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"Problems in Zinc-Iron Plating with Trivalent Chromate"

Current question:

July 14, 2021

Q. I'm having trouble achieving SST of 240 hours and red rust of 480 hrs with ZINC IRON trivalent white with 13-15 micron thickness. Also, component is heat treated with carbo nitriding, so carbon remains in ID even after cleaning and acid pickling. Please suggest what to do?

Ankit saini
- Rohtak, Haryana ,India

Closely related historical postings, oldest first:


Q. We've been trying to achieve jet black color on alkaline zinc-iron coated parts using trivalent chromium passivation. After about a month our supplier decided that we should re-make up the bath using KOH rather than NaOH to have jet black color with trivalent black chromates. We are hesitant to do so. Does anyone have any experience on zinc-iron baths and trivalent black passivation?

metal finishing - Istanbul, Turkey


A. Based in our experience if you cannot achieve a full black colour it is because iron concentration is low, try increasing the iron content of the bath. I've never heard of KOH in such baths, can you explain the theory on this one.

Guillermo Luna
- Mexico City, Mexico


A. Hello Sena,

Using NaOH, or KOH does not help for dense black appearance in black passivation , trivalent non silver black passivation doesn't gives dense blackish appearance as itself but if you use a black sealant which will increase the salt spray life as well the appearance is jet black.

praveen kumar
Praveen Kumar
    plating process supplier
Mumbai, India



A. Dear Mr Sena Kiray,

I must admit, I have not come across the KOH chemistry, but if yr supplier recommends it he must have his reasons. Why don't you invite him to list them here ?

Stick with the advice from Guillermo ; its either the iron too low or the iron complexor or both. Also the life of the passivation bath may be over.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser
Bengaluru, Karnataka, India

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October 22, 2013

Q. We are facing a white patch or hazing/blooming problem in zinc-iron plating-black passivation. The haziness found after passivation before top-coat. Please help me to solve this problem.

Divahar Selvaraj
- Chennai, India

October 24, 2013

A. Hello Divahar,

If you tell us how do you do the zinc iron process (from how the parts get to you, how do you clean them, if you do analyse your bath, etc.) and a photograph of the issue may help us to help you!

Hope you can solve your issue... Regards,

Daniel Montañés
- Cañuelas, Buenos Aires, Argentina

July 16, 2014

Q. Hi,

We currently have a zinc iron process, with a trivalent chrome passivation.

We have noticed that certain parts get 'white spots' that appear after a few hours after plating.

We are not sure what these are.

The parts do pass our salt spray standard (200 hours).

Any ideas on what these are or what causes them?


Rochan Mehta
- Ranipet, Chennai, India

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