"Red rust" refers to the state in which iron oxidizes upon contact with water and air, causing it to rust red.

Generally, the term "rust" refers to this red rust. On the other hand, rust that forms when zinc oxidizes is white, so it is called "white rust."

Red rust is a form of corrosion caused by the oxidation of iron, which leads to quality deterioration, such as a reduction in the strength of iron products. Additionally, red rust can adhere to or make close contact with other uncorroded iron parts, becoming a cause of "contact rust," which spreads corrosion, so caution is required.