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Anodising: titanium

When we hear the word 'anodising', it is generally a process we associate with aluminium components. Most of us will be familiar with decorative coloured anodic finishes and the much thicker 'hard' anodising treatments that are often applied to mechanical parts. However, a small number of other materials may be anodised; these are zinc, magnesium and titanium.

The anodising of titanium is certainly less widespread than anodising aluminium, but it can fulfil an important role in the successful use of some titanium components. Titanium is a strange material, but has quite a lot of desirable properties for certain applications. For example, it is quite strong in tension, has low density, has low modulus and low thermal conductivity.

However, what sets titanium back for many uses is its poor surface behaviour. In any sliding application, the surface of titanium is very quickly damaged. It is an excellent material to use for threaded fasteners, but there is a risk of the surface becoming damaged at extremely low levels of stress, especially when the female thread is also titanium.

This is a behaviour that titanium shares with austenitic stainless steels, and the phenomenon is referred to as 'galling'. Where stress levels are modest, an anodic surface treatment changes the behaviour of the surface enough to allow it to be used successfully. The anodising treatment affects only a very thin layer, and is often combined with further surface treatments which are aimed at providing a low-friction surface. If these further treatments wear or otherwise degrade during installation or use, then the anodising prevents any damage to the titanium substrate. For surface protection purposes, the anodising process takes place in a caustic electrolyte.

Another use of titanium anodising is for decorative purposes. In the decorative anodising of aluminium, the anodic film is colourless, but is quite porous and readily accepts dyes, of which a wide range exists, in hues of varying degrees of tastefulness. Titanium anodising has an infinite number of shades available, but these rely on the phenomenon of interference rather than any chemical dye process following the anodic treatment.

In the same way that steel components change colour at around the 200 C mark and continue to exhibit changing colours until around 340 C, owing to the changing thickness of the oxide layer, the same applies to titanium. Where a coloured decorative film is produced, the anodising process takes place in a weakly acidic electrolyte.

The final colour of the oxide film produced depends on the terminal voltage during processing, and it is the adjustability of this voltage which allows the shade to be 'tuned'. However, the exact shade can't be guaranteed from piece to piece, and while the number of shades is theoretically infinite, the range of colours is not. Decorative anodic films on titanium range from magenta to blue. It is possible to produce the same effect, although with less control, by heating the surface in air.

Written by Wayne Ward

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