What is anodising, which material is it most commonly used on, and what advantage does it offer?

• A. A mechanical process of roughening the surface; used on steel; improves adhesion by mechanical interlock.
• B. A thermal treatment that hardens the surface of copper alloys; improves wear resistance.
• C. An electrolytic process to thicken the oxide layer, commonly used on aluminium; it increases corrosion resistance and allows colour/finish control.
• D. A vapor deposition technique for applying a protective coating to titanium; improves hardness.

Answer: (C)

Anodising is an electrolytic oxidation process that thickens the oxide layer on a metal surface. It is most commonly used on aluminium because aluminium readily forms a protective oxide, and anodising grows that layer much thicker to enhance its properties. The thicker oxide is harder and greatly improves corrosion resistance, and because the oxide is porous, dyes or sealants can be absorbed to produce different colours and finishes. The metal being anodised acts as the anode in an electrolyte bath, driving the formation of the oxide layer. This isn’t a mechanical roughening process, a heat treatment for copper alloys, or a vapor-deposition coating—those describe different surface treatments.