Re: Do bronze propellers get "brittle"?
A little elementary metallurgy here: Most BRASS is primarily copper and zinc alloy with other trace metals added to modify basic properties--like free machining, casting, etc.
Most BRONZE is an alloy of copper and TIN with other metals added in trace amounts to modify properties such as casting etc. Propellers are made of BRONZE, not brass simply because as stated above, water will set up a galvanic reaction within the metal and the zinc will simply go away as a sacrificial anode to the copper, eventually weakening the prop to the point of failure. Tin, being closer to copper in electrical valence will not set up so great a reaction.
Naval Bronze is another alloy with (I think) Monel metal and other alloys in it to make it more corrosion resistant than regular Bronze.
Embrittlement is a completely different process not related to corrosion. The marina has it wrong, but is probably correct in that the propellers need to be replaced: Bronze will over time corrode, not as quickly nor as completely as brass and yes, the bronze can with age and use become embrittled and cracked. So in either case, they still would need to be replaced.
As an aside, one of the reasons the Titanic sank was because her substandard quality steel plates became embrittled in the cold north Atlantic water. Yes, Embrittlement of metals is a well known metallurgical process.
