Bauxite

Aluminium oxide is an electricalinsulatorbut has a relatively highthermal conductivity(30 Wm⁻¹K^−1[1]) for a ceramic material. In its most commonly occurring crystalline form, calledcorundumor α-aluminium oxide, its hardness makes it suitable for use as anabrasiveand as a component incutting tools.^[5]

Aluminium oxide is responsible for the resistance of metallic aluminium to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of alumina (4 nm thickness) forms on any exposed aluminium surface.^[6] This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The alumina generated by anodising is typically amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.

Aluminium oxide is completely insoluble in water. However it is an amphoteric substance, meaning it can react with both acids and bases, such as hydrochloric acid and sodium hydroxide.

Al₂O₃ + 6 HCl → 2 AlCl₃ + 3 H₂OAl₂O₃ + 6 NaOH + 3 H₂O → 2 Na₃Al(OH)₆

Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminium oxide is on EPA's Toxics Release Inventory list if it is a fibrous form.^[7]