anodizing in vacuum

[Gerard Luppino]

Roger Smith recently wrote:
 
>Presumably, the only surfaces which can outgas are the warm ones. 
>Our dewar interiors are polished Aluminium, and while they are
>explicitly not anodized, they will of course be oxidised.  Anodized
>surface scan adsorb/absorb sig-nificant amounts of water which is
>hard to pump out and will later outgas.   Perhaps we should only
>open dewars in dehumidified air!
>Does anyone reading this coat treat internal surfaces in any way 
>to reduce this effect? Has anyone switched to alternate materials
>because of it?
 
I seem to recall that Gerry Luppino told me that they do indeed
anodize the insides of their dewars, with good success.
 
How about this, Ger ??
 
--John Geary

------

Indeed -- we have successfully used dewars that had several anodized
parts in the vacuum.  In particular, some had gold anodized inner
walls of the outer vacuum jacket.  It turned out that these do adsorb
more crud and need to be pumped on longer to before cooling, but once
they are clean, they stay clean so long as they aren't exposed to
warm moist air upon venting.  We find that short exposure to humid
room air is not catastrophic, SO LONG AS THE INITIAL VENTING IS TO
DRY NITROGEN.  I suspect that if the dewar is vented to dry nitrogen,
then N2 is adsorbed rather than water.  After venting, the dewar can be
opened and worked on and exposed "moist" air for periods of hours with
little effect on the pump down time.  If, however, the dewar is vented to
moist air, the pump down time is greatly increased.  Our standard procedure
for venting is to use the biol off from LN2 as our source of "pure" dry N2
and this works quite well.
 
Nevertheless, we now routinely remove polish the inside surfaces of our dewars,
which makes them easier to clean (and they look nicer!). 
 
Gerry Luppino