What is "rust" commonly referred to in relation to iron?

"Rust" is commonly referred to as a corrosion product of iron, which typically forms when iron reacts with oxygen and moisture over time. This process results in the formation of iron oxides, which give rust its characteristic reddish-brown color. The term "corrosion product" encompasses all such oxidation reactions that lead to degradation of the metal.

In the context of iron, rust is specifically a type of corrosion that severely affects the material's structural integrity. When iron is exposed to elements such as water and air, it undergoes a series of electrochemical reactions that result in rust formation. This corrosion not only deteriorates the appearance of the metal but also weakens it, leading to potential failure of structures and components made from iron.

The other options describe related concepts but do not precisely capture the essence of rust. While an oxide layer may form during rusting, "rust" is more accurately defined as the specific corrosion product resulting from iron oxidation. A protective coating typically refers to materials used to prevent rust, and a metal byproduct could imply leftover material from a process, which does not apply to the formation of rust in this context.