Show me documentation where it states it is okay to use co2 as a brazing purge gas.
Do a search "co2 corrosion carbon steel".
CO2 is not an inert gas.
Maybe my terminology was a little strong. Unless somethings have changed, I read in one of my books never to introduce co2 into refrig systems. Having pulled high vacuums on the shells of many large cryogenic vessels, I can verify that it is much easier & faster to remove nitrogen with a vacuum pump than it is co2, air or any gas with a moisture content. Boiling off moisture is what delays busting below 500mi. I needed to get below 100mi on perlite tanks, and 10mi on super insulated mylar tanks.
CO2 will also react to high heat such as it does in the GMAW.
Read about it here.