Electropolishing Stainless

[rwm]

I am making some geometrically complex constructions out of 304. Polishing after welding is a challenge due to the geometry. I thought I would try electropolishing.
My test setup failed miserably. Here is a piece of hex after a few minutes in the bath:

As you may see, the exposed area actually got duller with an etched look. The original is shinier.
My solution was about 45% phosphoric acid. I ran a voltage of about 3v initially with the work piece on the + electrode. When that looked poor I uped the volts to 5v without improvement.
What am I doing wrong here? Anyone have experience with this?

Robert

 

[homebrewed]

From an earlier thread on electropolishing (found here), it sounds like some form of mechanical "scrubbing" is needed to get a polished surface. Does the geometry of your construction prevent that?

 

[rwm]

Maybe I should have continued that old thread? I tried this by immersion which is something I saw on YT. The geometry would make any mechanical polishing difficult. Some people are using a 12v charger. Do I need a higher voltage?
Robert

Edit: I tried 12v. No improvement.
R

 

[rwm]

I found this but I can't say it makes sense:


Always annoying to me how people think they can make up their own abbreviations! Note the voltage is in Astronomical Units. I have no idea what current "density" is. Could that be current divided by surface area?
Robert

Actually this is better info with the same curve.

R

 

[hman]

"a.u." might possibly indicate "arbitrary units." Nevertheless, without numbers to go by, both of those charts are just about useless.

Way back in grad school I used to electropolish stainless steel parts for ultra-high vacuum systems. The usual method was to wrap some cloth around the "hot" electrode, dip the cloth in the juice, and run it slowly across the surface. I did use the immersion technique a time or two, but most of the parts were much too large to immerse. Wish I could recall the voltages and current levels, or even what kind of power supply we used. Always got a very nice, shiny surface on stainless. Theoretically, the process preferentially attacked the high spots of the surface. Smoothing them down made the surface more even.

I do recall that the solution was a pretty deep green (possibly from dissolved chromium salts), and if I ever got any on my Levis, I'd have a nice hole next time I washed them PPE was almost unheard of back then, especially for (expendable) grad students.

 

[homebrewed]

Etch/plating results are often heavily affected by physical conditions like bath temperature, agitation, aeration and brushing. Having said that, I don't have any direct experience with electropolishing. Your best bet is to continue your online search to look for more specifics regarding current density (amps/square inch, cm, foot etc.), bath conditions and type/concentration of chemicals. Typically you adjust the voltage to get the current density you want, so the actual voltage really depends on the item(s) you want to polish.

The quality of electroplated surfaces is affected by small amounts of additives -- surfactants, chealating agents, and so on, so it is possible that some as-yet-undetermined additives may be important when it comes to getting a nice, shiny electropolished surface, too.

Some preliminary searching did find this article, which does provide some specifics regarding current density. It also suggests that simple immersion + anodic polishing works, once the proper etch conditions are present. The Wikipedia article about electropolishing indicates that the bath chemicals (primarliy sulfuric and phosphoric acids) must be concentrated enough to significantly increase the viscosity of the solution, to promote diffusion-limited polishing action (which helps to preferentially remove high points). One thought that immediately occurs to me is the possibility of achieving the same thing with less nasty chemicals by adding something friendlier which would simply increase the viscosity -- perhaps polyvinyl alcohol or the like. Maybe even a concentrated sugar solution could do the job.

No matter what approach you use, you will end up with some nasty chromium compounds that need to be neutralized before you dispose of them. Hexavalent chromium is particularly bad, but I think that cobalt salts can be almost as nasty. @RJSakowski mentioned the use of iron filings to take care of that, in the earlier thread I found.

 

[rwm]

I tried this again at 2v and ran it for about an hour. I got no polishing effect. I did get a nice gray satin finish similar to bead blasting. This would be good to clean up weld discoloration but not for polishing. My conclusion is that electropolishing is a scam and is not really used in industry. All the photos are faked. They probably have overseas child labor doing the polishing so they call it "electropolishing!"
My solution is pretty viscous compared to water so I don't think that is an issue. I could add a surfactant but I doubt that is the issue. I could try straight 85% H3PO4?
Robert

 

[hman]

Perhaps some of the info in this article will help. See Table 1 for electrolyte composition, voltage and current density:

http://www.nmfrc.org/pdf/psf2003/030308.pdf

Note that they suggest voltages up to 18V and current densities between 46 and 465 amps per square foot for stainless.

 

[rwm]

Thank you! I can easily add some sulfuric acid. I can try a higher voltage. My work piece is rather small and hard to figure the surface area but I will try.
Robert

 

[hman]

I think your "figure of merit" will be voltage. When we were electropolishing in grad school we never gave a thought to current. The figures in the chart are probably there to let you know how large a power supply you might need. But as you said, yours is a very small part. So amperage should not be any kind of problem.