Letter 12008

Creating brass on bbl plated zinc castings 

We are barrel plating zinc die castings with copper nickel,using a strike. We are getting blisters that all have a brass deposit on the base metal when cut open. The blisters appear to be casting blisters, but any testing to raw castings will not produce blisters, yet the blister is under a layer of zinc. Is is possible that the plating process is creating brass? This is only happening on round parts. Any ideas would be greatly appreciated.

John M. Mahaney
- East Berlin, PA, USA

The brass you are seeing is normal due to diffusion of the copper into the zinc die casting. The copper plate should be heavy enough that you still have a true copper layer. Many times blistering in zinc die cast is due to the surface preparation of the casting and/or lead in the casting. Although you may be told that there is no lead in the casting, often some is and it comes to the surface. Use of a 10% fluroboric acid predip after usual acid pickel is helpful at removing lead from the surface so that the deposit does not blister in the areas where lead is present.

Gene Packman
- Syosset, NY

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