Pickling and bright dipping of copper

A. Hi Aaron. You must plate onto metal, you can't reliably plate onto oxides. In turn, this means that you must remove those oxides, and apparently the sulfuric acid isn't cutting it.

We don't suggest brands or sources in this forum ( huh? why?). EPI is a supporting advertiser of this site; please consider mention of their products as an advertisement rather than a recommendation of a particular brand. But I have seen sulfuric acid plus stabilized hydrogen peroxide successfully used as a copper prep several times, and it strikes me as simpler & safer than the alternatives. I'm not aware of bifluoride being useful or needed unless the alloy contains silicon.

But process changes should be tested in the lab before being implemented on the production line. So maybe pull a few parts after cleaning, pickle them in beakers in the plating lab, and then return them to the line for tin plating. As long as you have good rinsing between the acid dip and the tin plating I would not expect the addition of an oxidizing agent to the acid dip to adversely affect the subsequent process.

Luck & Regards,


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

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 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

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 sulfuric & 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.

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, you don't re-invent copper mining and smelting technology.

Similarly, metal finishing and the supply of metal finishing chemicals has been a well-developed industry for over a century, so I suggest that you treat the issue of bright dipping like you treated the copper metal: Find companies offering 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 over the decades, plus troubleshooting hints.

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

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,

A. Hi again Aravinth. You can certainly do a bit of jar testing of citric acid if you wish, but my opinion is that it will play no useful role in pickling or tarnish prevention for brass.

citric acid is used for passivation of stainless steel, not brightening or tarnish prevention. The tarnish which forms on stainless steel is essentially 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 based or chromate based tarnish preventer will extend the period of brightness. If electrical conductivity is not required, a lacquer or clearcoat can be applied.

Good luck.

Regards,

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.

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 knowledge, an oxidizing solution. Pages 166-168 of the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] explain the process of bright dipping copper and brass, concluding with mention of proprietary stabilized peroxide-sulfuric acid dips as a possible substitute for nitric and chromic acids.

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

Regards,

A. Hi again Aravinth. Shops in the USA almost always buy their metal finishing processes as proprietaries rather than home-brewing them, so I have only seen the sulfuric acid plus stabilized hydrogen peroxide approach as a proprietary product. 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 a vendor about its availability in your area.

Regards,

Hi Aravinth. Glad to hear that sequential dips in sulfuric acid and in hydrogen peroxide worked, although that's not quite what I suggested. Good!

As for researching the long term impacts of your process changes, please return the favor by coming back to us with what you find out -- trying to teach is a great way to learn! This is a public forum, not a consultancy, and after 4 rounds of responses to your problem, apologies but we must move along because we have hundreds of other readers wanting their problems posted and replied to. Good luck.

Regards,

A. Hi Arun. Sulfuric acid plus hydrogen peroxide is well commercialized as part of a proprietary descaler and bright dip sequence for copper ... for example, see the technical data sheet on E-prep 222 at the top of the page. There are not many free non-commercial metal finishing papers these days; most academic papers are behind elsevier.com, academia.edu, and similar paywalls, or part of conference proceedings you must pay for -- but you can try a search on scholar.google.com for "peroxide pickling copper". Good luck.

Regards,

A. Hi Ted
Academia.edu only has a paywall for premium services.
You can search, read or download for free.
It is a vast database of academic papers with a considerable section on electroplating.
Many of these are academic research papers and it would be a mistake to believe that they all represent stable practical processes.