Cover the entire crusty area on the aluminum piece with baking soda.
Pour a thin stream of white vinegar over the baking soda. There should be an immediate, fizzing reaction. The baking soda and vinegar fizz will break apart the crust on the aluminum and make it easy to wash away.
Scrub the area using a steel wool pad, using small circular motions over the crust until it breaks away from the aluminum.
Rinse the area with hot water until all baking soda and crust remnants have been washed away.
Buff the aluminum dry using a soft cloth.