Sorry. Aluminium will ONLY corrode if the
"pure" unprotected alloy is exposed to oxygen. This can be an issue if Ali is violently exposed to heat and shear exposing the "pure" unoxidised metal. It will spontaniousely combust like magnesium does and can be very difficult to stop once burning.
We all know Ali is a passivating metal. Meaning that it spontaneousely produces a surface oxide that becomes a barrier to oxygen and thus stops the oxidation process from continuing. "Coating or sealing the surface" as you originaly said, even if it meant the removal
of the oxide somehow, would still sheild the Ali from oxygen and prevent oxidation just as the natural fim does. However, if the "alien" film is scratched, the imdeiadte surface below will instantly oxidise and be protected.
Notice I am using the term oxidation and not corrosion
. Although the two are the same, I consider (rightly or wrongly) that corrosion is more of an electrolytic errosion issue than oxidation. And this is the most likely culprit of the orginal situation.