This lesson explains how metals corrode, the electrochemical processes involved, and the different methods used to prevent or reduce corrosion.

1. Core Concepts (Short Notes)

21.1 What Is Corrosion?

Corrosion is the gradual destruction of metals due to chemical or electrochemical reactions with the environment.

Most common form: Rusting of iron.

21.2 Conditions Needed for Rusting (Iron)

Rusting requires:

• Oxygen

• Water

Presence of salts or acids accelerates rusting.

21.3 Electrochemical Nature of Corrosion

Rusting is a redox reaction occurring via miniature electrochemical cells on the metal surface.

At the anode (oxidation): Fe → Fe²⁺ + 2e⁻

At the cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻

Formation of rust: Fe²⁺ + OH⁻ → Fe(OH)₂ → Fe₂O₃·xH₂O (rust)

21.4 Factors Increasing Corrosion

• Moisture

• Electrolytes (salt water)

• Acids

• Impurities in metals

• Contact between different metals (galvanic corrosion)

2. Methods of Preventing Corrosion

21.5 Barrier Methods

Prevent oxygen and water from reaching metal.

• Painting

• Greasing and oiling

• Plastic coating

• Enamelling

21.6 Galvanizing

Iron coated with a layer of zinc.

• Zinc acts as a sacrificial anode.

• Even if coating is scratched, zinc corrodes instead of iron.

21.7 Sacrificial Protection

More reactive metal (Zn or Mg) is attached to iron structures.

• Used in pipelines, ship hulls, underground tanks.

• Reactive metal oxidizes → protects iron.

21.8 Cathodic Protection

Iron is made the cathode in an electrical setup.

• Prevents oxidation.

• Common in large storage tanks & pipelines.

21.9 Alloying

Forming alloys with corrosion-resistant metals.

• Stainless steel: iron + chromium + nickel.

• Chromium forms a protective oxide layer.

3. Tips & Tricks for Exams

• Remember: rusting is electrochemical, not just oxidation.

• Moisture + oxygen are essential.

• Zinc protects iron even when scratched.

• Sacrificial protection works only with more reactive metals.

• Stainless steel is rust-resistant due to chromium oxide layer.

• Galvanic corrosion occurs when two different metals contact in an electrolyte.

4. Important Points to Remember

• Rust is hydrated iron(III) oxide.

• Corrosion can be slowed or stopped by controlling exposure to oxygen/water.

• Protective methods rely on isolating metal or making it the cathode.

• Reactivity series helps determine sacrificial metals.

Corrosion has major economic and safety impacts.