Why is sludge magnetic? I thought rust lost its magnetic properties? If its not rust, what is it? – limescale has no magnetism.
It seems that the sludge is magnetic as it is not rust, due to it being oxidised in a reduced oxygen/slightly alkali environment. So testing the copper pipes for reaction with the magnet would suggest lots of sludge. However on the radiator it's utter tripe, as they're 99x out of 100, steel and the magnet would stick.
Nope still don't get it. Yes the water will have some iron(II) and iron (III) in it but if it is alkaline then they will come out of solution as the hydroxides, this precipitate is not magnetic. This reaction will be preferential to any reduction the iron ions may undergo.
If those conditions gave iron then we wouldn't be spending billions a year on smelting plants we'd just have massive radiators keeping the world warm and dropping out iron.