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QUESTION: 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?
ANSWER: 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 +/- 25F.
Any possible inclusions, defects, outgassing will show up as spalling
in the coating.
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