Corrosion of Metals

Redox reactions play a significant role in everyday life, one of the most common examples being the rusting of iron. New vehicles and iron objects appear shiny, but over time, they lose their lustre and develop a reddish-brown layer called rust. The chemical formula of rust is Fe₂O₃·H₂O. This process of rusting is a type of corrosion, which gradually damages metals due to oxidation by air, water, or other environmental factors.

• Rusting is the corrosion of iron when it reacts with moisture and oxygen, forming a reddish-brown layer (rust).
• Corrosion is a general process where metals deteriorate due to oxidation caused by air, moisture, or chemicals. It affects various metals, such as iron, copper, and silver.
• Copper reacts with moist air and carbon dioxide, forming a greenish layer of copper carbonate (CuCO₃), reducing its lustre.
• Silver reacts with hydrogen sulphide (H₂S) in the air, forming black silver sulphide (Ag₂S), tarnishing silver objects.
• Aluminium forms a thin protective layer of aluminium oxide (Al₂O₃), preventing further corrosion.
• Corrosion is a type of redox reaction where metals oxidise in the presence of air, water, or chemicals. In the case of rusting, iron loses electrons (oxidation) and reacts with oxygen and moisture, forming rust.

Rusting

1. Aim: To study the conditions necessary for rusting of iron.

2. Requirements: four test tubes, four small iron nails, a rubber cork, boiled water, oil, salt water, and anhydrous calcium chloride.

3. Procedure

• Take four test tubes and place them in a test tube stand.
• In the first test tube, add boiled water and cover it with a layer of oil.
• In the second test tube, add salt water.
• In the third test tube, keep only air.
• In the fourth test tube, place anhydrous calcium chloride and seal it with a rubber cork.
• Put a small iron nail in each test tube and leave them undisturbed for a few days.

To study rusting

4. Observation

• The nail in the second test tube (salt water) rusted the most.
• The nail in the first test tube (boiled water with oil) did not rust.
• The nail in the third test tube (only air) did not rust.
• The nail in the fourth test tube (with anhydrous calcium chloride) remained unchanged.

5. Conclusion: Rusting occurs only when both air and water are present. The presence of salt (electrolytes) accelerates the rusting process. If either air or water is absent, rusting does not take place.

The process of rusting of iron occurs through an electrochemical reaction, where different areas of the iron surface act as an anode and cathode. This is demonstrated in the Iron Nail Rusting Experiment under different conditions.

Step 1: Oxidation at the Anode

At the anode, iron loses electrons and forms Fe²⁺ ions.
Fe (s) → Fe²⁺ (aq) + 2e⁻

Step 2: Reduction at the Cathode

At the cathode, oxygen from the air reacts with hydrogen ions (H⁺) and gains electrons to form water.
O₂ (g) + 4H⁺ (aq) + 4e⁻ → 2H₂O (l)

Step 3: Formation of Rust

The Fe²⁺ ions migrate and react with water, getting further oxidised to form Fe³⁺ ions. These Fe³⁺ ions combine with water to form hydrated iron oxide (rust).
2Fe³⁺ (aq) + 4H₂O (l) → Fe₂O₃·H₂O (s) + 6H⁺ (aq)

Effects of corrosion:

Corrosion is a natural process that damages metals over time, leading to

• Weakened metal structures, affecting buildings, bridges, rail transport, ships, etc.
• Economic losses are due to frequent repairs and replacements.
• Safety hazards, as corroded structures may collapse.

Effects of corrosion