Carbon conducts electricity, true, but that has nothing to do with corrosion. The material has to be capable of participating in an electrolyitic reaction, i.e, giving up or receiving electrons from another substance., usually in the presence of water. I've never heard of elemental carbon being involved in an electrolytic reaction. It reacts, of course, but needs heat and an oxidixzer to get started (combustion).
If salt water gets trapped between rubber and an aluminum tank, you could get corrosion, but the only role the rubber would play would be holding the water on the surface of the tank.