Correct Response: C. When welding some metals such as aluminum and magnesium, it is very important to protect the high-temperature metal from the oxygen and water vapor that are in the air. For this reason, an inert shielding gas is used (typically a combination of helium and argon). The most common welding process for these metals is gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding. Shielded metal arc welding (A) is mostly used for welding steel and iron. It is not commonly used with nonferrous metals such as aluminum and magnesium because the shielding gas, which is given off by the flux, does not tend to be extensive enough to protect these metals from oxidizing. As the name implies, oxyacetylene welding (B) uses a combination of pure oxygen and acetylene gases. The presence of pure oxygen precludes the use of this welding technique with aluminum and magnesium because these metals will react with the oxygen, creating oxides that interfere with the welding process. Since carbon arc welding (D) is carried out without any mechanism to shield the metal from the oxygen and water vapor in the air, it is not appropriate for welding aluminum and magnesium.
