Corrosion of Metals: Rusting of Iron

Objective

State the conditions required for the rusting of iron and steel to form hydrated iron(III) oxide.

What is Corrosion?

Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with their environment. Rusting is a specific type of corrosion that occurs on iron and its alloys, like steel.

Rusting of Iron

Rusting is the process by which iron reacts with oxygen and water to form hydrated iron(III) oxide ($Fe_2O_3 \cdot nH_2O$). This reddish-brown coating is commonly known as rust.

Conditions Required for Rusting

For iron and steel to rust, the following conditions must be present:

• Presence of Water: Water acts as an electrolyte, facilitating the electrochemical reactions involved in rusting.
• Presence of Oxygen: Oxygen is an essential reactant in the oxidation of iron.
• Presence of an Electrolyte: An electrolyte is a substance that conducts electricity. Salt (like sodium chloride - $NaCl$) is a common electrolyte that accelerates rusting. Other electrolytes can include acids and bases.

The Electrochemical Process of Rusting

Rusting is an electrochemical process involving oxidation and reduction reactions. Iron atoms lose electrons (oxidation) and oxygen gains electrons (reduction). This transfer of electrons creates a flow of electric current within the metal, accelerating the process.

Diagram of the Rusting Process

Summary Table

Condition	Role in Rusting
Water	Acts as an electrolyte, facilitating ion movement.
Oxygen	Reacts with iron to form iron(III) oxide.
Electrolyte (e.g., Salt)	Increases the conductivity of water, accelerating the rusting process.