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topic 5422

Electropolishing defects





A discussion started in 2000 and continuing through 2020 so far.
Adding your Q. / A. or Comment will restore it to our busy Current Topics page

2000

Q. My name's Nick Matthews and I would appreciate some help on the following problem we are experiencing:

We have our stainless steel (316L) tube units (vary in diameter between 600 and 1600 mm in diameter) electro-polished by a sub-contractor. Occasionally we observe small imperfections on the electro-polished tube wall which can be best described as 'comets'. i.e. they have a small head from which emanates a straight 'tail'. The head is approx. 0.2mm and the tail is typically 5mm in length. The tails are all in the same direction, as if they are 'escaping' from the surface and rising to the surface of the electro-polishing tank.

Has anyone else experienced this problem and more importantly identified the cause ?

Thanks
Nick

Nick Matthews
- Glos, UK


2000

A. Book knowledge rather than 'hands-on', but I think this may what is called "gas streaking" due to excessive current density in that area. Certainly a closer look would be needed though before insisting that this rather than poor mechanical polishing is the real culprit.

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


2000

A. Dear Nick, I used to run a lot of Electropolish of 304 303 types of stainless, our problem was much as you're talking, In that scenario, although the Manufacturer was buying 304 /303 type of stainless it was not "pure" enough (back then they called it electropolish grade(, we sometime would get these "comets" in straight streaks or misc depending on how this material was formed. The reason for the "tails" the pit or head of the "comet" is were the contaminate was at its most concentration and did not mix thoroughly

Chris Snyder
plater - Charlotte, North Carolina


2003

A. Hi Nick,

It seems that due to an locally high concentration of one of the SS316L elements, that area is more active then the other. The comet shape you described is a result of gas bubbles formed at the active area.

You may try to reduce voltage (current) or circulate the acid so gas bubbles will evacuate rapidly from surface.

By the way, which polishing acid do you use?

Oded Nissan
- Neve Ilan, ISRAEL



2000

Q. Hi. My name is Jacqueline Tong. I was reading the earlier message, and I have the same problem. We electropolish stainless steel 316L implant plates, and occasionally we get straight scratches or water stains-like marks on the surfaces. The problem seems to show up at random.

I am trying gather information on all possible causes:

1. If the problem is due to 'gas streaking', how should we correct this?
2. If it is due to contaminants or inclusions in the metal, how can they be identified.
3. Can waxy residues from previous surfaces treatments cause the problem?
4. Can other environmental factors, such as temperature, humidity, etc., be the cause?

Any help will be greatly appreciated. Thanks.

Jacqueline Tong
- Canada


2000

Q. Sorry,

I'm not able to help with the above problem, but I have a similar issue. We are electropolishing 316L Stainless steel. Occasionally we find a 'dry grey' contamination on the surface of the steel. It seems to be a mixture of phosphates and sulphates. Just wondering if anyone else has experienced this problem. If so, how can it be fixed?

Ivan Mooney
- Ireland


2000

A. To Jacqueline Tong and Ivan Mooney:

Electropolishing defects can be caused by a wide variety of problems in the substrate, the preparatory processes, and in the electropolishing procedures. It is generally not possible to pinpoint the cause of any specific complaint by remote control.

Regards,

Ed Bayha
Metal Coating Process Corporation - Charlotte, North Carolina


2000

affil. link
"Electroplating Engineering Handbook"
by Larry Durney
from Abe Books
or
info on Amazon

A. Jacqueline, Ivan,

Most of the "must have" texts have pretty good treatment of electropolishing of stainless steel. The Electroplating Engineering Handbook perhaps being the most thorough. The AESF has an excellent illustrated lecture (slides & text) on electropolishing.

If you can send along some photos of the problems you are encountering you can probably get some better targeted responses. For example, one evidence of gas streaking is the path of the disturbance or "streak" compared to the way the part is racked. One evidence of contaminants would be their location relevant to the machining operations. One evidence of waxy residues would be a water break before electropolishing. Poor rinsing could leave grey salts on the work.

Ed Bayha is surely correct that it can be difficult to troubleshoot a process remotely, and sometimes impossible. His implication that one needs a certain level of experience & training before they can troubleshoot effectively is a good point too.

But on the other hand, the exact same thing can be said about every metal finishing process we discuss here ... and after many years of such discussions, and receiving thousands of letters of thanks, we are certainly not about to to concede that posting trouble-shooting questions here is futile :-)

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



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



Problem with Electropolishing SS410 material

2005

Q. Dear all,
I'm a process engineer in a plating house in Singapore. Our original process is capable of electropolishing SS300 series materials. But recently our customer want to do EP on SS400 material(micro clamp). These micro clamp after EP turns black. Can anyone share your experience with me if you have done success EP on similar materials and parts?

Our current EP setting is: %80 phosphoric acid, 15% ethylene glycol, and 5% tributylamine, at 60 °C rotation for 2 minutes. Our bath change frequency is 1 week or max 2 week, and phosphoric acid will be toped up everyday.( For voltage, I tried 2 volt to 6 volt, it seems higher voltage causes blacker surface. But I don think lower voltage is good, as it does not really get the expected cleaning effect).

Another problem with this part: as the part is very small and very easy to adhesive to each other, so the overlapping problem is very serious. After we used some dummy to separate them, the result is still not satisfactory. We don't have the condition to do jigging process, and I already adjust the barrel rotation speed to max. Is there any other condition I can change to improve the overlapping problem?

Your suggestions will be greatly appreciated, and I will share with you any improvement on my side.

Bob Wang
Electropolish, plating house - Singapore



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



November 18, 2008

Q. We are a manufacturer of stainless steel (17-4 PH and 455) instruments for orthopedic industry. We are trying to get a smooth and even surface finish on stainless steel parts after electropolishing. Our current sequence of process is Machine, Heat Treat, Clean, Glass bead and Electropolish. This sequence is causing surface unevenness and staining problems.

I spoke to a lot of suppliers and they gave me feedback that this may be because of residual heat treat scales and uneven glass bead surface. The media that we use for glass bead is glass silica with size of 100-150 microns.

Can someone please give me a feedback if the root cause is the glass bead media/process or uneven residual heat treat scales?

Is there any solution to obtain a smooth and even surface finish after glass bead and EP? PS: We do not want to change the sequence to EP and then glass bead.

Any kind of suggestions will be appreciated.

Thanks

Divya Gowdar
Quality Engineer - Piscataway, New Jersey


simultaneous November 21, 2008

A. Divya

Precipitation hardening alloys can form oxide films from HT operations that can interfere with subsequent electropolish operations if not removed. How are you cleaning after heat treat? Are you glass beading the parts as a means to remove the scale? You did not say 'shiny' to describe your finish. Is the glass bead to provide a texture (additionally)? Experience has shown that 1 in 100 who do bead blasting, know how to do it correctly. If you are electropolishing a textured surface and trying to just brighten it, a poor texture will show shadows, glove prints, and other 'stains' that detract from the finish.

A solution to a textured and electropolished surface is to methodically and uniformly texture the surface. Consider automating the process.

Willie Alexander
- Colorado Springs, Colorado


A. Divya, I'd look at both areas you mention. If your company performs both heat treat and blast, you may have more direct control over ruling one or the other out. Glass Bead blasting is straight forward, but several parameters should be noted: Media size and conformance to spec, working mix "beads in blaster" conformance to media spec, pressure, distance to part, nozzle, time, classifier performance if reusing media, etc...also you may find that many "offshore" glass medias have poor life in blast units and breakdown prematurely leaving inconsistent cleaning results. Regards,

Tim Deakin
North Tonawanda, New York


December 4, 2008

A. Maybe your glass bead creating a compression stresses, which causes a more active sites in your metal .
Try changing blasting angel or better to change to diffused fine aluminum oxide .
Good luck .
P.S. please feedback if it does help !

Cair Shishani
Khair Shishani
aircraft maintenance - Al Ain, UAE


January 4, 2009

A. I do not have any direct experience of electropolishing surfaces blasted with glass beads. However surfaces with heat treatment oxide are not electropolished well, the surface ends up matt and blotchy.
The oxide may be removed chemically. A solution of 100 gms/l sodium hydroxide and up to 100 gms/l potassium permanganate at 90 °C for 1 to 3 hours degrades the oxide. Afterwards the work usually has a thick brown layer (containing manganese dioxide) which must be removed in inhibited hydrochloric acid. This process might have to be repeated to remove all the heat treatment oxide.
If the glass beads are pushing oxide into the surface I doubt whether electropolishing stands any chance of success.

Nick Clatworthy
- Whitstable, Kent, UK


January 5, 2009

A. Try adding an initial blast with 120-240 mesh aluminum oxide. This will remove residual oxides from HT and will leave a surface that is easier to inspect visually for uniformity before going to the glass beads, specially for intricate parts or not-so-skilled operators.
Caution: this step will remove some metal, so, parts with very tight tolerances may not be suitable.

Guillermo Marrufo
Monterrey, NL, Mexico


January 6, 2009

adv.
Send us few parts after heat treatment. our electrolyte does not require oxides removal before electropolishing.

anna_berkovich
Anna Berkovich
Russamer Lab
supporting advertiser 
Pittsburgh, Pennsylvania 
russamer labs banner



June 25, 2019

Q. Last 3 weeks I have a problem to electropolish straight surfaces.
I get stains on the surface.
It's hard to see that on pictures, but I marked the stains as good as I could.

What can I do?

5422-1

yakov agarunov
- israel



Shadows in electropolishing of 304 SS

March 25, 2020

Q. Hello,

I've encountered a problem in electropolishing of bowl-like pressed stainless steel sheets.
The bowl is 25cm in diameter and has 12mm hole in center by which it is fixed on Ti rack.
Inflexion of the bowl is about 4cm.(see photo)

5422-2

Above the fixture in the inner part of the bowl, is a shadow after electropolishing. I found out by measuring the current in that area that there is about 1/3 of the current density then elsewhere, which is why it probably leaves shadows.
Distance between electrodes is 12 cm on both sides.
I've tried stirring the solution with bubbling, but it does not help much, probably because bubbles can't reach inner part of the bowl.
Bowl is fixed vertically, as both sides need to be electropolished.
Part is etched and cleaned before polishing.

My thought is that the H bubbles from anode are decreasing conductivity, so current is lower than needed.
Still, it is in polishing zone, as polishing it for longer time (about 2 times as long) removes the shadow.

Parameters of the process:
Bowl surface 9dm2, Current per part 60A -> 6.6 A/dm2
Sulphuric (96%)-phosphoric acid (85%) 33:67, plus added glycerine 5%. Mixed it few days ago, so water is not a problem.

Anyone has any idea how to fix that?
Funny part is that old electrolyte did not leave shadows, but it is more than 2 years ago since I mixed it and can't remember the composition I used.
Btw. given electrolyte has minimal polishing current of 0.8 A/dm2, which is the best I've ever mixed. Though it's a bit slower polishing than more sulphuric mixtures.

To simplify the question: Is there any way to improve polishing "depth" without adding helper cathodes?

Marek Babor
- Brno, Czech Republic



Electropolishing 303 SS

August 25, 2020

Q. My name is Patrick and I had some large 304 SS parts bead blasted to remove pitted rust. They are now being electropolished but are showing signs of inconsistent finish as well as a light layer of black stuff. Attached is a couple of pictures.

5422-3a

5422-3b   5422-3c  

This can be wiped off with a paper towel but I am being told it is a result of the blasting process. Any thoughts on how to prevent or how to clean?

Patrick Daniel
- Dothan, Alabama



September 11, 2020

Q. Last 2 weeks I have a problem to electropolish surfaces, like a blister defects, but roughness is normal.

5422-4

What can I do? Material ss 316

KALYAN DUTTA
- Pune India



September 30, 2020

Q. We are a metal forging company since 1940'S. We have been forging 316 SS for an extensive time. Recently, we are having parts returned for black spots appearing after the electropolishing process. How can we tell if the electropolishing process is causing the issue or if it's our forging process? What possible issues can cause these black blemishes and how can we find the root cause?

Lisa Venette
- Tonawanda New York



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