As the water remains in the system the only corrosion is initial use and a small amount for evaporation replacement. This can be overcome by central heating corrosion inhibitor.
It would be a good idea to add a pressure release valve to the vent pipe especially if the expansion tank could be subject to freezing conditions.
If a tapping was taken from the top and bottom of the barrel and run through the coil of a hot water cylinder it could supply household hot water. Well that's the theory.
The only problem would be getting the exhaust heat through the barrels with enough surface area to heat the mass.