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December 16 - December 23, 2013

What is the risk of damaging a paint film by inspecting a repaired area more than twice with a high-voltage holiday detector?



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Selected Answers

From Tom Swan of M-TEST on December 19, 2013:
If the coating is the specified thickness and if the proper voltage is used, the number of times that you do a high voltage test should not be significant in maintaining the structural integrity of the coating. If the voltage is set too high or the coating has thin spots, then you may have a problem. I have been in several situations where the testing has been done more than once and no problems occurred. Now, if the first person does the testing at 10Kv and the second person decides that maybe 15Kv or 16Kv is better, you may have a problem.

From William slama of International Paint/Ceilcote Products on December 18, 2013:
The purpose of high voltage holiday detection (spark testing) is to detect holes through a lining or coating. The holes (holidays) can then be repaired so that a corrosive environment cannot find a free passage to the substrate. This is done by imposing an electrical stress across the lining to a conductive substrate (usually carbon steel) and using a voltage such that a spark will easily arc if a hole exists through the lining.

 In order to ensure that an arc will form, the most typical recommendation is to use approximately 100 volts per mil based on the nominal thickness of the lining. This insures that there will still be enough voltage to jump an air gap at the maximum lining thickness. Both alternating and direct current testers accomplish the same result. It should be noted, however, that in some cases AC will produce more corona and may give erroneous readings, particularly if there is surface contamination.

 Because this process puts a relatively high electrical stress on the lining and since the lining has a finite dielectric strength, areas where the lining is thin or where a void exists but does not go all the way through will usually break down electrically, causing (burning) a hole all the way through to the steel, resulting in a visible strong spark. When you spark test, in addition to finding existing holes, you will actually make new holes where the dielectric strength of the lining is exceeded. For this reason, spark testing should be done only once, after completion of the lining (or area) and before immersion or contamination of the lining. Where equipment is lined prior to final erection, it is advisable to spark test only after erection so that any lining damage due to handling will be discovered at that time. Continued lining cross-linking, temperature cycles, and mechanical stresses of the substrate can change the lining's dielectric strength without having any actual pinholes (holes through to the substrate).

Actual chemical or water exposure definitely has been shown to reduce the dielectric strength of a lining/coating. High voltage inspection has caused serviceable, functioning linings to be burned through repeatedly, simply burning new holes in an otherwise good lining. Repeated spark testing can burn new holes in a monolithic lining. References: NACE RP0188-90, Standard Recommended Practice for Discontinuity (Holiday) Testing of Protective Coatings; ASTM D4787-93, Standard Practice for Continuity Verification of Liquid or Sheet Linings Applied to Concrete Substrates; ASTM D5162-91, Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coating on Metallic Substrates.

From Parviz Babayev of Caspian Pipe Coating on December 17, 2013:
High voltage may destroy the integrity of paint film. As a result, the coating loses its isolation property.

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Tagged categories: Holidays


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