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Galv coating: Correlation between Elcometer reading and microscopic thickness measurement

We use Elcometer thickness gauges to measure galvanized coatings on our steel tube and pipe. These are meticulously calibrated using the plastic shims provided by the manufacturer against a ground, flat mild steel test piece.

The problem is we seem to get constant under-reading on our Elcometer galv coating thickness measurement. When compared to acid strip results and microscopic analysis a coating reading between 55 and 70 microns are in effect closer to 100 microns. This causes and increase in zinc consumption as we try to meet the specs.

I read the Australia Galv Association has published some guidelines on this:

Calibration curves for corrected readings and additional information on these important non-destructive testing methods are available free of charge from Galvanizers Association of Australia.

My suspicion is that the gauge is "fooled" by the varying iron content in the different layers, resulting in a thinner reading than the actual coating thickness. Anyone had a similar problem? Is there a simple fix?

Thanks!

Francois de Villiers
Galvaniser - Johannesburg, South Africa
September 27, 2011

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September 27, 2011

Sir:

I published an article in Metal Finishing (Parts I and II) which consisted of a few hundred thousand measurements by 20 or so magnetic thickness measuring devices. The best results were using "banana gauges" manufactured in the USA and Germany. I suggest you get a copy of my article and your question will be fully answered.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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Ed. note: "Magnetic Thickness Gauges & Standards", March and April 1994.

Thank you Dr Cook. I have tried buying pdf's from www.sciencedirect.com but it seems their Metal Finishing archive only goes back to 2006. Is there another way to obtain these articles?

Francois de Villiers
- Johannesburg, South Africa
September 30, 2011

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September 30, 2011

Hi, Francois.

I can't speak for libraries in South Africa, but here in the USA any mid-sized library has arrangements with the major lit search companies to obtain such documents for you.

Or search google scholar for the document. I see a couple of services from which it can be obtained. It was $12.80 on one that listed prices (www.refdoc.fr/Detailnotice?cpsidt=11206258&traduire=en). Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Thank you Ted! I found the articles at refdoc and had them delivered to my mailbox a couple of minutes later. You just have to believe the www is one of the modern wonders of the world.

Francois de Villiers
- Johannesburg, South Africa
October 12, 2011