If you wash them out with dishsoap and hot water, then ventilate it's just fine to weld a used tank.
You can also fill them with the same gas a mig welder uses and they are safe then too.
use to get carbon tetrachloride and it would completely neutralize the gas fumes. But now thanks to the epa, u can`t buy it anymore, but there is a more friendly replacement, I think.
