why is rust sometimes considered bad and sometimes good? isn’t all rust eventually bad?
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Your traditional rust peels, because it “swells” (it’s crystal structure is larger than iron is ot increases in volume), bonds poorly to the iron underneath and is brittle. This is bad, because all it does is expose more iron to oxygen, leading to more rust, until all iron is eaten away.
But it’s possible for an oxide layer to not change volume a lot (there are acceptable limits) and be well bonded to the metal underneath. It’s still brittle, but it’s really hard (as ceramics do) and it’ll “heal” if damaged (sacrifice a bit of exposed metal). Because it’s strongly bonded, it won’t peel off, and it’s hardness and chemical inertness (oxides are really stable) it acts as a protective barrier instead.
Aluminium has this naturally. Steels can be treated to have it (blueing for example). Some special steels naturally create a better rust layer (it’s still your traditional orange) like this, that doesn’t peel.
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