Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Tools & Tips: Holiday Detection on Coated Steel & Concrete Substrates

By Tom Swan
Published: May 30, 2019
Key Takeaways

A coating inspector offers insider tips on using low voltage and high voltage testers to detect coating pinholes (holidays) on painted metallic substrates and on nonconductive concrete.

Source: Tinker and Rasor

I'm often asked questions about holiday testing or, as the Europeans call it, porosity testing. Of course, pinholes in a newly applied coating can leave the substrate exposed to the environment and vulnerable to corrosion. It is the coating inspector’s job to detect and document these defects in the coating. (Get a copy of our Coating Failures and Defects Guide.)


There are two main categories of holiday detectors: low voltage (sponge) testers and high voltage (JEEP) testers. According to almost all of the coating standards, you may use a low voltage tester for coating thicknesses of up to 20 mils (500 µm). If thicker than that, go to a high voltage tester. High voltage testers can be used down to about 10 mils, but I generally do not recommend it if you can use the low voltage tester.

Using Low Voltage (Sponge) Testers

Sponge testers are relatively simple to use. Ground them to the surface and run the damp (wet but not dripping) sponge over the surface. While the coating standards limit the use of sponge testers to 20 mils coatings, I have run a sponge tester side by side with a high voltage tester on coatings up to 50 mils and have found the same holidays.


On thicker coatings (10 to 20 mils), use a surfactant or wetting agent to make sure that the water doesn’t "bridge over" the holidays due to surface tension.

Here’s another trick not found in any of the standards that I use when holiday testing small areas such as the inside of a valve: Grab the sponge tester in one hand and use your other hand with a damp finger to find any pinholes. On some models you might get a small shock, but you are an inspector so you can take it. Note: This tip is only for a low voltage tester.

A sponge tester generally costs $375 to $450 for a battery-powered detector. ASTM standards require 67.5 volts. ISO standards use 9 volts or 90 volts. Several low voltage testers on the market have user-selectable voltages that will do all three voltages.


High voltage testers are generally in the $2,800 to $5,000 range and the features can vary widely. It is best to work with someone knowledgeable when purchasing one to make sure that you get the model and features you want. Most high-voltage testers will top out at 35,000 to 40,000 volts, but 90% of users will rarely need over 10,000 volts. (For more on tool selection, see How to Select the Best Value Monitoring and Testing Tools.)

Choosing a High Voltage Tester

There are two types of high voltage holiday testers: direct DC and pulse types.


Direct DC is generally less expensive and works well on dry coatings. Because the DC tester puts out a constant current as it goes over the surface of the coating, it can impart a current into the coating that will back-feed to the wand causing detection of a "false" holiday. There is a sensitivity knob on these meters to adjust the milliamp setting that will trigger a "beep" from the meter. For most coatings, the default setting will generally work; however, if you are getting "beeps" (or Jeeps) from the meter without a spark, you may need to change the setting.

Pulse-type meters are more versatile in that they can be used in damp environments even with moisture condensation on the coatings. The electrical pulses are generated between 20 and 60 cps. Each electrical pulse is "on" for a time period of 20 to 200 microseconds. They do not require the sensitivity knob. While the pulse-type meter has the advantage of being able to work in damp conditions, both types of meters will work under most circumstances with proper training.

Tips on Testing Coated Concrete

Using a high voltage detector on steel or conductive substrates is generally straightforward. When it comes to testing for holidays on coatings on concrete, it can get a little tricky.

I do not have any experience in very dry, desert environments, but generally there will be enough moisture and salts present in concrete to make it conductive. When testing pipelines, inspectors sometimes just drag the ground cable along on the ground since both soil and concrete conduct electricity. But a proper earth ground is always best.

To create a proper ground for holiday testing concrete, I generally recommend the following steps:

  1. If available, ground the detector to rebar in the concrete.
  2. If rebar is not available, steel is often bolted directly to the concrete. You can ground the detector to the bare steel.
  3. If testing a slab with no steel rebar, drive a piece of rebar into the ground to at least the depth of the slab and immediately next to the slab. It helps if you make sure that the soil is wet. Use this as your ground.

An alternative grounding method is to place a 2"x2" piece of ordinary metallic window screen wire flat upon the concrete surface. Place wet sandbags over the entire metallic surface and connect the ground wire to the screen wire. The wet bags placed upon the screen wire ensure intimate contact of the screen wire against the concrete surface. This grounding method is usually sufficient for either low voltage "wet sponge" testers, or high voltage "spark-type" holiday detectors.

Check the electrical ground of the detector by touching the brush to the bare concrete substrate and observe the audible signal. No audible signal means inadequate grounding and a better ground must be obtained or the signal sensitivity increased.

As with metal, first follow the specs; next, go to product data and application bulletins; and then go to the coating manufacturer.

If you still do not have a voltage setting, do not use the settings recommended for steel. The best way is to find several areas representative of the coating to be tested at varying distances from the ground. Create a holiday and determine the voltage necessary to create a spark, twice the distance across the coating as the coating is thick.

For example, if the coating thickness measures 100 mils, the spark should "jump" 200 mils across the surface then down to the substrate. If the resistance increases as you move away from the ground, you will need to increase the test voltage or, preferably, move the ground point.

Several concrete coating manufacturers are now making conductive primers. By using one of these primers, you can follow the procedures for holiday testing metallic substrates. However, since they cost more than standard primers, they are seldom used unless a holiday-free surface is critical.

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Written by Tom Swan | Manager, M-TEST

Tom Swan
Thomas Swan is Manager of M-TEST (Montipower–Test Equipment & Specialty Tool), a distributor of coating inspection, welding inspection and non-destructive testing equipment. He has been involved in the corrosion industry since 1977, specializing in chemical processes, water chemistry, and paints and coatings.

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