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Improving our electropolishing of 316L junction boxes

We electropolish our junction boxes after annealing to remove oxides layers. We use grade 316L for boxes fabrication and our electropolishing bath contain H2SO4 & H3PO4. I want to know what actions-reaction happen in the bath and parameters that affect, to improve this process.

Thank you for your help.

Ghasem Babaki
explosion proof junction box - shahsavar, Mazandaran, Iran
2003

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2003

It's obviously difficult to suggest improvements without extensive details of what you are doing, Mr. Babaki, and it takes a whole chapter in a book to briefly address the reactions, but ASM's Metals Handbook Volume 5, the Metal Finishing Guidebook, and the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] all have such chapters. The AESF (NASF) has an "Illustrated Lecture" on electropolishing.

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

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2003

Dear Sir

I have received your letter and decided to describe our processes. We use 316L to manufacture boxes and their doors. Boxes are cubic that are open in one face and be closed by doors. First by deep drawing we form a box shape then for stress relieving we shall anneal them at 900 to 1000 centigrade. For removing oxide layer that is made during the heat treatment we immersed them in electro polishing bath for nearly 40 min and then rinse and exposure to open air for a while. The bath contains H2SO4+H3PO4 (at 400-900 Amps that depend to the size). We can not achieve uniform quality for all the boxes that belong to same batch. There is a sign like rainbow in some area while other sections are bright. Sometimes some area is snowy. By the way, is the passivation that is acquired in open air , suitable to protect corrosion? Shall we use nitric acid for passivation? May we use citric acid for this purpose replace of nitric acid?

Thank you for your consideration.

Regards.

F.Farzan
quality control - Shahsavar, Mazandaran, Iran

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Electropolishing is a better passivation process than passivation itself; after you have electropolished your parts, you are done--you do not need to passivate them again in either nitric or citric acid . The "snowy" areas are probably what I would call "etched"; there are several possible causes, including excess water in the solution, but if the etching is in low current density areas, the current density is probably too low in that area. You may need auxiliary cathodes to reach into the boxes to maintain current density in the inside. I don't know the cause of the rainbow effect, but hopefully another reader will. I still feel you need to get a book or two about electropolishing.

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

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Electropolishing 316L gives you a very excellent surface and corrosion resistance. Passivation after electropolishing gives you the best surface and corrosion resistance. If your application is just ornamental the electropolish is good enough, but if you need an orthopedic or semiconductor finish you will normally need to passivate afterwards to get the chrome oxide ratios required in these industries. I have heard some people claim that they know how to get the high chrome oxide ratios with just EP, but I have not seen it.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2003

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2003

I want to know how I can measure our bath contents. I mean a chemical method to indicate composition of the mixture in the bath. I want to determine what percentage of H₂SO₄ and H₃PO₄ are in the finishing bath in each check.

thanks for your help

regards

P.S titration by methyl orange [on eBay or Amazon] and phenolphthalein [on eBay & Amazon] are suitable?

farzan
- shahsavar mazandaran Iran