The oxide layer on aluminium protects the metal, but rust does not protect iron in the same way. Why not?
Question 1 Report
The oxide layer on aluminium protects the metal, but rust does not protect iron in the same way. Why not?
Answer Details
The difference lies in the physical nature of the two oxide layers. Aluminium oxide forms as a thin, continuous film that grips the metal tightly, so it seals the surface and blocks further attack. Rust, hydrated iron(III) oxide, is porous and flaky, and it lifts off the surface as it builds up.
Once the rust falls away, fresh iron is exposed to water and oxygen again, so corrosion continues layer by layer until the object is eaten through.
Rust is a solid, not a gas, and it does not change back into iron on its own; extracting iron from its oxide requires reduction in a blast furnace. Describing rust as impenetrable contradicts the observation being explained.
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