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"Score" marks on electropolished 316

2004

Dear sirs,

We recently installed an electropolishing bath consisting of a 100 gal. tank (316 stainless steel) heated to 65 degrees c.(149 f.). I'm not sure of the exact make-up of the solution, its made by a spanish chemical co. and is called e-262. For cathodes we are using 304 sheets of s.s.the power source is 500amp s.c.r unit. We installed this as the pieces we fabricate are stainless 316 custom one-off for the mega-yacht industry, usually of very complex shapes, almost impossible to polish manually. We have been delighted with the results obtained, however, where there are holes, or projections, or lips on the workpiece, we find deep vertical "score" marks, or ridges. This also occurs where faces are angled to face each other. This is the only electropolishing unit in the balearics, and information like this is very difficult to come by in spain generally, so I hope you may be able to help.

Many thanks,

Lincoln Berrange
technician-marine fabrication - Palma, Mallorca, Balearic Islands, Spain

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I am not sure if I am reading correctly, however....

Electropolishing is a funny process in that the current density has to be very high, and if it falls off you get etching instead. If these 'score' marks or 'ridges' are on low current density areas, they may be etching. Your scores may be 'trails' of hydrogen bubbles. It may not be easy, but it is possible to use additional anodes near these low current density areas to increase the current and get out of the etching range.

Unfortunately, it is also true that electropolishing can reveal rolling marks, strain lines, inclusions, etc. If this is the problem the solution is more difficult, although some electropolishing process chemistries may reduce the obviousness of those features. What you might want to do is to send a few troublesome sample parts to an established and well qualified electropolishing shop. That way you would at least know whether it is your technique that can be improved, or if the material cannot be electropolished without these marks.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003