Process description: Anodizing in surface industry

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Anodising

The anodising of metals is an electrolytic surface oxidation process which enhances the natural aptitude for the metal to oxidise; coatings can be 1000 times thicker than the natural layer. Aluminium is the most important material to be anodised, with alumina (Al2O3) formed at the surface: magnesium, titanium, tantalum and niobium are also anodised in smaller quantities.

Aluminium is normally (90% of cases) anodise in sulphuric acid electrolyte. For special applications, aluminium may be anodised in many different types of process solution: phosphoric acid, sulphuric/oxalic acids, sulphuric/salicylic acids and chromic acid electrolytes.

A wider variety of current forms and wave shapes may be used for the process, depending on the solution selected and the purpose of the anodic film. Direct current (DC), alternating current (AC), and DC with superimposed AC, are all used in various processes.

The workpiece or substrate to be treated is made anodic. During the anodising process the negatively charged anion migrates to the anode where it is discharged with a loss of one or more electrons. The metal reacts with the oxygen of the anion and a layer of oxide forms on the surface.

Figure 2.9 shows examples of decorative anodising options and Figure 2.10 shows a typical anodising line layout.

Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.


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