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Spotting, Pits and Assorted Defects on Zinc Die Casts

How can one avoid spots, blisters and assorted defects when plating zinc die casts with high porosity?
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Q. How can we avoid spots, blisters and assorted defects when plating zinc die casts with high porosity? –S.K.

A. This problem has been around since the dark ages of electroplating. Sadly, there are no “silver bullet” solutions. Even waving your iPhone over the parts will not solve the issue! In any event, here are some possible causes and cures for these problems:

  • Defective Castings: The castings may, contain pits and cold shots. When the castings are buffed, the cold shots and pits will open up and create small pores. The zinc die casts typically have a thin, skin-like, surface on them. Over-polishing/buffing of the castings prior to plating can damage that skin, exposing pores.
  • Dirty Castings: A cleaner specially formulated for zinc substrates should be used for cleaning the castings. Stronger cleaners tend to open pores.
  • Contaminated Cleaners: If the cleaner is contaminated with copper, immersion copper deposits will form on the surface of the casting and blisters will form.
  • Improper Acid Dips: Many people use 0.5 percent sulfuric acid, but there are other materials available. Five percent fluorboric acid will do a better job, but I recommend using proprietary acid dips.
  • Overcleaning of the Die Casts: Overly active surface cleaning prior to the copper strike can cause problems. An intermetallic zinc/copper compound is formed, resulting in blisters. Improper cleaning of the castings is usually the cause of over cleaning.
  • Spotting Out: This has a number of causes, including porosity, trapped plating solution in the pores, ammonia and moisture. While there are no absolute cures for this problem, make sure that the first rinse after plating is at the same temperature or higher than the plating bath temperature. Sometimes alternate hot and cold rinses will “pump” out the pores.

 

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