For a non-pressurised flat plate solar collector, what working fluid would be best, based on fluid cost and corrosion of the galvanised steel in the system? The pump internals and the pipes and valves will be pvc. Ignore freezing for the moment, as it can be drained. The internet seems to be weighted towards polypropylene glycol and water, which, if i am not mistaken, is the normal car “this won’t kill your dog” antifreeze, and i don’t have data on it’s corrosivity towards the zinc galvanising.
My system uses the “safe” antifreeze. It’s mostly copper though so I don’t know about the galvanized part.
From the American Galvanizer’s Association…
How Corrosive is Propylene Glycol to Hot-Dip Galvanized Steel?
Not so much. They also have a page about water itself.
Note that propylene glycol has a higher viscosity than ethylene glycol, increasing the pump head.