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What is the difference between Chromate and Dichromate plating

Our company has many hydraulic tubes and pins zinc plated. I would like to know the difference between Chromate and Dichromate plating.

Kurt Opelt
- Port Washington, Wisconsin

If you were talking to a chemistry prof, the difference would be that chromate is CrO4 whereas dichromate is Cr2O7.But if you are talking to a plater, for some reason 'dichromate' often means yellow chromate to them; so by contrast 'chromate' will mean clear or blue chromate. But it's not really the right way to specify things and may lead to disputes.

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

Thank you for your response.

To clarify the question, which of the following specifications is correct?

ZINC PLATE PER ASTM B633 -85 SC 1 (.0002 THICK MIN.) TYPE 2 (YELLOW CHROMATE).

ZINC PLATE PER ASTM B633-85 SC 1 (.0002 THICK MIN.) TYPE 2 (YELLOW DICHROMATE).

Kurt Opelt
- Port Washington, Wisconsin

Well, in that context chromate and dichromate really mean the same thing, in that they mean a yellow chromate coating from a hexavalent chromium solution.

So, rather than confusing the person you are writing the spec for, I would use whichever wording you find in B633, which I believe will be "chromate". Unfortunately (argh!), my copy of B633 is in a computerized fax format I received from ASTM before I changed my fax software, and I've just spent an hour but can't decode it :-)

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

There is a technical difference, as Ted said, but the two terms are pretty much interchangeable. I doubt a plater is going to ask you if you want "chromate" or "dichromate." You can strive for technical correctness, but it would also be fine if you treat them as the same thing.

Tim Neveau
Rochester Hills, Michigan

Hello Kurt!

I'd accept either, since I am so lazy that I don't distinguish between 'chromate' and 'dichromate'. I mean, it's clear what you want, isn't it? You want an electrodeposited zinc coating, service condition 1 meaning at least 5 microns thick, and you want it type II supplementary colored chromate coating, per ASTM B201-80 (called out by the ASTM B633 spec). And you want it yellow.

If you want to get picky, the wording of the ASTM specs, or at least my 1998 and 2000 versions, use only "chromate" and not "dichromate".

Hope this helps!

Lee Gearhart
metallurgist - E. Aurora, New York

Dear respondents,

I often wondered the difference as well. You may feel the difference as trivial, however, I am restoring a 73 Corvette and having that correct plating is of EXTREME importance. Thanks!

Rich Cousineau
- Athol, New York

I appreciate that, Rich, but the thing is, we're not talking about two very similar but slightly different finishes, we're only talking the semantics/language of specifying one finish.

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

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