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topic 5432

Crystals in Ammonium Based Acid Zinc Bath


We have just converted our zinc cyanide plating bath into ammonium based acid zinc plating.

When we had black spots on the plated products, our vendor said it was iron contamination and mild ones need to be treated with zinc dust. After experiencing more of this, they recommended hydrogen peroxide.

Right after adding hydrogen peroxide and skimming out the orange mud, we added new brighteners and carriers and plate again.

And during plating, we observed that there are a lot of white triangular shaped crystal precipitating. Mostly wrapping around the cooling pipes. Can anyone explain to us what this crystals are? Thanks in advance

Andrew Kaw
- metro manila, philippines

1 of 2 simultaneous responses (2000)

Sounds like Boric acid precipitating out.

George Shahin
George Shahin
Atotech - Rock Hill, South Carolina

2 of 2 simultaneous responses (2000)

I assume that what you mean by converted was making up a new plating electrolyte. The white crystalline material can either be a "salt out" of some of the brightener organics or a precipitated chloride salt. Does the salt dissolve in warm water? If so, analyze for chloride. If it appears to be primarily chloride I would attempt to redissolve into the bath. If it require hot water to redissolve, also try dissolving in alcohol. If this works, it may be a brightener organic and the matter should be discussed with your chemical supplier.

Gene Packman
process supplier - Great Neck, New York


George Shahin's answer is valid for Potassium Chloride based baths that use Boric Acid for conductivity and as a buffer. Generally, Boric Acid is not used in baths that contain ammonium chloride. What I did not point out in my previous reply was to ask you how much ammonium chloride (or total chloride) the electrolyte contains and what temperature the bath operates at and lowest temperature the bath may be at. It is entirely possible that at a lower temperature and high chloride contents you may have exceeded the solubility of the chloride salt.

Gene Packman
process supplier - Great Neck, New York


Dear Mr. Packman: Thank you very much for your replies. We put 200 grams of ammonium chloride per liter of water and operate at temperature between 43 to 50 °C. Please do take note that we experience this crystal only after we put zinc dust for iron contamination treatment. Thank you very much.

- phil


Now it gets more interesting. Zinc dust treatment is generally used for removal of heavy metals e.g. cadmium not typically for removal of iron contamination. A much simpler method is to add hydrogen peroxide (35% diluted in half, do not use 3.5% "drug store" material as this may have additional materials added. Usually, when added to a working plating solution, I add 500 - 1000 ml per thousand gallons of plating solution as a starting point. If necessary, larger quantities may be used but you must not over add since excess peroxide will keep the bath from plating for about 24 hours. It is necessary to continuously filter the solution to remove the precipitated iron or it will re-dissolve (a simplification). Generally, a filter media of 20 to 50 micron is adequate with pump capacity of two tank volumes per hour when pumping through a partially coated filter media.

Gene Packman
process supplier - Great Neck, New York

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