wire.
High Voltage – Procedure
High voltage testing uses electricity to detect any defects in the coating. It will check for any discontinuities in the coating film by going all the way into the substrate such as pinholes, fisheye, scratch, low film thickness and any other flaws etc.
The test is carried out by using brass wire brushes, steel wire brushes, Carbon impregnated steel wire brushes and rolling springs.
To carry out the test firstly, ensure the battery is charged fully, if the battery is low then erroneous results will occur. Also, if the probe is adjusted too high the results will not be accurate either.
Do not pass the probe too fast as it will miss the smaller defects or holidays.
Review the required voltage setting requirements by contacting the paint manufacturer and by referring to standards.
The rule of thumb for voltage settings is 4volts per micron.
Whilst testing connects the ground wire to the structure, select the right electrode by attaching the probe with a power wire. Turn the tester on, test first by touching against the ground and then pass the electrode over the substrate of the dry coating at approximate rate of 0.3 m/s (1 ft/s) using a single pass by beginning from top to bottom.
When any holiday is found instant beep will be heard, mark the area to repair and to re-test before backfilling.
Holiday Testing Paint
Coating depends mainly on the environment of that area to which the piping surface would be exposed hence paint is selected based on this criteria.
The underground terrains mainly consist of dissolved salts and moisture resulting in a severe corrosive atmosphere as compared to above-ground conditions.
The underground (UG) is better protected by thick coatings or rather a high-performance coating hence, Fusion Bonded Epoxy (FBE) 3-Layer Polyethylene (3LPE) are better suited for underground coating.