Corrosion and Its Prevention

1. Introduction

Corrosion is the gradual deterioration of a material (usually metals) due to chemical or electrochemical reactions with its environment.

• Common example: Rusting of iron
• It leads to loss of strength, appearance, and functionality

2. Types of Corrosion

A. Dry Corrosion (Chemical Corrosion)

4

• Occurs due to direct chemical reaction between metal and gases
• No electrolyte required
• Common at high temperatures

Example:
Oxidation of metals in furnaces

B. Wet Corrosion (Electrochemical Corrosion)

4

• Occurs in presence of moisture (electrolyte)
• Involves formation of anode and cathode regions

Example: Rusting of iron

3. Mechanism of Rusting of Iron

4

Electrochemical Reactions:

At Anode:$$Fe \rightarrow Fe^{2+} + 2e^-$$Fe→Fe2++2e−

At Cathode:$$O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$$O2​+2H2​O+4e−→4OH−

Final Product:$$Fe(OH)_2 \rightarrow Fe_2O_3 \cdot xH_2O \; (Rust)$$Fe(OH)2​→Fe2​O3​⋅xH2​O(Rust)

4. Types of Electrochemical Corrosion

1. Galvanic Corrosion

4

• Occurs when two dissimilar metals are in contact
• One acts as anode (corrodes)

2. Concentration Cell Corrosion

4

• Due to difference in concentration of electrolyte or oxygen

3. Pitting Corrosion

4

• Localized corrosion forming small pits or holes
• Very dangerous and hard to detect

4. Stress Corrosion Cracking (SCC)

4

• Occurs due to combined effect of stress and corrosive environment

5. Factors Affecting Corrosion

• Nature of metal (purity, composition)
• Environmental conditions (moisture, temperature)
• pH value
• Presence of impurities
• Oxygen availability

6. Prevention of Corrosion

A. Protective Coatings

1. Painting and Varnishing

4

• Acts as a barrier between metal and environment

2. Galvanizing

4

• Coating of zinc on iron
• Zinc acts as sacrificial metal

3. Electroplating

4

• Thin layer of another metal deposited using electricity

B. Cathodic Protection

1. Sacrificial Anode Method

4

• More reactive metal corrodes instead of base metal

2. Impressed Current Method

4

• External power source is used to prevent corrosion

C. Material Selection

• Use corrosion-resistant materials (e.g., stainless steel)

D. Environmental Control

• Reduce moisture and oxygen
• Use inhibitors

7. Effects of Corrosion

• Loss of material strength
• Economic losses
• Structural failure
• Safety hazards

8. Advantages of Corrosion Control

• Increases life of components
• Reduces maintenance cost
• Improves reliability and safety

---