Electropolishing with citric acid
A discussion started in 2002 but continuing through 2017(2002)
Q. Does anybody have experience of electropolishing stainless steel and/or other metals using citric acid as one of the constituents of the electrolyte.Mark Ambrose
Electrosheen - Clevedon, North Somerset, UK
A. I haven't used the system, but I am aware of one with the following composition:
Citric acid - 55-60% by weight;
sulphuric acid - 15-20% by weight;
water - remainder;
Temp: - 83-93 °C;
Current:- 100-250 ASF
Apparently austenitic stainless steels can be run at slightly lower temperatures than ferritic stainless steel.
This recipe was published in "Surface Finishing of Stainless Steels", published by the Stainless Steel Development Association in London back in the 1960's.
I have periodically wondered how good it is, but suspect it has some major drawbacks because it doesn't seem to be in wide use now. With the environmental lobby both in Europe and the US, if it was as good as the sulphuric/phosphoric systems, I would have thought it would have superseded them ... However, I may be wrong!
R&D practical scientist
Chesham, Bucks, UK
June 15, 2017
Q. Hello, I found this thread probably is old, but I'm currently working in the implementation of electro polishing for SS 304 in my work, I did an internet search and bought the Electroplating Engineering Handbook as recommended in multiple occasions.
We initiated doing lab trails with a phosphoric/sulfuric acid solution recommended in the Metal Finishing Guidebook
Phosphoric acid 63%
Sulfuric acid 15%
We ran some parts and achieved good results.
Then we tried Trevor's suggested solution of citric acid and also got good results. I attached some pictures so you can see the samples.
Sample 1 phosphoric acid -- Sample 2 original part -- Sample 3 citric acid
I have one question about the cathode, should this be made similar to the form of the part or it could be in the wall of the tank as in the anodize process? The parts are cubic banquets.
- Chihuahua, Mexico
A. Thanks Yohands!
I've personally never seen conforming (shaped) anodes used for electropolishing. If anyone has, please chime in.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
June 20, 2017
A. I've seen lots of conforming cathodes used for electropolishing. Inside tubes, tight bends, inside corners, complex shapes.
Usually made of lead, just like hard chrome.
Jeffrey Holmes, CEF
July 6, 2017
I'm still working on the implementation of the electropolishing process; doing some testing. We electropolished a part that came up with several defects on the same part that you can see in the pictures (we use the citric acid solution):
Temperature 160-180 °F
10 minutes electropolishing
stainless steel cathode
current 100 Amps/sq ft
Probably not the exact concentration of the citric acid due to solidification in the bottom of the tank.
Im looking for some guidance maybe this defect also happens with the phosphoric process and the solution of the problem could be similar?
I will post the results after the implementation of your suggestion.
KAT - Chihuahua, Mexico
August 16, 2017
A. Hi, Rey..
You should add Tergitol 1 ml/l to the solution and decrease current density!
- Bekasi, Indonesia
October 23, 2017
A. Yohands Rey was asking about Citic Acid treatment of 304 Stainless Steel. This is a late reply because I just now came by and then saw this question. I had been looking for the same information so as to clean 304SS after using 75% Silver solder + flux paste. Here is my summary of info I had collected this past week from several metal-business and enameling websites (the caps are mine, for emphasis to myself, but I decided to just leave them that way), so I hope this is still useful!:
For the Austenitic Stainless Family (which includes 304 and 316) you would use 10% BY WEIGHT of Citric Acid in distilled water, heated to 150 °F - immerse part(s) for 20 minutes, to 30 minutes MAXIMUM. You must also make sure the surface is METICULOUSLY CLEANED and well-rinsed with *distilled* water, THEN passivate, rinse, and dry.
Laboratory tests have indicated that citric acid passivation procedures were more prone to "flash attack" (black pitted corrosion) than nitric acid procedures. Factors causing this attack included excessive bath TEMPERATURE, excessive immersion TIME and ESPECIALLY bath CONTAMINATION, which is why the metal must be thoroughly cleaned BEFORE the acid treatment.
- Richmond, Texas USA
November 2, 2017
A. It's an interesting question. I've had lots of people ask me if my citric acid product can be used for electropolishing. I always encourage them to give it a try and let me know. Only one ever came back to me with a good report, and he achieved it by adding sulfuric acid to the citric.
I'm not sure replacing a phosphoric-sulfuric mix with citric-sulfuric is any real advantage as far as reducing the use of hazardous acids. The sulfuric is just as bad if not worse than the phosphoric.
Stellar Solutions, Inc.
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