What is Rust?

An essential aspect of the use of metallic iron is the possibility of rusting or corrosion. Rust is a hydrated form of a compound known as iron(III)oxide. Rust is a general term that defines a series of iron oxides (red oxides). The process is formed by the reaction of iron with oxygen in the presence of air moisture or water. The rust formula is approximately Fe2O3 x H2O however, the exact amount of water in the formula is variable.​ However, there is no physical process to obtain the iron from the rust. What is the Chemical Formula of Rust?

The chemical formula of a compound is defined as the symbolic representation of the composition of a compound. The chemical formula for rust is Fe2O3 and is commonly known as ferric oxide or iron oxide. 

The final product is a series of chemical reactions simplified below as- The rusting of the iron formula is simply 4Fe + 3O2 + 6H2O → 4Fe(OH)3.

The rusting process requires both the elements of oxygen and water. The process is usually accelerated by activities such as contact with less-active metals, acids, strains in the iron, and the presence of rust itself. 

What is the Equation for Rusting?

Rusting of iron involves an increase in iron oxidation, accompanied by a loss of electrons. Rust consists mainly of two different iron oxides. These vary in the iron atom's oxidation state- Iron(II) oxide or ferrous oxide with +2 oxidation state and Iron(III) oxide or ferric oxide with +3 oxidation state. 

The rusting of the iron formula is a series of chemical reactions which is as follows-

  • Fe(OH)2 ⇌ FeO + H2O

  • 4Fe(OH)2 + O2 + xH2O → 2Fe2O3.(x+4)H2O

  • Fe(OH)3 ⇌ FeO(OH) + H2O

  • FeO(OH) ⇌ Fe2O3 + H2O

The series of rusting equations results in a reddish-brown deposit called rust, formed over a piece of iron when exposed to moist air for some time. The rusting of iron undergoes a chemical change that cannot be obtained back as pure iron by reversing the conditions.


Rust is a reddish-brown flaky coat present on the metal. Technically, rust is a Hydrated Iron (III) Oxide whose chemical formula is Fe2O3 x H2O. The rusting of the iron formula is represented by  4Fe + 3O2 + 6H2O → 4Fe(OH)3. Rusting can be prevented by galvanization, painting, and the application of grease.

FAQs on Rust Formula

1. What is Rust?

Rust is an orangish-reddish layer, primarily composed of ferric hydroxide and ferric oxide and created by oxidation. Rust is usually formed on the surface of iron when exposed to any air, moisture, or water.  The chemical formula of rust is Fe2O3 x H2O with a variable amount of water content. Unlike iron and steel, aluminium does not rust or corrode due to the protective layer of aluminium oxide which prevents the contact of air from the metal and hence the aluminium layer does not flake off like rust.

2. How does Water cause Rust?

When the oxygen atom splits, water makes the element iron react with oxygen. This causes the iron to lose electrons while oxygen gains electrons during the early stages of rusting. The ferrous and ferric ions respond to ferrous hydroxide, ferric hydroxide, and hydrogen with water to form rust. There have been several experiments to prove the same. When a nail is placed in a container containing air and water, it will rust in no time. But if the nail is placed in water with a layer of oil on the water surface, it blocks the entry of air and prevents rusting. Similarly, if the container with the nail contains calcium chloride and is covered by a lid, it removes the water vapour from the air and avoids rusting.

3. How can Rusting be Prevented?

Preventing the contact of water and air with iron objects can prevent the metal from rusting. Some prevention methods include painting to create a barrier, applying grease or oil as protection or lubrication, and electroplating with chromium. The most effective prevention tool is galvanization which prevents water and air from reacting with the metal substance. The use of calcium chloride cuts out the water vapour in the air and prevents rusting. Oiling can either slow down the process or prevent contact with water or air on the iron.

4. Which product speeds up rusting?

We all know that iron and steel,  in contact with air and water, can cause rusting. But certain elements speed up the process of rusting. For example, dissolving salt in water accelerates the process by lowering the electrical resistance of water. Oxidation means loss of electrons from metal atoms and the faster the flow of electrons from iron to oxygen, the faster will be the rusting. Even corrosive acids increase the conductivity of metals which speeds the rusting process.

5. Does rust spread across the metal?

The presence of oxygen, water and electrolytes leads to the iron being oxidized and hence rusted. It is further accelerated by the presence of salt and acids. But physically speaking, the rusting process does not spread like an infection. The oxidisation takes place independently based on the conditions surrounding the iron. If the iron is kept clean and dry, the protected layer of the iron will either not rust or will do so very slowly as compared to the wet metal.