Wurt is not that acidic which is why you can use copper and brass fittings in brewpots. Beer gets more acidic as it ferments and the acidity shows on the inside of any brass fittings, like brass faucets you may have installed in your kegerator. And BTW, modern brass is a alloy of copper and zinc. The anodes are zinc, not magnesium. Magnesium would react violently with water.
Interesting that in the first post of this entry https://www.homebrewtalk.com/f170/element-rust-no-more-235704/, the magnesium anode is not reacting violently with the water. I think you're misinformed regarding the reactivity of solid magnesium in water. http://www.lenntech.com/periodic/water/magnesium/magnesium-and-water.htm
Perhaps you are thinking about sodium or potassium?
http://www2.uni-siegen.de/~pci/versuche/pics/natrium3.jpg