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Help ! Blistering after baking

 

We are zinc plating in a chloride zinc process small parts that have a heat treatment process. Parts look great after plating, but we bake for 4 hours at 200 °C and the end the parts have very small blisters in the surface. Parts appear clean before plating. What can be the cause of these blisters? Can to much brightener cause this effect?

Thank you in advance for your help.

Enrique Segovia
- Monterrey, Mexico

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There are several possible answers to your problem and without actually seeing the part it is hard to surmise them all. Assuming you are plating over mild steel without welds, etc. if the blister is one of zinc from zinc you would probably have a mechanical problem such as current interruption (making and breaking connection, etc resulting in a laminated deposit). Second possiblity is that a residue has remained on the part after the preplate cycle. A third possibility is that oil or oily substances floating on the surface of the zinc tank has caused the same effect as a residue from the preplate process. Finally, I have seen this problem in baths with very high brightener contents or out of balance brightener components. If you feel the brightener is high, carbon filter with a heavy carbon pack (not carbon tubes or discs) and peroxide or permanganate treat as you would for iron. If you think maybe your additives are out of balance, add carrier (or it may be called ! wetter or starter) up to 25% of make up and see if it is helpful.

I hope this helps some.

Gene Packman
process supplier - Great Neck, New York