Re: vacuum outgassing of hot water sealed anodized aluminum?

"Grant Kiehne" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
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_En
hance_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


Any compound containing OH groups will lose water when heated in high
vacuum. Even at room temperature, the rate of loss of water may be enough to
prevent you getting a high vacuum. You also need to consider whether the
coating re-adsorbs water vapour when the system is open to the atmosphere.
Unless you have a compatibility problem with other materials, stainless
steel would be a better material for very high vacuum systems.

Bevan