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• Corrosion


  Destructive and unintentional attack on a metal, which is electrochemical and ordinarily begins at the surface
• Corrosion has significant economic impact, with approximately 5% of an industrialized nation's income spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions
• Consequences of corrosion


  • Rusting of automotive body panels and radiator and exhaust components
• Oxidation reaction


  Chemical reaction in which there is transfer of electrons from one chemical species to another, where metal atoms lose or give up electrons
• Reduction reaction


  Chemical reaction in which electrons generated from oxidation are transferred to and become a part of another chemical species
• Anode


  Site at which oxidation takes place
• Cathode


  Location at which reduction occurs
• Electrochemical reaction


  Consists of at least one oxidation and one reduction reaction, where the total rate of oxidation must equal the total rate of reduction
• Oxidation of zinc in an acid solution

  • Zn → Zn2+ + 2e-
  • 2H+ + 2e- → H2 (gas)
  • Zn + 2H+ → Zn2+ + H2 (gas)
• Oxidation (rusting) of iron in water containing dissolved oxygen


  • Fe + 1/2O2 + H2O → Fe2+ + 2OH- → Fe(OH)2
  • 2Fe(OH)2 + 1/2O2 + H2O → 2Fe(OH)3
• Galvanic couple


  Two metals electrically connected in a liquid electrolyte, where one metal becomes an anode and corrodes while the other acts as a cathode
• Standard half-cell


  A pure metal electrode immersed in a 1 M solution of its ions at 25°C
• Standard hydrogen electrode


  Reference cell consisting of an inert platinum electrode in a 1 M solution of H+ ions saturated with hydrogen gas at 1 atm and 25°C
• Measured cell voltages represent differences in electrical potential, with the standard hydrogen electrode arbitrarily chosen as the reference point
• With knowledge of the types and an understanding of the mechanisms and causes of corrosion and degradation, it is possible to take measures to prevent them from occurring
• Measures to prevent corrosion and degradation

  • Change the nature of the environment
  • Select a material that is relatively nonreactive
  • Protect the material from appreciable deterioration
• Ceramic materials are relatively resistant to deterioration, which usually occurs at elevated temperatures or in rather extreme environments
• Degradation of polymeric materials

  May occur through dissolution when exposed to a liquid solvent, or by absorption of the solvent and swelling; electromagnetic radiation (primarily ultraviolet) and heat may also cause alterations in their molecular structures
• Standard half-cell


  A pure metal electrode immersed in a 1 M solution of its ions and at 25°C (77°F)
• Electromotive force (emf) series

  A ranking of standard half-cells according to measured voltage, used to determine corrosion tendencies of metals
• The standard hydrogen electrode is the reference cell in the emf series
• Standard electrode potentials


  • Au3+ + 3e- → Au (+1.420 V)
  • O2 + 4H+ + 4e- → 2H2O (+1.229 V)
  • Pt2+ + 2e- → Pt (~+1.2 V)
  • Ag+ + e- → Ag (+0.800 V)
  • Fe3+ + e- → Fe2+ (+0.771 V)
  • O2 + 2H2O + 4e- → 4(OH-) (+0.401 V)
  • Cu2+ + 2e- → Cu (+0.340 V)
  • 2H+ + 2e- → H2 (0.000 V)
  • Pb2+ + 2e- → Pb (-0.126 V)
  • Sn2+ + 2e- → Sn (-0.136 V)
  • Ni2+ + 2e- → Ni (-0.250 V)
  • Co2+ + 2e- → Co (-0.277 V)
  • Cd2+ + 2e- → Cd (-0.403 V)
  • Fe2+ + 2e- → Fe (-0.440 V)
  • Cr3+ + 3e- → Cr (-0.744 V)
  • Zn2+ + 2e- → Zn (-0.763 V)
  • Al3+ + 3e- → Al (-1.662 V)
  • Mg2+ + 2e- → Mg (-2.363 V)
  • Na+ + e- → Na (-2.714 V)
  • K+ + e- → K (-2.924 V)
• Oxidation


  Reaction where a metal loses electrons and is converted to a positively charged ion
• Reduction


  Reaction where a metal gains electrons and is converted from a positively charged ion to the metallic state
• Electrochemical cell potential calculation

  1. Oxidation of metal M1

  2. Reduction of metal M2

  3. Overall cell potential ΔV0 = V02 - V01
• Nernst equation


  Equation that relates electrochemical cell potential to solution concentrations and temperature
• The galvanic series ranks the relative reactivities of metals and alloys in seawater
• Most metals and alloys are subject to oxidation or corrosion to some degree
• Gold and platinum are noble metals that may exist in nature in the metallic state
• Corrosion penetration rate (CPR)

  Thickness loss of material per unit of time, calculated using weight loss, density, area, and time
• Corrosion rate


  Expressed in terms of current density and the number of electrons involved in the ionization reaction
• Polarization


  Displacement of an electrode potential from its equilibrium value due to a non-equilibrium condition
• Overvoltage


  The magnitude of the polarization, expressed in volts or millivolts relative to the equilibrium potential
• Corrosion rate


  Expression relating corrosion rate and current density
• Polarization


  Displacement of each electrode potential from its equilibrium value
• Overvoltage (η)


  Magnitude of the displacement of each electrode potential from its equilibrium value, expressed in terms of plus or minus volts (or millivolts) relative to the equilibrium potential
• Activation polarization


  • Reaction rate is controlled by the one step in the series that occurs at the slowest rate
  • An activation energy barrier is associated with this slowest, rate-limiting step
• Exchange current density (i0)

  Current density from the equilibrium between oxidation and reduction reactions, determined experimentally and varies from system to system
• Concentration polarization


  • Reaction rate is limited by diffusion in the solution
  • A depletion zone may be formed in the vicinity of the interface because the reactants are not replenished at a rate sufficient to keep up with the reaction
• Concentration polarization is not normally rate controlling for oxidation reactions