Electropolishing: theory, intro, need equipment

A. There are several electropolishing technical articles promoting electropolishing over mechanically polishing of stainless steel. You may want to do keyword searches on: electropolishing.

All the research I've done, seem to indicate that electropolishing is capable of polishing flat stainless steel surfaces to: a Ra of 2-4 microinches, if the materials is not flat as in tubing the Ra is not as well defined. Mechanically mirror polished stainless stainless mentioned in some of the electropolishing technical paper is supposed to have a Ra of 4-10 microinches. This may be true based on some of the samples, I've seen, but our mirror polished stainless sheets have a Ra of 0.729 nanometer and RMS of 0.748 nanometer. Below a micron!

The bottom line is: it depends on the stainless steel mills quality. While electropolishing and mechanical polishing can smooth out the surface roughness. The starting substrate's surface roughness, porosity, etc. plays a big part in the final surface roughness quality.

Regards,

"Electroplating Engineering Handbook"
by Larry Durney

on AbeBooks

or eBay or

Amazon

(affil links)

A. Hi Herbert,

Articles about electropolishing can be found in the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] , edited by Larry Durney, the Metal Finishing Guidebook, and ASM's Metals Handbook. Of these, the Electroplating Engineering Handbook has the best chapter on electropolishing in my opinion. In the days before Powerpoint, AESF had a great 35mm slide show and accompanying booklet on the subject; if you belong to a local AESF chapter, a friend may have a copy. Good luck.

Regards,

A. Whenever talking about electropolishing I mention that about 10 years ago the Metal Finishing published about 8-10 articles written by DR FAUST. Try to find them-they are great. If you are not able to trace them, let me know, I have them somewhere in our library. Sara

A. Many of the finishing industry texts cover electropolishing, but the best coverage of the topic is probably in the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] .

1. Refers to smoothness or surface profile;
2. Probably alludes to the fact that there mechanical polishing "plows" the surface and can bury contaminants in a way that electropolishing doesn't;
3.) Similarly, you'll get no 'folded over' gas pockets;
4.) Perhaps more theoretical than practical, I'd bet.

A. Janine:

"Micro surface improvement" refers to the smoothing of the surface accomplished with electropolish. Many manufacturers are using electropolish for microbiological reasons as it quite simply gives the bugs less places to hide.

The "means of inspection" probably refers to the fact that EP will expose any defects in the material or welding procedures. Secondarily, it stress relieves the alloy by reducing hydrogen embrittlement.

Minimal outgassing would refer to the activity in the electrolyte when the part is being electropolished. Traditional EP "off gasses" and the effect can be very deleterious.

Increased heat transfer probably refers to the electropolishing of stainless heat exchanger surfaces which benefit from the increased flow rate after treatment.

A. Hi, Janine:

1. "Micro-inch improvement" refers to the capability of electropolishing to reduce the Ra or surface roughness. A good electropolishing system can reduce Ra by about 50% without getting your parts out of the size tolerance.

2. Electropolishing is sometimes used as a metallographic method of inspecting for flaws metallic crystal structure. Most of this work is done on the laboratory level, but there are some industrial applications.

3. "Outgassing" refers to the tendency of the processed metal to release gaseous compounds under vacuum.

4. Electropolishing smoothes the metallic surface and simultaneously produces a re-structured surface oxide with a lower heat transfer coefficient.

Regards,

A. Hello Vimal. Many of the standard electroplating texts have chapters on electropolishing, including ASM Metals Handbook, and the Metal Finishing Guidebook, but the chapter in the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] is especially good.

Electropolishing does not apply to carbon steels, but to stainless steel, especially 316SS, as well as aluminum, copper and a few other materials.

A. We've appended your inquiry to an existing thread that addresses it, Mr. Gopikrishnan. But the answer runs about 20 pages long and depends, as you say, on which metals you are electropolishing, but can be found in the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] referenced above. Best of luck!

A. Hi, Andrew.

The solution is not water, it is a very strong and viscous acid with very little water in it. It it sometimes straight sulfuric acid, or a mix of sulfuric and phosphoric acid, or a proprietary mix designed to extend the life of the bath.

Do not use perchloric acid; it is horribly dangerous.

The reason people tend to not detail it may be that there are many variations, and that some are secret proprietaries, but it's mostly because the solution composition is only 1% of the issue. Temperature, current density, freedom from water, racking to preclude problems from gas pits, etc., are critical.

I would suggest that you try to find a copy of the Electroplating Engineering Handbook (it's surely in the library), which has a great 20-page intro into the various solutions available as well as the critical issues. Good luck.

Regards,

A. Hi, Andrew.

Yes, a welding machine provides DC current, but not the clean well-regulated easily adjustable power you want. Best of luck, but I really don't think you will be successful at electropolishing with neither the correct equipment nor an authoritative reference. It isn't easy; you'll want everything going for you not against you :-)

Luck and Regards,