what makes deionized water so corrosive?

Answer 1

Deionized water is NOT particularly corrosive.

However, de-ionizing water removes those ions that normally act as buffers. That means that any contaminant or leachate that gets in the water will cause a much larger change in the pH than would occur in typical buffered groundwater.

So, water that is deionized can easily become low or high pH (acid or caustic) corrosive.

Answer 2

Water is naturally weakly acidic. Dissolved metal ions increase the ph of water (less acidic). If you remove these ions, the water will have a tendency to attack metals to replace those ions.