Many metals ate electrodeposited on aluminum alloys to obtain various decorative and functional finishes. The deposits applied for decorative finishes include chromium, nickel, copper, brass, silver, and gold, as well as alloys and oxidized modifications of these. In addition, silver is applied to improve surface conductivity; brass to facilitate adhesion of rubber, copper, nickel or tin for assembly by soft soldering; chromium to reduce friction and impart wear resistance, zinc to lubricate threaded parts, and tin to reduce friction on bearing surface.
It has long been recognized that aluminum and its alloys require specific surface preparation for successful electro deposition. Such special treatment is necessary because of the high position of aluminum in the electromotive force series and because of its relatively impervious and rapidly formed natural oxide film. Many procedures for plating on aluminum have been presented in some literature, most of them falling into one of the following categories:
1. Surface roughening or roughening plus metal deposition by immersion before electroplating.
2. Anodic oxidation followed by electrode position in the pores of the coating.
3. Direct zinc plating before plating with other metals.
4. Immersion deposition of zinc before plating with other metals.
5. Immersion of tin or nickel before plating with other metals.
The zinc immersion procedure was long considered the most practical and economical of the various processes and accordingly, has seen the greatest commercial application. In general, immersion process require less time and equipment, are less critical to control and have wider applicability than other known methods.