Hard anodising

Hard anodising refers to a number of low-temperature processes that produce thick (50-100 pm) and dense oxide layers. They have better abrasion resistance than the best treated steels, and their electrical insulation properties are on a par with those of porcelain. 

Hard anodising is used in the electrical and mechanical industries. The friction coefficient is reduced by a number of impregnation products lanolin, teflon, molybdenum disulphide, etc. Given the thickness of these layers, it may be necessary in some cases to allow for dimensional variations after anodising. 

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Decorative self-coloured films can also be produced in sulphuric acid under conditions intermediate between normal and hard anodising. 

The hardness and abrasion resistance of anodic coatings have never been easy properties to measure, but the development of a British Standard on hard anodising has made this essential. Film hardness is best measured by making microhardness indents on a cross-section of a film , but a minimum film thickness of 25 tm is required. For abrasion resistance measurements, a test based on a loaded abrasive wheel , which moves backwards and forwards over the film surface, has improved the sensitivity of such measurements. 

Hard anodising is a process where the anodising parameters are changed, leading to a much thicker, harder, abrasion- and corrosion-resistant film. The film formed is dark in colour, so black is the only sensible film colouring. 

Fig. 4. Current-voltage-time relationships during anodisation, demonstrating progression from mild to hard anodisation conditions (left) and mUd-hard-mild conditions (right).

Electrochemical chromic acid anodising Electrochemical phosphoric acid anodising Electrochemical sulphuric acid anodising Electrochemical hard anodising Electrochemical boric acid/sulphuric acid anodising Chemical sol-gel procedures Activated plasma... 

As already indicated, it is generally accepted that the oxide film produced either by conventional sulphuric acid anodising or by hard anodising in sulphuric acid, does not give the optimum surface structure for structural adhesive bonding, which is why these techniques seem to be rarely used in the aerospace industry. 

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