Anodizing converts the surface of aluminum to aluminum oxide, also called Alumina. The volume resistivity of Aluminum Oxide is greater than 10^14th power, and so it is a good insulator. Perhaps it would be reasonable to model the structure as pure aluminum covered by an insulating dielectric. The dielectric constant of aluminum oxide is about 9.8. The depth of the insulating dielectric is variable depending on the anodizing process, but a good starting value might be .0006 inches.