Monday, February 16, 2009

Electrolytic Brightening of Metal

A radically different chemical immersion brightening process is available for super purity aluminum and its magnesium alloys, under the name of the Erftwerk process. This utilized a solution of nitric acid and ammonium biflouride. A minute trace of lead is also essential to the process. The solution is operated at about 60oC in hard rubber or PVC tanks. The reaction is extremely vigorous, but the fumes are less unpleasant than with phosbrite; 1 to 2 m immersion only is necessary. The product of the reaction is aluminum fluoride, which precipitates, so that although the solution becomes exhausted it can be continuously regenerated by addition of ammonium bifluoride and occasionally of nitric acid. The process is therefore fairly cheap to operate, and has been widely used in Germany as a component part of the process of producing bright anodized motor can trim parts.

In these chemical brightening processes the driving force is provided by the chemical energy from the dissolution of the metal in the acid. In electrolytic brightening the driving energy is supplied by an external current. The work piece are made the anode, i.e. they are connected to the positive side of the current source. This has the advantage, compared with chemical brightening, of being more controllable, and dealing with metals which do not dissolve vigorously in acids, but the disadvantage of requiring more plant (current source, meters, hangers, etc.) and particularly of a varying action on outstanding and re-entrant areas. Comparatively high current densities have to be used, often over 200 A/ft2.

Electrolytic brightening has achieved considerable industrial importance for stainless steel (usually for the austentic nickel chromium 18:8 alloy) which is a hard and difficult metal to polish mechanically. Sulfuric phosphoric acid mixture are used, sometimes with the addition of glycerine or chromic acid. Somewhat similar solutions are also used successfully in industry for aluminum alloys. An alkaline electro brightening process for super purity and high purity aluminum alloys (the Brytal process) utilized a hot alkaline solution of sodium phosphate and sodium carbonate at a modernate current density of about 10 A/ft2.