Which type of oxide forms quickly when metals or alloys are hot, especially during welding?

The correct answer is aluminum oxide. When aluminum and its alloys are heated, especially during processes such as welding, they rapidly form a protective layer of aluminum oxide on their surface. This occurs because aluminum reacts readily with oxygen, particularly at elevated temperatures. This oxide layer serves as a barrier that prevents further oxidation and protects the underlying metal from corrosion.

Aluminum's high affinity for oxygen means that it will quickly generate this oxide layer even in the presence of moisture or air, and this characteristic is crucial to consider during applications like welding where clean metal surfaces are necessary for proper adhesion and structural integrity. The adherence and properties of aluminum oxide also play a significant role in the welding process, as it can affect the quality of the weld if not properly managed.