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"Required Current Density for Electropolishing"

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Current question and answers:

April 18, 2022

Q. Hello,

I work in a factory manufacturing surgical needles for sutures. We use SS302 and 301 in our manufacturing line. One of the finishing steps is electropolishing process. The goal of implementing the EP line is to deburr, brighten, passivate and finally to improve the performance (lower penetration force). Our products on which we perform EP are bent, can either have round shaped body with taper point, or triangular shaped body with triangular cutting edge tip.

My question is regarding how EP works on parts with complex shape and how to master them. I understand that EP removes the inconsistencies on the surface by measuring the difference of current density (higher current density on surface peaks results to higher rate of removal).

Our workpieces don't have the most complex shape, but we still have difficulty in understanding how electropolishing attacks the tip of the needles. For example, we notice that when there is burr on the pointy tip, EP could not remove it. If there is burr on the cutting edges, EP could remove it to an extent. Another thing that we notice as well is the tip becomes much more fragile after EP. It is as if the tip becomes thinner but the extreme point stays the same. Is EP highly dependent on the positioning of the workpiece? Our workpiece is placed flat on the anode, so the tip is pointing perpendicularly towards the cathode and the anode.

I tried to be as detailed as I can without the message being too long, so feel free to ask me if I need to clarify anything.

Aimran Nadzmi
- Samut Prakan, Thailand
^


Electropolishing effect on pointy parts

May 3, 2022

Q. Hello,

We perform electropolishing on small bent suture needles used in surgeries and operations. Our bath is around 40% H3PO4 and 40% H2SO4.
We have problem concerning the quality of the tip of the needles that it becomes too fragile after EP.
Our process team from our mother company insists that the diameter loss be between 6 to 12 microns, to prove that the needles have gone through EP.
Conforming to these instructions, the tips are still as fragile at the low extreme (6 microns of diameter loss) as it is at normal setting (around 9 to 10 microns of diameter loss). By fragile, I mean that the tip easily become crooked during manipulation of the needles after EP. It seemed as if more material is dissolved around the tip rather than the tip itself (difficult to verify this as we are dealing with really small parts).
Therefore, I was wondering how does EP attack sharply curved points. I read somewhere that electrical charges tend to accumulate on sharply curved areas. How does this affect the EP? Does it dissolve the extreme tip at the same rate as the other parts of the surface?
Do you think by changing positioning of the needle we can have different effects? I.e. having the needle pointing towards the cathode.
Or do you think it correlates with the type of bath that we use?
Thank you in advance for your support.

Aimran Nadzmi [returning]
- Samut Prakan, Thailand
^


May 3, 2022

A. Samut,
It all has to do with how electric current flows off the microscopic features of the piece while immersed in an electrolyte bath. You are correct in that the current flows more off the "peaks", but the tricky part is learning how that works when you have a macroscopic feature like the needle tip. As you said, the tip gets thinner rather than being solely dissolved down from the point.

I'm not well versed in this other than to know it's very complicated. In many respects, electropolishing is as much an art as a science, being able to craft the electrode arrangement and such in order to control current density as required for the shape of the piece.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner
^


electropolishing needles
May 4, 2022

thumbs up sign Ray,

Thank you for your input. It is interesting that you talk about electrode arrangement. I just noticed an error in my previous question. Actually, the tip of the needle is parallel to the electrodes.

Q. In theory, if I put multiple cathodes around the workpiece it could work no? Or an electrode with a concave shape that covers until the front of the tip.
We will try to do some tests of having the needles pointing towards the cathode to see the effect.

Besides that, do you know about the accumulation of charges in corners or at pointy areas? (see photo)
I am having trouble understanding how this will affect EP.. Logically I thought it would help dissolution at the tip but it is not the case.

Aimran Nadzmi [returning]
- Samut Prakan, Thailand
^

May 10, 2022

Q. 'Frosted mark' after EP?

Hello,
After EP, we found some white traces on our workpieces. Our quality control team calls them frosted mark.
At first I thought it must be a contaminant from a previous process, so I noted all the possible contaminants (grease, hydraulic oil from our stamping machine, a water displacing agent) and tested them without success in recreating this defect.
I also tried to reduce the iron concentration in our EP bath but we still have this defect on some of the following batches.
The problem appears randomly but I fear that it will occur more frequently in the future.
For info, our workpieces are small stainless steel wires of around 1 mm in diameter. We work with a 1:1 H3PO4/H2SO4 bath. Our workpieces are pre cleaned with a type of hydrocarbon solvent to remove oil, grease and any organic contaminant prior to EP.

55467-2b   55467-2a  

Could you confirm the defect resembles to what we would normally call frosted defect? If so, do you have any more ideas or clues on the possible cause?
Thank you in advance.

Aimran Nadzmi
- Samut Prakan, Thailand
^


55467-3
May 2022

A. Hi Aimran.

'Etching' is a common defect in electropolishing. Although you say that the problem is random, is it possible that it effects low current density areas rather than high current density ones?

Luck & Regards,

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




Previous closely related Q&A's, oldest first:

August 4, 2010

Q. We have a business doing Photo Chemical Milling (Acid Etching) and fine laser cutting (down to 20 microns). As part of this, we have an Electropolishing Bath, the solution is made from phosphoric and sulphuric acid. Typically, we electropolish 304 stainless that is about 500 mm x 500 mm (20" x 20") and 0.005" thick. We have mixed success with this, and current is usually 100 - 150 amps. It is adjusted, dependent on the actual size in reference to a chart we were given by the guy that installed the bath. He is no longer around.

I have been trying to find a reliable guide to the required current density. I have the Canning Handbook, which seems to indicate a far higher current density than we are presently using.

Now to the specifics of my current problem:
I need to polish an item with a surface area of 3400 mm2 (5 square inches) and I need to know what current to apply and for how long. I need a very high quality result and cannot afford to damage the items to be polished. Could someone please advise either/both the required and recommended current density and/or the appropriate formula?

Readers may also be interested to note our experience with air agitation of the electropolishing bath...
Our air supply for the factory and laser passes through an air dryer and we almost never get any moisture at the various filter/drain points in the distribution system. Surprisingly though, when we commenced air agitation of the phosphoric/sulphuric mixture, our tank liquid volume has grown by a significant amount - about 10% in 2 months. This can only be from moisture from the air supply! That said, I will be replacing the current mixture with a new solution before attempting to polish the above mentioned item. (This leads to another question - why do some polishing solutions recommend a percentage of water be added to the phosphoric/sulphuric mixture when others specifically advise to keep water out of this solution?)

Jim Cove
Metal cutting and finishing - Burleigh, Gold Coast, Australia
^


August 12, 2010

A. Hi Jim,

Starting with the first question: yes, you seem to have a too low CD for your electropolishing activity. If typically you use 100 - 150 A for a surface area of 50 dm2, you will only have 2 - 3 A/dm2 where an average of 10 A/dm2 would be a much more "normal" value.

Concerning the water coming into your electrolyte: I always learned that the concentrated sulfuric acid has hygroscopic properties and so slowly attracts water from the ambient atmosphere. Maybe your air supply contains water or, by applying air agitation, you increase the contact area between your solution and the outside air, thereby enhancing the influx of moisture.

From theory, the solution works best with as little water in it as possible. If the water concentration becomes too high, instead of polishing, you will get an etching of the surface and in the worst situation, you will even get a grain boundary attack. By increasing the CD, you will come back in an electropolishing current density range, however this has a limit as well and your operation window becomes smaller and smaller.

Why people add water? Maybe to stabilise the concentration and so the working conditions, as inevitably it will contain water after some period of time.

The 3400 mm2 part:
If your 2 - 3 A/dm2 worked always OK in the past, that will give you a nice starting point. In your situation, not knowing if the geometry is complex or not, I would calculate with an average CD of 5A/dm for a complex geometry and if it's only a plain sheet I would go for 10 A/dm2, which will mean a current of 1,7A only for this part of 3400 mm2 in the complex form and 3,4 as plain sheet.
Please make sure that you have a suited rectifier for a job like this, you cannot use your usual one! The reading will be too inaccurate. Use a 5 or 10 A max. rectifier.

The question for the time is a nice one, but unfortunately this is, together with your remark that you need "a high quality result" impossible to answer scientifically. You should know the actual roughness now and have to define the required roughness after electropolishing. If we assume that the electropolishing process will dissolve the surface with the same parameters as for Ni-deposition, you will dissolve 0,205 µ/Aminute/dm2. This will mean that after 10 minutes at this 1,7 A/dm2 you will have a situation that on average 3,5 you is dissolved and that should be more than enough to obtain the optimum result possible for the complex part. Double the CD and use 5' for the simple part. Longer electropolishing will make no sense.

Make sure that you enter the solution with either a dry part or remove excessive water first before entering and then dip the part in the solution turn it around a few time and switch on the rectifier.

So this is what I should do in your place and before the start, I would cross my fingers for a moment too, in order to exclude Murphy and O'Toole (Murphy: What can go wrong, will go wrong, O'Toole: Murphy's law always occurs at the most unwanted moments).
In order to minimise your risk, you could spend some time in additional experimenting with similar base material and investigate for example your current density range. Higher CD's are the standard (Even up to 25 A/dm2).

By the way, I assume that the temperature of your electrolyte is somewhere in the range of 60 - 80 °C? If not, go for the lower current densities you used to apply!

Best regards and Success, always eager to hear if you succeeded with the job.

Harry.

Harry van der Zanden
Harry van der Zanden
- Budapest, Hungary
^



July 8, 2014

Q. Respected sir/ madam
I srinivas Ratod am studying about electropolishing and am confused about the calculation of the voltage and current. Could you please help me how to find the voltage and current of a tube? Specifications are (41.5 OD x 41ID and 152 mm length). Please reply as early as possible.
Thanks and Regards,

Srinivas Ratod
Quality Engineer - Bangalore, India
^


July 2014

A. Hi Srinivas. Harry has offered good advise on current density, saying about 10 A/dm2 (93 A/ft2) is "normal", and up to 25 A/dm2 (230 A/gt2) may be doable. This concurs well with sources such as the Electroplating Handbook, which says 50-500 A/ft2. I would figure 14 A/dm2 (150 A/ft2) is a good number. Voltage is whatever is required to sustain that amperage considering the conductivity of the solution, and will depend on cathode placement and the conductivity of the chosen electropolishing solution, but 6-12 V would be pretty normal.

The electricity will follow the path of least resistance, so there will be little polishing on the ID if you use tank anodes.

Regards,

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



Ed. note: Pretty please,
No abstract questions!

March 12, 2021

Q. What are specific electropolishing parameters?
Like volt
Amp
Temperature.
Time

Santosh bhamare
- Satara Maharashtra
^


March 2021

A. Hi Santosh. Amperage and voltage are discussed reasonably deeply on this page already. Temperature is probably not lower than 45 °C, but 60-80 °C is more typical. Time is completely dependent on how much material you want to remove -- 3 to 4 minutes might be right for metallographic work, 20 minutes is probably more typical for production, and even more time than that is not unusual.

"Specific" answers require specific questions. What are you electropolishing and why please? Thanks!

Luck & Regards,

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

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