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topic 4795

Electro-polish - 'Comets'


My name's Nick Matthews and I would appreciate some help on the following problem we are experiencing:

We have our stainless steel (316L) tube units (vary in diameter between 600 and 1600 mm in diameter) electro-polished by a sub-contractor. Occasionally we observe small imperfections on the electro-polished tube wall which can be best described as 'comets'. i.e. they have a small head from which emanates a straight 'tail'. The head is approx. 0.2mm and the tail is typically 5mm in length. The tails are all in the same direction, as if they are 'escaping' from the surface and rising to the surface of the electro-polishing tank.

Has anyone else experienced this problem and more importantly identified the cause ?


Nick Matthews
- Glos, UK


Book knowledge rather than 'hands-on', but I think this may what is called "gas streaking" due to excessive current density in that area. Certainly a closer look would be needed though before insisting that this rather than poor mechanical polishing is the real culprit.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


Dear Nick, I used to run a lot of Electropolish of 304 303 types of stainless, our problem was much as your talking, In that scenario,all though the Manufacture was buying 304 /303 type of stainless it was not "pure" enough, back then they called it electropolish grade, we sometime would get these "comets" in straight streaks or misc depending on how this material was formed. The reason for the "tails" the pit or head of the "comet" is were the contaminate was at its most concentration and did not mix thoroughly

Chris Snyder
plater - Charlotte, North Carolina


Hi Nick,

It seems that due to an locally high concentration of one of the SS316L elements, that area is more active then the other. The comet shape you described is a result of gas bubbles formed at the active area.

You may try to reduce voltage (current) or circulate the acid so gas bubbles will evacuate rapidly from surface.

By the way, which polishing acid do you use?

Oded Nissan
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