Citric acid or white vinegar will descale stainless without harm if you dilute and keep the contact time short and i mix one cup vinegar with two cups water or dissolve one tablespoon citric acid in half a liter bring it to a boil, then switch off and let it sit about twenty minutes. Swirl, pour it out, and wipe the interior with a soft bottle brush or a non scratch sponge. Rinse two or three times, then boil a full kettle of plain water once and discard. Works great.
With hard water I do this every two to four weeks if I use the kettle daily. If the chalk comes back fast, weekly is fine. Avoid bleach and abrasives and do not leave acid in there overnight.
