Both processes use cover gas to protect the weld from oxidation. In the case of MIG welding, which stands for Metal Inert Gas, an inert gas is used in the process: this carries the current more easily, as it does not react with the weld pool.
An inert gas is also used for the TIG welding process; the acronym, in fact, stands for Tungsten Inert Gas. The most commonly used inert gases are argon and helium, which are perfect for deoxidising certain materials such as aluminium.