Isn't softened water more corrosive than hard water?

No, unfortunately, the properties of naturally soft water are often confused with those of softened water. Naturally soft water is usually aggressive since it is low in all dissolved minerals not just the hardness minerals. A water softener merely exchanges the hardness minerals for sodium ions but doesn't change the characteristics of water associated with corrosion.

From the EPA study evaluating the effects of household softening on home plumbing materials, "the ion exchange softened water did not show a pattern of higher metal leaching from plumbing materials. Furthermore, the water softener did not have a detrimental effect on the significant water quality parameters that influence metallic solubility and the rate of corrosion: pH, total inorganic carbon, dissolved oxygen, chlorine, temperature and ortho-phosphate".

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