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Paint blistering on zinc plating - Why?

We have recently had a very serious problem where we had 8 mm (5/16") thick bright drawn mild steel bar zinc plated and clear passivated.

It was then powder coated using polyester powder. The surface of the paint came up with 3 mm (1/8") blisters which could be scrapped off.

We had self colour mild steel from the same steel batch powder coated using the same route. This trial piece did not blister.

Our powder coater claimed that they did not know why the blistering occurred but stated that they have this problem from time to time. Because of this they use a particular zinc plater with whom, they claim, the blister problem does not occur.

Any comments please?

George Startin
Buyer - Cannock, U.K.
April 5, 2009

When things are zinc plated they are almost universally chromate conversion coated, George. For RoHS and safety, the world has switched from well established, almost generic, hexavalent chromate coating to a bunch of highly proprietary trivalent chromate and chrome-free alternatives. This has caused widespread adhesion failures that are difficult to scientifically control because the treatments are different one from the other, the plating shop may switch from one to the other, the shop doesn't want to reveal quite what they are doing, and the coating supplier doesn't either :-)

The result is that if you have found a powder coating shop and they want to use a particular plating shop, you pretty much have to let them or exonerate them from adhesion failures.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 9, 2009