This is the kind of bs info that the internet experts love to spread on forums. The only “built in” protection aluminum has is IF it is Alclad. If there is the tiniest scratch that alclad is compromised. Drill a hole and guess what, you’ve compromised the alclad. In reality the aluminum will out last you if untreated, but the big guys like Boeing prime everything. In critical areas they are assembled with wet primer. All external skins on pressurized aircraft have sealant applied. Built in protection indeed.