Deionized water has no calcium or magnesium in it. If you racall in the 60's radiators would collect white residue. This usually was the calcium and magnesium deposits. Oddly they are deposited more readily at higher temperatures. Corrosion is the reaction of oxygen with metals. In today's radiators with cooant resevoirs the oxygen is most likely boiled off and vented so it can't be a bad actor.

I have used deionized water and antifreeze in my vehicles since 1983 without the first problem. (Total of 500,000+ miles)

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1991 350 SDL
114,000 miles