Final answer:
A backing gas is used to prevent root oxide contamination by providing a protective atmosphere around the molten weld pool, thus preventing the formation of oxides during the welding process.
Step-by-step explanation:
A backing gas is often used in welding processes, particularly when welding metals such as stainless steel, to prevent oxidation on the underside of the weld. The backing gas accomplishes this by providing a protective atmosphere for the molten weld pool. Its primary function is to prevent root oxide contamination which occurs when the root of the weld is exposed to oxygen in the air. By filling the space with an inert or semi-inert gas (typically argon, helium, or a mixture), oxidation is minimized, ensuring a clean, high-quality weld. This layer creates a barrier that effectively shields the molten metal from exposure to O₂, thus preventing the reformation of oxides like Fe₂O₃ during the weld process.