PhotoS: Courtesy of TNEMEC COMPANY, INC.
This article is a follow-up to “Are You Ready for a Holiday? Industry Misconceptions Around Testing for Discontinuity,” which was co-written by the author and published in the September 2022 issue of JPCL.1 The original article covered the theory behind holiday testing, the basics of performing holiday testing and the issues with industry standards based on published testing in a recent peer-reviewed paper. In this article, the author discusses how high-voltage holiday testing standards are currently being updated and the assessment of any risks associated with these updates.
As a refresher from the original article, discontinuity testing—commonly referred to as holiday testing—is the process of using specialized electrical equipment to detect and identify discontinuities in a coating or lining system. This specialized electrical equipment utilizes an electric current to identify areas where the conductive substrate is exposed to the atmosphere. It is critical that coating systems that are placed in highly corrosive environments be free of discontinuities as those areas can rapidly corrode and lead to premature coating failure.
High-voltage holiday testing works by utilizing high voltages (~3,000 V to 35,000 V) to initiate dielectric breakdown of the air gap between the equipment probe/brush and conductive substrate while not causing dielectric breakdown of the coating. This test method only finds areas where the coating is missing, and air is present. These are known as holidays. Local variations in coating thickness and imperfections in the electrode (brush) can require the use of voltages above the theoretical dielectric strength of air. The previous article discussed a series of experiments performed to find the practical voltages required to detect holidays consistently, at a range of coating thicknesses. These experiments and associated findings led the way to the updating of NACE SP0188 and other industry standards.
A new version of NACE SP0188 was published on January 26, 20242. Unlike most revisions of AMPP/NACE standards, this revision of SP0188 includes multiple significant changes to the use and content of the standard that users and specifiers should be aware of.
In the Rationale, new wording was added that notes the reason for the major updates:
“Significant shortcomings were seen in the field when this standard was utilized, resulting in many coating systems passing the high-voltage testing standard but failing prematurely in the field. A peer-reviewed paper was presented at the AMPP Annual Conference + Expo 2022 that provided test data and utilized Paschen’s Law and the breakthrough voltage of air to provide additional data for this.”
Updates to the Significance and Use for the high voltage testing section notes background information on the operation as explained above:
“High-voltage holiday detectors operate at an electrode voltage that is higher than the dielectric strength of air at the measured coating thickness, but lower than the dielectric strength of the coating. In this manner, electrical breakdown (and an alarm on the high-voltage holiday detector) will not occur when the electrode is in contact with the coating but will occur if there are any areas where the coating is missing and only air is present.”
One of the notable changes is that the standard now reflects that high voltage holiday testing can be used for coating systems with a dry film thickness (DFT) down to 10 mils (250 microns). Previously, the standard recommended that high voltage holiday testing only be used for systems with a DFT above 20 mils (500 microns).
The most significant change to the standard is the change to the recommended voltage settings. The previous recommended voltage suggestion table was based on anecdotal evidence instead of scientific reasoning. The new Voltage Setting reflected in the revised standard is based on the findings as presented by the original article’s authors at the AMPP 2022 annual conference in San Antonio3. The work by the authors studied over 200,000 holiday tests on various coatings at thicknesses from 10 mils to 400 mils and concluded that the experiment voltages closely matched the trend of Paschen’s law within a range of tolerance.
The revised voltage settings can be found by multiplying the dielectric strength of air calculated using Paschen’s law by 1.5 times plus 1,500 V. This modification to Paschen’s law allows for the accounting of irregularities in the electrode probe (brush), substrate, and environmental conditions.
The voltage setting of the new standard is recommended to be calculated based on average dry film thickness (DFT) in accordance with ASTM D7091 or ASTM D6132 or in accordance with SSPC-PA 2 or SSPC-PA 9. The following equation outlines the new method of calculating the testing voltage:
V=1500+ 1.5*[170+2.48d+58√d] (microns)
V=1500+ 1.5*[170+63d+293√d] (mils)
Where:
V = The test voltage (volts)
d = the coating thickness
In Appendix A of SP0188-2024 there is a reference table with pre-calculated voltages for a variety of coating thicknesses to allow for quick reference as needed.
Since the new voltage settings have increased significantly compared to the previous voltage settings, statements were added to the NACE SP0188 standard to warn users of the risk of damaging the coating system if the dielectric strength of the coating is exceeded by the testing. While some specialty and highly filled coatings may have lower dielectric strength, common coatings should not be affected by this concern. With that said, let’s look at the potential for damaging common high performance coatings and lining systems when using the new recommended voltage settings.
As coatings and linings are never perfectly uniform in thickness, some areas may have lower DFTs and therefore lower dielectric strengths. This can introduce a risk of damaging the thinner areas of the coating system if voltages are set for higher thicknesses that are found elsewhere on the coating system. A discussion regarding the theoretical aspects of dielectric properties of materials and their dependence on material thickness is a prudent starting point.
Previous assumptions in the coatings industry led to the belief that the dielectric strength of a material has a linear relationship with the thickness of the material. While this may be true for materials or air gaps with thicknesses/distances greater than 1 meter, it is not a valid statement for the thinner thickness ranges that are traditionally found with protective coatings. Instead, in most cases, as a coating decreases in thickness, the dielectric strength (V/mil) of the coating increases.
A quick review of some material properties proves this point. By combining data from a master’s thesis by John Harvey Gustafson on “The dielectric strength of some anti-corrosive paints” with different dielectric strength values of 12 industrial epoxy and polyurethane coating systems, we are able to get a good picture of the average dielectric strength for a range of industry protective coatings4.
Figure 1 shows the dielectric strength of several epoxies and polyurethane coating systems applied over a range of thicknesses. Using power trending, we can estimate the breakdown voltages for the materials across the range of thicknesses that will be tested. Breakdown voltage is the measured point where an electric circuit is completed due to the material/coating breaking down and becoming a conductor. While this trend provides a method of analysis for the “average” material, it is acknowledged by the author that other materials will have varying degrees of dielectric strength.
Fig. 1: Dielectric strength of various epoxy and polyurethane coatings at a range of applied thicknesses.
Table 1 displays the estimated “Burn-Through Thickness” based on the SP0188 voltages determined from a system average thickness. For example, for a coating system that has an average thickness of 20 mils assessed with average dielectric strength properties, there is a chance to burn through the coating if a low spot of 4.4 mils is found during testing. While this may sound concerning, coating experts would advise a location with a 4.4-mil coating thickness will lead to premature failure and issues, to the substrate; so, the breakdown of the coating and subsequent detection of a holiday in this location may be warranted.
Table 1: Estimated “Burn-Through Thickness” (per SP0188 Voltages)
In summary, there have been significant changes made to the NACE SP0188 standard, “Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates.” The most significant change that users need to be aware of is regarding the recommend testing voltages for the high voltage section.
While many users may have concerns with the increased testing voltages, a review of dielectric properties of 13 protective materials provides assurances that there is a very low risk of damaging compliant coating systems while performing high voltage holiday testing. If there is concern on a project, consultation with the coating manufacturer for further information on the coating properties should be completed. If it is concluded that testing voltages are to be decreased due to the unique properties of the selected coating system, it must be noted to the owner that there is an increased chance of not identifying existing holidays due to the lower voltages and that it may decrease the coating system service life.
The author hopes that the update to NACE SP0188 and other industry standards to testing voltages will help ensure consistent discontinuity detection and improved coating system longevity. As a member of the coatings industry, readers can help implement these changes by ensuring that the latest version of NACE SP0188 is specified and used while spreading the word about the changes to your colleagues.
Cameron Walker, Corrosion Program Coordinator
ICE Dragon Corrosion Inc.
Cameron Walker is a Corrosion Program Coordinator for ICE Dragon Corrosion Inc., a consultancy specializing in corrosion management plans and programs in Toronto, Ontario, Canada. Walker has extensive experience with coating specification, applications, inspections, selections and failure analysis. He is currently coordinating and building a best practices corrosion management program while supporting corrosion engineering efforts. He is a NACE-certified Coating Inspector (Level 2) and holds a Bachelor of Applied Science and Masters of Engineering in Chemical Engineering from Queen’s University.
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