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topic 31148

Pickling procedure for copper


(2001)

Q. We are making copper washers. Is there a defined method to pickle copper washers so that it shines and does not become blackish.

Vikram Kirad
gaskets & components - Pune, Maharashtra, India


simultaneous (2001)

A. I would recommend using a peroxide-based bright dip followed by a triazole containing corrosion inhibitor. This will provide some tarnish resistance, but won't last forever. To keep the parts bright for long periods of time, a clear chromate finish or a lacquer sealer may be what is needed.

Dan Brewer
chemical process supplier - Gurnee, Illinois


(2001)

A. What you are looking for is a bright dip, not a pickle. The bright dip has many formulations, but will typically contain some nitric acid and some chromium 6. I would suggest finding several formulations from several sources and try each of them in the lab. The exact copper alloy and temper will make a difference as to which one will work best.

James Watts
- Navarre, Florida


(2004)

A. We also manufacture copper washers. Pickling won't solve your problem. Copper due to the material nature gets oxidised quickly when exposed to atmosphere. The best way to have a highly concentrated sulphuric & nitric acid bath, then it has to be rinsed with water (preferably soft water). Then it is dipped in a solution (Copper finish solution).

Thanks.

Karwar Industries
- Mumbai, India



(2004)

Q. I am a student trying to make a copper-water heat-pipe. The copper tube has etched in rectangular grooves running in the axial direction. The aspect ratio of each groove is 0.5 mm x 0.5 mm. In the course of my testing, it came to my attention that the copper surface trapped a lot of gas which evolve out into the evacuated heat-pipe. After reading a bit, I learned that heat-pipe manufacturers typically pickle the copper in a nitric/phosphoric acid (50%-50%) solution. I guessed that this solution had the effect of cleaning and passivating the surface and that the passivation likely decreased the porosity of the oxide layer and the associated trapped gases.

On my first try, I first cleaned a piece of copper measuring 50 mm x 25 mm x 1 mm by dipping it into NaOH followed by HCl. Afterwards, I submerged this copper into a 15cc mixture of nitric acid (50% acid - 50% DI water) and phosphoric acid (50% acid - 50% DI water). The dipping was done in room temperature, and the beaker was hand-twirled to produce a swirling motion. After 3 minutes, the resulting copper was removed from the beaker and sequentially dipped into 3 beakers of DI water for rinsing purpose. The whole rinsing process lasted around 10 seconds. The piece was then immediately dried with a paper towel. The piece was pink in color and looked a little rough similar to having been sandblasted.

On my second try, I placed another copper piece with same measurement and NaOH/HCl precleaning into a 400 cc mixture of nitric/phosphoric acid. Again, the dipping occurred in room temperature, but without the swirling. After 2 minutes, the copper was removed and subjected to an extended rinsing cycle of 1 minute. Afterwards, the piece was left to air dry. Upon inspection, I found the piece to have streaks of blue-ish/red-ish marks. There were also a few pink spots, but most of the surface had the typical copper color (reddish/yellow). The surface also looked a little rough, but was also noticeably more shiny than in my first try.

The following are now my questions:
1. What is the function of the nitric acid and why the mixture with phosphoric acid?
2. What is the passivation layer supposed to be?
3. How much solution am I supposed to use for the dimensions mentioned above?
4. Is the pickling process self-limiting or is there a requisite time duration?
5. Is agitation or heating required during the pickling process?
6. For my objective, do I wish to have surfaces similar to that produced under the first trial or the second?
7. What are the bluish/reddish streaks?
8. Why did the surface in the first trial turn up pink while the second trail turn up reddish-yellow?

Any help or pointers would be greatly appreciated.

Samuel Lee
student - Kowloon, Hong Kong


(2004)

A. Hello, Samuel. Students like yourself want to deeply understand their projects, and that's admirable. But even still ...

When you decide to make a heat pipe, you start with a piece of copper metal, not by re-inventing copper mining technology and smelting.

Similarly, metal finishing and the supply of metal finishing chemicals has been a well-developed industry for a century, so I suggest that you treat the issue of bright dipping like you treated the copper metal: Find out what companies offer packaged processes for bright dipping copper, and buy the process from them; they will include technical data sheets with operating conditions that have been demonstrated to work, and troubleshooting hints.

Nitric acid based solutions are usually reserved for cleaning up very heavy scale rather than simple chemical brightening. I would investigate a proprietary sulfuric acid - peroxide dip designed for bright dipping, and then follow with a dilute sulphuric acid dip if necessary, and then treat with a proprietary benzotriazole copper preservative. Further info can be found in the Metal Finishing Guidebook but, as I say, a proprietary process will probably be faster, easier, and better rather than a home-brew of acid and oxidizers. Good luck.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey



Pickling brass with citric acid

February 8, 2018

Q. Hi, I am doing my final year project on pickling process in a company. They use Nitric acid for removal of carbon contents (pickling) that is scale removal process in brass material. We would like to implement and suggest citric acid for pickling process. But I have doubts whether citric acid will do scale removal process for brass. Please do give some ideas and help me.

Aravinth Kanagaraj
4th year Student, Sastra University - Trichy, India


February 2018

A. Hi Aravinth. I am not familiar with the use of citric acid for pickling or bright dipping brass; my understanding is that simple mineral acids will not brighten brass because oxidizers are required. But we have attached your inquiry to a thread where we've suggested that a proprietary mix of sulfuric acid and inhibited hydrogen peroxide might be able to be used for bright dipping of copper and brass. In this case the hydrogen peroxide provides the oxidizing power, which obviates the need for nitric acid.

Remember that theoretical knowledge like this is to guide you in jar-testing of a revamped process ... you don't redesign a production process without testing :-)

Luck & Regards,

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


February 8, 2018

Q. I heard that for stainless steels citric acid is better so is it better for brass? While using nitric acid the brass material tends to tarnish and shining is lost in 4 weeks. Which acid can I use for pickling of brass.

Aravinth Kanagaraj
Sastra University , Thanjavur - Trichy, India


February 2018

A. Hi again. The tarnish which forms on stainless steel is rather transparent, which is why it can maintain its shine; the tarnish which forms on brass is brown-ish, which is why the shine is lost. A dip in a sodium benzotriazole tarnish preventer will probably extend the period of brightness. If electrical conductivity is not required, a lacquer or clearcoat can be applied. Good luck.

Regards,

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


February 12, 2018

A. Brass tarnish isn't really terrifically difficult to remove, citric acid is one among many things that do so effectively. This generally leaves the surface shiny and new-looking. I'm not sure if that counts as a "bright dip" or if that term refers specifically to something else.

Note that this does nothing to prevent the tarnish from coming back, that's where you need a clear lacquer or something similar.

Though I don't think the terms pickling or scale are typically used in reference to brass tarnish, so I'm tempted to think you are talking about something else.

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


February 2018

A. Thanks Ray. I suppose, but don't personally know, that if someone has a formerly bright brass object which has become tarnished, citric acid can remove that tarnish, restoring the bright finish. But to get a bright finish on brass in the first place requires, to my limited knowledge, an oxidizing solution. Pages 166-168 of the "Electroplating Engineering Handbook" [link is to product info at Amazon] explain the process of bright dipping copper and brass, concluding with mention of proprietary stabilized peroxide-sulphuric acid dips as a possible substitute for nitric and chromic acids.

Have you ever added peroxide to citric acid? I wonder whether such mixes could work as bright dips as described in the EEH.

Regards,

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


February 13, 2018

Q. Hi,
Is it good to pickle brass with sulphuric acid plus hydrogen peroxide? As nitric acid is used currently in a company to remove the oxide layers of brass, which I don't want to follow. If I use sulphuric acid + hydrogen peroxide will it affect brass? And in what concentration are these two acids used?

Aravinth Kanagaraj
4th year Student, Sastra University - Trichy, India


adv.
31148
February 2018

A. Hi again Aravinth. I have only seen the sulphuric acid plus stabilized hydrogen peroxide as a proprietary product, not as a home-brew. But I have seen it in successful use on copper a few times; and the vendor's data sheet says it can also be used on leaded brass (I don't know for sure whether it can be used on unleaded brass, and if not why not). I suggest that you contact the vendor about its availability in your area.

Regards,

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


February 14, 2018

Hi,
Thank you Ted Mooney sir. I followed the acid suggested by you sir that is dipping brass in water + sulphuric acid. Then dipping the component in hydrogen peroxide and then rinsing and dipping in sodium dichromate (gold coating) and then rinsing. The final result is good sir. But i want to know whether these acids will attack brass in future. If attacked what can I do sir?. And will it compensate the nitric acid cost, toxic everything. The company is using nitric acid now.I suggested and showed the result by dipping it in your suggested acid but I want to know any problems with this acids and some reasons.

Aravinth Kanagaraj [returning]
Sastra University , Thanjavur - Trichy, India


February 2018

thumbs up sign Hi Aravinth. Actually, I think I suggested that a proprietary mix of stabilized peroxide and sulphuric acid be used, rather than sequential dips in sulfuric acid and in hydrogen peroxide. But if your idea worked, it worked I guess. Good!

As for researching long term impacts of these process changes, you'll have to do that yourself (this site is for mutual help with quick suggestions and pointing you towards reference materials; we receive a hundred inquiries a day, and after 3 responses to one person about a single issue, sorry, I have to sign off, wish you well, and move along. Maybe other readers can help you; you can encourage it by trying to help them -- and trying to teach is a great way to learn. Good luck.

Regards,

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



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