Alternative pickling solutions for copper & brass

ASM Metal Handbook
9th Edition, Vol. 5
"Surface Cleaning, Finishing & Coating"

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A. Hi Marcello. 35% sounds very high to me for sulfuric acid; 15-20% is more conventional for pre-plating activation, but it depends on exactly what you mean by pickling (the actual condition going into pickling, the desired condition coming out).

Hydrogen peroxide is an oxidizing agent, and to my experience is often used for brightening copper or brass after pickling rather than being used in the pickling. But here in the USA, finishing shops tend to use proprietary processes rather than formulating from commodity chemicals (for example, peroxide is expensive and it deteriorates quickly, so we usually purchase it as a proprietary with effective inhibiting and stabilizing agents already in it).

ASM Metals Handbook vol. 5, "Surface Finishing, Cleaning, and Coating" has a nice 17-page chapter on 'Cleaning and Finishing of Copper & Copper Alloys' ⇨

I don't know how big your brass parts will be, but can you use a pickling pot and do some experimentation before committing to designing the process line?

Luck & Regards,


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

A. Hi Marcello,
We use a solution of sulfuric acid at 40 to 50 mls/l and sodium dichromate at 22.5 to 30 g/l.
It works fine at ambient temperature and 3-5 mins immersion.
Best regards,
Mark

A. I wonder in the context what "pickling" means, what the goal is. Are we looking to create an etched surface finish? Just remove tarnish?

Just removing the tarnish can be done with citric acid.

A. What are you trying to accomplish? Why don't you wish to use an acidic product? Would you be willing to use one which contained carcinogens? Would you be willing to use one which was highly corrosive but was alkaline? What do you need?

A. Dear Edward,
Most jewelers and metalsmiths use heated "Sparex No.2" ⇦this on eBay or Amazon [affil links] safety pickle for precious and copper base metals. It is mixed with water and used hot. A heated crock pot is ideal for this solution as it has a lid to slow evaporation. It can be purchased at most larger jewelers supply houses in 2-1/2 lb containers. "Sparex 2" is dry granular sodium bisulphate and is mixed with water for use.

Good luck & Best regards,

? Hi Dale. We appended your inquiry to an earlier thread which suggests a proprietary acid salt. But we need to ask why. Does the sulfuric not reliably deoxidize the brass? Does it not leave the brass bright enough? Might phosphoric acid work, or do you have phosphate discharge limits that would be a problem? Thanks.

Regards,

A. Hi Dale. Please try the dry acid salt 'safety pickle' mentioned by Jim; but here's the thing, and just my personal opinion ...

There are materials which really ought to be avoided when possible:

- cyanide, which will always be difficult to treat and which offers strong potential for fatally poisoning someone if there is a knowledge slip-up anywhere
- hydrofluoric acid, which can cause horrible life-threatening burns from a small exposure, and which can be anesthetizing to conceal it's early damage
- chromic acid, which is carcinogenic, and dries to a powder which tends to spread everywhere
- cadmium, lead, and mercury because they are biocumulative poisons such that any amount at all is considered bad.

While sulfuric acid can be dangerous, especially concentrated acid in drums at full strength, everything in the world is dangerous -- which leaves the question about what is safer and in what way, and what new troubles might it bring, like disrupting waste treatment.

Regards,

A. Hi again. You're probably right. I was sprayed on the head with concentrated sulfuric acid when an operator felt that one-handed operation of a drum pump would be okay; I was fortunate enough to have a clear-headed co-worker get me instantly washed down, so it burned enough to hurt but no scars; but it could have been much worse!

Dry acid salts like sodium bisulphate have been suggested, as that is the conventional replacement for sulfuric acid, although they're operated hot, and probably more people in metal finishing shops have been severely injured by hot solutions than by acids. Acid salts, being dry, address some of your safety concerns, and one of the hydrogen atoms has been pre-neutralized with sodium, addressing some other safety concerns.

Phosphoric acid, or mixes of phosphoric & glycolic acid are commercially sold as brass cleaners. Sulfamic acid is the principal ingredient in many copper & brass polishes; and citric acid ⇦this on eBay or Amazon [affil links] , operated hot, might have merit. So I think it's possible to get away from sulphate as the anion if that's wanted, but those other anions have the possibility of chelating or complexing dissolved metals and causing waste treatment issues that we don't have with sulphates.

I doubt that you'll find affordable highly concentrated liquid acids of any anion that are safe in the way we are talking about anyway. So, short of dry acid salts, one further potential solution is to buy and store the sulfuric acid at a lower strength like battery acid strength if you can find it affordably and pure enough to not mess up your processing or wastewater treatment (usually lead isn't a concern for battery acid but is a concern for metal finishing plants_.

Best of luck!

Regards,