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April 18, 2008

Zinc Immersion

The zinc immersion process, however, was not widely applied until later, when pertinent work was reported by Bengston, Meyer, Ehrhardt and Gutrie. More recently there have been contributions by Saubestre and Morico.

When a properly conditioned aluminum alloy article is immersed in an alkaline zincate solution at about 25oC the oxide film present on the surface dissolve and as soon as any underlying aluminum is exposed, it also starts to dissolve and is immediately replaced by an equivalent weight of zinc. When the aluminum surface is completely covered with an extremely thin layer of zinc, action in this solution virtually ceases.

For best performance of the plated part in service, it is important that the zinc immersion film as possible, as long as there is enough to permit satisfactory electrodeposition. The two most important factors determining the amount of zinc deposited by immersion are the particular alloy being processed and the conditioning treatment employed. Time, temperature, and concentration of the immersion bath also affect the amount of zinc formed. The weight of zinc deposited should be of the order of 1.6 to 4.7 mg/dm2 (0.1 - 0.3 mg/in2), and generally it is desirable to limit the weight to not over 3.1 mg/dm2 (0.2 mg/in2).

A basic zinc immersion bath that has performed well over the years consist of:

400 - 500 g/l NaOH
80 - 100 g/l ZnO

Likewise, proprietary sodium zincate solutions of this general type are available commercially and are capable of producing satisfactory result.