An Introduction to Pinholes and Holidays – Porosity Detection
Whether you are coating a pipeline or an oil tank; a ballast tank or a flat roof; a filing cabinet or an appliance - there are many parameters that must be monitored or controlled to ensure the coating performs over the lifetime of the product. One of these parameters is how well the coating has “covered” the surface, and if there are any “flaws” in the coating which could lead to a premature coating failure or corrosion.
Corrosion is caused by two things – a steel substrate, and oxygen, while contaminants such as water can accelerate the process. The coating is there to protect the steel from oxygen and contaminants. A flaw in the coating can leave the substrate poorly protected, or in some cases completely exposed.
In the protective and industrial coatings industry, there are a number of coating flaws to be aware of. These include:
Runs & Sags: Where the coating moves under gravity, typically caused by over coating and low paint viscosity, leaving thin areas of cover;
Cissing: Where the coating does not stick to the substrate properly due to surface contamination from moisture, grease, or oil; leading to surface breaks in the coating that expose the substrate;
Cratering: Where air or solvent bubbles are released from the surface when the coating has partially cured, and the coating does not flow to cover the void, leaving small craters that only thinly cover the substrate;
Pinholes: Similar to cratering, air or gas bubbles burst which create small or minute holes that the coating does not cover; but unlike cratering, pinholes go all the way to the substrate, leaving it fully exposed;
Over Coating: If too much coating is applied, the top dries out quicker than the bottom, creating a skin which then shrinks as the coating beneath it dries out, cracking the skin;
Under Coating: If a coating is too thin, the peaks of the surface profile are left uncovered or thinly coated, which leads to rust spots, also known as rust rash
These flaws are referred to as Holidays, Discontinuities, or Pinholes within the coatings industry; and are often very small or invisible to the naked eye – which is where flaw detectors come in.
There are, essentially, three flaw detection methods:
1. The Low Voltage Pinhole, or Wet Sponge Technique such as the Elcometer 270, is for testing insulation coatings less than 500μm (20mils) thick on conductive substrates, and is ideal for powder coatings and other applications where you do not wish to damage the coating. However, this method only detects pinholes that go directly to the bare substrate.
Pinholes are important to detect because they are a small hole which goes all the way to the substrate that acts like a capillary tube, drawing moisture drops down to the steel through capillary action – where liquid can flow through narrow spaces without gravity - accelerating the process of corrosion.
2. The High Voltage or Holiday Detection method is where a high voltage current is applied to a probe, which is passed over the coated surface.
The voltage used is dependent on the dielectric strength and thickness of the coating being tested, or the test method or standard you’re working to; but typically the voltage should be high enough so that in areas where the coating is electrically weaker due to a flaw, there is sufficient voltage to breakdown the gap between the probe and the substrate. When this breakdown occurs, the current flows through the substrate, and back into the unit via a grounding cable, setting off an alarm to signal a flaw has been detected.
This allows you to detect flaws that don’t go all the way down to the substrate, as well as voids within the coating.
However, it is vital that the voltage isn’t set too high, as this could breakdown and damage the coating, causing the unit to alarm when there are no flaws, and creating a flaw in the process. Testing with bright blue sparks is a clear sign the voltage is too high.
3. The UV Pinhole technique - such as the Elcometer 260, which makes use of a fluorescent coating which is applied to the surface as a base coat. Once the second coat is applied, any pinholes fluoresce when you shine a UV light on them, revealing their location