Mostly on a closed cooling system you are protecting from freezing . . . as far as corrosion, not as much of a concern. I run -100˚ F AF through the open side of my closed cooling system. There is a sea water strainer, sea water pump, the oil coolers, the HE itself, and the exhaust elbows.
I used to run the -50˚ F stuff, but found every year the sea water strainer (plastic) was cracked (from freezing). Once I switched to the -100˚ f stuff no more cracked sea water strainers.
It seems that the -50˚ F AF rating is for a pipe burst . . . however at about -20˚ F the softly frozen AF was enough to crack the bowl on the sea water strainer.