Blisters on Belt Buckles

Question:

We manufacture belt buckles that are diecasted using the zinc alloy, zamac3. We electroplate these with cyanide copper+acid copper+other decorativeplating.

To protect the plate, we have recently installed an electrophoratic lacquer plant. Curing the lacquer at 266°F, we invariably get blistering in the plating, resulting in peel-off and exposure of the zinc base of the product. What is causing this peeling? O. S.

Answer:

You are facing one of the age-old problems when plating zinc diecasts. If the surface of the diecast has been cleaned too thoroughly, you may have exposed many very small pores. When the part is copper plated in the strike bath copper gets into these pores and forms a copper/zinc intermetallic compound. This intermetallic is brittle and will break away from the surface. The end results are blisters and peeling. When you heat the part to 130°C you accelerate this process. You can examine the surface of the buckle after heating the part. It should have a pebble-like appearance under a microscope.

A less aggressive cleaning step prior to plating may solve your problem.