Dana,
I agree with Andy's comments above and add one further suggestion. Since it looks like corrosion caused the pitting, it is a good idea to check the entire cooling system. The radiator might need a flushing to get any stuff out. Corroded aluminum turns to a chalky white stuff. Typically pretty soft so it isn't gonna damage anything, but could block the radiator passages reducing its effectivity. A good cleaning never hurts. Then use a top brand coolant or just plain distilled water and a bit of water pump lubricant.
Eric