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Paint Adhesion to clear chromate over zinc

Our shop is painting steel parts that have clear zinc with chromate applied by outside platers. It has been our experience that we need to let the plated parts "cure" for 48 hours, otherwise we experience adhesion problems with the paint. Is this a common occurrence ? I have been told that the plating needs to "outgas" and conversely that the plating needs to absorb moisture from the air. We have good results if we wait the 48 hours.

Jim Schamber
Subcontract Manager for Plating - San Jose, CA USA
August 22, 2009

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First of two simultaneous responses -- August 25, 2009

The chromate is initially a gell that needs to gradually dehydrate. I have seen in the past where there was no painting for 24 hours.Your paint is evidently not compatible with the hydrated chromate.
Choices are wait 48 hours, switch chromate or switch paint.
If you are in a massive rush, you might try a few test pieces where you dry them overnight at about 90F in an air conditioned room.
The wait 48 hours sounds like the cheapest way out with the info that you gave.

James Watts
- Navarre, Florida

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Second of two simultaneous responses -- August 25, 2009

Dear Mr Schamber,

A passivation film on zinc requires 24 hours to cure fully.
It can vary depending on the humidity and other conditions in your area and go upto 48 hours.

The way to circumvent this is to phosphate the zinc plating
thereby almost removing the passivation prior to powder coating or painting.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India