sacrificial anode is what it is called and are found on boats or boat motors (hi in zinc so that corrosion attacks this instead of other components) . i can't see them coroding any faster than any other alaminium componant on your car concidering an anode can take a number of years to corrode in salt water and an ally radiator would have now where as much zinc as an anode.

i can't see insolating the radiator a possibility as water is quite a good conductor.

i would buy one in a hart beat