Chromium Plating on Aluminum

1) Degrease with an organic solvent.
2) Soak clean.
3) Rinse thoroughly.
4) Dip in a 50% by volume nitric acid. 
Note: For silicon-containing alloys, a sodium bifluoride dip may be required instead of the straight nitric acid dip. (Add a bit of sodium bifluoride to the nitric acid dip. Be very careful with fluoride-containing solutions.)
5) Rinse thoroughly.
6) Apply a zincate by dipping in the zincate solution for about 1 min. You’re best off using commercially available zincate solutions.
7) Rinse thoroughly.
8) Strip the zinc coating by dipping in 50% by volume nitric acid.
9) Rinse thoroughly.
10) Repeat the zincating step, step 6.
11) Rinse thoroughly.
12) Dip in sulfuric acid dip, sulfuric acid 5 mL in 1 L of water.
13) Rinse thoroughly.
14) Strike in a copper cyanide or non-cyanide strike bath. Usually 3–5 min in the bath is adequate.
15) Rinse thoroughly.
16) Buff and polish if necessary.
17) Degrease with an organic solvent and electro-clean cathodically for 30 sec in a cleaner suitable for copper.
18) Dip in a 50 mL/L sulfuric acid dip.
19) Rinse thoroughly.
20) Plate with bright nickel.
21) Rinse thoroughly.
22) Chrome plate as required (usually 2–3 min).
Like most plating procedures, you will no doubt have to tweak this procedure to fit your own specific needs.