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DIY - Electropolishing

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The_Apprentice

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#1
Yes, electropolishing, NOT electroplating.

I have been debating on finding a better way to polish my stainless pieces made on my mini-lathe. Using abrasives and compounds seems to be too much work. LOL

I have also been looking at different solutions for a DIY method instead of having to deal with other companies to do this for me. It seems the simplest method I've found explained is here:


I was debating on using this as a starting point.

I am curious what others with any expereince in electropolishing have to say here.

I have heard rumors if you can get sanding up to 600 grit on any piece, using e-polish will convert the finer scratches to a mirror. Well, I will believe it maybe when I see it.
 

hman

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#2
Back when I was a grad student (early '70s), we used to electropolish stainless steel vacuum system parts. Beautiful finishes, and it would indeed "clean up" sanded surfaces. Electropolishing preferentially removes high spots, so treated surfaces are smoothed out.

The downside (unbeknownst to me at the time) is that, because SS contains chromium, electropolishing can form the notorious "hexavalent chromium" compounds. Bad stuff, and difficult to dispose of legally. I'd urge you to do some research, and take all the proper precautions before undertaking this venture.
 

quickcut

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#3
Will this also work on aluminium ? Hope fully without toxins
 

RJSakowski

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#4
Back in my student days and later as an analytical chemist for a major battery company (1960's - 70's) we use chromic acid to clean laboratory glassware. There were two variations on the formula, one using potassium dichromate and the other using chromium trioxide.. They would be used to clean glassware until deemed ineffective and the spent solution would be dumped down the drain.

The latest standard for hexavalent chromium in drinking water is 10 ppb. How times have changed!

Given the electrolysis process used for electro-polishing, I would think it highly likely that hexavalent chromium would be formed. Deactivating the stuff in ppb levels is a challenge due to the dilution. Current trends are to remove it using ion exchange columns and treat the concentrate to reduce it to the safe trivalent form by reduction with ferrous sulfate.
 

chips&more

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#5
I wonder if blasting with walnut shells. Or some other media? Would be a good enough substitute?
 

RandyM

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#6
I wonder if blasting with walnut shells. Or some other media? Would be a good enough substitute?
I have for years, entertained vibrator cleaning and polishing. I think it is less harsh than blasting depending on the media used. Still thinking about it. ;)
 

chips&more

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#7
I have for years, entertained vibrator cleaning and polishing. I think it is less harsh than blasting depending on the media used. Still thinking about it. ;)
Yes, like a bullet casing vibrator. Not as fast a process as phosphoric acid though. But safer...Dave
 

cathead

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#8
Chrome bumpers on vehicles all but disappeared after the EPA was established back in the 70s. One can get exposed to
hexavalent chromium by welding on stainless steel or even by grinding carbide inserts.
 

The_Apprentice

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#9
Bad stuff, and difficult to dispose of legally. I'd urge you to do some research, and take all the proper precautions before undertaking this venture.
And this is particularly why I am looking into it deeper here :) I had seen only one person mention an issue with it earlier, so I was not fully sure if it had just been an alarmist.

If the DIY is that toxic to the environment, with no easy work-around, I suppose I could look into the larger companies here who do e-polishing for a price... I hear it's a growing field, but still I'd have to travel an hour away or two if I can't find someone even more local here. Will keep digging around.

If I am very lucky, it would be nice to not have to even bother with emery cloth again after turning down a cut on a lathe.
 

hman

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#11
Tony -
Here's a screen shot from the link you provided:
Screen Shot 2018-04-12 at 3.09.32 AM.jpg
Given the high rotational speed required to get "very high G-forces," and the masses involved (not just the parts and polishing media, but also the strength required of the barrels to resist the G-forces), I'd say it would be difficult to scale this process down for home shop use. Durn!
 

vtcnc

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#12
Tony -
Here's a screen shot from the link you provided:
View attachment 264776
Given the high rotational speed required to get "very high G-forces," and the masses involved (not just the parts and polishing media, but also the strength required of the barrels to resist the G-forces), I'd say it would be difficult to scale this process down for home shop use. Durn!
We have the HZ-12 in our plant. It's about a $15,000 unit. The barrels are about 3/8" thick wall steel containers and are lined with urethane. We run our stainless steel parts in these for about 45 minutes and large burrs and edges from stampings are completely gone and parts have a uniform finish.
 

RJSakowski

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#13
After I posted in April, I did some experimenting with hexavalent chromium (aka chromium VI or Cr VI). It is easily reduced to chromium III by the addition of iron filings. Chromium III (in small amount) is not harmful and is in fact an essential part of our diet. It is used as treatment for type 2 diabetes by enhancing insulin effectiveness.

If you have concerns about disposing of any solution which you feel may contain hexavalent chromium, throwing in a handful of iron filings or turnings and allowing it to sit for a few days should take care of it.
 

hman

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#14
Great news! Thanks.
... not that I plan to do any electropolishing in the foreseeable future, but it's always good to know that there's a safe, easy means of detoxifying the waste.
 

homebrewed

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#15
Hexavalent chromium is an extremely powerful oxidizing agent. I recall a demonstration of this during a fire training session at work. The trainer made a small pile of chromium trioxide crystals, then -- from a safe distance -- sent a stream of acetone at it from a squirt bottle. The acetone instantly burst into flame! This was done outside on a paved parking lot. The point being Cr+6 is very happy to go to a lower oxidation state, and that's why something like (fresh) iron filings works to reduce CR+6 down to something less toxic.

The demo was done back when regulations on things like Cr+6 were considerably looser than they are now. A demo using the same stuff just isn't happening these days.

Even now, chromium trioxide is a component of some types of metallurgical etches. Getting rid of the spent etch solutions costs more than the fresh chemicals. You can't store them in glass because many also contain hydrofluoric acid, which attacks glass. And plastic bottles exposed to it eventually develop pinhole leaks.....or worse.
 
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