Blisters on Zinc Die Casts

Q. We have blisters more often than not when plating zinc die cast nameplates, although the blisters do not appear on all parts. I have skimmed the tanks and have no oil on them. I’m stumped. My boss believes the parts have been sanded too deep and pores in the zinc have been opened.—P.J.

A. The problem you describe is a recurring problem with plating zinc castings. Without all the details of your process I can only give you some general areas to investigate. There are a number of possible causes for the blisters:

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1) The die castings were improperly prepared, and the “skin” normally formed on the surface of the die cast has been damaged by improper sanding and polishing. This will expose pores that can trap cleaning and plating solutions. The best way to solve this problem is to review the process used to finish the die castings. Perhaps an overzealous sander/finisher has to be reigned in. Better rinsing of the die castings will also help as rinsing “pumps” the pores and can help remove solutions that otherwise might be trapped in them. Alternating warm and cold rinses should be used, with the first rinse being slightly warmer than the cleaner or plating bath.   

2) The die castings were improperly cleaned. Strong alkaline cleaners can attack the die casting and open pores.

3) The cleaner may be contaminated with other heavy metals. Copper, if present in the cleaner, can form an immersion deposit on the die castings, and this type of deposit can cause blistering. You can obtain specially formulated cleaners from plating supply vendors.    

4) Acid dips can cause problems also. If you are using the classic 1-2 percent sulfuric acid dip, you might consider a dip based on 10-15 percent fluoboric acid. Acetic acid and proprietary acid salts can also be used. The classic sulfuric acid dip requires good time control, while the other acid dips are less dependent on time.