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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 ⇦this on eBay or Amazon [affil links] 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
2000