July 14, 2008

Degreasing Small Parts

Degreasing action for normal parts need some time and 5 min or more immersion is usual; the action is most rapid in an almost boiling solution, but too high temperature has the disadvantage that the articles dry off when removed before they can be rinsed. Dislodgement of the grease can be encouraged by evolving hydrogen gas electrolytically at the surface by passing a current between the steel tank acting as anode (positive) and the articles as cathode (negative). The magnitude of the current is less important than a copious evolution of gas; 50 to 100 A/ft of cathode urea is suitable, for which a voltage of 4 to 6 V suffices.

For small and intricate parts the degreasing action of both organic solvents and aqueous alkaline solutions can be assisted by ultrasonic action. This consist of electronically generated alternate pulses of compression and rarefaction in the liquid, at a frequency above that of the highest pitched audible sound, e.g. 100.000 c/s. The rare faction pulses are so rapid that they generate bubbles of vacuum on the liquid at the metal surface, and particularly in holes, crevices and recesses. These collapse equally suddenly during the compression pulses, in fact they implode. The resultant violent disturbance at the metal surface greatly assists in the degreasing action.

After degreasing, the article must be rinsed very thoroughly to remove all traces of emulsified oils and soaps, which would be precipitated by subsequent acid dips. The articles will then show an unbroken film of water over the whole surface, and should not be allowed to dry off.

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