Why demineralized water is corrosive?

Answer 1

To remove the minerals from raw water, the raw water has to undergo several process during which energy has to be input to it. As a general rule, any substance with higher than normal energy level tries to come to a stabilized state by releasing the excess energy (compare with the electrons imparted with higher energy level, which try to return to lowest possible state for stabilization, or something having high potential energy due to elevation which tries to return to the minimum possible elevation to attain a stable state or to make it simple, remember: HE THAT IS DOWN, NEEDS FEAR NO FALL!). That is the reason which makes demineralized water highly unstable and reactive. It tries to react with metals to lose energy and stabilize.

Answer 2

That depends on conditions. Demineralised water can be a scavenger - it picks-up trace impurities very readily. This is related to dissolved oxygen rather than the water itself. It is a particular problem in boilers using demineralised feed water - without deaeration or treatment chemicals to remove dissolved oxygen, boiler corrosion is far more severe than when using standard tap/mains/mineralised water.