What is rust?

What exactly is rust?

Chemically, it is the state in which metals react with oxygen in the air and become oxidized.

The general image of rust is red rust.

Considering the chemical meaning above:

Even stainless steel with its beautiful, shiny luster can rust.

Strictly speaking, this means that the chromium in the stainless steel has rusted (oxidized).

The beautiful luster is due to the oxide film formed by the oxidation of chromium and other elements contained in stainless steel.

This oxide film is stable and does not oxidize the underlying metal.

The compounds in stainless steel rust first, protecting the main component, iron, from rusting.

Red rust is generally recognized as rust,

In this state, even the underlying metal (iron in the case of stainless steel) has oxidized.

(Strictly speaking, it has reacted with water molecules as well as oxygen)

Unlike chrome rust (oxide coating), this rust peels off easily and can easily penetrate deep into the metal.

Because it peels off easily, it destroys the chromium oxide coating from the inside.

Rust = red rust.

This is a condition in which oxidation of the underlying iron begins, destroying the oxide coating of chromium and other materials.