Minimum amount of antifreeze

I just got my cooling system leaks fixed and was noticing how much lower the temp gauge is when running straight water. It never gets cold where I live so just about any amount of antifreeze is sufficient protection from freezing. Antifreeze contributes very little to raising the boiling point, the radiator cap takes care of that. I'm mainly concerned with the corrosion protection. What is the minimum mixture of antifreeze necessary? I have s 22R with an aluminum head and an iron block.