Formula for Blue Chromate (Passivation) for Zinc Plating

A. Hi Brian. Sianturi's posting was from over 8 years ago so please 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,

A. Hi Usama. Sorry, but abstract inquires very rarely draw any reader replies, and Sianturi did not respond to Brian so is unlikely to respond :-(

As mentioned, toxic, carcinogenic, sodium dichromate is not looked at favorably anymore. Please do not apply it to any jewelry or anything in contact with the skin, and check into RoHS regulations before considering exporting it.

Luck & Regards,

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 our readers, don't know the formulations for the newer generation of chromates as they are all trade secrets not public domain knowledge.
We'll post references to books & published papers if anyone offers them but, sorry, we can't print actual formulations on this semi-anonymous site for fear of crowd sourcing industrial espionage. Good luck.

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.

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 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,

A. Hello Joe. What you are looking for is 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.

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 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,

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. 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,

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

Regards,

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^ ^{(Sorry, I forgot for a moment that we were talking about chromates for zinc plating, not for aluminum)}. Then you will know that the coating complies with the requirements of the 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,