Monday, December 29, 2008

Chemical and Electrolytic Brightening

Chemical and Electrolytic Brightening of Metal

Most metals can be superficially by chemical or electrochemical attack in specially formulated solutions; this process is widely used for providing the final lustrous flat surface on metal objects. However, it has only achieved industrial importance in the treatment of brass, stainless steel and, particularly, aluminum. Although these processes are frequently referred to as chemical or electrolytic polishing, this is a misnomer. They will not remove even minor irregularities of surface level or scratches in the way that mechanical polishing will. The actual effect is to convert an already smooth surface which is matt into a brilliantly lustrous one. Even so, unless the metal alloy is very homogenous, some parts become more severely attacked than others and the reflectivity of the surface after brightening may be lustrous, but will not yield sharp reflected images.

A simple process of this kind has been for brass article under the name of bright dipping. In this the degreased articles are dipped in a mixture of nitric and sulphuric acids containing a small but critical addition of hydrochloric acid. Cuprous chloride is relatively insoluble in this solution, so that when the brass is vigorously attacked by the nitric acid a very thin resistant film of cuprous chloride is formed on the surface, particularly in hollows. This delays the attack there, so that the surface becomes leveled on a micro scale. i.e. it becomes lustrous. Greatly superior results are obtained from solution containing sulphuric, nitric and phosphoric acids; the most effective formulations are different for brass and for aluminum. Since these solutions are protected by the licenses, under the trade name of Phosbrite. The solutions are used hot; they are very vigorous in action and evolve unpleasant fumes, so that fume extraction is necessary. The solutions become exhausted fairly speedily in use and are relatively expensive, because of the phosphoric acid content. Nevertheless, in the course of a 2 to 5 m dip they provide a very satisfying luster on sheet material, pressings and simple assemblies, thereby obviating expensive mechanical polishing.

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