The blackening of the inner wall of a new aluminum kettle after boiling water is mainly related to the oxidation of aluminum, the effect of minerals in the water, and improper cleaning methods.
Aluminum Oxidation: Aluminum is a chemically reactive metal. When it comes into contact with air or water, an oxide film naturally forms on its surface, which is usually black or yellowish-brown. Especially in humid environments, such as when the kettle is not dried promptly after washing, the oxidation process is accelerated, making the inner wall more prone to blackening. This oxidation reaction is a common chemical change of aluminum in the natural environment and is one of the fundamental factors causing the kettle's inner wall to blacken.
The Effect of Minerals in Water: The mineral composition of the water also has a significant impact on the blackening of the aluminum kettle's inner wall. Iron salts in the water react chemically with aluminum to form black iron(III) oxide (Fe3O4). Specifically, when the water contains iron salts (such as ferric chloride), because aluminum is more chemically reactive than iron, aluminum displaces iron ions from the iron salts. The resulting elemental iron adheres to the kettle wall in the form of black particles, thus blackening the inner wall of the kettle. Furthermore, during the heating process, limescale precipitated in the water mixes with aluminum oxide to form a black deposit, further exacerbating the blackening of the inner wall.
Improper Cleaning Methods Inappropriate cleaning methods are also a significant cause of blackening inside aluminum kettles. Using dishwasher detergents, acidic or alkaline substances to clean aluminum kettles will damage the original oxide film on the surface. While this oxide film may seem ordinary, it provides some protection for the aluminum kettle. Once damaged, the kettle becomes more susceptible to reaction with air, water, and minerals in the water, leading to blackening of the inner wall. Additionally, using steel wool or other hard objects to clean aluminum kettles will scratch the kettle walls, damaging the oxide film and making the kettle more prone to blackening.