I have been getting poor circulation through my pool heater, so I took the
plumbing apart and found that the inlet and outlet pipes (metal) are rusted
something awful. The pipes are about 2 inches in diameter, but have so much
rust built up inside that there is less than a half inch of space.

Replacing the pipes (and cleaning the heat exchanger, etc.) is easy enough,
but I'm wondering if the new pipes will rust as well? In other words, is
this just a necessary evil over time (15 years in this case) or is there
some way to prevent this? Some other kind of pipe? Or maybe someone didn't
maintain the chemicals well?

Thanks,
Joe