Well, coolant will actually accumulate an electrical charge over time. The engine block is used as a ground for the electrical system, so there is a small current flowing through the metal. When antifreeze is electrically charged, it is corrosive to some engine components. Mostly, it will start eating away at aluminum parts...

Corrosion is an extremely complicated problem. For starters, if you use an aluminum radiator with a cast iron block motor, you've created a battery, and aluminum is the sacrificial anode. As long as the coolant contains water (distilled, demineralized, or straight from the tap), the aluminum will corrode no matter what mixture of coolant you use. If the aluminum alloy isn't uniform, you can also get localized corrosion from mini batteries created in just the aluminum, or these localized areas become the "preferred" anode and corrode first, which is what may have caused your pinhole leak.

This is the same galvanic corrosion you get when you bolt steel to aluminum, and the joint just gets wet. About the only way to totally prevent this would be to use a battery to reverse the polarity of the cooling system's Al-Fe battery, but I've never seen anyone do this.

The other purpose of antifreeze additives is to prevent hard scale deposits. Beside the loss of heat rransfer from a scale build-up, you can also get localized corrosion under the scale.

My recommendation: Always use either distilled or demineralized water to minimize scale build-up. Always add a corrosion inhibitor package, either what comes in antifreeze or something like Water Wetter. If you have an aluminum radiator and a cast iron block or head, recognize that you don't have the most trouble-free system.