Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

Chromate conversion coating on silver plating

Q. Dear all,
We are electroplaters from Pune. Currently we are doing silver plating on aluminium with chrome based passivation process. But when we want to re-plate a part it has cloudy uneven finish like the chrome passivation layer on the part was not removed. I have tried caustic, and HCl, but it doesn't work. What should I do?

A. Hi, Patwardhan. Back in the old days when I entered this industry, before today's focus on sounder environmental practices, defective zinc plated parts would simply be run through the line again. The chromate would come off in the electroclean tank which was obvious because the tank would quickly become the color of lemon icing :-)

Electrify that caustic tank of yours and I'm quite confident all of the chromate will come off in it (which may or may not solve the problem of a cloudy finish).

Luck & Regards,

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

⇩ Related postings, oldest first ⇩

Q. WE ARE AN IMITATION JEWELRY MANUFACTURER IN HONG KONG. FREQUENTLY, WE ARE PRODUCING JEWELRY IN SILVER PLATED. HOWEVER, THE SILVER COLOUR WILL TURN YELLOW AFTER 3 TO 4 MONTHS. CAN ANYONE HELP ME ON THIS MATTER? OR CAN ANYONE INTRODUCE SOME ANTI-TARNISH AGENT TO ME?

THANK FOR YOUR HELP!

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Ed. note: Non-chromate alternatives are offered and discussed on thread 1977 and on thread 3755, or search the site.

A. There are commercial electrolytic chromate passivations for silver. Try your local supplier.

Sara Michaeli
Tel-Aviv-Yafo, Israel

A. AN ELECTROLYTIC CHROMATE FOR SILVER, MADE BY ENTHONE CO. ALSO AN IMMERSION COATING WITH A PRODUCT CALLED TARNIBAN. I HOPE I COULD BE OF SOME HELP.

JOE LASPADA

A. Alan,

Try Degussa ATB 328

Mark Cheng
- Hong Kong

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Ed. note: We've been unsuccessful at googling for this, and believe that Mr. Cheng probably made a typographical error.

Chromating of silver jewelry

Hi, We produce nickel-free costume jewelry using our in-house tin/lead and zinc casting machines and have a problem with the shelf life of our plated products. After silver or gold plating, our surface finish is remarkably bright and just the way we want it, but there is no way we can make the finish last. People have suggested that we use Clear Electrophoretic Lacquers over our final layers of gold or silver plating, but we do not want to get into the complication of starting this separate process.

I was in Korea a few days back and noticed that after silver plating the plater takes it to a bath which is yellow in color and has stainless steel anodes.

On passing current there is no deposition of any sort on the plated part and after passing the current and taking out the piece the finish of the pieces is the same as it was before it was plated, but what I understood from the plater was that the plated part does not tarnish so easily after that and is made up of a mixture of three chemicals. The plater was unable to explain to me the contents of the bath because of our language restrictions since I do not speak Korean and the plater spoke extremely limited English.

I am attaching a picture of the bath with this mail so if anyone finds it familiar then I would appreciate some more information on the bath. Thanks,

A. The anti tarnish process that you noticed is a electrolytic chromate process. It is used for gold, silver and electroless nickel. Ask your supplier about it. It is for sure cheaper than clear coat although less effective.

Sara Michaeli
Tel-Aviv-Yafo, Israel

A. Hi. Roger's inquiry is over a decade old by now, so I would update it by noting that chromate treatments are probably inappropriate for jewelry because chromate is considered a carcinogenic toxin. Probably there is very little chromate compound on the jewelry, but there really should be none. It is possible that the process Roger observed was being applied to electronic components rather than jewelry; and again, there is no "right" anti-tarnish treatment for all situations.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 2013

Multiple threads merged: please forgive chronology errors :-)

Q. Tom Pullizzi answered a related question some time ago regarding a fishing lure (thread 1312), but all advise welcome obviously !

Does silver plate (over nickel on aluminium) accept a chromate conversion coating, i.e., Alodine 1200 or Iridite 14.2? Does the conversion coating itself damage the plating in any way or does it provide corrosion protection in a similar way to on aluminium? I have seen it mentioned in passing that clear coatings are sometimes used to prevent tarnishing.

In the previous thread regarding the fishing lure Tom mentioned that a chromate conversion coating over the silver would improve paint adhesion to the silver surface. We have had problems with this in the past and have had to lightly bead blast the silver plated surface to achieve paint adhesion. Would a chromate coating negate the requirement for an etch primer ?

I would appreciate any advise or expansion on the above subject.

Thanks in advance.

A. Iridite 17P is the proper Iridite product for chromating silver. We use it on all silver plating calling for chromated silver per QQ-S-365, either Grade A or B, which ever is appropriate for the chromated callout.

Milt Stevenson, Jr.
Syracuse, New York

Q. Milt,

Thanks for the reply. The ass'y in question is aluminium with areas of selective silver plating. After various operations the aluminium section is Alodined (1200). At the moment the assemblies are locally dipped in the Alodine bath, would the Alodine 1200 adversely effect the silver if the entire ass'y were to be dipped, silver and all ?

A. Iridite 17p, 17, 7p, and 7 will all leave a chromate protective film on silver, but Iridite 18 was the data sheet recommended chromate for silver.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Silver plate in contact with conversion coated aluminum

Q. I need information regarding the potential problems (if any) when using a silver plated berryllium-copper spring in contact with a conversion coated aluminum part. We are not subjected to a salt atmosphere, but are subjected to high humidity and temperatures from -40 °C to +105 °C. I know (or think I know) that the silver will tarnish, but aside from that I don't know if there are other potential problems I need to be aware of. The application is for a check valve (air) for an aircraft electronics cooling system.

The current valve has an un-plated spring, but a new manufacturer of the spring has used a silver plated wire rather than the un-plated wire for the spring as shown here.

Thanks for any help you can provide.

A. I am not going to respond to your basic question however let me reveal that one of the chromate suppliers that has been absorbed into another conglomerate had one of the leading products for chromating aluminum. They also had a product for chromating silver electroplate AND although the marketing name was different, the manufacturing formula was exactly the same; therefore I would suggest that if you chromate the silver in the same tank that you chromate the aluminum you will have one very good corrosion resistant system.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

A. Having plated radar / sonar components for Navy applications, I've seen several aluminum/silver corrosion cases. The fact that this is a spring, I'm guessing that they'll be some wearing - silver plated copper will wear away the chromate in short order and you're looking at a high voltage aluminum-silver dissimilar connection.

Being that the environs will be non-salt may be some blessing -- however, with swing in temperature given the humidity, there will be electrolyte available to accelerate whatever millivolt potential arises from the aluminum-silver couple. I would certainly try electroless nickel plating the aluminum component over chromating assuming that anodizing is out of the question due to electrical or heat conductivity concerns.

Anodically speaking - GOOD LUCK.

Milt Stevenson, Jr.
Syracuse, New York

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  Wow! Two outstanding but non-contradictory ideas. Thanks Robert! Thanks Milt! Sometimes our helpful readers make me proud of this forum :-)

Regards,

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

Silver Plating with a Chromate Finish

Q. We are a small plating company with a customer requesting silver plating with a chromate finish. We use silver cyanide for our silver plating process and we were wondering if we use the same process to do this as we do on our zinc chromate line.

A. NO, instead use the aluminum chromate on silver. Robert H Probert Robert H Probert Technical Services Garner, North Carolina A. Generally you should have a customer specification as to what type of chromate is required. Typically the chromate used on silver plate is a mild passivate and may be a formula with an older "multi purpose" chromate, a chromate also used for clear on cadmium or yellow on zinc (at a low concentration). Do not use the same solution if you already have one on your zinc line, you must make up fresh solution to avoid contamination. The specification has been for hexavalent chromate and I am not sure if any trivalent materials will passivate silver except for those intended for aluminum. Gene Packman process supplier - Great Neck, New York A. Iridite for silver sold by Macdermid, Waterbury CT Don Baudrand Consultant - Poulsbo, Washington (Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]            and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])

Multiple threads merged: please forgive chronology errors :-)

Anti-tarnish sealant for silver plating

Q. First excuse my English. I am looking for a product to prevent oxidation of silver plating, antioxidant to avoid yellow spots appearing on imersion. Where it is or how to make this product because in Brazil there is no such technology?

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Silver Anti-tarnish Strips

on eBay or

Amazon

(affil links)

A. Luciano,
There is a product called Tarniban 51 which does a great job preventing tarnishing of silver. Technic, Inc is the supplier. I am pretty sure they have a distributor in your country.

Mark Baker
Fellow Plater - Syracuse, New York

Multiple threads merged: please forgive chronology errors :-)

Q. Hi.
We are doing Jewelry items with silver.
So I need to know the best anti-tarnishing method for silver Jewelry?
Currently we are using chromate base anti-tarnishing agent. Is it okay to use chromate base anti-tarnishing agents for Jewelry?

A. Hi, Deshaka. I don't know all the environmental standards for your area, nor the jewelry industry regulations for wherever you are selling these items. However, my personal opinion is that carcinogenic hexavalent chromate does not belong on jewelry, and I would not even want trivalent chromate on jewelry.

A clear coating or a flash of rhodium plating sounds like a better much idea to me. Good luck.

Regards,

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

Multiple threads merged: please forgive chronology errors :-)

! Dear all,
I've read all comments about anti-tarnish of silver, but can't justify a complete answer.
Actually here is how I plated silver on brass chain which has a bright stone on it, when I applied clear coat the stone was cloudy after dipping, and chain was somewhat hard that's no issue.
Sodium dichromate: people are using it here for silver anti tarnish with current. There is no other easy method to seal silver plating and its tarnishing.

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Hi Mustanser. Sorry, but I can't agree. Sodium dichromate (hexavalent chromium) is carcinogenic and should not be used on jewelry. If you are certain that all traces have been converted to metallic chromium, that could be a different story. Rhodium plating is a much better answer in my opinion.

Best Regards,

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

Q. Any ideas about the chromate bath solution, anodes, and current to passivate silver coating for electric elements?
Thank you

A. Hi mohammad. Go to patents.google.com and search for US1782092A or, if you or a local library have it, there is an excellent subchapter on the subject in Biestek & Weber's "Electrolytic and Chemical Conversion Coatings", which includes the following as one suggestion:

• sodium thiosulphate ⇦this on eBay or Amazon [affil links] 20 g/l
• potassium chromate 20 g/l
• pH 7-10
• temperature 20-95 °C
• 1 A/dm^2, 3 V

I'm guessing here, but I imagine that stainless steel anodes will be fine. Good luck.

Regards,

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

Q. Thank you very much dear ted.
Unfortunately I couldn't find the book you suggest but I read the patent. May I ask another question?

What do you think is the best way to make silver plate (electrodeposition of silver on copper) anti-tarnish?

Electrical conductivity is important for us because we are working on electrical elements.

thank you very much

A. Hi again. You can rule out clear coats because they're non-conductive. And I think it's time to start avoiding chromates when practical. Rhodium plating the silver might be the best answer. If interested in that, please follow up on one of our 'rhodium plating on silver' threads.

Regards,

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

A. 1. Without a "barrier" between the copper and silver, then the copper will come thru and tarnish the top of the silver.

2. Then if you have a barrier (nickel or palladium), you may retard the corrosion of the silver with an "aluminum chromate" Class 3 (thin) on the silver. One of the popular chromate products on the market uses the exact formula, and a different operating data sheet, for corrosive protection of silver and still conduct low voltages. Cannot name proprietaries here.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

A. Instead of chromate you can use Al2O3 or TiO2 coating (or MgO or BaO):

8 gms titanium oxosulphate TiOSO4
1 lit water
pH 0,8
stainless steel anode
10 minutes 0,05 A/dm2
20 c°C temp.

Hope it helps and good luck!

Goran Budija
- Zagreb, Croatia

Q. We use Iridite 18P as a tarnish resistance coating but since been discontinued. No one can tell me what the replacement chemical is. Is there anyone who knows the replacement or alternative solution? Must be chromate base for use in old specification.

A. Any aluminum chromate will be the same thing, the one you mentioned was an aluminum chromate, just with a label change to sell for silver protection.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina