Aluminum is first plated with electroless zinc, then copper plated. Once you have that, you can put on almost any plate you want. The nickel has brighteners that smooth over fine scratches and make it shiny. The chrome is in a very thin layer that improves corrosion resistance and gives it a blue cast that people like better than the nickel's yellow.
It is also possible to anodize some alloys and plate directly on that. I experimented with it a bit and was able to plate tin directly on an aluminum casting. The bond was so strong that I could solder a wire to it and in a pull test, the wire broke before the solder came loose from the aluminum.
I'm not sure that it was actually anodizing. It may have been similar to the process I use to plate beryllium copper. Using an 8% sulfuric acid electrolyte, I set the voltage between the ion discharge voltage between the part and a titanium cathode. That unplates the beryllium atoms and leaves a pure copper surface that I plate silver on. I think it is doing a similar conditioning to the aluminum.
Bill
