Whereas normal sulphuric anodising can give anodised finishes for either functional or aesthetic purposes, hard anodised anodic films are typically specified for their functional value, as the films formed are dark in colour, leaving black as the only sensible possibility for colouring the anodic film.
By changing the process parameters for anodising, a much thicker, harder, abrasion and corrosion resistant film can be built up. The quality of the film is very dependant on alloy composition and factors such as the sealing process, which is beneficial for the fatigue resistance and corrosion resistance, but reduces the abrasion resistance.
The high film thickness can cause tolerance problems that can be largely avoided by prior consideration of machining dimensions, plugging holes and masking of areas that are required to be film-free. The masking may be by means of a resistive paint, by applying adhesive films, or bunging holes with threads or close tolerance bores thereby preventing the area underneath access to the electrolyte.
For enhanced corrosion protection and for anti-galling properties Redditch Anodising can apply a PTFE coating to the hard anodised film. This PTFE coating is not heat treated in any way so does not have any deleterious effects on the hard anodic film.