Coating Holidays & Pinholes: Chinks in the Armor
Defects in a coating film, however small, can ultimately leave the metal substrate vulnerable to corrosion, thus negating the purpose of the protective coating.
Traditionally, armor is considered only as strong as its weakest point, highlighting the significant dangers of any subsequent breach. A chink in one’s body armor poses life-threatening consequences, not unlike the vulnerabilities caused by a breach in a protective coating.
Within the coating industry, these defects are referred to as holidays, which are areas of the substrate that have not been properly coated, subsequently creating flaws in the coating film. Holidays include misses, inadequate coating thicknesses, and inclusions and pinholes, all of which register as anomalies.
Common Causes of Holidays
The occurrence of holidays can be attributed to applicator error or to a lack of knowledge in the coating application process. This includes knowledge of ambient environmental conditions, which can affect and overall hinder the application. (Be sure to read Defining Service Requirements & Environmental Factors for Coating Specification for more information.)
A common environmental cause of holidays is imperfect coating viscosity, which can undermine the success of the overall application. Extreme cold or hot temperature conditions can cause a coating to become either too viscous or too fluid, which makes it difficult to properly apply. Subsequently, the coating forms sags and curtains, ultimately leading to misses or skips within the coating structure. The addition of an anti-sag agent may help prevent sagging.
During application, the characteristics of the coating can also increase the likelihood of holidays occurring once the coating has been cured. Primarily, insufficient or incorrect mixing of the coating components can result in areas that won’t cure, leading to disbondment and other coating defects. Overmixing the coating prior to application can combine too much air with the product, creating a significant number of air bubbles.
Predominantly, pinholes are formed when air bubbles rise to the surface of the coating and burst, and are not resealed by the coating itself. These minuscule holes can be simply limited to the top layer of the coating, or continue directly through two coats, exposing the bare substrate.
When brush-coating a surface, applicators must ensure that two layers are applied. Following the base layer, a second coat must be applied perpendicular to the original, thereby covering over any potential holidays that are present. If this method isn’t used, existing flaws such as pinholes may not be covered.
Coating errors also occur in difficult-to-access areas where it is difficult to apply the coating evenly across the substrate. In perfect practice, applicators and coating inspectors should use mirror-access methods to detect and repair coating holidays and pinholes that occur in hard-to-reach locations.
Equipment and Methods of Holiday Detection
Since most coating defects cannot be visibly spotted, special inspection equipment and techniques are used to locate coating anomalies.
Essentially, coatings are insulative products that protect the conductive metal beneath. Holiday detectors use electricity to locate film anomalies by passing a current over the coating to create a closed circuit with the substrate beneath. An anomaly can be identified by using both high and low-voltage instruments; however, each must be matched specifically with the application, depending upon several variables.
For instance, environmental conditions must be suitable for effectively holiday testing the surface, especially with regard to the surface conditions. A dry and cured coating is essential for positive test results, otherwise trapped solvents or uncured areas still present within the coating can provide false readings.
Typically, either the low-voltage wet-sponge test or the high-voltage spark test is used, with each possessing distinctly different capabilities in terms of functionality. (See Tools & Tips: Holiday Detection on Coated Steel & Concrete Substrates for more information.) Obviously, the significant difference concerns the voltage of the two pieces of equipment; however, this does impact on the type of coating that each can be used for.
Manufacturers’ guidelines state that the low-voltage wet-sponge testers are only recommended for coatings with a maximum dry film thickness of 500µ (microns). For coatings applied at thicknesses exceeding 500µ, high-voltage detector equipment should be used (assuming that the coating is compatible).
Figure 1. High-voltage spark testing in action.
Another significant difference concerns how these instruments detect the presence of holidays. The low-voltage equipment utilizes water as a conduit for electricity to connect with areas of exposed substrate. In contrast, the high-voltage equipment is able to bridge the gap between the tester and conductive material, simply through the air.
There are also considerable safety differences, in that the wet sponge system operating at a low-voltage maximum of 90V cannot harm the operator or the coating; whereas if not operated correctly, the high-voltage version operating up to 60,000V can seriously damage both the operator and the coating being tested.
Common Problems in Holiday Detection
One of the biggest challenges in holiday detection concerns voltage settings and dielectric strength. Every coating possesses a certain dielectric strength, which determines the specific voltage that a coating can withstand before it begins to break down. In addition to setting the correct voltage, choosing between high and low-voltage testers is crucial when using this type of detection equipment.
Ian Wade, a seasoned NACE certified coatings inspector, says the speed of inspection can also adversely impact the coating. “When using the DC high-voltage spark tester, the inspector must not move the probe too slowly across the coating in a single pass, as this can create holidays. Per NACE standard SP0188, the minimum speed must be at least 1 ft/s (0.3 m/s). Similarly, with the low-voltage holiday detector, the user must ensure that full contact with the coating is continuous and not move the probe too fast or too slow,” Wade explained.
“Moreover, the user will have to ensure that they do not oversaturate the sponge, as this can lead to erroneous results,” Wade further noted. “Likewise, the same problem will arise if the sponge is not saturated enough (1 part wetting agent to 128 parts water). Check the grounding of the equipment by making contact on a bare spot on the substrate with the sponge and repeat periodically to verify grounding.”
Figure 2. The use of a surfactant improves the performance of low-voltage wet-sponge tests.
As highlighted, sometimes the pinholes are so minute that the passage of water from the low-voltage tester, through to the conductive surface underneath, is restricted. In these instances, applicators can use a surfactant to lower the surface tension of the water. With reduced surface tension, the solution is able to penetrate the pinhole and enables the current to find the coating defect.
Some types of coatings contain metallic fillers to enhance coating performance. The presence of metals must be acknowledged before any testing is conducted. When an inspector is working with metallic-filled coatings that may be semi-conductive, a low-voltage test method is preferred, such as the wet-sponge technique.
If the coating does not contain metals, high-voltage equipment can be used to determine if the applied coating has holidays or other defects.
Application techniques can be controlled by following specific coating guidelines, procedures and requirements. To achieve the best coating possible—void of potential defects such as holidays and pinholes—NACE standard SP0188 should be followed. (More suggestions are discussed in 5 Coating Defects That Can Be Avoided By Adhering To Coating Specs.)
As always, safety is paramount when using equipment of this nature. NACE guidelines include certain measures to protect the safety of the coating application and inspection personnel. Inclusive of electrical safety checks, when conducting high-voltage tests, it is necessary to carry out full assessments of the testing environment to ensure that there are no potentially explosive elements or conditions. Moreover, complete electrical grounding of the substrate is vitally important to avoid electrical shocks.
Restoring the Armor
Solutions for repairing holidays are completely dependent upon whether a particular coating can be renewed. If the lining is at the maximum coating thickness, then another coating cannot be applied on top of the existing holiday. In this case, the area around the coating is initially delineated with the edges feathered in toward the defect. Once completed, the area can be frost-blasted before a replacement coating can be applied.
On the other hand, if the coating is under the maximum thickness, another layer can be applied over the defective area. By simply roughening the surface prior to a successive application, the holiday can be covered and successfully eliminated.
After the repair is complete, another holiday test must be conducted in order to establish that the repair has been successful. Notably, the area of repair is the only place that should be tested, in order to prevent excessive stress on the coating’s integrity.