Specific gravity testing and replacement of electropolishing electrolyte

A. Hi Rudolph. The phosphoric acid / sulfuric acid solution is presumably per patent number US2334698, which seems to be expired. It is very high in acid, low in water, and viscous -- but sorry I do not have the viscosity numbers for you.

Luck & Regards,

A. Hi Gulhan. We added your question to a thread which already offers that info. Commonly 70 parts sulfuric to 30 parts phosphoric, or 75 to 25. More detail on the ratio is offered on this thread, and additionally on thread 40576.

But it probably should be mentioned when people ask for the ratio, that the exact ratio is probably not the key to successful electropolishing. I know of one major shop which electropolished without any phosphoric acid, and the Electroplating Engineering Handbook offers a dozen or so different formulations for the electropolishing solution. If you can, please present your actual situation. Books are best for abstract & theoretical questions; public forums where experienced readers are presented with actual situations are best for troubleshooting.

Luck & Regards,

A. We here at our company use a commercial product; 50 % vol. Phosphoric acid, 40 % vol. sulfuric acid and 10 % vol. of some secret stuff. We have been supplied with these figures:

At 20 °C:

• 2% Fe: 1,77 g/mL
• 3% 1,78 g/mL
• 4% 1,79 g/mL
• 5% 1,80 g/mL

and so on. It may depend on the composition of your electrolyte, your amps, temperature, polishing time and so forth.

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

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.

A. It isn't quite clear to me whether you've previously electropolished to your satisfaction with this solution, Sayed. So it is hard to recommend that you simply change your solution formulation, when the problem could instead be improper cleaning, poor current control, insufficient agitation, water in the solution, or several other things. Nonetheless, a number of solution formulations are included in the Electroplating Engineering Handbook. Good luck.

A. Excessive time indicates inadequate current density, most likely from inadequate fixturing. Cathodes should conform to the basket geometry, both inside & out. Try a frame of copper strip 1/4" thick by 3/4" wide (wide enough to drill bolt holes), with coarse copper mesh for the cathode surface. Remove from the EP solution when not in use. The basket (anode) fixturing must also allow sufficient current.

The EP solution with 7% Diethyleneglycolmonobutylether is listed in the Metal Finishing Guidebook, so should work, but how is it analyzed? I suggest a solution 60-65 wt% phosphoric + 15-20% sulfuric + 20-24% water, with acids determined by titration & specific gravity. A bit of surfactant (0.05-0.1 %) aids wetting and reduces gas bubble problems.

Search this site for more information on electropolishing.

A. Sayed,
I am a Technician at a reputable electropolishing company. From my experience there is only one thing that causes pitting during the polishing process, that is running the part in the acid bath for too long. If you're not getting a good polish in a short amount of time you are: Not running sufficient volt's/amperage for the part you are running, or your cathode (negative copper field) is not in close enough proximity of the part to polish effectively, or you are racking your part so that gasses are trapping between the part and the cathode (causing what we refer to as gassing, which will make a part look as if it never polished), or you could have an improper chemical balance in your EP fluid.
There are endless things that can cause a part to not get shiny but only one that causes pitting and that as I said above is from polishing a part for way too long.

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,

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" [affil. link to book on 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,

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,

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,

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.

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

A. Hello Jeffrey,

From my experience, the current density does get limited after a certain concentration of metal ions is achieved. We analyze the iron concentration in the bath monthly and we need to change a part of the bath and replace it with 'virgin' acid to get the metal concentration back in the possible work range. What we noticed when current density gets limited is that we need to apply higher voltage to get the diameter spec but in which case the EP becomes too aggressive and we have pits and impacts on the workpiece.

A. Hi cousin Vignesh. As suggested to BV, please review the 21-page chapter in "Electroplating Engineering Handbook" [affil. link to book on Amazon] the Electroplating Engineering Handbook for a quick introduction to electropolishing. If you need more thorough info than that, there are two books on the subject, referenced previously on this page. If you have specific questions, please get back to us with them. Thanks. Good luck.

Regards,

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.