The manual for my dishwasher says to refill salt just before running a wash cycle, because if any grains of salt spill onto the stainless steel interior it will corrode. If it runs right away, no issue because the salt is quickly dissolved, diluted, and flushed.
So then I realized when I cook pasta I heavily salt the water (following the advice that pasta water should taste as salty as the ocean). But what happens when I leave that highly salty brine in a pot, sometimes for a couple days to reuse it? Does that risk corroding the pots?
Well it’s not actually clear to me whether the soft water is to protect the dishwasher, or to make cleaning more effective. Soft water dissolves soap better which makes it more effective in cleaning. It also means I can use powdered detergent (which is cheaper than liquid detergent, but in hard water powder doesn’t perform as well). Soft water has the down side that it’s actually /more prone/ to corrosion than hard water (at least according to youtube plumbers). So I’m tempted to conclude the built-in water softener is just for cleaning effectiveness.
My understanding is that every mole of water hardness (mostly Ca^2+) reacts with two moles of soap to produce soap scum. So using softened water (replacing Ca^2+ with 2Na^+) allows you to use less soap and reduces soap scum buildup.