Corrosion is often a result of anions in the water. But even in the absence of any ions in the water, the iron is likely to rust.
I recommend cleaning off the rusting area to remove any loose material, dry it off well, and then coat the rusting areas with silicone seal. That will isolate the ferrous metals from the water and REDUCE their opportunity to rust. It is not perfect, but does work well enough in my system. Remember that the metal never gets much above boiling temperature, so there is no worry with the silicone burning up.