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What is that black layer on the stainless steel surface?

Hi,

I am working in a metal etching company, got a question to ask and hope someone can tell me. There's a difference type of Stainless Steel like 301, 302, 304, 316 and 430 etc. After the metal been etched with ferric chloride [on eBay or Amazon], the 301 stainless steel surface covered with a layer of black fine powder, but not on 430. Can anyone tell me what is that black powder thing.

Thanks a lot.

Albert Tsang
- England, U.K.

A complete guess here! The black stuff is most likley carbon as the ferric does not dissolve carbon.

Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England

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