Question

Why does corrosion always happen layer by layer?

Answer 1

Corrosion happens when the metal reacts with oxygen or moisture in the atmosphere to form its metal oxide.
So the reaction ocuurs in atomic levels.
The top layer atoms of the metal react first to form their corresponding metal oxides.Then the mext layer.
You need to keep in mind the fact that the oxygen or other corroding agents dont mix or get inside the metal, since its a solid.The reaction ocuurs only layer by layer.
Once the top layer is corroded, the layers beneath it is exposed to the corroding agents, and eventually the second layer corrods.
This is how the metal corrods.
So we can say that corrosion always happen layer by layer