Atomarc,
Distilled water is corrosive. Since there are no ions in the water, the water will "grab" ions from the surrounding material in order to reach a more balanced state. The reason a radiator company might recommend using distilled water MIXED with antifreeze is to eliminate any possible interaction between the antifreeze and dissolved ions in the water. The action of adding the antifreeze with all of its additives, including corrosion inhibitors, eliminates the natural corrosivity of distilled 100% pure water.
In short, distilled water isn't corrosive mixed with antifreeze, but it is corrosive in itself.
