Q&A: Electropolishing 304 stainless steel, or alternatives to it

Other problems on this thread:


Probable cause: Corrosion on non-immersed area above the solution level.



Probable cause: Needed an auxiliary cathode in the recessed area.

A. Hi Jayavel. The oldest and most conventional solution is perhaps phosphoric-sulfuric acid, originally Battelle U.S. Patent 2,334,698 from 1938: Bath 1 is 63% phosphoric, 15% sulfuric, 22% water. There are many other formulations, including proprietaries which overcome the weaknesses of this solution, but there is no "best" without your weighted list of what parameters you want the "best" to most closely match.

You probably know this, but there will be a lot involved in successful electropolishing of water bottles besides choosing an electrolyte.

Luck & Regards,

A. Hi, V. Electropolishing is an electrolytic procedure which operates with the opposite polarity of electroplating to smooth and brighten stainless steel by taking advantage of the fact that any high points (asperities) experience the highest current density and can consequently be preferentially dissolved into the solution. Some dairy & pharmaceutical folks, boating accessory suppliers, etc., feel that electropolished stainless steel is the only acceptable material for some applications.

Practically speaking, however, many variables are involved ... and etching rather than polishing will occur unless everything is right: correct solution, proper current density, no excessive water in the solution, etc., etc. The Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] has a 21-page chapter which is a great summary of those things, but it's is hard to further summarize it, or condense it all into a public forum response.

Many of the "must-have" textbooks, including The Metal Finishing Guidebook, and ASM Metals Handbook Vol. 5 can introduce you to it as well.

For deeper coverage there is a well-known book on the subject of electropolishing by W.J. Tegart, and another by P.V. Shchigolev.

In the days before Powerpoint, AESF had a great Illustrated Lecture / Slideshow about it; one of your friends in the plating industry may have a copy. Please get back to us with any questions you may have. Thanks.

Regards,

A. SS304 is one of the easiest stainless steels to electropolish. There are numerous commercial electrolytes around that will do an excellent job. They are generally based on phosphoric acid, usually with sulfuric acid. It can also be done with a citric acid based system. Talk to your chemical supplier.

A. The basic chemicals are sulfuric acid and ortho-phosphoric acid. However, there are many additives that will improve the performance of the bath. It would be foolhardy to specify these operational details without knowing what is available. I suggest you consult the references that Ted gave above and decide what you can obtain. Then follow the instructions in the references.

A. Hello G.,

We appended your inquiry to a question which answers it. Hopefully this will get you started, and you will come back with any specific difficulties. Good luck!

Regards,

A. Hi Judy. Electropolishing will only occurs at sufficient current density and with good agitation to constantly deliver fresh solution. Without it you'll get etching rather than polishing, so your source is correct that simply dropping a complex item with I.D.s, etc., into an electropolishing tank will not work.

I believe you need an electropolishing shop with large enough tanks and the time to give attention to the jigging of this part with auxiliary cathodes, special agitation, and whatever else is required to electropolish it despite its peculiarities.

A. Large electropolishing tanks can handle your size part. Ted is correct in saying that proper cathoding can produce a uniform appearance on a complex shaped part.

A. The simplest and easiest way is to get it electropolished by someone who knows what he is doing. If that is not possible, there is one other way that is not very labor intensive and will give you what you need. You can rig an electric wand using the right chemistry and equipment with enough power to give you the speed you need. This is being done quite effectively to welds by a number of companies.

A. I am not sure that there is ever a "best", Azimah, but the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] has an excellent chapter on electropolishing. If your school and town lack that book, the subject is also covered in the Metal Finishing Guidebook and ASM Metals Handbook, Vol. 5, Surface Engineering. Consulting accepted sources, and having real information available, will probably prove to be a better way to do student research than to rely on a stranger on the internet's claims regarding best electrodes, electrolyte, temperature and DC supply. Good luck.

A. Dana,

adv.
If you switch to the Universal Electrolyte (the one I supply to you for 420 stainless steel electropolishing), then there will be no rusting areas. 304 steel polishes well in this electrolyte (but in different regimes). I am in town right now and can deliver electrolyte if you need.

A. It's funny, one stain is above the EP line, meaning this part did not enter your electrolyte ... still you are in no doubt, that your process is to blame?

To make sure the problem is in your polishing line, try to take some parts, degrease those parts and leave the parts to dry, see if they get stains also.

A. If one uses conventional electropolishing solution, fumes from the electrolyte can contact the metal surface above the electrolyte level surface and impact the metal above the polishing line. Do not forget that the whole metal part is under current.

adv.
Our electrolyte does not have this problem. That is why we successfully use it for partial polishing or for reel-to reel continuous electropolishing, when wire or tape or conveyor exit the electrolyte tank while under electrical current, and enter the rinsing tank.

A. You've got a point, but conventional electrolyte consists of sulfuric and phosphoric acid; neither tend to evaporate at normal operation temperature, but splashes from hydrogen bubbles might be an opportunity. Still, my first shot would be would look for iron contamination of the work piece.

Thank you, everyone, for your responses. I actually was able to find the problem. We run these parts through CIP 100 w/ an ultrasonic, and somehow the sweep on the ultrasonic got moved from its standard position and the parts weren't being cleaned properly. We had to scrap out a bunch of parts, but at least we found the problem and got it resolved.

A. Hi Neil. Is the objective to buy and use them or to formulate them? What substrate do you wish to electropolish -- stainless steel? Unfortunately I think that some of our understanding of electropolishing is empirical, and our successes are not really based on deep understanding of first principles theory :-)

Regards,

A. My post above about electro-polishing the inside of the my casted part was resolved by adding a Cathode inside the cavity.