The propagation stage aluminiumoxidisesinto aluminiumions at the bottom of the pit. A reduction of either water or hydrogen occurs in contact with the metal on a site outside of the pit. With either reduction reaction the pH on the outside of the pit will increase to give an alkaline pH.The aluminiumion will form a film of aluminiumchloride or aluminiumoxychloridein the pit and stabiliseit. After a while the aluminiumchloride will hydrolyseinto aluminiumhydroxide. This leads to a decrease in the pH value to a more acidic environment, which increases the corrosion rate within the pit