Letter 21052

Painting and heating of zinc-plated details [Texas] 

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WE are currently processing assemblies using cold rolled sheet material. These require plating per ASTM B-633 [link is to spec at TechStreet]-85 TYPE IIFe/Zn 8. After this process one side of the details is masked for powder coat painting, cure tempature for the powder coat is 400 degrees. With this tempature the zinc plating gets cloudy and has a white tint.This is not acceptable to our customer.What can we do to avoid this situation.

RICK CARTER
- ROUND ROCK, TEXAS, USA

Perhaps I'm missing something, but I would have thought it would be more practical to do the powder coating before the plating.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Sounds to me like you are dehydrating the chromate in your baking of the powder paint. The gold color is imparted in a post plating operation called chromating. The chromates are what really provide your corrosion resistance by forming a hydrous (spelling?) chromium barrier on the zinc. Heating above a certain temperature will dehydrate the coating and the chromate will no longer protect the parts. Visually it appears as you have described.

Bill Grayson
- Santa Cruz, CA

WHAT WILL HAPPEN TO THE PAINT WHEN IT GOES THRU THE ZINC PLATING PROCESS? WILL IT DEGRADE THE FINISH AND DURABILITY OF THE POWDER PAINT? THANK YOU

RICK CARTER
PARADIGM METALS INC. - ROUND ROCK, TEXAS USA

That depends on a number of factors, and may require some development work. But what I can tell you is that people do make products like this by doing the powder coating first and then plating. Powder coating can be very resistant to all plating chemicals.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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