After the surface of an aluminum alloy article has been conditioned and a zinc immersion deposit has been formed, other metals can be plated on this surface by any of the methods suitable for plating on zinc. One important factor must always be observed. The zinc deposit is extremely thin and any plating treatment that penetrates the zinc layer and attacks the underlying aluminum will result in a poor deposit.
Ordinarily, it is advisable to apply a suitable copper strike over the zinc immersion layer before other metals are deposited. Silver, brass, zinc, nickel, or chromium, however, may be deposited on the zinc immersion layer provided the plating procedures are suitable for plating over zinc.
Other immersion treatments may require a different type of strike, as for example, the bronze strike of the Alstan process.
The Copper Strike
For applying a copper strike before plating with other metals, a Rochelle copper cyanide solution of the following composition is recommended:
Copper cyanide, CuCN 40 g/l
Total Sodium cyanide, NaCN 50 g/l
Sodium carbonate, Na2CO3 30 g/l
Rochele salt, KNaC4H4O6. 4 H2O 60 g/l
Free sodium cyanide 4 g/l
This strike solution is generally employed at a temperature of 38 to 43oC and a pH of 10.2 to 10.5. electrical contact should be made before immersion of the article into the bath, and an initial high current density for about 2 min at this current density, the current may be reduced to 1.3 A/dm2 and deposition continued for an additional 3 to 5 min, depending on the thickness desired. After this strike, the work can be transferred to other standard plating solution for further plating. Probably the greatest source of difficulty for current density is evenly to all parts of the work. This allows deposition by immersion to occur and leads to blistering and poor adhesion of the electroplate. As the operating temperature of the strike bath increases, the time before deposition by immersion decreases, and the necessity for rapid application of current becomes more and more important.