>> but have never had any problems or issues with rusting.
That's great! Surprising to me, but great.
I know they protect propellers against seawater by connecting some zinc to the bronze propeller. The zinc corrodes instead, because it's more chemically active which makes it the "anode", and protects the bronze by turning it into the "cathode". Or I may have those two words reversed!
Maybe (wild guess here) the one that is rusting would slow down or stop if you set a piece of zinc under one foot.
