February 5, 2008

Anodizing

The anodizing takes into account the composition of the solution, the alloy and the service requirement. To maintain the required temperature cooling arrangements in the case of sulfuric acid anodizing and heating solution is agitated by means of compressed air which is introduced trough a perforated coiled pipe arranged along the bottom of the vat. The cathodes used in anodizing baths, especially those of sulfuric acid, is very great. It is therefore almost unnecessary to use auxiliary cathodes when anodizing of the interior of pipes. If the length of the pipe exceed ten time the diameter, an internal cathode must be used.

When anodizing different component, the work being treated at any time in the anodizing bath should be of the same alloy. This ensures best conditions for production of a satisfactory film. The anodic film form on aluminum and its alloys grow at different rates and their electrical resistance is also different. Simultaneously anodizing of different materials may, therefore, result in poor quality films being obtained on some of them. Precise control of the anodizing conditions is of great importance when anodizing work very small dimensional tolerances. As a result of the conversion of the metal to the oxide, the dimensions of parts are altered by anodizing. At first there occurs an increase in size followed by a decrease if anodizing is prolonged. The temperature of the solution also plays an important part and its control during the whole of the anodizing process is essential if dimensional accuracy is to be achieved.



After anodizing the work is washed in running water to remove traces of the anodizing solution. This washing should be very thorough because any sulfuric acid left on the parts will destroy the oxide film. Further operations depend on the process that has been used for anodizing and the purpose for which the parts have been anodized. Parts that have been anodized in chromic acid for corrosion protection are un-jigged, dried and inspected. They are then sent for application of the primer paint coating. In the case of sulfuric acid anodizing, the parts after washing are subjected to chromate sealing before primer coating. When sulfuric acid anodizing is done for decorative purpose, the parts, after anodizing and washing are dyed. The dying should be done within 40 minutes of anodizing and during this interval the parts should be stored under cold water and should not be touched by fingers otherwise finger marks will be imprinted which will be visible more prominently after dying.

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