Which material, once oxidized, is known to corrode further?

Iron, when oxidized, forms iron oxide, commonly known as rust. This rust does not provide any protective barrier; instead, it continues to allow moisture and air to reach the underlying metal, leading to further corrosion. Oxidation of iron is not just a surface phenomenon; it promotes additional corrosion as the layer of rust produces weak points that can lead to extensive degradation over time.

Aluminum, on the other hand, behaves differently upon oxidation. When aluminum is exposed to oxygen, it forms a thin layer of aluminum oxide on its surface. This oxide layer is protective and prevents further corrosion of the underlying metal. Therefore, aluminum does not corrode further once oxidized.

Copper also forms an oxide layer when it oxidizes, typically developing a patina that can actually protect the underlying metal from further corrosion.

Thus, iron is the only material listed that, once oxidized, is known to corrode further, which makes it the correct answer to the question.