Plastic Impellers are the answer!
My opinions only folks!
At one stage several manufacturers had the genius idea that to avoid the iron bacteria problem, they would make the internals out of plastic rather than steel.
I almost fell over laughing when a rep told this and kept a straight face. Needless to say, what ever the materials are for the impellors, the iron bacteria problem is one that requires management and maintenance. The easiest solution is generally to clean the pump and bore with BoreSaver Ultra C in a managed timely interval. In other words before the pump fails!