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पाठ्यक्रम

Prevention of Corrosion

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  • Key Points: Prevention of Corrosion
Maharashtra State Board: Class 10

Coating with a Protective Layer

Corrosion is a natural process that deteriorates metals due to environmental factors like moisture, oxygen, and chemicals. To protect metals, especially iron, various methods are used to prevent rusting and extend the lifespan of metal objects. The rate of corrosion can be reduced by isolating metals from direct contact with air and moisture.

A layer of paint, oil, grease, varnish, or enamel is applied to block air and moisture from reaching the metal surface.

  1. Painting and Varnishing: Metals like iron gates, railings, and furniture are coated with paint or varnish to prevent rust. However, if the paint gets scratched, rusting starts underneath.
  2. Greasing and Oiling: It is used for moving parts of machines, gears, and bicycle chains to prevent rusting by reducing moisture contact.
  3. Enamelling: A glassy layer fused onto metals to provide long-term corrosion resistance.

Limitations:

  • If the coating gets scratched or damaged, rusting can begin underneath.
  • It requires regular maintenance and reapplication over time.
Maharashtra State Board: Class 10

Metal Plating (Galvanisation, Tinning, Electroplating)

A non-corrodible metal is coated onto corrodible metals like iron or steel to prevent corrosion.

1. Galvanisation: A thin layer of zinc is applied to iron or steel to prevent rusting. Examples include shiny iron nails and pins. Zinc corrodes before iron because it is more electropositive, but once the zinc layer wears off, iron starts rusting.

Galvanising Process

2. Tinning: Molten tin is coated on brass or copper utensils (also called kalhaee) to prevent corrosion. This protects against the greenish poisonous layer that forms on copper and brass, especially when in contact with acidic foods like buttermilk or curry.

3. Electroplating: A less reactive metal (silver, gold, or chromium) is deposited onto a more reactive metal using electrolysis. Examples include silver-plated spoons, gold-plated jewellery, and chromium-plated taps.

Electroplating

Limitations:

  • Over time, the protective layer wears off, exposing the metal to corrosion.
  • The plating process can be expensive.
Maharashtra State Board: Class 10

Anodisation (Formation of Oxide Layer)

In anodisation, metals like aluminium and copper are coated with a thin and strong oxide layer using electrolysis.

  • The oxide layer is uniform, strong, and durable, preventing further oxidation.
  • Protection can be increased by making the oxide layer thicker.

Example: Anodised aluminium cookware forms a protective Al₂O₃ layer, preventing oxidation and corrosion.

Limitations: Only applicable to certain metals like aluminium and copper.

Anodization

Maharashtra State Board: Class 10

Alloying (Corrosion-Resistant Alloys)

Metals are mixed with other metals or nonmetals to create alloys, which are stronger and more resistant to corrosion.

  • It provides permanent protection against corrosion.
  • Improves the strength and durability of metals.

Examples,

  1. Bronze (90% copper + 10% tin): Used for statues and sculptures, resistant to sun and rain.
  2. Stainless Steel (74% iron + 18% chromium + 8% carbon): Does not rust or stain, making it ideal for utensils, construction materials, and surgical instruments.
  3. Coins made from alloys: Modern coins are made from corrosion-resistant alloys to prevent wear and tear.

Limitations: Expensive and difficult to separate metals once alloyed.

Coins made from various alloys

CBSE: Class 10
Maharashtra State Board: Class 10

Key points: Prevention of Corrosion

  • Alloys are homogeneous mixtures of metals (or metals with non-metals) with different properties than pure metals—often harder, less conductive, and with lower melting points.
  • 24-carat gold is soft, so 22-carat gold (an alloy with copper or silver) is used in jewellery to improve strength.
  • Solder, an alloy with a low melting point, is used to join electrical wires.
  • Corrosion prevention methods include painting, galvanising, anodising, electroplating, and alloying.
  • Galvanising, tinning, anodisation, and electroplating all involve protective coatings to prevent corrosion, while alloying enhances durability and resistance.
 
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