Electro-polishing pipe bores

A. Hi Amy.
I am unfamiliar with wand-type electropolishing, so I can't help much, but it is well-known that attempting to electropolish at a current density which is too low will cause etching, so that is what I would expect to be the problem.

If your picture illustrates the full length of the component, it seems that tank electropolishing with an internal anode would be relatively easy and would give more consistent results.
Luck & Regards,


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

A. Hi again,
My experience is in metal finishing rather than machining, but if the problem is apparent after grinding, I'm not understanding why that is considered acceptable. Isn't honing or another machining step available?
Electropolishing can remove tiny burrs & asperities but cannot (within my understanding) smooth out waves. The analogy I have always heard and found is that electropolishing can remove the clods in a plowed field, turning the field into ocean swells, but will not remove those swells.

I'm not understanding what is causing the verticality of the striated finish if it's not etching. I don't know whether the logistics involved are workable, but I'd suggest sending a sample ground part to an electropolishing shop to see what happens when reliable and proven electropolishing is done on parts received from machining in this condition. Good Luck,


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

A. Dear Henry:

Pitting problems are difficult to trace by remote control. The problem can be caused by any step in the operation, including the choice of the metal, the mechanical finishing, the cleaning and descaling, the physical electropolishing setup, and the condition of the electrolyte. We do trouble-shooting of electropolishing problems by a process of elimination, not by speculation.

A. Ed: Yes it is difficult to troubleshoot a process from a distance, even with the parts sitting right in front of you, let alone from photos of the defects, let alone from a verbal description of the problem without even a photograph. Still, if we believe in process of elimination, there should be a suggested step 1

Henry: There are some good troubleshooting hints in the electropolishing chapter of the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] , edited by Larry Durney. And his book, "Trouble in Your Tank" is the industry's definitive guide to methodical troubleshooting. Good luck.

A. The problems you describe could be related to the method used to manufacture the tubing, the electropolishing process used, or a combination of the two.

A. Let me answer your questions one by one.

Please inform, is it a straight tube, welded or seamless. If seamless, has it been cold Drawn / Pilgered?

1. The Pitting is due to high current and the inclusions in the stainless Steel that has created Galvanic Cell.

2. I have to see the sample. Can't answer.

3. It can be if it is a Pilgered Tube where the reduction of OD and wall thickness is of different factor. We call it 'Q' Factor.

4. It seems that the pipe is Seamless as during Direct Extrusion, there is "ORANGE PEEL" surface generated due to use of Glass as lubricant during extrusion. Further, if it was Cold drawn / Pilgered, the reduction percentage was not sufficient to remove the "ORANGE PEEL" surface.

A. Hi Tom. Although problems can sometimes be encountered in electropolishing (as in any process), if done right it is the perfect answer. It will smooth the tubing, keep it reflective, and powerfully deter rust.

Regards,

A. Hi, Jatin. Please see the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] as it has an excellent chapter on electropolishing of stainless steel, including many different acid formulations. But all of those many different acids have been used successfully, and I would guess it much more likely that your problem is in low current density, inadequate solution movement, insufficient cleaning, or excess water in the acid than that you managed to discover an unworkable acid formulation :-)

Still, tell us what formulation you are using because I suppose anything is possible, as well as th type of stainless steel, and whether you are doing OD only, ID only, or both ID and OD. Thanks.

Regards,

A. Jatin, there are many different types of stainless steels and many different types of electropolishing solutions for them. The basic formulation for stainless steel is a mixture of phosphoric and sulfuric acids; the major solutions are based on sulfuric acid contents of between 15% and 60%, with between 30% and 63% orthophosphoric acid, the remainder is water. The technology is relatively easy, but you do need to know what stainless steel you are treating! There are other solutions available, but they may not be as good as the sulfuric acid/phosphoric acid mixes. By the way, it is very difficult to successfully electropolish 400 series stainless steels.

On Trevor's comments:
Many years ago, I used a proprietary solution on Jetheat which is a modified 410 SS with excellent results. I tried to use the same parameters on a friends 17-4PH and nearly ruined it.

Different solutions work better than others on certain alloys.

See if you can find the series by Charlie Faust in Metal Finishing magazine, probably in the late 80's time frame. He was the accepted expert at that time. It will be a big help, if you can get it.

A. The results you say you get: are you talking about the inside of the tube? Usually the inner area of a tube are low-current density sites. depending of the size of the tube you can use auxiliary electrodes, if this is not possible you can use higher current density and stirring.

A. I have very successfully demonstrated to a large US Corporation, the electropolishing of the ID of tubing, which is what I presume to be the nature of your enquiry.

I would like to raise a few points which might assist you with your problems. Gassing, seems always something that affects the surface of tubing. To reduce the incidence of this, short lengths can be processed vertically in the tank but should you have long lengths of tubing, then look to pumping the electrolyte through the ID during processing.

Then of course, you have to look at the parameters of your process and examine the current density to determine that your amperage is delivering what is necessary to achieve the results you are seeking

Test the efficacy of your results by looking at the chrome to iron ratio and surface profile after electropolishing.

To offer a more qualified answer we need more information regarding lengths, whether or not you are referring to the ID and the ultimate use of the tubing.

A. Hi Hanumant. As you can see from this thread, successful electropolishing of stainless steel requires a significant knowledge base; and even with that general knowledge base, the electropolishing of the inside diameter of a 20-foot long pipe requires knowledge, experience, and a proper plant, including an internal anode arrangement as well as an efficient method of pumping solution through the pipe.

Please tell us what alloy this pipe is. And can you please very briefly describe the facility you have on hand (length & depth of electropolishing tank, rectifier rating?) and your prior electropolishing experience (biggest pieces you've electropolished, any ID electropolishing experience?), so readers have a good starting point for trying to help you? Thanks!

Regards,

A. Hi Ashish. Large tanks are often electropolished on the inside by filling them with solution. That may be less practical for these smaller containers. They can surely be electropolished, but probably not with a single racking because the inside will require both an auxiliary anode and air agitation; and the logistics of having open end up during polishing, but open end down as the parts are trnsferred from tank to tank means that a lot of human effort and intervention will be required.

Luck & Regards,

A. Hi JAISHANKAR. I suppose we could list substrate defects, equipment inadequacies, chemical inaccuracies, and operational errors as possibilities. It would probably be better for you to describe your situation and the specific stain via photographs. There are tiny white specs, black specs, whole areas of frostiness, water stains from not using DI water in your final rinse, etc. Tegart or Shchigolev might offer pretty good infromation.

Luck & Regards,