Aluminum is actually a very active metal, in the sense that it oxidizes very quickly. This feature, while a weakness for most metals, is actually the key to its ability to resist corrosion. When oxygen is present (in the air, soil, or water), aluminum instantly reacts to form aluminum oxide. This aluminum oxide layer is chemically bound to the surface, and it seals the core aluminum from any further reaction. This is quite different from oxidation (corrosion) in steel or other metals, where rust puffs up and flakes off, constantly exposing new metal to corrosion. Aluminum’s oxide film is tenacious, hard, and instantly self-renewing.