There are a few different types of anodising. In Graph, we do ‘sulphuric acid anodising’ – the parts are put into a sulphuric acid bath and an electric current is passed through to make the aluminium oxide grow on the surface. Depending on the temperature of the bath and the voltage of the current, the size of the holes or pores can be changed. Hard anodising is carried out at a lower temperature and with a higher voltage than clear anodising. This makes the pores smaller and the cells are packed more tightly together, which gives a harder coating. Because the pores are smaller and the cells are closer together, this means that hard anodising changes the colour of the part. The colour depends on the alloy or type of material. Clear anodising uses higher temperatures and lower voltage; this results in a more porous, less hard-wearing coating that is suitable for different applications.
Because clear and hard anodised coatings have different properties, they are used for different applications. Hard anodising is more expensive to carry out than clear anodising (because the temperature must be kept lower and the voltage is higher, plus it takes longer to get a thicker coating).