Final answer:
In welding, the purpose of a backing gas is to prevent root oxide contamination by creating a protective barrier around the molten metal, preventing it from being oxidized by atmospheric oxygen.
Step-by-step explanation:
A backing gas is predominantly used in the welding process to:
d) Prevent root oxide contamination. In welding, a backing gas creates a barrier that prevents the molten metal from exposure to oxygen, which could otherwise oxidize the metal back to its oxide form, such as turning iron (Fe) back to iron oxide (Fe2O3).
While backing gases play a role in other aspects of the welding process, such as preventing root burn-through, reducing root concavity, and supporting molten root metal, their main purpose is to protect the weld area from contamination by atmospheric gases, especially oxygen. Similar to how a sacrificial anode protects metal from corrosion in a galvanic cell, the backing gas ensures that the integrity of the weld is not compromised by oxidation.