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Cheaply electropolishing stainless steel

Q. We make parts for sail boats by welding stainless steel and are looking for a way to electropolish these pieces. I wondered if we could use a 12 volt battery and reverse plate the stainless and wondered what liquid the bath should be.

Chuck Flett
boatbuilder - Knoxville, Tennessee
2003

A. Hi Chuck. If it's worth doing...

The bath will be a very high concentration of strong acid. Most shops use a mixture of phosphoric acid plus sulfuric acid. But, it is very unlikely to work unless done with extreme attention to proper detail. Applying electricity to stainless steel will cause it to etch rather than electropolish unless the conditions are exactly right: maintaining acid concentration and freedom from water, correct elevated temperature, generous air agitation, racking to avoid air pockets, correct current (not too low),thorough cleaning first. Not to mention that you would need wastewater treatment and EPA permits to do this.

Either manually polishing the items or sending them to an electropolishing shop is more promising.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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Some time back a tig welder who worked in the fore river shipyard told me they connected a doorbell transformer to a cotton swab/wand rig with an embedded wire dipped in an acid connected the wire embedded in the swab to one side of the transformer connected the other side of trans. to a wire attached to a spot near the weld and swabbed the heat stain away...there is a portable rig that has a power supply and some sort of a similar swab rig for doing this... using a proprietary liquid. quite expensive for ME.

My question is: 1. is the latter an effective way to clean ss welds I tig weld thin walled tubing small rods and light gauge stainless... historically have taken pieces to a commercial electropolisher however it is major enterprise to get the stuff there now that I live in Maine.... have tried a battery charger [on eBay or Amazon] with some of the above proprietary solution making a small tank with a lead anode? cathode? then hooking the piece to the other side of the charger it works moderately well on little stuff will fit.... I mechanically polish the ss before welding so my concern is cleaning the welds I read your response and wonder how critical the process is.. as I recall the e polisher had a big tank a power source can't remember seeing any heating coils I hope I am clear can you advise me?

Thanks,

William Barriss Martin
sculptor - Thomaston, Maine
2003

Ed. note: Removal of heat/welding discoloration is one thing, but actual "electropolishing" is something else entirely.

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Hi,

I usually get rid of harsh discoloration after heating the metal with phosphoric acid.
First I clean it well with a 50% lukewarm sodium hydroxide water solution. before and after the weld. I do this before welding so the heat mark comes out to be even and fingerprints or other oils don't make it patchy and uneven.

After cleaning with caustic, using gloves I rinse it very well to remove any remaining caustic.

I create a bath of hot (80° C) concentrated phosphoric acid and a small amount of EDTA (1 gm per liter) as a chelator to keep the oxidation in solution therefore removing the discolorations. I leave my projects in the solution for a long time with a stirrer.

The above method is very effective for smaller projects and all the chemicals mentioned can be found on eBay . All but the EDTA are inexpensive also. EDTA is a very important aspect to it though.

Aarash Navabi
- Boston, Massachusetts, USA
July 1, 2011