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Black crust forming on stainless steel?

2006

My son is doing a science project on the corrosion of stainless steel in certain liquids; when put into bleach ⇦ in bulk on eBay or Amazon [affil link] a black crust is forming on the screws.

can any one tell us what it is?

Thanks

Steve W.
school project - Boynton Beach, Florida, USA

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2006

Is this an '18-8' stainless steel such as 304?
I.e., a non-magnetic stainless.

If so, it contains nickel and I will venture a guess that the black material is nickel oxychloride, NiOCl, perhaps in hydrated form.

Another possibility is magnetite, Fe3O4, which can be attracted to a magnet.
Let us know if this helps or you have additional info.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
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