How to know if a part has been processed with black oxide or black zinc

Hi,

Our local plating supplier has sent us a part that he says is black zinc. We did a small test submerging in salty water for 3 days and in some parts the black came off. I suspect that the part has black oxide instead of black zinc, is there a test (chemical) that we can use to be sure of this?

Thank you,
Alejandro Jiron
Consumer - Cartago, Costa Rica

Dear Alenjandro Jiron

Its easy to check the parts with zinc-black. Just dip a part in Concentrated HCl, You will get vigorous gassing(or Stripping) if it is zinc black.
Shetty
- Bangalore, India
2006

Dear,

Oxide coating are remove in dilute cyanide solution while zinc black do not.
GULSHAN SACHDEV
- MORADABAD U.P. INDIA
2006