The main reason why the aluminum kettle turns black is that the water contains some iron salts, which will be dissolved during heating. After chemical reaction with aluminum, iron will be produced, which will be attached to the aluminum kettle. Therefore, it is reasonable for the kettle to turn black.
Many people who boil water in aluminum pots will have such doubts. The pots just bought will turn black after burning water several times. I don't know whether drinking such water is harmful to the body. In fact, this is a normal phenomenon. The substances dissolved in water at high temperature are attached to the aluminum pots.
General water looks very clean, but in fact, many chemicals will be dissolved and separated during heating, such as calcium salt, magnesium salt and iron salt. Iron salt is the main substance that makes the aluminum pot black. This is because aluminum is more active than other substances. When it encounters iron salt, chemical reaction will occur to replace iron, This iron will adhere to the aluminum pot, and it makes sense to turn black.
But generally, if it turns black, there are three main factors. First, the water contains more iron, second, the water is boiled for too long, and then the aluminum pot is new, because there will be a layer of alumina on the surface of the old aluminum pot, which will hinder the chemical reaction between aluminum and iron salt.