Metal finishing Q&As since 1989

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Dichromate vs. "Dachromate"

We are supplying a 12.9 alloy steel screw to a customer and they call for "Black Dachromate" finish on their print. Is there a difference between "black dachromate" and black dichromate?

Ron Sanderson
distributor - Hopkinton, Massachusetts
2007

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Zinc electroplated parts would normally have a dichromate passivation layer, which may be black. I have not heard the term "dachromate", but it might possibly be a trade name. More likely it's a misspelling, but a misspelling of what -- dichromate or "dacromet" (a proprietary zinc-rich dip-spin coating)?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

How does "dacromate" compare to class 3 galvanizing for corrosion resistance. Do you have salt spray comparisons?

Dave Knapp
Manufacture - Mineral Point, Wisconsin
June 2, 2009

Hi, Dave. Are you sure about that word "dachromate"? Where do you see it? I have never heard of it and suspect it's a misspelling of something else, but that leaves us blindly going off in several wrong directions, depending on what the original word was :-)It's hard to beat the real-world corrosion resistance of heavy hot-dip galvanizing, though, so that's what I'd bet on.

Remember that salt spray tests are only a QA test to see that a process hasn't gone south. They are not intended to predict real life, and they don't! The corrosion mechanism is completely different. The corrosion resistance of galvanizing is largely due to the formation of glassy complex carbonate reaction products which can't form in a salt fog chamber where there is no appreciable fresh air. Don't try to compare the corrosion resistance of different coatings via salt spray hours; it doesn't work.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 3, 2009