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Speckled Granular appearance when Electropolishing Cast 316 (CF8M)

I recently electropolished some cast 316 parts (CF8M)for an application in the architectural field. The outcome was quite unexpected. Instead of a nice shiny finish, the part exhibited a speckled granular appearance. Although its not what we were looking for in this application, it might be attractive in others. Has anyone ever seen this? Does anyone know how this happens? Does this change the corrosion resistance or strength properties of the material?

Any help would be greatly appreciated.

Michael Tardie
mechanical engineer - Westford, Massachusetts
2007

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Yes we have seen this before. Particularly on cast 304 or 316 marine cleats and anchors. These usually come in with a mirror polished finish.
If this mirror-polished cast stainless is then subsequently electropolished it will "expose" the crystalline surface (depending on the amount of time in the EP tank) It is a very neat looking finish and some customers actually like it. We have never experienced a product being returned due to corrosion resistance failure of the exposed crystalline surface.
I would say no it does not compromise the structural integrity.
The simple answer to "how" this happens is as stated above the electropolishing removes a minute amount of the surface layer to expose the crystallized cast metal beneath it.
I will try to post some pictures and would like to hear Bill Reynolds scientific more in depth explanation. He would probably better explain how? metallurgically this happens.

Cliff Kusch
electropolishing shop - North Vancouver, British Columbia, Canada
September 24, 2009