Summary for ammonium chloride zinc bath crystals Discussion
Crystals in ammonium based acid zinc bath were discussed after a shop converted from zinc cyanide to ammonium chloride acid zinc and began seeing white triangular deposits, mostly on cooling pipes, after treating black spots for suspected iron contamination. George Shahin noted that boric acid guidance applies to potassium chloride baths, not generally to ammonium chloride systems, and asked about chloride level and operating temperature, since low temperature plus high chloride can exceed solubility. After the poster reported 200 g/L ammonium chloride and 43–50 °C operation, Gene Packman said the crystals appeared after zinc dust treatment, which he said is generally used for heavy metals like cadmium, not iron. He recommended hydrogen peroxide instead, using 35% peroxide diluted 1:1, with typical starting additions of 500–1000 mL per 1000 gallons, warning that too much can stop plating for about 24 hours. He also stressed continuous filtration so precipitated iron does not re-dissolve.
Q. 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
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. 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 plated again.
And during plating, we observed that there are a lot of white triangular shaped crystals precipitating. Mostly wrapping around the cooling pipes. Can anyone explain to us what this crystals are?
Thanks in advance
andrew kaw
metro manila, philippines
 | A. 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 requires 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 A. Sounds like Boric acid precipitating out.
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A. 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
Syosset, New York
Q. 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.
andrew kaw [returning]
metro manila, philippines
A. 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