Variations on a method of surface treatment for aluminium were developed and first used in aluminium processing in the 1930s, particularly in Germany; they are known as anodic oxidation or simply anodising. The process is based on electrolysis and was thus referred to in the early days as the "eloxal process" (electrolytically oxidised aluminium). The aluminium component is suspended in a bath of dilute acids and made the positive electrode (the "anode"). Stainless steel or lead, for example, acts as the negative electrode (the "cathode"). If one applies a voltage to the electrodes, gaseous hydrogen forms at the cathode and gaseous oxygen at the anode (the component).
This oxygen reacts with the aluminium to form aluminium oxide, which forms an artificial oxide layer on the surface of the component (or possibly strengthens the natural layer). This oxide layer consists of two layers. To some extent, the primary or barrier layer (so-called because of the absence of any porosity) dissolves again chemically on the outside in the acid bath in such a way that a surface layer is formed having fine pores that are perpendicular to the surface. The total thickness of the oxide layer reaches its upper limit when growth and dissolution are in equilibrium, which depends on the bath, current density and temperature.
The various processes used differ in the electrolyte composition and temperature as well as the electrical parameters. Anodising is carried out in small or large baths depending on the size and number of the components; of importance is the continuous anodising of strip.
- The d.c. sulphuric acid process uses direct current and sulphuric acid at 18-22 °C, 12-20 volt direct-current voltage and current densities up to 2 amperes per square decimetre. The largest layer thickness of approximately 25 thousandths of a millimetre is reached after about an hour and the layers are colourless to yellowish.
- Electrolytic colouring is carried out using alternating current in baths to which metal salts have been added (for example copper salts for red colouration, tin sulphate for bronze tones).
- Hard anodising is carried out at 0 °C, in order to retard the re-dissolution of the aluminium oxide, and at higher voltages and current densities, in order to achieve layer thicknesses of up to 150 thousandths of a millimetre; colouration is grey to black.
- The resistance of the oxide layer is optimised by "compaction" with boiling water or steam, whereby the aluminium oxide of the surface layer is transformed to aluminium monohydrate and the pores become sealed. The various properties of the artificial oxide layer are used in a specific manner in many applications:
- The permanent metallic lustre, for example, is used by bright-finish alloys.
- Porosity is of use in colouring (whereby the colourant deposits in the pores), for example for the use of aluchroming in art or for applying light-sensitive layers to photographic plates.
- The high hardness and abrasion resistance, particularly of hard anodising, coupled with the good adhesion (the oxide layer is not a coating but has grown out of the base metal), is utilised, for example, for cylinder faces in mechanical engineering.
- Façade cladding and windows in building construction are examples of the good colour fastness and corrosion resistance.
- The electrical insulation effect is used in electrical engineering.