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302 Stainless Steel Etching

Please could you tell me what would be the best way to measure the thickness of the oxide layer on 302 stainless steel. Also I would like to know what may cause brown tarnish, surface pitting and a blue residue (visible under the optical microscope) after etching with ferric chloride ⇦this on eBay or Amazon [affil links] .

Thanks for any help.

Pete Anniss
alloys - Kingsbridge, Devon, UK
2001

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Two methods are commonly used for measuring the thickness of oxide films on stainless steels - Auger electron spectroscopy and x-ray photoelectron spectroscopy. These techniques perform very surface sensitive chemical analysis concurrent with ion sputtering to slowly remove material from the sample surface. A plot of surface composition versus depth from the surface is obtained, from which the oxide layer thickness can be determined. I am assuming that you are seeking to measure the thickness of the typical oxide layer for passivated stainless steel - not for a thick oxide coating.

Larry Hanke
Minneapolis, Minnesota
2001