17.6 Corrosion

Corrosion is a sneaky destroyer of metals, eating away at structures and objects we rely on daily. It's an electrochemical process where metals lose electrons, like iron turning to rust. Understanding this helps us protect our stuff from falling apart.

We've got tricks to fight corrosion, like painting metals, mixing them with other elements, or using sacrificial anodes. Each method has its strengths and weaknesses, depending on what we're trying to protect and where it's located.

Corrosion and Its Prevention

Electrochemical process of corrosion

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• Involves the oxidation of a metal, which is the loss of electrons from a substance
• Common example is iron rusting, where iron ($Fe$) is oxidized to iron(II) ions ($Fe^{2+}$) at the anode: $Fe \rightarrow Fe^{2+} + 2e^-$
  • Oxygen ($O_2$) is reduced at the cathode: $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$
  • Overall reaction: $2Fe + O_2 + 2H_2O \rightarrow 2Fe(OH)_2$
  • $Fe(OH)_2$ further oxidized to form hydrated iron(III) oxide ($Fe_2O_3 \cdot xH_2O$), known as rust
• Requires four components: anode (site of oxidation where metal ions are released), cathode (site of reduction where electrons are consumed), electrolyte (solution that allows ion movement, such as water with dissolved salts), and electrical connection (allows electrons to flow from anode to cathode)

Methods of corrosion prevention

• Protective coatings act as a barrier between the metal and the environment
  • Examples include paint, plastic, and ceramic coatings
  • Prevent exposure to oxygen and moisture, slowing or preventing corrosion
• Alloying involves mixing a metal with other elements to improve its corrosion resistance
  • Examples include stainless steel (iron alloyed with chromium and nickel) and brass (copper alloyed with zinc)
  • Alloying elements can form protective oxide layers or change the metal's reactivity
• Cathodic protection connects the metal to be protected (cathode) to a more easily corroded metal (anode)
  • Two types: sacrificial anode (more reactive metal like zinc or magnesium connected to metal to be protected) and impressed current (external power source supplies electrons to metal to be protected, making it a cathode)
  • More reactive metal corrodes preferentially, protecting the metal of interest
  • Selection of sacrificial anodes often based on the galvanic series, which ranks metals by their electrochemical potential

Effectiveness of prevention techniques

• Protective coatings suitable for a wide range of applications, from small objects to large structures
  • Effectiveness depends on coating material, application method, and environment
  • Require regular maintenance and reapplication to maintain protection
• Alloying effective for applications requiring intrinsic corrosion resistance and suitable for harsh environments where coatings may fail
  • May be more expensive than using pure metals
  • Alloying elements can affect other properties, such as strength and ductility
• Cathodic protection effective for large structures (pipelines, storage tanks, ship hulls)
  • Sacrificial anodes are simple and require no external power, but need periodic replacement
  • Impressed current systems more complex and require constant power supply, but offer adjustable protection
  • Less effective in low-conductivity environments (freshwater, dry soil)

Types of Corrosion

• Uniform corrosion: Even degradation across the metal surface
• Localized corrosion: Concentrated in specific areas
  • Pitting: Formation of small holes or cavities in the metal surface
  • Stress corrosion cracking: Combination of tensile stress and corrosive environment leading to cracks
• Passivation: Formation of a thin protective layer on some metals, reducing further corrosion