Originally Posted by KyDan
Some have posted that antifreeze in the water heater can react with the
tank and cause corrosion.
I don't know if that's true but I don't plan to find out either!
Whenever you have a situation where there are two dissimilar metals and a conductive liquid, galvanic corrosion will occur. In the Suburban water heater, the tank is steel and some of the fittings are brass. The water being heated is the conductive liquid. Therefore, the steel tank will corrode if nothing is done to prevent it.
That is the purpose of the sacrificial anode. It is magnesium which is more negative than the steel tank. Thus, the anode will corrode away instead of the tank. The anode is cheap and easy to be replaced every few years. See Galvanic anode - Wikipedia, the free encyclopedia
RV antifreeze is propylene glycol, which is much less conductive than water. Therefore, the water is more likely to corrode when in use heating water then when it is filled with antifreeze.
That being said, I can't see why anybody would waste 6 gallons of antifreeze by filling the water heater with it. The normal process is to drain the heater and then bypass it before adding antifreeze to the system.