Copper Strike on Zinc Diecasts

Question:

We are experiencing problems with our copper strike on zinc diecasts. A black porous layer forms under the copper strike. Varying the amount of cleaning and chemical etching doesn’t seem to affect the results. Do you have any ideas as to what is wrong? P.M.

Answer:

Problems with the plating of zinc diecasts seem to be one of those issues that just don’t go away! A few basic ideas should be kept in mind. The purpose of the copper strike when plating zinc diecasts is not to activate the surface of the part but to seal the part so more chemically aggressive plating baths can be used on the part. Because of the nature of the zinc diecast, you basically have a porous part with a thin “skin” covering the pores. This means that you can’t be too aggressive in cleaning or etching the part. If you overdo it, you run the risk of exposing these pores, trapping solutions in these pores and causing staining and blistering of your plated deposit. The parts should enter the strike bath “live,” otherwise you run the risk of a non-adherent, immersion deposit forming on the diecast.

If the above general principles are followed, what else might cause your problem? The copper strike can and does become contaminated with zinc after a period of use. Heavy contamination with zinc can give you a non-adherent layer on the surface of the diecast.

The cleaning process can also cause problems. If the parts are not thoroughly rinsed after cleaning, materials from the cleaning step can form complexes with zinc. These complexes can contaminate your copper strike bath and give you non-adherent deposits.

In conclusion, your copper strike bath may need replacement or carbon treatment. Determine the zinc concentration. Make sure the concentrations of the various components in the strike bath are correct.