Corrosion (संक्षारण) is the gradual destruction of metals due to chemical reactions with the environment, such as oxygen, moisture, acids, or salts. The most common example is the rusting of iron. According to NCERT, corrosion causes damage to bridges, vehicles, pipelines, machines, and household items. Therefore, preventing corrosion is important to increase the life of metals and reduce economic losses.
What is Corrosion?
Corrosion is the process in which a metal reacts with substances present in the environment and gets converted into undesirable compounds.
Example:
Iron reacts with oxygen and water to form rust.
Rust = Hydrated Iron(III) Oxide
Why is Corrosion Prevention Important?
- Increases the lifespan of metal objects.
- Reduces maintenance costs.
- Prevents accidents caused by weakened structures.
- Protects industrial equipment.
- Saves natural resources and energy.
Main Methods to Prevent Corrosion
1. Painting
Painting creates a protective layer over the metal surface that prevents contact with air and moisture.
Examples:
- Iron gates
- Bridges
- Railings
- Water tanks
Advantages:
- Simple and economical.
- Easy to apply.
- Available in different colors.
2. Oiling and Greasing
Oil or grease forms a protective barrier that prevents air and water from reaching the metal surface.
Common Uses:
- Bicycle chains
- Machine parts
- Tools
- Vehicle components
Advantages:
- Easy to apply.
- Effective for moving parts.
- Reduces friction as well.
3. Galvanization
Galvanization is the process of coating iron or steel with a layer of zinc.
The zinc layer protects the iron from rusting.
Uses:
- Water pipes
- Iron sheets
- Buckets
- Fencing wires
Why Zinc?
Even if the coating is scratched, zinc protects iron because zinc is more reactive than iron.
4. Electroplating
Electroplating involves depositing a thin layer of another metal on the surface of a metal object using electricity.
Examples:
| Base Metal | Coating Metal |
|---|---|
| Iron | Chromium |
| Iron | Nickel |
| Copper | Silver |
| Brass | Gold |
Advantages:
- Prevents corrosion.
- Improves appearance.
- Increases durability.
Examples in Daily Life:
- Bathroom taps
- Bicycle handles
- Jewelry
5. Alloying
An alloy is a mixture of a metal with one or more elements.
Many alloys are more corrosion-resistant than pure metals.
Examples:
| Alloy | Components | Corrosion Resistance |
|---|---|---|
| Stainless Steel | Iron + Chromium + Nickel | Very High |
| Brass | Copper + Zinc | High |
| Bronze | Copper + Tin | High |
Example:
Stainless steel utensils do not rust easily because chromium forms a protective layer.
6. Plastic Coating
A plastic layer is applied over metal surfaces to prevent contact with moisture and oxygen.
Examples:
- Wire insulation
- Metal furniture
- Storage racks
Advantages:
- Water resistant.
- Attractive appearance.
- Durable protection.
7. Tinning
Tinning is the process of coating iron or steel with a thin layer of tin.
Examples:
- Food cans
- Storage containers
Advantages:
- Prevents rusting.
- Safe for food storage.
8. Cathodic Protection
In this method, a more reactive metal is attached to the metal that needs protection.
The reactive metal corrodes first and protects the main metal.
Common Metals Used:
- Zinc
- Magnesium
Applications:
- Underground pipelines
- Ship hulls
- Water heaters
Comparison of Corrosion Prevention Methods
| Method | Protective Material | Common Uses |
|---|---|---|
| Painting | Paint | Gates, bridges |
| Oiling/Greasing | Oil or grease | Machines, tools |
| Galvanization | Zinc | Iron sheets, pipes |
| Electroplating | Chromium, nickel, silver | Taps, jewelry |
| Alloying | Chromium, nickel, zinc | Stainless steel products |
| Plastic Coating | Plastic | Wires, furniture |
| Tinning | Tin | Food cans |
| Cathodic Protection | Zinc, magnesium | Pipelines, ships |
NCERT Key Points
- Corrosion damages metals through chemical reactions with the environment.
- Rusting is a common form of corrosion in iron.
- Moisture and oxygen are essential for rust formation.
- Galvanization is one of the most common methods to prevent rusting of iron.
- Stainless steel is corrosion-resistant due to alloying.
- Protective coatings prevent direct contact between metal and the environment.
Quick Revision Notes
- Corrosion = Slow destruction of metals.
- Rusting is corrosion of iron.
- Painting blocks air and moisture.
- Oiling and greasing protect machine parts.
- Galvanization uses zinc coating.
- Electroplating deposits a protective metal layer.
- Alloying improves corrosion resistance.
- Tinning uses a tin coating.
- Cathodic protection uses sacrificial metals.
Practice Questions
Very Short Answer Questions
Q1. What is corrosion?
Answer: Corrosion is the gradual destruction of a metal due to chemical reactions with its surroundings.
Q2. What is rust?
Answer: Rust is hydrated iron(III) oxide formed on iron in the presence of oxygen and moisture.
Q3. Which metal is used in galvanization?
Answer: Zinc.
Q4. What is alloying?
Answer: Alloying is the process of mixing a metal with other elements to improve its properties.
Q5. Name one corrosion-resistant alloy.
Answer: Stainless steel.
Short Answer Questions
Q1. Why is painting used to prevent corrosion?
Answer: Painting forms a protective layer that prevents oxygen and moisture from reaching the metal surface.
Q2. How does galvanization protect iron?
Answer: A zinc coating prevents air and moisture from contacting iron and also protects it if scratched.
Q3. What is electroplating?
Answer: Electroplating is the deposition of a thin layer of another metal on a metal surface using electric current.
Fill in the Blanks
- Rusting occurs in the presence of ______ and ______.
Answer: oxygen, water - Galvanization involves coating iron with ______.
Answer: zinc - Stainless steel is an example of an ______.
Answer: alloy - Food cans are often protected by ______.
Answer: tinning - Oiling and greasing prevent contact with ______ and moisture.
Answer: air
Multiple Choice Questions (MCQs)
1. Rusting is a form of:
A) Melting
B) Corrosion
C) Distillation
D) Condensation
Answer: B) Corrosion
2. Which metal is used in galvanization?
A) Copper
B) Silver
C) Zinc
D) Tin
Answer: C) Zinc
3. Which method uses a thin layer of another metal?
A) Alloying
B) Painting
C) Electroplating
D) Oiling
Answer: C) Electroplating
4. Stainless steel is:
A) Pure iron
B) Non-metal
C) Alloy
D) Compound
Answer: C) Alloy
5. Which is commonly used on machine parts?
A) Painting
B) Oiling and greasing
C) Tinning
D) Alloying
Answer: B) Oiling and greasing
6. Which coating is used for food cans?
A) Zinc
B) Chromium
C) Tin
D) Nickel
Answer: C) Tin
7. Corrosion mainly occurs due to:
A) Light only
B) Sound only
C) Chemical reactions with surroundings
D) Magnetism
Answer: C) Chemical reactions with surroundings
8. Which alloy is highly resistant to corrosion?
A) Iron
B) Aluminium
C) Stainless Steel
D) Lead
Answer: C) Stainless Steel
9. Cathodic protection commonly uses:
A) Gold
B) Silver
C) Zinc or Magnesium
D) Copper
Answer: C) Zinc or Magnesium
10. Which of the following acts as a barrier against moisture?
A) Painting
B) Plastic coating
C) Oiling
D) All of these
Answer: D) All of these
Leave a Reply