Is it in context ot TIG welding? pipes? when you do it, then the Argon (usually, but may be some other inert gas too) is not only fed through the TIG nozzle, but also fed to the inside of the pipe so (with the ends closed) so that the molten metal (of the weld) is not exposed to air and hence liable for oxidation. It is in the inert gas atmosphere like the top layer (facing the nozzle). This is a common practice for the pipeline TIG welding.
As already mentioned, typically Argon is used. The gas that comes from the TIG torch is your shield gas. The gas that you run into the inside of the pipe is the purge gas if you don't get a full purge you'll get what they call "sugaring" which is oxidation and it looks like black sand typically this happens on stainless.
One must be very careful not to completely close of the ends of the pipe. A tiny hole should be available (1/16" or so) to allow the gas to escape. Without the hole, the last little bit of the root pass will "suck back". Also only a very low flow (5-10 cfh) should be used so that the external shielding gas is not blown away. A low pressure flow meter should always be used.
What is Back Purging?
Re: Definition of "back purging"?
