Metal finishing Q&As since 1989

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Removal of chrome from Alkaline zinc plating

2005

An present I am working at a Zinc plating company in Thailand
Now, I found a problem of impurity in Zinc plating chemical: Cr 1.07 ppm in plating chemical.
After plating the part appears to have Black colour at high current
I need to know How to decrease chrome in plating chemical.


thank you very much for all suggestions

Anek Jangsri
plating company - Thailand

It would be helpful to know what type of zinc electrolyte you are using.

Gene Packman
process supplier - Great Neck, New York
2005

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Anek.

It comes from your racks, and that will stay in the zinc bath and will show defect on the part you plate on.

I don't know the enlisch name on it to solve yor problem but we use sodiumdisulfite. or you can use saccarin to get the hexavelet chrome to be trivlalent. I understand that you use hexavalent chromates.
Regards,

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
2005

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Simply restric chrome entry in to the plating bath as Mr. Anders rightly said, it comes from the jigs.Do not wait till it enters the bath. Kill it before that.

T.K. Mohan
plating process supplier - Mumbai, India
2005

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2005

H1!


I think that You will be able to "put" chromium from the bath only by "drag-out".
You can really help yourself by putting natrium-dithyonit or natrium disulfit in bath; those two will help you to reduce chromium(VI). But during next day or two, it wil oxidize again in Zn-generator tank....

So first step is to prevent chromium to enter into the Zn bath (increase concentration of acid in pickling, alkaline in degreasing, increase of time for pickling and degreasing, increase amount of water for rinsing in pretreatment). This all will be helpfull to prevent entering chromium into Zn bath.

Second step is reducing the chromium (maybe every second day?, or when troubles with black spots are noticed) with natrium disulfit or dithionyte.

Third step is to follow the first step and repeat the second step until the concentration of chromium is low enough...

My opinion is, that is very difficult to lower the chromium from alkaline Zn bath, due to the oxidation proces in Zn generator tank (redox reaction of Zn dissolwing). In acid bath, that is not problem at all.

Best regards!


Dimitrij


P.S.
Maybe I am completely wrong... It will be helpful if you notify us, if you, and how you have solved the problem.

Dimitrij Us
- Kranj, Slovenija (EU)