Corrosion

📚 Key Concepts

🔹 Real-Life Example

That reddish-brown coating on old iron nails is rust – iron’s reaction with oxygen and moisture in air. Corrosion costs billions of dollars annually in infrastructure damage. But notice how aluminum cans don’t rust like iron – aluminum forms a protective oxide layer that prevents further corrosion!

Corrosion: The gradual destruction of metals by chemical reaction with their environment, especially oxygen and moisture.

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🧪 Rusting of Iron

🔸 Conditions Required

  1. Oxygen (from air)
  2. Water/Moisture
  3. Both together (neither alone causes rusting)

🔸 Chemical Process

  • Step 1: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
  • Step 2: Fe(OH)₃ → Fe₂O₃·H₂O (hydrated iron oxide – rust)
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🔸 Why Rusting is Harmful

  • Makes iron weak and brittle
  • Causes structural damage
  • Economic losses (bridges, buildings, ships)

🔍 Prevention Methods

🔸 Physical Barriers

  1. Painting: Prevents air and moisture contact
  2. Oiling/Greasing: Creates waterproof layer
  3. Plastic coating: Complete isolation

🔸 Galvanization

Process: Coating iron with zinc

Why it works:

  • Zinc is more reactive than iron
  • Zinc corrodes first, protecting iron
  • Even if coating breaks, zinc still protects iron

Uses: Galvanized iron sheets, buckets, pipes

🔸 Alloying

Stainless Steel: Iron + Chromium + Nickel

  • Properties: Rust-resistant, strong, shiny
  • Uses: Kitchen utensils, surgical instruments, architecture