Sunday, December 14, 2008

Phosphating Step

In the operation of the process, the work must first be descale and degreased, but strongly acid or strongly alkaline cleaners leave the surface in a condition which encourages a coarsely crystaline phospate deposit, so these are usually avoided. The articles are then arised and hung in the phosphating solution. As the process is non electrolytic, close spacing is possible, and small parts can be treated in baskets or perforated containers. The solution is usually operated at 80 oC or above; treatment times of 3 to 20 m are usual. After phosphating the parts are rinsed, but residues of the solution inavertently left in joint, erevices, etc., are not corrosive. A short further dip in 0,5 % chromic acid solution is often given for further passivation, with or without a final rinse.

Since very considerable numbers of parts are processed, and these are often very large, e.g. complete car bodies, or very small, phosphating plants are frequently mechanized. In the motor car industry "Rotto-Dip" Plants are used in which complete car bodies, each skewered on a horizontal shaft, are passed through an elaborate automatic phosphating plant, and finally dipped in a bath of priming paint, suitably drained, and then stoved. The bodies are rotated continuously to ensure that every area is treated and that solution is not trapped in pockets.

Although the design and formulation of the phosphating process is complex, its control can be achieved by simple titrations with standard alkali solutions, using different indicators. The results are usually expressed as 'points', and additions of an acid phosphate mixture are made, as necessary to restore these to some determined value.

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