Unit 7.1: Corrosion Resistance

Definition

Corrosion Resistance is the ability of a material to withstand deterioration caused by chemical or electrochemical reactions with the environment.

In simple terms, it is the ability of a material to resist corrosion (rusting or chemical attack).

What is Corrosion?

Corrosion is the gradual destruction of a metal due to chemical reaction with its surroundings such as:

• Oxygen

• Moisture

• Acids

• Salts

• Industrial gases

The most common example is rusting of iron.

Example

When iron is exposed to moisture and oxygen, it forms iron oxide (rust).

Chemical reaction:

$$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

Rust weakens the metal and reduces its life.

Meaning of Corrosion Resistance

A material with high corrosion resistance:

• Does not easily react with the environment

• Maintains its strength and surface

• Has longer service life

Example:

• Stainless steel

• Aluminum

• Titanium

These materials form protective oxide layers that prevent further corrosion.

Factors Affecting Corrosion Resistance

1. Chemical Composition

Alloying elements can increase corrosion resistance.

Examples:

• Chromium

• Nickel

• Molybdenum

Chromium in stainless steel forms a protective oxide layer.

2. Environment

Corrosion rate increases in:

• Humid environments

• Salt water

• Acidic atmosphere

• Industrial pollution

Example:

Coastal areas cause faster corrosion due to salt in air.

3. Surface Condition

Rough or damaged surfaces corrode faster.

Smooth and coated surfaces resist corrosion better.

4. Temperature

Higher temperatures accelerate chemical reactions.

Therefore corrosion increases at higher temperatures.

Methods to Improve Corrosion Resistance

1. Protective Coatings

Applying coatings to prevent contact with environment.

Examples:

• Painting

• Varnishing

• Plastic coating

2. Galvanizing

Coating iron or steel with zinc.

Zinc protects the metal from rusting.

Example:

• GI sheets

• Water pipes

3. Alloying

Adding elements to improve resistance.

Example:

• Stainless steel (iron + chromium + nickel)

4. Cathodic Protection

Metal is connected to a more reactive metal which corrodes first.

Example:

• Magnesium blocks used in pipelines and ships.

Engineering Importance

Corrosion resistance is important in many industries.

Applications:

• Bridges

• Pipelines

• Ships

• Automobiles

• Chemical plants

• Boilers

• Storage tanks

Good corrosion resistance reduces maintenance cost and increases service life.

Common Corrosion Resistant Materials

Exam-Focused Points

Very important for JE / AE mechanical exams:

• Corrosion = deterioration of metal due to chemical reaction.

• Rusting of iron is the most common example.

• Corrosion resistance means ability to resist corrosion.

• Stainless steel resists corrosion due to chromium oxide layer.

• Galvanizing uses zinc coating.

• Cathodic protection uses sacrificial anode.

Example Competitive Exam Questions

Question: What is corrosion resistance?
Answer: The ability of a material to resist deterioration caused by chemical or electrochemical reactions with the environment.

Question: Why does stainless steel resist corrosion?
Answer: Because chromium forms a protective oxide layer on the surface.

Question: What metal is used in galvanizing?
Answer: Zinc.

Question: Give one example of corrosion.
Answer: Rusting of iron.

Quick Revision Summary

• Corrosion = chemical deterioration of metals.

• Rusting of iron is a common example.

• Corrosion resistance = ability to resist corrosion.

• Stainless steel resists corrosion due to chromium.

• Protection methods include:

  • Coatings

  • Galvanizing

  • Alloying

  • Cathodic protection.