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Black Zinc and Blackening turn out greenish.

I have problem with black zinc and blackening. What are difference of black zinc and blackening?
Why it turn out to greenish and rusty when keep in long term condition.(room temperature)?
How to avoid this from happen?

Zakaria Abdullah
Quality Technician - Penang, Malaysia
February 23, 2011

Hi, Zakaria.

Black zinc would generally be a shorthand for zinc electroplating followed by black chromate conversion coating. Blackening would generally be slang for black oxide coating of steel. As you see, the terms can be too loose to be useful though. You must determine what the spec is and what process is supposed to be done before you can begin troubleshooting it. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 24, 2011

Can you give the detail the process of black zinc?

Zakaria Abdullah
- Penang, Malaysia
February 28, 2011

March 1, 2011

Hi, again.

Zinc plating with black chromate usually involves electrocleaning, acid dip, zinc electroplating, black chromate dip, with rinses in between -- plus, sometimes,additional cleaning or acid treatment, sometimes a nitric acid dip between plating and chromating, sometimes a sealer or post treatment aften chromating. Processes cannot be offered in the abstract, however, but only in relationship to the specific parts; for example, some parts would be sensitive to hydrogen embrittlement, which can demand altering the sequence.

A green coloration sometimes happens because the chromating process uses light-sensitive silver-based compounds.

You might consider zinc-iron plating because silver-free chromates can still deliver a good black color on zinc iron

It sounds like you might profit from a zinc plating consultant because whole books are written on these topics. At the least, call in a reputable supplier for a visit with you. Good luck.

Regards,

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

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