Eloxal anodising describes the (el)ectrolytic (ox)ydation of (al)uminium. The process closes the pores of materials and improves the corrosion capability.
Functional and decorative
Eloxal is a coating for the functional or decorative refinement of aluminium materials. The aluminium oxide layer is built up with direct current in a sulphuric acid electrolyte at approx. 18-20 °C. The workpiece is connected as an anode. In the course of the treatment, the surface of the workpiece is transformed into an aluminium oxide layer.
The porous (dense), hard and wear-resistant surface created in this way is firmly bonded to the base material. Special sealing processes can close the pores and greatly improve corrosion resistance.
Different electrolyte parameters give anodically produced oxide layers different properties and thus meet the most diverse requirements. The appearance (i.e. colour and gloss) and properties of the oxide layers (e.g. hardness, corrosion resistance and wear resistance) depend on the composition of the base material, the alloy additives and the surface treatment. The Eloxal layer is resistant to chemically neutral substances in the range of pH = 5 to pH = 8. With the appropriate layer thickness and sealing, the layer even resists acidic or weakly alkaline attacks for a short time.
The hardness of the Eloxal layer is influenced by the material composition – particularly by the alloy components. Hardnesses from 250 up to 350 HV are possible. The Eloxal layer considerably increases the wear resistance of the aluminium.
The best resistance is achieved by sealing in hot water or nickel acetate.
The layer thickness to be produced depends on the intended use, the requirements placed on the layer and the subsequent stress. For most applications, layer thicknesses of 5 to 25 μm are produced. The oxide layer grows 1/3 out of the metal and 2/3 into the metal. If possible, only materials in Eloxal quality should therefore be used! This property must be taken into account in the design of the components with regard to close fits. However, it is also possible to influence this so that dimensional accuracy of close fits can be guaranteed.
The coatings exhibit a colourless, glass-like and transparent oxide layer on pure aluminium. In the case of aluminium alloys, the components may acquire a somewhat cloudy, yellowish or even dark oxide layer, depending on the material composition. If desired, the layers can also be coloured.
Almost all aluminium alloys can be anodised using the Eloxal process. Various alloying elements such as silicon, manganese, copper and zinc can cause clouding or colouring of the oxide layer. For decorative use, not only the aluminium alloy but also the manufacturing method and the heat treatment applied are of critical importance.
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