Corrosion – Types, Prevention, and Case Studies-GRK

 

Corrosion – Types, Prevention, and Case Studies

1. Introduction to Corrosion

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

👉 In simple terms:
Corrosion = metal + environment → damage (rust, decay, weakening)

Examples

  • Rusting of iron
  • Tarnishing of silver
  • Green coating on copper (patina)

2. Importance of Studying Corrosion

  • Reduces strength and life of materials
  • Causes economic loss (billions annually)
  • Leads to accidents and failures (bridges, pipelines)
  • Affects appearance and performance

3. Types of Corrosion

Corrosion is mainly classified into:

A. Dry Corrosion (Chemical Corrosion)

  • Occurs without moisture
  • Direct reaction between metal and gases

Example

  • Oxidation of iron at high temperature:

    4Fe+3O2→2Fe2O34Fe + 3O_2 → 2Fe_2O_3

Characteristics

  • Occurs in high-temperature environments
  • Forms oxide layer

B. Wet Corrosion (Electrochemical Corrosion)

  • Occurs in presence of moisture/electrolyte
  • Most common type

Example: Rusting of Iron

Electrochemical reactions:

  • Anode:

    Fe→Fe2++2e−Fe → Fe^{2+} + 2e^-

  • Cathode:

    O2+2H2O+4e−→4OH−O_2 + 2H_2O + 4e^- → 4OH^-
  • Final product:
    Rust (Fe₂O₃·xH₂O)

4. Forms of Corrosion

1. Uniform Corrosion

  • Occurs evenly over the surface
  • Example: Rusting of steel

2. Galvanic Corrosion

  • Occurs when two different metals are connected in an electrolyte
  • More reactive metal corrodes

👉 Example: Iron + Copper in water

3. Pitting Corrosion

  • Localized corrosion forming small pits or holes
  • Dangerous because it is hard to detect

4. Crevice Corrosion

  • Occurs in confined spaces (joints, bolts)

5. Intergranular Corrosion

  • Occurs along grain boundaries of metals

6. Stress Corrosion Cracking (SCC)

  • Combination of stress + corrosive environment
  • Leads to sudden failure

7. Erosion Corrosion

  • Caused by fluid flow + corrosion
  • Example: Pipelines

5. Factors Affecting Corrosion

A. Environmental Factors

  • Moisture (humidity)
  • Temperature
  • Oxygen concentration
  • pH value
  • Presence of salts

B. Material Factors

  • Purity of metal
  • Surface condition
  • Alloy composition
  • Mechanical stress

6. Methods of Corrosion Prevention

1. Material Selection

  • Use corrosion-resistant materials
    Example: Stainless steel, aluminum

2. Protective Coatings

a) Metallic Coatings

  • Galvanizing (zinc coating)
  • Electroplating

b) Non-Metallic Coatings

  • Paints
  • Enamels
  • Plastics

3. Cathodic Protection

Metal is made cathode → prevents corrosion

Types

  • Sacrificial anode method (Zn, Mg used)
  • Impressed current method

4. Anodic Protection

  • Metal is made anode and controlled

5. Corrosion Inhibitors

  • Chemicals added to reduce corrosion
  • Example: Chromates, phosphates

6. Proper Design

  • Avoid water accumulation
  • Avoid sharp corners and crevices
  • Ensure drainage

7. Environmental Control

  • Reduce humidity
  • Remove oxygen
  • Control temperature

7. Case Studies of Corrosion

Case Study 1: Silver Bridge Collapse

Cause

  • Stress corrosion cracking in steel eyebar

Result

  • Bridge collapsed → 46 deaths

Lesson

  • Importance of inspection and corrosion control

Case Study 2: Aloha Airlines Flight 243 incident

Cause

  • Corrosion fatigue due to salty environment

Result

  • Aircraft roof failure mid-flight

Lesson

  • Corrosion in aircraft structures must be monitored

Case Study 3: Flint Water Crisis

Cause

  • Corrosion in pipes released lead into water

Result

  • Health crisis

Lesson

  • Importance of corrosion inhibitors in water systems

Case Study 4: Oil & Gas Pipelines

Cause

  • Internal corrosion due to moisture and gases

Result

  • Leakage, explosions

Prevention

  • Cathodic protection
  • Coatings
  • Regular inspection

8. Advantages of Controlling Corrosion

  • Increases life of structures
  • Improves safety
  • Reduces maintenance cost
  • Enhances efficiency

9. Disadvantages of Corrosion

  • Material failure
  • Economic loss
  • Environmental pollution
  • Safety hazards

10. Conclusion

Corrosion is a major engineering problem affecting industries such as:

  • Construction
  • Automotive
  • Aerospace
  • Marine

👉 Proper understanding of:

  • Types
  • Causes
  • Prevention methods

is essential to ensure durability, safety, and cost efficiency.

11. Viva Questions (Important)

  1. What is corrosion?
  2. Difference between dry and wet corrosion
  3. What is galvanic corrosion?
  4. Define cathodic protection
  5. What are corrosion inhibitors?
  6. What is pitting corrosion?
  7. Causes of rusting of iron
  8. Methods to prevent corrosion
  9. Explain stress corrosion cracking
  10. Give real-life examples of corrosion

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