Wiki-Rust

It refers to the red or black corrosion that often occurs on the surface of metals. The oxide or other compounds formed by the oxidation-reduction reaction of unstable atoms on the metal surface with oxygen or water are the substances that cause rust. In particular, if a steel knife is not properly maintained, rust can occur on its surface. Although small amounts of rust on a knife are not harmful to the human body, it can affect the taste and odor of food, and can therefore affect the quality of cooking. Therefore, frequent maintenance is necessary when using knives made of rust-prone steel.

Related websites
How to Prevent Rust on Japanese Knives | KIREAJI
Maximize Your Japanese Knife's Lifespan: Essential Care Tips | KIREAJI