Ultrapure Water Corrosion

I was told by a sagacious professor of chemistry that ultrapure, de-ionized water will corrode stainless steel. You can't blame me for wondering why. I always pictured water as a passive solvent. But "H2O" in its purest form can indeed be corrosive. Copper, glass, plastic, and stainless steel will all slowly lose molecules to the pure liquid. Highly insoluble metals, i.e. pure tin, are the only materials immune to water's alluring properties. But, if the gaseous oxygen is removed from the water before it is stored, you're ok to use whatever typical storage container you want. I still don't know why this happens, but I imagine it has to do with huge osmolarity forces coupled with the chiseling action of electrons on dissolved O2 molecules.