Why does chromium not rust

Galvanized steel is technically a coated material, but it is worth mentioning here. Galvanized steel is a carbon steel that would most likely rust if it did not have one or more layers of zinc applied to it.

The zinc layer acts as a sacrificial metal for the steel. This means that the zinc layer will combine with the oxygen more readily than the iron in the steel will.

This creates a zinc oxide layer that prevents the formation of iron oxide, thus eliminating the possibility of rust forming. However, if the zinc coating is damaged, or if the galvanized steel is placed in extreme environments, the zinc coating may be rendered ineffective and the steel will rust.

The formation of white rust does not necessarily damage the material or the zinc coating. Copper , brass , and bronze do not rust for the same reason as aluminum. All three have a negligible amount of iron in them. Therefore no iron oxide, or rust, can form. However, copper can form a blue-green patina on its surface when exposed to oxygen over time. They have their own forms of corrosion, such as pitting that can occur in stainless steel or the blue-green tarnish found on oxidized copper.

Iron and Chromium Stainless steel is an alloy made up of iron and at least Some are better at resisting rust than others, but all can rust. Chromium in stainless steel resists rust by forming a thin layer of chromium oxide on the surface of the steel. Free iron is iron that is separate from the alloy, and therefore not protected by the chromium oxide. The potential sources of free iron are many. The important things are: Free iron on the surface of the stainless steel can rust.

The chemical changes that happen during corrosion can be shown as follows:. Picture 3. How stainless steel resists corrosion. If iron and chromium are exposed to oxygen, it is the chromium that reacts to form an oxide. Due to its high chromium content, stainless steel forms a very thin layer of chromium III oxide as soon as the surface is exposed to the oxygen in the atmosphere. The layer of oxide is so thin that the metal can still shine through it, but it is thick enough to prevent the oxygen and water attacking the metal underneath and so no corrosion takes place.