Why circulator on heatpump Armstrong S-25 BF Cast Iron In-Line Pump, 1/12 hp breaks every year?
One possibility (and it's just that - a possibility, a guess, not definitive) is that your system leaks, and makeup water is being constantly introduced by an automatic fill valve.

The problem with that is that new water comes with oxygen dissolved in it, and oxygen rusts cast iron. So if your pump plugs up with rust, and fails, that might be your cause - as you have not provided any information on the symptoms of failure or state of the failed pumps, it remains a guess. If the failures are normally at the start of heating season, that's another clue in this direction - lots of time for the pump to rust while not spinning all summer.

If that is the case, a stainless steel pump might be the solution to your problem, though another solution would be to stop the leaks.

Systems with cast iron pumps and no leaks just rust out the oxygen that's in the initial fill water, and then rust no more, as there's no more oxygen.