Contrary to popular belief, acid anodising of titanium which thickens the oxide film confers only a minimum improvement to wear resistance. The anodic film also serves to reduce the inward diffusion of oxygen at elevated temperature and of hydrogen under conditions of galvanic charging. Many electrolytes are effective; 80% phosphoric acid + 10% sulphuric acid + 10% water produces a sound coherent film with potential raised from 0 to 110v over ten minutes. Galling can be significantly reduced by acid anodising for example on threaded components, by conjunction with compressive surface treatment and with a dry film lubricant. Ti-6Al-4V bolts used on the successful Heidrun riser had an epoxy polyamide molydisulphide coating applied over a peened and anodised surface.
Thicker oxide films able to withstand relatively higher loadings are produced by alkaline anodising processes such as TiodizeR. Long established alkaline base proprietary finishes such as CanadizingR control the oxide film thickness and density so that one or more of a series of dry film lubricants may be applied and ‘locked-in’ to the surface. Canadized coatings were used successfully to prevent galling at the joints and in the drive shaft of the titanium core sample drill tubes used in NASA’s exploration of the moon.
Colour versus Anodised Film Thickness
Progressive thickening of the oxide film by anodizing results in colour changes due to refraction of light. This effect has been used selectively for component identification, but more imaginatively for colouring titanium sheet to produce ‘paintings’, and ornamental jewellery. 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.