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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2007

Chromate Breakdown at High Temperature

Q: I have been told that the chromate over zinc plating breaks down or is destroyed at high temperatures.  These are parts that are painted. I'm looking for the temp. that this process starts and documentation on this process that can be forwarded to our customers.

A: Specification MIL-C-5541E Class 1A,  "Chemical Conversion Coatings on Aluminum and Aluminum Alloys" that states the following:

6.11 Temperature effects on corrosion protection. Unpainted conversion coatings will commence losing corrosion resistance properties if exposed to temperatures of  60°C(140°F) or above, during drying, subsequent fabrication , or service. In  general, as temperature and exposure times increase, the corrosion protection o f unpainted conversion coated parts decreases. The reduction is believed to result from the coating dehydrating and the resulting insolubility of the chromates within the coating.

In the past I have asked chemical suppliers how this affects parts that are already painted before they are exposed to the elevated temperatures. I'm not sure that I ever received a definitive answer. The statement in the MIL specification clearly warns that the fabricator shouldd not raise the drying or curing temperature of the chromate conversion coating (CCC) above 140°F before applying  paint or powder coating. Since powder coatings and baking enamels are always cured at temperatures well above 140°F (in fact, 250°F - 400°F) I assume that CCC does not significantly loose its properties, otherwise nobody would use them under powders. I suggest that you call the technical staff at vendors, such as Henkel, Oakite-Chemetall and others to validate my answer.


Ron Joseph

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