[syntaxing]

Not a chemist but a MechE. How is aluminum insanely reactive? Aluminum oxide is significantly less prevalent than iron oxide (aka rust). Even on a sheet metal part, you might get a couple microns worth of aluminum oxide. Hell, we purposely anodize it so we get more aluminum oxide.

[sclarisse]

You anodize it to get more because the raw material is so reactive that it forms a thin layer of oxide more or less instantly. That keeps more oxygen out, but it’s too thin and can be easily scratched.

[BostonEnginerd]

Aluminum is extremely reactive. It self-passivates with Aluminum oxide very quickly. The difference between rust and AlO3 is that the AlO3 adheres well to the base metal and doesn't flake off like rust does.