The chamber is going to be around 6" in diameter and 12" in length,
wall thickness is going to be 5mm. Its purpose is to hold data disks
for long term storage. The chambers are going to be placed into fire
safes and the safes will be placed into underground bunkers.

Would plating the Stainless with gold help prevent corrosion?

The chambers will be filled with nitrogen gas to help preserve the
disks. What would you recommend for a seal? It cannot be any substance
such as rubber due to outgassing. I need something that makes a good
seal and does not deteroriate rapidly. The chambers will be opened
periodically, around every 50 years to refresh the disk media.

Can you also recommend a good machine shop in the Phoenix, AZ area? I
need a place that can weld all the pieces together.

Thank you for your help.