Spotting, Pits and Assorted Things on Zinc Diecasts

Q. Every so often we get zinc die casts with high porosity. The results are spots, blisters and assorted defects. As anybody will tell you plating on materials with high porosity is a real “bear.” Do you have any suggestions that might help in solving this problem? S. K.

 

A. This is a problem that has been around since the year one. Sadly, there are no “silver bullet” solutions. Here are some of the causes and some possible cures for these problems:

• The castings may be defective. The casting may contain pits and cold shots. When the castings are buffed, the cold shots and pits will open up and create small pores.
• Over polishing/buffing of the castings prior to plating. The zinc diecasts typi- cally have a thin skin-like, surface on them. If the buffing step is too vigorous, the skin will be damaged and additional pores will be exposed.
• Cleaning of the castings. A cleaner specially formulated for zinc substrates should be used. Stronger cleaners tend to open pores.
• Contaminated cleaners. If the cleaner contains copper, immersion copper deposits will form on the surface of the casting and blisters will form.
• Improper acid dips used. Classically ½% sulfuric acid is used but there are other materials available. Five percent fluorboric acid will do a better job and there are proprietary acid dips that work well. I recommend using proprietary acid dips.
• “Overcleaning” of the diecasts. This is usually due to overly active surface prior to the copper strike. An intermetal- lic zinc/copper compound is formed and the end result is blisters. Improper cleaning of the castings is usual cause overcleaning.
• Spotting out. “Spotting out” has a number of causes including porosity, trapped plating solution in the pores, ammonia and moisture. There are no sure cures for this problem but be sure the first rinse after plating should be at the same temperature or warmer than the plating bath temperature. Sometimes alternate hot and cold rinses will “pump” out the pores.