Formula for Blue Chromate (Passivation) for Zinc Plating

A. Hi Sufyan

Although it's a hard book to find, Biestek & Weber ⇨
has a 124-page chapter on Chromating that explains every aspect of the old-fashioned chromates.

• A simple formula they offer is 180 g/l sodium dichromate and 6 ml/l of sulfuric acid.
• They also mention the formula 150g/l sodium dichromate, 20 g/l sulfuric acid, and 20 g/l nitric acid, which will produce lighter, clearer, coatings.

... but they also present 36 other formulations for just the basic ingredients before going on to discuss numerous accelerators and other addition agents  🙂

Rinsing in hot water leaches out both some of the color and some of the corrosion resistance.

You have not told us why you want to know, and it's difficult to determine how best to distill 124-pages down to a couple of paragraphs when we don't know your situation, but we probably should advise of two general facts in order to not mislead other readers --

1). Chromating has been highly proprietary since at least the 1950's. It's one thing to form a coating on a surface in the lab for an investigation, but quite another thing to rapidly form robust, reliable coatings on actual components in a production environment. In my own career in electroplating, involved with hundreds of plating shops, I never encountered a single installation which used a home-brew chromate conversion coating rather than an off-the-shelf mixture developed and optimized over the decades by plating process suppliers.

2. Since roughly the late 1990's there has been terrific worldwide pressure to switch from these older toxic and carcinogenic hexavalent chromates to newer trivalent formulations. European RoHS (Reduction of Hazardous Substances) requirements, adhered to by most suppliers around the world, mean that hexavalent chromating is infrequently done anymore ... and robust trivalent chromating is completely proprietary and also requires zirconium, silane, or other proprietary topcoats to deliver acceptable performance.

Good Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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A. Your formula may be just as good as any other for a non-proprietary, Will, although some sulfuric acid is a good idea as well. Here in the U.S. nobody (well, virtually nobody) mixes their own chromates -- they've been proprietary since the mid 1950's. So, sorry but the formulations for the newer generation of chromates are all trade secrets not public domain knowledge.
We'll post references to books & published papers if anyone offers them of course. Good luck.

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

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.

"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber

( Rarely avail. for purchase, but see library holdings on Worldcat.org)

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A. Hi Will. If your interest is in understanding the general chemistry involved in chromating, Biestek and Weber's "Electrolytic and Chemical Conversion Coatings", ISBN 0901994782, has a 124-page chapter which covers the chemistry exhaustively.
It was translated from the Polish in 1976, and has 36 "synopsis" formulas. Still, it was published in 1976, before the modern trivalent chromate conversion coatings. Best of luck!

Regards,

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

A. Hello Joe. ASTM B201 addresses chromates, but you might be looking for a zinc plating specification. You can find U.S. military specs, ASTM and SAE/AMS specs, and others -- good starting points may be ASTM B633 or AMS2402. Good luck.

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

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 consider purchasing trivalent chromating solutions from the suppliers as it is the way forward unless you can make truly substantial financial investment 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, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Sianturi,
All chrome-VI blue passivations can give you 24 hours salt spray at best. If you want better protection, you will have to change to one of the modern chrome III proprietary passivations. Good luck!
Best regards
Trudy

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 because these specs are multiple-page detailed documents and you can't certify accordance with the "general thrust of them", you have to certify compliance to every word of them. Good luck!

Regards,

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

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

Regards,

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

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.

But 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 exactly 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 question of whether one brand is actually "better" than another just leads us down the rabbit hole :-(

Regards,

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

A. Hi Brian, hi Usama.

Sianturi's posting was from 9 years ago so please don't hold your breath waiting for him, but maybe someone else will help. Biestek & Weber does not include all 4 of those in any of its 36 formulations, but it does offer 150g/l sodium dichromate, 20 g/l sulfuric acid, and 20 g/l nitric acid.

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 modern trivalent formulations like most of the world  🙂

Check RoHS requirements before considering coating parts with hexavalent chromate.

Luck & Regards,

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