kfh227 wrote:Hope I got this right. I went to an engineering school a long time ago and a mechanical engineer told me.
Close enough to correct for me.
The difference is rust is
hydrated iron oxide. The approximate formula is Fe2O3.32H2O, the number of oxygen molecules being some what variable.
Anyway, the result is a huge increase in mass and volume from all those extra atoms attaching to the iron.
So it flakes, and the surface of the iron remains vulnerable to further attack from water and oxygen.
Aluminium on the other hand, unless exposed to lots of water, oxidises to Al2O3 almost immediately it when exposed to the air.
That oxide layer stays put, passivating the metal. It will only continue to oxidise to any significant degree is something damages/removes the layer.