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Aluminum Anodizing

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February 6, 2008

Anodizing Plant

Anodizing is usually carried out in stationary vats similar in construction to those used in electroplating. Vats to be used for chromic acid anodizing are built of carbonized with a steam or water jacket for heating the solution. A perforated steel coil carried on insulating supports is placed on the bottom of the vat for supply of compressed air for the agitation of the bath. Before being fed into the bath, the compressed air must be freed from dust and oil by passing trough special filters. For the automatic for the temperature of the solution, a temperature regulating device is provided in the steam heating circuit of the bath, the device being connected with the valves which control the supply of steam.

The vats used in sulfuric acid process are made of steel or wood and are lined inside with acid resistant materials, chiefly lead, vinyl plastics, asbestos vinyl composition, rubber on tiles on an acid resistant content backing. The sulfuric acid process is characterized by an evolution of heat, and cooling of the solution is therefore essential. It the vat is made of steel and lined with lead, cooling can be arranged by providing a water jacket. If the vat is made of wood and lined with a material having low thermal conductivity (vinyl plastics, tiles and the like), the cooling is effected by means of lead coils arranged inside the vat and through which cold water is circulated. Water chiller are recommended for intense cooling, the coolant in such cases consisting of an aqueous solution of sodium chloride. The presence of the salt however, leads to corrosion of the water pipes, in addition to which, should the cooling coil become defective, entry of the sodium chloride into the anodizing solution renders the latter unfit for use. It is therefore, preferable to use 40 percent solution of potassium carbonate as a coolant which is less corrosive and any accidental entry of the coolant into the anodizing solution will become apparent by the vigorous evolution of carbon dioxide.

Freon gives good result and is handled in coils made of cooper clad with a lead copper-aluminum alloy from outside. As in the case of chromic acid, the sulfuric acid solution is agitated by means of compressed air. Anodizing vats are equipped with ventilating outlet along the edges. Special plants are used for anodizing of aluminum strip and wire. In this plants, the material being anodized in drawn continuously through the anodizing bath over various pulleys, and the bending to which it is subjected in the process produced some improvement in the elasticity of the oxide film. For anodizing selected areas of the surface of the part, brush anodizing or spray anodizing method using a special gun to spray the anodizing solution may be used. The solution employed in a 3-5 percent solution of chromic acid using either alternating or direct current at 110 volts. The direct current source in sulfuric acid anodizing may consist of three selenium rectifiers rated at 200 ampere, 6 volts connected in series to give 18 volts voltage. For alternating current anodizing, the current is supplied by a transforming having primary voltage of 220 volts or 440 volts and a tapped secondary giving 20, 24 and 30 volts.