Carbon fibre is electrically conductive. Carbon & aluminium are almost at opposite ends of the galvanic scale; given half a chance they WILL react badly, with the aluminium coming out second best. Carbon bars or seatpost in aluminium frame are relatively inert because the polyurethane coating over the carbon acts as an isolating layer (unless it's scratched deeply enough to expose raw carbon). Alloy posts in carbon frames are bad news, because there is direct contact with the carbon fibre unless it's really thoroughly coated with an isolating compound of some sort. If you're mixing carbon & metal components, you're much better off with titanium; they're very close on the galvanic scale, so reactiveness is minimal.