Aluminum is a base metal that spontaneously oxidizes in contact with oxygen (and moisture) from the air. In the process, the aluminum is covered with an oxide skin of aluminum oxide. This natural oxide layer can protect the underlying aluminum against corrosion to a limited extent under stress-free conditions.
However, the natural oxide layer is thin and vulnerable. Damages in the natural oxide layer reduce the corrosion resistance, because there is no completely closed oxide layer on the metal. Therefore, if no or incomplete passivation takes place, the aluminum is insufficiently protected against corrosion. Through the passivation process a protective layer (chromate or conversion layer) is formed, which forms a bond with the aluminum.