Letter 062

Heat resistance of anodized aluminum

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Heat resistance of anodized aluminum surfaces   I am looking for information about the effects of elevated temperatures (300-350 °F) on the integrity of anodized coatings in the 1-2 mil range. Will crazing, cracking. dustiness (ablation) result from the dehydration of the coating? Can someone confirm this hypothesis - or have knowledge of the opposite?   Thanks for your help.  

Wolf Penzel

Can you give us a little more data on the application?

'The Properties of Electrodeposited Metals and Alloys' by William H. Safranek notes that the coefficient of linear expansion of the anodic coating is about one fifth that of pure aluminum and that 'This, obviously, is the reason for the susceptibility of coatings to crazing when significant temperature changes are encountered'.

Maybe one person's 'crazing' is another person's 'dustiness'?

Ted Mooney, P.E.  finishing.com Brick, NJ

Perhaps - the parts are an assembly of steel and anodized aluminum to which a plastic component is molded. Part temperature reaches the values indicated, and inspectors have objected to 'dustiness' of the anodized coating. It is probable that the appearance is a result of the heating (particularly since it is not evident prior to the molding operation), but may also be 'flash' or dust from the molding/flash removal process, as yet unanalyzed.   Thanks for your prompt reply.

Wolf Penzel

I'm hard coating a part with 2 mil hard anodize that I want to heat to 300 degrees C without cracking. Is this possible? What alloys would work best? If it's not possible, how high could I go before cracking occurs?  

Jim Laughlin
anodizing shop

As you can see, we found a related letter and added your inquiry to it, Jim. Anodized coatings, whether thick or thin, are brittle. They crack under mechanical deformation; and they crack from temperature changes because the coefficient of expansion is so much lower than that of aluminum.  

I don't know for a fact that you can't go to 300 degrees C without cracking (and the stove-topable hard coated frying pans must approach that temperature) but I would be surprised if you could. Further, I'd be very surprised if the coefficient of expansion of any aluminum alloy differed significantly from any other, so I don't think the alloy matters a lot for this particular consideration. If the part is susceptible to fatigue failure, note that these cracks act as stress risers and reduce the fatigue strength of the component.   The Surface Treatment and Finishing of Aluminum & Its Alloys can probably offer you all the detailed info you want.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

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