Plating ZAMAC-27 Castings

Q. We are trying to develop a plating process for Zamac-27 castings. So far, we have had little success. In every process we have tried, the cyanide copper plate blisters or does not adhere to the base material. Can you suggest a suitable process for solving this problem? D.S.

 

A. Zamac-27 is a zinc/aluminum alloy that contains approximately 27% aluminum. This is a newer material that is stronger and harder than the traditional zinc alloys used for casting.

You do not specify which processes you have tried so far but before blaming the processes you are testing, have you determined whether the castings are of good quality. If the parts have not been cast properly, you may have excessive porosity that can cause blistering and peeling. Sometimes it is not the plater’s fault!

Assuming the castings are of good quality here is a process that works well with other Zamac alloys:

 

1. Degrease
2. Electroclean cathodically
3. Rinse thoroughly
4. Acid dip in a sulfuric acid solution consisting of 4–6 mL of concentrated acid in one liter of water. 
   Parts should be dipped for 30–60 seconds.
5. Rinse thoroughly
6. Strike in a cyanide copper strike. You must go into the plating bath live. A current density of 
   20–25 asf should be used for two min followed by a current density of 10–15 asf for 5–7 
   min.
7. Rinse thoroughly

 

 

At this point, the parts should be dipped in a 10 mL/liter sulfuric acid dip followed by a thorough rinse. The parts can than be plated in an acid copper bath followed by whatever steps the final finish requires.

In step 4 above a proprietary acid salt can be substituted for the sulfuric acid dip.