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)

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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

vacuum outgassing of hot water sealed anodized aluminum?
 

My guess is that it is a combination of water trapped in "sealed" pores
that can migrate out when things get hot enough, plus water adsorbed on
the surface since even sealing the surface leaves behind a pretty
respectable surface area compared to the simple geometric surface area,
plus water of dehydration when you get hot enough but I would think this
would take several hundred degrees C. Obtaining a given base pressure
is a matter of balancing pumping speed, outgassing rate per unit area,
and total surface area. I doubt if you are going to get a definitive
answer without doing some experiments of your own with samples of
anodized materials. You don't need three digits of precision, so if you
have access to almost any high vacuum system that reaches 10-6 torr or
better you can just do a simple comparison test. Pump the chamber down
empty and plot pressure versus time. Then put in as much anodized
material as you can, leaving lots of room between sheets. For example,
if you have a 12" ID x 12" tall bell jar, make up a wire rack so you can
stand 5 8"x10" sheets with an inch or so between each sheet. Have the
racks in for the empty pumpdown, as well. Get some 16 ga (0.06" thick)
aluminum sheet sheared to size, find a local anodizer, get it anodized
with the process you want to test, and record the pumpdown curve. Any
standard vacuum text (I like O'Hanlon) will discuss outgassing rates and
pumpdown curves and give you enough information to analyze your data.
We pay about $20 for sulfuric acid and $35 for hardcoat, per plate, when
we get 18"x30"x0.16" plates done in batches of 100, so you should be
able to get yours done for under $100. Go nuts and test stainless steel
too, just to compare your values with the literature :-). Have fun and
call it research :-). Once you get the outgassing rate within an order
of magnitude you can look at your proposed vacuum system and see if you
are anywhere close on conductance and pumping speed, and go forward from
there.

--
Regards,
Carl Ijames [Only registered and activated users can see links. Click Here To Register...]