Original text - Engels - Teknik data

Al2O3, hard oxide existing in different modifications, of which korund has got the highest hardness. Al2O3 forms spontaneously on aluminium surfaces upon exposure to air, protecting the underlying Al from further oxidation. The layer and its protecting effect can be strengthened by anodization. Al2O3 surfaces can be etched in a plasma by Ar bombardment or employing halogen containing gases