We are having problems with zinc die-cast parts. These components develop air bubbles after baking. Our plating cycle is as follows:
1) Hot soak cleaner with ultrasonic
2) Hot cathodic cleaner
3) 2% hydrofluoric acid
4) 6-8 microns cyanide copper
5) 10-12 microns bright nickel
6) 4-5 microns satin nickel
7) Flash gold
8) Electrophoretic lacquer
9) Baking at 125C for 10 min
Please advise us if our cleaning cycle needs to be improved. S.G.
I am not sure of the function of your parts but find it curious that you have a satin nickel prior to your bright nickel. For corrosion protection, a semi-bright nickel is usually deposited first, followed by a bright nickel for appearance.
You may have already done this (and that is why you are writing to me), but you should first confirm that you have a cleaning problem. Take one of the areas that has developed an air bubble after baking, section through it carefully and prepare a metallurgical mount of it through the failure point. By examining a cross section of the failure, you will be able to pinpoint the exact location of initiation of the failure. If the bubble is coming from the surface of the zinc casting, you have confirmed that it is a cleaning problem. It also may be possible that it could be a plating problem as evidenced through inter-plate adhesion problems. If you do not have the resources to perform this service, you can contact a metallurgical laboratory that would be able to perform it.
If you have confirmed the problem to be cleaning at this stage, I would have a couple of suggestions. Is your cathodic cleaning tank contaminated? In cathodic electrocleaning, you make the part the cathode, so charged species may "plate" onto the surface. This tank needs to remain clean in order to minimize this effect. Alternately, you could try a final current reversal (anodic clean) in order to remove any of these contaminants from the surface after cathodic cleaning.
The hot soak cleaner should be adequately inhibited with sodium metasilicate in order to minimize base metal attack and oxidation of the surface. The part will then require very thorough rinsing in order to remove any residual cleaner, especially silicates. The hydrofluoric acid will be effective for removal of residual silicate but may not be the best choice overall. A 10% sulfuric acid at room temperature may be more effective. Additionally, I would contact your supplier of plating solutions and ask if they would suggest alternative acids prior to the cyanide copper plating.
In general, it appears that you have a good overall approach to cleaning and pretreatment of your parts. It could simply be that the casting is outgassing, which the cleaning process will not have any effect on. The only solution to this would be to bake the parts prior to start of the pretreatment and plating cycles.