Aluminium oxide

Al2O3, existing in a cubic or a rhomboedric ("corundum") cristallic structure. In nature no metallic Aluminium is existing but only the Aluminium oxide mineral. Aluminium is generated by chemical reduction of the mineral. Metallic Aluminium in the open atmosphere is allways coated by an Aluminium oxide layer. Aluminium oxide is a very hard and solid mineral and protects the Aluminium mechanically and from additional corrosion.

Aluminium surfaces can be etched by a Halogene or an Argon plasma to remove the oxide layer. This may be necessary to provide electrical contact for bonding or sufficient adhesion for painting and coating.