The metal reacts with oxygen in the atmosphere to form an oxide film on the surface

The iron oxide formed on ordinary carbon steel continues to oxidize, which expands the corrosion and finally forms holes. The carbon steel surface can be guaranteed by electroplating with paint or oxidation resistant metals (e.g. zinc, nickel and chromium), but as is known, this protection is only a thin film. If the protective layer is damaged, the steel below begins to rust.