Re: anodized aluminum for high vacuum?

"Grant Kiehne" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> Which [anodizing] would be best...........

Unless specified otherwise by your client, I'd go for an oxalic/sulfuric
anodize coat, A-8625, type 3, ~ 1.5-2 mils, followed with a NiAc2/Cr2O7
sealing step, then dehydration by baking for 12 hrs at 375 +/- 25°F.
Any possible inclusions, defects, outgassing will show up as spalling
in the coating.

Take care,
hanson

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

> 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