How does alloying with chromium prevent rusting?

Alloying with chromium forms a protective layer of chromium oxide that prevents rusting.

When iron is alloyed with chromium, the chromium reacts with oxygen in the air to form a thin, stable layer of chromium oxide on the surface of the metal. This layer is invisible and adheres tightly to the metal surface, preventing oxygen and moisture from reaching the underlying iron. This is how chromium prevents rusting, which is essentially the oxidation of iron.

The chromium oxide layer is self-healing. If it gets scratched or damaged, new chromium atoms from the alloy will react with oxygen to repair the layer. This ensures that the protective barrier is maintained, even under harsh conditions or after physical damage.

The amount of chromium in the alloy can also affect the level of rust resistance. Typically, stainless steel, which is highly resistant to rust, contains at least 10.5% chromium. The higher the chromium content, the better the rust resistance. However, other factors such as the presence of other alloying elements and the conditions the metal is exposed to can also influence rust resistance.

In summary, alloying with chromium enhances the rust resistance of iron by forming a protective chromium oxide layer. This layer acts as a barrier that prevents oxygen and moisture from reaching the iron, thereby preventing rusting. The self-healing nature of the layer also ensures long-lasting protection.