Don't know if they do or don't, but supposedly if you only use water, sorrosion begins IMMEDIATELY. Read an article from C&D that said using just water in a new cooling system and running 35 laps on a racetrack, when drained, already had red flakes in it. Then again, it was the Zerex lead chemist saying this, so take it with a grain of salt. However, when I was in high school, my friend's POS Mustang had a hole in the radiator and he found it more cost effective to just keep refilling it with water than to get the hole fixed or replace the radiator. So even though it was constantly getting fresh water, twice/thrice daily, the stuff coming out would often have contaminants.