electrolytic rust removal

[pcmacd]

Hey guys. I know this is seriously off topic, but I also understand that there are many chemists that haunt this site.

I am attempting to remove rust from old military rifle barrels electrolytically, using 20 Mule Team Borax as an electrolyte, and some rather fancy programmable DC power supplies.

Even though I have an engineering background, I am always horribly confused about what goes where in electrolysis.

I connected my power supply (p/s) NEGATIVE terminal to the BARREL (cathode) of the rifle, with the POSITIVE terminal connected to a mild steel anode. I took appropriate measure such that the anode never directly contacted the cathode.

This seemed to remove significant rust from a 150 old rolling block rifle, as evidenced by the green and brown sludge that came out the barrel during the process.

What is bewildering me is that others in various net forums are claiming that OPPOSITE polarity is required? Not tried that yet, but can somebody who knows much more about electrochemistry than I chime in here?

On the web in find instructions for both the barrel as the cathode, and the barrel as the anode!

I get it that current flows from negative to positive, but what about the other electrons in the pool?

That is what just sucks me brain out!!!

 

[ajw22]

Sounds like the type of science experiment @JoyfulNoise enjoys.

 

[JoyfulNoise]

What you are attempting to do is to ELECTROPOLISH the metal. In an electropolishing setup, the work piece is always the ANODE (+) while the cathode can be any number of different conductors. There are as many varied combinations of anode & electrolyte as there are metals to electropolish and so what you are attempting to do can get very complicated. However, I will point out that, as the kids say nowadays, "you're doing it wrong..." - steel electropolishing should always be done at low pH and typically using a concentrated acid mixture of phosphoric and sulfuric acid. Doing it a high pH is going to evolve a lot of hydrogen at the work piece which will not only fail to properly reduce the metal surface but also likely cause hydrogen embrittlement. Hopefully this rifle barrel will never actually be used....

The first step in any electropolishing process is to physically CLEAN the metal surface of any rust, smut or dirt. This usually requires a stiff brush, a cleaning solution (typically containing a surfactant) and, if necessary, a very light polishing clothe. Then the steel surface should be PICKLED in concentrated hydrochloric acid to reduce any oxides. The pickling process can take several minutes to several hours depending on how heavy the build up is. Finally the part should be rinsed (multiple times) and dried. Once the surface is prepped, you then attach the work piece to the positive (+) anode and your cathodes (-) to the negative terminal. Depending on the geometry of the part, it is often good practice to have multiple cathodes surrounding the workpiece. This will cut down on current density variations. Electropolishing is typically done at a constant voltage to avoid overcurrents and "burning" the part. You want to apply enough voltage to remove any remaining oxides and begin polishing (dissolving) the metal but not so much voltage that you electrolyze the water in solution thus forming hydrogen gas. If you are getting lots of hydrogen evolution at the work piece, your voltage is set too high and that will tend to cause surface roughness variation well above tolerance. Concentrated sulfuric and phosphoric acid mixtures are used because they are very viscous and high conductivity solutions that help to remove metal at a decent rate without evolving too much hydrogen gas. If the part is stainless steel, the current will eventually reduce in magnitude as the chromium reforms the passivation layer. Low grade steels will tend to over etch and so you must visually inspect the workpiece. After the piece is sufficiently polished, you want to immediately remove the part and rinse it throughly in flowing deionized water baths. Finally, if passivation is required, one typically finishes the part in a concentrated nitric acid soak to help fully form the oxide passivation layer (and, of course, rinse and dry the part when you are done).

If the work piece is attached to the cathode, you will get plating of metal onto the surface. But what likely is happening is not polishing or plating but oxygen evolution essentially blowing off all the junk on the surface. By doing it in reverse, you risk plating out contaminants onto the surface which will make the mar the workpiece.

Electropolishing is a very complicated process and so you should investigate it a little more so you don't damage anything you value. There are metal workshops out there that will electropolish to order but you usually have to give them a large enough order or wait a long time until they can add your part to a batch process. Look up "metal finishing services" and you might be surprised what you find...these guys are local to me and they do some pretty cool custom work (including rifle barrels....) -

High Grade Polishing — Capital Metal Finishing

www.cmf-capitalmetalfinishing.com

 

[pcmacd]

Thanks for your detailed response.

 

[pcmacd]

...

 

[pcmacd]

JoyfulNoise? Here's another.

I have digested what you told me earlier, I hope.

What troubles me is that I see all manner of internet posts using calcium carbonate to electrolytically remove rust from firearms barrels, among other things.

Some say the barrel should be the anode. Others claim it should be the cathode.

What is truly weird is that all of these posters seem to get results.

I just can't wrap my head around the polarity for removing rust with calcium carbonate as the electrolyte?

The information is just so all over the place.

~~~~~
I cleaned this particular barrel a couple of days ago with an ammonium acetate / lead acetate / distilled water mix to take out lead.

Around 3 volts at approximately 50 milliamperes.

The barrel was the anode.

It took out the lead, and all manner of rust. Not all of it, just lots.

• I mean, the residual solution was clearly full of rust.

~~~~~
What about using white vinegar as the electrolyte?

~~~~~
Please advise.

Thank you, sir.

mac