I use the MAG flakes as they seem to be superior to the other types (two) I have tried and they store well.
MgCl is less corrosive to metal and concrete than CaCl or ordinary salt, but I do find that my arena drag gets a light layer of rust on it. It doesn’t concern me as it would be slightly rusted by now anyway. I just rinse it off now and then.
If you have kick boards in your arena then the steel of your building will be quite safe.
I haven’t watered my indoor in almost a year and the footing is nicely moist as well as completely dust free.