Aluminum Oxidation, should I anodize? 1

[wchowe]

Hi Guys,
I have an interesting problem. I have an aluminum (6061-T6) tube that I run gas through. I also have optics in the gas stream. I have been clear anodizing the tube to seal the surface and prevent oxidation. What I have found is that the aluminum, anodized, outgases( oils, water vapor)slowly over time, this contaminates my optics continually. I believe if I do not anodize the tube the outgasing will happen quickly and I will end up with clean optics and gas. The question, is will the aluminum (unanodized) oxidize to the point where I end up with a white powder, that will break loose and get carried by the gas stream onto the optics?
Thanks,
Bill

 

[Metalguy]

What is the gas? Anodizing produces a hard Al2O3 layer a few microns thick-very tough and highly unlikely to "break loose".

 

[kenvlach]

1) What is the temperature range? This is important for judging both the reactivity of your gas, and whether the difference in CTE between Al and anodize could cause cracking of the anodize.
2) Anodized aluminum can liberate some water when heated, but definitely not oil. Are you using a mechanical vacuum pump w/o a cold trap? Back diffusion of oil vapor could contaminate the anodize & be very difficult to desorb without solvent.
3) I have seen lots of expensive analytical equipment, 'big machine' physics, etc. IMHO, the best material for your application is electropolished stainless steel tubing. Wrap heating tape around the outside to aid in vacuum degassing.

 

[hacksaw]

apart from contaminated gases, you may be experiencing catalytic reaction with your gas stream and producing the deposits

anodized aluminum is extremely pourous (depending on the thickness, purity of the alumnum and the anodizing process)and will soak up everything in the shop where it was produced.

the way you inject the gas into your tube and pick it up will influence how much "litter" your optics will see. it is very easy to create a lot of un-intended circulation in optical cells. care flow path design is suggested.

 

[wchowe]

The gas is a mixture of N, He, and CO2. The temperature is around 85 degrees F. The oil that is outgassing, is probably some small amount left over from the machining our extrusion process. I believe I have to stay with the Al tube, because of design considerations. Yes, it is under vaccuum. What is a cold trap? Is it a super cooled surface to freeze out the water vapor? The question is, will I be better off with or without anodizing?
Thanks,
Bill

 

[hacksaw]

if you anodized the aluminum properly there could not be oil left (at least where the anodizing was effective). sounds like you need to document what you actually have.

are you using water pumped or oil pumped gases?

can believe that you are dealing with a vacuum system and haven't heard of a cold trap...

 

[kenvlach]

For this application, don’t anodize. While properly sealed, clean anodize will not outgas significantly at 85 F (unless UHV, ultra hard vacuum), it will retain contamination such as cutting oil and pump vapor condensate (unless oil-less pump or properly cold trapped).

For a clean, smooth surface on the interior of your aluminum tube: Polish as fine as possible, degrease, deoxidize (your anodizer knows how) [DO NOT ETCH], rinse in DI water and dry in warm air (e.g., 120 F). The aluminum will form an amorphous, invisible, protective oxide film of ~30 nm thickness.

A cold trap (often, liquid N₂-cooled) is a device used in vacuum systems to prevent vacuum pump vapor from contaminating the system, and sometimes, to prevent reactive vapors from reaching the pump. A cold trap can also be used to remove moisture from H₂ streams, etc. Where liquid N₂ is not available, a mixture of dry ice and acetone can be used. You will need a valve between the trap and the system as well as one between the trap and pump.

Are you using analytical grade cylinder gases, or something that needs filtering, such as welding grade or process gas? How are you preparing the desired gas mixtures?

Also, check the o-ring seals and vacuum/lubrication grease in connections and valves, etc.

 

[wchowe]

We are using as pure of gases as we can get. I am interested in your comment on "Also, check the o-ring seals and vacuum/lubrication grease in connections and valves, etc." Are there cetain greases that are best? are there certain seal material that will help? I was using silicone O-rings, but are switching to Viton. The gas does get heated in the process up to around 450 deg F. In the tube, however, it is cool!
Thanks,
Bill

 

[hacksaw]

aluminum and o-rings potentially at 450 F, thats really good