< ELECTROPLATING
ELECTROPOLISHING
Electropolishing is carried out by attaching the metal cast to an ANDOE, then passing a DC current over from the opposing CATHODIC terminal. In contrast to the ambient temperature metallic salts used for electroplating, the electrolyte for electropolishing is preheated. Both the temperature of the bath and composition of the contained electrolyte are varied according to the nature of the metal or alloy being polished; though the bath is generally heated to a point somewhere between 176-356°F (80-180°C).
The electrolyte for aluminium polishing is composed of phosphoric acid and ethylene glycol. For polishing copper alloys, the electrolyte is composed of copper cyanide, zinc cyanide and potassium cyanide. Many other metals and alloys may also be successfully electropolished. Casts and fabrications formed in a pre-polished stainless steel, which may have become locally discoloured (for instance through welding), can be selectively electropolished by applying a weak oxalic acid solution (10%) to the affected area, earthing the workpiece with a copper strip, then attaching a DC welding transformer as a means of current supply (the stainless steel design itself, effectively functions as the anodic terminal) [ref].
Electropolishing is often carried out as a preliminary process prior to electroplating, though it can also often used as a finishing technique in it’s own right. Unlike the preparatory machining of a cast for manual or mechanical polishing, electropolishing does not remove any material from surface of the cast. This effectively means that a cast with a pitted surface sent for electropolishing will remain pitted after treatment (albeit now polished). As with electroplating, certain cast shapes and forms are unsuitable for electropolishing. Advice on the potential limitations of all these processes should be sought early on during the design stage, well before commencing work on the master pattern.
ANODISING >
|
|
