Anodising is an electrolytic process for producing thick oxide coatings, usually on aluminium and its alloys. The oxide layer is typically 5 to 30µm in thickness and is used to give improved surface resistance to wear and corrosion, or as a decorative layer.
In the electrolytic process, the components to be treated are made an anode in a dilute acid solution. Oxidation occurs at the component surface, resulting in the formation of a coherent oxide film that is very adherent to the underlying metal substrate. The majority of anodising is done on aluminium and its alloys. Other materials which can be anodised include magnesium and titanium alloys.
Before anodising can be undertaken, the aluminium alloy surface needs to be given a pre-treatment. This pre-treatment will influence the final appearance and properties of the anodised coating. The types of pre-treatment can range from mechanical processes such as abrasive polishing, to chemical treatments such as chemical brightening or electrolytic polishing. In addition, any machining, drilling or welding of the component should be done before anodising.
Three types of electrolyte solution are commonly used with the anodising process. The first is a 10-15% solution of sulphuric acid at 25°C. This electrolyte gives a coating formation rate of about 25µm/hr. The second electrolyte solution is a mixture of sulphuric acid and oxalic acid at 30°C. This gives a higher coating formation rate of about 30µm/hr. The third electrolyte is 10% chromic acid at 38-42°C, which gives a film formation rate of about 15µm/hr. These conventional anodised coatings are porous and clear, and are normally used with dyes for decorative coating.
'Hard anodising' refers to the preparation of thicker oxide coatings, about 25-100µm, with a higher hardness of typically 500-900 HV and are used to give a wear resistant surface to aluminium alloys. This is achieved by using a sulphuric acid/oxalic acid mixture at higher concentrations and at lower temperatures of about 0-10°C. The coatings produced are grey to black in colour and are non-porous. Not all aluminium alloys can be anodised to give hard anodised coatings. The 5xxx and 6xxx series alloys respond well to hard anodising, whereas 2xxx alloys and other alloys, including casting alloys with high copper and silicon content, do not. For these higher silicon and copper-containing alloys, the anodised layer tends to be porous and of low hardness.
Hard anodising is often the lowest cost, wear resistant coating that can be applied to aluminium alloys and is particularly suitable for protecting against low stress abrasion. As a result, hard anodised coating is often used with aluminium components in sliding systems. These coatings also have some use for protecting aluminium components that experience fluid-assisted wear, slurry erosion, solid particle erosion and liquid erosion. The anodised coating is also resistant to many chemicals, with the exception of alkalis. Anodising is not used for impact wear applications due to the brittle nature of the coating.
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