Untreated aluminium has very good corrosion resistance in most environments. This is primarily because aluminium spontaneously forms a thin but effective oxide layer that prevents further oxidation.
Aluminium oxide is impermeable and, unlike the oxide layers on many other metals, it adheres strongly to the parent metal. If damaged mechanically, aluminium’s oxide layer repairs itself immediately.
This oxide layer is the main reason for aluminium’s good corrosion properties and use in many applications. The layer is stable in the general pH range 4 – 9. In strongly acid or alkaline environments,
aluminium normally corrodes relatively rapidly.