anodized aluminum for high vacuum?

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

"Grant Kiehne" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ] m...

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" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ] m...

On 25 Oct 2003 06:30:32 -0700, in sci.chem.coatings you wrote:

Hi Grant,


Correct.


You should perform the anodization with your required layer thickless,
then followed by a sealing of the pores with superheated steam. To
prevent water being trapped, I would heat the whole part up to 380K
while treating it with steam.


I would seal it, definitely. And I would make a small test with an
electrically heated surface while steaming it. If the sealing is still
successful, the amount of water being trapped should be much lower

Unsealed anodized alumina is a very good catalyst
support material because of its high inner surface area. Its surface
is strongly acidic and it will definitely soak with water when exposed
to air. You should pay good attention to the electolyte used for
anodization: H2SO4 as electrolyte will cause very narrow pores of
roughly 5-10 nm, oxalic acid will have approximately 20-40 nm pores
and phosporic acid will have approximately 100. The former will have
the highest inner surface area, the latter the lowest and the oxalic
is somewhere between both. Therefore, I'd suggest (for your
application) you give phosphoric acid a try.

If you are not happy with anodic oxidation, you should give ANOF a
try. Thats a german abbreviation for a method best translated as
"ANodic Oxidation applying Spark Discharges". These sparks with their
high temperature cause a very hard alpha-alumina layer (nearly
ceramic) having no pores. I've once got a layer at my former
university for catalytic tests. The layer was inert, so I wanted to
get the catalyst by cooking / dissolving it with diluted HNO3. It was
resistant to diluted (20%) and hot HNO3 for 48h. If you need more
information about this method, I can give you a very competent contact
person at my former university. He has forgotten more about aluminum
and coating methods than I have learned in my life ;-)

Greets from Germany and kind regards,

Ansgar
--
The two most common things in universe are hydrogen and bureaucracy.

Thanks, Ansgar,

You suggested:


I've done some preliminary investigation into this method. A start-up U.S.
company called Microplasmic ([Only registered users see links. ]) has a patent on such a
process (I found it on [Only registered users see links. ]). It is commercially available under
the trade name CeraFuse from Ceramic Coatings Technologies
([Only registered users see links. ]). The process yields a hard, dense alpha-alumina layer
covered by a gamma-alumina layer that is buffed off. With buffing only, the
surface of the alpha-alumina is quite rough (~120 microinch), but it can be
diamond grit polished like a piece of sapphire down to the desired
roughness. I am interested in ~32 microinch or better. Reportedly, there
are some pores due to the micrplasmic discharge (sparks).

Reportedly, the CeraFuse process has been applied to aluminum vacuum chamber
surfaces, although it is not clear if one can consider it suitable for high
vacuum or uhv application.

Any academic or commercial contacts you can provide would be helpful. Might
there be non-U.S. vendors who can apply a similar process? I wonder if the
Microplasmic process is patent-protected outside the U.S.?

Cheers,

Grant


"Ansgar Kursawe" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

Am Sun, 26 Oct 2003 12:36:50 GMT, Grant Kiehne postete:


Great patent strategy. That method is much older than the german
reunification. My former collegue worked during his PhD thesis on that
Method and that was at the beginning of the 80'ies (in the former GDR)
;-)


Good name, ANOF coated aluminum feels and looks like ceramic. Even if
it is not of your special interest, it can be quite colorful i.e. with
small amounts of Co salts used in the electrolyte.


Yes, there are a few pores and no, they do not make trouble. I've
never experienced a better protective coating for Al than this.


Yes, there is a scientific life even outside the US and sometimes they
hold patents. As far as I heard it is patent protected in Germany by a
company making coatings for Titanium implantates. If you already have
a company on hand to coat your material, I'd really go for this
coating.

Btw, it is not really difficult to build your own coating machine. All
you need is a receipe for an electrolyte, a single 400-2000V DC
sawtooth generator having some power and a stainless steel or glass
vessel having a steel-ring electrode. If you switch the lab lights
off, the sparking vessel looks really great ;-)

Kind regards,

Ansgar Kursawe

--
The two most common things in universe are hydrogen and bureaucracy.

in article [Only registered users see links. ], Ansgar Kursawe at
[Only registered users see links. ] wrote on 10/26/03 2:47 AM:


Except for thickness, what are the important differences between the surface
of aluminum oxidized in air with respect to anodized aluminum? From what I
understand, clean aluminum surfaces in air are immediately oxidized.

Bill

Am Sun, 26 Oct 2003 18:41:41 GMT, Repeating Decimal postete:



The porosity, the well defined thickness, the optical quality, the
improved sturdiness and finally the acidity.


Yes. If you have a clean Aluminum surface (i.e. by etching with NaOH),
just dip it into concentrated H3PO4. This forms AlPO4 at the surface.
This passivation looks and is a bit better than an uncontrolled
re-oxidation at the air, but it cannot replace a good anodization.

Kind regards,

Ansgar Kursawe
--
The two most common things in universe are hydrogen and bureaucracy.

For those interested in further investigation, here is a list of commercial
outfits that claim to offer plasma electrolytic oxidation of aluminum parts:

[Only registered users see links. ]

[Only registered users see links. ]

[Only registered users see links. ]

[Only registered users see links. ]

[Only registered users see links. ]

Grant

"Grant Kiehne" <[Only registered users see links. ]> wrote in message
news:CdPmb.20098$[Only registered users see links. ].prodigy.c om...
U.S.
the
be
chamber
high
Might
the

Grant Kiehne wrote:

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

josh halpern

"Josh Halpern" <[Only registered users see links. ]> wrote in message
news:WNFob.21487$[Only registered users see links. ]...
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.
Have fun, Josh,
hanson