You're talking about corrosion by electrolysis.
Water (tapped or distilled) and electric current always causes electrolysis.
To avoid this you've to use always the proper coolant at 50/50 (like Honda Type 2) analogy that contains silicate-free corrosion inhibitors and replace it according to the scheduled maintenance.
Also all the electrical connections and cables must be in good condition.
You can easily check this one by using a voltmeter. If you can see some mV through the radiator on the voltmeter then you've problem...
Another patent is to use a cap with a sacrificial magnesium anode (the more sensitive metal is attacked first). It's heavily used on boats but I'm not sure how effective is in a vehicle or if it's not causing any problems to the closed circuit of a vehicle. (On boats it's in the open sea).
In general we never use anything else than distilled water because we will destroy the water pump and other components with the dissolved minerals and salts that contains and they're corrosive...
Google it for more details...
I have a sacrificial zinc anode on boat motor and a outdoor furnace. I never heard of magnesium used before. A simple Google and your post gave me a more to read.
Thanks.