While it is sometimes held that the calcium carbonate scale that hard water deposits on the interior of pipes acts like a protective layer, there's no evidence for this claim. Water that has been completely demineralized and stripped of all total dissolved solids (known as TDS), can act as a corrosive, but that's not a water softener does, since it replaces calcium cations with sodium cations. Studies conducted by the Water Quality Association have concluded that water softeners do not contribute to the erosion of copper pipes.