anodized aluminum for high vacuum?

Does anyone have experience using anodized aluminum parts in a high vacuum or ultra-high vacuumm (UHV) system? I am investigating using
some relatively large-area anodized aluminum parts in a system that must achieve ~5x10^-8 Torr (cool) after baking at 135 deg. C for ~12 hrs. The alloys are 6061-T4 and 6063-T5. I am familiar with the standard preparation of aluminum for vacuum service of machining and hot basic solution etch, followed by hot air bake. This yields a fresh native oxide on the aluminum surface. My application requires a thicker oxide layer (microns).
I have been warned that generally the anodic oxide layer is a porous, hygroscopic material that is a poor coating for high vacuum service, due to its high outgassing rate. Yet, anodized aluminum does seem to be necessary for certain vacuum applications such as systems with corrosive gases/plasmas. Is there a type of anodization that would be a practical (but perhaps imperfect) coating for high vacuum service? For example, would a thin (0.0001"/2.5 micron), sulfuric acid clear (Class 1) anodized coating be o.k.? Which would be best, low-temp. hard-coat (Type III) or room-temp standard (Type II)? Should the coating be sealed and if so, by what method, hot water? Or does the sealing process just trap water that will later outgas?
Altenatively, has anyone tried the Cerafuse (Microplasmic) aluminum anodization process on parts for high vacuum service? Any problems with outgassing? Other problems?
Thanks,
Grant
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Grant Kiehne wrote:

Generally one uses stainless steel for high vacuum. Al might be ok for 10-8
josh halpern

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Josh,
See for example:
http://www.atlasbimetal.com/
Atlas sells aluminum UHV vacuum parts and chambers with with stainless steel flanges. Generally, I think you are correct about using stainless for UHV. But there are some applications for which aluminum is better suited due to technological and/or economical constraints.
Grant

10-8

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.