When iron is added to brass it produces hard, tough alloys. One of these is delta
metal ( 55% copper, 41% zinc, 1%–3% iron, and fractional percentages of tin and manganese, ) which can be forged, rolled, or cast and is used for bearings, valves, portlights
, and ship propellers.
Bronze is an alloy of copper,tin, zinc, phosphorus, and sometimes small amounts of other elements. Bronzes are harder than brasses. Most are produced by melting the copper and adding the desired amounts of tin, zinc, and other substances.
The properties of the alloy depend on the proportions of its components. Aluminum
bronze has high strength and resists corrosion; it is used for bearings, valve seats, and machine parts
. Leaded bronze, containing from 10% to 29% lead, is cast into heavy–duty bushings and bearings. Silicon bronze is used for telegraph wires and chemical containers. Phosphor bronze is used for springs.
(borrowed from njscuba.net)
here is a chart to help.
Bottom line is, bronze is stronger, brass is just as corrosion resistant. IMHO, run it until it starts to fail, and replace with bronze.