Brass can be applied directly on a zinc immersion film from a standard brass plating solution operated at 26 to 32oC and 1 A/dm2. Consequently, it can also serve as a "strike" before plating with other metals. Which high speed brass baths, a prior Rochelle copper or standard brass strike may be necessary.
Chromium plating on Aluminum
Bright decorative chromium plating procedure has also been employed on aluminum alloys in conventional manner over the required copper and nickel undercoats. Similarly, hard chromium deposits can be applied over the copper strike by employing the necessary hard chromium plating bath.
Another decorative chromium plating procedure has also been employed on aluminum alloys for many years. This consists of plating at 18 to 21 oC and 15 A/dm2 directly on the zinc immersion film. The deposit produced in this manner is slate gray and approximately 2.5 to 5.0 µm thick, but can be buffed to an attractive metallic luster by using chromium buffing compounds. For maximum ease of buffing the chromium, it is important that the plating temperature not exceed 21 oC.
In like manner, hard chromium or high temperature soft chromium, in moderate thickness, can also be plated directly on the zinc film by entering the chromium bath with the current applied. For heavy deposits or entering the chromium bath with the current applied. For heavy deposits or for complicated shapes, however, it may prove desirable to employ a copper.
Silver can be deposited directly on the zinc immersion coating by following the silver plating procedure employed for steel. This involves the use of two preliminary strikes at 1.6 to 2.7 A/dm2 and making contact before the work enters the solution. More consistent results, however, especially in production work, are obtained by employing a Rochelle copper strike in place of the first silver strike. The second silver strike and silver plating then follow in the usual manner.
Zinc can be plated over the zinc immersion layer from either acid or cyanide solution at about 25oC, again applying the current (1-2 A/dm2) before immersion in the plating solution. For heavy deposits from high speed baths, it may be desirable to employ a copper strike for best adhesion.