Corrosion Mechanism Chapter 1

Corrosion Mechanism

• Corrosion requires four basic elements: anode, cathode, path, and electron flow
• The anode is where oxidation occurs, and the cathode is where reduction occurs

Corrosion of Steel

• The corrosion mechanism for carbon steel involves two half-reactions:
  • Anode: Fe → Fe2+ + 2e
  • Cathode: O2 + 2H2O + 4e → 2OH-
  • Overall reaction: Fe2+ + 2OH- → Fe(OH)2 (insoluble hydroxide, white-green)

Rust Formation

• Fe(OH)2 can further react to form different types of rust:
  • Fe(OH)3 (black magnetite)
  • γ-Fe3O4 (brown rust)
  • γ(FeOOH) (yellow rust)
  • Fe2O3 (reddish-brown rust)

Controlling Corrosion

• Methods to control corrosion include:
  • Material selection and design
  • Corrosion inhibitors and monitoring
  • Coatings and linings
  • Cathodic protection

Damage Mechanisms

• Damage mechanisms are processes that result in asset degradation, including corrosion
• Some damage mechanisms occur before or after an asset is put into service
• Examples of damage mechanisms include:
  • Hydrogen attack, including:
    • High-temperature hydrogen attack (HTHA)
    • Hydrogen embrittlement (HE)
    • Hydrogen blistering
    • Hydrogen-induced cracking (HIC)
    • Stress-oriented hydrogen-induced cracking (SOHIC)