Sulphuric acid anodising produces a protective anodised layer on aluminium. This layer can be further treated in the tank to produce a variety of colours.
The Natural Clear Finish
Sulphuric acid anodising produces a clear slightly translucent coating. If left unsealed and untreated, the coating will have a colour determined by the aluminium alloy. The coating on high purity aluminium alloys will have a silver-white appearance. Some of the aluminium casting alloys have higher silicon contents and these will produce a grey or even yellow-brown coloured coating.
Organic Dye Colouring
Organic dyes are absorbed by the porous oxide layer creating a vast array of colours including: black, gold, red, blue, green and violet. The majority of organic colours are not UV stable, therefore subject to fading when exposed to the elements outdoors. However, there are many colours that are specifically designed for use externally and the very bright colours such as red and blue are generally better suited for interior use.
Electrolytic Two-Step Colouring
Electrolytic colouring is a two-step process which firstly deposits metallic salts into the pore base of the anodised layer using an AC current after anodising. The time that the current is passed determines the depth of the colour. The resulting range of tones are all very stable outdoors and are typically used for architectural aluminium sections. If you are looking for Sulphuric acid anodising, see www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising.
Alloy Grade and Colour Shift
Series 1000 and 6000 alloys anodize and hold colour well. Series 2000 alloys (which contain copper) tend to produce a yellow/brown base colour that can interact badly with light colours. It is essential to specify the correct alloy grade when ordering anodized parts.
How Sealing Affects Final Appearance
After colouring the porous oxide layer with dye or electrolytic colouring, the pores of the layer are closed by a seal. The appearance of the coloured layer is affected by the sealing process. Hot deionised water sealing slightly increases the colour intensity of the layer. A mid-temperature nickel acetate seal provides very good colorfastness and has little effect to increase the colour intensity. For applications where it is crucial to obtain a consistent colour for all items in a batch, the sealing method and temperature must also be specified.
Getting the finish right on aluminium parts to be used outdoors is not a matter of chance: it involves matching the appropriate alloy with the correct anodizing process, the appropriate colouring process and finally the appropriate sealing process.
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