Surface treating mild steel for hard vacuum

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norlesh
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I understand mild steel is very bad at out gassing so is never usually considered for hard vacuum applications. But if the decision was constrained by other factors would it be possible to apply a surface treatment or coating to the internal walls of the vessel - am I correct in assuming that as long as the chambers structure is up to the task of withstanding the external pressure any internal surface skin only needs to withstand the small non-accumulative forces of individual atoms 'attempting' to out gas from the inner thickness of the walls?

The 'other factors' constraining my decision to use mild steel is I have an old fire safe with 0.23 m3 internal space that I would like to turn into some sort of vacuum deposition chamber Oh and this thing is located on the second floor of my building and probably weighs in at a tonne so it's not like I can send it to get plated ;).
 
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Well my initial goal is to be able to do aluminium coating of optics - still looking for a reliable number on what pressure is required for this. The upper/lower limit of UHV seems like a good target to shoot for which according to wikipedia starts at 10-7 Pa.

As for heating this thing, the walls are 850 mm thick - not sure if its all steal or if there is some kind of refractory in between but either way it would take obscene amounts of energy to raise the temperature. Perhaps I could accelerate the out gassing by initiating a low pressure arc to irradiate the walls with UV.

Maybe vaporizing a couple of hundred grams of a suitable substance (2.4 m2 surface area) on the inside could work assuming it didn't fuse the door permanently closed that is.