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Clear zinc plating discoloring after painting

Hi,

I am the quality manager of a sheet metal fabricator, that provides plating and painting as subcontracted services to our customer. We make some parts that are clear zinc plated, then powder coated and silk screened. The problem is that we have some parts that were rejected by the customer because the plating that is visible, (masked off), after paint is discolored. It is dull, dark brown, or iridescent with shades of purple, green, and yellow. This discoloration was not noticed until after the painter pre-baked the parts for 1 hour at 350 degrees F. The powder coating cure cycle is 1 hour at 400 degrees F. The painter tells me that this is their normal process, and has been for the 7 years that they have been painting these parts.

The plater has been plating these parts for about a year, using either/both an acid or alkaline plating process. We have not seen this type of discoloring before, and are at a loss to explain it to the customer. Both plater and painter are claiming that it is the others' problem, and I am not satisfied with either of their answers. I am not an expert on either process and need to be able to answer our customer on the why's and how do we prevent this from happening again. Please let me know what the causes could be, and/or what questions I should be asking.

Thank You,

Cindy Christensen
Metal Fabrication - Rosemount, Minnesota, USA
2004

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2004

There are times when I say "I'm glad I'm not in the middle of that one."

I see two immediate possibilities:
1. the powder coater overheated the parts on this run and isn't owning up,
2. the plater changed his zinc bath or chromate for this particular run, and isn't owning up.

In which case it's time to break out the polygraph machine.

Chromated finishes really shouldn't be exposed to 350-400 degrees, but it is done. So one more possibility is that this particular batch was pre-baked before the 24 hours usually allowed for a chromate film to dry. Try to latch on and believe in that one because if the problem was actually one of the other two things it won't happen again anyway, and you won't solve it without making one of your two subcontractors a liar, and probably both :-)

But another reader may have a better idea.

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

The zinc plating is not thick enough or properly applied. As a result the steel under the plating is oxidized giving a color to the parts. Zinc will not discolor when heated, steel will.

Jonathan Flicher
- Chicago, Illinois
2004