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Humidity Effects on Sealed Anodized Coating

I am interested in how humidity effects the mechanical properties of anodized coatings. Is it true that once the anodized layer is sealed, then humidity should not impact coating performance? Or will the sealed anodize surface still swell a small degree when the humidty is high and the temperature is high (100-115F), introducing internal stress?

Les Apgar
BFGoodrich - Troy, Ohio
1998

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1998

First let's generally agree on two different types of sealing methods for anodic coating. Hydrothermal sealing is the conversion of the aluminum oxide to a trihydrated Al2O3 which causes the coating to expand and closes the outer surface of the pore. Mid and low temperature sealing generally seals by the deposition of a material into the pores effectively sealing them.

If an unsealed, anodized part is placed in the normal atmosphere (some humidity and above freezing temperatures) the coating should hydrate and seal itself in some time frame. Also, a sealed part should continue to hydrate the coating beyond that which was done in the sealing tank.

I would expect this to be the case for a hydrothermally sealed part in a high humidity environment. I would also expect the induced stress from continued hydration to be minimal. Excessively high temperatures may cause the coating to crack (craze) due to the difference in expansion of Al with respect to Al2O3. I don't have information on the effect of the mid/low temperature seals.

Lee Branch
Richmond, Virginia

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