Although I don't have experience using it on aluminum, I would not be afraid of trying a mild acid on the aluminum. You can always limit the exposure time and check to see for any damage on the aluminum. Some soap would be a good idea too. Scale deposits are usually layered deposits and may require surfactants to help loosen and clean the surface. Nalco Chemical, Betz and some of the other water treatment companies used to have cooling system kits that were stronger than most store bought systems. Just an idea