50058

Black spots on Zn plated and trivalent chromated (bright) parts

September 22, 2008

Hi,
We are faced with black spots problem on Zn plated (15~20 microns) & trivalent chrome passivated (bright) parts. Black spots are appearing on surface within 24 hrs in salt spray test. Zn plating is getting done locally. They are following the following process: first components are cleaned with kerosene (scrubbing with saw dust), dipping in HCL, water rinsing, Zn plating, water rinsing, passivation, water rinsing, drying.
I do not know much about plating. Can anyone help me to get out of this problem? Is trivalent yellow passivation better?

Kunadha Raju
Production Manager - Hyderabad, India

Hi, Kunadha. I don't think kerosene is an adequate cleaner (plus, of course, it's dangerously flammable). I think your plating shop should alkaline clean the parts. I'm not confident that that is the direct cause of the black spots (it could be iron contamination or something else), but no sense fine tuning the process until it's roughly tuned with proper alkaline cleaning instead of kerosene. Good luck.

Regards,

Ted Mooney, P.E.  finishing.com Brick, New Jersey

HELLO SIR,
Are you doing acid zinc or cyanide zinc plating?.And what trivalent passivation you are doing yellow or blue?.tell clearly.

Shoban Kesarapu     plating chemicals supplier Secunderabad, A.P., India