Magnesium forms a protective layer of magnesium hydroxide naturally however chlorides attack it. If the component is a magnesium alloy (guessing the Honda was) then any chlorides in the water (tap water) will attack the metal
. Even if coated any small defect in the coating will make the corrosion very severe at the location of the fault. Advise to keep coolant as free as possible from chloride by using demin water is correct. Draining the coolant is also wise but any trapped water will continue to cause corrosion so drain when engine is hot and remove hoses etc to allow water to steam out.
If attempting repairs the advice is not to use wire brushes or other iron based implements to remove the corrosion and prepare the surface for repair as the iron can cause further corrosion under the repair. Magnesium was not the smartest choice of material.....but is sure is light!
My 87 510 Husky has magnesium cancer of the barrel and the water pump flat surface (the pump casing is plastic) but since its 24 years old and not holed yet it should last my ownership
Later models reverted to aluminum alloy.
