The removal of hardness does not effect corrosion.
Is softened water corrosive?
We will turn to our American friends for their advice on this as they have many more water softeners installed than we do.
No. The suggestion that ion exchange water softeners promote corrosion is a misperception and the suggestion to use scale build up as a corrosion control method is inappropriate as proven by the EPA. The conclusions were drawn from an EPA pilot study conducted by Thomas J. Sorg and Michael R. Schock of the EPA's Drinking Water Research Division as project manager and principal investigator. Simply put, the removal of hardness with an ion exchange water softener does not affect the factors which cause or even accelerate corrosion. Corrosion is caused by a change in the pH or carbon dioxide concentration, the dissolved oxygen concentration, or the total chemical concentration of minerals. None of these factors is affected by water softeners.