They are "dis-similar metals" from an electrochemical standpoint and if there is an electrolyte available (like water) they form a battery and the aluminum is preferentially corroded.

Early carbon tubed bikes with aluminum lugs often failed at the joints from this problem. The manufacturers soon learned to wrap a thin layer of fiberglass around the carbon tubes to act as an insulator. A carbon seatpost in an Aluminum frame, or vice-versa can cause the same type of corrosion resulting in a VERY stuck seatpost.