I think its the dissimilar metals at work that is actually causing the damage. You've got copper, brass, cast iron, steel, galvanized freeze plugs, etc. With this older stuff you really do need to change the coolant every 2-3 years to prevent corrosion. I know that the big trucks have a stick that they can use to check the ph of the coolant and adjust it accordingly. GM solved some of this problem when they went to aluminum radiators with the plastic tanks in the 80's, but the heater cores were still a problem.