Blisters on nickel plated zinc diecastings..copper undercoat missing
I am electroplating Nickel on Zinc Pressure Die Castings. The process sequence involves, polishing & buffing, Ultrasonic cleaning, electrocleaning, rinse, 5% sulphuric acid dip, rinse, cyanide copper 10 to 15 microns, rinse acid neutralizer, rinse, nickel 10 to 15 microns, rinse , dry. After this next day these parts are again Ultrasonically cleaned and rinsed and finally electrophoretic lacquered 20 microns and is cured at 180 °C for 20 minutes. Now the problem is that some of these pc,s get blisters and on peeling off these blisters there is no sign of copper either on the part or under the peeled off plated foil. Where does the copper go? Is it getting fused with the zinc die castings if yes then how thick is enough to withstand this temperature. The pressure die castings are very porous (micro) and some pinholes can also be detected. On doing cross hatch test after plating the adhesion is perfect even after coarse filing it doesn't come off but after curing at 180 °C some square's come off. The parts if preheated at the above mentioned temp also get some blisters but they are very tiny compared to what I get when they are electroplated and lacquered.Nippinder Singh
- Delhi, INDIA
I believe the copper alloys with the zinc, forming a brittle alloy at the interface. I suggest that your try 20-25 microns of copper plating. I have had success with 10 microns of copper followed by tin and fused at 500 F for a few seconds. The long time of 20 minutes allows greater diffusion of copper into the zinc.
Consultant - Poulsbo, Washington
(Don is co-author of the book "Plating on Plastics")