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Pits in polished surface

Pits in polished surface

Pits in polished surface

I am polishing concave optical surfaces in stainless steel
and have been experiencing problems with pits or pull outs on the surface.
I have looked into this in some detail and have found out that the problem lies in the material.
After having the metal anaylised, the metallurgist told me that the material contains what he descibed as duplex sulphides.
I was wondering if anyone could tell what duplex sulphides were and if there could be any proven link between these and the pits, or if anyone has any other explainations as i'm at a loss.

RE: Pits in polished surface

Very well polished surface have good corrosion resistence. But the inclusions in stainless steels can produce powerful micro galvanic cells in presence of corrosion medium. It will be reasonable to use better grades of stainless steels like AISI 304L or AISI 316L.

RE: Pits in polished surface

Sulphides occur as solid precipitates in some stainless steels.  They are easily etched and "attacked" by many of the surface preparation solutions used in metallography, so they might also be attacked by the solutions you are using in your polishing process.  Look at using materials with extremely low or non-existent sulphur in the metallurgy.  You might look at heat treatment as a means of reducing the presence of the sulphide inclusions.  Not sure if that will work in all materials, however.  

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