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