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Baking aluminum parts that have been conversion coated per Mil-C-5541

Will baking aluminum at 300 °F. that has been conversion coated per MIL-C-5541 deteriorate the conversion coating, and if so, what will the results look like?

Homer D. Black
Precision Machine Shop - Tempe, Arizona, USA
2005

Read the Mil Spec and you will see that the baking temperature is limited to 140 °F (I prefer 120 °F ), to prevent dehydration of the water bearing gel coating. The appearance will be dried out and cracked and flaking off.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

300 °F will certainly destroy any benefits from the conversion coating. The coating will dehydrate and, if you look under a microscope, you will see that the coating has cracked where it has shrunk. The drying process also "locks in" the chromate and so the coating no longer offers corrosion protection.

Current wisdom is not to expose the coating to anything over 140 ° F, and this should only be for short periods (maybe up to half an hour). Recently, I have had my doubts about going as high as 140 F as I think this still causes some dehydration of the coating. I currently recommend that our treatment shops do not go over 120 F.

In any case, check with the manufacturers for their recommended maximum exposure temperature (and then for good measure knock 10 deg F off to give you a safety margin).

Brian Terry
Aerospace - Yeovil, Somerset, UK

A. It's been awhile since I've done any chromating, as I recall it's not recommended to go above 150 °F with cured conversion coatings. At 300 F, I would imagine that the conversion coating would be powdery.

Marc Green
anodizer - Boise, Idaho
2005

Green Powder Coating Tape

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A. If powder coating is what you are referring to "baking", the chromate conversion coating does not dehydrate in the curing process in the absence of air. Even though the substrate temperature is exposed to heat of 325 °F or higher, the powder coating protects the substrate from dehydrating by shielding it from air. Those areas not coated with powder are normally protected by masking devices (tape, plugs, etc.) that perform the same function as the powder coating with respect to shielding the substrate from air.

Caleb Hammons
- Warner Robins, GA, USA
2006

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