Question

Welders use helium (He) and argon (Ar) gases to blow over the metal during the welding process. Why would they want to use these gases?

Answer 1

Such gases are used for the same reason that chemists use them to protect reaction mixtures: both helium and argon are quite inert (unreactive) and they form a protective atmosphere around the welding site.

Explanation:

In arc welding, a large potential difference is imposed between the arc tip and the welding site: the welding site becomes HOT, and melts. In the absence of inert gas, the welding site would rapidly oxidize; in the presence of inert gas, the arc tip (usually tungsten) and the welding site are protected from aerial oxidation.

Inorganic chemists regularly use argon gas for this reason. To protect reactive chemicals from oxidation. Argon and helium, as (reasonably) inert gases, are supremely unreactive, and tend not to be oxidized. Of course, under the high temperature conditions of arc welding, oxidation would be very rapid in the absence of the inert gas.

Argon is a good gas for both welders and chemists inasmuch as it is denser than air (i.e. argon is heavy!), and tends to displace the reactive gases.