you guys are nuts if you think fresh water = no corrosion. Rust is only one form of corrosion, but anything iron (ie, steel) is likely done for if critical...like crank bearings.
The dissolved oxygen content and pH of freshwater DEFINITELY allows significant iron / steel corrosion to occur. The pH would have to be greater than about 9.5 for the formation of ferric hydroxide to slow things down.
As a rough order of magnitude, you're probably in the ballpark of losing .001" / month, probably more depending on dissolved oxygen content.