Why do aluminum copper and zinc form more protective surface layers than iron?

The premise that aluminum, copper, and zinc form "more protective" layers than iron is accurate because these metals develop compact, adherent oxide films that act as a barrier to further oxidation. In contrast, iron forms rust—a porous, non-protective oxide layer that allows moisture and oxygen to reach the underlying metal, thereby promoting continued corrosion.1

The effectiveness of these surface layers depends on their physical structure:

• Aluminum, Copper, and Zinc: These metals form stable, compact, and adherent oxide layers. Once this film covers the surface, it effectively limits further chemical attack by isolating the metal from the environment.1
• Iron: The corrosion product of iron, known as rust, is porous and flaky. Because it does not adhere tightly or form an impermeable seal, it fails to prevent the ongoing electrochemical reactions that degrade the metal.1

To protect iron and steel from this process, engineers often use barrier protection methods, such as galvanization, which applies a layer of zinc to provide both a physical barrier and sacrificial protection.1