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topic 11047

Help with specific gravity testing of electrolyte and replacement of electrolyte


A discussion started in 2001 but continuing through 2019

2001

Q. Our firm has been electropolishing stainless steel wire for quite a few years. The process is rather archaic and I am trying to improve the process somewhat.

One such problem that I am trying to solve is that the electrolyte is never tested, it just "loses effectiveness" and the whole lot is thrown out. I have heard mention that testing of specific gravity and metal ion contamination should occur and the electrolyte should be topped up and parts replaced (but not the whole lot).

Can someone please explain specific gravity (in relation to how an electrolyte works) and also some tests that should indicate how effective the electrolyte is and when it needs to be replaced.

We currently use a Phosphoric (81%) and Sulphuric acid bath with the following concentrations: 2.5L phos, 880 mL sulphuric plus 620 mL of water.

The parts that we electropolish are fine wire to small rods between 0.3mm and 5 mm diameter.

Thanks,

Kayte Worrall
- Melbourne, Victoria, Australia


2001

A. While it is possible to gain some measure of control by using specific gravity, I think you should look at using titration of the solution to accurately determine both acids.

For Sulfuric-Phosphoric mixtures, you can use this procedure:

1) Pipette 20 ml of bath solution into a 100 ml volumetric flask. Fill to the mark with DI water.

2) Pipette 10 ml of this diluted solution into a 125 ml Erlenmeyer flask or beaker, and add 20 ml of water.

3) Add 2 drops of methyl orange indicator solution, and titrate with 1.0 N NaOH to the yellow endpoint. Record the amount of NaOH as volume A.

4) Pipette a second 10 ml of the solution from step 1 into another 125 ml Erlenmeyer flask or beaker, and add 20 ml of water.

5) Add 2 drops of phenophthalein indicator solution, and titrate with 1.0 N NaOH until a persistent pink color appears. Record this volume of NaOH as volume B.

6) Calculate volume C = B - A

%(v/v) Sulfuric Acid (66° Be') = (A - C) x 1.36
%(v/v) Phosphoric Acid (85% or 59.2°Be') = C x 3.4

David Sugg
- Thorndale, Pennsylvania


2001

A. If you take the specific gravity with a hydrometer that reads satisfactory in that range e.g., 1.4 -1.7, after make up,then maintain at that SG with additions of the same mix as make up as loss will be from dragout. A drop in SG will indicate excess water from drag-in or absorption. There will be a slight increase due to some metal dissolving.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia


Electropolish specific gravity problems

July 25, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Our electropolish tanks fall out of specific gravity range during the summer months probably due to humidity. We try to raise the tank temperatures to burn off the excess water but once we put the tank back in production it falls out in a couple of days. We can't remove much solution and replace it because we have designated iron content levels. Does anyone have any ideas?

Jay Bells
- Denver, Colorado


July 2018

The Electrolytic and Chemical Polishing of Metals in Research and Industry
by W.J. Tegart
from AbeBooks
or Amazon

A. Hi Jay. If air conditioning or de-humidifying isn't a practical answer, you probably can't do much about the absorption. But as Geoff reminds us, just because absorption is worse in summer and puts you over the edge, doesn't mean that drag-in in playing no part of course. Maybe it's practical to slightly reduce the drag-in, maybe with a warm water rinse?

Regards,

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



Retaining some portion of electrolyte when dumping electropolishing bath

May 16, 2019

Q. Hello everyone,
I was gathering some information on the aging of electrolyte bath used for Electropolishing in industry.
Would someone please let me know if there is a standard value for the amount of charge that has to be maintained per unit volume in order to achieve a quality controlled approach for EP?

BV Krish
- Montreal, Quebec, Canada


May 2019

A. Hi BV. If you're just gathering general information and have no personal experience yet, you won't find a quicker or better path to learning about electropolishing than the "Electropolishing" chapter by Dean Ward [dec.] in the "Electroplating Engineering Handbook" [link is to product info at Amazon]. In 20 pages it covers all aspects of Electropolishing.

There are "finite life" and "infinite life" electropolishing baths. For the "infinite life", you must periodically decant or otherwise remove the precipitated salt sludge from the bottom of the tank, and I'm sure there is no 'standard amount' to discard (you discard the 'sludgy' part and retain the 'non-sludgy' part and that is always going to be a matter of balance, judgement, and how much sludge you find there.

For the "finite life" type of solution it is true that in some metal finishing operations it is conventional to retain some portion of the old solution to serve as a "seed" when dumping it, but I don't know if that is commonly done with all electropolishing solutions, none, or some.

Hopefully someone with experience in this will chime in on that ... but as we ceaselessly note, readers tell us they're very drawn to helping people with actual situations where there is back-&-forth from which they can learn, but are mostly uninterested in one-directional technology transfer and abstract questions. Good luck.

Regards,

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


May 17, 2019

Q. Hello Ted,
I am a Research student and my work is on EP.
I am in the process of studying the electrolyte ageing in the industry; if they use any kind of parameters to judge the electrolyte ageing.
And I was wondering if the industry has a standard value for the amount of charge that has to be maintained for a unit volume of electrolyte for perfect electropolishing, and below which the electrolyte will no longer deliver desirable results and thus, it had to be replaced fully or decanted.
Thanks,

BV Krish [returning]
- Montreal, Quebec, Canada


May 2019

A. Hi again, BV. Can you give me a couple of synonyms for 'charge'? I previously thought I understood your question but, apologies, now I have no idea what you are asking or talking about :-(

Metal finishing solutions can almost always be replenished and 'juiced up' as time & tide consume their active ingredients. You don't dump because you run out of acid or other solution component, you dump because the contaminants, even if only water, have become problematic. Electropolishing solutions build up in metal if they are 'finite life' and must be dumped due to the metal buildup; they must be decanted or filtered or whatever to remove the metal if they are the 'infinite life' type. You are asking for a 'standard value' for a concept that I don't even understand :-)

Sorry, and Regards,

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


May 17, 2019

Q. Hello Ted,
Currently, I am studying the ageing of electrolyte as part of research in Electropolishing.
I was trying to figure out if the industries use any kind of standard parameters to evaluate the ageing of electrolyte, i.e., to determine if the electrolyte is fit/ unfit for prolonged use, like by measuring the charge density or current density.
Thanks.

BV Krish [returning]
- Canada, Quebec, Montreal


May 2019

A. Hi once more. Your question is posted and we'll see if it gets any responses. As explained in the reference that I suggested, you must electropolish at a fixed current density (probably about 150 ASF) because if the current density drops, you get etching instead of polishing. So people will generally set their power supply to deliver that much current density. I suppose it's possible that the current density could drop below that despite their effort, and signal the need to replace the solution, but that is not something I have personally seen, so I doubt that it's 'standard'.

Have you had an opportunity to visit an electropolishing shop or maybe to do some small-scale electropolishing in the lab? It has even been done in shotglass-size processing tanks and smaller. It's difficult to suggest realistic ways to improve an operation based on one single parameter if you don't yet truly understand it :-)

Regards,

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


simultaneous May 18, 2019

Q. Hello Ted,

Thank you so much for your reply.

Electrolytic and Chemical Polishing of Metals
by P.V. Shchigolev
from AbeBooks

I could find the book "Electroplating Engineering Handbook" in the library, but only the second edition.
I have requested my Research supervisor if it's possible to order one from Amazon :)

Recently, I was going through the research paper, Electropolishing of surfaces: theory and applications and I came across this paragraph:

"Other factors that affect the electropolishing process includes pretreatment of the metal surface, orientation of the workpiece in electropolishing bath, choice of the cathode material, electrode spacing, bath age, electropolishing time, etc. The pretreatment is critical to the electropolishing process. For instance, if the sample surface roughness is too large, a high voltage and long electropolishing time should be chosen in order to improve the surface finish quality. The electropolishing bath age is also a big concern during the process. This is because that it is a standard practice in industry to reuse the electrolyte to keep a low profile of cost and minimise the detrimental effect to the environment.However, as time goes on, it is commonly seen that the limiting current density decreases after a certain course of time due to the consumption of the cations. Thus, it is critical to guaranty a preferable chemistry for electropolishing and the bath age should be tuned to an optimistic value."

"The limiting current density decreases due to the consumption of cations". That's when I started searching for this "optimistic value".

I never visited an electropolishing shop, but I have performed a couple of experiments in our lab to understand the influence of various parameters. I have been working with EP for almost a year now. Yes, I perform EP even in small beakers. I used to throw away the solution after each use or after each set of experiments. And I was almost running out of acids.

This is when I thought of studying the ageing of electrolyte, to see how it affects the final finish or quality, and to understand how the industry deals with the ageing of electrolyte.

Thanks,

BV Krish [returning]
- Montreal, Quebec, Canada


May 17, 2019

A. If you're running the conventional phosphoric/sulfuric electropolish bath:

Heat the bath during downtime to evaporate the water it absorbs from the air.

Introduce parts dry.

Operate the bath at the highest temperature which produces good work - probably about 100-120 °F.

If you don't have time for a heat/cool cycle, cover the bath when not in use to reduce water absorption.

Remove sludge whenever it interferes with parts.

You need not be concerned with bath chemistry beyond specific gravity. Regular removal of the sludge along with additions of fresh acids will be all that is necessary.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina


May 21, 2019

Q. Hello Jeffrey,
Thanks for the input.
I am trying to figure out the ways to determine the ageing, i.e., at which point to decide that the electrolyte solution has aged and now it has to be replaced/decanted.
Industries would definitely have some kind of judgement parameters to evaluate the solution ageing.
Would you please let me know if you have some kind of knowledge on this?
Thanks,

BV Krish [returning]
- Montreal, Quebec, Canada


A. Hello again, BV:

Let me try again. I have many, many years experience electropolishing, parts ranging from fractions of an ounce to multiple ton weight.

Assuming you are using the common 3:1 phosphoric/sulfuric bath.

After a fairly extended period of use, the metals will begin to form sludge on the bottom of the tank. This sludge causes no problem until it becomes deep enough to interfere with use of the tank.

At that time, the sludge is removed, and the volume made up with fresh acid.

To repeat - there is no testing/measurement of aging. The bath never ages out. It is self regulating through the formation of sludge which is removed periodically.

The above process can be continued eternally. There is never any need to replace the liquid portion of the bath. There is no analytical control beyond measuring specific gravity. If the specific gravity is low, heat the bath to drive off the excess water. The specific gravity will never be too high as dissolved metals are removed by settling to the bottom as sludge.

I repeat: the electropolish solution is never replaced. Not ever. As it is used the excess dissolved metals drop out as sludge. The sludge is removed when it becomes deep enough to interfere with processing, and new acid is added to restore volume.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina


May 23, 2019

thumbs up sign Hello Jeffrey,
Thanks for the reply, helped to understand the industrial practice to handle the electrolyte.
Regards,

BV Krish [returning]
- Montreal, Quebec, Canada



May 17, 2019

Q. Hi, I am vignesh willing to start electropolishing company. Will you guide me?

Vignesh surendran
- chennai,tamilnadu,india


May 2019

A. Hi cousin Vignesh. Please read the chapter which I suggested to BV and get back to us with your questions. Good luck.

Regards,

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


May 29, 2019

A. One last thing to add to this discussion of maintaining an EP bath in good condition is the importance of keeping the cathodes themselves clean. They will, at least in my experience with copper cathodes, build up a layer of sludge on the surface which needs to be removed regularly with a stiff brush (polypropylene or polyethylene bristles will stand up to the solution just fine) to keep the tank functional and predictable.

rachel_mackintosh
Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont


sidebar2 June 26, 2019

Q. Hi,
I am vignesh. I would like start a electroplating business in chennai india. I need your help by suggesting me which electroplating will be more profitable and least costliest.

Vignesh surendran [returning]
- chennai,tamilnadu, india


June 2019

A. Hi Vignesh. Whichever one you can do with zero rejects and zero defects will be the most profitable and least costly. It would be a mistake to enter a business you know nothing about; please try to take a job with an electroplating or electropolishing jobshop for at least a few months first, while taking an inventory of your own past experiences and your interests. Best of luck.

Regards,

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



What gets dissolved in the electrolyte solution during Electropolishing of Stainless Steel Grade 316?

May 21, 2019

Q. Hello Everyone,
Would someone here please let me know what gets dissolved in the electrolyte solution during Electropolishing of Stainless Steel Grade 316?
Thanks.

June 21, 2019

Q. Hello Everyone,

Would someone here help me with this question:
What happens to the Electrical Conductivity of the electrolyte when the electropolishing solution ages?
Will it increase or decrease?
I am considering a mixture of Sulphuric acid, Phosphoric acid and water.

Thanks.

June 28, 2019

Q. Hello all,

What are the factors responsible for the diminishing quality of the final finish when the electropolishing bath begins to sludge?

Thanks.

Krishna B V [returning]
Student - Montreal, Quebec, Canada

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