Theories of Corrosion

Importance

Corrosion is a crucial topic for examinations and knowledge.
Two main theories:
- Dry or Chemical Corrosion
- Wet or Electrochemical Corrosion

Occurs under dry conditions.
Caused by direct attack of chemicals, e.g., atmospheric gases like oxygen, chlorine, hydrogen sulfide.
Known as dry corrosion due to dry conditions.
Involves a direct attack of chemicals on metal surfaces.

Oxidation Corrosion (by Oxygen Gas)
- Direct attack of oxygen gas on metal surfaces under dry conditions.
- Occurs at both low and high temperatures.
- Metals affected:
  - Low Temperature: Alkaline and alkaline earth metals (first and second group elements).
  - High Temperature: All metals except noble metals (silver, gold, platinum).
- Mechanism involves creation of a corrosion interface, leading to oxidation.
Corrosion by Other Gases
Liquid Metal Corrosion

Mechanism:
- Corrosion interfaces are created, allowing metal oxidation.
- Metal loses electrons, reacting with oxygen to form metal oxide (corrosive product).
Types of Metal Oxides Formed:
1. Stable Metal Oxide:
  - Examples: Lead, aluminium, copper, chromium.
2. Unstable Metal Oxide:
  - Dissociates back into ions, metals do not undergo oxidation corrosion.
  - Examples: Silver, gold, platinum.
3. Volatile Metal Oxide:
  - Evaporate in fractions, creating surface heterogeneity.
  - Example: Molybdenum oxide.
4. Porous Metal Oxide:
  - Contains pores, allowing further oxygen penetration, creating heterogeneity.
  - Examples: Alkaline and alkaline earth metals, lanthanides.

Stable Oxides prevent further corrosion.
Unstable/Volatile Oxides may not allow corrosion but can lead to wet corrosion due to created heterogeneity.
Porous Oxides can lead to significant corrosion due to easy oxygen penetration.

Understanding oxidation corrosion is crucial for students.
Wet or electrochemical corrosion is a major threat due to its destructive nature.
Future discussions will cover corrosion by other gases.