vacuum outgassing of hot water sealed anodized aluminum?

Folks,

 

This is a follow-on to my recent post, "anodized aluminum for high vacuum?", on these same newsgroups.

 

Does anyone understand vacuum outgassing of hot water sealed anodized aluminum?  I am looking  for a physical/chemical model to explain what is going on here.  The "party line" is that anodized aluminum is not suitable for high and ultra-high vacuum applications because it is porous and traps water that is subsequently released into the vacuum system.  Yet, I have read that an anodic oxide layer can be sealed with hot water.  When the water temperature is greater than 80 deg. C, the reaction goes like this:

 

Al2O3 (anodic oxide)  + H2O ---> 2AlO(OH) (boehmite)

 

(ref.: http://www.metalast.com/technology/Research_PDF/Technical_Reports/Sealing_Enhance_Perf.pdf)

 

So, if the pores are sealed with boehmite, reducing/eliminating the porosity, where does the water come from when the coating is vacuum baked?  Does boehmite dissociate back into water at high temperature?  Or is the boehmite filling the pores also a high surface area material from which water desorbs with vacuum baking?  Perhaps the sealing process results in a coating that is boehmite plus some entrapped water that outgasses during vacuum baking?

 

In summary, I am looking for a physical/chemical model of vacuum outgassing of anodized aluminum sealed with hot water.

 

Grant