Anodising is used to increase surface hardness, wear resistance and dielectric strength of metals such as Aluminium, Titanium and Magnesium. Anodising can also provide a good base for painting or colouring, creating an extremely stable but porous oxide layer on the surface of the metal.And there you were thinking it was a decorative finish!
Anodising is very simply an oxidation process, as is the corrosion of steel into iron oxide (rust). However, the oxides of aluminium, titanium and magnesium are highly stable ceramic compounds which are integral to the metal surface, they will not flake or peel. Anodising is the process of submerging the metal in an acidic solution, and applying a voltage to promote the absorption of oxygen into the surface. In the case of anodising aluminium, this causes the formation of the hard ceramic compound, Aluminium Oxide.
This is by far the most widespread process and varies from Chromic Anodising in the chemical composition of the solution and process variables such as the level and method of application of voltage. A range of oxide layer thicknesses can be achieved as can effective colouring, most commonly achieved by the application of a secondary colouring treatment. Film thicknesses in aluminium are in the range of 5-25 Microns, with 10-15 required for effective colouring using dyes.
Anodising is largely understood as a finishing process as it provides mechanical and decorative property enhancement of a component largely in the finished state. However, anodising can be used very effectively as a pre-treatment for painting processes – providing micro-pores for the paint to adhere to. In the case of Magnesium anodising, it is almost exclusively used to provide a solid base for further painting. The porous and low density surface is not anywhere near as protective as the oxide layers produced on aluminium and titanium.
Laser marking can remove the oxidised layer of material locally, when combined with colour anodising this can create a high contrast between the base material and the coloured anodised layer. This can be used for decoration, such as the marking of a logo, or more functional purposes such as part numbering or marking.