Desmutting (Topic: 14062)

During the etching operation a black smut layer may be left on the aluminium surface. This smut tends to be heavier the lower the purity of the aluminium and the more highly alloyed it is, and it is particularly heavy on copper-based alloys. It consists of particles of oxide, intermetallics, silicon, etc., which are insoluble in the alkaline solution, and in general are quite loosely held on the surface. Smut is usually removed by a dip in an acid solution, most commonly 25ñ50 vol% nitric acid, used at ambient temperature. This removes normal smut layers, including those on high copper containing alloys, quite quickly, and a dip of 3ñ5 minutes is usually sufficient to give a clean surface.

For alloys mentioned in this manual, desmutting solutions based on sulphuric acid (spent anodising electrolyte), with special additives is widely used, but addition of 1ñ5 vol% of nitric acid is also common. This desmutting step is also called deoxidisation step, or neutralisation step due to neutralisation of the caustic soda by the nitric or sulphuric acid. Such a step does not affect the aluminium surface.