Anodisation is the electrolytic oxidation of aluminium, a process used for the surface treatment of aluminium. During this process, an oxidic protective layer is created on the metal by means of anodising. In contrast to galvanisation, the protective layer is not applied but an oxide layer is created due to the conversion of the metal surface. This layer is permanently connected to the aluminium because it consists of the metal itself.
The oxide layer which is created by the aluminium itself is only a few nm thick. The anodising process creates a layer with a thickness of 5-25 µm which provides improved corrosion protection and which can be dyed in different colours.
Before the process can be started, the aluminium parts must be degreased and pickled using a caustic soda solution. Subsequently, the workpieces can be anodised with or without dye. In doing so, the pre-treated layer is dyed using different colours and then, it is sealed by means of hot water.
Normally, aluminium is anodised in sulphuric acid or oxalic acid electrolytes by use of direct current. This process is called anodisation because the metal is connected as an anode. The counter electrode usually consists of a material which is not affected by electrolytes.
Now, the pores have to be sealed in order to prevent the accumulation of corrosive materials. To do so, the anodised workpiece is placed in hot purified water. During this process, the aluminium oxide reacts with water, developing aluminium oxide hydrate. Because the pores absorb the water, they are narrowed and closed.
During anodisation, the formation of the oxide layer is influenced by many factors. Amongst others, these factors include the selection, concentration and temperature of the electrolyte, the type of current, current density, as well as the voltage and duration of the treatment. Changes in temperature also have a direct influence on the anodised layer. In some cases, the electrolytes contain additives which can influence the properties of the oxide layer. This way, it is possible to improve the resistance and look of the layer by adding additives. Due to the aluminium oxide, the anodised layer is, other than aluminium itself, not electrically conductive.
During anodisation, it is possible to achieve a hardness between 200 and 400 HV; however, a hardness of up to 600 HV is also possible when using the hard anodising method. In this case, the layer will be thicker and more brittle. This may cause cracks which is why it is necessary to finalise the workpiece before anodisation in order to prevent damage.