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April 24 - April 28, 2017

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

Selected Answers

From Ben Mitchell of Coating and Lining Inspections. on July 5, 2017:
I spent more years than I care to remember manufacturing and servicing high voltage porosity detectors. Over those years I have performed countless soak tests on units which involved leaving a unit turned on while sitting on a coated test plate, usually at more than ten time the recommended voltage for that dft. Quite often, the testing was performed over one shift (8-10 hours), but soak tests of several days were not unheard of. Add to this that the test plates were also holiday tested thousands of times a year and regularly exposed to maximum output voltages. I don't recall ever seeing a holiday created on a test plate from this testing except when intentionally running extreme voltages over thin coatings. Anyone following recognized standards to match the test voltage to the dft would not see the same result. In my experience the coating has never been damaged by multiple passes of the porosity detector. Multiple passes is more likely to find a greater number of coating defects, particularly when testing in the field, due to factors such as poor condition of brushes/coils, skipping over defects and operator issues such as speed of movement and and lack of overlap. The catch to this is that, yes, some coatings in particular are susceptible to damage from high voltage porosity testing but these are generally coatings that aren't really suited to this test in the first place, such as those with conductive pigments or additives, but I am sure the manufacturers data would specify if this is the case anyway.

From Warren Graves of Warren Graves Coating Consultant inc on April 30, 2017:
I mistakenly identified the IEEE as IEEA and used the wrong descriptors for the organization. IEEE is the Institue of Electrical and Electronic Engineers. I apologize to the members.

From Warren Graves of Warren Graves Coating Consultant inc on April 29, 2017:
Holiday testing, when done according to industry standards, has been done on multiple millions of square feet of pipe and tank linings without damaging the coating, including repaired areas. Generally speaking, epoxies have a dielectric strength of 500 volts per mil so if the repair was done properly, to the corrrect thickness, then it should be as good as the original application. ASTM D5162 allows 1,500 volts for 8 -12 mils and as much as 25,000 volts for 201 -250 mils. None of these voltages will damage the coating. And none of the standards actually prohibit going over the coating a second time as long as the electrode is moving. They only state that you perform a single pass when attempting to detect a holiday. I asked a manufacturer of holiday testing equipment if they had done any tests for breakdown of coatings. His answer was that they have applied 36,000 volts to a piece of pipe coated with FBE (16 - 20 mills) for 24 hours with no effect on the coating. The IEEA (International Electrical Engineers Association) test materials, including epoxies, for their resistance to dielectric breakdown. One study showed that after 20 years, there was almost no effect on the epoxy in electrical service when compared to a recently applied epoxy in similar service. Finally, has Mr. Schomburg every tried to see any spark, or the optical effect of his suggested use of OAP, when testing pipeline coatings outdoors on a sunny day.

From Gordon Cameron of GSC PAINTING INSPECTION LTD on April 28, 2017:
There is a likely risk of breaking the film of the coating, and you could affect the dielectric strength of the coating.

My suggestion is to use primers with OAP (optically active pigments) in their formulation and forget the destructive holiday detector.

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Tagged categories: Holidays; Meters and gauges; Paint defects; Quality Control; Quality control

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