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topic 3113

Formula for Blue Chromate (Passivation) for Zinc Plating

Current question and answers:

March 3, 2021

Q. Hi Sianturi, regarding blue chromating, can you please provide the specific amounts of the four chemical elements (sodium dichromate, sodium sulphate, sodium fluoride and nitric acid) and the procedure to combine? Thanks.

brian bensen
- Orange California
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March 2021

A. Hi Brian. Sianturi's posting was from over 8 years ago so don't hold your breath waiting for him, but maybe someone else will help.

But you should not be using toxic, carcinogenic sodium dichromate unless there's a very good reason to be sticking with hexavalent chromate rather than switching to trivalent like the rest of the world :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey




Previous closely related Q&A's starting in:

1999

Q. I need to know a make up formula for blue/clear zinc passivate solution ...

I have one: 5 grams Sodium Dichromate and 25 mls Nitric acid per litre, 25 seconds immersion time. But this is from a pretty old source and I wanted to check on the current formulae. Thanks.

Will Hamlyn
London, UK


1999

A. Your formula might be just as good as any other for a non-proprietary, Will. Here in the U.S. nobody (well, virtually nobody) mixes their own chromates -- they've been proprietary since the mid 1950's. Sorry but I, and most of us here, don't know the formulations for the newer generation of chromates as they are trade secrets not public domain knowledge, and we couldn't print them if we did know them. Good luck.

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


1999

Q. That's as may be, but I wish to ... so can anybody be a little more helpful as to the chemistry involved? Thanks.

Will Hamlyn [returning]
London UK


1999

A. Dear Will, a chromate composed only of sodium dichromate and nitric acid may be no better than a nitric acid bright dip which brightens zinc & cadmium electrodeposits, i.e., there may be no residual zinc salts left on the "converted" surface of the zinc deposit. The dichromate may be useful in passivating any unplated steel which may exist in internal areas or as porosity on the plated surface.

See Chemical Conversion Coatings by Robert F. Ayres, 46th Annual Technical Proceedings of the American Electroplaters' Society, 1959 for a good discussion. Mr. Ayres also has references listed at the end of the paper including some patents. Contact the Society (www.nasf.org) for reprints, and search patents for other additives.

tom pullizzi monitor
tom pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania 



affil. link
"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber
from Abe Books
or
info on Amazon

A. Hi Will. Again, all current formulas are trade secrets because tens or hundreds of millions of dollars was invested to develop RoHS-compatibility along with good corrosion resistance. But if your interest is in understanding the chemistry involved in chromating, Biestek and Weber's "Electrolytic and Chemical Conversion Coatings", ISBN 0901994782] has a 123-page chapter which covers the chemistry exhaustively =>
It was translated from the Polish in 1976, so doesn't give "current" formulas though. Best of luck.

Regards,

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



2003

Q. Dear Everybody, I'm new in here! Can anybody can tell me, if I applied the blue zinc as a passivating material in our screw for the connector, what standard do I need to follow? Please help!

Joe [last name deleted for privacy by Editor]
- Hong Kong


2003

A. Hello Joe. What you are looking for is a zinc plating specification. You can find U.S. military specs for this, ASTM and SAE/AMS specs, and others -- a good starting point may be ASTM B633 [affil. link to spec at Techstreet] or AMS2402 [affil. link to spec at Techstreet] . Good luck.

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



November 11, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. My field is zinc electroplating. I made blue chromating by myself; it's mixed by four chemical elements, i.e., sodium dichromate, sodium sulphate, sodium fluoride and nitric acid. The result of the finishing is very blue, but it fails in Salt Spray Test. It can only achieve 24 hour with white rust, <5%. It should be achieve 48 hours to meet the standard. Do you have any suggestion which part I should decrease or increase? Thanks.

Sianturi Asmer
- Bekasi, Indonesia


November 13, 2013

A. Hi Sianturi. 24 hours is the expected number for traditional blue chromates. As previously mentioned, I don't know the formulas for superior formulations because in general they are retained as trade secrets rather than public domain knowledge. But you should be aware that chromates based on hexavalent chromium (sodium dichromate) are now forbidden worldwide in whole industries (automotive for example) and on all materials in many countries (RoHS, WEEE, and other standards). Please purchase trivalent chromating solutions from the suppliers as it is the only way forward unless you can make truly substantial investments in development efforts. If you will be staying with hexavalent chromate, the previously mentioned book by Biestek and Weber will surely help you. Good luck.

Regards,

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



November 14, 2013

Q. I WOULD LIKE TO KNOW THE DIFFERENCE BETWEEN ASTM B633 [affil. link to spec at Techstreet] AND AMS2402 [affil. link to spec at Techstreet] STANDARD REQUIREMENTS

KURINGI ANAND
- CHENNAI, TAMILNADU, INDIA


November 21, 2013

A. Hi Kuringi. ASTM specs are published by the American Society for Testing Materials and AMS specs are published by the Society of Automotive Engineers. Unfortunately there really isn't a good answer to your question. These specs are multiple-page detailed documents and you can't certify accordance with the general thrust of them, you have to have the spec in your possession and certify compliance to every word of it. Good luck.

Regards,

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


May 25, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I want to know about blue chromate conversion coating formula. I mean chromate passivating solution which is used after Zinc plating.

Ghasem Ebrahimi
- Karaj, Iran


May 2014

A. Hi Gaseem. As you see, we appended your inquiry to a thread which may answer it for you. Good luck.

Regards,

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



October 4, 2018

Q. Hi all, I understand that the composition / formula is a trade secret and the chemical formula changes between the suppliers, but will there be a difference of the substances on the component itself?

Jitendra Desai
Component Engineer - Chennai, Tamilnadu, India


October 2018

Chromate conversion coatings on electroplated zinc and cadmium

A. Hi Jitendra. You start by deciding whether the chromate should be hexavalent or trivalent, then whether it needs to be low resistance vs. maximum corrosion resistance. With those two decisions in mind, you specify compliance with a particular specification, for example MIL-DTL-5541 Class 3 or Class 1A^ (I forgot for a moment that we were talking about chromates for zinc plating, not for aluminum). Then what you will know is that the coating complies with the requirements of the specification.

But sure, even still there might be some differences: a thick-film trivalent chromate that complies and a thin-film trivalent chromate plus topcoat that complies are not precisely the same thing (as we find out when we apply them in ways that are not strictly part of the specification, for example subjecting them to some arbitrary cleaning cycle).

And the ultimate question of whether one brand is actually better than another in any way just leads us down the rabbit hole.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

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