I've heard that said from time to time but don't have a clue to whether or not it is true. I went to the web and did some searching and found one decent article on a study done by NASA. I figured they are as reliable as any. The gist of it is that the common metals like aluminum, boron-steel, stainless steel, etc. are not affected. A few metals like copper and tungsten are corroded by DI water and shouldn't be used in nuclear reactors. Unfortunately, cast iron is not on their list of tested materials so your question remains unanswered. One point to remember is that these tests were done with total immersion for several weeks at a temperature of 100o
F. If you are using DI for a brief pre-wash the time scale doesn't seem long enough to incur significant damage. If you want to read the NASA article yourself, go to:http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690015779_1969015779.pdf
I'm puzzled as to why DI water would be noticeably more corrosive than tap water. It does become slightly acidic due to dissolved CO2
from the air, but it's a little surprising for that to have such a big effect. The chemical potential of iron ions will of course be lower if they are at lower concentration, so DI is "hungrier" for them, just as your engineer said. Unless the concentration in the tap water were high, it's hard for me to see why that would have more than a minor effect on the corrosion rate. But maybe we're missing something. Have you considered taking small samples of your iron and leaving them for a while in pots of DI and tap water to see if there's any difference in corrosion? We'd love to hear the results. Mike W.
(published on 08/22/2011)