Process description: Sealing following anodizing
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- Sealing following anodising
Sulphuric acid anodising is normally followed by a sealing process. Sealing improves the corrosion and stain resistance of the oxide layers. It also prevents organic dyes from leaching out and improves the light fastness. Sealing may be carried out in hot or cold processes.
Hot sealing
With hot sealing, the pores in the oxide layer are closed by hydrating the aluminium oxide to boehmite: the pores are close by the increased volume of the boehmite structure. The sealing process is carried out by dipping the anodised parts in hot or boiling (minimum 95 – 96°C) deionised water for three minutes per μm thickness. Sealing with steam achieves the same effect.
Cold sealing
Sealing methods at lower temperatures have been developed. So-called mid-temperature sealing processes are also available operating at about 60°C. These are not based on the hydrothermal conversion of aluminium oxide for closing the pores, but by the use of nickel-salts, such as the fluorite or silicate. In the past, in the European market, there was some concern about long term-properties such as light fastness and corrosion resistance. However, such processes have now been proven and accredited for outside use.
There are also processes working at 25 – 35°C. Advantages with cold processes are lower energy consumption and shorter process times.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
