Other related surface-finishing techniques

Electrochemical variations of several other surface finishing methods are used to a limited extent and these will be considered in this section. 

This technique is used to remove grease and oil from metal surfaces. The medium is generally 5—10% caustic soda at 60—80°C and the surface is made cathodic (30—100 mA cm ). The principles of operation include (i) a local increase in pH where the grease is sited and hence an increase in the rate of hydrolysis to water-soluble products, (ii) the evolution of hydrogen gas bubbles which rise to the surface and take organic particles with them, i.e. electroflotation. 

In electrochemical pickling the objective is to remove rust and other oxides and sometimes to produce a uniform etch to produce adhesion of deposits. The surface is made anoidc in an acid medium, e.g. H2SO4 or FeCla. 

These processes produce a uniform layer of a passivating insoluble salt at the metal surface and are widely used as a preliminary treatment of steel, e.g. in car body manufacture. As the names suggest, the salts are either a phosphate or a chromate. 

In some circumstances, the rate of formation of this layer is enhanced by carrying out the coating electrochemically with the surface as the anode. Commonly, however, the layer is sprayed on and dried. 

Typically —200 to —300 V are used and the current at a stationary surface will drop from 70 m A cm to 5 m A cm in the steady state after a few seconds. It then takes 2-5 min to build up a paint layer of 20 5 pm. Application techniques vary but the most common strategy is to dip, sometimes aided by simultaneous spraying of the electrolyte. 

Electropolishing is a process used to produce metal articles with a highly reflective mirror finish it U closely related to anodizing. 

The most common metal to be electropolished is aluminium, but the procedure is also used for steel, brass, copper and nickel/silver alloy. 

The article to be polished is made at the anode with a current density in the range 100-80DmAcm in a bath of phosphoric acid at 60 C for a period of 5-10 min. The cathode material is not critical and may be steel, copper and lead. The mechanism of electropolishing is thought to involve both selective anodic dissolution (the potential distribution will favour corrosion of the surface at peaks rather than in troughs) and oxide film formation. The conditions are somewhat more forcing than those used in anodizing. 

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Toughened polystyrene and other related plastics like acrylonitrile-butadiene-styrene can be extruded to produce opaque, coloured sheet for thermoforming, and as profiles and pipe. Techniques are available by which good surface finish may be obtained, but under typical conditions the extruded surfaces will be matt or satin . 

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