Oxide layer. First of all, judging from the material of the thermostatic kettle, the situation you mentioned appears at the bottom of the kettle, which may be because the raw materials for making the kettle contain other ingredients, not entirely stainless steel; Secondly, it is possible that the heating part at the bottom of the lake is too close to the bottom of the pot. Due to the high temperature generated during heating and the cooling effect of water, the chemical properties of the constant temperature pot have changed. If finely polished with sandpaper, the black shift can be removed. Although this problem occurs, it will not affect the use for the time being. As long as the temperature of water reaches boiling, sterilization can be achieved.