Why does chromium in stainless steels make them more corrosion resistant in many environments than plain carbon steels?

Plain carbon steels corrode when they come into contact with an oxidizing environment (usually in the presence of water and oxygen). The steel surface forms an oxide film, called rust, which can weaken the material's structure, and may even lead to equipment failure or structure collapse.

Chromium is added to steel to form stainless steel (typically, at least 10.5% chromium content). Chromium is highly reactive with oxygen, and it forms a thin, continuous, passive layer of chromium oxide on the surface of the steel when exposed to air or water. This transparent layer acts as a barrier, separating the underlying metal from the corrosive environment.

When stainless steel is exposed to air or water containing oxygen, the chromium in the steel reacts with the oxygen to form chromium oxide (Cr2O3). This oxide layer is tightly adhering to steel's surface and very stable, preventing further reaction between the steel and the environment.

One unique feature of the chromium oxide layer is its ability to self-heal. If the layer is damaged or removed, the exposed chromium atoms in stainless steel can react with oxygen in the environment to form a new layer of chromium oxide, continuing to protect the steel from corrosion.

The presence of chromium and its ability to form a stable oxide layer increases the corrosion resistance of stainless steel in many environments. While plain carbon steels are vulnerable to corrosion in most environments (water, acids, salts, etc.), stainless steels perform better under various conditions, due to the protective chromium oxide layer. This makes stainless steels suitable for a wide range of applications, such as chemical plants, automotive industry, and medical equipment, where corrosion resistance is crucial. To sum up, chromium in stainless steels forms a stable, self-healing chromium oxide layer on the surface, which acts as a protective barrier and prevents the steel from corroding in many environments. This is why stainless steels have greater corrosion resistance compared to plain carbon steels.