Tarnishing of brass and copper plating

A. Hi A H.

Unfortunately your problem is so very general -- a two-word description of defects (no pictures), apparently involving 4 different finishes, and on unspecified substrate material, with no process details offered -- that I doubt that anyone can offer direct and immediate help.

Please send well focused pictures to mooney@finishing.com, tell us the substrate, pick one of the four finishes and tell us the process sequence, and give us some history (have you been operating fine until recently, or have you just opened your shop and not produced defect-free parts yet). Thanks!

Luck & Regards,

? Is this tarnish taking place between processes or is the plating both brass & copper being sent as it is ?

? Do you apply any anti tarnish at post-treatment process?

A. As pointed out in the other replies do you have the problem during or after plating? If after plating one must first be sure that a thick enough plate has been deposited and it is as non porous and fine grained as possible. It should also be thoroughly rinsed. Then either treat with a proprietary anti tarnish (or 1 ounce/gal chromic acid) or lacquer.

A. Dear Mr Sen,

The best non proprietary anti tarnish I have used is 20 gms per litre of Sodium Dichromate in Dm Water .

regards

A. Benzotriazole ⇦ this on eBay or Amazon [affil links] and toltriazole are very good post treatment products for copper and brass to prevent tarnish without adversely effecting visual characteristics of the part.

A. Thank you very much Mr. VAHANWALA for your information. I however observed blackish deposition when tap water (having hardness 700 ppm approx). The same process provides satisfactory deposition when soft water or D.M. water is used. Regarding post treatment did you observe any effect of little amount of sulfuric acid along with dichromate?

Dear Dr Sen,

Will try out the sulfuric acid as you suggest.

Regarding sourcing of the water, we use a little device which is a magnet covered in PP and is to be connected online. This is supposed to convert hard water into soft water by breaking down the crystals resulting in the formation of CaCo3 molecules, etc.

The difference between hard and soft water therefore we have not observed.

regards

A. Economical Post Treatment for preventing Tarnishing of copper & Brass parts produced using Copper / Brass Strips, produced by Stamping / Press operations.

Our company was having the problem of Brass Terminals getting tarnished over a period of time (3-4 months).

The problem was eliminated by applying a 5% solution of a chemical called ANTAR (manufactured by Prosol Chemicals, Hyderabad, India), mixed with a rust preventive-cum lubricant - CASTROL, when applied by dipping / spraying on the strip to be treated & allowing it to dry for a few minutes at room temperature , solved the problem, without affecting any electrical properties / showing any visual change in the appearance of the finished terminals.

Trials were conducted by subjecting the treated products to temperatures up to 75 °C at 95 % humidity for over 15 days. The untreated products showed clear tarnishing, while the treated ones remained as bright as the original brass strip. Now we have introduced this as a regular process prior to stamping operation and we have NOT had any customer complaints due to Tarnishing of our Terminals. Of course there was a very nominal increase in cost. The chemical did contain Benzotriazole -- the usual constituent of most Anti-Tarnish compounds for Brass & Copper. The process is in operation since the last one year.

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Ed. note: We neither recommend nor slam specific proprietaries in this forum ( huh? why?), but we know from Mr. Vishwanathan's several earlier postings on other threads that he was having a serious problem for a long time and are pleased that he found that a benzotriazole-based chemical plus a lubricant/rust preventative solved his problem. We thank him for his generosity in sharing his solution!

A. Hi Arpit. There is no permanent solution to tarnish, but a dip in benzotriazole will provide a major improvement, a dip in chromate is probably even better if the toxicity and color change is not a problem. And benzotriazole followed by lacquer or clearcoating, or a lacquer or clearcoat with benzotriazole is probably the 'most permanent'.

Please carefully detail your exact situation. Thanks!

Regards,

A. Ted's answer is very good. I would just like to add that the trivalent chromates will also provide good shelf-life protection without the toxicity issues. Circuit board manufactures use an anti-tarnish coating based upon dilute organic acids that also would work, and are heavy metal-free. Entek is one of these.

A. Hi Jon. You are certainly welcome to investigate the benzotriazole literature as deeply as you wish, and if other readers can help you with its application and implementation, great!

But speaking just for myself, a mechanical engineer who has acquired a little book knowledge of metal finishing chemistry, when I suggest 'benzotriazole' for a given situation, it's much like suggesting zinc plating for another situation. To me 'benzotriazole' doesn't mean chemically pure sodium benzotriazole, it means a proprietary formulation of benzotriazole-rich copper preservative operated in accord with the technical data sheet supplied by the process vendor :-)

I've seen it applied on copper electroformed recording masters and on copper plated grounding rods, and perhaps another application or two ... but always as a preservative solution purchased from a plating process supplier, complete with technical data sheet, material safety sheet, and technical assistance as required for its implementation :-)

Regards,

A. Hello again, Jon. The benzotriazole product I saw was offered by Atotech and was added to water in a dip tank and used in the fashion of a passivating final rinse. I believe it operated at room temperature and the immersion time was short, perhaps 20 seconds. I would expect that Macdermid and Henkel and other major suppliers offer it as well. But problems arise if people start offering testimonials for one brand over another. If you can find the answer you need in technical articles or patents, or people can relate their experience for you, that's fine too.

Regards,

A. Hi Magaan. Short of a non-conductive coating like a brass lacquer or other clearcoat, you probably are not going to exceed the tarnish resistance of a good chromate coating. But I don't know how we would know from here whether your home-brew is optimally formulated and and is being optimally operated. Have you tried talking to the suppliers regarding proprietary formulations and their operating instructions?

Regards,

A. Magaan,
Clearly whoever gave you that formula for "passivating" copper and brass didn't know what they are talking about.

The tarnish on copper/brass/bronze is fairly easy to remove with even a mild acid, but the tarnish itself IS the passivation (defined as the metal atoms on the surface reacting with other atoms available in the environment until they no longer want to react with anything).

Not everything is stainless steel where the passive layer looks nice and very similar to the raw, zero valence condition of the metal, or aluminum where the passive layer forms so quickly that you seldom even see the zero valence metal.

Much like corrosion resistance on mild steel is typically achieved via some manner of coating, tarnish protection on brass and bronze is usually achieved via a clear lacquer coating.

 Hi Ray. I can't quite agree with you -- the problem being semantics. Although passivation of stainless steel has a clear meaning, and has clear specifications, 'passivation' with regards to some other metals is far more "slang-y". Chromate conversion coatings for copper, brass, zinc, & cadmium are frequently called 'passivation' whether we like it or not. Oftentimes copper, brass, and electroless nickel plated surfaces must remain electrically conductive and they are dipped in chromate conversion coatings to deter tarnish which would be insulative.

There are, I suppose, 4 cases for brass and copper finishes:
- finishes which must be at least somewhat conductive but appearance isn't critical, for which chromate conversion coatings are often used;
- finishes which must be conductive and a bright appearance matters a lot, for which benzotriazole is often used;
- finishes that can be non-conductive, for which some sort of clearcoat is used with or without benzotriazole; and
- finishes which are simply allowed to tarnish.

Regards,