Aluminium anodised using the sulphuric acid process can be produced in a number of different finishes, each providing specific benefits for the part’s service. Here we describe the main anodising finishes available and for what they are used.
Clear or Natural Finish
Clear or Natural Finish – A transparent oxide layer is formed that reveals the aluminium beneath. This finish is suitable for parts that require a corrosion resistant surface and where appearance is not important. As with all anodized surfaces, the aluminium must be sealed to prevent contamination entering the pores of the oxide layer.
Dyed Colour Finishes
The open pore structure of the anodized surface can be penetrated by dye before the anodizing is sealed. A wide range of colours including black, blue, red, gold and many others can be produced. The depth and colour intensity of the dye is dependent upon the thickness of the anodic layer and the concentration of the dye used. Sulphuric acid anodized components are commonly used on consumer products and architectural items, which are subject to longer term exposure and where the colour may eventually fade under prolonged UV exposure.
Bright or Decorative Finishes
Parts that have been chemically brightened or electropolished prior to anodising have high-gloss surfaces. This surface finish is preserved by the sulphuric acid anodising process and is typical of lighting reflectors and trim components. Such parts must be handled with extreme care as they are highly susceptible to scratching and will display any such mark very clearly.
Matte or Etched Finishes
An alkaline etch is carried out on the anodised surface before the part is finished to produce a uniform Matte or Matt finish. Such a surface finish disguises minor surface irregularities and does not produce glare. It tends to have a consistent appearance from part to part.
Sealed Versus Unsealed Finishes
Sealing – in hot deionised water, or in a nickel acetate bath – will close the pores and lock in dye, to provide the highest corrosion protection. Anodising that is left unsealed is often specified where the anodised surface is to be coated or where it is to be bonded using adhesives. All surface finished products are covered by recognised surface engineering finishing standards which define between sealed and unsealed products. There is more on Sulphuric acid anodising at //www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising/.
Know your finish before specifying a process to save time and avoid expensive rework.
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