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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 ⇦ bleach/sodium hypochlorite in bulk on eBay or Amazon [affil links] 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

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.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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