Description of the etching and descalling process

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Etching

Descaling and pickling are not normally applied to aluminium processing because the natural oxide on aluminium is very thin, unlike that of steel. Aluminium oxide removal may not be necessary as part of a pretreatment but as a consequent.

The etching of aluminium and its alloys is usually carried out in sodium hydroxide solutions, fluorides being added as necessary. Zincate treatment is necessary for good adhesion prior to autocatalytic or electrolytic metal plating. This solution is based on sodium hydroxide containing <20g/l zincates.


Alkaline etching of aluminium:The most frequently used method for etching aluminium is aqueous solutions of caustic soda with or without other additives. It can be used for general cleaning purposes where oxide, grease of subsurface detritus may be removed. With more prolonged etching, it can produce stain or matt finish for nameplates or decorative architectural work; or for deep engraving and chemical milling. It is a relatively cheap and easy method of etching but the details can be exceedingly complex.

Solutions for decorative etching may contain from 4 – 10% or more caustic soda, used at 40 – 90 °C, possibly with a wetting agent to disperse grease and give a light foam blanket.

The most common are “long-term use” etch processes operated on a never-dump basis. These processes contain special additives, which stabilize the aluminate solution. Etch rate must be controlled to get a constant etch finish. Important parameters are: temperature, free sodium hydroxide, aluminium and additive content.

Alkaline etches with increased etch rated are used to remove the anodised film from aluminium jigs before the next jigging.

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


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