Description of the colour anodizing process
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Colour anodising on aluminium
Aluminium can be coloured in many shades and colours in conjunction or after sulphuric acid anodising and is normally referred to as „colour anodising“. Normally colouring methods are identified as “dip dyeing”, “electrolytic dyeing” or “two-stage colour anodising”, “self-colouring process”, or “integral colour process”. There are four methods.
- Immersion colouring
This is the most widely used colouring method with a large range of dyes giving good light resistance. The anodised aluminium workpieces or substrates are immersed in a water-based organic or inorganic dye solution before sealing. The pores in the oxide layer adsorb the dye and then sealing is carried out to achieve durability. For colouring the oxide, thickness depends on the type of product and on the colour required. For a good corrosion protection and weather resistance, at least 15 μm minimum is needed.
- Electrolytic colouring
The anodised aluminium is placed in an acid solution containing metal salts and an alternating current (AC) is applied. This deposits oxide of the other metal which penetrates 1 – 5 μm deep into the pore of the alumina, with the residual oxide layer remaining on top. The film obtains a colour characteristic of the metal salts used. The metal salt most commonly used in Europe is tin. For special applications, nickel, cobalt and copper can be used. Tin sulphate gives shades from light bronze to black depending on the process time, of 0.5 to 15 minutes. A major use is for architectural panels. Electrolytic and immersion colouring can be combined for form new shades.
- Interference colouring
Interference colouring is a special technique based on the electrolytic colouring principle. The appearance is produced by interference effects between two light-scattering layers: the electrochemically deposited metal layer at the bottom of pores and the aluminium oxide/aluminium interface beneath.
- Integral colouring
With integral colouring, the aluminium oxide layer is coloured itself during the anodising process. Colouring occurs either by anodising in a solution of special organic acids or by normal anodising in sulphuric cid of special aluminium alloys with substances that are not oxidised such as Al-Si or Al-Fe-Mn. The oxide layer appears with a colour ranging from light bronze through dark bronze to black, depending on the layer thickness. This technique has almost entirely been replaced by electrolytic colouring.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
