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Will Vibrofinishing Close the Pits Created in Pickling of Stainless Steel?

Q. We do pickling of SS 304 components. The components are similar to pins having dia. approx. 3 mm. Pickling makes the surface white (similar to aluminum), but we can observe microscopic pits due to the pickling process when we observe under 16X. Is there a method of removing or closing these pits, maybe by vibro tumbling? If so what should we put in the vibro along with the components? Please note that after pickling we do barrel polishing but these pits do not close, hence can we polish in vibro machines to polish the components and at the same time close the pits? Any suggestion will be welcome.

Greg Warne
- Mel, Australia

A. I don't like the idea of trying to correct a problem with a secondary operation when the first process failed. I'd try to correct your chemistry. Mass finishing will only be able to modify what already exists. Pits, to me, indicate a deep depression and to even out these recesses is an SOB without a lot of time, material, and effort. It can be done.

tony kenton
AF Kenton
retired business owner - Hatboro, Pennsylvania

Q. Thanks for your response. I am not sure what chemistry you are referring to, is it chemistry of SS 304L which we use or of the pickling solution. Can you suggest a pickling solution and time whereby a batch of 5 - 10 kgs can be pickled without creating micro pits. Also you have said that mass finishing can close the pits or even them out. We have a 300 litre vibro bowl. What media can we use to mass finish and polish these components

Greg Warne [returning]
- Australia

A. Hmmmmm. Well, without seeing the parts, I'd go to the largest, coursest ceramic media possible and just run the h--- out of it. This will eventually leave your parts looking like a piece of galvanized metal. You didn't say anything about tight tolerances or the final appearance as being critical. There is also a chemical accelerator and media that might also work.

tony kenton
AF Kenton
retired business owner - Hatboro, Pennsylvania

A. Hi Greg. I agree with Tony's first response. You can't over-pickle components to the extent of creating pits, and then try to close those pits with tumbling. Further, I don't think he implied that closing the pits was practical, but emphasized how difficult it would be.

However, processing problems like these are tough to solve in the abstract, so asking what the proper pickling solution concentration and immersion time should be probably won't work. Rather, please tell us what pickling agent you use, at what strength and for how long, and hopefully someone can suggest an adjustment that will offer enough pickling activity to get the job done, but not so much that it pits the material this way. Please believe that pitting your stainless with the pickling solution is not the right approach. Good luck.


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

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