Electropolishing of aluminum and stainless steel

A. Many electropolishing solutions contain phosphoric and sulfuric acids. Various formulas are given in the in the Metal Finishing Guidebook article on Electropolishing

There are many vendors with proprietary solutions; this is a good way to begin as you get the correct solutions and procedures for your particular alloy(s). Also, they will advise as to the most suitable alloys and run sample parts. Click the Chemicals link below, then search for electropolish. At least one of these vendors has a non-environmentally hazardous product.

A. I would like to recommend the following formulation to you: Glycolic acid 55%, sulfuric acid 30%, water 15%. The formula is free of phosphate and absolutely environmentally friendly. Of course you can obtain excellent result.

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Ed. note: It is kind of Herbert to address the readers as 'advisor', but we don't want people to be misled. This is NOT a consultation service; it's a public forum where everyone is expected to give as good as they get. Every reader knows something, and a great way to learn is to try to teach ... so we're waiting to hear your answers as well as your questions :-)

A. Things are, I'm sure, different in Brazil than the U.S., Herbert. But the idea of running a process like electropolishing in a garage is a dangerous one. There is a tendency to think that the selection of the exact electropolishing formulation is the key to success, but it's not. So I'd try Benlogo's suggestion.

Pieter, if you don't find the guidance you seek in the previously mentioned article in the Metal Finishing Guidebook, there is a great chapter on electropolishing in the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] . Voltage has very little to do with anything at all, but maintaining amperage in the 'polishing range' rather than the 'etching range' is a critical element. Good luck.

Merlin, to my knowledge carbon steel cannot be electropolished.

A. There are many different electropolishing solutions for stainless steel. The most extensive conveniently available list is in The Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] . There are also electropolishing chapters in the Metal Finishing Guidebook and ASM Metals Handbook, Vol. 5, Surface Engineering. The AESF (NASF) has an illustrated lecture on electropolishing. Dozens of formulas have worked, but if you focus on finding the best magical chemical formula, you are misunderstanding the subject and are therefore unlikely to achieve success.

A. Dear Mr Shoney,

Sorry for not replying after visiting your factory on 6th January.

My concern with suggesting a chemical process for brightening and passivating SS is that your products are used in Surgeries and chromic containing chemistries that are carcinogenic; are not a good idea according to me.

I would suggest that you continue your mechanical buffing, cleaning operations with its difficulties and conclude the process with an Ultrasonic Cleaner. Maybe anyone else here may suggest a safe process.

adv.
There is an alternative to chromic-acid-based electropolishing. Non-acid saline based electropolishing will polish to mirror finish 300- and 400-series stainless steel. There will be no need for additional passivation step.

A. 40% sulfuric acid, 60% phosphoric acid
So 11 Volts
The object to electropolishing is anodic-- For cathode use Stainless steel 316L 1:1
Distance between electrodes 2 inches.
For hanger use copper.

Time 7 minutes or by the quality of electropolishing.

Best regards

This formulation comes from the excellent book Metal Finishing Guidebook and Directory.
Phosphoric acid 40%
Lactic acid 46.5%
sulfuric acid 13.5%
The cold phosphoric and lactic acids are put into the tank first and then stirred to mix. Lastly the sulfuric acid is added and stirred. Heat is generated and the temperature will rise to about 65 °C.
sulfuric acid and lactic acid must not be mixed first because there is a chemical reaction which destroys the lactic acid and generates too much heat. 10% water is acceptable and reduces the tendency for the mixture to char and become black.
60 - 90 °C operating temperature is suitable. About 9 volts is typical.
The reason for using this formulation rather than the sulfuric phosphoric mixture is that I found that the high sulfur content of free machining stainless steels is less liable to be pitted. Metal removal rates are low and the polish is excellent. Maintenance does not require chemical analysis of the composition, just replace drag out with new solution.

A. For optimum, it is going to depend on the alloy and exactly what you need. Several books have good suggestions.

"Electroplating Engineering Handbook"
by Larry Durney

on AbeBooks

or eBay or

Amazon

(affil links)

I agree with James completely. I have seen straight sulfuric acid used in a very capable high production shop although a substantial mix of phosphoric acid is more common, and there are dozens of mixes. But a starting point is to really understand what electropolishing is about, and a great article on the subject is in the Electroplating Engineering Handbook .

It is problematic to try to learn the 'right' formula and the 'right' operating conditions, without much knowledge of things practitioners should probably understand about it, like the need for a clean surface, the problems of water accumulation, getting from the 'etch' range to the bright polishing range, why the parts probably won't get as bright as mechanically polished parts, etc.

Regards,

A. Read this...
http://www.euro-inox.org/pdf/map/Electropolishing_EN.pdf

This was of immense help when I was finding out the requirements for an electropolishing plant

A. Hi folks. There's a lot of good info on this page, but there has also been great progress in the last several years on "no dump" extended life electropolishing chemistry, and processes which use less toxic chemistry. Read the suggested books, articles, and websites, etc. ... but make contact with the proprietary vendors like Russamer Lab [a finishing.com supporting advertiser] to learn about what's new before making final commitments.

Regards,

? Hi Puneet. What material are you electropolishing and what solution are you using? With that information, a guess might be possible. But these days there are "never dump" solutions, and a number of proprietary approaches as well.

Regards,

A. Hi Muliana. Electropolishing of aluminum is probably not a good hobby, and electropolishing of "metal" might not even be possible, depending on the kind of metal. You can read about the topic in textbooks and get a general introduction to it, but for motorcycle parts it will probably be cheaper, safer, faster, and much less to learn if you simply mechanically polish them, which is what "everybody" does. Please start with letter 800, "Aluminum motorcycle polishing -- rims, wheels, swingarm, frame". Good luck.

Regards,

A. Hi Adnan. I'm not familiar with that particular formulation, but dozens of different formulations have been successfully used, and I have no reason to think that your formulation would be unworkable when so many similar ones work. Where did you get the formula?

No part of your item polishes? Does it etch or does nothing happen? What current density are you using? Have you been electropolishing successfully and it suddenly stopped working, or are you just beginning your research and have never successfully electropolished anything before? Are you sure the surface is clean to the point of waterbreak-free?

If you can get access to the previously suggested Electroplating Engineering Handbook, it has an excellent chapter on electropolishing. We'd like to help, but if our response is limited to just suggesting a different formulation, odds are strong that it won't help much.

Regards,

Usually available on eBay;

sometimes on
AbeBooks or Amazon

(affil links)
free pdf is currently available from academia.edu

A. Hi again cousin. I run this site and am somewhat familiar with most of the technologies we discuss; but I do not pretend to be heavily versed in all of them. Thus, I think 304V stainless steel is simply 304 which has undergone an extra vacuum purification, so I don't think it should electropolish any differently ... but I don't actually have any experience to make that claim with confidence. PH-17 is a precipitation hardened stainless, and my understanding is that it therefore requires blasting with glass beads or aluminum oxide before electropolishing.

As I've said countless times on this page, numerous solutions have been successfully used, and I don't feel there is a "best", but this letter includes many postings with formulations, and letter 46041 includes another which the people posting them felt were best or at least good. As mentioned, the Electroplating Engineering Handbook and the Metal Finishing Guidebook have additional formulations. Books may be hard to come by in your area, but simply dismissing the need for books is no good at all. You can find still more in patent databases, such as www.google.com/patents/US2334698
Do not consider perchloric acid in any electropolishing formulation; it is tragically explosive.

"Etching" is undesirable in the sense I was using it, and often indicates that the current is too low or the water content too high.

I don't know what you mean by '7 AMSI'. Maybe you mean 1 Amp/ sq. in. for 7 minutes? The current density must be high enough, probably need at least 1 Amp/ sq. in., maybe twice that, or you will get etching.

It is difficult to function without a good library of metal finishing books and without a proprietary solution vendor to assist you, but at least download the Metal Finishing Guidebook. Good luck.

Regards,

A. Adman

I would try lowering the current density. Maybe even as low as 2 amps per square inch.

A. Adnan

Yes, try electro polishing at 2 amps per square inch (290 asf) or even a little lower. Your 7 amps per square inch could be near the upper limits. Then try in between to find what works best. Time will depend on the starting condition of the surface. Rougher surfaces will take longer than smoother but you will still get a shiny finish first. Smoothing requires longer times for more metal removal.

Hi Luvex. At least 4 electropolishing formulations are already printed on this one page, with references to authoritative sources for additional information, and suggestions that you consider proprietary formulations, and that you skip chromic acid. Is there a particular reason that you want to use a mix of sulfuric, phosphoric, and chromic acids even though you don't know a suitable ratio?

There is a lot to the topic, and I suggest you do some preliminary research into it before falling into an assumption that an exact ratio of three acids is the key to electropolishing--dozens of different formulations have been used, and I've already mentioned that 100% sulfuric - 0% phosphoric - 0% chromic is one such workable possibility. If you don't have books on hand yet, you can start with the on-line version of the Metal Finishing Guidebook. Best of luck!

Regards,

adv.
I would like you to inform that we have managed to electropolish to mirror finish (no burrs whatsoever) on small pieces of aluminum alloy. So far we do not see any limitations on what aluminum type we cannot electropolish. We are still in the test mode, and are experimenting only on small parts so far.

I think this electropolishing method is suited mostly for high-value parts, for example in electronic industry or airspace, where the level of deburring is extremely important.

A. Hello Mahesh. There are modern proprietary formulations of "never dump" chemistry and other higher tech solutions. But if you want to stick with simple mixes of mineral acids: there are few that absolutely won't work. There are a dozen listed in the Electroplating Engineering Handbook, and I have even seen pure sulfuric acid used in a high production electropolishing shop. Your proposed mix is widely used and accepted, but it is important that you use concentrated sulfuric acid because even fairly small amounts of water in the solution can cause etching.

Regards,

A. Hi again, Mahesh. Sorry, but I don't know the answer to that, nor even whether that is an accepted mix. But a good starting point is the books mentioned on this page. The EEH offers lots of patent numbers you can search. If you have no city or university library with metal finishing books accessible to you, and you can't find enough info in the on-line version of Metal Finishing Guidebook which we previously suggested, then please use a free patent search like
https://www.google.com/patents/ to search for "stainless steel electropolishing solution" or other terms which suit your needs. Best of luck.

Regards,

A. Hi Rajiv. If you're not using DI water for your final rinse, you probably should. It's also possible to add corrosion inhibitors .

But if this wire rope is being stored under the same roof where chemical processing is done, it can be a very corrosive environment. Good luck.

Regards,

A. Hi Dana. The 1st Edition was 1955, the 4th (and current) Edition was 1984. I don't recall ever seeing a 2nd or 3rd Edition, although one of them apparently was 1971 :-)

Sorry but it has been so long since I've seen the 1st Edition that I don't remember whether it has the Electropolishing chapter. The 4th Edition definitely does -- I just checked mine.

Regards,

A. 75% phosphoric/25% sulfuric is the usual formulation It may work better for you

  Jeffrey, many thanks for your guidance will update you in due course.

Best Regards

John