Notes on Corrosion and Its Control

CHAPTER 7: CORROSION AND ITS CONTROL

  • It is preferable to control corrosion rather than prevent it, as complete elimination of corrosion is impractical.
INTRODUCTION
  • Most metals, except noble metals (Au, Pt), exist in combined forms like oxides, carbonates, etc.
  • Metals in their pure form from extraction processes are in an excited, high-energy state.
  • When exposed to the environment (gases, moisture, liquids), metals tend to revert to a more stable state through corrosion.
  • Corrosion: The deterioration and loss of metallic materials due to unwanted chemical or electrochemical reactions that begin at the metal's surface.
    • Example: Rusting of iron forming Fe3O4, or green film of copper carbonate on exposed copper.
  • Financial impact of corrosion estimated at 2-2.5 billion dollars annually worldwide.
DRY OR CHEMICAL CORROSION

  • Occurs through direct chemical action with environmental gases (e.g., O₂, H₂S) on metal surfaces.

  • Types of Dry Corrosion:

    1. Oxidation Corrosion:
      • Occurs due to direct oxygen interaction, especially at high temperatures.
      • General reaction:
        2M + O₂
        ightarrow 2M^{+} + 2e^{-}
      • Metal oxidizes, forming a protective oxide scale or allowing further oxidation.
      • Stability of the oxide layer is crucial:
        • Stable: Adheres well, prevents further oxidation (e.g., Al, Sn, Pb).
        • Unstable: Decomposes, prevents oxidation (e.g., Au, Ag, Pt).
        • Volatile: Leaves surface exposed, leading to rapid corrosion (e.g., MoO3).
        • Porous: Allows access of oxygen to underlying metal, causing continuous corrosion.

  • Pilling-Bedworth Rule: Indicates whether oxide layer is protective or porous based on volume considerations.

  • Corrosion can also occur through gases like SO₂ and Cl₂, influenced by the degree of protective films formed.

  • Liquid Metal Corrosion: Occurs at high temperatures via flowing liquid metal, which can penetrate or dissolve the solid metal.

WET OR ELECTROCHEMICAL CORROSION
  • Takes place when conductive liquids contact metals, with separate anodic and cathodic areas.
  • Anodic: Oxidation/metal dissolution occurs.
  • Cathodic: Reduction takes place (e.g., formation of OH⁻ or H₂ gas).
    • Net result: Electrolytic movement and product formation.
MECHANISM OF WET OR ELECTROCHEMICAL CORROSION
  • Involves electron current flow and reactions at anodic and cathodic areas.
    • Anodic reaction:
      M
      ightarrow M^{n+} + ne^{-} (oxidation)
    • Cathodic reaction:
      O₂ + 2 H₂O + 4 e^{-}
      ightarrow 4 OH^{-} (reduction)
    • The overall process leads to corrosion at anode and accumulation of rust near cathode.
GALVANIC (OR BIMETALLIC) CORROSION
  • Occurs when dissimilar metals are electrically connected in an electrolyte, leading to corrosion of the more anodic metal.
  • Example: Zinc dissolving while copper is protected.
CONCENTRATION CELL CORROSION
  • Arises due to heterogeneous distribution of ion concentrations or aerial differences.
    • Usually occurs in partially submerged metals due to differential aeration.
  • Example: Zinc corrosion occurring more in submerged areas with less oxygen.
PITTING CORROSION
  • Characterized by localized corrosion creating pits, often due to breakdown of protective films.
    • Initiated by surface imperfections.
STRESS CORROSION
  • Involves tensile stress in a corrosive environment, leading to cracks under otherwise negligible general corrosion.
    • Specific corrosive agents impact different metals (e.g., ammonia on brass).
FACTORS INFLUENCING CORROSION
  • Temperature: Higher temperatures increase reaction rates, promoting corrosion.
  • pH Levels: Acidity (pH < 7) tends to be more corrosive; amphoteric metals behave differently in alkali solutions.
  • Oxygen Concentration: Variances create concentration cells aiding corrosion.
METHODS OF CORROSION CONTROL

  1. Cathodic Protection: Uses sacrificial anodes to protect structures from corrosion.

    • Sacrificial Anode Method: Connect a more active metal to absorb corrosion.
    • Impressed Current Method: An external electric current is applied to negate corrosion currents.

  2. Protective Coatings: Shield surfaces from corrosive environments using anodic (sacrificial) or cathodic coatings.

  3. Application of Paints: Utilization of paints to create protective layers - comprising pigments, vehicles, and thinners with specific drying characteristics.

APPLICATIONS OF PROTECTIVE COATINGS
  • Galvanizing: Coating with zinc to protect steel.
  • Tinning: Coating with tin for non-toxic applications.
IMPORTANCE OF UNDERSTANDING CORROSION
  • Engineers must grasp corrosion mechanisms to minimize its adverse effects, directly impacting safety and financial resources.