Aluminium [ from Wikipeadia ]
Pure aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in many environments. Some aluminium alloys, however, do not form the oxide layer well, and thus are not protected against corrosion. There are methods to enhance the formation of the oxide layer for certain alloys. For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and peroxide alternating with deionized water. The nitric acid and peroxide oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.[9]
Generally, there are two main ways to passivate aluminum alloys (not counting plating, painting, and other barrier coatings): chromate conversion coating and anodizing. Alclading, which metallurgically bonds thin layers of pure aluminium or alloy to different base aluminium alloy, is not strictly passivation of the base alloy. However, the aluminum layer clad on is designed to spontaneously develop the oxide layer and thus protect the base alloy.
Chromate conversion coating converts the surface aluminum to an aluminum chromate coating in the range of 0.00001-0.00004 inches in thickness. Aluminum chromate conversion coatings are amorphous in structure with a gel-like composition hydrated with water.[10] Chromate conversion is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys.
Anodizing is an electrolytic process that forms a thicker oxide layer. The anodic coating consists of hydrated aluminum oxide and is considered resistant to corrosion and abrasion.[11] This finish is more robust than the other processes and also provides electrical insulation, which the other two processes may not.