This argument is total BS. I'm an engineer at a nuclear power plant. Corrosion of nuclear safety related steam piping and of the pressure vessel and internal components is a HUGE concern for us, obviously, as there is not just a huge capital investment, but public safety involved as well.

We constantly run a portion of our coolant water through a series of filter / demineralizers. Disolved ions in water CAUSE corrosion. Our license regulates us to shut down if the ion content of our water gets past a very conservative limit, measured by the conductivity of the water.

Those that make this argument don't understand the mechanisms of corrosion. Metal surfaces exposed to a water environment develope a passive corrosion layer, very thin, that actually protects the metal underneith. One of the toughest and best protecting oxide layers is aluminum oxide BTW. Anyway, we go to great lengths to protect this passive corrosion layer by carefully controlling the pH (slightly toward the acidic side of neutral) and mineral content.

The other issue, addressed here, is that minerals form hard deposits on heat exchanger surfaces called scale, which degrades the efficiency of the heat exchanger.

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