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topic 26448

Problems Black Chromating on Zinc Plating

A discussion started in 2003 but continuing through 2019


Q. Hi,

Zinc plating (10-15 micron thickness) with Silver based Black passivation appears black on the first day of production. The same component turns Dark olive green in three to four days time. What is the prime reason for failure? Is there any process improvement to be made to make the process consistent? We use only deionised water, Chrome concentration is also regularly analyzed. As of now we apply NC lacquer. Even then, the color fading remains the same, but the corrosion resistance improves to 400 hrs for NSS test.

M.Kanagavel Rajan
Plating-process Engineer - Coimbatore, Tamilnadu, India


A. Hi, Rajan. This is a very common problem due to the light sensitivity of the silver salts used in black chromating. Please consider switching to zinc-iron plating which will permits a silver-free black chromate that is color stable. Good luck.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

October 10, 2012

Q. Sir, after black trivalent passivation on zinc plating, our lacquer gets skips, i.e., it gets removed after rubbing

Mulane Bahiru
- Nashik, Mahrashtra, India

October 12, 2012

Q. Detail procedure of black trivalent passivation and lacquer coating.

bahiru mulane
- nashik, India

October 15, 2012

Hi Bahiru. Specific questions often get good answers in this forum, but short, abstract requests for information that fills whole chapters in plating books usually don't. We have to give people a reasonable starting point :-)

Please try to detail what you already know how to do, and explain whether you are talking about chromating on zinc plating, and whether you do barrel or rack plating. If you can purchase a proprietary chromate and lacquer, you'll receive this required technical service from the vendor. Good luck!


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

White spots develop on black zinc plating

January 20, 2019

Q. Sir,

We have this problem regarding formation of white spots in our black zinc items. The problem is that after process, the spots are not evident but after prolonged storage white spots begin to appear. We also conducted some testing in which we subject the items into humidity testing with the parameters of 75 °C and 80% Relative humidity, the result is after 24 hours testing white spots become visible. When we store the items in our storage area it took roughly 1 month for the spots to become visible. Our zinc bath is alkaline zinc the chromate conversion chemical is trivalent. The spot often occurs in the edges. Can you give me some possible advice on where to look at so we could troubleshoot our problem? Or is the condition of the storage playing a major role regarding the formation of thewhite spots?

Thank you very much.

Stephen Oliver
- Southeast Asia

January 2019

? Hi Stephen. Please email pics to for posting here. One person's white spots are another reader's pits, salt stains, or whatever. Thanks.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

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