Anodising is an electrolytic (electrochemical) surface-conversion process most commonly applied to aluminium and its alloys. The component to be treated is made the anode (the positive electrode) in a bath of acidic electrolyte, typically dilute sulphuric acid, while a suitable cathode completes the circuit. When direct current is passed, oxygen is liberated at the workpiece surface and reacts with the metal itself.

The key chemical change is therefore the controlled formation of a thick, hard oxide layer on the surface — for aluminium this is a porous aluminium-oxide (Al₂O₃) film that grows out of the parent metal rather than being deposited from the bath. This oxide film is far thicker and more durable than the thin natural oxide, greatly improving corrosion and wear resistance; its porous structure can also absorb dyes, after which the pores are sealed. So the correct answer is formation of a thick oxide layer.

• Removal of the oxide layer is the opposite of anodising; stripping or cleaning off oxide is done by pickling/etching, whereas anodising deliberately builds oxide up.
• Deposition of metallic ions describes electroplating, where metal ions from the solution plate onto the cathode; in anodising no metal coating is added — the surface is converted to its own oxide, and the part is the anode, not the cathode.
• Dissolving the metal away would describe electrochemical machining or excessive etching, not the protective oxide growth that characterises anodising.

Hence anodising mainly produces the formation of a thick oxide layer.