Titanium has a tendency to gall when in rubbing contact with itself and other surfaces. A number of techniques have been developed to engineer the titanium surface and overcome this problem:
• Ion Implantation
• Shot Peening
As a surface treatment, anodising, which thickens the natural titanium oxide film confers only a minimum improvement to wear resistance. Where components are not subjected to continual wear, which will cause the layer to be removed, anodising provides a simple cost-effective solution.
Galling can be significantly reduced for example on threaded components by acid anodising in conjunction with shot peening and dry film lubricant.
The anodic film also serves to reduce the inward diffusion of oxygen at elevated temperature and of hydrogen under conditions of cathodic charging.
Thicker oxide films able to withstand relatively higher loading are produced by alkaline anodising processes. The deposition of low frictional co-efficient polymers simultaneously with the growth of the anodic film provides both surface hardness and optimum corrosion resistance and lubricity.
Many electrolytes are effective for the basic anodising process. A sound coherent film can be produced in a solution of 80% phosphoric acid and 10% sulphuric acid, with 10% water, with potential raised from 0 to 110V over 10 minutes.
Progressive thickening of the oxide film by anodising results in a surface colour change due to the refraction of light. This effect has been used imaginatively to produce paintings, ornamental jewellery and large architectural panels. Various electrolytes are suitable but 3-5% trisodium phosphate in distilled water is commonly used. Colours produced vary according to the electrolyte and the applied voltage.
Anodising has also been used as a means of removing surface iron contamination – for example as a final clean up operation on titanium fabrications. It is less effective however than a five minute pickle in a nitric/hydrofluoric acid. Traces of iron have been found to remain even after 20 minutes anodising.