Why does iron turn flaky instead of just getting a thin coating?
Your bike chain and a magnesium ribbon both react with oxygen, but they do not behave the same way afterwards. With some metals, you get a surface layer that acts like a sealed phone case. With iron, you get damage that keeps spreading.
Simple oxidation happens when your metal combines with oxygen and makes an oxide just on the outside, often as a thin surface layer.
A classic example is magnesium burning: 2Mg(s) + O₂(g) → 2MgO(s). You would see a bright white flame, and the metal ends up with a white solid, magnesium oxide, MgO.
That surface product can sometimes help your metal. In magnesium, and especially in aluminium, the oxide can be packed tightly enough to slow down more reaction underneath. So oxidation is not always a disaster.
Corrosion is when your metal is gradually eaten away by reactions with its surroundings, not just oxygen alone but things like water, acids, and dissolved salts too.
Instead of stopping at the surface, the damage can move inward. That is why corroded metal can lose strength, develop holes, and fail in real structures.
For iron and steel, this kind of damage is called rusting. The reddish-brown material is hydrated iron(III) oxide, written as Fe₂O₃·xH₂O. The surprising part is that rust does not protect the iron underneath well, so the process keeps going.
Think of it like a cracked screen protector versus a proper waterproof case. A protective oxide layer is like the case: it blocks more damage. Rust is like a protector that flakes off and leaves new parts exposed, so your metal keeps getting attacked.
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