Wood grain anodising (Topic: 14123)

A process which becomes possible with vertical anodising is a differential anodising process known as "Starmite", which gives a "wood-grain" pattern on the anodised extrusions. This process involves a multistage anodising treatment in which the extrusions are first degreased and etched normally. They are then AC anodised in an alkaline electrolyte, which often contains a weak acid capable of barrier layer formation. Such an electrolyte might, for example, contain sodium metaborate and boric acid, and anodising would be carried out at a current density of 2 A/dm2 for 15 minutes. Counter electrodes of stainless steel are used.

During this treatment a "wood-grain" pattern develops on the extrusion surface. This is apparently caused by gassing effects which influence the rate of anodising in different areas, so in order to produce the effect along the length of the extrusions they must be processed vertically. Having produced the wood-grain effect, the profiles are etched briefly in caustic soda, presumably to remove the barrier layer of oxide, and then anodised conventionally in a sulphuric acid electrolyte. The profiles are then usually coloured, either electrolytically or using conventional dyeing methods prior to sealing. In Japan electrophoretic sealing is used.