Re: anodized aluminum for high vacuum?

hanson wrote:
> 
> "Josh Halpern" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
> > 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.   This yields a
> > >fresh native oxide on the aluminum surface.  My application requires a
> > >thicker oxide layer (microns).
> > >Grant
> > >
> > Generally one uses stainless steel for high vacuum.
> > Al might be ok for 10-8
> > josh halpern
> >
> Yo, Josh, old bud,
> Easy on the that "generally". I think that all non-load bearing,
> non-ablating space vehicle skins are made from Aluminum,
> 2024, 6061 or 7075, about the thickness of a beer can.
> Most of it is not even anodized, but bare, maybe CrO4/SiF6 treated.
> The pressure in space at 130 - 140 miles up is about 10e-6 torr.
> 250 miles up at MIR we have a vacuum of about 10e-8 torr, and
> at the Hubble Space Telescope at about 370 miles up the pressure
> is near 10e-9 torr. Alu appears to serve quite well.

A hard vacuum system is not about survival, it is about outgassing. 
Aluminum is notorious for outgassing hydrogen.  Forever.  Large scale
electrolytic aluminum production typically sparged elemental chlorine
through the molten product to purge hydrogen by reaction.  Castings
would otherwise solidify with serious voids.

One encounters a similar problem with copper and silver.  The molten
metals are incredible sponges for oxygen.  Thomas Edison is said to
have been bothered by a copper king at a party.  He asked for a nice
trophy - a cast cubic foot of copper weighing its proper theoretical
density.  If you can get cast copper to 90% of its theoretical
density, you are having an exceptionally fine day (or do vacuum
casting).  Quality copper objects are usually forged.

-- 
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