In fact, I don't believe that hot water in the copper pipes in a home corrode very quickly. Even a hot water heater that has a sacrficial anode to prevent rust does so to prevent the steel tank from rusting -- it's not so much to prevent the copper pipes. In fact, steel has a greater tendency to rust when it is near copper or brass.

So it is possible that if steel is used in spa or pool heaters then rusting could occur, but I think that in these heaters only copper is exposed to the water, not steel.

I certainly want to know if I'm wrong about this or if I'm missing something.

Richard