Why is it that iron plates rust and crumble, whereas aluminium plates do not?

 Rust formation is actually oxidation of the metal. The exposed surface of a metal plate gets oxidised –– for example, iron to iron oxide and aluminium to aluminium oxide. In aluminium, this oxide stays on the surface and prevents further oxidation beneath its surface. Thus, it forms a protective layer on the metal surface. In the case of iron, the oxide does not have the ability to form a protective layer, so iron keeps getting oxidised and eventually crumbles.