[shirleyquirk]

Metallic aluminium is insanely reactive and any Aluminium exposure will always be in the form of oxides or hydroxides. It's disingenuous to suggest that it is inert in this form, especially given that the point of the exercise is active, complex, catalysed photochemistry.

[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.