Re: anodized aluminum for high vacuum?

Grant Kiehne wrote:

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

josh halpern

> 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
>