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March 5 - March 9, 2018

What is the correct gauge type and probe for measuring coating thickness of a super duplex stainless steel system with epoxy and polyurethane?

Selected Answers

From David Zuskin of Indepedent on March 16, 2018:
I suggest using a dedicated, non-ferrous probe versus the auto- sensing ferrous/non-ferrous probes.

From Tom Swan of M-TEST on March 15, 2018:
To add to the above answer, the reason the FN type gauges get confused is some of the lower grades of SS, especially out of China, have enough magnetism to fool the gauge into thinking it is ferrous, so the gauge tries reading in F mode. The N lock prevents this from happening. As an alternative, but slightly more expensive, some of the newer UT gauges will read coating thickness over metal whether the substrate is carbon steel or SS. The various carbon content in the SS will not effect the coating thickness readings.

From David Lemke of Team Industries, Inc. on March 14, 2018:
Regardless of the type of stainless steel (SS), a type 2 gage with a non-ferrous eddy current lock must be used. Then the gage must be zeroed using the same material (and blasted if that is how the substrate has been prepared) onto which the coating has been applied. All carbon content is not the same in the various types of SS. Even with the non-ferrous lock on, the gage will read different mil thicknesses from type to type of SS while the film thickness is exactly the same. I have had gages that are supposed to automatically select the type of material go bananas because of the amount of carbon in the SS and the gage didn't know what to do. Also, the carbon content in some of the duplex SS is not always evenly distributed in the manufacturing process. This will in turn show an erroneous (many mils off) reading and then less than an inch away show a reading that corresponds with the rest of the readings. As far as type of probe, there are different ways the probes are wired and the jury may be out. I have had discussions with one gage manufacturer and they were not sure which one works the best. Not a lot of research has been done with the different types of probes and which one works best with this or that type of SS. Our company has seen more SS and types of SS being coated than ever before and it will remain that way into the future. Most of the time SS was not painted in the past and taking DFT measurements on SS is going to be new for a lot of folks.

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Tagged categories: Coating / Film thickness; Coating/Film Thickness; Dry Film Thickness (DFT); Duplex coatings; Epoxy; Polyurethane; Quality control; Quality Control; Stainless steel

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