This question is quite vague. It would be helpful if you could provide more information on the environment to which the component will be exposed. Items to include:
1. Temperature-- minimum and maximum?
2. Presence of humidity or water? What is the pH?
3. Presence of chemicals? Sodium chloride? Acids? Bases?
4. Is the component dynamically loaded? Is it subjected to impact, wear, or steady-state loading?
5. Is it necessary for the component to still be electrically conductive?
6. Will the aluminum component be in contact with another metal?
Coatings for aluminum castings are generally either polymer paints (epoxy, polyurethane, polyester, etc.) or anodic deposition coatings (aluminum oxide that may or may not be subsequently sealed). Maximum corrosion protection is usually obtained by anodizing the surface, sealing this anodized layer, and then applying relatively thick powder coatings as the final barrier layer. However, impact, fatigue, and wear are all factors that may favor another solution.