April 22, 2008

Modified Zinc Immersion

Zinc immersion process of the first method can be modified by the following procedure. A modification of the standard zincates solution has been developed for use on both wrought and cast products, giving the following advantages:
  1. More uniform coverage by subsequent plating baths.
  2. Greater operating range for the double immersion surface conditioning treatment.
  3. Improved resistance to corrosion when followed by other deposits.
The modified of zinc immersion solution is prepared in the usual way by intimately mixing the zinc oxide (ZnO) and sodium hydroxide (NaOH), dissolving in water, and cooling to 25oC. Before the bath is diluted to final volume, a water solution of ferric chloride (FeCl3. 6H2O) and Rochele salt (KNaC4H4O6. 4H2O) is added. Composition of this bath is as follows:
  • 525 g/l NaOH
  • 100 g/l ZnO
  • 1 g/l FeCl3. 6 H2O
  • 10 g/l KNaC4H4O6. 4H2O
It should be operated at room temperature for immersion times on the order of 30 sec to 1 minute.

Other modified immersion solution have employed copper and nickel together with the zinc. The proprietary Bondal process, involving a nickel containing solution such as describe in Leloup's patent, has received considerable commercial attentions, especially in Europe.

Dilute Zinc Immersion

Another variation has been developed for applications where rinsing and drag out are problems, as, for example, on automatic equipment. This variation consist of reducing the bath viscosity by lowering the concentration of the principle component. At the same time, a low coating weight must be maintained by a closer control of operating condition and by addition agents. Two typical dilute baths, which may be prepared in the usual manner, are as follows:

Chemicals Composition

                     NaOH    ZnO   KNaC4H4O6.4H2O   FeCl3. 6 H2O  NaNO3
Bath I, g/l       50          5                 50                                2                1

Bath II, g/l     120        20                50                                2                1

Bath II will provide a greater zinc reserve for high production work with only a small sacrifice in rinsing and drag out properties. When using these dilute solutions the temperature must be maintained at 21 to 24oC and the immersion time must not exceed 30 sec.

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